CD4 and infection: In addition, we found that phosphorylation of CD3ζ (Tyr83) (Fig. 6K), ZAP70 (Tyr319) (Fig. 6L), and STAT1 (Ser727) (Fig. 6M), as well as the expression of T-bet (Fig. 6N), was reduced in lung CD4+ T cells of CD4–TREM-2 KO mice upon MHV-A59 infection.