Sialophorin, a surface sialoglycoprotein defective in the Wiskott-Aldrich syndrome, is involved in human T lymphocyte proliferation

Receptors and ligands implicated in human T cell costimulatory processes

It is well established that full activation of T cells that recognize antigens requires additional signals. These second signals are generated by the interaction of costimulatory ligands expressed on antigen presenting cells with their receptors on T cells. In addition, T cell activation processes are negatively regulated by inhibitory costimulatory pathways. Interaction of members of the B7 and the TNF superfamilies with members of the CD28 and TNF-R-superfamilies plays major roles in costimulatory processes. However, a large number of molecules that do not belong to these families have been reported to be involved in the generation of T cell costimulatory signals. In addition to well-defined costimulatory pathways, where both receptors and ligands are known, there are many T cell surface molecules that have been described to generate a second signal under certain experimental conditions, f.i. when ligated with antibodies. Furthermore there are several ligands that have been shown to positively or negatively modulate T cell activation by interacting with as of yet unknown T cell receptors. Here we give a comprehensive overview of molecules that have been implicated in human T cell activation processes and propose criteria that define genuine T cell costimulatory pathways.

CD43 controls the intracellular growth of Mycobacterium tuberculosis through the induction of TNF-alpha-mediated apoptosis

Establishment of Tuberculosis infection begins with the successful entry and survival of the pathogen within macrophages. We previously showed that macrophage CD43 is required for optimal uptake and growth inhibition of Mycobacterium tuberculosis both in vitro and in vivo. Here, we explore the mechanisms by which CD43 restricts mycobacterial growth in murine macrophages. We found that although M. tuberculosis grows more readily in resting CD43-/- macrophages, priming of cells with IFN-gamma returns the bacterial growth rate to that seen in CD43+/+ cells. To discern the mechanisms by which M. tuberculosis exhibits enhanced growth within resting CD43-/- macrophages, we assessed the induction of inflammatory mediators in response to infection. We found that absence of CD43 resulted in reduced production of TNF-alpha, IL-12 and IL-6 by M. tuberculosis-infected macrophages. We also found that infected resting, but not activated CD43-/- macrophages, showed decreased apoptosis and increased necrosis. Exogenous addition of the pro-inflammatory cytokine TNF-alpha restored control of M. tuberculosis growth and induction of apoptosis to CD43+/+ levels. We propose that CD43 is involved in the inflammatory response to M. tuberculosis and, through the induction of pro-inflammatory mediators, can regulate apoptosis to control intracellular growth of the bacterium.

CD43 is a ligand for E-selectin on CLA+ human T cells

The recruitment of memory T cells from blood into tissues is a central element of immune surveillance and adaptive immune responses and a key feature of chronic cutaneous inflammatory diseases such as psoriasis and atopic dermatitis. Human memory T cells that infiltrate skin express the carbohydrate epitope cutaneous lymphocyte-associated antigen (CLA). Expression of the CLA epitope on T cells has been described on P-selectin glycoprotein ligand-1 (PSGL-1) and associated with the acquisition of both E-selectin and P-selectin ligand functions. In this report, we show that CD43, a sialomucin expressed constitutively on T cells, can also be decorated with the CLA epitope and serve as an E-selectin ligand. CLA expressed on CD43 was found exclusively on the high-molecular-weight (125 kDa) glycoform bearing core-2-branched O-linked glycans. CLA+ CD43 purified from human T cells supported tethering and rolling in shear flow via E-selectin but did not support binding of P-selectin. The identification and characterization of CD43 as a T-cell E-selectin ligand distinct from PSGL-1 expands the role of CD43 in the regulation of T-cell trafficking and provides new targets for the modulation of immune functions in skin.

Expression characteristics and stimulatory functions of CD43 in human CD4+ memory T cells: analysis using a monoclonal antibody to CD43 that has a novel lineage specificity

We have used HSCA-2, an mAb that recognizes a sialic acid-dependent epitope on the low molecular mass (approximately 115-kDa) glycoform of CD43 that is expressed in resting T and NK cells, to examine the expression characteristics and stimulatory functions of CD43 in human CD4+ memory T cells. Having previously reported that the memory cells that respond to recall Ags in a CD4+ CD45RO+ T cell population almost all belong to a subset whose surface CD43 expression levels are elevated, we now find that exposing these same memory T cells to HSCA-2 mAb markedly increases their proliferative responsiveness to recall Ags. We think it unlikely that this increase in responsiveness is a result of CD43-mediated monocyte activation, especially given that the HSCA-2 mAb differs from all previously used CD43 mAbs in having no obvious binding specificity for monocyte CD43. Predictably, treatment with HSCA-2 mAb did not lead to significant recall responses in CD4+ CD45RO+ T cells, whose CD43 expression levels were similar to or lower than those of naive cells. Other experiments indicated that the HSCA-2 mAb was capable of enhancing the proliferative responsiveness of CD4+ memory T cells that had been exposed to polyclonal stimulation by monocyte-bound CD3 mAb and could also act in synergy with CD28 mAb to enhance the responsiveness of CD4+ T cells to CD3 stimulation. Taken together, these findings suggest that the CD43 molecules expressed on CD4+ memory T cells may be capable of enhancing the costimulatory signaling and hence providing accessory functions to TCR-mediated activation processes.

Overexpression of leukocyte marker CD43 causes activation of the tumor suppressor proteins p53 and ARF

CD43 or leukosialin is a transmembrane sialoglycoprotein, whose extracellular domain participates in cell adhesiveness and the cytoplasmic tail regulates a variety of intracellular signal transduction pathways involved in cell proliferation. CD43 is abundantly expressed on the surface of hematopoietic cells, but CD43 expression is also frequently found in the tumor cells of nonhematopoietic origin. In the early stages of some tumors, the accumulation of tumor suppressor protein p53 has been described. Here, we show that the expression of CD43 causes the induction of functionally active p53 protein. Moreover, we found that the activation of p53 by CD43 is mediated by tumor suppressor protein ARF. The coexpression of CD43 and ARF in ARF-null mouse embryonic fibroblasts resulted in programmed cell death, but that was not the case when CD43 alone was expressed in these cells. These data provide the first evidence of the connection between p53- and CD43-dependent pathways.

hnRNP-K and Pur(alpha) act together to repress the transcriptional activity of the CD43 gene promoter

