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Naturally processed heterodimeric disulfide-linked insulin peptides bind to major histocompatibility class II molecules on thymic epithelial cells.

We determined whether disulfide-linked insulin peptides that are immunogenic in vitro for CD4+ T cells bind to major histocompatibility complex class II in vivo. Radiolabeled recombinant human insulin (rHI) was injected into BALB/c mice, and processed rHI peptides bound to I-Ad molecules on different thymic antigen-presenting cells were characterized. The A6-A11/B7-B19 and A19-A21/B14-B21 disulfide-linked I-Ad-bound rHI peptides were isolated from thymic epithelial cells but not dendritic cells. While both thymic epithelial cells and dendritic cells present rHI to HI/I-Ad-specific T cells, these antigen-presenting cells do not present the reduced or nonreduced forms of the disulfide-linked rHI peptides. Thus, a naturally processed disulfide-linked peptide can bind to major histocompatibility complex class II in vivo. The potential role of these peptides in immunological tolerance is discussed.