Evaluation of degranulation and cytokine production in natural killer cells from spondyloarthritis patients at single-cell level

Background: A pathogenetic role of natural killer (NK) cells has been hypothesized in spondyloarthritides (SpA), but still conflicting data exist. Aim of this study was to investigate, in a cohort of SpA patients, the peripheral blood NK cell number, the cytokine production (IFNγ and TNFα), and the cytotoxic activity on target cell stimulation. Moreover, we aimed to evaluate the expression of KIR3DL1, an inhibitory receptor binding HLA class I alleles, including HLA B27 strongly associated with SpA, between different NK cell subsets. Methods: We enrolled 21 SpA patients and 21 healthy controls. Multicolor flow cytometry was used to analyze the cytokine levels triggered by activating receptors, the degranulation as CD107a surface expression on target cell stimulation, the number and KIR3DL1 expression on peripheral blood NK cells. Results: When stimulated with P815 cells preincubated with antibodies against activating receptors, NK cells produced IFNγ to a lesser extent respect to healthy controls (P = 0.0012). No differences were found in TNFα production and NK cell degranulation in patients and controls. We observed a higher frequency of KIR3DL1 positive NK cells from SpA patients than in controls (P = 0.02). Similar results were obtained in NK cell subsets. Conclusions: In our SpA patients, we observed a reduced IFNγ production, which may be explained by the elevated frequency of KIR3DL1 expressing NK cells, while the other parameters were similar to those of healthy controls. These results may support the role of NK cells in the pathogenesis of SpA, although more data are needed to substantiate these observations.