INTRODUCTION
The diversity and functionality of CD8+ T cells
recruited during an epitope-specific response is thought to arise
through the contribution of antigen density1, antigen
sensitivity2, TCR gene biases3, and
other factors, but the precise contributions are incompletely
understood. There are further constraints which shape the diversity of
the final repertoire based on structural features at key conserved
residues4 and consideration of a T-cell clone’s
pre-existing frequency in the naïve repertoire5,6although this may be overcome by repeated antigen re-exposure through
vaccination7. Typically, these factors conspire to
generate epitope-specific responses consisting of a range of unique TCR
arrangements (clonotypes)8, with a distinct affinity
for their cognate peptide in the context of MHC (pMHC). Notably however,
epitope-specific populations have been observed to comprise few or
single unique clonotypes during infection with human immunodeficiency
virus9 and Mycobacterium
tuberculosis 10. Such monoclonal populations may have
arisen through a selective advantage intrinsically conferred by its
specific TCR clonotype.
The affinity of the TCR-pMHC interaction may be measured by both its
association (kon) and dissociation
(koff) constants, which collectively contribute to its
host cell’s activation potency11. One such method for
the measurement of the koff rate utilises dissociable
HLA multimers, so-called MHC Streptamers, which allow the real-time
tracking of dissociation of monomeric TCR-pMHC complexes by flow
cytometry12-14. Notably, CD8+ T cell
clonotypes with lower koff constants (or practically,
high-affinity clones with extended dissociation times) have improved
protective capacity when used for adoptive transfer13and high affinity neoantigen-specific clones are more effective at
slowing tumor growth15. By contrast, low-affinity
clonotypes are preferentially expanded long-term during chronic
cytomegalovirus infection16, may be better suited for
providing responses optimised against antigens with relatively higher
abundance17, and provide the flexibility for
recognising and responding to mutated epitopes18.
Taken together, these studies provide evidence that the affinity of
TCR-pMHC interactions likely influence the functional capacity of
epitope-specific populations.
In this study, we characterised the phenotype and affinity of a
naturally-derived monoclonal CD8+ T cell population
arising from an individual with acutely resolving infection that we had
reported on previously19. We examined three hepatitis
C virus (HCV)-specific CD8+ T cell populations from
two individuals and profiled each population’s respective repertoire
diversity, phenotypic characteristics, and dissociation affinities at
the single-cell level. The koff rate-derived affinity of
the monoclonal TCR clonotype was the strongest out of all tested
sequences, although clonotypes from a polyclonal population targeting
the same epitope also displayed surprisingly strong affinities. Notably,
analysis of the transcriptional signatures from each population revealed
associations between epitope specificity, affinity and molecular
phenotype. Our findings contribute to an improved understanding of the
heterogeneity of TCR repertoires generated by viral infections and offer
insights into how affinity may shape the phenotype and expansion of T
cells at the epitope-specific level.