PRF1 and COVID-19: To explore a possible relationship between IL-22R1 expression by myeloid cells and the functional activity of NK cells or T lymphocytes in COVID-19 patients, the percentages of IL-22, IL-17 and IFN-γ-containing lymphocytes as well as the density of expression for granzyme B and perforin was analyzed by flow cytometry on PBMC, both in the absence and presence of in vitro stimulation.