Phosphorylation of the major leukocyte surface sialoglycoprotein, leukosialin, is increased by phorbol 12-myristate 13-acetate

Buprenorphine decreases the CCL2-mediated chemotactic response of monocytes

Despite successful combined antiretroviral therapy, ∼ 60% of HIV-infected people exhibit HIV-associated neurocognitive disorders (HAND). CCL2 is elevated in the CNS of infected people with HAND and mediates monocyte influx into the CNS, which is critical in neuroAIDS. Many HIV-infected opiate abusers have increased neuroinflammation that may augment HAND. Buprenorphine is used to treat opiate addiction. However, there are few studies that examine its impact on HIV neuropathogenesis. We show that buprenorphine reduces the chemotactic phenotype of monocytes. Buprenorphine decreases the formation of membrane projections in response to CCL2. It also decreases CCL2-induced chemotaxis and mediates a delay in reinsertion of the CCL2 receptor, CCR2, into the cell membrane after CCL2-mediated receptor internalization, suggesting a mechanism of action of buprenorphine. Signaling pathways in CCL2-induced migration include increased phosphorylation of p38 MAPK and of the junctional protein JAM-A. We show that buprenorphine decreases these phosphorylations in CCL2-treated monocytes. Using DAMGO, CTAP, and Nor-BNI, we demonstrate that the effect of buprenorphine on CCL2 signaling is opioid receptor mediated. To identify additional potential mechanisms by which buprenorphine inhibits CCL2-induced monocyte migration, we performed proteomic analyses to characterize additional proteins in monocytes whose phosphorylation after CCL2 treatment was inhibited by buprenorphine. Leukosialin and S100A9 were identified and had not been shown previously to be involved in monocyte migration. We propose that buprenorphine limits CCL2-mediated monocyte transmigration into the CNS, thereby reducing neuroinflammation characteristic of HAND. Our findings underscore the use of buprenorphine as a therapeutic for neuroinflammation as well as for addiction.

CD43 interaction with ezrin-radixin-moesin (ERM) proteins regulates T-cell trafficking and CD43 phosphorylation

CD43 interaction with ERM proteins regulates CD43 phosphorylation and T-cell migration. CD43 phosphorylation can also drive CD43 localization in T-cells independently of ERM association.

Cell polarization is a key feature of cell motility, driving cell migration to tissues. CD43 is an abundantly expressed molecule on the T-cell surface that shows distinct localization to the migrating T-cell uropod and the distal pole complex (DPC) opposite the immunological synapse via association with the ezrin-radixin-moesin (ERM) family of actin regulatory proteins. CD43 regulates multiple T-cell functions, including T-cell activation, proliferation, apoptosis, and migration. We recently demonstrated that CD43 regulates T-cell trafficking through a phosphorylation site at Ser-76 (S76) within its cytoplasmic tail. Using a phosphorylation-specific antibody, we now find that CD43 phosphorylation at S76 is enhanced by migration signals. We further show that CD43 phosphorylation and normal T-cell trafficking depend on CD43 association with ERM proteins. Interestingly, mutation of S76 to mimic phosphorylation enhances T-cell migration and CD43 movement to the DPC while blocking ERM association, showing that CD43 movement can occur in the absence of ERM binding. We also find that protein kinase Cθ can phosphorylate CD43. These results show that while CD43 binding to ERM proteins is crucial for S76 phosphorylation, CD43 movement and regulation of T-cell migration can occur through an ERM-independent, phosphorylation–dependent mechanism.

CD43 processing and nuclear translocation of CD43 cytoplasmic tail are required for cell homeostasis

The sialomucin CD43 is highly expressed on most hematopoietic cells. In this study, we show that the CD43 ectodomain is shed from murine granulocytes, mast cells, and T cells, but not from macrophages. To study the significance of CD43 shedding, we constructed 2 CD43/34 chimeras in which the CD43 membrane-proximal or transmembrane domain was swapped with the corresponding domain from CD34 that is not shed from cells. Viability of cells that normally shed CD43 was negatively affected when forced to express either of the 2 CD43/34 chimeras, but toxicity was reduced when cells coexpressed wild-type CD43. The CD43 cytoplasmic tail (CD43ct) was found to translocate into the nucleus, and inhibition of either its nuclear translocation or its release by gamma-secretase was proapoptotic. Involvement of CD43 in regulation of apoptosis is consistent with our findings that CD43ct was modified by small ubiquitin-like modifier-1 and was colocalized with promyelocytic nuclear bodies. CD43-deficient cells exhibited reduced levels of promyelocytic nuclear bodies and had increased sensitivity to apoptosis induced by growth factor withdrawal or T-regulatory cell suppression. Taken together, our data indicate an essential function of CD43 processing and nuclear localization of CD43ct in cell homeostasis and apoptosis.

