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\begin{document}

\title{Emerging patterns of regulatory T cell function in tuberculosis}



\author[1]{Asma Ahmed}%
\author[1]{Annapurna Vyakarnam}%
\affil[1]{Indian Institute of Science}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
Tuberculosis (TB) is one of the top 10 causes of mortality worldwide
from a single infectious agent and has significant implications for
global health. In 2018, 1.5 million people died from TB worldwide and
440,000 of those were from India. The WHO End-TB strategy aims to reduce
TB deaths by 95\% and new TB cases by 90\% by 2035, with a call for more
basic research on TB pathogenesis and immunity. A major hurdle in the
development of effective TB vaccines and therapies is the absence of
defined immune-correlates of protection. In this context, the role of
regulatory T cells (Treg), which are essential for maintaining immune
homeostasis, is even less understood. This review aims to address this
knowledge gap by providing an overview of the emerging patterns of Treg
function in TB. The review also provides a comprehensive critical
analysis of the key features of Treg cells in TB; highlights the
importance of a balanced immune response as being important in TB and
discusses the importance of probing not just Treg frequency but also
qualitative aspects of Treg function as part of a comprehensive search
for novel TB treatments.%
\end{abstract}%



\sloppy


\textbf{Regulatory T cells: a brief introduction}

Extensive experimental evidence shows that it is not only important to
mount an effective immune response but equally crucial to efficiently
control it. A vital cog in the immune regulation machinery is a class of
CD4\textsuperscript{+} T cells, termed as regulatory T cells or Treg
cells. Treg cells have been extensively studied particularly in
autoimmune disorders, as potential therapeutic targets. Tregs can be
broadly classified as (a) thymic or tTregs/natural or nTregs, which
originate in the thymus and, (b) induced or iTregs/peripheral or pTregs,
which develop in the periphery during T cell activation. Development of
nTregs is influenced by signal strength {[}1,2{]} co-stimulatory CD28
signaling, ICOS/ICOSL interactions, and thymic stromal lymphoprotein
(TSLP) {[}3-5{]}. Transcription factor FoxP3 is a key regulator of Treg
development, maintenance and suppressive function {[}6-10{]} and its
expression in
CD4\textsuperscript{+}CD25\textsuperscript{hi}FoxP3\textsuperscript{+}tTregs
is positively regulated by IL-2 {[}11{]}, transcription factors NFAT,
STAT5 and Smad3 {[}12-14{]} and negatively regulated by PI3K, Akt and
mTOR {[}15{]}. However, FoxP3 expression can also be induced upon
exposure to non-self antigens in
CD4\textsuperscript{+}FoxP3\textsuperscript{-} conventional T cells,
which then differentiate into FoxP3\textsuperscript{+} Tregs known as
pTregs/iTregs by a process regulated by cytokines TGF\textbf{-} \selectlanguage{greek}β \selectlanguage{english}and
IL-2 {[}16,17{]}, suboptimal CD28 mediated co-stimulation {[}18{]}, and
sub-optimal TCR triggering {[}19, 20{]}. While nTregs maintain tolerance
and homeostasis systemically, pTreg/iTregs are crucial for dampening
over-exuberant antigen specific immune responses locally {[}21{]}.
Inducible T regulatory type 1 or Tr1 cells, a sub-class of
pTregs/iTregs, mediate suppressive effects via the immune-regulatory
cytokine IL-10 {[}22{]}; cell surface markers CD49b and LAG3 promote Tr1
differentiation {[}23{]}; and IL-27, IL-6, IL-21, IL-10, immature DCs
and plasmacytoid DCs promote Tr1 expansion {[}24-28{]}. nTreg
suppression mechanisms can be contact-dependent or -independent (Figure
1). A breakdown in Treg suppression can occur due to (i) reduction in
Treg frequencies, (ii) loss of Treg immunosuppressive capacity or due to
(iii) resistance acquired by effector T cells (Teff) to Treg mediated
suppression, with impact on a variety of clinical conditions (Table 1
and 2). This review focuses on how these mechanisms may contribute to
disease in the context of tuberculosis (TB).

\textbf{Tuberculosis}

\emph{M. tuberculosis} (Mtb) the causative agent of TB is well-adapted
to the human host, such that it can lie dormant for several years
(latent TB), sometimes a life time, without causing disease. Only 5-10\%
of subjects infected with Mtb progress to disease during their life.
Activation of latent TB can be due to several reasons among which HIV
co-infection is a major pre-disposing factor {[}68{]}. Although, Mtb is
spread through aerosols and replicates in lung epithelial cells, it can
also replicate in lymph nodes, bones, stomach, kidneys and other organs
causing extra-pulmonary TB. In extreme cases \emph{Mtb} can be
systemically disseminated precipitating a potentially fatal condition
known as miliary TB. Upon entering the host through aerosol, Mtb bacilli
are taken up by alveolar macrophages by phagocytosis facilitated by cell
surface receptors, e.g. toll like receptors (TLR), C-type lectin
receptors (CLR), scavenger receptors (SR), complement receptors (CR) and
Fc receptors {[}reviewed in 69{]} and replicate in macrophages in the
lung parenchyma. Primed DCs traffic to the lymph node and trigger
activation of adaptive immune cells, which are recruited to the lung and
gradually an organized structure, the granuloma, begins to form, which
comprises a caseous, necrotic core with replicating bacilli, surrounded
by an inner ring of epithelioid interlocked macrophages, neutrophils and
foam cells and an outer ring of T cells, B cells and NK cells {[}70{]}.
The resolution of infection within the granuloma relies on host immune
responses, which can potentially be impacted by Treg cell function.
Indeed, the role of Tregs has been studied in the context of early acute
stage of Mtb infection and the chronic phase of infection with evidence
from mouse, primate and human studies, as summarized below.

\textbf{Tregs in TB} : Acute phase of infection: an analysis of animal
model studies suggests early expansion to be detrimental. Mice models
highlight the impact of Tregs on TB to be phase specific with Treg
frequencies inadvertently high in the acute phase, which is detrimental
for infection control {[}71-76{]}. Aerosol infection of mice with
mycobacteria leads to activation of CD4\textsuperscript{+} Teff cells by
infected DCs in the pulmonary lymph node at approximately day 11 and
subsequent expansion and accumulation of CD4\textsuperscript{+}
(effector and regulatory) T cells in the lungs by day 14-21 {[}71{]}.
Significant disease associated lung pathology and cfu (colony forming
units) burden can be observed at day 14-21 and this period can be
classified as the early phase of infection in mice {[}71, 74, 75{]}.
Time points subsequent to this, e.g. 4-7 wks post infection can be
classified as late stages of infection {[}71, 74, 75{]}. Whilst
timelines for early and late phases can vary with multiplicity of
infection, in general 50-200 Mtb cfu results in increased Treg
frequencies in lung and pulmonary lymph node at 10-21 days which is
maintained till 60-127 days post infection {[}73,76{]}. This early
expansion was found to be deleterious to emerging protective anti-TB Th
responses {[}72-74, 76, 77{]}. Depletion of Tregs in C57BL/6 mice by
systemic administration of anti-CD25 3 days prior to infection with BCG
resulted in enhanced culture filtrate protein (CFP) specific
IFN\selectlanguage{greek}γ\selectlanguage{english}\textsuperscript{+} and
IL-2\textsuperscript{+}CD4\textsuperscript{+} cells in lungs and spleen
of BCG-infected mice 14 days post infection suggesting that presence of
Treg cells hinders appearance of protective Th1 responses {[}74{]}.
Also, adoptive transfer of CD25\textsuperscript{+} Treg into Mtb
infected mice leads to reduced frequencies of Mtb-specific Teff cells in
the lungs at 14-17 days post aerosol infection {[}72{]}. Importantly,
absence of protective Th1 responses due to expansion of Tregs leads to
increased bacterial burden in the acute phase {[}73{]}. However, this
dampening effect of Tregs on protective immune responses is transient
and not evident in later stages of infection {[}71, 74, 75{]}. Depletion
of CD25\textsuperscript{+} Treg had no effect on cfu burden or lung
pathology in BCG or Mtb Erdman infected mice at days 21 and 44 post
infection {[}74{]}. Similar results were reported in another study,
where Treg depletion in Mtb Erdman or Kurono infected DBA/2 mice reduced
cfu at 2 weeks post infection but had no effect on bacterial burden or
pathology subsequently, at 3 and 5 weeks {[}75{]}. It has now been
demonstrated in mice that Mtb-specific Tregs are culled via IL-12 driven
expression of T-bet by 32 days post infection; T-bet being known for its
pro-apoptotic effects {[}71{]}. How Mtb infection drives this early
expansion of Mtb-specific Tregs, which is beneficial to the pathogen
remains to be elucidated.