CD43 is an abundant, heavily glycosylated molecule expressed specifically on the surface of leukocytes and platelets. When leukocytes are at rest, CD43 acts to prevent both homotypic and heterotypic interactions. However, during leukocyte activation CD43 expression is repressed, facilitating the intercellular contact required for chemotaxis, phagocytosis, aggregation, adhesion to endothelium, and transendothelial migration. Consequently, CD43 repression plays a vital role both in innate and acquired immunity. Here we report that a dramatic down-regulation of CD43 mRNA levels occurs during activation of the leukocytic cell line K562. This repression coincides with repression of the transcriptional activity of the CD43 gene promoter. We have determined that heterogeneous nuclear ribonucleoprotein K (hnRNP-K) and Pur(alpha) act together to mediate repression of the CD43 promoter during K562 activation. The hnRNP-K molecule and Pur(alpha) bind single-stranded DNA. Therefore, exposure of single-stranded structures within the CD43 promoter probably plays a major role in effecting CD43 repression.

Engagement of CD43 enhances human immunodeficiency virus type 1 transcriptional activity and virus production that is induced upon TCR/CD3 stimulation

Human immunodeficiency virus type 1 (HIV-1) transcriptional activity is regulated by several cytokines and T cell activators. CD43 (sialophorin) is a sialoglycoprotein expressed on the surface of a wide variety of blood cells including T lymphocytes. Several studies have shown that CD43 ligation induces proliferation and activation of human T lymphocytes. We were thus interested in defining whether CD43-mediated signaling events can modulate the life cycle of HIV-1. We demonstrate here that CD43 cross-linking potentiates HIV-1 promoter-driven activity and virus production that is seen following the engagement of the T-cell receptor (TCR).CD3 complex. This effect is independent of the CD28 co-stimulatory molecule and is mediated by both NF-kappaB and NFAT transcription factors. A number of signal transducers known to be involved in the TCR/CD3-dependent signal transduction pathway, including p56(lck), p36(lat), and SLP-76, as well as capacitative entry of calcium, are crucial for the noticed CD43 co-stimulatory effect. Calcium mobilization studies indicate that a synergy is occurring between CD43- and TCR/CD3-mediated signaling events leading to an augmented calcium release. These data suggest that CD43 can be seen as a co-stimulatory cell surface constituent that can modulate HIV-1 expression in T lymphocytes.

CD43 polarization in unprimed T cells can be dissociated from raft coalescence by inhibition of HMG CoA reductase

Movement of T-lymphocyte cell surface CD43 is associated with both antigen activation of T-cell clones and chemokine induction of T-lymphocyte motility. Here, we demonstrate that CD43 movement away from the site of T-cell receptor ligation occurs in unprimed CD4(+) T cells as well as T-cell clones. The T-cell receptor (TCR)-dependent movement of CD43 in unprimed T cells is associated with a polarized morphology and CD43 accumulation at the uropods of the cells, unlike that reported for primed T cells. The polarization of CD43 has a requirement for Src kinases and occurs in conjunction with lipid raft coalescence. Thymocytes and T-cell hybridomas, cells that have altered responses to TCR activation and lack lipid raft coalescence, do not polarize CD43 as readily as unprimed T cells. The movement of CD43 depends on the cholesterol biosynthetic pathway enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. Blockade of this enzyme can specifically prevent CD43 redistribution without affecting cell shape polarization. The likely mechanism of this alteration in CD43 redistribution is through decreased protein prenylation because the cholesterol-dependent lipid rafts still coalesce on activation. These findings suggest that the polarization of cell shape, lipid raft coalescence, and CD43 redistribution on T-cell activation have signaling pathway distinctions. Dissecting out the relationships between various stages of molecular redistribution and lymphocyte activation may facilitate fine-tuning of immunologic responses.

Adhesion via CD43 induces Syk activation and cell proliferation in TF-1 cells

The effect of adhesion via CD43 (leukosialin, sialophorin) on cell proliferation and phosphorylation signaling were examined in a growth factor-dependent hematopoietic progenitor cell line, TF-1. TF-1 cells promptly resulted in death after withdrawal of growth factors. However, the viable cell number increased when TF-1 cells were cultured on anti-CD43 monoclonal antibody-coated plates. In this case, sustained activation of protein tyrosine kinase Syk and extracellular signal-regulated kinase (Erk) 1/2 were detected. Overexpression of exogenous Syk on TF-1 cells by the adenovirus vector system induced enhancement of the cell proliferation accompanied with enhancement of the Erk activation by a dominant-positive effect. The signal transducer and activator of transcription (STAT) 5 seemed not to be associated with the CD43-mediated cell proliferation. These results indicated that adhesion via CD43 induces the proliferation of TF-1 cells in the absence of growth factors in part by Syk-dependent Erk 1/2 signaling.

Reduced CD43 expression on the neutrophils of MDS patients correlates with an activated phenotype of these cells

CD43 (also known as leukosialin and sialophorin) is a surface sialoglycoprotein expressed at high levels on most leukocytes implicated in adhesion, antiadhesion, and activation/proliferation mechanisms. We studied the expression of this molecule on the leukocytes of patients with myelodysplastic syndromes (MDSs) in an effort to detect acquired deficiencies of this molecule. We used immunofluorescence flow cytometry in analyzing whole blood and isolated neutrophils from 49 MDS patients, 33 men and 16 women aged 33 to 85 years (median, 75 years), and 18 healthy individuals aged 35 to 80 years (median, 72 years). According to French-American-British classification criteria, 13 patients had refractory anemia, 18 had refractory anemia with ringed sideroblasts, 9 had refractory anemia with excess of blasts, 4 had refractory anemia with excess of blasts in transformation to acute leukemia, and 5 had chronic myelomonocytic leukemia. We found decreased expression of CD43 on the neutrophils of these patients, and we correlated this finding with the activation status of these cells as it is defined by their phenotypes. We studied the expression of CD11b, CD18, CD35, CD67, CD69, CD44, and CD53 molecules known to be changed in the activated form of neutrophils. CD43 expression correlated positively with CD53 and CD44 expression and negatively with CD11b, CD18, CD35, CD67, and CD69 expression. Additionally, increased levels of soluble vascular cell adhesion molecules were detected in these patients, suggesting endothelial cell activation. In conclusion, we believe that the decreased expression of CD43 on the neutrophils of MDS patients is associated with activation of these cells and is probably due to cleavage of the molecule from the cell surface and that the same mechanism is possibly responsible for the parallel down-regulation of CD44 and CD53.

