Reduction of peripheral natural killer cells in patients with SAPHO syndrome.

OBJECTIVES: Little is known about the roles of peripheral immune cell subsets in synovitis, acne, pustulosis, hyperostosis and osteitis (SAPHO) syndrome. Up to now, just a few studies have focused on this issue. We aimed to analyse the distribution and phenotype of T cell subsets and natural killer (NK) cells in the peripheral blood of patients with SAPHO syndrome.

METHODS: The proportion and absolute counts of circulating immune cells were assessed in 19 patients diagnosed as SAPHO syndrome and 19 healthy controls. CD4+T cell subsets were also analysed in 9 untreated SAPHO patients and 9 healthy volunteers by flow cytometry.

RESULTS: The proportion and absolute counts of NK cells were significantly reduced in SAPHO patients in comparison with the controls (proportion, 10% vs. 18%, p<0.001; absolute counts, 231/μl vs. 307/μl, p=0.014). Conversely, the proportion and absolute counts of Th17 cells in untreated SAPHO patients were significantly higher than that in the healthy controls (proportion, 1.49% vs. 0.93%, p=0.004; absolute counts, 14.36/μl vs. 5.14/μl, p<0.001). Similarly, Th17/Th1 cells were significantly increased (proportion, 0.45% vs. 0.33%, p=0.024; absolute number, 5.47/μl vs. 1.98/μl, p<0.001), but there was no significant difference between the percentage and number of Treg cells in patients with SAPHO syndrome and healthy controls. Thus, the ratio of Th17/Treg was increased in SAPHO patients (0.68 vs. 0.17, p=0.004).

CONCLUSIONS: Our data suggested that the immune inflammation in SAPHO patients may be related to the depletion of NK cells and the imbalance of Th17 and Treg cells. A reduction of peripheral NK cells may exacerbate the disease progression by not being inhibited Th17 cells.