The tetraspanin family member CD151 forms complexes with integrins and regulates cell adhesion and migration. While CD151 is highly expressed in megakaryocytes and to a lesser extent in platelets, its physiologic role in platelets is unclear. In this study, we investigate the physical and functional importance of CD151 in murine platelets. Immunoprecipitation/ Western blot studies reveal a constitutive physical association of CD151 with integrin ␣ IIb ␤ 3 complex under strong detergent conditions. Using CD151-deficient mice, we show that the platelets have impaired "outside-in" integrin ␣ IIb ␤ 3 signaling with defective platelet aggregation responses to protease-activated receptor 4 (PAR-4) agonist peptide, collagen, and adenosine diphosphate (ADP); impaired platelet spreading on fibrinogen; and delayed kinetics of clot retraction in vitro. This functional integrin ␣ IIb ␤ 3 defect could not be attributed to altered expression of integrin ␣ IIb ␤ 3 . CD151 ؊/؊ platelets displayed normal platelet alpha granule secretion, dense granule secretion, and static platelet adhesion. In addition, CD151 ؊/؊ platelets displayed normal "in- IntroductionIntegrin ␣ IIb ␤ 3 maintains an inactive conformation on the surface of resting platelets until it is converted to its high-affinity state by agonist induced "inside-out" signaling via G protein-coupled or tyrosine kinase-linked pathways. Once activated, integrin ␣ IIb ␤ 3 can bind its soluble ligands, fibrinogen, and von Willebrand factor (VWF). Subsequently, "outside-in" integrin ␣ IIb ␤ 3 signaling events induce integrin clustering that leads to cytoskeletal reorganization and postoccupancy events including clot retraction, platelet spreading, and platelet aggregation. 1 The importance of integrin ␣ IIb ␤ 3 has been documented in Glanzmann thrombasthenia (GT) patients, who have bleeding complications despite a normal number of platelets. GT patients have dysfunctional platelets, which display quantitative or qualitative defects in integrin ␣ IIb or ␤ 3 . This gives rise to an integrin ␣ IIb ␤ 3 -mediated platelet defect, where the platelets fail to aggregate, bind fibrinogen, or retract fibrin clots. 2,3 This phenotype has been recapitulated in a complete ␤ 3 knock-out (KO) mouse model, which shows the cardinal features of Glanzmann thrombasthenia. 4 The concept that transmembrane receptors including integrins can form multimolecular complexes with tetraspanin-4 superfamily members (TM4SF) was first demonstrated by the ability of integrin ␣ IIb ␤ 3 to associate with a tetraspanin superfamily member, CD9, in stimulated platelets. 5 Subsequently, biochemical associations between tetraspanin superfamily members and other transmembrane receptors have been reported and clarified based upon associations in the presence of detergents with differing stringencies. 6 The tetraspanin superfamily member CD151 (also known as platelet endothelial tetraspan antigen-3/PETA-3) is expressed on the surface of platelets, megakaryocytes, endothelial cells, activated T lymphocytes, Schwann cel...

Carcinoembryonic antigen cell adhesion molecule-1 (CEACAM1) is a surface glycoprotein expressed on various blood cells, epithelial cells, and vascular cells. CEACAM1 possesses adhesive and signaling properties mediated by its intrinsic immunoreceptor tyrosine-based inhibitory motifs that recruit SHP-1 protein-tyrosine phosphatase. In this study, we demonstrate that CEACAM1 is expressed on the surface and in intracellular pools of platelets. In addition, CEACAM1 serves to negatively regulate signaling of platelets by collagen through the glycoprotein VI (GPVI)/Fc receptor (FcR)-␥-chain. ceacam1 ؊/؊ platelets displayed enhanced type I collagen and GPVI-selective ligand, collagen-related peptide (CRP), CRPmediated platelet aggregation, enhanced platelet adhesion on type I collagen, and elevated CRP-mediated alpha and dense granule secretion. Platelets derived from ceacam1 ؊/؊ mice form larger thrombi when perfused over a collagen matrix under arterial flow compared with wild-type mice. Furthermore, using intravital microscopy to ferric chloride-injured mesenteric arterioles, we show that thrombi formed in vivo in ceacam1 ؊/؊ mice were larger and were more stable than those in wild-type mice. GPVI depletion using monoclonal antibody JAQ1 treatment of ceacam1 ؊/؊ mice showed a reversal in the more stable thrombus growth phenotype. ceacam1 ؊/؊ mice were more susceptible to type I collagen-induced pulmonary thromboembolism than wild-type mice. Thus, CEACAM1 acts as a negative regulator of platelet-collagen interactions and of thrombus growth involving the collagen GPVI receptor in vitro and in vivo. (Blood.