Altered expression of CD43-hexasaccharide isoform on peripheral T lymphocytes from HIV-infected individuals

OBJECTIVE

To examine if peripheral T lymphocytes from HIV-infected individuals show abnormalities in the surface expression of CD43, the major sialoglycoprotein of leukocytes.

DESIGN

A series of 86 HIV-positive individuals was studied. The subjects, grouped by their peripheral CD4 cell count, were in different stages of the disease as defined by the Centers for Disease Control and Prevention (CDC).

METHODS

Peripheral leukocytes and isolated lymphocytes were examined by double and triple immunofluorescence flow cytometric and Western blot analyses with monoclonal antibodies, which discriminate between CD43 isoforms.

RESULTS

We found elevated percentages of the surface expression of CD43-hexasaccharide isoform on T lymphocytes from 82 out of 86 individuals tested. Increasing percentages are progressively found in CDC groups 1, 2 and 3 patients. The expression of the molecule is remarkably biased towards the CD8 cell subpopulation. The percentage of cells bearing human leukocyte antigen-DR locus molecules (HLA-DR) is also augmented. Two subsets expressing T305 have been identified: a minor subset that co-expresses HLA-DR and T305; and a second population formed by the majority of T305-positive cells, which lack surface HLA-DR. Finally, we found CD43 bands with altered electrophoretic mobility in lysates from peripheral lymphocytes from all HIV-positive individuals tested.

CONCLUSION

The augmented expression of CD43-hexasaccharides and the observed cellular distribution suggest an important regulatory role for this molecule in HIV-specific responses.

Identification and cloning of a CD43-associated serine/threonine kinase

CD43 is expressed on most hematopoetic cells and has been shown to regulate the activation and adhesion of T cells. We have cloned a serine/threonine kinase that can interact with the cytoplasmic domain of CD43. This protein is expressed in multiple tissues, including lymphoid cells. Analysis of the subcellular localization reveals it to be present in both the nucleus and the cytoplasm of the cell. The identification of this protein suggests that CD43 may mediate its biologic effects through activation of a kinase cascade, resulting in the regulation of cell growth.

CD43-mediated signals induce DNA binding activity of AP-1, NF-AT, and NFkappa B transcription factors in human T lymphocytes

Although numerous reports document a role for CD43 in T cell signaling, the direct participation of this molecule in cell activation has been questioned. In this study we show that CD43 ligation on human normal peripheral T cells was sufficient to induce interleukin-2, CD69, and CD40-L gene expression, without requiring signals provided by additional receptor molecules. This response was partially inhibited by cyclosporin A and staurosporine, suggesting the participation of both the Ca(2+) and the protein kinase C pathways in CD43 signaling. Consistent with the transient CD43-dependent intracellular Ca(2+) peaks reported by others, signals generated through the CD43 molecule resulted in the induction of NF-AT DNA binding activity. CD43-dependent signals resulted also in AP-1 and NFkappaB activation, probably as a result of protein kinase C involvement. AP-1 complexes bound to the AP-1 sequence contained c-Jun, and those bound to the NF-AT-AP-1 composite site contained c-Jun and Fos. NFkappaB complexes containing p65 could be found as early as 1 h after CD43 cross-linking, suggesting that CD43 participates in early events of T cell activation. The induction of the interleukin-2, CD69, and CD-40L genes and the participation of AP-1, NF-AT, and NFkappaB in the CD43-mediated signaling cascade implicate an important role for this molecule in the regulation of gene expression and cell function.

CD43 gene expression is mediated by a nuclear factor which binds pyrimidine-rich single-stranded DNA

CD43 is a leukocyte-specific surface molecule which plays an important role both in adhesion and signal transduction. We have identified a site spanning nucleotides +18 to +39 within the human CD43 gene promoter which in vitro is hypersensitive to cleavage by nuclease S1. Repeats of this region are sufficient to activate expression of a heterologous promoter in CD43-positive cell lines. Two nuclear factors, PyRo1 and PyRo2, interact with the hypersensitive site. PyRo1 is a single-stranded DNA-binding protein which binds the pyrimidine-rich sense strand. Mutation analysis demonstrates that the motif TCCCCT is critical for PyRo1 interaction. Replacement of this motif with the sequence CATATA abolishes PyRo1 binding and reduces expression of the CD43 promoter by 35% in Jurkat T lymphocytic cells and by 52% in the pre-erythroid/pre-megakaryocytic cell line K562. However, this same replacement failed to affect expression in U937 monocytic cells or in CEM T lymphocytic cells. PyRo1, therefore, exhibits cell-specific differences in its functional activity. Further analysis demonstrated that PyRo1 not only interacts with the CD43 gene promoter but also motifs present within the promoters of the CD11a, CD11b, CD11c and CD11d genes. These genes encode the alpha subunits of the beta2 integrin family of leukocyte adhesion receptors. Deletion of the PyRo1 binding site within the CD11c gene reduced promoter activity in T lymphocytic cells by 47%. However, consistent with our analysis of the CD43 gene, the effect of this same deletion within U937 monocytic cells was less severe. That PyRo1 binds preferentially to single-stranded DNA and sequences within the CD43 and CD11 gene promoters suggests that expression of these genes is influenced by DNA secondary structure.