Binding of the Streptococcus gordonii DL1 surface protein Hsa to the host cell membrane glycoproteins CD11b, CD43, and CD50

Infective endocarditis is frequently attributed to oral streptococci. The mechanisms of pathogenesis, however, are not well understood, although interaction between streptococci and phagocytes are thought to be very important. A highly expressed surface component of Streptococcus gordonii, Hsa, which has sialic acid-binding activity, contributes to infective endocarditis in vivo. In the present study, we found that S. gordonii DL1 binds to HL-60 cells differentiated into monocytes, granulocytes, and macrophages. Using a glutathione S-transferase (GST) fusion to the NR2 domain, which is the sialic acid-binding region of Hsa, we confirmed that the Hsa NR2 domain also binds to differentiated HL-60 cells. To identify which sialoglycoproteins on the surface of differentiated HL-60 cells are receptors for Hsa, intrinsic membrane proteins were assessed by bacterial overlay and far-Western blotting. S. gordonii DL1 adhered to 100- to 150-kDa proteins, a reaction that was abolished by neuraminidase treatment. These sialoglycoproteins were identified as CD11b, CD43, and CD50 by GST pull-down assay and immunoprecipitation with each specific monoclonal antibody. These data suggest that S. gordonii DL1 Hsa specifically binds to three glycoproteins as receptors and that this interaction may be the initial bacterial binding step in infective endocarditis by oral streptococci.

Signaling through CD43 regulates CD4 T-cell trafficking

The mucin-like protein CD43 is excluded from the immune synapse, and regulates T-cell proliferation as well as T-cell migration. While the CD43 cytoplasmic domain is necessary for regulation of T-cell activation and proliferation, the mechanism via which CD43 regulates trafficking is not well defined. To investigate whether CD43 phosphorylation regulates its function in T cells, we used tandem mass spectrometry and identified Ser76 in murine CD43 as a previously unidentified site of basal phosphorylation. Interestingly, mutation of this single serine to alanine greatly diminishes T-cell trafficking to the lymph node, while CD43 exclusion and CD43-mediated regulation of T-cell proliferation remain intact. Furthermore, the CD43 extracellular domain was also required for T-cell trafficking, providing a hitherto unknown function for the extracellular domain, and suggesting that the extracellular domain may be required to transduce signals via the cytoplasmic domain. These data reveal a novel mechanism by which CD43 regulates T-cell function, and suggest that CD43 functions as a signaling molecule, sensing extracellular cues and transducing intracellular signals that modulate T-cell function.

Quantitative phosphoproteomic analysis of the tumor necrosis factor pathway

Protein phosphorylation has become a focus of many proteomic studies due to the central role that it plays in biology. We combine peptide-based gel-free isoelectric focusing and immobilized metal affinity chromatography to enhance the detection of phosphorylation events within complex protein samples using LC-MS. This method is then used to carry out a quantitative phosphoproteomic analysis of the tumor necrosis factor (TNF) pathway using HeLa cells metabolically labeled with 15N-containing amino acids, where 145 phosphorylation sites were found to be up-regulated upon the activation of the TNF pathway.

Shedding and gamma-secretase-mediated intramembrane proteolysis of the mucin-type molecule CD43

CD43 is a transmembrane molecule that contains a 123-aminoacids-long cytoplasmic tail and a highly O-glycosylated extracellular domain of mucin type. Endogenous CD43 expressed in COLO 205, K562 and Jurkat cells revealed a membrane-associated, 20 kDa CD43-specific cytoplasmic tail fragment (CD43-CTF) upon inhibition of gamma-secretase. This fragment was formed by an extracellular cleavage, as it was not accumulated after treating cells with 1,10-phenanthroline, a metalloprotease inhibitor. When CD43 was transfected into HEK-293 cells expressing dominant-negative PS1 (presenilin-1), the CD43-CTF was accumulated, but not in cells with wild-type PS1. Owing to its accumulation in the presence of a non-functional PS variant, it may thus be a novel gamma-secretase substrate. This CTF is formed by an extracellular cleavage close to the membrane, is a fragment that can be concluded to be a substrate for gamma-secretase. However, the intracellular gamma-secretase product has not been possible to detect, suggesting a quick processing of this product. During normal growth the CTF was not found without gamma-secretase inhibition, but when the cells (COLO 205) were very confluent the fragment could be detected. The intracellular domain of CD43 has previously been shown to contain a functional nuclear localization signal, and has been suggested to be involved in gene activation. From this and the present results, a novel way to explain how mucin-type molecules may transduce intracellular signals can be proposed.

