Engaging CD19 or target of an antiproliferative antibody 1 on human B lymphocytes induces binding of B cells to the interfollicular stroma of human tonsils via integrin alpha 4/beta 1 and fibronectin

Tetraspanins CD81 and CD82 facilitate α4β1-mediated adhesion of human erythroblasts to vascular cell adhesion molecule-1

The proliferation and terminal differentiation of erythroid progenitors occurs in human bone marrow within erythroblastic islands, specialised structures consisting of a central macrophage surrounded by developing erythroid cells. Many cell-cell and cell-matrix adhesive interactions maintain and regulate the co-ordinated daily production of reticulocytes. Erythroid cells express only one integrin, α4β1, throughout differentiation, and its interactions with both macrophage Vascular Cell Adhesion Molecule-1 and with extracellular matrix fibronectin are critical for erythropoiesis. We observed that proerythroblasts expressed a broad tetraspanin phenotype, and investigated whether any tetraspanin could modulate integrin function. A specific association between α4β1 and CD81, CD82 and CD151 was demonstrated by confocal microscopy and co-immune precipitation. We observed that antibodies to CD81 and CD82 augmented adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 but not to the fibronectin spliceoforms FnIII_12-IIICS-15 and FnIII_12–15. In contrast, different anti-CD151 antibodies augmented or inhibited adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 and the fibronectin spliceoform FnIII_12-IIICS-15 but not to FnIII_12–15. These results strongly suggest that tetraspanins have a functional role in terminal erythropoiesis by modulating interactions of erythroblast α4β1 with both macrophages and extracellular matrix.

The small interferon-induced transmembrane genes and proteins

Interferon-induced transmembrane (IFITM) genes are transcribed in most tissues and are with the exception of IFITM5 interferon inducible. They are involved in early development, cell adhesion, and control of cell growth. Most IFITM genes are activated in response to bacterial and viral infections, and the exact host immune defense mechanisms are still unknown. Elevated gene expression triggered by past or chronic inflammation could prevent spreading of pathogens by limiting host cell proliferation. Accordingly, induction in cells with low basal protein levels is sufficient to drive growth arrest and a senescence-like morphology. On the other hand, loss of IFITM levels in cancer is correlated with pronounced malignancy; thus, these genes are considered as tumor suppressors. However, several cancer cells have deregulated high levels of IFITM transcripts, indicating a tumor progression stage where at least one of the interferon-controlled antiproliferative pathways has been silenced. Phylogenetic analyses of the protein coding genomic sequences suggest a single interferon-inducible gene in the common ancestor of rodents and primates. Biological functions studied so far may have evolved in parallel, and functional characterization of IFITM proteins will provide insight into innate immune defense, cancer development, and other pathways.

The follicular versus marginal zone B lymphocyte cell fate decision

Bone marrow-derived B cells make an important cell fate choice to develop into either follicular B cells or marginal zone B cells in the spleen, which depends on signalling through the B cell receptor, Notch2, the receptor for B cell-activating factor and the canonical nuclear factor-kappaB pathway, as well as signals involved in the migration and anatomical retention of marginal zone B cells. Recent information discussed in this Review reconciles some of the controversies regarding the role of the B cell receptor in this cell fate decision and a clearer picture has also emerged regarding the anatomical location of ligands for Notch2 in the spleen. This cell fate decision could provide mechanistic insights that are relevant to other commitment events in lymphocytes.

Germinal center structure and function: lessons from CD19

The germinal center is a critical locus in the production of protective immunity, but its function is poorly understood. Studies of mutant forms of CD19 revealed differences in signaling in different compartments inside the germinal center, and structural findings indicate a selective role in the interaction with follicular dendritic cells in the GC. Loss of these signals leads to surprising changes in germinal center B cells that challenge previous models of GC function.

Functional implications of tetraspanin proteins in cancer biology

Human tetraspanin proteins are a group of 33 highly hydrophobic membrane proteins that can form complexes in cholesterol-rich microdomains, distinct from lipid rafts, on the cell surface in a dynamic and reversible way. These complexes are composed of a core of several tetraspanin proteins that organize other membrane proteins such as integrins, human leukocyte antigen (HLA) antigens and some growth factor receptors. Although most tetraspanin proteins have been studied individually, tetraspanin proteins and their complexes can have effects on cellular adhesion and motility, interactions with stroma or affect signaling by growth factors, and for most of them no ligand has been identified. Functionally these proteins have been mostly studied in cells of lymphoid lineage, but they are present in all cell types. Data is also available for some tumors, where some tetraspanins have been identified as metastasis suppressors, but their significance is still not clear. Some of their implications in tumor biology and the areas that deserve further study are outlined.

IFITM/Mil/Fragilis Family Proteins IFITM1 and IFITM3 Play Distinct Roles in Mouse Primordial Germ Cell Homing and Repulsion

The family of interferon-induced transmembrane protein (Ifitm/mil/fragilis) genes encodes cell surface proteins that may modulate cell adhesion and influence cell differentiation. Mouse Ifitm1 and -3, which are expressed in primordial germ cells (PGCs), are implicated to have roles in germ cell development, but the specific functions have been unclear. Our results show that Ifitm1 activity is required for PGC transit from the mesoderm into the endoderm, and that it appears to act via a repulsive mechanism, such that PGCs avoid Ifitm1-expressing tissues. In contrast, Ifitm3, which is expressed in migratory PGCs, is sufficient to confer autonomous PGC-like homing properties to somatic cells. These guidance activities are mediated by the N-terminal extracellular domain of the specific IFITM, which cannot be substituted by that of another family member. Complex homo- and/or heterotypic intercellular interactions among various IFITMs in PGCs and neighboring cells may underpin coordinated germ cell guidance in mice.

