Memory B cells are poised for robust antigen presentation by HLA-DO

Abstract

Abstract The B cell is a uniquely efficient APC due to expression of BCR for antigen uptake and delivery to MHCII-enriched compartments (MIIC), and key regulatory molecules, invariant chain (Ii), HLA-DM and HLA-DO. All professional APCs express Ii and DM, whereas expression of DO is limited to B cells, certain DCs and thymic medullary epithelia. DO levels change dramatically in B cells entering or exiting the germinal center (GC). To determine effects of DO on other components of the MHCII pathway, we delineated the intracellular distribution of DO, DM, MHCII, and class II-associated CLIP in T2 cells with or without DO or DM expression using structured illumination microscopy. The distribution at steady state shows a very restricted lysosomal destination (LAMP1+) for DO and DM, and dual destinations (surface and LAMP1+) for CLIP and MHCII. DO expression increased overall CLIP compared to DM+DO-cells, but almost doubled the percent of CLIP co-localized with LAMP1, establishing peptide exchange potential (PEP) within MIICs, but not any other compartments or the cell surface. PEP enhancement associated with DO expression is likely due to its regulation of DM activity, as neither the overall CLIP level nor the percent of DR in MIICs increased. We then evaluated the extent of PEP in distinct primary B cell populations isolated from human tonsils, including memory (CD38−/IgD−), naïve (CD38−/IgD+), and GC (CD38+/IgD−) B cells. The measurement shows PEPMemory (significantly) > PEPNaïve > PEPGC−, although the percent of CLIP among all class II peptides is naïve > memory >> GC. These results argue that DO expression in memory B cells equips them for more efficient APC function upon re-encounter with antigen.