\emph{Chronic phase: animal model studies show loss in Treg frequency or
failure to recruit Tregs to site of infection can be detrimental.} In
contrast to the detrimental role of Tregs in the early / acute stage of
infection in murine models\textbf{,} several studies in mouse and
primate models highlight a potentially beneficial role for Tregs in the
chronic phase of infection. Comparison of TB disease progression and
pathology in TB resistant and TB susceptible mouse strains showed TB
resistant mouse strains to have higher Treg frequencies and consequently
less TB induced lung pathology in the chronic phase of the disease
{[}78, 79{]} compared to TB sensitive mice, which recruit significantly
fewer Tregs to the lung {[}79{]}. Interestingly infecting TB sensitive
C3HeN/FeJ mice with \emph{M. maserensis} (environmental mycobacterium)
resulted in a boost in Treg frequencies with a reduction in lung
pathology and improved survival {[}78{]}. These observations have been
corroborated in non-human primate models of TB infection, where
cynomolgus macaques infected with 25 cfu of Mtb Erdman can either
develop active TB or establish latency {[}80{]}. In this experimental
system it was observed that macaques that developed latent TB had higher
basal pre-infection Treg frequencies compared to animals that develop
active disease {[}80{]}. In a separate study, IL-2 administered either
pre- or post-Mtb infection in macaques resulted in Treg frequency
expansion, which in turn led to reduced bacterial burden and TB induced
pathology, suggesting that expansion of Treg cells in later stage of
chronic TB infection can help control excessive TB induced inflammation
{[}81{]}.

Human studies: In contrast to animal model studies where changes in
circulating Treg frequency can impact infection levels, reports of Treg
frequencies in human TB are varied. Some studies show an increase in
peripheral Treg frequencies in TB {[}55-58{]}. However, our study
{[}61{]} and others {[}59, 60{]} found no differences in peripheral Treg
frequencies between pulmonary TB patients and healthy controls. This
disparity may be linked to differences in markers used for Treg
delineation, which vary and can include, CD4 and CD25 {[}55, 56{]}; CD4,
CD25 and FoxP3 {[}57, 59{]}, a combination of CD4, CD45RA/CD45RO, CD127,
CD25 and FoxP3 to identify memory Tregs {[}60, 61{]} or
CD4\textsuperscript{+}CD127\textsuperscript{lo}CD25\textsuperscript{+}FoxP3\textsuperscript{+}CD45RO\textsuperscript{+}Ki67\textsuperscript{+}to
identify activated Treg cells {[}60{]}. Beyond variation in markers used
for definition, a further limitation of only tracking Treg frequency to
define Treg function in a disease like TB, is the impact of trafficking;
thus Treg frequencies have been shown to be higher at the site of
infection in the broncheoalveolar lavage compared to that in the
peripheral blood of pulmonary TB subjects {[}55, 82{]}.

Chronic phase: novel qualitative studies of Treg function in humans
highlight emergence of Treg resistant T effectors in chronic TB.

In contrast to the many studies that have probed Treg frequency, few
human or animal model studies have analyzed qualitative aspects of Treg
function in TB. Some studies have shown that Treg cells from pulmonary
TB patients retain their capacity to suppress autologous Teff cells
{[}83-85{]}. However, data from our laboratory shows that autologous
suppression mediated by
CD4\textsuperscript{+}CD45RA\textsuperscript{-}CD25\textsuperscript{+}CD127\textsuperscript{lo}memory
Treg cells isolated from subjects with pulmonary TB in south India is
significantly compromised {[}61{]}. By testing isolated Tregs from
healthy controls on Teff isolated from TB subjects and vice versa, we
demonstrated this impairment is not due to the loss of suppressive
potential of Treg cells isolated from TB subjects; instead it is due to
the effector cells from TB subjects acquiring resistance to Treg
mediated suppression {[}61{]}. Thus,
CD127\textsuperscript{lo}CD25\textsuperscript{+} Treg cells from TB
subjects were effective in suppressing Teff from healthy controls but
not those from TB subjects; conversely, Treg isolated from healthy
controls effectively suppressed autologous Teff but failed to suppress
Teff from TB subjects {[}61{]}. Phenotypic analysis of the Treg
resistant Teff isolated from TB subjects highlighted the presence of a
significant proportion of highly activated cells that expressed HLA-DR
and CD38; depletion of the HLA-DR\textsuperscript{+} subset in
particular, restored sensitivity of HLA-DR- Teff to autologous Treg
suppression, thereby confirming that resistance of Teff from TB subjects
to Treg mediated suppression was due to the presence of
HLA-DR\textsuperscript{+} cells {[}61{]}. The expansion of
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cells in TB is driven
by infection as anti-tubercular (anti-TB) treatment reduced the
frequencies of HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cells
{[}61, 86, 87{]}; indeed we have shown that anti-TB treatment by
dampening the frequency of HLA-DR\textsuperscript{+} cells restores T
effector cell sensitivity to autologous Treg cell mediated suppression
{[}61{]}. Consequently, measuring
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cell frequency can
potentially be used to monitor treatment responses and predict efficacy
of treatment {[}87{]}. In this context, our observation that
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T effectors isolated
from subjects with pulmonary TB become resistant to Treg mediated
suppression provides a mechanistic basis for how the expansion of
HLA-DR\textsuperscript{+} T effectors may be detrimental in TB (Figure
2, 61).

The observations of the emergence of Treg resistant Teff in TB is
consistent with data from other chronic inflammatory conditions,
particularly, autoimmune disorders (Table 2).
CD161\textsuperscript{+}Th17 cells enriched in the synovial fluid of
rheumatoid arthritis patients are resistant to Treg mediated suppression
and their depletion restores suppression in \emph{in vitro} cultures
{[}65{]}. A similar phenomenon of the emergence of suppression resistant
effectors has been reported in systemic lupus erythromatosis {[}88{]},
multiple sclerosis {[}89{]}, type-1 diabetes {[}63, 90{]} and juvenile
idiopathic arthritis {[}91{]}, with potentially varying mechanisms
underpinning such resistance. In multiple sclerosis, it was attributed
to high Teff cell derived granzyme B {[}89{]}; in type-1 diabetes due to
downregulation of TGF\selectlanguage{greek}β\selectlanguage{english}RII on Teff cells and consequently reduced TGF\selectlanguage{greek}β
\selectlanguage{english}mediated suppression {[}90{]} and in juvenile idiopathic arthritis, like
TB, due to expansion of activated
CD69\textsuperscript{+}HLA-DR\textsuperscript{+} Teff cells which were
Treg suppression resistant {[}91{]}.

\textbf{Mechanisms that underpin Treg dysregulation in TB}

(i) Evidence for exaggerated expression of cytokines that counterbalance
Treg function

It is well recognized that proinflammatory cytokines can suppress the
generation and function of Treg cells. By directly comparing the
transcriptome of Treg resistant HLA-DR\textsuperscript{+} effector
CD4\textsuperscript{+} T cells isolated from TB subjects with that of
the Treg sensitive fraction depleted of
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cells, we provided
first evidence that HLA-DR\textsuperscript{+} T effectors from TB
express a number of pro-inflammatory cytokines including IL-2, IFN\selectlanguage{greek}γ,
\selectlanguage{english}CSF2, IL-17A and IL-22 (Figure 3). This exaggerated cytokine profile was
noted in T effectors stimulated with both Mtb antigen as well as
polyclonal stimulation {[}61{]}. Both IFN\selectlanguage{greek}γ \selectlanguage{english}and IL-17A, although crucial
for Mtb control {[}92, 93{]}, are also recognized to counter-regulate
Treg development and function and their exaggerated expansion therefore,
could be one mechanism for Teff resistance to Treg control {[}94-96{]}.

