Modulation of Antigen Presentation and B Cell Receptor Signaling in B Cells of Beige Mice

Abstract

Binding of Ag by B cells leads to signal transduction downstream of the BCR and to delivery of the internalized Ag–BCR complex to lysosomes where the Ag is processed and presented on MHC class II molecules. T cells that recognize the peptide–MHC complexes provide cognate help to B cells in the form of costimulatory signals and cytokines. Recruitment of T cell help shapes the Ab response by facilitating isotype switching and somatic hypermutation, and promoting the generation of memory cells and long-lived plasma cells. We have used the beige (Bg) mouse, which is deficient in endosome biogenesis, to evaluate the effect of potentially altered Ag presentation in shaping the humoral response. We show that movement of the endocytosed Ag–BCR complex to lysosomes is delayed in Bg B cells and leads to relatively poorer stimulation of Ag-specific T cells. Nevertheless, this does not affect Bg B cell activation or proliferation when competing with wild-type B cells for limiting T cell help in vitro. Interestingly, Bg B cells show more prolonged phosphorylation of signaling intermediates after BCR ligation and proliferate better to low levels of BCR cross-linking. Primary Ab responses are similar in both strains, but memory responses and plasma cell frequencies in bone marrow are higher in Bg mice. Further, Bg B cells mount a higher primary Ab response when competing with wild-type cells in vivo. Thus, the intensity and duration of BCR signaling may play a more important part in shaping B cell responses than early Ag presentation for T cell help.

Molecular Control of B Cell Activation and Immunological Synapse Formation

“…In addition to the antigen internalization, endocytosis from the synapse might function as a way to remove 'used' effector molecules, similarly to T cells (80). Indeed, recent studies demonstrate that the BCR continues to signal after internalization and, furthermore, once inside the cell, activates a distinct set of kinases as compared to the signaling at the plasma membrane (82,83). Also exocytic vesicles have been described in the IS and the release of their acidic content and hydrolytic enzymes to the cell-cell interface within the synapse has been proposed to facilitate the antigen extraction from the APC (84) (Figure 3).…”

The Transmembrane Adaptor Protein SCIMP Facilitates Sustained Dectin-1 Signaling in Dendritic Cells

“…Antigen Presentation Assay-For the antigen-presentation assay, we modified a method published previously (47). CD4 ϩ T cells isolated from lymph nodes of OTII mice using negative selection (CD11b, CD8, CD19-FITC) on AutoMACS were cultured for 2 days in RPMI medium supplemented with 100 ng/ml anti-CD3 antibody and 2 units/ml IL2 for 48 h. B cells negatively selected on AutoMACS were fed for 1 h with 10 mg/ml NP-ovalbumin at 37°C, washed with PBS, and cultured in a ratio of 1:1 with CFSE-stained T cells.…”

B Cell Receptor Signaling

“…This results in normal calcium fluxes, but elevated and prolonged BCR signaling to Erk and p38, and enhanced antibody responses in vivo (Chatterjee et al 2012). This results in normal calcium fluxes, but elevated and prolonged BCR signaling to Erk and p38, and enhanced antibody responses in vivo (Chatterjee et al 2012).…”