Tetraspanins: molecular organisers of the leukocyte surface

Tetraspanins are a large superfamily of cell surface membrane proteins characterised by their four transmembrane domains. They are expressed in a wide variety of cell types and have functional roles in processes, such as cellular adhesion, motility, activation and tumour invasion. Leukocytes express Collapse

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MESH Headings

• Animals
• Antigen Presentation
• B-Lymphocytes/immunology
• Cell Membrane/immunology
• Humans
• Integrins/immunology
• Leukocytes/immunology
• Lymphocyte Activation
• Membrane Proteins/chemistry
• Membrane Proteins/genetics
• Membrane Proteins/immunology
• Mice
• Mice, Knockout
• Signal Transduction
• T-Lymphocytes/immunology

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Grants Collapse

Affiliation(s)

Jacqueline M Tarrant The Walter and Eliza Hall Institute, 1G Royal Parade, 3050, Parkville, Victoria, Australia
Lorraine Robb
Annemiek B van Spriel
Mark D Wright

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The CD81 Tetraspanin Facilitates Instantaneous Leukocyte VLA-4 Adhesion Strengthening to Vascular Cell Adhesion Molecule 1 (VCAM-1) under Shear Flow

Leukocyte integrins must rapidly strengthen their binding to target endothelial sites to arrest rolling adhesions under physiological shear flow. We demonstrate that the integrin-associated tetraspanin, CD81, regulates VLA-4 and VLA-5 adhesion strengthening in monocytes and primary murine B cells. CD81 strengthens multivalent VLA-4 contacts within subsecond integrin occupancy without altering intrinsic adhesive properties to low density ligand. CD81 facilitates both VLA-4-mediated leukocyte rolling and arrest on VCAM-1 under shear flow as well as VLA-5-dependent adhesion to fibronectin during short stationary contacts. CD81 also augments VLA-4 avidity enhancement induced by either chemokine-stimulated Gi proteins or by protein kinase C activation, although it is not required for Gi protein or protein kinase C signaling activities. In contrast to other proadhesive integrin-associated proteins, CD81-promoted integrin adhesiveness does not require its own ligand occupancy or ligation. These results provide the first demonstration of an integrin-associated transmembranal protein that facilitates instantaneous multivalent integrin occupancy events that promote leukocyte adhesion to an endothelial ligand under shear flow.

Human CD81 directly enhances Th1 and Th2 cell activation, but preferentially induces proliferation of Th2 cells upon long-term stimulation

BACKGROUND

CD81, a cell-surface protein of the tetraspanin superfamily, has been shown to costimulate T cell activation in murine T cells, and is involved in development of Th2 immune responses in mice.

RESULTS

Here it is shown that stimulation of CD81 on human T cells can enhance T cell activation by antigen or superantigen, causing an increase in the early activation marker CD69, and increasing the number of cytokine-producing and proliferating T cells. Interestingly, CD81 costimulates cytokine production by T cells producing both Th1 and Th2 cytokines. Although human CD81 is highly expressed on non-T as well as T cells, CD81 costimulation appears to act directly on T cells. Pre-incubation of purified T cells with anti-CD81 antibody is sufficient to increase T cell activation, while pre-incubation of non-T cells is not. However, long-term polyclonal stimulation of T cells by anti-CD3 antibody, in the presence of CD81 costimulation, biases T cells towards the production of IL-4 and not IFNgamma. This is accomplished by a preferential proliferation of IL-4-producing cells.

CONCLUSION

Thus, signalling through CD81 on T cells costimulates both Th1 and Th2 cells, but increases the number of Th2 cells during long-term activation.

Complexes of tetraspanins with integrins: more than meets the eye

The transmembrane proteins of the tetraspanin superfamily are implicated in a diverse range of biological phenomena, including cell motility, metastasis, cell proliferation and differentiation. The tetraspanins are associated with adhesion receptors of the integrin family and regulate integrin-dependent cell migration. In cells attached to the extracellular matrix, the integrin-tetraspanin adhesion complexes are clustered into a distinct type of adhesion structure at the cell periphery. Various tetraspanins are associated with phosphatidylinositol 4-kinase and protein kinase C isoforms, and they may facilitate assembly of signalling complexes by tethering these enzymes to integrin heterodimers. At the plasma membrane, integrin-tetraspanin signalling complexes are partitioned into specific microdomains proximal to cholesterol-rich lipid rafts. A substantial fraction of tetraspanins colocalise with integrins in various intracellular vesicular compartments. It is proposed that tetraspanins can influence cell migration by one of the following mechanisms: (1) modulation of integrin signalling; (2) compartmentalisation of integrins on the cell surface; or (3) direction of intracellular trafficking and recycling of integrins.

FPRP, a major, highly stoichiometric, highly specific CD81- and CD9-associated protein

CD81 and CD9, members of the transmembrane-4 superfamily (TM4SF; tetraspanins), form extensive complexes with other TM4SF proteins, integrins, and other proteins, especially in mild detergents. In moderately stringent Brij 96 lysis conditions, CD81 and CD9 complexes are virtually identical to each other, but clearly distinct from other TM4SF complexes. One of the most prominent proteins within CD81 and CD9 complexes is identified here as FPRP, the 133-kDa prostaglandin F(2alpha) receptor regulatory protein. FPRP, a cell-surface Ig superfamily protein, associates specifically with CD81 or with CD81 and CD9, but not with integrins or other TM4SF proteins. In contrast to other CD81- and CD9-associating proteins, FPRP associates at very high stoichiometry, with essentially 100% of cell-surface FPRP on 293 cells being CD81- and CD9-associated. Also, CD81.CD9.FPRP complexes have a discrete size (<4 x 10(6) Da) as measured by gel permeation chromatography and remain intact after disruption of cholesterol-rich membrane microdomains by methyl-beta-cyclodextrin. Although CD81 associated with both alpha(3) integrin and FPRP in 293 cells, the alpha(3)beta(1).CD81 and CD81.CD9.FPRP complexes were distinct, as determined by immunoprecipitation and immunodepletion experiments. In conclusion, our data affirm the existence of distinct TM4SF complexes with unique compositions and specifically characterize FPRP as the most robust, highly stoichiometric CD81- and/or CD9-associated protein yet described.