CD43 has a functional NLS, interacts with beta-catenin, and affects gene expression

CD43 is a transmembrane molecule with a highly O-glycosylated extracellular domain of mucin type. It is a normal constituent of leukocytes and found in colon adenoma, but not in normal colon epithelia. Here it is shown that the cytoplasmic tail of CD43 contains a functional bipartite nuclear localization signal directing it to the nucleus. The intracellular domain of CD43 interacts with beta-catenin and causes an upregulation of the beta-catenin target genes c-MYC and CyclinD1. The present results suggest that CD43 can be involved in nuclear signaling and via beta-catenin interaction be involved in cell proliferation.

Costimulation of host T lymphocytes by a trypanosomal trans-sialidase: involvement of CD43 signaling

Trans-sialidase is a membrane-bound and shed sialidase from Trypanosoma cruzi, the protozoan parasite responsible for Chagas disease. We investigated the role of soluble trans-sialidase on host CD4+ T cell activation. Trans-sialidase activated naive CD4+ T cells in vivo. Both enzymatically active and inactive recombinant trans-sialidases costimulated CD4+ T cell activation in vitro. Costimulation resulted in increased mitogen-activated protein kinase activation, proliferation, and cytokine synthesis. Furthermore, active and inactive trans-sialidases blocked activation-induced cell death in CD4+ T cells from T. cruzi-infected mice. By flow cytometry, inactive trans-sialidase bound the highly sialylated surface Ag CD43 on host CD4+ T cells. Both costimulatory and antiapoptotic effects of trans-sialidases required CD43 signaling. These results suggest that trans-sialidase family proteins are involved in exacerbated host T lymphocyte responses observed in T. cruzi infection.

Alteration of leukocyte motility on plasma-conditioned prosthetic biomaterial, ePTFE, via a flow-responsive cell adhesion molecule, CD43

The physiologic determinants of leukocyte migration on vascular prosthetic biomaterials remain poorly understood, despite their relevance to the control of periprosthetic infection. Using hemodynamic exposure of human polymorphonuclear leukocytes adherent to expanded polytetrafluoroethylene (ePTFE) in vitro, we investigated the role of fluid shear in regulating leukocyte migratory behavior on plasma-adsorbed, prosthetic vascular biomaterial. The presence of flow at a wall shear stress of 25 dyn/cm(2) increased the degree of leukocyte displacement along the flow direction without altering the degree of overall cell attachment. Moreover, plasma-ePTFE elicited a lower overall degree of displacement under flow in comparison with untreated ePTFE. We further probed the molecular level regulation of leukocyte migratory responses under flow through the immunocytochemical quantification of specific leukocyte adhesion molecules and determined that CD43, a cell adhesion molecule, was upregulated via flow exposure for leukocytes adherent to plasma-ePTFE, whereas basal levels of CD43 expression were not significantly altered on untreated ePTFE. When flow-exposed, adherent leukocytes were incubated in the presence of substrate immobilized anti-CD43 immunoglobulin, the degree of cell displacement along flow was found to be significantly enhanced on plasma-ePTFE. Quantification of the cell population redistribution under flow using a modified random motility model, indicated that the incorporation of anti-CD43 on plasma-ePTFE led to a significant increase (243 +/- 60%) in the cell dispersion coefficient, mu(D), whereas only a minimal increase (61 +/- 30%) was detected on non-adsorbed ePTFE. Overall, our results suggest that flow exposure can induce the migration of leukocytes adherent to prosthetic materials in a substrate-dependent manner. An important implication of our study is that, although biomaterials exposed to plasma intrinsically passivate leukocyte migration even under hemodynamic conditions, it may be possible to promote cell motility by targeting a specific, flow-responsive, adhesion molecule.

