Generation of specialized CD4 subsets: TFH and ThCTL, requires signals from local antigen & infection during the effector phase.

Abstract Certain key specialized tissue-restricted CD4 effector subsets such as cytotoxic T cells (ThCTL) in the infected tissue and follicular helper T cells (TFH) in secondary lymphoid organs reach their peak responses after other CD4 effectors (Th1, Th17). Using 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, a phase we have termed the late Ag checkpoint. We used TcR transgenic CD4 donor T cells and host mice in which Ag presentation is restricted to specific APC, as well as addition of various APC subsets at the checkpoint, to study which APC drive TFH vs. ThCTL generation. While tissue-restricted antigen presentation is crucial, several APC subsets were able to present Ag effectively to drive TFH and ThCTL generation. However, we found that host infection during the late Ag checkpoint was needed for optimal generation of donor ThCTL and TFH subsets. This contrasts with most CD4 memory generation that is not dependent on infection. Thus multiple CD4 fate decisions are made at the late Ag checkpoint. Both optimal specialized effector and most memory generation require continuing Ag presentation, while further tissue-specific effector differentiation requires local Ag presentation and we think, signals from pathogen recognition pathways (PRR). We suggest that to induce optimum protective immune responses, vaccines and immunotherapies may need to be designed taking into account these requirements for additional Ag presentation and PRR pathways at the late Ag checkpoint.