TREM-2–Fc fusion protein, used to block the interaction between TREM-2 and M protein, reduced the phosphorylation of CD3ζ (Tyr83) and ZAP70 (Tyr319) in CD4+ T cells of patients with COVID-19 upon pseudovirus–M protein stimulation, whereas control IgG did not affect the phosphorylation of CD3ζ/ZAP70 (Fig. 4, C and D). Here, CD4 is linked to COVID-19.