Moreover, increased IL-21 promotes diabetogenesis in KRc-transgenic NOD mice and enhances the expression of Tfh-associated molecules, such as inducible T cell costimulator, programmed cell death 1 (PD-1), and CXCR5 (32), consistent with previous observations that memory T cells from patients with T1D express elevated levels of Tfh cell markers (21, 22). Here, PDCD1 is linked to type 1 diabetes mellitus.