PRF1 and COVID-19: Although CD56dim NK cells were only minimally activated in a Transwell setting, we found that CD56bright CD16− NK cells had strongly increased expression of Granzyme B and Ki-67 after Transwell coculture with monocytes from severe COVID-19 patients (Fig. 6D), although these cells did not undergo an increase in Perforin expression in a Transwell setting (Fig. 6D).