Identification of polymorphonuclear leukocyte and HL-60 cell receptors for adhesins of Streptococcus gordonii and Actinomyces naeslundii

Interactions of oral streptococci and actinomyces with polymorphonuclear leukocytes (PMNs), mediated by sialic acid- and Gal/GalNAc-reactive adhesins, respectively, result in activation of the PMNs and thereby may contribute to the initiation of oral inflammation. Sialidase treatment of PMNs or HL-60 cells abolished adhesion of Streptococcus gordonii but was required for adhesion of Actinomyces naeslundii. The same effects of sialidase were noted for adhesion of these bacteria to a major 150-kDa surface glycoprotein of either PMNs or undifferentiated HL-60 cells and to a 130-kDa surface glycoprotein of differentiated HL-60 cells. These glycoproteins were both identified as leukosialin (CD43) by immunoprecipitation with a specific monoclonal antibody (MAb). Adhesion of streptococci and actinomyces to a 200-kDa minor PMN surface glycoprotein was also detected by bacterial overlay of untreated and sialidase-treated nitrocellulose transfers, respectively. This glycoprotein was identified as leukocyte common antigen (CD45) by immunoprecipitation with a specific MAb. CD43 and CD45 both possess extracellular mucinlike domains in addition to intracellular domains that are implicated in signal transduction. Consequently, the interactions of streptococci and actinomyces with the mucinlike domains of these mammalian cell surface glycoproteins result not only in adhesion but, in addition, may represent the initial step in PMN activation by these bacteria.

Signaling Through CD43 Induces Natural Killer Cell Activation, Chemokine Release, and PYK-2 Activation

Natural killer (NK) cell activation is the result of a balance between positive and negative signals triggered by specific membrane receptors. We report here the activation of NK cells induced through the transmembrane glycoprotein CD43 (leukosialin, sialophorin). Engagement of CD43 by specific antibodies stimulated the secretion of the chemokines RANTES, macrophage inflammatory protein (MIP)-1, and MIP-1β, which was prevented by treatment of cells with the specific tyrosine kinase inhibitor genistein. Furthermore, signaling through CD43 increased the cytotoxic activity of NK cells and stimulated an increase in the tyrosine kinase activity in antiphosphotyrosine immune complexes of NK cell lysates. PYK-2 was identified among the tyrosine kinase proteins that become activated. Hence, PYK-2 activation was observed after 20 minutes of CD43 stimulation, reached a maximum after 45 to 60 minutes, and decreased to almost basal levels after 120 minutes of treatment. Together, these results demonstrate the role of CD43 as an activation molecule able to transduce positive activation signals in NK cells, including the regulation of chemokine synthesis, killing activity, and tyrosine kinase activation.

Structural Requirements for CD43 Function

The regulation of T cell activation and adhesion by CD43 (leukosialin, sialophorin) has been thought to be mainly a function of the large size and negative charge of the extracellular domain of the protein. In this work, we demonstrate that the cytoplasmic tail is both necessary and sufficient for the negative regulatory effect of CD43 on cell-cell adhesion. Expression of mutant CD43 proteins in primary T cells from CD43-deficient mice demonstrated that the antiproliferative effect of CD43 is also dependent upon the cytoplasmic tail. In contrast, Ab-mediated costimulation through CD43 does not require the intracellular domain of CD43. These data demonstrate that CD43 primarily serves as a negative regulator of T cell activation and adhesion, and that this is mediated not exclusively by passive effects of the extracellular domain, but requires participation of the cytoplasmic tail, perhaps through interactions with the cytoskeleton, or alternatively, active regulation of intracellular signaling pathways.

Biochemical Characterization and Molecular Cloning of a Novel Endothelial-Specific Sialomucin

We have generated rat monoclonal antibodies (MoAbs) against cell surface antigens of the mouse endothelioma cell line bEND.3. Three antibodies (V.1A7, V.5C7, and V.7C7) were selected, all of which recognize a 75-kD antigen on bEND.3 cells and bind selectively to endothelial cells in cryostat sections of mouse tissues. A cDNA for the antigen was isolated from a bEND.3 pCDM8 expression library by using transient expression in COS-7 cells and immunoselection with the three MoAbs. This cDNA coded for a novel, type I membrane protein of 248 amino acids with an extracellular domain rich in threonine and serine residues (35%). The protein is sensitive to O-sialoglycoprotein endopeptidase, indicating that it belongs to the class of sialomucin-like proteins. Therefore, we suggest the name endomucin. Treatment of isolated endomucin by sialidase and O-glycosidase reduced the apparent molecular weight to 45 kD and abolished binding of all three antibodies, indicating that carbohydrates are directly or indirectly involved in the formation of the antibody epitopes. Immunohistological analysis of all examined mouse tissues showed that endomucin is an endothelial antigen found in venous endothelium as well as in capillaries, but not on arterial endothelium. Interestingly, high endothelial venules of peripheral and mesenteric lymph nodes as well as of Peyers’s patches were negative for staining with the three MoAbs.

