CD4 and COVID-19: We observed that TREM-2 was predominantly expressed in infiltrated CD4+ T cells that had infiltrated the lungs of patients with COVID-19, while in the lungs of healthy controls, TREM-2+CD4+ T cells were almost undetectable, and TREM-2 was mainly expressed in other types of cells, hypothesized to be alveolar macrophages and epithelial cells (Fig. 1I).