Immunofluorescence microscopy detected the localization of TREM-2 on the surface of peripheral CD4+ T cell from patients with COVID-19, while no colocalization of TREM-2 with CD4 was observed in healthy controls, consistent with the extremely low level of TREM-2 on CD4+ T cells from healthy volunteers (Fig. 1, F and G). Here, TREM2 is linked to COVID-19.