We investigated the role of the hematopoietic-specific tetraspanin superfamily member, TSSC6, in platelet function using wild-type mice and TSSC6-deficient mice. TSSC6 is expressed on the surface of murine platelets and is up-regulated by thrombin stimulation, indicating an intracellular pool of TSSC6. Immunoprecipitation/Western blot studies reveal a constitutive physical association of TSSC6 with the integrin ␣ IIb ␤ 3 complex under strong detergent conditions. In vivo evaluation of hemostasis by tail bleeding revealed increased bleeding time, volume of blood lost, and evidence of tail rebleeds in TSSC6 null mice, indicating unstable hemostasis. Using ex vivo techniques, we showed that TSSC6-deficient platelets exhibited impaired kinetics of clot retraction, platelet aggregation at lower doses of PAR-4, and collagen and platelet spreading on fibrinogen in the presence of normal integrin ␣ IIb ␤ 3 expression. TSSC6-deficient platelets showed normal alpha granule secretion, normal "insideout" integrin ␣ IIb ␤ 3 signaling (fluorescein isothiocyanate [FITC]-fibrinogen and JON/A binding), and normal platelet adhesion on fibrinogen. Furthermore, we show that absence of platelet TSSC6 affects the secondary stability of arterial thrombi in vivo upon vascular injury. These data demonstrate that TSSC6 appears to regulate integrin ␣ IIb ␤ 3 "outside-in" signaling events in platelets and is necessary for stability of arterial thrombi in vivo. (Blood.

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This study supports a new concept where the opposing functions of the tetraspanins CD37 and CD82 may coordinate changes in migration and Ag presentation during dendritic cell (DC) activation. We have previously published that CD37 is downregulated upon monocyte-derived DC activation, promotes migration of both skin and bone marrow–derived dendritic cells (BMDCs), and restrains Ag presentation in splenic and BMDCs. In this article, we show that CD82, the closest phylogenetic relative to CD37, appears to have opposing functions. CD82 is upregulated upon activation of BMDCs and monocyte-derived DCs, restrains migration of skin and BMDCs, supports MHC class II maturation, and promotes stable interactions between T cells and splenic DCs or BMDCs. The underlying mechanism involves the rearrangement of the cytoskeleton via a differential activation of small GTPases. Both CD37−/− and CD82−/− BMDCs lack cellular projections, but where CD37−/− BMDCs spread poorly on fibronectin, CD82−/− BMDCs are large and spread to a greater extent than wild-type BMDCs. At the molecular level, CD82 is a negative regulator of RhoA, whereas CD37 promotes activation of Rac-1; both tetraspanins negatively regulate Cdc42. Thus, this study identifies a key aspect of DC biology: an unactivated BMDC is CD37hiCD82lo, resulting in a highly motile cell with a limited ability to activate naive T cells. By contrast, a late activated BMDC is CD37loCD82hi, and thus has modified its migratory, cytoskeletal, and Ag presentation machinery to become a cell superbly adapted to activating naive T cells.

Regulatory T cells (Tregs) must express appropriate skin-homing adhesion molecules to exert suppressive effects on dermal inflammation. However, the mechanisms whereby they control local inflammation remain unclear. In this study we used confocal intravital microscopy in wild-type and Foxp3-GFP mice to examine adhesion of effector T cells and Tregs in dermal venules. These experiments examined a two-challenge model of contact sensitivity (CS) in which Treg abundance in the skin progressively increases during the course of the response. Adhesion of CD4+ T cells increased during CS, peaking 8–24 h after an initial hapten challenge, and within 4 h of a second challenge. At these time points, 40% of adherent CD4+ T cells were Foxp3+ Tregs. CD4+ T cell adhesion was highly dependent on ICAM-1, and consistent with this finding, anti–ICAM-1 prevented Treg adhesion. Skin TGF-β levels were elevated in skin during both challenges, in parallel with Treg adhesion. In the two-challenge CS model, inhibition of ICAM-1 eliminated Treg adhesion, an effect associated with a significant increase in neutrophil adhesion. Similarly, total CD4+ T cell depletion caused an increase in adhesion of CD8+ T cells. Because Treg adhesion was restricted by both of these treatments, these experiments suggest that adherent Tregs can control adhesion of proinflammatory leukocytes in vivo. Moreover, the critical role of ICAM-1 in Treg adhesion provides a potential explanation for the exacerbation of inflammation reported in some studies of ICAM-1–deficient mice.

