Vitamin D supplementation induces CatG-mediated CD4þ T cell inactivation and restores pancreatic b-cell function in mice with type 1 diabetes

Abstract

Type 1 diabetes (T1D) is a chronic autoimmune disease accompanied by the immune-mediated destruction of pancreatic b-cells. In this study, we aimed to explore the regulatory effects of vitamin D (VD) supplementation on pancreatic b-cell function by altering the expression of bioinformatically identified cathepsin G (CatG) in T1D mice. A T1D mouse model was established in nonobese diabetic (NOD) mice, and their islets were isolated and purified. Pancreatic mononuclear cells (MNCs) were collected, from which CD4þ T cells were isolated. The levels of interleukin (IL)-2, IL-10, tumor necrosis factor-a (TNF-a), and interferon-c (IFN-c) in the supernatant of mouse pancreatic tissue homogenate were assessed using ELISA. Immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labelin (TUNEL) staining were conducted to evaluate the effects of VD supplementation on pancreatic tissues of T1D mice. The pancreatic b-cell line MIN6 was used for in vitro substantiation of findings in vivo. VD supplementation reduced glucose levels and improved glucose tolerance in T1D mice. Furthermore, VD supplementation improved pancreatic b-cell function and suppressed immunological and inflammatory reactions in the T1D mice. We documented overexpression of CatG in diabetes tissue samples, and then showed that VD supplementation normalized the islet immune microenvironment through downregulating CatG expression in T1D mice. Experiments in vitro subsequently demonstrated that VD supplementation impeded CD4þ T activation by downregulating CatG expression and thereby enhanced pancreatic b-cell function. Results of the present study elucidated that VD supplementation can downregulate the expression of CatG and inhibit CD4þ T cell activation, thereby improving b-cell function in T1D.