CD4 and COVID-19: Coculturing MILD COVID-19 patient exosomes with CD4+ T cells and autologous monocytes as primary APC source caused a significant increase in CD4+ T-cell HLA-DR protein levels (Figure 3G), an effect that was totally abolished when we blocked MHC-II, confirming the requirement of cross-presentation for efficient CD4+ T-cell activation and indicating a peculiar antigen cross-presentation activity of MILD COVID-19 exosomes.