PRF1 and COVID-19: In contrast to the cytotoxic lymphocytes, CD4+ T cells displayed no or only very limited levels of GrB, perforin, TNF-α, or IFN-γ secretion in COVID-19 patients (Figure 1D and Supplementary Figure S2), in line with the distinct role of cytotoxic lymphocytes for the clearance of pathogen-infected cells through the granule pathway that differs from the canonical role of CD4+ T cells.