Biochemical Characterization and Molecular Cloning of a Novel Endothelial-Specific Sialomucin

We have generated rat monoclonal antibodies (MoAbs) against cell surface antigens of the mouse endothelioma cell line bEND.3. Three antibodies (V.1A7, V.5C7, and V.7C7) were selected, all of which recognize a 75-kD antigen on bEND.3 cells and bind selectively to endothelial cells in cryostat sections of mouse tissues. A cDNA for the antigen was isolated from a bEND.3 pCDM8 expression library by using transient expression in COS-7 cells and immunoselection with the three MoAbs. This cDNA coded for a novel, type I membrane protein of 248 amino acids with an extracellular domain rich in threonine and serine residues (35%). The protein is sensitive to O-sialoglycoprotein endopeptidase, indicating that it belongs to the class of sialomucin-like proteins. Therefore, we suggest the name endomucin. Treatment of isolated endomucin by sialidase and O-glycosidase reduced the apparent molecular weight to 45 kD and abolished binding of all three antibodies, indicating that carbohydrates are directly or indirectly involved in the formation of the antibody epitopes. Immunohistological analysis of all examined mouse tissues showed that endomucin is an endothelial antigen found in venous endothelium as well as in capillaries, but not on arterial endothelium. Interestingly, high endothelial venules of peripheral and mesenteric lymph nodes as well as of Peyers’s patches were negative for staining with the three MoAbs.

CD43, a molecule with multiple functions

To initiate a specific immune response, lymphoid cells integrate a variety of signals generated through the orchestrated interaction of multiple cell surface molecules with their counter-receptors. As a result of the specific recognition of the antigen through antigen-specific receptors, and of the monitoring of their particular environment through the so-called coreceptor molecules, lymphoid cells go through elaborate processes of maturation and activation, contributing to the plasticity and sensitivity of immune response. CD43 is the major sialic acid rich protein on the surface of lymphocytes. However, the specific roles of this protein in different lymphoid cells under normal physiological conditions remain largely unknown. In this review we will mainly focus on the recent advances concerning the functions of this molecule as a coreceptor of different lymphoid cells as well as on the participation of this molecule in different pathologies.

Cloning and expression of cDNA for a human Gal(beta1-3)GalNAc alpha2,3-sialyltransferase from the CEM T-cell line

Complementary DNA encoding a human Gal(beta1-3)GalNAc alpha2,3-sialyltransferase type II (hST3Gal II) was cloned from a CEM T-cell cDNA library using a 23-base oligonucleotide probe. The sequence of this probe was established on the basis of a slightly divergent sialylmotif L that was obtained by polymerase chain reaction with degenerate oligonucleotide primers based on the conserved sialylmotif L of mammalian Gal(beta1-3)GalNAc alpha2,3-sialyltransferases. It was thus confirmed that a short oligonucleotide probe may be sensitive and highly specific. The nucleotide and amino acid sequences of hST3Gal II show, respectively, 56.3% and 49.3% similarity to hST3Gal I [Kitagawa, H. & Paulson, J. C. (1994) J. Biol. Chem. 269, 17872-17878] and 88.1% and 93.7% similarity to murine ST3Gal II [Lee, Y. C., Kojima, N., Wada, E., Kurosawa, N., Nakaoka, T., Hamamoto, T. & Tsuji, S. (1994) J. Biol. Chem. 269, 10028-10033]. hST3Gal II mRNA was highly expressed in heart, liver, skeletal muscle and various lymphoid tissues but not in brain and kidney. A soluble form of hST3Gal II expressed in COS-7 cells was tested in vitro for substrate specificity and kinetic properties. Asialofetuin and asialo-bovine submaxillary mucin appeared better substrates for hST3Gal II than for its murine counterpart as previously reported [Kojima, N., Lee, Y.-C., Hamamoto, T., Kurosawa, N. & Tsuji, S. (1994) Biochemistry 33, 5772-5776]. In previous studies, we have shown hyposialylation of O-glycans attached to two major lymphocyte CD43 and CD45 cell surface molecules in human-immunodeficiency-virus-1(HIV-1)-infected T-cell lines. Since comparable levels of hST3Gal I and hST3Gal II mRNA and enzymatic activity were observed in parental and HIV-1-infected CEM T-cell lysates, the sialylation defect associated with HIV infection of this cell line is probably due to a mechanism different from a simple altered catalytic activity of these sialyltransferases.

