DPP4 and type 1 diabetes mellitus: Based on preclinical and clinical studies conducted in the field of autoimmune diseases and solid organ and cell transplantation, the present narrative review aims to describe the rationale behind the investigation of combination therapy with vitamin D plus DPP-4 inhibitors (DPP-4i) [18] as a potential adjuvant treatment strategy to enhance the efficacy of novel beta-cell replacement therapies for T1D.