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

Abstract

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. IntroductionReorganization of proteins at the interface between the T cell and the antigen-presenting cells (APCs) leads to the formation of the immunologic synapse. 1 Once a specific T-cell receptor has recognized the major histocompatibility complex (MHC)-peptide complex, MHC class II-mediated signals influence antigen-presenting function, adhesion, apoptosis, and cytokine production of the APC. 2 While MHC class II molecules do not harbor any known signaling motifs in their cytoplasmic domains, they can act as signal transducers, leading to activation of protein tyrosine kinases, phospholipase C␥, protein kinase C, and mitogen-activated protein kinases pathways. [3][4][5][6][7][8][9] Thus, MHC class II-mediated signals are likely to be at least partially dependent on interactions with other APC surface molecules. They also can be influenced by their compartmentalization into specialized membrane microdomains 10 such as glycosphingolipid-enriched microdomains, also called lipid rafts, 11 which are considered as platforms where signaling events are generated. MHC class II molecules are constitutively present in lipid rafts from murine and human B-cell lines, 12 monocytic cell lines, 13 human monocytes, 14 and tumor cells, 15 and the integrity of these microdomains is necessary for the transmission of MHC class II-mediated signals. 16 We reported that engagement of CD38 or MHC class II molecules on monocytes induced phosphorylation of common cytosolic substrates with marked additive effects between the 2 activation pathways. 17 CD38 plays an important role in activation and adhesion processes of monocytes. 18 CD38 ligation regulates the response to respiratory-burst activators 19 and enhances MHC class II and B7 expression in m...

CDw78 Defines MHC Class II-Peptide Complexes That Require Ii Chain-Dependent Lysosomal Trafficking, Not Localization to a Specific Tetraspanin Membrane Microdomain

“…While MHC-II isolated with anti-CDw78 mAb certainly are bound to tetraspanins, we conclude that this does not indicate a requirement for any particular MHC-II-tetraspanin interaction for CDw78 reactivity. In agreement with this conclusion are recent data identifying MHC-II tetraspanin complexes in monocytes that are CDw78 Ϫ (18). The authors speculate that these complexes are present in CDw78-independent, tetraspanin-containing microdomains and go on to show that MHC-II-CD9 complexes are present in detergent-insoluble microdomains that behave like lipid rafts (18).…”

“…By contrast, there is only limited data regarding the importance of MHC-II association with tetraspanins and/or tetraspanin microdomains. MHC-II have been shown by many groups to bind to individual tetraspanin family members (6,(12)(13)(14)(15)(16)(17)(18)(19). Recently Kropshofer and colleagues (2,6) have reported that a MHC-II epitope, termed CDw78, specifically recognizes MHC-II associated with tetraspanins.…”

“…MHC-II has long been known to associate with specific tetraspanin family members (6,(12)(13)(14)(15)(16)(17)(18)(19); however, the biological significance of this association remains unclear. Pulse-chase studies revealed that the association of MHC-II with the tetraspanin CD82 does not occur until the MHC-II-associated chaperone invariant chain (Ii) is released from newly synthesized MHC-II in lysosomelike Ag-processing compartments (12).…”

MHC Class II Antigen Presentation by Dendritic Cells Regulated through Endosomal Sorting

“…Perhaps pMHCII uses the clathrinindependent carrier (GLIC)/GPI-AP-enriched early endosomal compartment (GEEC) pathway, an endocytosis route that is also used by lipid raft-associated molecules like GPI-linked proteins and cholera toxin B (Mayor and Pagano 2007;Howes et al 2010). Furthermore, tetraspanin-enriched microdomains have been implicated in clustering and routing of MHCII from the plasma membrane (Hammond et al 1998;Engering and Pieters 2001;Kropshofer et al 2002;Zilber et al 2005;Poloso et al 2006;Unternaehrer et al 2007). Tetraspanins belong to a family of proteins that span the membrane four times and are palmitoylated, a posttranslational modification that stimulates lateral partitioning into protein/lipid domains (Kovalenko et al 2004;Yáñez-Mó et al 2009).…”

S100A4 and Uric Acid Promote Mesenchymal Stromal Cell Induction of IL-10+/IDO+ Lymphocytes

“…Surprisingly, and in contrast with blocking experiments with anti-S100A4, rhS100A4 protein by itself inhibited the proliferation of MSCs in a dosedependent manner (Fig. 4B), suggesting that other factors within the lysates buffer the S100A4, perhaps synergistically with HMGB1 or other chemotactic proteins such as CXCL12 (32)(33)(34)(35), acting to promote MSC proliferation. Similarly, UA inhibited proliferation of MSCs in a dose-dependent manner at concentrations up to 300 mg/ml.…”