MHC class II antigen-processing chaperone H2-O shapes CD4 T cell receptor repertoire

Abstract

Abstract HLA-DO (DO) is a MHC class II-encoded, nonpolymorphic heterodimeric protein in humans (known as H2-O in mice). It is expressed only in thymic medulla, B lymphocytes and some subsets of dendritic cells. Although DO is found to form a stable complex with HLA—DM (DM), its physical function remains unknown. Some studies have shown that it inhibits the activity of DM to edit peptide presentation on MHC II molecules while others reported that the DO can up- or downmodulate presentation of certain peptides in association with DM on HLA-DR4 molecules. To solve this controversy, our lab recently proposed a model suggesting that the DO facilitates the binding of DM-resistant peptides and inhibits the binding of DM-sensitive peptides to the HLA-DR1 molecules in vitro. Based on this model, we expected to see lower density of pMHC expression on mTEC cells in thymus of H2-O-KO mice, which will lead to faulty negative selection and different CD4 TCR repertoires as compared to H2-O-WT mice. By developing a “Prime-Restim” strategy based on the principle of mixed lymphocyte reaction, we observed the proliferation of primed H2-O-WT CD4 T cells in responding to repeated in vitro stimulations by antigen presenting cells from H2-O-KO mice and vice versa. The results demonstrate that in the absence of H2-O, the peptide repertoire of MHC II in the H2-O knockout mice is different from the peptide repertoire of MHC II in the H2-O-WT mice. It establishes a role for DO in regulation of epitope selection for presentation by MHC class II molecules in vivo. These findings support the idea that the expression of DO in thymic medulla might lead to better thymic deletion of self-reactive CD4 T cells.