KLRK1 and COVID-19: Interestingly, IL-6, that is present in large excess in sera of patients with COVID-19, may down-regulate NKG2D on NK cells, leading to impairment of NK activity.23,24 In keeping with a NK dysfunctional phenotype, IFNγ secretion by NK cells was impaired in COVID-19 patients, particularly in the CD56bright subset.