Emerging role for NK cells in the pathogenesis of inflammatory arthropathies

Natural killer (NK) cells are large, granular lymphocytes devoted to the defense against microbial agents and cancer cells, traditionally recognized as an important arm of the innate immunity, even if more recent data underpin a role also in the responses of acquired immunity. Several studies have led to ascertain that NK cells are involved in the pathogenesis of many immune-mediated diseases, where they may exert both protective and pathogenic roles. In particular, the CD56(bright) NK cell subset, showing immunoregulatory properties, has been found to accumulate in tissue sites of inflammation, such as the skin lesions in psoriatic patients and the synovial membrane in rheumatoid arthritis (RA) patients. In this latter disease, while data on the number of NK cells are still controversial among the different studies, more consensuses exist on the impaired activity of these cells. In another group of inflammatory arthritides, the spondyloarthropathies (SpA), the presence of peculiar allotypes of the killer cell immunoglobulin-like receptors (KIR) superfamily, coding for molecules expressed on NK cells, seems to modulate the susceptibility to this group of diseases, especially ankylosing spondylitis and psoriatic arthritis. Interestingly, in vitro studies showed that NK cells of patients with inflammatory arthropathies might produce pro-inflammatory Th1 cytokines; furthermore, they are involved in bone damage, interact and activate different cell types such as monocytes, dendritic cells and resident fibroblast-like synoviocites cells, thus creating and/or maintaining the inflammatory response. Certainly, these features encourage more research on the role of NK cells in the pathogenesis of inflammatory arthropathies, which could be essential to define potential new therapeutic strategies.