iTr35 cells: a novel immunoregulatory subset involved in bone loss in rheumatoid arthritis and clinical implications

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by chronic inflammation and progressive bone destruction. While classical regulatory T (Treg) cells are known to inhibit osteoclastogenesis, the role of non-classical subsets such as inducible IL-35-producing regulatory T cells (iTr35) in RA-related bone loss remains unexplored.

This prospective case-control study aimed to evaluate the expression of iTr35 cells in RA patients and assess their association with bone mineral density (BMD) and serum bone turnover markers.

Peripheral blood mononuclear cells (PBMCs) were collected from 34 untreated RA patients and 34 age- and sex-matched healthy controls. Flow cytometry was used to measure the frequencies of iTr35 (CD4⁺Foxp3^-Ebi3⁺IL-12p35⁺), conventional Tregs, Teff, and naïve CD4⁺ T cells. BMD at the lumbar spine and femoral neck was determined using DXA, while serum levels of TRAP5b, CTX-I, and osteocalcin were measured. Multivariate regression was applied to assess the associations between iTr35 frequency and bone-related parameters.

RA patients exhibited significantly decreased frequencies of circulating iTr35 and Treg cells, along with increased Teff. iTr35 levels were strongly negatively correlated with BMD at the lumbar spine (r = -0.6162, p = 0.0001) and femoral neck (r = -0.6421, p < 0.0001). Moreover, iTr35 frequency was inversely associated with TRAP5b and CTX-I, and positively correlated with osteocalcin.

These findings identify iTr35 as a novel immunoregulatory T cell subset potentially involved in protecting against bone loss in RA. This study highlights the significance of iTr35 in osteoimmune regulation and suggests its therapeutic potential in preventing skeletal damage in autoimmune diseases.