Protein-protein Interaction In The Intracellular Pathway That Leads To MHC Class II Presentation Of Peptide On the Surface Of The B Cell

The highly specialized system of adaptive immune responses necessitates the surface expression of major histocompatibility complexes (MHC) Class II on antigen presenting cells, such as B cells. MHC class II proteins bind antigenic peptides for recognition by a sub-group of lymphocytes, known as CD4+ T helper cells. However, the protein-protein interactions that occur within the intracellular pathway leading to peptide presentation by MHC Class II are not well understood. We investigated these interactions using lysates of immortalized human B cell lines. Extending previous findings that HLA-DM co-precipitates with IgG antibody (Strohman et al., manuscript submitted, 2012), we further investigated whether HLA-DM (a peptide-loading catalyst for class II), HLA-DO (inhibitor of DM), HLA-DR (MHC class II), and MHC Class I (negative control) co-precipitate with IgM antibody. Co-precipitating proteins were analyzed via SDS-PAGE and immunoblotting. We expect HLA-DM to co-precipitate with IgM, similar to our findings with IgG. HLA-DO, though in complex with a subset of DM molecules, is not expected  to co-precipitate with DM/IgM complexes.  Based on findings by Sherman et al., 1995 that HLA-DR binds to peptides in the presence of HLA-DM, we expect that when HLA-DM co-precipitates with IgM, HLA-DR will co-precipitate as well. If the results correspond with expectations, they will provide further insight into interactions that occur in the intracellular compartments of B cells and how overall immune response is generated.