CD4 and infection: These observations suggest that the airway niche promotes survival and/or maintenance of recruited CD4+ T cells (perhaps via MHC-II plus the costimulatory CD40 and/or CD86 of the club and multiciliated cells) to form CD4+ TRM cells, while the parenchymal niches instigate contraction of CD4+ T cells (perhaps via LEC MHC-II plus coinhibitory molecules like PD-L1) during resolution of infection.