Type 1 diabetes (T1D) is characterized by dysregulated self-reactive immune responses that result in destruction of the insulin producing beta-cells. Two surface receptors referred to as CD226 and T cell immunoreceptor with Ig and ITIM domains (TIGIT) have been identified as regulators of immune cell function. They are expressed by CD4+, CD8+ T cells, and Natural Killer (NK) cells that are critical in beta-cell destruction. CD226 and TIGIT compete for the same ligands and have divergent effects in immune regulation. Our group and others have identified that co-stimulatory molecule CD226 promotes inflammation whereas co-inhibitory molecule TIGIT inhibits inflammation. Importantly, a CD226 single nucleotide polymorphism (SNP, rs763361) is associated with the risk of multiple autoimmune diseases, including T1D. This project evaluates the impacts of CD226 and TIGIT in T1D development and immune cell regulation utilizing non-obese diabetic (NOD) mouse models for T1D. CD226- or TIGIT-deficient NOD mice are generated using CRISPR/Cas9 technologies and genotyped by TaqMan real-time PCR. The levels of immune cell infiltration to islets in prediabetic (12-week old) mice deficient in CD226 are lower compared to prediabetic wild-type (WT) controls. Further, frequency of T1D incidence in CD226 knockout mice is significantly lower, indicating CD226 is important for T1D development. However, TIGIT knockout mice show similar disease onset and incidence compared to WT mice. A more thorough investigation of the impacts of the CD226 receptor on CD4+, CD8+ T cell, and NK cell phenotype, activation, and function will augment its potential implementation as a biomarker and therapeutic target of T1D.