Synovial fluid NKT cells display different properties compared to peripheral blood NKT cells in rheumatoid arthritis

Abstract

Natural killer T cells (NKT) are a population of regulatory T cells that co-express an invariant T cell receptor as well as NK cell markers. Several studies have shown that NKT cells are decreased or dysfunctional in autoimmune conditions such as insulin-dependent diabetes mellitus, systemic sclerosis, systemic lupus erythematosus and multiple sclerosis. Significant therapeutic effects of a-GalactosylCeramide (a-GalCer), a synthetic antigen of NKT cells have therefore been implicated to participate in the regulatory immune mechanisms controlling autoimmunity. However, their role in the pathogenesis of rheumatoid arthritis (RA) remains unclear. To this end, we studied the frequency, cytokine profile and heterogeneity of NKT cells in peripheral blood mononuclear cells (PBMC) of 23 RA patients and 22 healthy controls, which included paired PBMC-synovial fluid (SF) samples of 7 and paired PBMC-synovial tissue (ST) samples of 4 RA patients, respectively. Using flow cytometry, a decreased NKT cell frequency was observed in blood of RA patients compared to healthy controls. In addition, direct ex vivo ELISPOT analysis revealed a reduced IL-4/IFN-g ratio in NKT cells of RA patients. The invariant T cell receptor sequence was detected in paired SF and ST samples. NKT cells of all healthy controls, but only of 53.8% of the RA patients (responders) expanded upon in vitro stimulation. However, reactivity towards a-GalCer was observed in NKT cells isolated from SF of both responder and non-responder RA patients. Intracellular FACS analysis of the cytokine profile of CD4+ and CD4- PBMC derived NKT cell lines of RA patients revealed that both produced significantly less Il-4 compared to those of healthy controls. In contrast SF derived NKT cell lines displayed a Th0 phenotype comparable to that of healthy controls. These findings suggest that SF NKT cells are functional, even in patients with non-responding NKT cells in the blood. In conclusion, our data demonstrate that NKT cells are decreased and biased towards a Th1 phenotype in blood, but are not impaired in SF or RA patients. This indicates NKT cells that might be functionally related to resistance or progressiono rheumatoid arthritis.