CD4 Effectors Need to Recognize Antigen Locally to Become Cytotoxic CD4 and Follicular Helper T Cells [preprint]

T follicular helper (TFH) and Cytotoxic CD4 (ThCTL) are tissue-restricted CD4 effector subsets, functionally specialized to mediate optimal Ab production and cytotoxicity of infected cells. Influenza infection generates robust CD4 responses, including lung ThCTL and SLO TFH, that protect against reinfection by variant strains. Antigen (Ag) presentation after infection, lasts through the effector phase of the response. Here, we show that this effector phase Ag presentation, well after priming, is required to drive CD4 effectors to ThCTL and TFH. Using in vivo influenza models, we varied Ag presentation to effectors acutely, just at the effector phase. Ag presentation was required in the tissue of effector residence. We suggest these requirements contain unnecessary or potentially pathogenic CD4 responses, only allowing them if infection is uncleared. The results imply that providing effector phase Ag, would lead to stronger humoral and CD4 tissue immunity and thus can be applied to improve vaccine design

Pathogen Recognition by CD4 Effectors Drives Key Effector and Most Memory Cell Generation Against Respiratory Virus

Although much is known about the mechanisms by which pathogen recognition drives the initiation of T cell responses, including those to respiratory viruses, the role of pathogen recognition in fate decisions of T cells once they have become effectors remains poorly defined. Here, we review our recent studies that suggest that the generation of CD4 T cell memory is determined by recognition of virus at an effector “checkpoint.” We propose this is also true of more highly differentiated tissue-restricted effector cells, including cytotoxic “ThCTL” in the site of infection and TFH in secondary lymphoid organs. We point out that ThCTL are key contributors to direct viral clearance and TFH to effective Ab response, suggesting that the most protective immunity to influenza, and by analogy to other respiratory viruses, requires prolonged exposure to antigen and to infection-associated signals. We point out that many vaccines used today do not provide such prolonged signals and suggest this contributes to their limited effectiveness. We also discuss how aging impacts effective CD4 T cell responses and how new insights about the response of aged naive CD4 T cells and B cells might hold implications for effective vaccine design for both the young and aged against respiratory viruses

Cytotoxic CD4 development requires CD4 effectors to concurrently recognize local antigen and encounter type I IFN-induced IL-15

Summary: Cytotoxic CD4 T cell effectors (ThCTLs) kill virus-infected major histocompatibility complex (MHC) class II+ cells, contributing to viral clearance. We identify key factors by which influenza A virus infection drives non-cytotoxic CD4 effectors to differentiate into lung tissue-resident ThCTL effectors. We find that CD4 effectors must again recognize cognate antigen on antigen-presenting cells (APCs) within the lungs. Both dendritic cells and B cells are sufficient as APCs, but CD28 co-stimulation is not needed. Optimal generation of ThCTLs requires signals induced by the ongoing infection independent of antigen presentation. Infection-elicited type I interferon (IFN) induces interleukin-15 (IL-15), which, in turn, supports CD4 effector differentiation into ThCTLs. We suggest that these multiple spatial, temporal, and cellular requirements prevent excessive lung ThCTL responses when virus is already cleared but ensure their development when infection persists. This supports a model where continuing infection drives the development of multiple, more differentiated subsets of CD4 effectors by distinct pathways