CD4 and infection: Resident memory T cells may represent a first line defense against pathogens at sites of infection, whereas resident CD4 Treg cells may ensure tissue integrity by dampening T cell responses to self antigens and commensal bacteria antigens, and by controlling crosstalks between immune and non-immune cells6–8; for example, skin resident CD4 Treg cells crosstalk with hair follicle stem to modulate skin wound healing and hair regeneration9,10.