ACE2 and infection: We next tested human peripheral blood mononuclear cells (PBMCs), which do not express ACE2 (SI Appendix, Fig. S2D), and observed apparent cell-to-cell transmission for both SARS-CoV and SARS-CoV-2, yet no/low cell-free infection was detected, the latter being consistent with recently published results (51) (Fig. 4 E and F).