Group A Streptococcus (GAS) infections elicit both conventional and unconventional T cell responses, yet their immunological signatures remain incompletely understood. We investigated the activation and cytokine responses of CD4+, CD8+, and mucosal-associated invariant T (MAIT) cells from healthy UK adult donors following stimulation with fixed GAS and selected antigens SpyCEP, SpyAD, and MalE. Using activation-induced marker (AIM) assays, intracellular cytokine staining (ICS), ELISpot, and proliferation assays, we characterised the magnitude and specificity of these responses.
Among the T cell subsets tested, MAIT cells exhibited the highest frequencies of activation and IFN-γ responses to fixed GAS. They also produced substantial IL-17 in response to fixed GAS, SpyCEP and SpyAD. MAIT cell responses were largely driven by IL-12 and IL-18, highlighting the role of cytokine-mediated activation. CD8+ T cells demonstrated higher frequencies of IFN-γ, IL-2, and TNF production compared to CD4+ T cells, suggesting a dominant cytotoxic role. ELISpot confirmed robust antigen-specific IFN-γ secretion in response to GAS, and proliferation assays demonstrated robust expansion of GAS-reactive CD4+, CD8+, and MAIT cells. Notably, CD4+ T cells proliferated the most in response to SpyAD. We used a superantigen-negative strain of GAS, yet it elicited responses similar to the wild-type strain, suggesting that T cell activation in vitro is not solely driven by superantigen activity.
To further dissect the molecular mechanisms underlying GAS-specific T cell responses, we are conducting transcriptomic analyses of GAS-reactive T cells. This will provide deeper insights into their functional programming and potential implications for vaccine and therapeutic strategies targeting GAS infections.