H2-O, a MHC class II-like protein, sets a threshold for B-cell entry into germinal centers

Abstract

Upon antigen (Ag) encounter, B cells require T-cell help to enter the germinal center (GC). They obtain this help by presenting Agderived peptides on MHC class II (MHCII) for recognition by the T-cell receptor (TCR) of CD4 + T cells. Peptides are loaded onto MHCII in endosomal compartments in a process catalyzed by the MHCII-like protein H2-M (HLA-DM in humans). This process is modulated by another MHCII-like protein, H2-O (HLA-DO in humans). H2-O is a biochemical inhibitor of peptide loading onto MHCII; however, on the cellular level, it has been shown to have varying effects on Ag presentation. Thus, the function of H2-O in the adaptive immune response remains unclear. Here, we examine the effect of H2-O expression on the ability of Ag-specific B cells to enter the GC. We show that when Ag specific WT and H2-O −/− B cells are placed in direct competition, H2-O −/− B cells preferentially populate the GC. This advantage is confined to Ag-specific B cells and is due to their superior ability to obtain Ag-specific T-cell help when T-cell help is limiting. Overall, our work shows that H2-O expression reduces the ability of B cells to gain T-cell help and participate in the GC reaction.antigen processing | antigen presentation | H2-M | follicular helper T cells | HLA-DOA ntigen (Ag) processing and presentation on MHCII is central to T cell-dependent Ab-mediated immunity. During an immune response, Ag-specific naïve B cells recognize and internalize Ag via the B-cell receptor (BCR). The Ag is subsequently transported to late endosomes and lysosomes, where it is degraded into peptides, some of which are loaded onto MHC class II (MHCII) (1). These MHCII-peptide complexes are then displayed on the cell surface for recognition by cognate CD4 + T cells. This recognition event allows B cells to obtain T-cell help from a rare population of T cells known as CD4 + follicular helper T cells (T FH ). The interaction between the Ag-specific B cells and T FH cells is essential for B-cell entry into the germinal center (GC) (2).Peptide loading of MHCII is catalyzed by the nonclassical MHCII-like protein H2-M (HLA-DM in humans) (3-5). H2-M, which localizes to endosomes (6), promotes the removal of class IIassociated invariant chain peptides (CLIP) from the peptidebinding groove of MHCII. H2-M also stabilizes empty MHCII molecules and edits the final MHCII-bound peptide repertoire that is displayed at the cell surface to ensure that only long-lived MHCII-peptide complexes are presented (7). Another nonclassical MHCII-like protein, H2-O (HLA-DO in humans), also localizes to endosomes in B cells (8). H2-O forms a complex with H2-M and has been shown to inhibit or modulate the ability of H2-M to catalyze and edit MHCII peptide exchange (9-12). Surprisingly, H2-O-deficient mice exhibit only minor abnormalities in Ag presentation (11, 13).Ag presentation is important for many aspects of immunity, including initiating the GC response. Admission of B cells into the GC is a competitive process that is dependent upon help from Agspecific T F...

TLR Agonists Downregulate H2-O in CD8α− Dendritic Cells

“…Thus, the maintenance of H2-O expression in B cells after challenge with TLR agonists would help to prevent B cells from presenting high levels of MHCII-peptide early in immune responses, thus promoting DC MHCII presentation and naïve CD4 T cell activation only by DCs. Support for this idea is provided by our recent studies showing that the presence of H2-O in naïve B cells reduces the ability of B cells to gain T cell help (14). Collectively, the studies presented here show the specific downregulation of H2-O in CD8α − DCs following activation and further highlight the importance of the developmental control of optimal MHCII presentation by DCs.…”

“…Recent evidence has indicated that H2-O/DO modulation of MHCII peptide presentation can have important biological consequences. H2-O expression has been shown to reduce the ability of B cells to gain T cell help and participate in the germinal center reaction potentially setting a threshold for B cell entry into germinal centers (14). Additionally, modulation of antigen presentation by DO expression in NOD DCs results in the prevention of the autoimmune disease, type 1 diabetes (15).…”

MHC Class II Antigen Presentation by Dendritic Cells Regulated through Endosomal Sorting

“…In immature DCs, DO is thought to increase the range of presented peptides, therewith facilitating tolerance induction to a broad range of self peptides (Yi et al 2010). DO expression is down-regulated in activated B cells and DCs, enhancing DMcatalyzed peptide loading under inflammatory conditions (Glazier et al 2002;Hornell et al 2006;Draghi and Denzin 2010;Porter et al 2011).…”

Mapping the HLA-DO/HLA-DM complex by FRET and mutagenesis

“…Constitutive overexpression of DO in dendritic cells blocks disease in the nonobese diabetic (NOD) mouse model of type 1 diabetes (18). Finally, DO expression in murine B cells limits their participation in the germinal center reaction through effects on antigen presentation (19). However, the full physiological function of DO is still unclear.…”