Cocaine-induced expression of the tetraspanin CD81 and its relation to hypothalamic function

CD81, a tetraspanin transmembrane protein involved in cell adhesion, was found by differential display to be upregulated in the nucleus accumbens of rat brain following acute cocaine treatment (four injections of 30 mg/kg every 2 h followed by 24 h withdrawal). Cocaine-induced expression of CD81 in adult rat brain was confirmed by quantitative real-time RT-PCR. Its expression in neurons and its function in the brain are unknown. In situ hybridization shows a neuron-specific expression pattern in brain regions functionally related to the regulation of cardiovascular function and fluid homeostasis. CD81 displays codistribution to galanin and, to a lesser extent, to vasopressin. These findings add to data that suggest a connection between the brain reward pathway and the centers regulating endocrine and autonomic functions, in relation to neurochemical, behavioral, and somatic consequences of drug abuse.

Crosslinking CD81 results in activation of TCRgammadelta T cells

CD81 is expressed on most cells and is associated with other glycoproteins, including CD4 and CD8, to form multimolecular membrane complexes. Crosslinking of CD81 on TCRalphabeta(+) T cells results in costimulatory signals that have been proposed to be mediated via CD4 or CD8. In this study, we show that CD81 is also expressed on TCRgammadelta(+)CD4(-)CD8(-) T cells. CD81 crosslinking greatly enhanced anti-CD3 activation of both TCRalphabeta(+) (CD4+ and CD8+) and TCRgammadelta(+) T cells with regard to IFN-gamma production. However, crosslinking of CD81 molecules on TCRgammadelta(+) T cells, in the absence of anti-CD3 stimulation, resulted in cytokine production and enhanced IL-2-induced proliferation, demonstrating that physical association with CD4 or CD8 is not necessary for CD81 signaling. In contrast, crosslinking of CD81 on TCRalphabeta(+) T cells, in the absence of anti-CD3 stimulation, failed to activate these T cells. These results suggest that CD81 signaling may be mediated via a different mechanism(s) in TCRgammadelta(+) versus TCRalphabeta(+) T cells.

Activated T lymphocytes bind in situ to stromal tissue of colon carcinoma but lack adhesion to tumor cells

It is not entirely clear which adhesion molecules are responsible for the site-directed traffic of T cells within the tumor microenvironment. The present study investigated whether colon carcinoma tissue and normal colon differ in the expression of functionally relevant molecules. In addition, we identified adhesion molecules involved in the binding of activated T cells onto colon carcinoma in situ. Malignant colon epithelium expressed few adhesion receptors, i.e. CD44 (HERMES), CD49b (integrin alpha2) and CD162 (PSGL-1), whereas the stromal compartment within colon carcinoma was positive for numerous binding molecules, e.g. CD44, CD49a (integrin alpha1), CD49e (integrin alpha5), CD51 (integrin alpha(v)), CD54 (ICAM-1), CD99 (MIC2) and CD162. Lymphocytes infiltrating tumor stroma contrasted with lymphocytes within normal colon interstitium by lacking CD28, CD154 (CD40L), CD56 (NCAM) and CD98 (4F2). Normal activated T cells bound to the lymphocyte-rich areas within the stroma of colon carcinoma using CD44, CD50 (ICAM-3), CD99, CD102 (ICAM-2) and CD162 on the T lymphocytes. We conclude that lymphocytes within colon carcinoma stroma may lack several functionally crucial cell surface molecules. We present a panel of adhesion molecules that could mediate the migration of activated T lymphocytes into the stroma of colon carcinoma.

Phemx, a novel mouse gene expressed in hematopoietic cells maps to the imprinted cluster on distal chromosome 7

Phemx (Pan hematopoietic expression) is a novel murine gene expressed in developmentally regulated sites of hematopoiesis from early in embryogenesis through adulthood. Phemx is expressed in hematopoietic progenitors and mature cells of the three main hematopoietic lineages. Conceptual translation of the murine Phemx cDNA predicts a 25-kDa polypeptide with four hydrophobic regions and several potential phosphorylation sites, suggestive of a transmembrane protein involved in cell signaling. The PHEMX protein is structurally similar to tetraspanin CD81 (TAPA-1), a transmembrane protein involved in leukocyte activation, adhesion, and proliferation. Phemx maps to the distal region of chromosome 7, a segment of the mouse genome that contains a cluster of genes that exhibit genomic imprinting. However, imprinting analysis of Phemx at the whole organ level shows that it is biallelically expressed, suggesting that mechanisms leading to monoallelic expression are not imposed at this locus. The human PHEMX ortholog is specifically expressed in hematopoietic organs and tissues and, in contrast to murine Phemx, undergoes alternative splicing. The unique mode and range of Phemx expression suggest that it plays a role in hematopoietic cell function.

Globotriaosyl ceramide modulates interferon-alpha-induced growth inhibition and CD19 expression in Burkitt's lymphoma cells

Previous studies have indicated that globotriaosyl ceramide (Gb3 or CD77) plays a role in alpha-interferon signal transduction and CD19-mediated homotypic adhesion in B cell lines derived from Burkitt's lymphoma. These roles for Gb3 may involve the proteins IFNAR-1 (subunit 1 of the interferon-alpha receptor) and CD19, respectively, both of which have potential Gb3-binding sites in their extracellular domains which resemble those of the verotoxin (Shiga toxin and Shiga-like toxin) B subunit. The majority of this work was performed using wild-type Daudi cells and a single, Gb3-deficient mutant cell line, VT500. In the present investigations, these and additional Daudi-derived cells with varying degrees of sensitivity to interferon-alpha were examined for Gb3 expression, interferon-induced growth inhibition and CD19 expression. The degree of interferon-induced growth inhibition and CD19 expression correlated with Gb3 expression in the various cell lines tested. In addition, reconstitution of the VT500 cell line with Gb3 but not other glycolipids partially restored the sensitivity of cells to IFN-induced growth inhibition. The degree to which reconstitution restored sensitivity to growth inhibition was similar to the results of previous studies in which Gb3 reconstitution restored sensitivity to verotoxin-induced cytotoxicity. These results demonstrate that Gb3 is specifically required for IFN-induced growth inhibition in Daudi cells and provide further evidence of a role for Gb3 in CD19 expression and function in these cells.