Leukosialin (CD43) is proteolytically cleaved from stimulated HMC-1 cells

Leukosialin (CD43), the major sialoprotein on circulating leukocytes, has been previously described to be down-regulated on neutrophils following activation with phorbol myristate acetate (PMA). The other single cells previously examined, blood lymphocytes, do not down-regulate CD43 when stimulated by PMA. Recently, we have characterized leukosialin on the human mast cell line HMC-1 and observed that leukosialin is down-regulated after stimulation with PMA. In the present study, we have investigated the mechanism of PMA-mediated down-regulation of CD43 on HMC-1 cells (subclone 5C6). PMA caused the release of soluble leukosialin (123 kD) during HMC-1 cell activation. The molecular weight of soluble leukosialin was nearly identical to that of the cell-membrane bound molecule, suggesting a cleavage proximal from the cell membrane. Inhibitors of serine proteases, like phenylmethylsulphonyl fluoride (PMSF), benzamidine and 3, 4-dichloroisocoumarin, blocked the PMA-mediated cleavage of CD43. In all experiments, the inhibition of CD43-down-regulation was dependent on the concentration of protease inhibitors. Treatment of HMC-1 cells with various proteases (trypsin, alpha-chymotrypsin, elastase, papain, nagarse) substantially decreased anti-CD43 binding capacity and caused the release of soluble leukosialin (116 kD) or its fragments into the supernatant. Pretreatment of HMC-1 cells with neuraminidases from Vibrio cholerae or Arthrobacter ureafaciens resulted in an increased sensitivity of CD43 against proteases, whereas the effects of PMA were not influenced. In conclusion, proteolytic cleavage of CD43 is described for the first time in a cell other than neutrophils, namely HMC-1 cells. Our results suggest that serine proteases are involved in the PMA-mediated down-regulation of leukosialin on HMC-1 cells.

Characterization of a CD43/leukosialin-mediated pathway for inducing apoptosis in human T-lymphoblastoid cells

The monoclonal antibody (mAb) J393 induces apoptosis in Jurkat T-cells. NH2-terminal amino acid sequence analysis identified the 140-kDa surface antigen for mAb J393 as CD43/leukosialin, the major sialoglycoprotein of leukocytes. While Jurkat cells co-expressed two discrete cell-surface isoforms of CD43, recognized by mAb J393 and mAb G10-2, respectively, only J393/CD43 signaled apoptosis. J393/CD43 was found to be hyposialylated, bearing predominantly O-linked monosaccharide glycans, whereas G10-2/CD43 bore complex sialylated tetra- and hexasaccharide chains. Treatment with soluble, bivalent mAb J393 killed 25-50% of the cell population, while concomitant engagement of either the CD3.TcR complex or the integrins CD18 and CD29 significantly potentiated this effect. Treatment of Jurkat cells with mAb J393 induced tyrosine phosphorylation of specific protein substrates that underwent hyperphosphorylation upon antigen receptor costimulation. Tyrosine kinase inhibition by herbimycin A diminished J393/CD43-mediated apoptosis, whereas inhibition of phosphotyrosine phosphatase activity by bis(maltolato)oxovanadium-IV enhanced cell death. Signal transduction through tyrosine kinase activation may lead to altered gene expression, as J393/CD43 ligation prompted decreases in the nuclear localization of the transcriptional regulatory protein NF-kappaB and proteins binding the interferon-inducible regulatory element. Since peripheral blood T-lymphocytes express cryptic epitopes for mAb J393, these findings demonstrate the existence of a tightly regulated CD43-mediated pathway for inducing apoptosis in human T-cell lineages.

