CD4 and infection: Since it was shown that CD4+ T cells can foster the development of lung CD8+ TRM cells during infection with influenza virus [137], it is conceivable that CD4+ TRM may also contribute to the generation of CD8+ TRM. Interestingly, CD4+ TRM outnumber CD8+ TRM in many tissues [23,123], suggesting a critical role for CD4+ TRM in tissue-specific immunity and barrier function.