Tissue resident memory CD4+ T cells are sustained by site-specific levels of self-renewal and continuous replacement

Tissue-resident memory T cells (TRM) protect from repeat infections within organs and barrier sites. The breadth and duration of such protection is defined at minimum by three quantities; the rate at which new TRM are generated from precursors, their rate of self-renewal, and their rate of loss through death, egress, or differentiation. Quanti-fying these processes individually is challenging. Here we combine genetic fate mapping tools and mathematical models to untangle these basic homeostatic properties of CD4+ TRM in the skin and gut lamina propria (LP) of healthy adult mice. We show that CD69+CD4+ TRM in skin reside for [~]24 days and self-renew more slowly, such that clones halve in size approximately every 5 weeks; and approximately 2% of cells are replaced daily from precursors. CD69+CD4+ TRM in LP have shorter residencies ([~]14 days) and are maintained largely by immigration (4-6% per day). We also find evidence that the continuous replacement of CD69+CD4+ TRM at both sites derives from circulating effector-memory CD4+ T cells, in skin possibly via a local CD69- intermediate. Our approach maps the ontogeny of CD4+ TRM in skin and LP and exposes their dynamic and distinct behaviours, with continuous seeding and erosion potentially impacting the duration of immunity at these sites.