8B4/20, A Private CD43 Epitope on Developing Human Thymocytes, Is Involved in Thymocyte Maturation

The 8B4/20 Ag is a 120-kDa molecule whose expression on human thymocytes varies according to the differentiation stage: high density on immature CD3−/low thymocytes, reduced density on CD3medium and double-positive thymocytes, and absent on CD3high and single-positive thymocytes and on circulating T cells. In this paper we present immunological and biochemical evidence demonstrating that 8B4/20 Ag is a variant of CD43. We show that 8B4/20-expressing molecules, which are a subset of the CD43 molecules on thymocytes, are heterogeneous in charge, suggesting varying sialylation levels. The 8B4/20 epitope was mapped to the peripherally exposed N-terminal region of CD43, and the 8B4/20 antigenic determinant was characterized by requirement for the sialic acid exocyclic polyhydroxyl side chain, a feature shared with ligands of CD22. Altogether, 8B4/20-CD43 expression pattern and biochemical characteristics suggest its participation in carbohydrate-based interactions in the thymus. We therefore used specific Ab to mimic putative 8B4/20 interactions with natural ligand and examined the effect on isolated thymocytes. Treatment with 8B4/20 had no effect on in vitro apoptosis of isolated thymocytes. In contrast, 8B4/20 ligation enhanced the conversion of isolated thymocytes to differentiated phenotypes. Increased numbers were found in 8B4/20-treated cultures of CD3high and single-positive thymocytes and decreased numbers of CD3−/low and double-positive thymocytes, strongly suggesting that engagement of 8B4/20 delivers a positive signal that favors completion of the thymocyte maturation program. The ability of 8B4/20 mAb to drive thymocyte maturation in vitro suggests that CD43 molecules bearing the 8B4/20 epitope participate in early events of thymic selection.

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.

The CBF.78 monoclonal antibody to human sialophorin has distinct properties giving new insights into the CD43 marker and its activation pathway

We confirm here the CD43 specificity of the CBF.78 monoclonal antibody (mAb) and compare its phenotypic and functional capacities to classical group-A mAbs (DFT1, MEM-59) and to 2 other CD43 mAbs (RDP/AD9, 161-46). It reacts with stable human CD43 transfectants in a sialic acid independent way and blocks completely cell binding of RDP/AD9 or 161-46 more or less but not DFT1 and MEM-59. Its distribution differs from all other CD43. B lymphocytes, but surprisingly the majority of granulocytes or monocytes are CBF.78 negative. CBF.78 is expressed on all T lymphocytes, but the number of CBF.78 molecules/cell is low and equally represented on resting T CD4 and CD8 cells. In comparison to naive T lymphocytes, CD45RO cells increase their CBF.78 epitopes much more than other CD43 epitopes. At a single cell level, confocal microscopy shows that CBF.78 can exist independently of other epitopes. CBF.78 is able to induce homotypic adhesion in different cell lines but not in peripheral blood lymphocytes and is unable to relocalise the targeted molecules. U937 cell line that is not agglutinated by CBF.78 (or RDP/AD9) undergoes a stronger adhesion with PMA, when this reagent is combined with this mAb. By itself CBF.78 is unable to activate T lymphocytes and to costimulate CD3 mAbs but partially blocks PMA. The phosphorylation of the tyrosine kinase p59fyn and p56lck, driven by CBF.78, is weak and almost blocked by PMA. Altogether these data support the hypothesis that there are at least 3 modes of interaction between PKC and CD43 pathways: each pathway is inhibitory towards the other but the CD43 one can also be synergistic.

A Common Signaling Pathway Via Syk and Lyn Tyrosine Kinases Generated From Capping of the Sialomucins CD34 and CD43 in Immature Hematopoietic Cells

The sialomucin CD34 is a useful marker for hematopoietic stem/progenitor cells. However, the role of CD34 remains poorly understood. Here we investigate the functions of CD34 and another sialomucin CD43 coexpressed on hematopoietic stem/progenitor cells. Stimulation of undifferentiated hematopoietic KG1a cells with anti-CD34 or anti-CD43 induced homotypic cytoadhesion, accompanied by formation of a long-lived cap of CD34 and CD43 respectively, which colocalized with F-actin. Stimulation with either antibody specifically increased tyrosine phosphorylation of the identical set of proteins of Lyn, Syk, pp60, pp69, and pp77 at the capping site. These events were similar to those observed in monocytic U937 cells ectopically expressing CD34. After stimulation of KG1a cells, coimmunoprecipitation of Lyn with pp69 and pp77 and of Syk with pp37 was detected in the membrane fraction. Blockade of antibody-induced cap formation by treatment with cytochalasin D leads to inhibition of tyrosine phosphorylation of Syk and pp77 and homotypic cytoadhesion. Moreover, normal human CD34+ bone marrow cells showed cap formation of CD34 or CD43 after stimulation. These results suggest that crosslinking of either CD34 or CD43 activates the same signaling pathway for cytoadhesion through Lyn, Syk, and the novel tyrosine-phosphorylated proteins within hematopoiesis.

A Common Signaling Pathway Via Syk and Lyn Tyrosine Kinases Generated From Capping of the Sialomucins CD34 and CD43 in Immature Hematopoietic Cells

The sialomucin CD34 is a useful marker for hematopoietic stem/progenitor cells. However, the role of CD34 remains poorly understood. Here we investigate the functions of CD34 and another sialomucin CD43 coexpressed on hematopoietic stem/progenitor cells. Stimulation of undifferentiated hematopoietic KG1a cells with anti-CD34 or anti-CD43 induced homotypic cytoadhesion, accompanied by formation of a long-lived cap of CD34 and CD43 respectively, which colocalized with F-actin. Stimulation with either antibody specifically increased tyrosine phosphorylation of the identical set of proteins of Lyn, Syk, pp60, pp69, and pp77 at the capping site. These events were similar to those observed in monocytic U937 cells ectopically expressing CD34. After stimulation of KG1a cells, coimmunoprecipitation of Lyn with pp69 and pp77 and of Syk with pp37 was detected in the membrane fraction. Blockade of antibody-induced cap formation by treatment with cytochalasin D leads to inhibition of tyrosine phosphorylation of Syk and pp77 and homotypic cytoadhesion. Moreover, normal human CD34+ bone marrow cells showed cap formation of CD34 or CD43 after stimulation. These results suggest that crosslinking of either CD34 or CD43 activates the same signaling pathway for cytoadhesion through Lyn, Syk, and the novel tyrosine-phosphorylated proteins within hematopoiesis.

