Among the public health relevant disorders, Type 1 Diabetes (T1D) is a degenerative disease affecting almost 2 million Americans. It is characterized by the loss of insulin-producing b-cells due to a T cell-mediated autoimmune response. The risk to develop T1D is HLA associated. HLA-DQ8-DR4 has been identified as the most prevalent HLA haplotype in the Caucasian T1D population. Although DQ8 has been demonstrated to be the primary genetic determinant of disease susceptibility, its predisposing effect is likely modulated by the expression of closely linked DR4 alleles. As one of hypotheses to explain the role of DR4 molecules in T1D etiology, the peptide competition model holds that DR4 competes to bind diabetogenic peptides with DQ8 and thus affects DQ8-restricted autoreactive CD4 T cell responses. However, the evidence of the competition is insufficient due to the lack of detection reagents and the difficulty of segregating the expression of DR4 from DQ8. In this study, we investigated the competition of peptides derived from Glutamic Acid Decarboxylase 65 (GAD65) - a putative b-cell autoantigen. A panel of DQ8-restricted T cell lines was generated to serve as detection reagents to evaluate the peptide occupancy of DQ8. After demonstrating that a single peptide derived from GAD65 could bind both HLA-DQ8 and HLA-DR4, we compared CD4 T cell responses elicited by antigen presenting cells expressing DQ8 alone with those expressing DQ8 and DR4 simultaneously. Results indicated that the co-expression of HLA-DR4 diminished DQ8-restricted T cell responses. In addition, distinct DR4 subtypes were demonstrated to affect DQ8-restricted T cell responses differently, suggesting the variable degrees of peptide competition potentials. Taken together, this study provides the evidence that DR4 is able to compete for peptides with DQ8. The outcome of this competition decreases DQ8-restricted CD4 T cell responses, which may hence contribute to a peripheral tolerance mechanism and explain the modulating role of DR4 to the DQ8-conferred T1D susceptibility
