Siglec‑15 dysregulation and its therapeutic implications in new‑onset type 1 diabetes 

Type 1 diabetes (T1D) is a T cell‑mediated autoimmune disorder characterized by the destruction of insulin‑producing β‑cells. Sialic acid‑binding Ig‑like lectin 15 (Siglec‑15) could inhibit T‑cell activation and suppress immune responses. However, the association between Siglec‑15 expression levels and T1D is largely unknown. Serum concentrations of soluble Siglec‑15 were quantified in a cohort comprising 34 individuals newly diagnosed with T1D and 21 healthy control subjects. A murine mesenchymal stem cell (MSC) line, C3H10 T1/2, was genetically engineered to stably express SIGLEC15 through lentiviral transduction. Non‑obese diabetic (NOD) mice were administered treatments with Siglec15‑expressing MSCs, control MSCs, or phosphate‑buffered saline. The present study evaluated diabetes incidence, blood glucose concentrations, the severity of insulitis and the composition of immune cell populations in the pancreatic lymph nodes and spleens. In the present study, measurement of human serum specimens by ELISA revealed a positive association between new‑onset T1D and soluble Siglec‑15 levels. In NOD mice, treatment with Siglec15‑MSCs resulted in a significant reduction in diabetes incidence, preservation of insulin‑positive islets and mitigation of insulitis. Flow cytometric analysis demonstrated an increase in CD4⁺effector memory T cells within the pancreatic‑draining lymph nodes of mice treated with Siglec15‑MSCs, while no significant alterations were observed in splenic T cell populations or the frequencies of regulatory T cells. The findings of this study underscore the potential of Siglec‑15‑overexpressing MSCs as a promising cell‑based therapeutic approach for T1D, primarily through the localized modulation of memory T cells within the pancreatic lymph nodes.