Modulation of Integrin Function in Hematopoietic Progenitor Cells by CD43 Engagement: Possible Involvement of Protein Tyrosine Kinase and Phospholipase C-γ

Attachment of cells to extracellular matrix components is critical for the regulation of hematopoiesis. CD43 is a mucin-like transmembrane sialoglycoprotein expressed on the surface of almost all hematopoietic cells. A highly extended structure of extracellular mucin with negative charge may function as a repulsive barrier to hematopoietic cells. However, some investigators have shown that CD43 has proadhesive properties, and engagement of CD43 has been reported to upregulate integrin-mediated cell adhesion in T cells. We found that cross-linking of CD43 with monoclonal antibodies (MoAbs) enhanced integrin 4β1 (very late antigen [VLA]-4) and 5 β1 (VLA-5)-dependent adhesion of human cord blood CD34+ cells to fibronectin. CD34+ CD38hi, but not CD34+CD38−/low cells responded significantly to the stimulus, suggesting that committed, but not stem and more immature progenitors are sensitive to CD43-mediated activation of integrin. To elucidate the molecular mechanism leading to integrin activation, we used the growth factor-dependent cell line MO7e. Cross-linking of CD43 induced tyrosine phosphorylation of several intracellular molecules including the protein tyrosine kinase Syk, the proto-oncogene product Cbl, and phospholipase C (PLC)-γ2 in MO7e cells. Moreover, protein tyrosine kinase inhibitor herbimycin A and PLC inhibitor U73122 both blocked CD43-induced enhancement of adhesion to fibronectin. These results indicate that signals mediated through CD43 may increase integrin affinity to fibronectin via a pathway dependent on protein tyrosine kinase and PLC-γ activation in hematopoietic progenitors.

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.

An antiserum raised against the recombinant cytoplasmic tail of the human CD43 glycoprotein identifies CD43 in many mammalian species

Leukosialin or CD43 is a heavily O-glycosylated transmembrane protein expressed on all cells of the haematopoietic cell lineage with the exception of red blood cells and mature B cells. This antigen has been identified in human, mouse and rat with monoclonal antibodies. Although orthologues of many human and rodent leucocyte cell surface antigens have been described in recent years, CD43, despite its abundance on human and rodent cells, remained uncharacterized in other vertebrate species. The comparison of CD43 amino acid sequences from human, mouse and rat indicated a high level of homology in the cytoplasmic domain. A serum, (p.aCD43cp) raised against the recombinant cytoplasmic tail of the human CD43, was shown not only to recognize human CD43, but it bound to putative CD43 orthologues in many mammalian species. CD43 was found to be expressed in the same leucocyte subpopulations and circumstantial evidence suggested that CD43 is also regulated similarly during leucocyte ontogeny in all species investigated. As CD43+ cells were readily observed in fixed tissues, the p.aCD43cp serum may be used as a reliable reagent for the verification of the haematopoietic origin of infiltrations and, used together with other reagents, for the serological characterization of normal and pathological lymphoid tissues and lymphoid infiltrations in experimental work and in animal disease.

Waltzing with WASP

Wiskott-Aldrich syndrome (WAS) is an inherited immune deficiency that is marked by eczema, bleeding and recurrent infections. The lymphocytes and platelets of WAS patients display cytoskeletal abnormalities, and their T lymphocytes show a diminished proliferative response to stimulation through the T-cell receptor-CD3 complex (TCR-CD3). The product of the WAS gene, WAS protein (WASP), binds to the small GTPase Cdc42. Small GTPases of the Rho family are crucial for the regulation of the actin-based cytoskeleton. WASP and its relative NWASP might play an important role in regulating the actin cytoskeleton. Since both WASP and NWASP have the potential to bind to multiple proteins, they might serve as a hub to coordinate the redistribution of many cellular signals to the actin cytoskeleton. In this review, the authors discuss the possible role of WASP/NWASP and of the newly described protein WIP, which interacts with WASP and NWASP, in coupling signals from the T-cell receptor to the actin-based cytoskeleton.

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.

Cutting Edge: TCR Signaling Induces Selective Exclusion of CD43 from the T Cell-Antigen-Presenting Cell Contact Site

CD43, a large highly glycosylated molecule, is arguably the most abundant molecule on the surface of T cells. Nevertheless, the function of CD43 remains unclear. Utilizing fluorescence microscopy, we find that CD43 is excluded from the T cell-APC contact site. This exclusion is Ag dependent since optimal CD43 exclusion requires Ag-pulsed APC, and since signaling through CD3, in the absence of any other receptor ligand interactions, can induce the modulation of CD43. These data suggest that CD43 may function as a barrier to nonspecific T cell-APC interactions that is removed as a result of T cell activation. Exclusion from the interaction site is a unique feature of CD43 and not universally found for all large highly glycosylated molecules since CD45 is not excluded. Thus, CD43 may represent a novel regulatory molecule on the T cell surface that can direct T cell interactions by changing its location on the cell surface.

In vivo overexpression of Core2 N-acetylglucosaminyltransferase prevents repopulation of the bone marrow with colony forming cells but fails to affect normal T cell development

UDP-GlcNAc:Galbet1 --> 3GalNAc-R beta1 --> 6N-acetylglucosaminyltransferase (Core2 N-acetyl-glucosaminyltransferase, C2GnT; EC 2.4.1.102) forms beta1 --> 6N-acetyl-glucosaminyl linkages in O-glycoproteins and creates branches for the addition of N-acetyl-lactosamine antennae. Changes in C2GnT activity have been associated with immune disorders, malignancies, and T-cell ontogeny. In this study, we used SCID (severe combined immune deficiency) mice to determine the effects of C2GnT overexpression on hemopoiesis, and in particular, on thymocyte development. BALB/c bone marrow cells transfected with C2GnT using the retroviral murine stem cell vector were used to repopulate SCID mice. Mice were analysed 3 weeks to 3 months after bone marrow transfer. Elevated levels of C2GnT activity in bone marrow, spleen, and thymus from mice repopulated with C2GnT transfected bone marrow cells indicated that C2GnT was overexpressed in recipient mice. In C2GnT repopulated mice, up to 50% of T cells showed an increase in CD43 130-kDa expression, compared with T cells from control animals, indicative of an elevated C2GnT activity in these cells. Furthermore, T-cell subset numbers appeared to be normal, suggesting that C2GnT overexpression did not alter T-cell ontogeny. Interestingly, C2GnT overexpression negatively affected the repopulation of myeloid cells. Only insignificant numbers of interleukin-3/granulocyte-macrophage colony stimulating factor (IL-3/GM-CSF) responsive bone marrow cells were found to be retrovirally transfected in C2GnT repopulated mice, whereas up to 50% of IL-3/GM-CSF responsive bone marrow cells were found to be retrovirally transfected in corresponding controls. These data indicate that in vivo overexpression of C2GnT negatively interferes with the myeloid differentiation pathway but does not affect T-cell development.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

Negative regulation of T cell homing by CD43

We report that the cell surface mucin CD43 acts as an anti-adhesin on T lymphocytes. CD43-deficient murine lymphocytes homed significantly more frequently to secondary lymphoid organs than wild-type cells. Intravital microscopy of peripheral lymph node venules revealed that CD43-deficient lymphocytes were twice as likely to tether, roll, and stick than wild-type cells. This effect was due to CD43 interference with the homing receptor, L-selectin, and was most pronounced in venules with low L-selectin ligand density. In vitro, CD43-deficient cells tethered to L-selectin ligands more efficiently and rolled more slowly than wild-type lymphocytes. Thus, CD43 exerts a negative regulatory effect on T cell trafficking by counterbalancing L-selectin-mediated adhesion.

