CD4 and infection: The expression of PD-1 on activated CD4+ T effector cells (CD3+CD4+CD44+CD62L−CD27−) (Figure 2A) and of PD-L1 on infiltrating monocytes (CD45+Ly6C+CD11b+) and resident macrophages (CD45+Ly6C−CD11b+F4/80+) (Figure 2B) were assessed in tissues from naïve WT mice and WT mice on days 12 and 35 post-infection (dpi).