\emph{(II)} \textbf{\emph{Evidence for altered expression of cell
surface molecules on Teff that are important for engaging with Treg
cells}}

Treg cells suppress via a variety of contact dependent and independent
mechanisms (Figure 1). Some of the key molecules shown to be involved in
promoting Treg suppression include CTLA-4, PD-1, CD39, CD73, PD-1,
PD-L1, LAG3 etc. Several lines of data show some of these molecules,
e.g. PD-1, PD-L1, CD39, CD73, LAG3 are altered in TB. PD-1 and PD-L1
expression is elevated in CD4\textsuperscript{+} T cells from TB
subjects compared to healthy controls {[}97{]}; CD39 and CD73 are
increased in lung parenchymal CD4\textsuperscript{+} T cells of Mtb
infected mice {[}98{]} and LAG3 is increased in granuloma of macaques
with active TB compared to those with latent TB {[}99{]}. However, the
function of these molecules has not been specifically studied in the
context of Treg suppression in TB. We provided some direct evidence for
a role for PD-1/PD-L1 and \selectlanguage{greek}β \selectlanguage{english}chemokine/CCR5 interactions. A comparative
RNA seq analysis of Treg suppression sensitive
(HLA-DR\textsuperscript{-} Teff) and suppression resistant (comprising
HLA-DR\textsuperscript{+} Teff \textsuperscript{+} and
HLA-DR\textsuperscript{-} Teff) cells isolated from subjects with
pulmonary TB showed elevated expression of PD-L1 and \selectlanguage{greek}β-\selectlanguage{english}chemokines in the
suppression sensitive HLA-DR\textsuperscript{-} Teff fraction (Figure
2). Also, blockade of PD-1/PD-L1 and \selectlanguage{greek}β-\selectlanguage{english}chemokine/CCR5 interactions
resulted in loss of Treg suppression in these cells pointing to the
importance of these pathways in maintaining Treg mediated homeostasis in
TB (Figure 2). PD-1/PD-L1 interactions and \selectlanguage{greek}β-\selectlanguage{english}chemokine/CCR5 interactions
have been previously implicated to play a role in Treg mediated
suppression {[}100, 101{]}. CCL3 and CCL4 secreted by Tregs serve as
chemoattractants for Teff cells. CD4\textsuperscript{+} Teff cells from
mice deficient in CCL3 and CCL4 fail to migrate and come in contact with
Treg cells {[}101{]}. Moreover, Tregs from type-1 diabetes patients are
deficient in CCL3 and CCL4 and this compromises their ability to
suppress {[}101{]}.

Apart from differences in PD-L1 and \selectlanguage{greek}β-\selectlanguage{english}chemokine levels (Figure 2), our
transcriptome analysis of Treg sensitive HLA-DR\textsuperscript{-} and
suppression resistant HLA-DR\textsuperscript{+} cells identified several
additional cell surface markers (CD46, TRAIL, TRAF1, TRAF3, FASLG, CD30,
SEMA7A) (Figure 3), some of which have previously been implicated in
Treg function {[}102-105{]}. Engagement of complement receptor CD46
results in suppression of bystander CD4\textsuperscript{+} T cells via
an IL-10 dependent mechanism {[}102, 103{]}. CD46 cross-linking also
suppresses mycobacteria specific CD4\textsuperscript{+} T cell responses
{[}106{]}. TRAIL is a regulator of T cell activation; its absence leads
to autoimmunity and reduction in Treg frequencies while its presence
dampens Th1 responses and boosts Tregs {[}104{]}. TRAF1 inhibits Th2
differentiation {[}107{]}; TRAF3 controls proximal T cell activation
events and its absence in mice leads to elevated thymus derived Treg
cell frequencies {[}105{]}; FASLG is a marker for T cell activation and
is expressed on Th1 cells {[}108, 109{]}; Sema7a and CD30 have been
implicated in Th1 and Th17 differentiation {[}110, 111{]}. However, a
role for these pathways impacting T effector function in TB remains to
be elucidated.

(III) Signal strength

A third potentially important consideration in the mechanisms that
underpin how activated, HLA-DR\textsuperscript{+} T effectors become
resistant to Treg cells may be linked to the quality and strength of the
primary signal that activates effector cells. The strength of activating
signal shapes the nature of the immune response, with high signal
strength leading to Th1 and low signal strength to Th2, Tfh and memory T
cell differentiation {[}reviewed in 112, 113{]}. Previous studies show
that effectors activated by a very strong signal strength become
refractory to suppression mediated by Treg cells- co-culture of human
CD25\textsuperscript{-} Teff and autologous CD25\textsuperscript{+} Treg
resulted in suppression only when stimulated with soluble anti-CD3 (weak
TCR signal) and not when activated with plate-bound anti-CD3 (strong TCR
signal) {[}114{]}. Whether this is pertinent in the context of TB
remains to be tested. What has been demonstrated from mouse studies is
that persistently activated CD4 T cells specific for the secretory Mtb
antigen ESAT6, which is expressed in abundance throughout infection,
fail to confer protection, whereas, CD4\textsuperscript{+} T cells
specific for an Mtb antigen that has more controlled expression, e.g.
Ag85B, can confer protection greater protection {[}115{]}. The failure
of ESAT6 specific cells to confer protection was linked to the fact that
these cells are more exhausted and terminally differentiated; i.e.
express higher KLRG1, lower CCR7, CD127 and CD62L; compared to Ag85B
specific cells {[}115{]}. Whether Treg resistant
HLA-DR\textsuperscript{+} Teff cells isolated from TB subjects arise due
to persistent antigen stimulation and bear markers of exhaustion remains
to be confirmed.

\textbf{Summary and future directions}

It appears now from studies in animal models and humans that in TB the
role of Tregs, both nTreg and antigen specific, has several dimensions.
While Tregs might delay the appearance of protective Th responses
especially during the early stages of infection; their function in the
chronic stage of TB disease is consistent with their known primary
function that is linked to control of exaggerated inflammation, which if
unchecked, can contribute to disease pathology {[}116, 117{]}. The
immune response in TB is clearly disordered and the theme of balance
between protective and pathogenic responses has been visited in the past
{[}118{]}. In fact, a balance between Th1/Th17 and immune-regulatory
responses is associated with better clearance of Mtb infection
{[}119{]}. In this context, the expansion of
HLA-DR\textsuperscript{+}Teff cells in TB is a likely marker for
inflammation associated with enhanced disease risk {[}120{]}. It has now
been demonstrated that this expanded subset exhibits resistance to
suppression mediated by natural Treg cells {[}61{]}. The putative role
of pro-inflammatory cytokines (IFN\selectlanguage{greek}γ, \selectlanguage{english}IL-17A, IL-2, CSF2 and IL-22),
\selectlanguage{greek}β-\selectlanguage{english}chemokines and PD-1/PD-L1 interactions in modulating T effector
resistance to Treg suppression in TB has been identified (Figure 2).
This calls for further analysis of the mechanisms that are important in
maintaining balance between inflammation and immune-regulation in TB.

\textbf{Acknowledgement}

We acknowledge the support to our work on Treg function in TB by a
Centre of Excellence grant to AV from the Department of Biotechnology,
Govt. of India (No BT/PR10394/MED/29/1128/2016). We acknowledge
additional funding for sample collection and analysis by EC HORIZON2020
TBVAC2020 and EC FP7 EURIPRED (FP7-INFRA-2012 Grant Agreement No.
312661) to AV. We also wish to thank Dr. Srabanti Rakshit for careful
reading and editing of the review.

\textbf{Figure Legends}

\textbf{Figure 1: Mechanisms of Treg mediated suppression.} Primary Treg
suppression mechanisms include (1) acting as sink for IL-2 due to
constitutive high expression of IL-2R and consequently depriving
effector T cells of the crucial cytokine {[}121{]}; (2) secretion of
immune-suppressive cytokines IL-10, TGF\selectlanguage{greek}β \selectlanguage{english}and IL-35 {[}122,123{]}; (3)
Granzyme-B dependent killing of target cells {[}124{]}; (4) inhibitory
signalling through binding of CTLA-4 on Tregs and CD80/86 on DCs and
reverse signalling via this interaction leading to elevated levels of
IDO in DCs which eventually deplete tryptophan and starve effector T
cells {[}125, 126{]}; (5) binding of LAG3 to MHC-II molecules on DCs
causing reduction in Ag presentation {[}127{]}; (6) suppression due to
interaction of PD-1 on Tregs and PD-L1 on target cells {[}61, 100{]};
(7) extracellular adenosine generated from ATP in concert by cell
surface CD39 and CD73 (ecto-5'-nucleotidase) interacts with A2AR on
effector T cells and suppresses their function by increasing cAMP levels
{[}128, 129{]}; (8) chemokines CCL3 and CCL4 secreted by Tregs bind to
CCR5 on effector cells triggering their migration and subsequent
suppression {[}61, 101{]}.

\textbf{Figure 2:} \textbf{A diagrammatic model which highlights the
difference in Treg suppression in healthy and latently infected
individuals and active TB subjects in context of expansion of
HLA-DR\textsuperscript{+}CD4\textsuperscript{+}memory T cells.}
Individuals infected with TB can either clear the bacteria, become
latently infected or come down with active TB disease. There is also a
possibility of reactivation of TB in latently infected subjects. The
reasons for this can be HIV co-infection, treatment with check-point
inhibitors like anti-PD-1, therapies such as anti-TNF for rheumatoid
arthritis etc. HLA-DR\textsuperscript{+} activated cells are low in
healthy and latently infected individuals and Treg suppression is good.
However, in active TB, HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T
cells expand and Treg mediated suppression becomes poor. The Treg
suppression pathways that are rendered inactive in TB are the PD-1/PD-L1
and \selectlanguage{greek}β-\selectlanguage{english}chemokine-CCR5 dependent. The reason for their becoming inactive
could be possible counter-regulation by IL-2, IL-17A, IFN\selectlanguage{greek}γ, \selectlanguage{english}IL-22 that
are secreted by the expanded
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cells.