Signal transduction via CD43 (leukosialin, sialophorin) and associated biological effects in human mast cell line (HMC-1)

CD43 has been shown to be involved in the regulation of cellular adhesion and activation of leukocytes, but its functional significance for mast cell biology has been poorly defined. We demonstrate here that mAb engagement of surface CD43 on human leukemic (HMC-1) mast cells initiates a signaling cascade which involves protein kinase C, while tyrosine kinases appear to play a minor role, as evidenced by effects of different kinase inhibitors on homotypic aggregation induced via CD43. Furthermore, administration of an activating anti-CD43 mAb is shown to induce and promote TNF-alpha- and to enhance IL-8-secretion from HMC-1 cells, but it does not initiate histamine, tryptase, or LTC4 release, suggesting that the intracellular pathways leading to aggregation and release of certain mast cell mediators are differentially regulated. Additionally, engagement of CD43 on HMC-1 cells leads to down-regulation of CD43 surface expression, implying that CD43 may be potentially involved in its own regulation.

Branched O-linked oligosaccharides ectopically expressed in transgenic mice reduce primary T-cell immune responses

Core 2 beta-1,6-N-acetylglucosaminyltransferase, C2GnT, is a key enzyme in O-linked oligosaccharide (O-glycan) biosynthesis and the resultant core 2 branch serves as a backbone for additional glycosylation to form oligosaccharide ligands such as sialyl Le(x). Since the expression of C2GnT is highly regulated during T-cell development and increases in pathological conditions such as the Wiskott-Aldrich syndrome, we have generated transgenic mice overexpressing C2GnT in the T-cell lineage. Surprisingly, T lymphocytes in the transgenic mice develop normally, but they exhibit a reduced immune response when assayed by delayed-type hypersensitivity, proliferation upon stimulation and cytokine production. Moreover, T lymphocytes from the transgenic mice adhere much less efficiently to ICAM-1 and fibronectin than do T lymphocytes from non-transgenic mice. These results indicate that overexpression of the core 2 branched O-glycans in T lymphocytes results in reduced immune responses due to impaired cell-cell interaction. Such an impaired immune response may be one of the causes for immunodeficiency in the Wiskott-Aldrich syndrome.

Anti-CD43 Inhibits Monocyte-Endothelial Adhesion in Inflammation and Atherogenesis

Recruitment of blood monocytes into tissues is a central event in the inflammatory response and in atherogenesis. The mechanisms leading to monocyte adhesion and migration through endothelium are not completely defined. We recently reported that MAb L11, against the leukocyte sialomucin CD43, blocks T-lymphocyte binding to lymph node and Peyer's patch high endothelial venules (HEV) and inhibits T-cell extravasation from the blood into organized secondary lymphoid tissues. We have now assessed the ability of L11 to inhibit monocyte-endothelial (EC) interactions and trafficking. L11 blocks binding of WEHI78/24 cells, a murine monocytoid cell line, to inflamed lymph node HEV and inhibits recruitment of monocytes and neutrophils to thioglycollate-inflamed peritoneum. Because monocyte adhesion to the endothelium and diapedesis in lesion-prone regions of the vasculature is among the earliest events in atherogenesis, leading to formation of lipid-laden foam cells, the ability of L11 to block monocyte recognition of aortic endothelial cells was assessed in a novel ex vivo assay of monocyte binding to intact rabbit aortic endothelium. Cholesterol feeding of rabbits induces enhanced aortic adhesiveness for monocytes and WEHI78/24 monocytoid cells, and this adhesion is inhibited by L11. The inhibitory effect of L11 is additive with that of a cocktail of anti–L-selectin and anti-α4 and β2 integrin monoclonal antibodies. Thus, CD43 represents a novel target for manipulation of monocyte recruitment in inflammation and atherogenesis.

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.

Leukosialin (CD43)-major histocompatibility class I molecule interactions involved in spontaneous T cell conjugate formation

Resting T cells spontaneously adhere in a selective manner to potent accessory cells, such as dendritic cells (DC) and lymphoblastoid B blasts (LCL). Here we demonstrate that leukosialin (CD43) and major histocompatibility complex class I molecules (MHC-I) might play a critical role in this process. T cell conjugate formation with monocyte-derived DC (md-DC) and LCL could be strongly inhibited by either preincubating T cells with Fab fragments of CD43 monoclonal antibody (mAb) 6F5 or by preincubating md-DC or LCL with MHC-I mAb W6/32. Intact CD43 mAb 6F5, in contrast to monovalent Fab fragments, enhanced T cell adhesiveness by transactivating CD2 binding to CD58 molecules. Interestingly, induction of this proadhesive signal via CD43 with intact 6F5 mAb was found to revert mAb W6/32-mediated inhibition of T cell conjugate formation. These observations indicated that CD43 cross-linkage mimics and monovalent mAb 6F5 inhibits interaction of T cell CD43 with a stimulatory ligand on opposing cells, presumably MHC-I. For the demonstration of direct physical interaction between CD43 on T cells and MHC-I-coated beads it was necessary, however, to ligate CD2 on T cells with a stimulatory pair of CD2 mAbs (VIT13 plus TS2/18). This suggests that CD2 ligation crosswise upregulates CD43 binding avidity for MHC-I and that both adhesion molecule pairs (CD43/MHC-I and CD2/CD58) act in concert to induce and mediate T cell conjugate formation with certain cell types.

