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Purpose of Review: Epigenetics is the study of inherited phenotype changes that do not involve in alteration of DNA sequence. Epigenetic regulation can be examined under three main headings and study methodologies for all three will be discussed: DNA methylation is the addition of a methyl (CH 3 ) group to DNA, histone modifications is a covalent post-translational modification of histones and non-coding RNAs are a group of functional RNA molecules that is copied from DNA but not converted into proteins. Epigenetic changes are being increasingly studied in the pathogenesis of most diseases including autoimmune and autoinflammatory diseases to shed light on the different phenotypes and disease courses. We have aimed to review the basic concepts in epigenetic studies and summarize the data for epigenetics in autoimmune and autoinflammatory rheumatic diseases., Recent Findings: Recent studies have assessed DNA hypomethylation in interferon-regulated genes in autoimmune diseases and in inflammatory pathways in systemic autoinflammatory diseases (SAIDs). Abnormal histone acetylation and methylation have been shown to be important in autoimmune diseases which was proven via effective targeted treatment trials against these pathways in mice models. miRNAs have an important role in the pathogenesis, and also, they can be used as diagnostic biomarkers in SAIDs (i.e., FMF, Behcet's disease) together with autoimmune diseases. Although the number of studies has increased over the years in parallel with the increase of interest in this field, we await further studies to improve the understanding and management of pediatric rheumatic diseases. Epigenetic studies in pediatric rheumatic diseases have enabled us to gain new information about disease pathogenesis, clinical heterogeneity, and prognosis. Further studies will help us define new diagnostic, prognostic, and therapeutic goals for rheumatic diseases.

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. [ABSTRACT FROM AUTHOR]