ICOS and infection: Surprisingly, CD4+ YFP+ GFP+ T cells from the spleen, liver, lung, blood, and Peyer's patches of mice on day 7 of infection displayed very few phenotypic differences; all populations expressed high levels of CD44, ICOS, PD-1, and Tigit; intermediate levels of KLRG-1 and Lag-3; and low levels of CD62L, CD69, CD25, and CD103 (Fig. 6A).