Cytotoxic CD4 Development Requires CD4 Effectors to Concurrently Recognize Local Antigen and Encounter Infection-Induced IL-15

Cytotoxic CD4 T cells (ThCTL) are tissue-resident effectors that enhance viral clearance by MHC-II-restricted cytotoxicity of infected cells. Using a model of influenza A virus (IAV) infection, we identify key factors that drive CD4 effectors to differentiate into lung-resident ThCTL. We find that, to become ThCTL, CD4 effectors must again recognize cognate antigen on antigen presenting cells (APC) within the lung. Different APC subsets can drive this transition, including dendritic cells, B cells, and to a lesser extent non-hematopoietic MHC-II+APC. CD28 co-stimulation is not required and can reduce ThCTL development. In contrast, T follicular helper cells (TFH) that are another specialized CD4 effector subset, require CD28 during this time. Optimal ThCTL generation also requires ongoing infection in the effector phase, that acts independently of antigen presentation. The mechanism involves production of Type I IFN, that induces IL-15 which acts to support further differentiation of CD4 effectors to ThCTL. The multiple spatial, temporal and cellular requirements for ThCTL generation from CD4 effectors described here would be expected to prevent cytotoxic CD4 responses in the lung after pathogen has already been cleared, while ensuring the development of potent lung-restricted ThCTL effectors when pathogen persists.