PDCD1 and infection: An inverse trend of TGF-β vs. PD-1 expression was observed with infection, which was altered to a positive correlation upon METH exposure in CD4 and CD8 T cells (CD4—LCMV vs. LCMV-METH–r = -0.0965 vs. r = 0.1949; CD8—LCMV vs. LCMV-METH–r = -0.06738 vs. 0.3093).