Reem Al-Daccak

Expression of functional major histocompatility complex class II molecules on HMC‐1 human mast cells

Mast cells hold a key position in the defensive mechanisms against exogenous intruders. In this study, we investigated whether human mast cells express functional major histocompatibility complex (MHC) class II molecules that can transduce endogenous signals and present staphylococcal enterotoxin A (SEA) to T cells. Similar to HMC‐1 human mast cell line, umbilical cord blood‐derived mast cells express HLA‐DR, ‐DP and ‐DQ molecules on their surface. MHC class II molecules expressed on HMC‐1 cells bind significantly the SEA (a natural MHC class II ligand), and their ligation with specific mAbs or with SEA, leads ultrastructural chages, suggesting their degranulation. Recognition of SEA‐bound MHC class II molecules on HMC‐1 mast cells by the T cell receptor of K25 cells, an SEA‐specific murine T cell hybridoma, triggers significant IL‐2 secretion by these T cell hybridomas. Hence, our data point out the expression of functional MHC class II molecules on human mast cells, reinforcing the implication of these cells in the defense mechanisms of acquired immunity. J. Leukoc. Biol. 64: 791–799; 1998.

MHC class II isotype-specific signaling complex on human B cells

The highly polymorphic human major histocompatibility complex (HLA) class II molecules are acknowledged as signaling receptors although their coupling to signaling pathways is not yet fully elucidated. In this study, we investigated how HLA class II can be coupled to protein tyrosine kinase (PTK) signaling pathway in B cells and whether there might be differences depending on HLA class II isotype. Using the human B cell line Ramos, we demonstrate that CD19 and CD20 are two HLA class II‐associated receptors that couple HLA class II to PTK signaling pathway where CD20 appears to be amajor component of HLA class II‐mediated activation of Src kinases. Both HLA‐DR and HLA‐DP co‐immunoprecipitate tyrosine‐phosphorylated proteins (p‐Tyr) whereas only activation through HLA‐DR increases the tyrosine phosphorylation of these proteins. Indeed, in contrast to HLA‐DR, cross‐linking HLA‐DP induces neither tyrosine phosphorylation nor homotypic adhesion, and induces ERK1/2 activation. Differential association of these isotypes with CD20 appears to be one of the mechanisms underlying their differential signaling. We provide an experimental evidence for a mechanism by which HLA class II molecules can be coupled to PTK signaling pathway and, underscores their isotypes differential signaling. Further investigation of these mechanisms is likely to provide new insights into how isotype specific MHC class II signaling can contribute to the regulation of the immune response.

CD40 associates with the MHC class II molecules on human B cells

The MHC class II and CD40 molecules are two major components of the immune system that are involved in cell‐cell interactions and signal transduction. Data obtained in the course of the present investigation show that these two molecules are physically associated on the surface of various human B cell lines and on normal tonsilar B cells. The CD40u2009/u2009MHC class II complexes were not detected on the germinal center B cell line Ramos. However, stimulation of these cells via CD40 or MHC class II triggered their association, suggesting that the formation of the complex is related to the activation status of the cells. The formation of these complexes did not alter the interaction of MHC class II molecules with one of their natural ligands, the staphylococcal enterotoxin A (SEA), as evidenced by the ability of SEA to bind MHC class IIu2009/u2009CD40 complexes. Cross‐linking of MHC class II or CD40 molecules leads to the association as well as the co‐association of both molecules to the NP‐49‐insoluble cellular matrix. Such association allowed us to demonstrate that only a fraction of these molecules can be physically associated on the cell surface. Based on previous observations and those presented here, it is highly possible that the CD40u2009/u2009MHC class II complexes may have an important role in signal(s) induced via both molecules and during Tu2009/u2009B cells interactions.

Involvement of zinc in the binding ofMycoplasma arthritidis-derived mitogen to the proximity of the HLA-DR binding groove regardless of histidine 81 of the β chain

Although our recent studies have provided the first evidence demonstrating the direct binding of Mycoplasma arthritidis‐derived mitogen (MAM) to MHC class II molecules, it is not yet established how MAM interacts with these molecules. Herein, we demonstrate that MAM binds preferentially and with high affinity to HLA‐DR molecules in a zinc‐dependent manner. MAMs affinity (25u2004nM) for HLA‐DR molecules is comparable to that of staphylococcal superantigens, and is slightly higher than that for murine MHC class II molecules expressed on the A20 B cell line (111u2004nM). The amino acid residues located between 14u2009–u200931 and 76u2009–u200990 of the MAM N‐terminus play a critical role in MAMu2009/u2009HLA‐DR interactions. Histidine at position 81 of the HLA‐DR β‐chain, known to be critical for binding of zinc‐coordinated superantigens, is not necessary for MAMu2009/u2009HLA‐DR interactions. The HLA‐DR residues involved in MAM binding are located in the proximal binding groove of the HLA‐DR molecule, where the nature of the peptide of the binding groove plays an important role in MAMu2009/u2009HLA‐DR interaction. This is the first detailed characterization of MAMs interactions with MHC class II molecules showing a mode of interaction with HLA‐DR distinct from that of other superantigens.

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

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) β 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 PKCβ activation nor for the induction of cell death. Indeed, the bulk of HLA‐DR‐activated PKCβ reside outside rafts. This is the first report showing that HLA‐DR‐induced PKCβ 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.