Chronic non-bacterial osteomyelitis and immune checkpoint molecules.

Objective: We aimed to investigate the plasma levels and cell surface expression of two checkpoint molecules, TIM-3 (T cell immunoglobulin and mucin domain-containing protein 3) and PD-1 (programmed cell death protein 1), in pediatric patients with chronic non-bacterial osteomyelitis (CNO).

Methods: Plasma samples of CNO patients were collected at diagnosis or during biologic agent treatment. Plasma levels of TIM-3 and PD-1 were measured using the sandwich enzyme-linked immunosorbent assay method, and the expression of the two immune checkpoint molecules on the cell surface was analyzed by isolating peripheral blood mononuclear cells by density gradient centrifugation technique.

Results: Twenty-seven patients with CNO (14 boys, 51.9%) and six healthy controls (3 boys, 50%) were enrolled in the study. There were no age differences between CNO patients and healthy controls (median age 14.5 vs. 13.5 years, respectively, p=0.762). Of the CNO patients, 18 were included at the time of diagnosis while 9 were receiving biologic treatment at enrollment. The median plasma PD-1 levels were significantly lower in the CNO group than in the healthy controls (p=0.011). However, no significant difference was found in the cellular expression of PD-1 and TIM-3 on CD3+CD4+ T cells in patients and healthy controls (p=0.083 and p=0.245, respectively). There was also no statistically significant difference in plasma TIM-3 levels of the patient and control groups (p=0.981).

Conclusion: CNO is an autoinflammatory disease, and overall, our results suggest that T cell exhaustion may not be significant in CNO. Further research is needed to find out whether the immune checkpoints are mainly associated with autoimmunity but not autoinflammation. Key Points • The median plasma PD-1 levels were significantly lower in the CNO group than in the healthy controls. • No significant difference was found in the cellular expression of PD-1 and TIM-3 on CD3+CD4+ T cells in patients and healthy controls. • Our results suggest that T cell exhaustion may not be significant in CNO pathogenesis.