PRF1 and COVID-19: However, dysregulation of NK cells has been observed, the cells exhibiting depletion and dysfunction in COVID-19 patients [26,27,28,33,34,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90] (Figure 1), accompanied by expansions of adaptive NK cells [26,77,79,89] and more mature perforin- and granzyme B-armed CD56bright NK cells [77,89], as well as the recruitment of CD34+DNAM-1brightCXCR4+ inflammatory precursors from the bone marrow [86].