Generation of tissue-restricted CD4 effectors (TFH and ThCTL) requires signals from local antigen & infection during the effector phase

Abstract Key specialized tissue-restricted CD4 effector subsets such as T helper cytotoxic cells (ThCTL) in the infected tissue and T follicular helper cells (TFH) in secondary lymphoid organs arise after other CD4 effectors (Th1, Th17) peak. In an influenza A virus (IAV) infection model, we find that generation of ThCTL and TFH require that CD4 effectors recognize specific Ag/MHC Class II, 5–7 days after initial infection. We restrict Ag presentation to specific APC subsets and add various APC via different routes, to study which APC and infection conditions drive TFH vs. ThCTL generation at the checkpoint. We find most MHC Class II-expressing APC subsets, including dendritic cells and B cells, are able to drive TFH and ThCTL generation. However, local Ag presentation is crucial. Strikingly, we find that continuing infection, even at this late effector phase, is needed for optimal generation of both subsets. This contrasts with CD4 memory generation, which in our previous studies, was not dependent on infection at this checkpoint. Thus, at the effector checkpoint, Ag presentation in the local tissue drives both optimal specialized effector and most memory generation, whilst signals from infection are required for optimum tissue-restricted effector differentiation. The infection signal can be provided by multiple PRR pathways. We suggest that vaccines will need to be designed to provide additional Ag presentation and the signals associated with infection at the effector checkpoint to induce optimum protective immune responses: to promote strong CD4 memory for broad, heterosubtypic protection and indirectly strong B cell response and memory leading to long-term humoral protection.