CD226 Knockout Inhibits Type 1 Diabetes via Impaired Thymocyte Development and Peripheral T Cell Activation

Background: Type 1 diabetes (T1D) results from autoreactive T cells destroying pancreatic β-cells. The costimulatory receptor, CD226, is a genome-wide association study-identified gene linked to susceptibility to T1D and other autoimmune disorders. CD226 is expressed primarily on NK cells and effector/memory T cells to promote T cell activation. While the role of CD226 in T1D pathogenesis remains unclear, blockade of CD226 has been shown to prevent experimental autoimmune encephalitis. Thus, we hypothesized that knockout (KO) of CD226 in non-obese diabetic (NOD) mice would prevent T1D by inhibiting T cell activation.

Methods: Disease incidence was monitored in CD226 wild-type (WT), heterozygous (HET) and KO littermates. We further characterized the cellular phenotypes leading to disease protection via analysis of insulitis, thymocyte development, and T cell activation in various organs of pre-diabetic female mice.

Results: Consistent with our hypothesis, KO of CD226 significantly reduced diabetes incidence in both females (KO vs. WT, p<0.05) and males (KO vs. WT, p<0.01; vs. HET, p<0.05) while CD226-HET showed similar kinetics of disease development to WT mice. Less insulitic infiltration was observed in CD226-KO as compared to WT mice (p<0.0001). CD226-KO mice showed decreased thymocyte numbers (p<0.05), accounted for mainly by fewer CD4+CD8+ T cells (p<0.05). The impaired thymocyte development may contribute to an imbalanced CD4+ to CD8+ T cell ratio (p<0.05) observed in the spleen of CD226-KO animals. Importantly, CD226-KO mice showed a decreased percentage of memory CD62L-CD44+CD8+ T cells in the pancreatic lymph nodes (p<0.01).

Conclusions: These data suggest that CD226 promotes T1D development by supporting thymic T cell development and CD8+ T cell activation in the periphery. Future work may elucidate whether CD226 promotes positive selection of autoreactive T cells as well as NK cell function. These studies may support future translational efforts to therapeutically block CD226 in individuals at-risk for T1D.