CD4 and infection: Alveolar macrophages in the lung have also been shown to express ACE 2, which may indicate that they too are susceptible to infection with SARS-CoV-2 and upon being infected will not only present viral epitopes on MHC I and MHC II for CD8+ and CD4+ recognition, but will also activate anti-viral IFN type I signaling (75, 76).