Signal transduction via MHC class II antigens on B lymphocytes

Class II MHC cytoplasmic domain-mediated signaling in B cells: A tail of two signals

In addition to their role in antigen presentation, class II MHC molecules also transmit signals to B lymphocytes. Class II MHC-mediated signals initiate a range of events in B cells, including induction of cell surface proteins, initiation of cell-cycle progression/proliferation, activation of or protection from apoptosis, and antigen-dependent plasma cell differentiation. Although various transmembrane signaling proteins associate with class II MHC molecules, the class II MHC cytoplasmic domains are essential for signals leading to increased intracellular cAMP and activation of protein kinase C (PKC). Although truncation and mutagenesis studies have provided considerable information about the cytoplasmic domain sequences required, how class II MHC molecules elicit cAMP and PKC activation is not known. Further, appropriate T-dependent B cell responses require intact cAMP and PKC signaling, but the extent to which class II MHC signals are involved is also unknown. This review details our current knowledge of class II MHC cytoplasmic domain signaling in B cells with an emphasis on the likely importance of class II MHC signals for T-dependent antibody responses.

MHC class II/CD38/CD9: a lipid-raft–dependent signaling complex in human monocytes

Despite a lack of signaling motifs in their cytoplasmic domain, major histocompatibility complex (MHC) class II molecules trigger a variety of intracellular signals that regulate antigen-presenting cell function. They thus may use associated effector molecules as demonstrated on B cells and dendritic cells. The starting point of this study comes from our previous work, which demonstrated that the ecto-enzyme CD38 is functionally linked to MHC class II molecules. We report that CD38 and human leukocyte antigen-DR (HLA-DR) are functionally and physically associated in lipid rafts microdomains of cellsurface monocytes and that the integrity of these domains is necessary for the HLA-DR and CD38 signaling events. Moreover, we identified the tetraspanin CD9 molecule as a partner of the CD38/HLA-DR complex and demonstrated that HLA-DR, CD38, and CD9 share a common pathway of tyrosine kinase activation in human monocytes. The analysis of conjugate formation between monocytes presenting superantigen and T cells shows the active participation of CD9 and HLA-DR on the monocyte surface. Together, these observations demonstrate the presence of a CD38 and HLA-DR signaling complex within tetraspanin-containing lipid rafts and the functional impact of their molecular partner CD9 in antigen presentation.

Intracellular phosphotyrosine induction by major histocompatibility complex class II requires co-aggregation with membrane rafts

Cross-linking MHC class II molecules human leukocyte antigen (HLA-DR) on the surface of THP-1 cells was found to induce their entry into the glycolipid-enriched membrane fraction of the plasma membrane. At the cellular level, this resulted in the synergistic co-aggregation of class II with cholera toxin, a marker of membrane rafts. The accompanying induction of intracellular protein tyrosine phosphorylation could be inhibited by treating cells with methyl-beta-cyclodextrin, a drug that chelates membrane cholesterol and thereby disperses membrane rafts. Signaling could also be inhibited by treating cells with the Src-family kinase inhibitor PP1. Together, these results show that the induced association of class II molecules with membrane rafts can contribute to their aggregation on the cell surface and mediate an association with intracellular protein-tyrosine kinases.

Persistent upregulation of MHC Class II antigen expression on T-lymphocytes from cats experimentally infected with feline immunodeficiency virus

A significant elevation in the percentage of CD4+ and CD8+ T-lymphocytes expressing major histocompatibility complex (MHC) Class II antigens was observed in the blood of cats shortly after they were experimentally infected with feline immunodeficiency virus (FIV). In addition to an increase in the relative proportion of T-lymphocytes expressing Class II antigens, there was an increase in the density of Class II antigens on the cell surface. These elevations were still evident at the completion of the 5 month study. A second group of cats that had been infected with FIV for almost 5 years, and with either normal or abnormally low levels of CD4+ T-lymphocytes, had similar elevations in MHC II expression, suggesting that such abnormalities are lifelong. Cats with chronic (2 year) feline leukemia virus (FeLV) infection or dual FIV/FeLV infections also showed similar alterations in MHC II expression on CD4+ and CD8+ T-lymphocytes, suggesting that these alterations were not FIV specific. Feline T-lymphocytes expressed more MHC II antigen and interleukin-2 (IL-2) receptor following stimulation in vitro with conconavalin A and IL-2, demonstrating that feline T-lymphocytes respond to activation signals in a manner similar to T-lymphocytes of other species. However, changes in MHC II expression on T-cells of FIV infected cats were not explainable by viral induced T-cell activation alone, because FIV infected cats with elevated MHC II expression did not have coincident elevations in IL-2 receptor expression.