Effect of mast cell derived mediators
LIF, OSM, CCL2, CCL3 and CCL4 were released both by high and low affinity IgE signalling, with CCL3 being the only mediator still differentially upregulated by low affinity stimulation 24 hours after activation. This is in contrast to comparable results from a murine response at six hours, where CCL2, CCL3 and CCL4 were predominantly induced by low affinity IgE (2). Possible explanations for this are the lesser difference in IgE affinity in our model (200-fold) than in the murine model (1600-fold), and the inbred nature of the murine model compared with the outbred human mast cell cultures derived from independent individuals. Moreover, the extreme difference in affinity of the murine system is due to different antigens, and not due to difference in antibody affinity for the same allergen, which is used in our system employing well characterized recombinant human IgE clones.
The affinity related differential mediator response was demonstrated to have functional consequence in the previously mentioned mouse model; the distinct signaling profile including IL-8 elicited from the high affinity stimulation accumulated more neutrophils than monocytes/macrophages whereas the low affinity stimulation recruited fewer neutrophils and more monocytes(2). Increased amounts of CCL2, CCL3 and CCL4 have been measured in BAL of patients with severe asthma compared with controls(19,20). Our data suggests that these chemokines could be derived from activated mast cells as part of their modulation of the late phase response.
The presence of neutrophils in induced sputum or BAL of patients with allergic asthma is associated with severe, chronic asthma (21), but the relationship of neutrophils to mast cell induced T2 disease is less clear. The murine study and our human data suggest that sputum neutrophilia may also be explained by high affinity IgE in addition to corticosteroid use or Th1/Th17 immunity (22). Neutrophils release elastase which activates matrix metalloproteinases (MPPs) and degrades extracellular matrix proteins such as collagens to contribute to the development of chronic airway inflammation (23). IL-8 recruits and activates neutrophils during the late phase reaction(24) and elevated levels of IL-8 have been found in BAL fluid from asthmatic patients(25). We observed a significant increase in IL-8 with high affinity IgE compared to low affinity IgE and baseline. Considering the strong effect of IL-8 on neutrophils, our results indicate that the presence of high affinity IgE antibodies may increase the number of neutrophils and thereby the risk for developing chronic allergic inflammation. The role of monocytes/macrophages in allergic asthma is less well understood (26). IL-13 is secreted by Th2 cells, mast cells and basophils(27). It induces IgE class-switch recombination in B-cells and activates, recruits and promotes survival of mast cells and eosinophils(28). Consistent with results from BMMCs(2) we observed a significant increase in IL-13 with high affinity IgE stimulation compared to low affinity IgE and baseline. (29) These observations suggest that mast cells may have a role in both neutrophil, eosinophil and granulocytic asthma, and that affinity of IgE for allergen may explain the shift from one endotype to another.