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Type 1 diabetes (T1D) is an autoimmune disease in which pathogenic lymphocytes target autoantigens expressed in pancreatic islets, leading to the destruction of insulin-producing β-cells. Zinc transporter 8 (ZnT8) is a major autoantigen abundantly present on the β-cell surface. This unique molecular target offers the potential to shield β-cells against autoimmune attacks in T1D. Our previous work showed that a monoclonal antibody (mAb43) against cell-surface ZnT8 could home in on pancreatic islets and prevent autoantibodies from recognizing β-cells. This study demonstrates that mAb43 binds to exocytotic sites on the β-cell surface, masking the antigenic exposure of ZnT8 and insulin after glucose-stimulated insulin secretion. In vivo administration of mAb43 to NOD mice selectively increased the proportion of regulatory T cells in the islet, resulting in complete and sustained protection against T1D onset as well as reversal of new-onset diabetes. The mAb43-induced self-tolerance was reversible after treatment cessation, and no adverse effects were exhibited during long-term monitoring. Our findings suggest that mAb43 masking of the antigenic exposure of β-cells suppresses the immunological cascade from B-cell antigen presentation to T cell–mediated β-cell destruction, providing a novel islet-targeted and antigen-specific immunotherapy to prevent and reverse clinical T1D. Article Highlights: mAb43, a zinc transporter 8 (ZnT8)–specific monoclonal antibody, provides lasting protection against autoimmune diabetes in NOD mice. The in vivo islet specificity of mAb43 enables targeted therapy to enhance safety. High-affinity binding of mAb43 to the extracellular surface of ZnT8 shields β-cells from antigenic exposure. β-Cell masking results in a localized increase in regulatory T cells in the pancreatic islet. Prolonged mAb43 treatment clears destructive insulitis, preserves β-cell mass, and reverses seroconversion of insulin autoantibodies in NOD mice. The immunological processes bridging the masking of specific cell-surface autoantigens and the broad suppression of polyspecific T-cell autoimmunity are still unclear. [ABSTRACT FROM AUTHOR]