Inhibition of Cell Adhesion by Antibodies to Arg-Gly-Asp (RGD) in Normal Immunoglobulin for Therapeutic Use (Intravenous Immunoglobulin, IVIg)

Intravenous immunoglobulin (IVIg) therapy is associated with a broad range of immunomodulatory activities. Several of the postulated mechanisms of IVIg action relate to the presence of antibodies to molecules relevant for regulation of the immune response. This article reports that IVIg contains antibodies to the Arg-Gly-Asp (RGD) sequence, and the attachment site of a number of adhesive extracellular matrix proteins, including ligands for β1, β3, and β5 integrins. Anti-RGD antibodies were identified in IVIg by enzyme-linked immunosorbent assay and by using the BIAcore (BIAcore, Uppsala, Sweden) technology. The affinity of anti-RGD antibodies to a synthetic RGD-containing peptide and to fibronectin (Fn) was found to be in the micromolar range. F(ab′)2 fragments specific for RGD were purified from IVIg by affinity chromatography. Anti-RGD F(ab′)2 antibodies inhibited adenosine diphosphate induced IIb/β3 integrin-mediated platelet aggregation and the adhesion of activated 4β1 integrin-expressing B cells to Fn. Adhesion of unstimulated platelets to fibrinogen (Fg) involving both the γ-chain dodecapeptide sequence and the RGD sequence was inhibited by anti-RGD antibodies. In addition, adhesion of thrombin-stimulated platelets to von Willebrand factor or Fg was completely inhibited by affinity-purified anti-RGD antibodies. Our results suggest that the presence of natural IgG antibodies to the RGD motif may contribute to the immunomodulatory and anti-inflammatory effects of therapeutic preparations of normal IgG.

Inhibition of Cell Adhesion by Antibodies to Arg-Gly-Asp (RGD) in Normal Immunoglobulin for Therapeutic Use (Intravenous Immunoglobulin, IVIg)

Intravenous immunoglobulin (IVIg) therapy is associated with a broad range of immunomodulatory activities. Several of the postulated mechanisms of IVIg action relate to the presence of antibodies to molecules relevant for regulation of the immune response. This article reports that IVIg contains antibodies to the Arg-Gly-Asp (RGD) sequence, and the attachment site of a number of adhesive extracellular matrix proteins, including ligands for β1, β3, and β5 integrins. Anti-RGD antibodies were identified in IVIg by enzyme-linked immunosorbent assay and by using the BIAcore (BIAcore, Uppsala, Sweden) technology. The affinity of anti-RGD antibodies to a synthetic RGD-containing peptide and to fibronectin (Fn) was found to be in the micromolar range. F(ab′)2 fragments specific for RGD were purified from IVIg by affinity chromatography. Anti-RGD F(ab′)2 antibodies inhibited adenosine diphosphate induced IIb/β3 integrin-mediated platelet aggregation and the adhesion of activated 4β1 integrin-expressing B cells to Fn. Adhesion of unstimulated platelets to fibrinogen (Fg) involving both the γ-chain dodecapeptide sequence and the RGD sequence was inhibited by anti-RGD antibodies. In addition, adhesion of thrombin-stimulated platelets to von Willebrand factor or Fg was completely inhibited by affinity-purified anti-RGD antibodies. Our results suggest that the presence of natural IgG antibodies to the RGD motif may contribute to the immunomodulatory and anti-inflammatory effects of therapeutic preparations of normal IgG.

PETA-3/CD151, a member of the transmembrane 4 superfamily, is localised to the plasma membrane and endocytic system of endothelial cells, associates with multiple integrins and modulates cell function

The Transmembrane 4 Superfamily member, PETA-3/CD151, is ubiquitously expressed by endothelial cells in vivo. In cultured human umbilical vein endothelial cells PETA-3 is present on the plasma membrane and predominantly localises to regions of cell-cell contact. Additionally, this protein is abundant within an intracellular compartment which accounts for up to 66% of the total PETA-3 expressed. Intracellular PETA-3 showed colocalisation with transferrin receptor and CD63 suggesting an endosomal/lysosomal localisation which was supported by immuno-electronmicroscopy studies. Co-immunoprecipitation experiments investigating possible interactions of PETA-3 with other molecules demonstrated associations with several integrin chains including beta1, beta3, beta4, (alpha)2, (alpha)3, (alpha)5, (alpha)6 and provide the first report of Transmembrane 4 Superfamily association with the (alpha)6beta4 integrin. Using 2-colour confocal microscopy, we demonstrated similar localisation of PETA-3 and integrin chains within cytoplasmic vesicles and endothelial cell junctions. In order to assess the functional implications of PETA-3/integrin associations, the effect of anti-PETA-3 antibodies on endothelial function was examined. Anti-PETA-3 mAb inhibited endothelial cell migration and modulated in vitro angiogenesis, but had no detectable effect on neutrophil transendothelial migration. The broad range of integrin associations and the presence of PETA-3 with integrins both on the plasma membrane and within intracellular vesicles, suggests a primary role for PETA-3 in regulating integrin trafficking and/or function.