\textbf{Figure 3:} \textbf{Treg suppression resistant total Teff cells
and Treg suppression sensitive HLA-DR- Teff cells have distinct
expression patterns with respect to certain cytokines and cell surface
receptors.} Total Teff and HLA-DR- Teff were sorted by flow cytometry
from 5 pulmonary TB patients. Cells were activated for different times
with anti-CD3/CD28 mitogenic beads, RNA was isolated and an RNA-Seq was
performed. Expression at each time was compared to baseline unactivated
control to arrive at a DEG list. A final DEG list was prepared applying
a cutoff of log2 fold change +/-2.5 and p \textless{} 0.05 (for details
on procedure and complete DEG list please see Ahmed A et al., 2018). The
DEG list was further mined to study expression of cytokines, chemokines,
activation markers, cell surface markers and transcription factors. A
summary of these results is shown. The numbers in boxes denote log2 fold
change for expression at 2, 24 and 96 hrs compared to expression at
baseline. For details of the complete DEG list, kindly see {[}61{]}

\textbf{References}

\begin{enumerate}
\tightlist
\item
  Moran AE, Holzapfel KL, Xing Y et al. T cell receptor signal strength
  in Treg and iNKT cell development demonstrated by a novel fluorescent
  reporter mouse. J Exp Med 2011; 208:1279-89.
\item
  Jordan MS, Boesteanu A, Reed AJ et al. Thymic selection of
  CD4\textsuperscript{+}CD25\textsuperscript{+} regulatory T cells
  induced by an agonist self-peptide. Nat Immunol 2001; 2:301-6.
\item
  Tai X, Cowan M, Feigenbaum L, Singer A. CD28 costimulation of
  developing thymocytes induces Foxp3 expression and regulatory T cell
  differentiation independently of interleukin 2. Nat Immunol 2005;
  6:152-62.
\item
  Nazzal D, Gradolatto A, Truffault F et al. Human thymus medullary
  epithelial cells promote regulatory T-cell generation by stimulating
  interleukin-2 production via ICOS ligand. Cell Death Dis 2014;
  5:e1420.
\item
  Watanabe N, Wang YH, Lee HK et al. Hassall's corpuscles instruct
  dendritic cells to induce
  CD4\textsuperscript{+}CD25\textsuperscript{+} regulatory T cells in
  human thymus. Nature 2005; 436:1181-5.
\item
  Fontenot JD, Gavin MA, Rudensky AY. Foxp3 programs the development and
  function of CD4+CD25+ regulatory T cells. Nat Immunol 2003; 4:330-6.
\item
  Brunkow ME, Jeffery EW, Hjerrild KA et al. Disruption of a new
  forkhead/winged-helix protein, scurfin, results in the fatal
  lymphoproliferative disorder of the scurfy mouse. Nat Genet 2001;
  27:68-73.
\item
  Gavin MA, Rasmussen JP, Fontenot JD et al. Foxp3-dependent programme
  of regulatory T-cell differentiation. Nature 2007; 445:771-5.
\item
  Lin W, Haribhai D, Relland LM, et al. Regulatory T cell development in
  the absence of functional Foxp3. Nat Immunol 2007; 8:359-68.
\item
  Wan YY, Flavell RA. Regulatory T-cell functions are subverted and
  converted owing to attenuated Foxp3 expression. Nature 2007;
  445:766-70.
\item
  Zorn E, Nelson EA, Mohseni M, Porcheray F, Kim H, Litsa D, Bellucci R,
  Raderschall E, Canning C, Soiffer RJ, Frank DA, Ritz J. IL-2 regulates
  FOXP3 expression in human CD4+CD25+ regulatory T cells through a
  STAT-dependent mechanism and induces the expansion of these cells in
  vivo. Blood. 2006 Sep 1;108(5):1571-9.
\item
  Tone Y, Furuuchi K, Kojima Y et al. Smad3 and NFAT cooperate to induce
  Foxp3 expression through its enhancer. Nat Immunol 2008; 9:194-202.
\item
  Burchill MA, Yang J, Vogtenhuber C et al. IL-2 receptor beta-dependent
  STAT5 activation is required for the development of
  Foxp3\textsuperscript{+} regulatory T cells. J Immunol 2007;
  178:280-90.
\item
  Feske S, M\selectlanguage{ngerman}üller JM, Graf D et al. Severe combined immunodeficiency due
  to defective binding of the nuclear factor of activated T cells in T
  lymphocytes of two male siblings. Eur J Immunol 1996; 26:2119-26.
\item
  Sauer S, Bruno L, Hertweck A et al T cell receptor signaling controls
  Foxp3 expression via PI3K, Akt, and mTOR. Proc Natl Acad Sci U S A
  2008; 105:7797-802.
\item
  Chen W, Jin W, Hardegen N et al. Conversion of peripheral CD4+CD25-
  naive T cells to
  CD4\textsuperscript{+}CD25\textsuperscript{+}regulatory T cells by
  TGF-beta induction of transcription factor Foxp3. J Exp Med 2003;
  198:1875-86.
\item
  Horwitz DA, Zheng SG, Wang J, Gray JD. Critical role of IL-2 and
  TGF-beta in generation, function and stabilization of
  Foxp3\textsuperscript{+}CD4\textsuperscript{+} Treg. Eur J Immunol
  2008; 38:912-5.
\item
  Semple K, Nguyen A, Yu Y et al. Strong CD28 costimulation suppresses
  induction of regulatory T cells from naive precursors through Lck
  signaling. Blood 2011; 117:3096-103.
\item
  Gottschalk RA, Corse E, Allison JP. TCR ligand density and affinity
  determine peripheral induction of Foxp3 in vivo. J Exp Med 2010 Aug
  2;207(8):1701-11.
\item
  Kretschmer K, Apostolou I, Hawiger D et al. Inducing and expanding
  regulatory T cell populations by foreign antigen. Nat Immunol 2005;
  6:1219-27.
\item
  Zeng H, Zhang R, Jin B, Chen L. Type 1 regulatory T cells: a new
  mechanism of peripheral immune tolerance. Cell Mol Immunol 2015;
  12:566-71.
\item
  Groux H, O'Garra A, Bigler M et al. A CD4\textsuperscript{+} T-cell
  subset inhibits antigen-specific T-cell responses and prevents
  colitis. Nature 1997; 389:737-42.
\item
  Gagliani N, Magnani CF, Huber S et al. Co-expression of CD49b and
  LAG-3 identifies human and mouse T regulatory type 1 cells. Nat Med
  2013; 19:739-46.
\item
  Jin JO, Han X, Yu Q. Interleukin-6 induces the generation of
  IL-10-producing Tr1 cells and suppresses autoimmune tissue
  inflammation. J Autoimmun 2013; 40:28-44.
\item
  Pot C, Jin H, Awasthi A et al. Cutting edge: IL-27 induces the
  transcription factor c-Maf cytokine IL-21, and the costimulatory
  receptor ICOS that coordinately act together to promote
  differentiation of IL-10-producing Tr1 cells. J Immunol 2009;
  183:797-801.
\item
  Levings MK, Gregori S, Tresoldi E et al. Differentiation of Tr1 cells
  by immature dendritic cells requires IL-10 but not
  CD25\textsuperscript{+}CD4\textsuperscript{+} Tr cells. Blood 2005;
  105:1162-9.
\item
  Yang Y, Ochando J, Yopp A et al. IL-6 plays a unique role in
  initiating c-Maf expression during early stage of CD4 T cell
  activation. J Immunol 2005;174:2720-9.
\item
  Wang H, Meng R, Li Z et al. IL-27 induces the differentiation of
  Tr1-like cells from human naive CD4\textsuperscript{+} T cells via the
  phosphorylation of STAT1 and STAT3. Immunol Lett 2011; 136:21-8.
\item
  Hashemi V, Farrokhi AS, Tanomand A et al. Polymorphism of Foxp3 gene
  affects the frequency of regulatory T cells and disease activity in
  patients with rheumatoid arthritis in Iranian population. Immunol Lett
  2018; 204:16-22.
\item
  Rossetti M, Spreafico R, Consolaro A et al. TCR repertoire sequencing
  identifies synovial Treg cell clonotypes in the bloodstream during
  active inflammation in human arthritis. Ann Rheum Dis 2017;
  76:435-441.
\item
