HLA-DO's effect on exogenous antigen presentation

In B cells, antigens internalized through the B cell receptor are processed throughout the endocytic pathway where peptides are loaded on MHCII. Mature B cells express the non-classical MHCII HLA-DO (DO) which conditionally blocks HLA-DM (DM) in a pH-dependent manner. DO is known to regulate the diversity of the self-peptide repertoire, but data on its influence on exogenous antigens is limited.

Here we investigated if DO influences exogenous peptide loading based on the pH of subcellular compartments. We generated variants of influenza hemagglutinin (HA) modified to be susceptible at different pHs, and tracked the presentation of a single HLA-DR1 (DR1) epitope. We found the absence of DO improves presentation of epitopes generated at a lower pH compared to a higher pH. We observed similar results with intact influenza viruses encoding the HA mutants suggesting that the impact of DO is greater for proteins processed at a lower pH and with a greater effect on peptide presentation later in the endocytic pathway.

We further identified naturally processed and presented DR1-bound flu peptides on B cells in the absence or presence of DO. The peptides were measured to have a range of sensitivities to DM-editing. Preliminary analysis suggests DO expression seems to promote presentation of DM-sensitive epitopes. Understanding the DM-dependent peptide-selection mechanism that DO employs on exogenous antigens could be important to understand its role in humoral responses to infections.