Transmembrane 4 superfamily protein CD151 (PETA-3) associates with beta 1 and alpha IIb beta 3 integrins in haemopoietic cell lines and modulates cell-cell adhesion

CD151 (PETA-3/SFA-1) is a member of the transmembrane 4 superfamily (TM4SF) of cell-surface proteins and is expressed abundantly both on the cell surface and in intracellular membranes by the haemopoietic cell lines M07e, HEL and K562. In the presence of mild detergent (CHAPS), CD151 co-immunoprecipitated with integrin alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1 and alpha IIb beta 3. The association of CD151 with alpha 4 beta 1 and alpha 5 beta 1 seemed to be constitutive, as it was not modified by treatment of M07e cells with cytokines that regulate integrin function by 'inside-out' signalling. CD151 also associated with other tetraspans in an apparently cell-type-specific fashion, as defined by its co-precipitation with CD9, CD63 and CD81 from M07e cells, but not from K562 cells, which express similar levels of these proteins. F(ab')2 fragments of monoclonal antibodies (mAbs) against CD151 caused homotypic adhesion of HEL and K562 cells that was dependent on energy and cytoskeletal integrity and was augmented in the presence of RGDS peptides. The adhesion was not blocked by function-inhibiting mAbs against beta 1 or beta 3 integrins, suggesting that cell-cell adhesion was not mediated by the binding of integrin to a cell-associated ligand. Furthermore, mAb CD151 did not affect adhesion of the cells to fibronectin, laminin, collagen or fibrinogen, which are ligands for alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1 and alpha IIb beta 3 integrins. Taken together, these results indicate that the ligation of CD151 does not induce the up-regulation of integrin avidity, but might act as a component of integrin signalling complexes.

CD19 is linked to the integrin-associated tetraspans CD9, CD81, and CD82

The CD19-CD21-CD81 complex regulates signal transduction events critical for B lymphocyte development and humoral immunity. CD81, a molecule with 4 transmembrane domains, member of the tetraspan superfamily, is engaged, together with other tetraspans such as CD9, CD53, CD63, and CD82, in multimolecular complexes containing beta1 integrins and major histocompatibility complex antigens. Here we demonstrate that two other tetraspans, CD82 and the early B cell marker CD9, are coimmunoprecipitated with CD19 from Brij97 lysates of B cell lines. Moreover, CD9 was coprecipitated from lysates of purified CD10(+) early B cells. These associations were confirmed by the cocapping of CD19 with CD9 or CD82. The CD9/CD19 association was disrupted in the presence of digitonin, contrary to the CD81/CD19 association, indicating that CD9 and CD81 interact with CD19 in different ways. The CD9/CD81 association is also disrupted in the presence of digitonin, suggesting that CD9 associates with CD19 only through CD81. To characterize the regions involved in the CD81/CD19 association, two reciprocal CD9/CD81 chimeric molecules were tested for the association with CD19, but none of them could be coprecipitated with CD19 in digitonin, indicating that the domain of CD81 responsible for its association with CD19 is complex. Finally, engagement of CD9 could induce the tyrosine phosphorylation of different proteins, including CD19 itself, suggesting that the CD9/CD19 association is functionally relevant. Thus, a physical and functional link is formed between the CD19-CD21-CD81 complex and the integrin-tetraspan complexes, which is dynamically modulated in the process of B cell differentiation.

Integrin associated proteins

Integrin cytoplasmic domains may interact directly with serveral different cytoskeletal proteins and intracellular signaling molecules. Also, integrins interact directly with other transmembrane structures, including transmembrane-4 superfamily (TM4SF) proteins. New evidence suggests that TM4SF proteins may act as linkers between extracellular integrin alpha chain domains and intracellular signaling molecules, such as phosphatidylinositol 4-kinase and protein kinase C.

Characterization of cDNAs encoding a new family of tetraspanins from schistosomes--the Sj25 family

The tetraspanins, also known as members of the transmembrane 4 superfamily (TM4SF), comprise an assemblage of surface antigens reported from mammalian and other vertebrate cells, from schistosomes, from fruit flies and from Caenorhabditis elegans. Tetraspanins are characterized by the presence of four hydrophobic domains, which are presumed to be membrane-spanning, and specific conserved motifs. A novel cDNA encoding a new tetraspanin, termed TE736 (tetraspanin 736), was isolated and characterized from the human blood fluke Schistosoma japonicum. Nucleotide and deduced amino acid sequences of the cDNA revealed that TE736 was similar to the previously characterized Sm23/Sj23/Sh23 species homologues and to Sj25/TM4 from schistosomes, and to other tetraspanins. Comparison of hydropathicity profiles of TE736, Sj25/TM4 and two novel tetraspanin-like sequences from S. mansoni showed that they contain four potential transmembrane domains like other tetraspanins. Sequence alignments showed there are four highly conserved, cysteine residues on the second predicted extracellular loop of TE736. A phylogenetic comparison of the relationship of approximately 30 tetraspanins from mammals and other groups revealed that TE736, Sj25/TM4 and the two sequences from S. mansoni formed an independent family of tetraspanins. We have termed these tetraspanins from schistosomes the Sj25 family. TE736 appeared to be encoded by a single gene and to be expressed in at least two life cycle stages of S. japonicum.

SFA-1/PETA-3 (CD151), a Member of the Transmembrane 4 Superfamily, Associates Preferentially with α5β1 Integrin and Regulates Adhesion of Human T Cell Leukemia Virus Type 1-Infected T Cells to Fibronectin

In this study we have analyzed the adhesion molecules associated with and the biologic function of SFA-1/PETA-3 (CD151) in human T cell leukemia virus type 1 (HTLV-1)-infected T cells and in freshly isolated adult T cell leukemia (ATL) cells using an anti-CD151 mAb. The anti-CD151 mAb coprecipitated α5β1 integrin from HTLV-1-infected T cells. Conversely, an anti-α5 integrin mAb coprecipitated CD151. The anti-CD151 mAb inhibited the adhesion of HTLV-1-infected T cells to fibronectin but did not have any effect on their adhesion to laminin, collagen type I, or collagen type IV. Moreover, antisense CD151 oligonucleotide-treated HTLV-1-infected T cells showed significant inhibition of adhesion to fibronectin. These findings showed that the CD151 molecule was associated with the α5β1 integrin molecule and that it enhanced α5β1 integrin-mediated adhesion to fibronectin. In addition, the expression levels of CD151, α4β1 integrin, and α5β1 integrin on ATL cells from lymph nodes of lymphoma-type ATL patients were significantly higher than those on circulating ATL cells from leukemia-type ATL patients. This suggests that the increased expression of these integrins may contribute to lymphoma formation through the adhesion of ATL cells to the extracellular matrix and dendritic cells, rather than contributing to transmigration.