  Zare HR, Habibagahi M, Vahdati A, Habibagahi Z. Patients with Active
  Rheumatoid Arthritis Have Lower Frequency of nTregs in Peripheral
  Blood. Iran J Immunol 2015; 12:166-75.
\item
  Beavis PA, Gregory B, Green P et al. Resistance to regulatory T
  cell-mediated suppression in rheumatoid arthritis can be bypassed by
  ectopic Foxp3 expression in pathogenic synovial T cells. Proc Natl
  Acad Sci U S A 2011; 108:16717-22.
\item
  Haas J, Hug A, Viehöver A et al. Reduced suppressive effect of
  CD4+CD25high regulatory T cells on the T cell immune response against
  myelin oligodendrocyte glycoprotein in patients with multiple
  sclerosis. Eur J Immunol 2005; 35:3343-52.
\item
  Azimi M, Ghabaee M, Moghadasi AN et al. Immunomodulatory function of
  Treg-derived exosomes is impaired in patients with relapsing-remitting
  multiple sclerosis. Immunol Res 2018; 66:513-520.
\item
  Kimura K, Hohjoh H, Fukuoka M et al. Circulating exosomes suppress the
  induction of regulatory T cells via let-7i in multiple sclerosis. Nat
  Commun 2018; 9:17.
\item
  Venken K, Hellings N, Broekmans T et al. Natural naive
  CD4\textsuperscript{+}CD25\textsuperscript{+}CD127\textsuperscript{low}regulatory
  T cell (Treg) development and function are disturbed in multiple
  sclerosis patients: recovery of memory Treg homeostasis during disease
  progression. J Immunol 2008; 180:6411-20.
\item
  Sambucci M, Gargano F, De Rosa V et al. FoxP3 isoforms and PD-1
  expression by T regulatory cells in multiple sclerosis. Sci Rep 2018;
  8:3674.
\item
  Kato R, Sumitomo S, Tsuchida Y et al.
  CD4\textsuperscript{+}CD25\textsuperscript{+}LAG3\textsuperscript{+} T
  cells with a feature of Th17 cells associated with systemic lupus
  erythematosus disease activity. Front Immunol 2019; 10:1619.
\item
  Jin X, Zhang C, Gong L, Li H, Wang Y, Li Q, Li H. Altered expression
  of CD39 on memory regulatory T cells in type 1 diabetes patients. J
  Diabetes 2019; 11:440-448.
\item
  Scherm MG, Serr I, Zahm AM et al. miRNA142-3p targets Tet2 and impairs
  Treg differentiation and stability in models of type 1 diabetes. Nat
  Commun 2019; 10:5697.
\item
  Hope CM, Welch J, Mohandas A et al. Peptidase inhibitor 16 identifies
  a human regulatory T-cell subset with reduced FOXP3 expression over
  the first year of recent onset type 1 diabetes. Eur J Immunol 2019;
  49:1235-250.
\item
  Kurup SP, Obeng-Adjei N, Anthony SM et al. Regulatory T cells impede
  acute and long-term immunity to blood-stage malaria through CTLA-4.
  Nat Med 2017; 23:1220-225.
\item
  Boyle MJ, Jagannathan P, Farrington LA et al. Decline of
  FoxP3\textsuperscript{+} Regulatory CD4 T cells in peripheral blood of
  children heavily exposed to malaria. PLoS Pathog 2015; 11:e1005041.
\item
  Tillu H, Tripathy AS, Reshmi PV, Cecilia D. Altered profile of
  regulatory T cells and associated cytokines in mild and moderate
  dengue. Eur J Clin Microbiol Infect Dis 2016; 35:453-61.
\item
  Jayaratne HE, Wijeratne D, Fernando S et al. Regulatory T-cells in
  acute dengue viral infection. Immunology 2018; 154:89-97.
\item
  Roider J, Ngoepe A, Muenchhoff M, et al. Increased regulatory T-cell
  activity and enhanced T-cell homeostatic signaling in slow progressing
  HIV-infected children. Front Immunol. 2019; 10:213.
\item
  Eggena MP, Barugahare B, Jones N et al. Depletion of regulatory T
  cells in HIV infection is associated with immune activation. J Immunol
  2005;174:4407-14.
\item
  Chase AJ, Yang HC, Zhang H et al. Preservation of
  FoxP3\textsuperscript{+} regulatory T cells in the peripheral blood of
  human immunodeficiency virus type 1-infected elite suppressors
  correlates with low CD4\textsuperscript{+} T-cell activation. J Virol
  2008; 82: 8307-15.
\item
  Xiao J, Zhang L, Dong Y et al. PD-1 Upregulation is associated with
  exhaustion of regulatory T cells and reflects immune activation in
  HIV-1-infected individuals. AIDS Res Hum Retroviruses 2019;
  35:444-452.
\item
  Younes SA, Talla A, Pereira Ribeiro S et al. Cycling
  CD4\textsuperscript{+} T cells in HIV-infected immune non-responders
  have mitochondrial dysfunction. J Clin Invest 2018; 128:5083-94.
\item
  Whibley N, Maccallum DM, Vickers MA et al. Expansion of
  Foxp3\textsuperscript{+} T-cell populations by \emph{Candida
  albicans}enhances both Th17-cell responses and fungal dissemination
  after intravenous challenge. Eur J Immunol 2014; 44:1069-83.
\item
  Pandiyan P, Conti HR, Zheng L et al.
  CD4\textsuperscript{+}CD25\textsuperscript{+}Foxp3\textsuperscript{+}regulatory
  T cells promote Th17 cells in vitro and enhance host resistance in
  mouse \emph{Candida albicans} Th17 cell infection model. Immunity
  2011; 34:422-34.
\item
  Kumar P, Misra P, Thakur CP et al. T cell suppression in the bone
  marrow of visceral leishmaniasis patients: impact of parasite load.
  Clin Exp Immunol 2018; 191:318-327.
\item
  Bhattacharya P, Ghosh S, Ejazi SA et al. Induction of IL-10 and TGF\selectlanguage{greek}β
  \selectlanguage{english}from
  CD4\textsuperscript{+}CD25\textsuperscript{+}FoxP3\textsuperscript{+}T
  cells correlates with parasite load in Indian kala-azar patients
  infected with \emph{Leishmania donovani} . PLoS Negl Trop Dis. 2016;
  10:e0004422.
\item
  Guyot-Revol V, Innes JA, Hackforth S et al. Regulatory T cells are
  expanded in blood and disease sites in patients with tuberculosis. Am
  J Respir Crit Care Med 2006; 173: 803-10.
\item
  Ribeiro-Rodrigues R, Resende Co T, Rojas R et al. A role for
  CD4\textsuperscript{+}CD25\textsuperscript{+} T cells in regulation of
  the immune response during human tuberculosis. Clin Exp Immunol 2006;
  144:25-34.
\item
  Chen X, Zhou B, Li M et al.
  CD4\textsuperscript{+}CD25\textsuperscript{+}FoxP3\textsuperscript{+}regulatory
  T cells suppress Mycobacterium tuberculosis immunity in patients with
  active disease. Clin Immunol 2007; 123:50-9.
\item
  Li L, Lao SH, Wu CY. Increased frequency of
  CD4\textsuperscript{+}CD25\textsuperscript{high} Treg cells inhibit
  BCG-specific induction of IFN-gamma by CD4\textsuperscript{+} T cells
  from TB patients. Tuberculosis (Edinb) 2007; 87:526-34.
\item
  Ghazalsofala R, Rezaee SA, Rafatpanah H et al. Evaluation of
  CD4\textsuperscript{+}CD25\textsuperscript{+}
  FoxP3\textsuperscript{+}Regulatory T cells and FoxP3 and CTLA-4 gene
  expression in patients with newly diagnosed tuberculosis in northeast
  of Iran. Jundishapur J Microbiol 2015; 8:e17726.
\item
  Zewdie M, Howe R, Hoff ST et al. Ex-vivo characterization of
  regulatory T cells in pulmonary tuberculosis patients, latently
  infected persons, and healthy endemic controls. Tuberculosis (Edinb)
  2016; 100:61-68.
\item
  Ahmed A, Adiga V, Nayak S et al. Circulating HLA-DR+CD4+ effector
  memory T cells resistant to CCR5 and PD-L1 mediated suppression
  compromise regulatory T cell function in tuberculosis. PLoS Pathog
  2018; 14: e1007289.
\item
  Ihantola EL, Viisanen T, Gazali AM et al. Effector T Cell resistance
  to suppression and STAT3 signaling during the development of human
