Monovalent ligation of the B cell receptor induces receptor activation but fails to promote antigen presentation

Abstract

We explored the role of antigen valency in B cell receptor (BCR) activation and rearrangement of intracellular MHC class II compartments as factors that contribute to the efficacy of antigen presentation. Using primary B cells that express a hen egg lysozyme (HEL)-specific BCR, we found that oligomeric HEL more efficiently promoted both BCR activation and internalization than did monovalent HEL, although monovalent HEL, unlike monovalent Fab fragments of anti-Ig, readily triggered the BCR. Nonetheless, oligovalent ligation positions the BCR in a membrane microdomain that is distinct from one engaged in the course of monovalent ligation, as judged by detergent extraction of the BCR. A ctivation of B cell receptors by specific antigen unleashes a cascade of downstream events which includes alteration of transcriptional programs that lead to cell proliferation and differentiation (1-3). B cells further use their B cell receptors (BCRs) to facilitate the capture of antigen, and in doing so, they improve antigen presentation via class II MHC molecules (4). The latter event is essential for B cells to receive help from antigen-specific CD4 ϩ T cells and to differentiate eventually into memory cells or plasma cells that secrete high-affinity antibody (5).Antigen-specific B cells present peptides derived from cognate antigens to CD4 ϩ T cells at concentrations far below those required for other B cells that lack an antigen-specific receptor (6). This highly efficient mode of antigen presentation is thought to be a consequence of the dual role of the BCR in the delivery of specific antigen to MHC class II-containing peptide-loading compartments and in signaling that leads to enhanced generation of immunogenic peptides (7,8).BCR-transduced signals promote efficient delivery of antigens through activation of Syk and B cell linker protein (BLNK) (9, 10). In addition, BCR-transduced signals induce physical and chemical remodeling of intracellular class II MHC-compartments (11). In the A20͞IIA1.6 mouse B lymphoma cell line, BCR activation by receptor cross-linking induced reorganization, fusion, and acidification of Lamp1-positive late endosomes where class II MHC molecules and invariant chain accumulate (12). In A20 B cells that express an anti-DNP IgM, BCR stimulation led to the transient intracellular accumulation of MHC molecules in newly formed multivesicular bodies, into which the peptide exchange catalyst H2-M was recruited (13).Although these studies were among the first to shed light on a role for BCR signaling in MHC class II-mediated antigen presentation, virtually all of the studies that describe behavior of MHC class II molecules in relationship to the activation of B cells have been performed in transformed cell lines. Moreover, stimulation of the BCR is usually accomplished either by cross-linking receptors with anti-Ig antibodies or by transfecting established cell lines to express BCRs of known specificity to enable their ligation in an antigen-specific manner, which then still involves extensive crosslinking...

BCR ubiquitination controls BCR-mediated antigen processing and presentation

“…The p62 UBA precipitates and WCLs were then probed for BCR IgM heavy chain ( Figure 1A, anti-IgM). The results demonstrate the presence of 2 different molecular weight forms of IgM heavy chain in the WCLs (which we [J.R.D., unpublished results, August 2006] and others 27 have determined represent different glycoforms of the IgM heavy chain) and show that the p62 UBA ubiquitin-binding protein selectively binds to the lower molecular weight heavy chain glycoform.…”

B Cell Activation Is Regulated by the Stiffness Properties of the Substrate Presenting the Antigens

“…The initiation of B cell activation has been shown to be sensitive to each type of these Ag presentation formats. For example, early studies extensively investigated how Ag valency can affect B cell activation (10)(11)(12) and the subsequent Ab responses (6)(7)(8). It is generally observed that multivalent Ags with high valency are more potent in inducing Ab responses as compared with monovalent Ags with low valency.…”

Inositol 1,4,5-Trisphosphate 3-Kinase B Is a Negative Regulator of BCR Signaling That Controls B Cell Selection and Tolerance Induction

“…In the IgHEL tg system, stimulation with soluble HEL is not sufficient to induce vigorous proliferation in vitro, perhaps reflecting the reduced valency of monovalent HEL or a less mature developmental status as reflected by high levels of surface IgM (16,22). Interestingly, Itpkb-deficient cells failed to proliferate in response to HEL to the extent observed in WT IgHEL tg B cells (Fig.…”