Treg-specific CD226 Deletion Reduces Diabetes Incidence in NOD Mice by Improving Regulatory T Cell Stability.

Co-stimulation serves as a critical checkpoint for T cell activation, and several genetic variants affecting co-stimulatory pathways confer risk for autoimmune diseases. A single nucleotide polymorphism (rs763361) in the CD226 gene encoding a co-stimulatory receptor increases susceptibility to multiple autoimmune diseases, including type 1 diabetes. We previously found that Cd226 knockout protected non-obese diabetic (NOD) mice from disease, but the impact of CD226 on individual immune subsets remained unclear. Our prior reports implicate regulatory T cells (Treg), as human CD226+ Tregs exhibit reduced suppressive function. Hence, we hypothesized that genomic Cd226 gene deletion would increase Treg stability, and Treg-specific Cd226 deletion would inhibit diabetes in NOD mice. Indeed, crossing NOD.Cd226-/- and a NOD Treg-lineage tracing strain resulted in decreased pancreatic Foxp3-deficient "ex-Tregs." We generated a novel Treg-conditional knockout (TregΔCd226) strain that displayed decreased insulitis and diabetes incidence. CD226-deficient pancreatic Tregs had increased expression of the coinhibitory counter-receptor TIGIT. Moreover, NOD splenocytes treated with a TIGIT-Fc fusion protein exhibited reduced T cell proliferation and IFN-β production following anti-CD3/CD28 stimulation. This study demonstrates that a CD226/TIGIT imbalance contributes to Treg instability in NOD mice and highlights the potential for therapeutic targeting this co-stimulatory pathway to halt autoimmunity.