Costimulatory Molecules CD226 and TIGIT Impact CD8+ T-Cell Activity and Function during Type 1 Diabetes

Investigations into the pathogenesis of type 1 diabetes (T1D) implicate antigen interaction and co-stimulation as key pathways in disease development. A non-synonymous polymorphism in CD226 (rs763361), that encodes for a co-stimulatory molecule, has been linked to risk for T1D. The CD226 activating pathway is constrained by TIGIT, the high-affinity co-inhibitory receptor T cell immunoreceptor with Ig and ITIM domains, which competes with CD226 for ligands CD112/CD155. We have previously demonstrated that CD226 facilitates CD4⁺ T effector function, whereas TIGIT potentiates regulatory T (Treg) cell activity. Here, we report that both receptors impact CD8⁺ T cell pathogenesis. Both CD226 and TIGIT are upregulated upon human CD8⁺ T cell activation and are divergently expressed by memory CD8⁺ T cell subsets. These results support our finding in the positive correlation between age and CD226 or TIGIT expression (P=0.0002 and P<0.0001, respectively). Interestingly, TIGIT is co-expressed with the transcription factors T-bet and EOMES, whereas ∼80% of cytokine producing cells express CD226, demonstrating the coordinated expression of these factors with differentiated T cell lineages. We next sorted CD8⁺ T cells based on CD226 and TIGIT expression and assessed effector activity as well as gene expression profile following in vitro activation. Using a multiplex assay, we showed that CD226⁺ cells produced cytokines, chemokines, and cytolytic proteins (e.g., IFNγ, MIP-1α, granzyme B, and perforin) in a manner independent of TIGIT. Importantly, CD8⁺ T cell cytotoxicity to human β-cell line βLox5 that express high levels of ligands CD112 and CD155 is attenuated by blocking CD226 and enhanced by hindering TIGIT through a granzyme B dependent mechanism. These data support this co-stimulatory axis as a potential therapeutic pathway in T1D that could either attenuate CD226 co-stimulation of effector T cells, or inversely support tolerance induction by TIGIT signaling on Tregs.