CD43 expression in a B cell lymphoma, WEHI 231, reduces susceptibility to G1 arrest and extends survival in culture upon serum depletion

CD43 is a major surface sialoprotein on hemopoietic cells, whose extracellular domain is heavily O-glycosylated. The functional role of CD43 in the hemopoietic system is not fully understood; however, it has been suggested that CD43 may have a role in cell-cell repulsion and in modifying T cell proliferation and activation. CD43 is expressed in immature B cells in the bone marrow, but not by peripheral B cells, except for B-1 B cells and plasma cells. To analyze the biological effect of CD43 in B-lineage cells, we transfected mouse CD43 cDNA into a CD43- B cell lymphoma, WEHI 231, and the growth and survival in culture were compared to those of a parental cell line, human CD8 transfectants, and CD43- revertants established from CD43+ clones. We observed that CD43 expression supported cell growth in culture upon serum reduction, whereas growth of CD43- cell lines was barely detected under this condition. CD43- cell lines accumulated in G1 phase of the cell cycle, and the numbers of viable cells were greatly reduced during culture upon serum depletion, whereas expression of CD43 reduced the susceptibility to G1 arrest and temporarily retarded the apoptotic process, which, in turn, resulted in an increase and maintenance of the number of viable cells in culture. The results suggest that CD43 may have some role in the survival and expansion of B-lineage cells. The biological effect of CD43 was initiated without stimulation by cross-linking and was significantly impaired by replacement of the extracellular domain by the human CD8 extracellular domain. The basis of these regulatory processes is discussed.

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.

Parallel pattern of expression of CD43 and of LFA-1 on the CD45RA+ (naive) and CD45RO+ (memory) subsets of human CD4+ and CD8+ cells. Correlation with the aggregative response of the cells to CD43 monoclonal antibodies

Human peripheral blood T cells form large aggregates when cultured in the presence of antibodies to the highly sialylated protein CD43. About 25% of the cells in such cultures do not aggregate, however, although virtually all T cells express CD43. To find out if these cells constitute a distinct subpopulation of T cells, we analysed the expression of CD43 and lymphocyte function-associated antigen-1 (LFA-1) and examined the aggregation induced by CD43 monoclonal antibodies (mAb) in CD4+ and CD8+ cells and in their CD45RA+ (naive) and CD45RO+ (memory) subsets, respectively. We found that CD43-stimulated CD8+ cells aggregated more rapidly and formed larger aggregates than CD4+ cells. Furthermore, whereas CD8+CD45RO+ cells formed compact clusters after some hours of incubation, a majority (about 75%) of the CD4+CD45RA+ cells remained as singles even after overnight culture. Flow cytometry analysis showed that the patterns of expression of CD43 and of LFA-1 on the different subsets were strikingly parallel to each other. Thus, CD4+ and CD8+ memory (CD45RO+) T cells expressed higher levels of CD43 than the corresponding naive cells, suggesting that increased levels of CD43 expression are, like LFA-1 expression, a marker of primed or recently activated cells. Immunoprecipitation and sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of 125I-labelled subsets showed that CD8+ cells expressed about twice as much CD43 as CD4+ cells. In a particular donor where the mean size of the cells in the four subsets was close to equal, CD4+ memory cells showed a 1.4-fold and CD8+ memory cells a twofold increase in CD43 compared to their corresponding naive populations. The propensity of memory T cells to extravasate may be facilitated by high expression of CD43.

Negative regulation of T-cell adhesion and activation by CD43

CD43 is a cell-surface sialoglycoprotein expressed by a variety of haematopoietically derived cells, including T lymphocytes. Earlier observations of defective CD43 expression by T lymphocytes from boys with the X-chromosome-linked Wiskott-Aldrich syndrome suggested the importance of CD43 in lymphocyte function. Subsequent studies have suggested that CD43 facilitates leukocyte adhesion and has a co-stimulatory role during T-cell activation. To define the physiologically relevant function(s) of CD43, we have generated CD43-knockout mice. We report here that CD43-deficient T cells from such mice show a marked increase in their in vitro proliferative response to concanavalin A, anti-CD3, the superantigen SEB and allostimulation. Additionally, CD43-deficient T cells show a substantial enhancement in homotypic adhesion and in their ability to bind different ligands, including fibronectin and the intercellular adhesion molecule ICAM-1. Vaccinia-virus-infected CD43-knockout mice mounted an augmented anti-vaccinia cytotoxic T-cell response compared with their wild-type littermates, yet developed an increased virus load. We conclude that CD43 negatively regulates T-cell activation and adhesion and is important for viral clearance.

CD43 and CD5 antibodies define four normal and neoplastic B-cell subsets: a three-color flow cytometric study

CD43 antibodies are best known for identifying normal and neoplastic T cells and a subset of neoplastic B cells in paraffin sections. The frequency of nonneoplastic CD43 + B cells in different reactive settings, the proportion of B-cell neoplasms with small CD43+ populations, and the relationship of CD43+ B cells was therefore studied in 138 specimens using three-color cytometric analysis. CD43 and CD5 defined four normal B-cell subsets (CD43 + CD5 +, CD43 + CD5-, CD43 - CD5+, and CD43-CD5-). A significantly greater proportion of CD43+ B cells was found in cord blood and putative HIV+ blood samples than in normal control bloods. B-cell neoplasms derived from each of these four B-cell subsets were identified, with CD43+ /CD5+ and CD43-/CD5- neoplasms being most common. The predominant B-cell population coexpressed CD43 alone in 2/39 neoplasms and CD5 alone in four. A minority of cases showed heterogeneous CD43 expression. The B cells in two of three posttransplant lymphoproliferative disorders coexpressed CD43. B cells showed weaker CD43 staining than did T-cells (relative fluorescence 0.38 +/- 0.29). These findings support the concept that CD43 expression by neoplastic B cells is not an aberrant finding. CD43 expression on normal and neoplastic B cells is independent of CD5 expression even though CD43 and CD5 are frequently coexpressed. CD43 expression by B cells sometimes might be underestimated in paraffin sections because it is much weaker than on T cells.(ABSTRACT TRUNCATED AT 250 WORDS)

CD43 is a murine T cell costimulatory receptor that functions independently of CD28

