Positive regulatory domain 1 (PRDM1) encodes B lymphocyte-induced maturation protein 1 (BLIMP1), also known as a master regulator of T cell homeostasis. We observed a negative relationship between Blimp-1 and IL-21 based on our previous data that Blimp-1 overexpression in T cells suppresses autoimmune diabetes while Blimp-1-deficient T cells contribute to colitis in NOD mice. Reanalysis of published data sets also revealed an inverse correlation between PRDM1 and IL21 in Crohn's disease. Here, we illustrate that Blimp-1 repressed IL-21 by reducing chromatin accessibility and evicting an IL-21 activator, c-Maf, from the Il21 promoter. Moreover, Blimp-1 overexpression-mediated reduction in permissive chromatin structures at the Il21 promoter could override IL-21-accelerated autoimmune diabetogenesis in small ubiquitin-like modifier-defective c-Maf-transgenic mice. An autoregulatory feedback loop to harness IL-21 expression was unveiled by the evidence that IL-21 addition induced time-dependent Blimp-1 expression and subsequently enriched its binding to the Il21 promoter to suppress IL-21 overproduction. Furthermore, intervention of this feedback loop by IL-21 blockade, with IL-21R.Fc administration or IL-21 receptor deletion, attenuated Blimp-1 deficiency-mediated colitis and reinforced the circuit between Blimp-1 and IL-21 in the regulation of autoimmunity. We highlight the translation of Blimp-1-based epigenetic and transcriptomic profiles applicable to a personalized medicine approach in autoimmune diseases.