  type 1 diabetes. J Immunol 2018; 201:1144-153.
\item
  Lawson JM, Tremble J, Dayan C et al. Increased resistance to
  CD4\textsuperscript{+}CD25\textsuperscript{hi} regulatory T
  cell-mediated suppression in patients with type 1 diabetes. Clin Exp
  Immunol 2008; 154:353-59.
\item
  Schneider A, Rieck M, Sanda S et al. The effector T cells of diabetic
  subjects are resistant to regulation via
  CD4\textsuperscript{+}FOXP3\textsuperscript{+} regulatory T cells. J
  Immunol 2008; 181:7350-355.
\item
  Basdeo SA, Moran B, Cluxton D et al. Polyfunctional, Pathogenic CD161+
  Th17 lineage cells are resistant to regulatory t cell-mediated
  suppression in the context of autoimmunity. J Immunol 2015;
  195:528-40.
\item
  Thorborn GS, Pomeroy L, Ishohanni H et al. Elevated effector cell
  sensitivity to Treg-cell suppression that is not associated with
  reduced Th17-cell expression distinguishes HIV+ asymptomatic subjects
  from progressors. Eur J Immunol. 2012; 42:138-46.
\item
  Elahi S, Dinges WL, Lejarcegui N, Laing KJ, Collier AC, Koelle DM,
  McElrath MJ, Horton H. Protective HIV-specific CD8+ T cells evade Treg
  cell suppression. Nat Med. 2011 Jul 17;17(8):989-95.
\item
  Ahmed A, Rakshit S, Vyakarnam A. HIV-TB co-infection: mechanisms that
  drive reactivation of Mycobacterium tuberculosis in HIV infection.
  Oral Dis. 2016; 22 Suppl 1:53-60.
\item
  Queval CJ, Brosch R, Simeone R. The Macrophage: A disputed fortress in
  the battle against \emph{Mycobacterium tuberculosis} . Front
  Microbiol. 2017; 8:2284.
\item
  Ramakrishnan L. Revisiting the role of the granuloma in tuberculosis.
  Nat Rev Immunol 2012; 12:352-66.
\item
  Shafiani S, Dinh C, Ertelt JM, et al. Pathogen-specific Treg cells
  expand early during \emph{Mycobacterium tuberculosis} infection but
  are later eliminated in response to Interleukin-12. Immunity 2013;
  38:1261-70.
\item
  Shafiani S, Tucker-Heard G, Kariyone A et al. Pathogen-specific
  regulatory T cells delay the arrival of effector T cells in the lung
  during early tuberculosis. J Exp Med 2010; 207:1409-20.
\item
  Scott-Browne JP, Shafiani S, Tucker-Heard G et al. Expansion and
  function of Foxp3-expressing T regulatory cells during tuberculosis. J
  Exp Med 2007; 204:2159-69.
\item
  Quinn KM, McHugh RS, Rich FJ et al. Inactivation of
  CD4\textsuperscript{+}CD25\textsuperscript{+} regulatory T cells
  during early mycobacterial infection increases cytokine production but
  does not affect pathogen load. Immunol Cell Biol 2006; 84:467-74.
\item
  Ozeki Y, Sugawara I, Udagawa T et al. Transient role of
  CD4\textsuperscript{+}CD25\textsuperscript{+} regulatory T cells in
  mycobacterial infection in mice. Int Immunol 2010; 22:179-89.
\item
  Kursar M, Koch M, Mittr\selectlanguage{ngerman}ücker HW et al. Cutting Edge: Regulatory T
  cells prevent efficient clearance of Mycobacterium tuberculosis. J
  Immunol 2007; 178:2661-5.
\item
  Ordway D, Henao-Tamayo M, Harton M, Palanisamy G, Troudt J, Shanley
  C,Basaraba RJ, Orme IM. The hypervirulent Mycobacterium tuberculosis
  strain HN878induces a potent TH1 response followed by rapid
  down-regulation. J Immunol. 2007 Jul 1;179(1):522-31.
\item
  Cardona P, Marzo-Escartín E, Tapia G et al. Oral administration of
  heat-killed \emph{Mycobacterium manresensis} delay progression toward
  active tuberculosis in C3HeB/FeJ mice. Front Microbiol 2016; 6:1482.
\item
  Kapina MA, Rubakova EI, Majorov KB et al. Capacity of lung stroma to
  educate dendritic cells inhibiting mycobacteria-specific T-cell
  response depends upon genetic susceptibility to tuberculosis. PLoS One
  2013; 8:e72773.
\item
  Green AM, Mattila JT, Bigbee CL et al.
  CD4\textsuperscript{+}regulatory T cells in a cynomolgus macaque model
  of\emph{Mycobacterium tuberculosis} infection. J Infect Dis 2010;
  202:533-41.
\item
  Chen CY, Huang D, Yao S et al. IL-2 simultaneously expands Foxp3+ T
  regulatory and T effector cells and confers resistance to severe
  tuberculosis (TB): implicative Treg-T effector cooperation in immunity
  to TB. J Immunol 2012; 188:4278-88.
\item
  Semple PL, Binder AB, Davids M et al. Regulatory T cells attenuate
  mycobacterial stasis in alveolar and blood-derived macrophages from
  patients with tuberculosis. Am J Respir Crit Care Med. 2013;187:
  1249-58.
\item
  Hougardy JM, Verscheure V, Locht C, Mascart F. In vitro expansion of
  CD4\textsuperscript{+}CD25\textsuperscript{high}FOXP3\textsuperscript{+}CD127\textsuperscript{low}/-
  regulatory T cells from peripheral blood lymphocytes of
  healthy\emph{Mycobacterium tuberculosis} -infected humans. Microbes
  Infect 2007; 9:1325-32.
\item
  Roberts T, Beyers N, Aguirre A, Walzl G. Immunosuppression during
  active tuberculosis is characterized by decreased interferon- gamma
  production and CD25 expression with elevated forkhead box P3,
  transforming growth factor- beta, and interleukin-4 mRNA levels. J
  Infect Dis 2007; 195:870-8.
\item
  Davids M, Pooran AS, Pietersen E et al. Regulatory T cells subvert
  mycobacterial containment in patients failing extensively
  drug-resistant tuberculosis treatment. Am J Respir Crit Care Med 2018;
  198:104-116.
\item
  Riou C, Berkowitz N, Goliath R et al. Analysis of the Phenotype of
  Mycobacterium tuberculosis-specific CD4\textsuperscript{+} T cells to
  discriminate latent from active tuberculosis in HIV-uninfected and
  HIV-infected individuals. Front Immunol 2017; 8:968.
\item
  Adekambi T, Ibegbu CC, Cagle S et al. Biomarkers on patient T cells
  diagnose active tuberculosis and monitor treatment response. J Clin
  Invest 2015; 125:1827-38.
\item
  Venigalla RK, Tretter T, Krienke S. Reduced
  CD4\textsuperscript{+}CD25\textsuperscript{-} T cell sensitivity to
  the suppressive function of
  CD4\textsuperscript{+}CD25\textsuperscript{high}CD127\textsuperscript{-/low}regulatory
  T cells in patients with active systemic lupus erythematosus.
  Arthritis Rheum 2008; 58:2120-30.
\item
  Bhela S, Kempsell C, Manohar M et al. Nonapoptotic and extracellular
  activity of granzyme B mediates resistance to regulatory T cell (Treg)
  suppression by
  HLA-DR\textsuperscript{-}CD25\textsuperscript{hi}CD127\textsuperscript{lo}Tregs
  in multiple sclerosis and in response to IL-6. J Immunol 2015;
  194:2180-18
\item
  Kawamoto K, Pahuja A, Nettles A et al. Downregulation of TGF-\selectlanguage{greek}β\selectlanguage{english}RII in T
  effector cells leads to increased resistance to TGF-\selectlanguage{greek}β-\selectlanguage{english}mediated
  suppression of autoimmune responses in type I diabetes. Autoimmunity
  2012; 45:310-9.
\item
  Haufe S, Haug M, Schepp C et al. Impaired suppression of synovial
  fluid CD4\textsuperscript{+}CD25\textsuperscript{-} T cells from
  patients with juvenile idiopathic arthritis by
  CD4\textsuperscript{+}CD25+ Treg cells. Arthritis Rheum 2011;
  63:3153-62.
\item
  Khader SA, Bell GK, Pearl JE et al. IL-23 and IL-17 in the
  establishment of protective pulmonary CD4+ T cell responses after