CD43-specific activation of T cells induces association of CD43 to Fyn kinase

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signal transduction pathways that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in CD43's signal transduction pathway are poorly understood. In the present report we show that activation of both purified T lymphocytes and Jurkat cells, through CD43 cross-linking with the anti-CD43 L10 monoclonal antibody, induced CD43 association to Fyn kinase. This association is mediated by the Src homology 3 (SH3) domain of Fyn, since a glutathione S-transferase-Fyn SH3 fusion protein was able to precipitate CD43 from lysates of CD43-activated T cells. A synthetic peptide containing the SH3 binding sites of p85, located within the amino acid sequence 300ERQPAPALPPKPPKP314, was able to inhibit binding of CD43 to Fyn as well as to the glutathione S-transferase-Fyn SH3 fusion protein. We also provide evidence that upon CD43 cross-linking, Fyn is tyrosine-phosphorylated in a time-dependent manner. Our results suggest that CD43 cross-linking on the T cell surface induces the interaction between CD43 and Fyn, presumably through the Fyn SH3 domain and a putative SH3 binding site in CD43, leading to Fyn tyrosine phosphorylation and signal propagation.

Lymphocytic CD43 and CD45 bear sulfate residues potentially implicated in cell to cell interactions

CD43 is a major heavily glycosylated lymphocyte surface molecule. It has been shown to play an important role in lymphocyte activation and cell-cell interactions. Here we demonstrate that in human activated lymphocytes and CEM T cells, CD43 is a sulfated molecule. We also observed that CD45, another lymphocyte surface glycoprotein, is a sulfated molecule. 35SO4(2-) incorporation would thus appear to be an appropriate labeling method for CD43 and CD45 visualization. Moreover, we show that the level of cell surface protein sulfation can modulate CD43-mediated homotypic aggregation induced by CD43 monoclonal antibodies. It is well known that glycoprotein sulfation is required for various recognition phenomena. Since there are numerous potential sulfation sites on CD43 and CD45, these residues could play an important role in regulating cell-cell interactions.

Carbohydrate structures of recombinant soluble lamp-1 and leukosialin containing sialyl Le(x) terminus

Recombinant soluble lamp-1 and soluble leukosialin can be produced from CHO cells which express sialyl Le(x) structures after stable transfection of fucosyltransferase-III. It was shown previously that those soluble lamp-1 and leukosialin are potent inhibitors for E-selectin-mediated adhesion of human colonic tumor cells (Sawada, R.; Tsuboi, S.; Fukuda, M. J. Biol. Chem., 1994, 269, 1425). In the present study, we have determined the amount of the sialyl Le(x) structure present in recombinant, soluble lamp-1 and soluble leukosialin. CHO cells were metabolically labeled with [3H]-galactose and recombinant soluble lamp-1 and leukosialin were purified from the spent medium. Glycopeptides containing N-glycans derived from lamp-1 were fractionated by sequential lectin affinity chromatography. Similarly, O-glycans released from leukosialin were fractionated by Bio-Gel P-4 gel filtration. The terminal structures of carbohydrate chains were determined by sequential digestion with specific glycosidases. The results clearly indicate that soluble lamp-1 contains much more sialyl Le(x) structure than soluble leukosialin. Considering that soluble leukosialin and lamp-1 are almost equally effective as inhibitors for E-selectin-mediated adhesion, the results strongly suggest that densely clustered O-glycans are better presenters for E-selectin ligands than N-glycans.

Transcriptional activation of human leukosialin (CD43) gene by Sp1 through binding to a GGGTGG motif

Human leukosialin (CD43) is expressed on the surface of hematopoietic cells in cell-type specific and differentiation-stage-specific manners. Previously we found that the sequence from -53 to -40 was critically involved in the promoter function [Kudo, S. & Fukuda, M. (1991) J. Biol. Chem. 266, 8483-8489]. A transient-expression assay using a chloramphenicol acetyltransferase reporter gene revealed that the promoter could confer a high basal transcriptional activity in both leukosialin-producing and non-producing cells. The transcription factor interacting with the promoter sequence was determined by DNase I footprinting and gel-mobility-shift assays. The nuclear extracts from both leukosialin-producing Jurkat cells and non-producing Hela cells showed a footprint on the 5' flanking region from -58 to -34. Gel-mobility-shift assays revealed that DNA-protein complexes were formed with both nuclear extracts, and these complex formations were inhibited by an oligonucleotide containing the Sp1-binding consensus sequence. Prior incubation of anti-Sp1 antibody with nuclear extracts in this assay resulted in the supershift of the band for the DNA-protein complex. In addition, the footprint produced by the purified Sp1 transcription factor was identical to those produced by nuclear extracts of Jurkat and Hela cells. The mutational analyses revealed that the binding affinities of Sp1 to mutated promoter sequences were parallel to the transcriptional activity of these promoter sequences. Transient expression analyses in Drosophila Schneider cells demonstrated that cotransfection with Sp1 expression plasmid increased the transcriptional activity. These results establish that Sp1 can bind to the promoter and positively regulates the expression of the leukosialin gene. Even the stable expression of CAT constructs in non-producing Hela cells showed high transcriptional activity. The leukosialin expression thus appears to be regulated by the unique mechanism, that is the repression of high basal transcriptional activity rather than the activation of the basal transcriptional level. Tissue-specific expression is probably achieved by suppression of the basal transcriptional activity in non-producing cells.

