TCR-like antibodies distinguish conformational and functional differences in two- versus four-domain auto reactive MHC class II–peptide complexes

Antigen-presenting cell-associated four-domain MHC class II (MHC-II) molecules play a central role in activating autoreactive CD4+ T cells involved in multiple sclerosis (MS) and type 1 diabetes (T1D). In contrast, two-domain MHC-II structures with the same covalently attached self-peptide (recombinant T-cell receptor ligands (RTLs)) can regulate pathogenic CD4+ T cells and reverse clinical signs of experimental autoimmune diseases. RTL1000, which is composed of the β1α1 domains of human leukocyte antigen (HLA)-DR2 linked to the encephalitogenic human myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide, was recently shown to be safe and well tolerated in a phase I clinical trial in MS. To evaluate the opposing biological effects of four- versus two-domain MHC-II structures, we screened phage Fab antibodies (Abs) for the neutralizing activity of RTL1000. Five different TCR-like Abs were identified that could distinguish between the two- versus four-domain MHC–peptide complexes while the cognate TCR was unable to make such a distinction. Moreover, Fab detection of native two-domain HLA-DR structures in human plasma implies that there are naturally occurring regulatory MHC–peptide complexes. These results demonstrate for the first time distinct conformational determinants characteristic of activating versus tolerogenic MHC–peptide complexes involved in human autoimmunity.