  vaccination and during Mycobacterium tuberculosis challenge. Nat
  Immunol 2007; 8:369-77.
\item
  Rosain J, Kong XF, Martinez-Barricarte R et al. Mendelian
  susceptibility to mycobacterial disease: 2014-2018 update. Immunol
  Cell Biol 2019; 97:360-67.
\item
  Caretto D, Katzman SD, Villarino AV et al. Cutting edge: the Th1
  response inhibits the generation of peripheral regulatory T cells. J
  Immunol 2010; 184: 30-4.
\item
  Diller ML, Kudchadkar RR, Delman KA et al. Balancing inflammation: the
  link between Th17 and regulatory T cells. Mediators Inflamm 2016;
  2016:6309219.
\item
  Ye ZJ, Zhou Q, Du RH et al. Imbalance of Th17 cells and regulatory T
  cells in tuberculous pleural effusion. Clin Vaccine Immunol 2011;
  18:1608-15.
\item
  Day CL, Abrahams DA, Bunjun R et al. PD-1 Expression
  on\emph{Mycobacterium tuberculosis} -specific CD4 T cells is
  associated with bacterial load in human tuberculosis. Front Immunol.
  2018; 9:1995.
\item
  Amaral EP, Machado de Salles \selectlanguage{ngerman}É, Barbosa Bomfim CC et al. Inhibiting
  adenosine receptor signaling promotes accumulation of effector
  CD4\textsuperscript{+} T cells in the lung parenchyma during severe
  tuberculosis. J Infect Dis 2019; 219: 964-974.
\item
  Phillips BL, Mehra S, Ahsan MH et al. LAG3 expression in
  active\emph{Mycobacterium tuberculosis} infections. Am J Pathol 2015;
  185:820-33.
\item
  Gotot J, Gottschalk C, Leopold S et al. Regulatory T cells use
  programmed death 1 ligands to directly suppress autoreactive B cells
  in vivo. Proc Natl Acad Sci U S A 2012; 109:10468-473.
\item
  Patterson SJ, Pesenacker AM, Wang AY et al. T regulatory cell
  chemokine production mediates pathogenic T cell attraction and
  suppression. J Clin Invest 2016; 126:1039-351.
\item
  Barchet W, Price JD, Cella M et al. Complement-induced regulatory T
  cells suppress T-cell responses but allow for dendritic-cell
  maturation. Blood 2006; 107:1497-504.
\item
  Kemper C, Chan AC, Green JM et al. Activation of human
  CD4\textsuperscript{+} cells with CD3 and CD46 induces a T-regulatory
  cell 1 phenotype. Nature 2003; 421:388-92.
\item
  Ikeda T, Hirata S, Fukushima S et al. Dual effects of TRAIL in
  suppression of autoimmunity: the inhibition of Th1 cells and the
  promotion of regulatory T cells. J Immunol 2010; 185:5259-67.
\item
  Yi Z, Lin WW, Stunz LL, Bishop GA. The adaptor TRAF3 restrains the
  lineage determination of thymic regulatory T cells by modulating
  signaling via the receptor for IL-2. Nat Immunol;15: 866-74.
\item
  Truscott SM, Abate G, Price JD et al. CD46 engagement on human
  CD4\textsuperscript{+} T cells produces T regulatory type 1-like
  regulation of antimycobacterial T cell responses. Infect Immun 2010;
  78: 5295-306.
\item
  Bryce PJ, Oyoshi MK, Kawamoto S et al. TRAF1 regulates Th2
  differentiation, allergic inflammation and nuclear localization of the
  Th2 transcription factor, NIP45. Int Immunol 2006; 18:101-11.
\item
  Brunner T, Kasibhatla S, Pinkoski MJ et al. Expression of Fas ligand
  in activated T cells is regulated by c-Myc. J Biol Chem 2000;
  275:9767-72.
\item
  Zhang XR, Zhang LY, Devadas S et al. Reciprocal expression of TRAIL
  and CD95L in Th1 and Th2 cells: role of apoptosis in T helper subset
  differentiation. Cell Death Differ 2003; 10:203-10.
\item
  Xie J, Wang H. Semaphorin 7A as a potential immune regulator and
  promising therapeutic target in rheumatoid arthritis. Arthritis Res
  Ther 2017; 19:10.
\item
  Sun X, Yamada H, Shibata K, et al. CD30 ligand/CD30 plays a critical
  role in Th17 differentiation in mice. J Immunol 2010; 185:2222-30.
\item
  Bhaumik S, Basu R. Cellular and molecular dynamics of Th17
  differentiation and its developmental plasticity in the intestinal
  immune response. Front Immunol 2017; 8:254.
\item
  Snook JP, Kim C, Williams MA. TCR signal strength controls the
  differentiation of CD4\textsuperscript{+} effector and memory T cells.
  Sci Immunol 2018; 3(25).
\item
  Baecher-Allan C, Viglietta V, Hafler DA. Inhibition of human
  CD4\textsuperscript{+}CD25\textsuperscript{+/high} regulatory T cell
  function. J Immunol 2002; 169:6210 -7.
\item
  Moguche AO, Musvosvi M, Penn-Nicholson A et al. Antigen availability
  shapes T cell differentiation and function during tuberculosis. Cell
  Host Microbe 2017; 21:695-706.
\item
  Sakai S, Kauffman KD, Sallin MA et al. CD4 T Cell-derived IFN-\selectlanguage{greek}γ \selectlanguage{english}plays
  a minimal role in control of pulmonary mycobacterium tuberculosis
  infection and must be actively repressed by PD-1 to prevent lethal
  disease. PLoS Pathog 2016; 12:e1005667.
\item
  Lyadova IV, Tsiganov EN, Kapina MA et al. In mice, tuberculosis
  progression is associated with intensive inflammatory response and the
  accumulation of Gr-1 cells in the lungs. PLoS One 2010; 5:e10469.
\item
  Orme IM, Robinson RT, Cooper AM. The balance between protective and
  pathogenic immune responses in the TB-infected lung. Nat Immunol 2015
  Jan;16(1):57-63.
\item
  Gideon HP, Phuah J, Myers AJ et al. Variability in tuberculosis
  granuloma T cell responses exists, but a balance of pro- and
  anti-inflammatory cytokines is associated with sterilization. PLoS
  Pathog 2015; 11: e1004603.
\item
  Fletcher HA, Snowden MA, Landry B et al. T-cell activation is an
  immune correlate of risk in BCG vaccinated infants. Nat Commun 2016;
  7:11290.
\item
  Pandiyan P, Zheng L, Ishihara S et al.
  CD4\textsuperscript{+}CD25\textsuperscript{+}Foxp3\textsuperscript{+}regulatory
  T cells induce cytokine deprivation-mediated apoptosis of effector
  CD4\textsuperscript{+} T cells. Nat Immunol 2007; 8:1353-62.
\item
  Bettini M, Vignali DA. Regulatory T cells and inhibitory cytokines in
  autoimmunity. Curr Opin Immunol 2009; 21:612-8.
\item
  Vignali DA, Collison LW, Workman CJ. How regulatory T cells work. Nat
  Rev Immunol 2008; 8:523-32.
\item
  Gondek DC, Lu LF, Quezada SA, et al. Cutting edge: contact-mediated
  suppression by CD4\textsuperscript{+}CD25\textsuperscript{+}regulatory
  cells involves a granzyme B-dependent, perforin-independent mechanism.
  J Immunol 2005; 174:1783-86.
\item
  Onishi Y, Fehervari Z, Yamaguchi T, Sakaguchi S. Foxp3+ natural
  regulatory T cells preferentially form aggregates on dendritic cells
  in vitro and actively inhibit their maturation. Proc Natl Acad Sci U S
  A 2008; 105:10113-8.
\item
  Fallarino F, Grohmann U, Hwang KW et al. Modulation of tryptophan
  catabolism by regulatory T cells. Nat Immunol 2003; 4:1206-212.
\item
  Huang CT, Workman CJ, Flies D et al. Role of LAG-3 in regulatory T
  cells. Immunity 2004; 21:503-13.
\item
  Deaglio S, Dwyer KM, Gao W et al. Adenosine generation catalyzed by
  CD39 and CD73 expressed on regulatory T cells mediates immune
  suppression. J Exp Med 2007; 204: 1257-65.
\item
  Bopp T, Becker C, Klein M et al. Cyclic adenosine monophosphate is a
  key component of regulatory T cell-mediated suppression. J Exp Med
  2007; 204:1303-310.
\end{enumerate}

