Promiscuous presentation and recognition of nucleosomal autoepitopes in lupus: Role of autoimmune T cell receptor α chain

T cells specific for nucleosomal autoepitopes are selectively expanded in lupus mice and these Th cells drive autoimmune B cells to produce pathogenic antinuclear antibodies. We transfected the TCR-α and -β chain genes of a representative, pathogenic autoantibody-inducing Th clone specific for the nucleosomal core histone peptide H4_71-94 into TCR-negative recipient cells. Although the autoimmune TCRs were originally derived from SNF1 (I-A(d/q)) mice, the transfectants could recognize the nucleosomal autoepitope presented by APC-bearing I-A molecules of all haplotypes tested, as well as human DR molecules. Competition assays indicated that the autoepitopes bound to the MHC class II groove. Most remarkably, MHC- unrestricted recognition of the nucleosomal peptide epitope was conferred by the lupus TCR-α chain even when it paired with a TCR-β chain of irrelevant specificity. Several other disease-relevant Th clones and splenic T cells of lupus mice had similar properties. The TCR-α chains of these murine lupus Th clones shared related motifs and charged residues in their CDRs, and similar motifs were apparent even in TCR-α chains of human lupus Th clones. The lupus TCR-α chains probably contact the nucleosomal peptide complexed with MHC with relatively high affinity/avidity to sustain TCR signaling, because CD4 coreceptor was not required for promiscuous recognition. Indeed, pathogenic autoantibody-inducing, CD4-negative, TCR-αβ⁺ Th cells are expanded in systemic lupus erythematosus. These results have implications regarding thymic selection and peripheral expansion of nucleosome-specific T cells in lupus. They also suggest that universally tolerogenic epitopes could be designed for therapy of lupus patients with diverse HLA alleles. We propose to designate nucleosomes and other antigens bearing universal epitopes 'Pantigens' (for promiscuous antigens).