Quantitative analysis of peptides from myelin basic protein binding to the MHC class II protein, I-A(u), which confers susceptibility to experimental allergic encephalomyelitis

Background: An important issue in autoimmune diseases mediated by T cells, such as experimental allergic encephalomyelitis (EAE), is the affinity of the disease inducing determinants for MHC class II proteins. Tolerance, either due to clonal deletion or anergy induction, is thought to require high-affinity interactions between peptides and MHC molecules. Low-affinity binding is compatible with the hypothesis that breaking tolerance to self proteins does nor have to occur for onset of disease. In contrast, a high-affinity interaction implies that an event leading to a breakdown of tolerance is central to the autoimmune process. Materials and Methods: Detergent-solubilized and af finity-purified I-A(u) was incubated with varying concentrations of a set of peptides from myelin basic protein and a biotinylated peptide agonist. The specific complexes were separated from excess peptide by capture on antibody-coated plates, and the affinity of the peptides was measured by adding europium-labeled streptavidin and measuring the resultant fluorescence. Results: The immunodominant and encephalitogenic determinant Ac 1-11, was shown to bind to I-A(u) relatively poorly (IC50 = 100 mu M), demonstrating that in this protein, immunodominance did not correlate with high-affinity binding. In contrast with the natural sequence. the ability of shorter analogs to induce EAE did correlate with their apparent affinity. Conclusions: The dominance of the natural determinant does nor arise from a high-affinity interaction with the MHC class II. molecule. This suggests that other mechanisms are operative and that the specific T cell for this peptide/MHC ligand is of high affinity.