\textbf{Table 1: A summary of clinical conditions both autoimmune (peach
highlighted) and infection (blue highlighted) where Treg frequency and
function are compromised.}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Sl. No.}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Clinical Condition}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Finding}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Reference}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
1.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Rheumatoid Arthritis (RA)\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Polymorphisms in FOXP3 gene associated with reduced frequency of Treg,
TGF\selectlanguage{greek}β \selectlanguage{english}and IL-10, in RA\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}29{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Increased circulating HLA-DR\textsuperscript{+}Tregs or
inflammation-associated Tregs which are suppressive but have similar TCR
repertoire as pathogenic CD4\textsuperscript{+} T cells\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}30{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Reduced frequencies of nTreg in patients with RA\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}31{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Tregs unable to suppress spontaneous generation of TNF\selectlanguage{greek}α \selectlanguage{english}in synovial
cells of RA patients due to reduced expression of CTLA-4 and LFA-1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}32{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
2.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Multiple Sclerosis (MS)\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
CD4\textsuperscript{+}CD25\textsuperscript{+} Treg cells/Treg-derived
exosomes from multiple sclerosis patients are inefficiently suppressive.
Circulating exosomes with significantly high miRNA let-7i in MS
patients, inhibit Treg function through an IGFR1 and TGFBR1
mechanism.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}33-35{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
CD25\textsuperscript{+}CD127\textsuperscript{low} Treg development and
function are perturbed. CD39\textsuperscript{+}FoxP3\textsuperscript{+}
memory Treg are diminished in MS patients. Expression of PD-1 is high on
theses Tregs in MS, suggesting possible exhaustion and compromised
function.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}36,37{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
3.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Systemic Lupus Erythromatosis (SLE)\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
CD25\textsuperscript{+}Lag3\textsuperscript{+} T cells, expressing FoxP3
and IL-17A, but not being suppressive are increased in patients with
SLE. The frequency of CD25\textsuperscript{+}Lag3\textsuperscript{+}
cells positively correlates with SLE disease activity.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}38{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
4.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Type 1 Diabetes\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Reduced suppressive function of Treg cells in Type 1 diabetes patients
possibly due to reduced CD39 expression on memory Treg cells.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}39{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Differentiation and stability of Tregs is impaired in Type 1 diabetes
through a miRNA-1423p dependent mechanism.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}40{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
FoxP3 expression declines with type 1 diabetes disease progression
suggesting loss in Treg function. The rate of loss is greatest in
Peptidase inhibitor -16 or Pi16\textsuperscript{+} Treg cells\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}41{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
5.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Malaria\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
FoxP3\textsuperscript{+}Treg cells increase in humans and mice during
blood stage malaria and hamper Th and Tfh-B cell interactions.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}42{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Frequency of FoxP3\textsuperscript{+} Tregs declines in children with
age in high exposure malaria settings.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}43{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
6.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Dengue\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Treg frequencies are higher in mild cases of dengue compared to moderate
cases and healthy controls.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}44{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Treg frequencies in acute dengue fever are high and most of the expanded
Treg population comprises of na\selectlanguage{ngerman}ïve Tregs with poor suppressive
potential.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}45{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
7.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
HIV\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
HIV infected paediatric slow progressors have higher Treg absolute
numbers with a suppressive phenotype compared to rapid
progressors.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}46{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Depletion of
CD4\textsuperscript{+}CD25\textsuperscript{hi}CD62L\textsuperscript{hi}
Tregs are depleted in HIV infection and this correlates with immune
activation.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}47{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
HIV+ elite suppressors maintain higher levels of Treg and lower immune
activation compared to progressors.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}48{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Frequency of PD-1\textsuperscript{+} Tregs increases in HIV and blockade
of the PD-1/PD-L1 pathway increases TGF-\selectlanguage{greek}β \selectlanguage{english}and IL-10 in
CD4\textsuperscript{+}CD25\textsuperscript{hi}CD127\textsuperscript{lo}Treg
cells.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}49{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Individuals who do not respond to ART have fewer dysfunctional Tregs
with defects in mitochondrial function compared to healthy controls and
HIV patients who respond to ART.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}50{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
8.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Candida infection\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{Candida albicans} infection in a mouse model drives expansion of
Tregs which corresponds with increased fungal burden. Expanded Tregs
suppress Th1 and Th2 but promote pathogenic Th17 responses.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}51, 52{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
9.\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Leishmaniasis\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Foxp3\textsuperscript{+}IL-10\textsuperscript{+} Treg cells are enriched
in bone marrow of visceral leishmaniasis patients with high parasite
load compared to those with low parasite load. Frequency of
CD4\textsuperscript{+}CD25\textsuperscript{hi}FoxP3\textsuperscript{+}
Treg cells correlates with parasite load in Kala-azar patients infected
with \emph{Leishmania donovani}.\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\emph{{[}53, 54{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Tuberculosis\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
While some studies have found Treg frequencies to increase in blood and
lungs of TB patients, others have found them to remain unchanged.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}55-61{]}}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 2: A summary of clinical conditions where Teff
susceptibility to Treg mediated suppression is altered.}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Sl. No}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Clinical Condition}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Finding}\strut
\end{minipage} & \begin{minipage}[b]{0.22\columnwidth}\raggedright\strut
\textbf{Reference}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
1.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Type I Diabetes\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Resistance of Teff cells to Treg mediated suppression via faster
activation of STAT3 signaling post TCR stimulation in Type-1 diabetes
patients\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}62{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Effector T cells from type 1 diabetes patients are resistant to
suppression mediated by CD4\textsuperscript{+}CD25\textsuperscript{+}
Treg cells.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}63, 64{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
2.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Rheumatoid Arthritis\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Synovial CD161\textsuperscript{+}Th17 cells are resistant to Treg
mediated suppression in rheumatoid arthritis patients\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}65{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
3.\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
HIV Infection\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
Increased sensitivity of CD4\textsuperscript{+}CD25\textsuperscript{-}
Teff cells to Treg mediated suppression in HIV\textsuperscript{+}
asymptomatic individuals compared to progressors. HLA-B*27 and HLA-B*57
restricted CD8\textsuperscript{+} T cells associated with protection
against HIV are not suppressed by Treg cells.\strut
\end{minipage} & \begin{minipage}[t]{0.24\columnwidth}\raggedright\strut
\emph{{[}66, 67{]}}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
4.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Tuberculosis\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} memory T cells which are
IFN-\selectlanguage{greek}γ\selectlanguage{english}\textsuperscript{hi}IL-2\textsuperscript{hi}IL-17\textsuperscript{hi}IL-22\textsuperscript{hi}
are resistant to Treg mediated suppression in TB patients.\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\emph{{[}61{]}}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1: Mechanisms of Treg mediated suppression.} Primary Treg
suppression mechanisms include (1) acting as sink for IL-2 due to
constitutive high expression of IL-2R and consequently depriving
effector T cells of the crucial cytokine {[}121{]}; (2) secretion of
immune-suppressive cytokines IL-10, TGF\selectlanguage{greek}β \selectlanguage{english}and IL-35 {[}122,123{]}; (3)
Granzyme-B dependent killing of target cells {[}124{]}; (4) inhibitory
signalling through binding of CTLA-4 on Tregs and CD80/86 on DCs and
reverse signalling via this interaction leading to elevated levels of
IDO in DCs which eventually deplete tryptophan and starve effector T
cells {[}125, 126{]}; (5) binding of LAG3 to MHC-II molecules on DCs
causing reduction in Ag presentation {[}127{]}; (6) suppression due to
interaction of PD-1 on Tregs and PD-L1 on target cells {[}61, 100{]};
(7) extracellular adenosine generated from ATP in concert by cell
surface CD39 and CD73 (ecto-5'-nucleotidase) interacts with A2AR on
effector T cells and suppresses their function by increasing cAMP levels
{[}128, 129{]}; (8) chemokines CCL3 and CCL4 secreted by Tregs bind to
CCR5 on effector cells triggering their migration and subsequent
suppression {[}61, 101{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2:} \textbf{A diagrammatic model which highlights the
difference in Treg suppression in healthy and latently infected
individuals and active TB subjects in context of expansion of
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} memory T
cells.}Individuals infected with TB can either clear the bacteria,
become latently infected or come down with active TB disease. There is
also a possibility of reactivation of TB in latently infected subjects.
The reasons for this can be HIV co-infection, treatment with check-point
inhibitors like anti-PD-1, therapies such as anti-TNF for rheumatoid
arthritis etc. HLA-DR\textsuperscript{+} activated cells are low in
healthy and latently infected individuals and Treg suppression is good.
However, in active TB, HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T
cells expand and Treg mediated suppression becomes poor. The Treg
suppression pathways that are rendered inactive in TB are the PD-1/PD-L1
and \selectlanguage{greek}β-\selectlanguage{english}chemokine-CCR5 dependent. The reason for their becoming inactive
could be possible counter-regulation by IL-2, IL-17A, IFN\selectlanguage{greek}γ, \selectlanguage{english}IL-22 that
are secreted by the expanded
HLA-DR\textsuperscript{+}CD4\textsuperscript{+} T cells.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
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\textbf{Figure 3:} \textbf{Treg suppression resistant total Teff cells
and Treg suppression sensitive HLA-DR- Teff cells have distinct
expression patterns with respect to certain cytokines and cell surface
receptors.} Total Teff and HLA-DR- Teff were sorted by flow cytometry
from 5 pulmonary TB patients. Cells were activated for different times
with anti-CD3/CD28 mitogenic beads, RNA was isolated and an RNA-Seq was
performed. Expression at each time was compared to baseline unactivated
control to arrive at a DEG list. A final DEG list was prepared applying
a cutoff of log2 fold change +/-2.5 and p \textless{} 0.05 (for details
on procedure and complete DEG list please see Ahmed A et al., 2018). The
DEG list was further mined to study expression of cytokines, chemokines,
activation markers, cell surface markers and transcription factors. A
summary of these results is shown. The numbers in boxes denote log2 fold
change for expression at 2, 24 and 96 hrs compared to expression at
baseline. For details of the complete DEG list, kindly see {[}61{]}

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