Thy 1+ dendritic epidermal cells but not Langerhans cells express Ly 48

Leukosialin (Ly 48) is a sialoprotein expressed by bone marrow-derived cells early in their development. To determine whether Ly 48 is expressed by specific subsets of epidermal cells (ECs) such as Langerhans cells, keratinocytes, or dendritic epidermal T cells (DETCs), we utilized flow cytometry to detect staining of ECs by a panel of four monoclonal antibodies (S11, S15, S7, and 3E8) that recognize two different epitopes of the leukosialin antigen. Approximately 2.0% of unfractionated ECs expressed Ly 48, as demonstrated by reactivity with the monoclonal antibodies S11 and S15. Similarly, immunoblots of unfractionated EC lysates revealed an antigen of 125 kD apparent molecular weight that reacted with the S11 monoclonal antibody. Enrichment or depletion of various EC populations indicated that DETCs are the only EC population that express Ly 48. Studies of long-term cultured DETC lines indicated a marked heterogeneity of expression of Ly 48 epitopes. The function(s) of Ly 48 on DETCs as well as T-cell receptor alpha/beta bearing mouse T cells remains to be determined.

Enhancement of T-cell activation by the CD43 molecule whose expression is defective in Wiskott-Aldrich syndrome

CD43 (sialophorin, leukosialin, leukocyte large sialoglycoprotein), a heavily sialylated molecule found on most leukocytes and platelets, was initially identified as a major glycoprotein of mouse, rat and human T cells. CD43 expression is defective on the T cells of males with the Wiskott-Aldrich syndrome, an X chromosome-linked recessive immunodeficiency disorder. Affected males are susceptible to opportunistic infections and do not respond to polysaccharide antigens, reflecting defects in cytotoxic and helper T-cell functions. Anti-CD43 monoclonal antibodies have a modest costimulatory effect on T cells, natural killer cells, B cells and monocytes, and one such antibody has been shown to activate T cells directly. To investigate a possible physiological role for CD43, a complementary DNA encoding the human protein was introduced into an antigen-responsive murine T-cell hybridoma. We observed that CD43 enhances the antigen-specific activation of T cells and that the intracellular domain of CD43, which is hyperphosphorylated during T-cell activation, is required for this function. We also found that antigen-presenting cells can bind specifically to immobilized purified CD43 and that the binding can be inhibited by liposomes containing CD43 as well as by anti-CD43 monoclonal antibodies.

Protein sequence and gene structure for mouse leukosialin (CD43), a T lymphocyte mucin without introns in the coding sequence

A partial cDNA clone for mouse leukosialin was isolated by use of a rat leukosialin cDNA probe. The mouse cDNA was then used to isolate genomic clones that corresponded to the two mouse genes detected in Southern blots. One gene encoded an open reading frame for the homologue of rat leukosialin and this gene was notable for the absence of introns within the coding sequence. A lack of introns has previously been observed for the human leukosialin gene (Shelley, C. S., Remold-O'Donnell, E., Rosen, F. S. and Whitehead, A. S., Biochem. J., submitted). The other mouse gene was an intronless pseudogene for a leukosialin-related sequence. The presence of only one functional gene that lacked coding-region introns established that molecular heterogeneity in mouse leukosialin could not arise from multiple genes or alternative splicing of exons. The sequence of mouse leukosialin suggested an extracellular segment with a high content of O-linked carbohydrate, as is the case in the rat and human. In addition the mouse molecule had one possible N-linked glycosylation site. The cytoplasmic domain of 124 amino acids was highly conserved between rodent and human leukosialins for the functional genes but not for the pseudogene. This suggests an important functional role for the cytoplasmic domain.