ICOS and infection: More strikingly, PD-1 and ICOS remained at significantly high levels in lung tissue resident Treg cells, as compared to other T cell subsets (Figure 6B, C, last two columns); PD-1 was rapidly downregulated while ICOS was up-regulated by 5 days post secondary infection, and adjusted back to the levels observed before re-infection by 19 days post secondary infection (Figure 6B, C) which are counter-correlated to the kinetics of lung tissue resident Treg cell expansion and IL-10 expression following secondary flu.