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...

Umbilical cord blood (CB) constitutes a promising alternative to bone marrow for allogeneic transplantation and is increasingly used because of the reduced severity of graft-versus-host disease after CB transplantation. We have compared the T-cell receptor β chain (TCRB) diversity of CB lymphocytes with that of adult lymphocytes by analyzing the complementarity determining region 3 (CDR3) size heterogeneity. In marked contrast to adult samples, we observed bell-shaped profiles in all of the 22 functional β-chain variable (BV) subfamilies that reflect the lack of prior antigenic stimulation in CB samples. However, the mean CDR3 size and BV usage were comparable between CB and adult samples. BJ2 (65%) segments were used preferentially to BJ1 (35%), especially BJ2S7, BJ2S5, BJ2S3, and BJ2S1, in both CB and in adult lymphocytes. We therefore conclude that although naive as reflected by the heterogeneity of the CDR3 size, the TCRBV repertoire appears fully constituted at birth. The ability to expand TCRB subfamilies was confirmed by stimulation with staphylococcal superantigens toxic shock syndrome toxin-1 and staphylococcal enterotoxin A. This study provides the basis for future analysis of the T-cell repertoire reconstitution following umbilical CB transplantation.