Active but not inactive granulomatosis with polyangiitis is associated with decreased and phenotypically and functionally altered CD56(dim) natural killer cells.

NK cell numbers correlate inversely with GPA activity. Reduced CD56(dim) NK cells in active GPA have an activated phenotype, which intriguingly is associated with profound deficiency in cytotoxicity. These data suggest a function for NK cells in the pathogenesis and/or modulation of inflammation in GPA. NK cell numbers, phenotype (CD16, CD69, NKG2C) or overall natural cytotoxicity are promising candidates to serve as clinical biomarkers to determine GPA activity.