Dynamics of MHC Class II-Activating Signals in Murine Resting B Cells

Abstract

MHC class II (MHC II) proteins are competent signaling molecules on APC. However, little is known about the mechanisms that control generation of their activating signals. Previous reports highlighted a number of factors that could affect the nature and outcome of MHC II signals, including the inability of MHC II ligation on resting vs activated murine B cells to induce mobilization of Ca2+. In the present study, we report that ligation of MHC II on resting murine B cells reproducibly induces mobilization of intracellular Ca2+ using both mAbs and cognate T cells as ligands. Mobilization of Ca2+ was independent of MHC II haplotype, isotype, or mouse genetic background. MHC II-mediated mobilization of Ca2+ is completely inhibited by inhibitors of src-like kinases and syk, and MHC II ligation increases overall tyrosine phosphorylation level. Moreover, MHC II ligation results in specific up-regulation of CD86. However, induction of these responses is dependent on the type of anti-MHC II Ab used, suggesting that epitope specificity and/or the nature of ligation is important. Moreover, we demonstrate that MHC II-derived signals are strictly regulated by the order and timing of BCR and CD40 signals, suggesting coordination of these signals preserves the integrity of early B cell priming events. Thus, the mode and the context of MHC II ligation influence generation of MHC II-derived activating signals in resting B cells. Based on these results, a new model that highlights the role of MHC II-activating signals in regulation of Ag presentation by B cells is proposed.

Differential Transmembrane Domain GXXXG Motif Pairing Impacts Major Histocompatibility Complex (MHC) Class II Structure

“…Previous studies established the Ia.2 epitope as a marker of a subset of lipid raft-resident I-A k class II molecules with unique signaling and antigen presentation capabilities (5,11). Lipid raft-resident Ia.2 ϩ class II molecules drive Src family kinase-dependent calcium signaling in mature B cells and, consistent with the work of Roche and co-workers (17)(18)(19), are critically important for B cell antigen presentation and formation of B cell-T cell conjugates (5).…”

“…It is possible that these two types of B cell-T cell interactions are mediated by the two distinct class II conformers described in this study. Based on the potent B and T cell stimulation properties of the Ia.2 ϩ class II conformer (5,11), it is most likely that antigen-derived peptides loaded onto Ia.2 ϩ class II molecules during B cell receptormediated antigen processing are responsible for the prolonged B cell-T cell interactions driven by cognate antigen. In contrast, Ia.2 Ϫ peptide-class II complexes bearing self-peptides may drive the short term "homeostatic" interactions seen in the absence of cognate antigen.…”

“…Class II Forms in the Endoplasmic Reticulum-Our previous studies established the Ia.2 epitope as a marker of a lipid raft-tropic I-A k MHC class II conformer that is able to drive B cell calcium signaling and enhanced antigen presentation to CD4 T cells (5,11). In addition, we reported that Ii is associated with both total and Ia.2 ϩ class II (5).…”

Myosin 1c Participates in B Cell Cytoskeleton Rearrangements, Is Recruited to the Immunologic Synapse, and Contributes to Antigen Presentation

“…It has been recognized that MHC-II molecules are immersed in DRMs, and their position allows them to initiate signaling events inducing maturation, proliferation, or cytokine production in B cells by recruitment and activation of Syk and Src family tyrosine kinases (39). The early events after MHC-II cross-linking in B cells, also includes calcium mobilization when using either monoclonal Abs or cognate T cells as ligands (40). It also has been observed in our spreading models that stimulation through MHC-II of activated B cells causes cytoskeleton rearrangements (1).…”

Antigen-B Cell Receptor Complexes Associate with Intracellular major histocompatibility complex (MHC) Class II Molecules

“…Moreover, the 11-5.2 mAb, which selectively binds M1-paired I-A k class II molecules, is highly effective at driving class II calcium signaling in B cells whereas pan-reactive mAb are not (30). This suggests that one consequence of association of internalized Ag-BCR complexes with M1-paired class II could be the transfer of BCR-derived CD79 signaling modules to M1-paired class II molecules, providing them with their unique signaling properties.…”