Sequences and expression of six new members of the tetraspanin/TM4SF family

Tetraspanins (or TM4SF) are expressed in a wide variety of species and regulate cell adhesion, migration, proliferation and differentiation. We have identified and sequenced six new members of the tetraspanin family, called Tspan-1-6, from human cDNA. Amino acid sequence analysis of the Tspans highlights conserved residues which may be critical to tetraspanin structure and function. The Tspans are differentially expressed in human tissues.

CD81 (TAPA-1): a molecule involved in signal transduction and cell adhesion in the immune system

CD81 (TAPA-1) is a widely expressed cell-surface protein involved in an astonishing variety of biologic responses. It has been cloned independently several times for different functional effects and is reported to influence adhesion, morphology, activation, proliferation, and differentiation of B, T, and other cells. On B cells CD81 is part of a complex with CD21, CD19, and Leu13. This complex reduces the threshold for B cell activation via the B cell receptor by bridging Ag specific recognition and CD21-mediated complement recognition. Similarly on T cells CD81 associates with CD4 and CD8 and provides a costimulatory signal with CD3. In fetal thymic organ culture, mAb to CD81 block maturation of CD4-CD8- thymocytes, and expression of CD81 on CHO cells endows those cells with the ability to support T cell maturation. However, CD81-deficient mice express normal numbers and subsets of T cells. These mice do exhibit diminished antibody responses to protein antigens. CD81 is also physically and functionally associated with several integrins. Anti-CD81 can activate integrin alpha 4 beta 1 (VLA-4) on B cells, facilitating their adhesion to tonsilar interfollicular stroma. Similarly, anti-CD81 can activate alpha L beta 2 (LFA-1) on human thymocytes. CD81 can also affect cognate B-T cell interactions because anti-CD81 increases IL-4 synthesis by T cells responding to antigen presented by B cells but not by monocytes. The tetraspanin superfamily (or TM4SF) includes CD81, CD9, CD37, CD53, CD63, CD82, CD151, and an increasing number of additional proteins. Like CD81, several tetraspanins are involved in cell adhesion, motility, and metastasis, as well as cell activation and signal transduction.

Regulation of endothelial cell motility by complexes of tetraspan molecules CD81/TAPA-1 and CD151/PETA-3 with alpha3 beta1 integrin localized at endothelial lateral junctions

Cell-to-cell junction structures play a key role in cell growth rate control and cell polarization. In endothelial cells (EC), these structures are also involved in regulation of vascular permeability and leukocyte extravasation. To identify novel components in EC intercellular junctions, mAbs against these cells were produced and selected using a morphological screening by immunofluorescence microscopy. Two novel mAbs, LIA1/1 and VJ1/16, specifically recognized a 25-kD protein that was selectively localized at cell-cell junctions of EC, both in the primary formation of cell monolayers and when EC reorganized in the process of wound healing. This antigen corresponded to the recently cloned platelet-endothelial tetraspan antigen CD151/PETA-3 (platelet-endothelial tetraspan antigen-3), and was consistently detected at EC cell-cell contact sites. In addition to CD151/PETA-3, two other members of the tetraspan superfamily, CD9 and CD81/ TAPA-1 (target of antiproliferative antibody-1), localized at endothelial cell-to-cell junctions. Biochemical analysis demonstrated molecular associations among tetraspan molecules themselves and those of CD151/ PETA-3 and CD9 with alpha3 beta1 integrin. Interestingly, mAbs directed to both CD151/PETA-3 and CD81/ TAPA-1 as well as mAb specific for alpha3 integrin, were able to inhibit the migration of ECs in the process of wound healing. The engagement of CD151/PETA-3 and CD81/TAPA-1 inhibited the movement of individual ECs, as determined by quantitative time-lapse video microscopy studies. Furthermore, mAbs against the CD151/PETA-3 molecule diminished the rate of EC invasion into collagen gels. In addition, these mAbs were able to increase the adhesion of EC to extracellular matrix proteins. Together these results indicate that CD81/TAPA-1 and CD151/PETA-3 tetraspan molecules are components of the endothelial lateral junctions implicated in the regulation of cell motility, either directly or by modulation of the function of the associated integrin heterodimers.

Reduced phagocytosis of apoptotic cells in malignant lymphoma

Efficient removal of lymphocytes undergoing programmed cell death (apoptosis) by macrophages plays an important role for the proper function of normal immune system. Furthermore, in malignant lymphoma, elimination of apoptotic tumor cells by phagocytes contributes to the anti-tumor immune response. It is unknown, however, whether macrophages in normal and malignant lymphoid tissues differ in their ability to recognize and remove apoptotic cells. Our present results demonstrate that normal and malignant lymphoid tissues differ according to the extent of the infiltration by macrophages. The highest densities of macrophages (p < 0.0001) were detected in diffuse large B-cell lymphoma, centroblastic (DLBCL-CB) and immunoblastic variants and Burkitt's lymphoma. The grade of the macrophage infiltration correlated with the proliferation rates of the tumors (p < 0.0001). Compared with normal lymphoid organs, malignant lymphoma contained lower percentages of apoptotic cells phagocytosed by tissue macrophages (p < 0.001). Of all lymphomas tested, mantle cell lymphoma and DLBCL-CB expressed the lowest percentages of phagocytosed apoptotic cells (p < 0.0001).

