TREM2 and infection: Based on the above findings, we considered that the cells traveled from state 1 through branch point 1, state 2, and then to branch point 2, representing clinical outcome progression of the disease, from early infection with SARS-CoV-2 to severe or recovery, during which the expression of representative marker genes including C1QA, CXCL2, and TREM2 in macrophages was changed following pseudo-time (Fig. 6b).