Costimulation mediated by the CD28 receptor has been shown to play an important role in the development of a vigorous T cell immune response. Nevertheless, CD28-deficient mice can mount effective T cell-dependent immune responses. These data suggest that other costimulatory molecules may play a role in T cell activation. In a search for other costimulatory receptors on T cells, we have characterized a monoclonal antibody (mAb) that can costimulate T cells in the absence of accessory cells. Similar to CD28 antibodies, this mAb, R2/60, was found to synergize with T cell receptor engagement in inducing proliferation. Independent ligation of CD3 and the ligand recognized by R2/60 results in T cell proliferation, suggesting that the two molecules do not have to colocalize to activate the R2/60 costimulatory pathway. R2/60 does not react with CD28, and furthermore, R2/60 costimulates in a CD28-independent fashion since the mAb costimulates T cells from the CD28-deficient mice as well as wild-type mice. Expression cloning of the R2/60 antigen identified the ligand as murine CD43. Together, these data demonstrate that CD43 can serve as a receptor on T cells that can provide CD28-independent costimulation.

Tissue-specific transcriptional regulation of human leukosialin (CD43) gene is achieved by DNA methylation

The expression of human leukosialin (CD43), a major sialoglycoprotein on the surface of hematopoietic cells, is regulated in cell lineage-specific as well as differentiation stage-specific manners. We have shown previously that transcription from the TATA-less promoter is mediated by the transcription factor Sp1, which binds to repeats of a GGGTGG motif in the 5'-flanking sequence. This regulatory region is ubiquitously functional in mammalian cells, providing a high transcriptional potential. No cis-acting element responsible for the specificity of this gene expression was revealed by extensive studies using transient as well as stable expression systems. Here, we demonstrate that DNA methylation plays a key role in leukosialin gene expression. Southern blot analysis of genomic DNAs from various human cell lines with methylation-sensitive and -insensitive restriction enzymes showed a tight correlation between gene activity and demethylation state of the 5'-region of the leukosialin gene. Consistent results were obtained from the same analysis of genomic DNAs from various human tissues. In addition, in vitro DNA methylation of the 5'-region drastically reduced transcriptional activity in a transient expression system. These results indicate that DNA methylation around the 5'-region of the leukosialin gene is required to shut off a high level of transcription. Thus, the tissue-specific expression of the leukosialin gene is constitutively achieved by alteration of DNA methylation.

Expression of the T antigen on a T-lymphoid cell line, supT1

We have measured glycosyltransferase activities of SupT1 cells, a T-lymphoid cell line shown to react with autoantibodies in the sera of many HIV patients. Since considerable alpha-N-acetylgalactosaminyl-transferase and beta 1, 3 galactosyltransferase activities were found in SupT1 cells, at least the O-glycan core 1 structure can probably be synthesized. FACS analysis using an anti-T monoclonal antibody showed expression of the T antigen (Gal beta 1-3 GalNAc). Glycoproteins with the T antigen were isolated by immunoprecipitation with the anti-T antibody from a SupT1 cell lysate labelled metabolically with 3H-glucosamine and then analysed by SDS-PAGE. It was revealed that the precipitate contained a glycoprotein with a molecular weight corresponding to that of leukosialin. O-glycans were prepared from the immunoprecipitate by alkaline-borohydride treatment and then fractionated on Bio-Gel P-2, GalNAcOH and Gal-GalNAcOH being identified inter alia. These results suggest that an anti-T antibody may be included in the autoantibodies found in HIV-1 infected individuals.

MEM-59 monoclonal antibody detects a CD43 epitope involved in lymphocyte activation

Previous studies on T cell activation via CD43 antigen stimulation were limited to the use of L10, a monoclonal antibody (mAb) recognizing a sialic acid-independent epitope on the CD43 molecule. Here we study the CD43 mAb MEM-59, which recognizes a neuraminidase-sensitive epitope on the CD43 molecule, for its ability to activate T lymphocytes. The antibody by itself is able to stimulate proliferation of peripheral blood mononuclear cells (PBMC) in a monocyte-dependent fashion, and to act synergistically with the mitogen phorbol 12-myristate 13-acetate. It is demonstrated that the monocyte dependence of MEM-59-induced proliferation of peripheral blood lymphocytes (PBL) cannot be attributed to cross-linking via Fc receptors on monocytes alone: F(ab')2 fragments of MEM-59 are at least as effective as intact IgG in the induction of PBMC proliferation. The effects of MEM-59 reported here are distinct in important ways from those reported for L10. Our proliferation data are extended by the observation that MEM-59 mAb induces mobilization of intracellular Ca2+ in PBMC and in the T cell line Jurkat, while the CD3/TcR-negative Jurkat derived-mutant J.TR3-T3.5 exhibits defective signaling compared to the parent cell line. Moreover, CD3 and CD43 are shown to be present jointly in a large complex in a mild detergent lysate of the T cell line HPB-ALL. These data indicate a physical and functional association between CD3/TcR and CD43 pathways, suggesting a role for CD43 as a co-stimulatory molecule in CD3/TcR signaling, especially in T cell-antigen-presenting cell interactions.

Disregulation of leukosialin (CD43, Ly48, sialophorin) expression in the B-cell lineage of transgenic mice increases splenic B-cell number and survival

Leukosialin (also known as Ly48, CD43, and sialophorin) is a major cell surface sialoglycoprotein found on a variety of hematopoietically derived cells. The precise function of this molecule is poorly understood but it has been implicated in cell proliferation and intercellular adhesion. We developed a transgenic mouse model to assess leukosialin's function in vivo. Our approach was to alter mouse CD43 (mCD43) expression in the B-cell lineage where it is tightly regulated, by expressing it in peripheral B cells where it is normally absent. To drive expression of leukosialin in mature B cells, the immunoglobulin heavy chain enhancer was fused to the mCD43 gene. mCD43-immunoglobulin heavy chain enhancer transgenic mice display splenomegaly due to increased numbers of B cells. Transgenic B cells show a striking increase in their ability to survive in vitro compared to B cells from nontransgenic control mice. This prolonged survival is reflected in a decreased susceptibility to apoptosis. These observations suggest that mCD43 plays an important role in the regulation of B-cell survival. The alteration of the temporal expression, or "disregulation," of a gene in transgenic mice provides a general strategy for elucidating the in vivo role of other molecules involved in cell signaling and adhesion.