Functional analysis of four tetraspans, CD9, CD53, CD81, and CD82, suggests a common role in costimulation, cell adhesion, and migration: only CD9 upregulates HB-EGF activity

Molecules of the tetraspan superfamily are engaged in multimolecular complexes containing other proteins such as beta 1 integrins and MHC antigens. Although their functions are not clear, they have been suggested to play a role in cell adhesion and migration, signal transduction, and costimulation. We have in this paper directly compared the functional properties of four tetraspans, CD9, CD53, CD81, and CD82. mAbs to any of these molecules were able to deliver a costimulatory signal for CD3-mediated activation of the T cell line Jurkat. CD82 mAbs were the most efficient in triggering this effect. Moreover, engagement of CD9, CD81, and CD82 induced the homotypic aggregation of the megakaryocytic cell line HEL, and inhibited the migration of this cell line. Similar results were obtained with the preB cell line NALM-6 using the CD9 and CD81 mAbs. The CD81 mAb 5A6 produced the strongest effects. Therefore, the tetraspans are recognized by mAbs which produce similar effects on the same cell lines. This is consistent with the tetraspans being included in large molecular complexes and possibly forming a tetraspan network (the tetraspan web). We also demonstrate that the tetraspans are likely to keep specific functional properties inside this network. Indeed, we have demonstrated that the human CD9 is able, like the monkey molecule, to upregulate the activity of the transmembrane precursor of heparin-binding EGF as a receptor for the diphtheria toxin when cotransfected in murine LM cells. Neither CD81, nor CD82 had such activity. By using chimeric CD9/CD81 molecules we demonstrate that this activity requires the second half of CD9, which contains the large extracellular loop, the fourth transmembrane region, and the last short cytoplasmic domain.

Identification of components of trans-Golgi network-derived transport vesicles and detergent-insoluble complexes by nanoelectrospray tandem mass spectrometry

Epithelial cells have to deliver newly synthesized proteins to the apical and the basolateral plasma membrane domains of the polarized cell surface. Sorting takes place in the trans-Golgi network and at least two vesicular carriers exist for apical and basolateral delivery. After immuno-isolation, the composition of these vesicle preparations was analyzed by two-dimensional (2-D) gel electrophoresis and detergent extraction. In this paper we compare the constituents of detergent-insoluble complexes in different cell lines of polarized or nonpolarized origin and present the identification of five previously uncharacterized proteins. We show that our protein identification strategy can be successfully applied to the problem of small hydrophobic proteins from organisms that have not been substantially sequenced. The high sensitivity of nanoelectrospray tandem mass spectrometry allowed us to identify two proteins that belong to the p23/p24 family of putative cargo receptors for vesicular trafficking. Furthermore we have mapped CD9 and CD81, two members of a large family of proteins consisting of highly hydrophobic four transmembrane proteins. In addition we have identified caveolin-2 as a constituent of basolateral transport vesicles. We have also extended our analysis of immuno-isolated vesicles to a more basic pI range and show that this region on 2-D gels is devoid of proteins. With these approaches and with the previously published data we have now identified most of the major low molecular weight proteins recovered in detergent-insoluble glycolipid-enriched complexes.

Generation of monoclonal antibodies to integrin-associated proteins. Evidence that alpha3beta1 complexes with EMMPRIN/basigin/OX47/M6

The alpha3beta1 integrin forms complexes with other cell-surface proteins, including transmembrane-4 superfamily (TM4SF) proteins (e. g. CD9, CD53, CD63, CD81, and CD82). To identify additional cell-surface proteins associated with alpha3beta1 integrin, a monoclonal antibody selection protocol was developed. Mice were immunized with integrin alpha3beta1-containing complexes isolated from HT1080 fibrosarcoma cells, and then 712 hybridoma clones were produced, and 95 secreted antibodies that recognized the HT1080 cell surface. Among these, 12 antibodies directly recognizing integrin alpha3 or beta1 subunits were eliminated. Of the remaining 83, 16 co-immunoprecipitated proteins that resembled integrins under non-stringent detergent conditions. These 16 included 15 monoclonal antibodies recognizing EMMPRIN/basigin/OX-47/M6, a 45-55-kDa transmembrane protein with two immunoglobulin domains. The EMMPRIN protein associated with alpha3beta1 and alpha6beta1, but not alpha2beta1 or alpha5beta1, as shown by reciprocal immunoprecipitation experiments. Also, association with alpha3beta1 was confirmed by cell-surface cross-linking and immunofluorescence co-localization experiments. Importantly, EMMPRIN-alpha3beta1 complexes appear not to contain TM4SF proteins, suggesting that they are distinct from TM4SF protein-alpha3beta1 complexes.

Negative regulation of Fc epsilon RI-mediated degranulation by CD81

Signaling through the high affinity receptor for immunoglobulin E (Fc epsilon RI) results in the coordinate activation of tyrosine kinases before calcium mobilization. Receptors capable of interfering with the signaling of antigen receptors, such as Fc epsilon RI, recruit tyrosine and inositol phosphatases that results in diminished calcium mobilization. Here, we show that antibodies recognizing CD81 inhibit Fc epsilon RI-mediated mast cell degranulation but, surprisingly, without affecting aggregation-dependent tyrosine phosphorylation, calcium mobilization, or leukotriene synthesis. Furthermore, CD81 antibodies also inhibit mast cell degranulation in vivo as measured by reduced passive cutaneous anaphylaxis responses. These results reveal an unsuspected calcium-independent pathway of antigen receptor regulation, which is accessible to engagement by membrane proteins and on which novel therapeutic approaches to allergic diseases could be based.

Impaired CD19 expression and signaling, enhanced antibody response to type II T independent antigen and reduction of B-1 cells in CD81-deficient mice

The tetraspanin CD81 is ubiquitously expressed and associated with CD19 on B lymphocytes and with CD4 and CD8 on T lymphocytes. Analysis of mice with disrupted CD81 gene reveals normal T cells but a distinct abnormality in B cells consisting of decreased expression of CD19 and severe reduction in peritoneal B-1 cells. CD81-deficient B cells responded normally to surface IgM crosslinking, but had severely impaired calcium influx following CD19 engagement. CD81-deficient mice had increased serum IgM and IgA and an exaggerated antibody response to the type II T independent antigen TNP-Ficoll. These results suggest that CD81 is important for CD19 signaling and B cell function.

