The impact of HLA-DM on peptide binding to MHC class II

Abstract

Recognition of peptides bound to class II major histocompatibility complex (MHCII) molecules by T cell receptors of CD4+ T cells initiates an adaptive immune response. Analysis of the antigen presentation pathway indicates that elements of the epitope selection process are critical to generation of the peptide repertoire presented to T cells. Antigen presentation by dedicated cells (APCs) involves the intracellular fragmentation of protein antigens by cathepsins, binding of the derived peptide epitopes to MHCII with the participation of the peptide-editing molecule HLA-DM (DM), and subsequent transport to the surface for recognition. This thesis focuses on the energetics and structural flexibility of the peptide-MHCII complex, and their correlation with DM-susceptibility, to identify the criteria associated with the selection of peptides by APCs for subsequent presentation to T cells. Using the human MHCII HLA-DR (DR), and peptides derived from influenza H3 HA305-318 as test system, it was observed that, in the absence of DM, stable peptide binding is not reached through independent contributions of single-point interactions, but is a distributive process that involves the peptide-DR groove dyad in its entirety highlighting the inherent flexibility of the binding process. Here, DM mechanism is investigated in its ability to impact structural flexibility of the complex. Analysis of release from and binding to DR of a gamut of HA-derived peptides at two different levels of pH reveals that structural stability is reduced as a consequence of DM function. The results indicate that the outcome of DM activity is favoring the endurance of complexes with limited structural flexibility.