Previous studies on the role of the tetraspanin CD37 in cellular immunity appear contradictory. In vitro approaches indicate a negative regulatory role, whereas in vivo studies suggest that CD37 is necessary for optimal cellular responses. To resolve this discrepancy, we studied the adaptive cellular immune responses of CD37 −/− mice to intradermal challenge with either tumors or model antigens and found that CD37 is essential for optimal cell-mediated immunity. We provide evidence that an increased susceptibility to tumors observed in CD37 −/− mice coincides with a striking failure to induce antigenspecific IFN-γ-secreting T cells. We also show that CD37 ablation impairs several aspects of DC function including: in vivo migration from skin to draining lymph nodes; chemotactic migration; integrin-mediated adhesion under flow; the ability to spread and form actin protrusions and in vivo priming of adoptively transferred naïve T cells. In addition, multiphoton microscopy-based assessment of dermal DC migration demonstrated a reduced rate of migration and increased randomness of DC migration in CD37 −/− mice.Together, these studies are consistent with a model in which the cellular defect that underlies poor cellular immune induction in CD37 −/− mice is impaired DC migration. Keywords: CD37 r Cellular immunity r Dendritic cells r Migration r TetraspaninsAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionAdaptive cellular immunity is initiated by presentation of foreign antigen by DCs to antigen-specific naïve T lymphocytes. DCs exist sparsely in peripheral tissues in a state specialized for antigen uptake and processing. However, upon pathogen encounter, DCsCorrespondence: Dr. Mark D. Wright e-mail: Mark.Wright@monash.edu transduce signals through pattern recognition receptors, leading to an increased expression of cell surface molecules and cytokines, and induction of DC migration from the periphery to draining lymph nodes (DLNs) via afferent lymphatic vessels. Thus, upon their arrival in secondary lymphoid organs, DCs are equipped to initiate adaptive cellular immune responses through their ability to * These authors contributed equally to this work.C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2013. 43: 1208-1219 Cellular immune response 1209 activate naïve antigen-specific T cells [1]. Despite the importance of DC migration from the periphery to DLNs, the roles of the numerous molecules that regulate this process are incompletely understood. One such molecule is the leukocyte-specific membrane protein CD37, a member of the tetraspanin protein superfamily. Tetraspanins molecularly organize cellular membranes by interactions with partner molecules, which they direct into regulated signal-transducing tetraspanin-enriched microdomains. The cellular processes regulated by tetraspanin-mediated molecular organization include proliferation, adhesion and migration [2,3]. In immune cells, many important cell s...

Deciphering the molecular basis of leukocyte recruitment is critical to the understanding of inflammation. In this study, we investigated the contribution of the tetraspanin CD37 to this key process. CD37-deficient mice showed impaired neutrophil recruitment in a peritonitis model. Intravital microscopic analysis indicated that the absence of CD37 impaired the capacity of leukocytes to follow a CXCL1 chemotactic gradient accurately in the interstitium. Moreover, analysis of CXCL1-induced leukocyte-endothelial cell interactions in postcapillary venules revealed that CXCL1-induced neutrophil adhesion and transmigration were reduced in the absence of CD37, consistent with a reduced capacity to undergo β2 integrin–dependent adhesion. This result was supported by in vitro flow chamber experiments that demonstrated an impairment in adhesion of CD37-deficient neutrophils to the β2 integrin ligand, ICAM-1, despite the normal display of high-affinity β2 integrins. Superresolution microscopic assessment of localization of CD37 and CD18 in ICAM-1–adherent neutrophils demonstrated that these molecules do not significantly cocluster in the cell membrane, arguing against the possibility that CD37 regulates β2 integrin function via a direct molecular interaction. Moreover, CD37 ablation did not affect β2 integrin clustering. In contrast, the absence of CD37 in neutrophils impaired actin polymerization, cell spreading and polarization, dysregulated Rac-1 activation, and accelerated β2 integrin internalization. Together, these data indicate that CD37 promotes neutrophil adhesion and recruitment via the promotion of cytoskeletal function downstream of integrin-mediated adhesion.