Differential expression of apoptosis, Bcl-x and c-Myc in normal and malignant lymphoid tissues

Bcl-x and c-Myc have an important role for the immune response by regulating the programmed cell death (apoptosis) of lymphocytes. Dysfunction of these selection processes can lead to the development of malignant lymphoma. The present study aimed at defining the differential expression of apoptosis, Bcl-x and c-Myc in normal and in malignant lymphoid tissues. Follicular centre lymphoma (FCL-F) and mantle cell lymphoma (MCL) contained the lowest apoptotic indices (AIs), whereas Burkitt's lymphoma (BL) had the highest AIs. The AIs correlated significantly with the growth rates of the tumours and with the extent of Bcl-x expression. Bcl-x was expressed in almost all BL cells, but in few tumour cells in FCL-F and in MCL. c-Myc, in contrast, was found in the majority of the tumour cells in FCL-F and in MCL, but not in BL. Whereas the extent of Bcl-x expression correlated positively with the growth rates, an inverse correlation was observed between the percentages of c-Myc-positive tumour cells and the growth rates of the tumours. We conclude that normal and malignant lymphoid tissues have a distinct pattern of apoptosis and that expression of Bcl-x and c-Myc in B cell lymphoma is differentially regulated.

Co-receptors of B lymphocytes

The past year has seen advances in our understanding of accessory membrane proteins that modulate the B cell response to antigen-receptor stimulation. The generation of complement receptor deficient mice has reinforced our appreciation of the importance of complement receptors in the B cell response to antigen. The association of inositol polyphosphate 5-phosphatase with FcgammaRIIB suggests another mechanism, in addition to recruitment of the phosphotyrosine phosphatase SHP-1, by which secreted immunoglobulin can limit further response to antigen. The in vivo function of CD22 in regulating the threshold of antigen-receptor signalling has been shown using CD22-deficient mice. Lastly, B cell receptor signalling in the B-1 subset of B lymphocytes has been demonstrated to be negatively regulated by CD5.

Human T lymphocytes bind to germinal centers of human tonsils via integrin alpha4/VCAM-1 and LFA-1/ICAM-1 and -2

Binding of T lymphocytes within the different compartments of the secondary lymphoid organs is crucial for the function of the cellular and the humoral immune response. It is still not known which adhesion molecules guide T cells to the distinct areas of the lymphoid microenvironment. In the current study an in situ adhesion assay was used to define the receptors for binding of T cells to human tonsils. The T cell lines Jurkat and MOLT-4 and normal, activated T cells were found to bind exclusively to germinal centers. Jurkat cells used the receptor pair integrin-alpha4 (VLA-4alpha)/VCAM-1, whereas activated MOLT-4 cells and normal T cells bound via both adhesion pathways, namely via integrin-alpha4/VCAM-1 and LFA-1/ICAM-1 and -2. It is suggested that these adhesion mechanisms are involved in the migration of T cells into the germinal centers of secondary lymphoid organs and that they influence the selection of B cells by apoptosis.

CD81 expressed on human thymocytes mediates integrin activation and interleukin 2-dependent proliferation

Lymphocyte recognition of antigen by the antigen-specific T cell receptor (TCR) and coreceptor complexes rapidly alters the cell's adhesive properties facilitating high avidity cell-ligand interactions necessary for lymphocyte development and function. Here, we report the expression of CD81 (target of antiproliferative antigen [TAPA]-1) on human thymocytes and the physical association of CD81 with CD4 and CD8 T cell coreceptors. Antibody ligation of CD81 on thymocytes promotes the rapid induction of integrin-mediated cell-cell adhesion via lymphocyte function-associated molecule-1 (LFA-1). Cross-linking CD81 is also shown to be costimulatory with signaling through the TCR/CD3 complex inducing interleukin 2-dependent thymocyte proliferation. These data suggest that a CD81-mediated pathway in thymocytes is involved in the regulation of both cell adhesion and activation.

Ligation of TAPA-1 (CD81) or major histocompatibility complex class II in co-cultures of human B and T lymphocytes enhances interleukin-4 synthesis by antigen-specific CD4+ T cells

We have previously shown that CD4+ T cells from allergic individuals are predisposed to producing interleukin (IL)-4 in response to allergens. IL-4 production could be modulated by antigen concentration as well as by the type of antigen-presenting cells (APC), with B lymphocytes inducing greater quantities of IL-4 than monocytes. Using this system we examined IL-4 synthesis after culture of CD4+ T cells with B cells, monocytes, or both, as APC in the presence of allergen and a monoclonal antibody against CD81 (TAPA-1), a member of the TM4 superfamily of proteins that regulates activation, proliferation and trafficking of B cells. Addition of anti-CD81 mAb during culture enhanced IL-4 synthesis by 2- to 70-fold over that using an isotype-matched control mAb. Furthermore, anti-CD81 mAb enhanced IL-4 synthesis in CD4+ T cells only when CD4+ T cells were cultured with B cells but not monocytes as APC, indicating that anti-CD81 mAb affected IL-4 synthesis in T cells via interactions with B cells. However, pretreatment of either population separately with anti-CD81 mAb prior to culture had no effect on subsequent IL-4 synthesis, suggesting a requirement for temporal or cooperative interactions between T and B lymphocytes. In addition, anti-CD81 mAb enhanced IL-4 production but reduced CD4+ T cell antigen-specific proliferation, demonstrating that IL-4 production and proliferation by CD4+ T cells were inversely related. Finally, mAb to major histocompatibility complex class II but not to anti-CD19 also enhanced IL-4 synthesis when B lymphocytes were used as APC. In all instances, enhancement of CD4+ IL-4 synthesis correlated with the presence of large cell aggregates in T-B lymphocyte cocultures. These results indicate that the capacity of B cells to induce IL-4 can be significantly enhanced by ligation of particular molecules on their surface and should aid in the design of treatments for diseases in which modulation of the cytokine profile would be beneficial.

Activation of B lymphocytes: integrating signals from CD19, CD22 and Fc gamma RIIb1

Three accessory membrane proteins, CD19, CD22 and Fc gamma RIIb1, alter signaling through membrane immunoglobulin of B cells by binding cytosolic proteins containing SH2 domains. Recent biochemical and genetic studies have shown that these receptors enable B cells to amplify responses to certain T-cell-dependent antigens (CD19), to restrict their response to T-cell zones of secondary lymphoid organs (CD22), and to dampen their response to antigens for which IgG is already available (Fc gamma RIIb1).