Signaling through HLA‐DR induces PKCβ‐dependent B cell death outside rafts

Abstract

Signals through HLA-DR molecules contribute to optimal activation of antigen-presenting cells (APC) during T cell/APC interactions participating in the generation of productive interactions, and to the induction of APC death, which has been postulated to play a role in the termination of the immune response. To understand how these molecules accommodate both cellular responses, we studied the not yet well-defined signaling events and the biochemical requirements for HLA-DR-mediated death. We demonstrate that in B cells the HLA-DR-activated protein kinase C (PKC) g is required for HLA-DR-mediated death whereas the HLA-DR-activated Src family of PTK is redundant. In contrast to HLA-DR-mediated activation of Src kinase Lyn, the aggregation of HLA-DR molecules in lipid rafts is not required for HLA-DR-mediated PKC g activation nor for the induction of cell death. Indeed, the bulk of HLA-DR-activated PKC g reside outside rafts. This is the first report showing that HLA-DRinduced PKC g activation is essential for the induction of B cell death via HLA-DR, and that these HLA-DR-mediated events do not require the integrity of rafts.

MPYS, a Novel Membrane Tetraspanner, Is Associated with Major Histocompatibility Complex Class II and Mediates Transduction of Apoptotic Signals

“…2c). This finding is consistent with results in previous studies, which showed that tyrosine phosphorylation and calcium flux are not required for the MHC-II-mediated death response of human B cells (14,16).…”

“…Thus, death induced by these Abs, and by extension T-cell antigen receptors, is thought to be mediated by MHC-II signaling (20). Consistent with this possibility, protein kinase C activation is reportedly required for MHC-II-mediated death in Raji human B-cell lymphoma, mature DCs, and activated THP-1 monocytes but not in primary human plasmacytoid DCs (2,12,14,27). MHC-II-mediated signaling of death also appears to occur independently of Src family kinase (14,27) and caspase activation (8,13) in Raji and Ramos cells.…”

The Pathway of Antigen Uptake and Processing Dictates MHC Class II-Mediated B Cell Survival and Activation

“…Ligation of type II complex in resting or anti--activated B cells by C4H3 mAb induced ϳ50% B cell loss; contrastingly, ligation of type I peptideclass II complexes failed to induce B cell to die. The recovery rate obtained with type II complexes in this study is comparable to HLA-DR (8,27) or peptide-loaded HLA-DR-induced B cell death (10). A more pronounced loss of B cells resulted from the MHC-TCR cognate interaction in our study (ϳ88%), and B cell loss was even higher than that observed in control samples at higher numbers of B cells (Fig.…”

“…CII signaling is also reportedly coupled to the protein tyrosine kinase-dependent signaling pathway in human B cells and cell lines (5,6). However, ligation of CII on human activated B cells results in cell death in vitro (7,8), while resting human B cells appear to be protected (9). More recently, ligation of peptide-loaded CII on EBV-transformed B cells has been shown to induce apoptosis following cognate MHC-TCR interaction (10).…”

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

“…In this regard, agents that induce an increase in cAMP, including anti-MHC II mAbs, have been reported to inhibit mitogen-or anti--induced B cell proliferation of resting B cells as well as mobilization of Ca 2ϩ (21,32), indicating that generation of cAMP is antagonistic to B cell activation and proliferation. Furthermore, MHC II-mediated activation of PKC␤, but not the Src family of PTK, has been linked to death of human B cells (33 can also be induced by Ca 2ϩ ionophores, which (in the concomitant presence of phorbol esters) stimulates B cell proliferation (34). These results suggest that there are different thresholds for elevation of either intracellular cAMP or Ca 2ϩ in resting mouse B cells that depends on the nature of ligation of MHC II by different mAbs.…”