Genetic susceptibility to many autoimmune diseases, including insulin-dependent diabetes mellitus (IDDM) is statistically linked to the HLA class II region of chromosome 6. However, a distinguishing feature of patients with HLA class II-linked autoimmune disease is an abnormally low density of conformationally correct, self-peptide filled HLA class I molecules on the lymphocyte cell surface. The transporters associated with antigen processing (Tap-1 and Tap-2) are essential for normal class I expression and presentation of intracellular peptides, and these genes are located within the HLA class II region. The aims of this project were to determine if Tap genes could be implicated in the defective class I expression associated with IDDM by using a novel Epstein-Barr virus (EBV)-mediated gene transfer system to introduce a cloned, normal Tap-2 or Tap-1 gene into B cell lines from normal and IDDM patients and analyzing the effect on conformationally dependent class I expression. The results show that Tap-2 gene transfer in B cells from 40% of randomly selected IDDM patients increased expression of conformationally correct, cell-surface class I molecules to levels comparable with similarly treated B cells from normal control individuals. B cells from another 40% of IDDM patients responded to Tap-1 gene transfer. These effects were specific because B cells from normal individuals did not respond to Tap-1 or Tap-2 gene transfer with increased class I expression, and B cells from IDDM patients responding to Tap-2 gene transfer did not respond to Tap-1 gene transfer and vice versa. Thus, these complementation studies identify distinct, non-overlapping subsets of IDDM patients whose class I defect in B cells can be reversed by Tap-1 or Tap-2 gene transfer. The increase in class I expression induced by Tap gene transfer is associated with a reduction in the number of peptide-empty class I molecules as demonstrated by the response to exogenous peptide loading. Furthermore, the increase in self-peptide filled class I molecules induced by Tap gene transfer into B cells from IDDM patients is associated with restored antigen presentation to autologous T cells. These studies conclude that Tap gene dysfunctions may contribute to the defect in class I phenotype and antigen presentation demonstrated by IDDM patients. Defective presentation of self-peptides by antigen presenting cells can lead to the failed T cell education and tolerance to self antigens evident in IDDM. These studies functionally identify HLA class II region genes that contribute to an immunologic defect in IDDM.