Type 1 diabetes (T1D) is a T cell-mediated autoimmune disorder in which beta-cell specific destruction leads to life-long exogenous insulin dependence. While islet transplantation can restore euglycemia in these patients, both allogeneic and autoimmune rejection of the islet graft pose challenges limiting graft longevity. CD4⁺ T cells play a major role in driving T1D autoimmune development and islet graft rejection. Recent discovery of hybrid insulin peptides (HIPs), a fusion of insulin peptides with other beta-cell granule containing peptides, were found to be cognate autoantigens for autoreactive CD4⁺ T cells in mouse models and in patients with T1D. HIPs are formed within the beta-cell during crinophagy, a process of protein degradation or turnover where insulin granules fuse with lysosomes. By reducing the formation of HIPs, autoimmune-mediated islet graft rejection may be delayed. Cathepsin L (CatL) is a lysosomal endopeptidase enzyme involved in protein degradation and is implicated in the generation of HIPs. Non-Obese Diabetic (NOD) mice lacking CatL (NOD.CatL^-/-) exhibited a delay in spontaneous T1D development. Immunophenotyping of islet-infiltrating immune cells from NOD.CatL^-/- mice displayed reductions in effector CD4⁺ T cells and tetramer-specific autoreactive BDC-2.5, BDC-6.9, and insulin T cells compared to NOD mice. We hypothesize that islets deficient in CatL will delay immune-mediated rejection due to the absence of antigenic HIPs following transplantation. CatL-deficient islets may improve islet graft survival due to reduced release of antigenic peptides and passenger CD4⁺ T cells when transplanted into recipient mice. Isolated islets (250 islets/recipient) from NOD.CatL^-/-, NOD.CatL^+/-, or NOD.Rag mice, were transplanted under the kidney capsule of streptozotocin-treated and spontaneously diabetic NOD mice and monitored for restoration and maintenance of euglycemia. Both NOD.CatL^-/- and NOD.CatL^+/- islets restored euglycemia similarly as NOD.Rag islets. Future studies will investigate the impact of CatL-deficiency on long-term graft survival and effects on immune cell infiltration and activation.