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The small cell lung cancer antigen cluster-4 and the leukocyte antigen CD24 are allelic isoforms of the same gene (CD24) on chromosome band 6q21.

Cluster-4 and CD24 cDNA's have recently been cloned from the small cell lung carcinoma (SCLC) cell line SW2 and from the erythroleukemia cell line K562, respectively. The only difference in the coding sequence, between cluster-4 and CD24 antigens is the substitution of a single base pair leading to a substitution of Val by Ala near the putative glycosylphosphatidylinositol (GPI) anchorage sites of the mature protein. Here we demonstrate that the nucleotide substitution which distinguishes the cluster-4 and CD24 antigen genes is due to an allelic polymorphism on chromosome band 6q21. In addition, we identified by Southern blotting and PCR of DNA from somatic human x hamster hybrid cell lines homologues of cluster-4/CD24 on the Y chromosome and chromosome 15. We suggest, however, that the gene on 6q21 is the active locus since the mRNA of cell lines always represents the allelic variants found on chromosome 6. The distribution pattern of this allelic polymorphism in SCLC cell lines and leukocytes of healthy donors did not reveal any obvious relationship with disease. However, it is noteworthy that homozygosity for cluster-4 was found in only one case whereas heterozygosity and homozygosity for CD24 both contribute up to 50% of the samples examined.

Alleles↗

Heat-stable antigen (CD24) as ligand for mouse P-selectin.

Heat-stable antigen (HSA)/CD24 is a cell surface molecule expressed by many cell types in the mouse. The molecule has an unusual structure because of its small protein core and extensive glycosylation. In order to study the functional role of the HSA-associated glycoconjugates we have isolated different forms of HSA. Using lectin analysis we provide evidence for extensive heterogeneity in carbohydrate composition and sialic acid linkage. Several HSA forms were recognized by mouse P-selectin-IgG but not E-selectin-IgG in ELISA. As expected, P-selectin-IgG also bound to L2/HNK-1-positive neural glycoproteins (L2-glycoproteins) and sulfatides but not to gangliosides and other control glycoproteins. The binding of P-selectin-IgG to L2-glycoproteins and HSA required bivalent cations. The reactivity to HSA was sensitive to sialidase treatment whereas the binding to L2-glycoproteins was not. Studies with alpha 2-6 sialytransferase indicated that alpha 2-6 linked sialic acid was not involved in the P-selectin binding to HSA. Surprisingly, an L2/HNK-1 specific antibody was found to cross-react with some HSA glycoforms and its binding correlated with P-selectin-IgG reactivity. L2/HNK-1-positive or L2/HNK-1-negative HSA glycoforms were also analyzed after coating to polystyrene beads. Only the L2/HNK-1-positive HSA coated beads were reactive with P-selectin-IgG and could bind to activated bend3 endothelioma cells expressing P-selectin whereas the L2/HNK-1-negative HSA beads did not. It is suggested that in its L2/HNK-1 modified form the HSA molecule on leukocytes could represent a ligand for P-selectin on endothelial cells or platelets.

Animals↗

Lack of CD24 antigen expression in B-lineage acute lymphoblastic leukemia is associated with intrinsic radiation resistance of primary clonogenic blasts.

The radiation sensitivity of primary clonogenic blasts from 27 children with immunologically classified CD2-CD5-CD7-CD19+slg- B-lineage acute lymphoblastic leukemia (ALL) was analyzed using leukemic progenitor cell (LPC) colony assays. Radiation survival curves of primary clonogenic blasts (ie, LPC) were constructed for each patient using computer programs for the single-hit multitarget as well as the linear quadratic models of cell survival. The D0 values ranged from 49 to 891 cGy (median, 239 cGy; mean +/- SE, 307 +/- 44 cGy) and the alpha values ranged from 0.000 to 2.047 Gy-1 (median, 0.156 Gy-1; mean +/- SE, 0.284 +/- 0.078 Gy-1). Patients were divided into groups according to sex, age, white blood cell count (WBC) at diagnosis, plating efficiency of primary bone marrow blasts, and immunophenotype. Patient sex, age, WBC at diagnosis, or in vitro plating efficiency was not associated with radiation resistance. Notably, freshly isolated primary clonogenic blasts from patients with a CD19+CD24-CD34+ composite immunophenotype (stage I B-cell precursor [BCP]) had 2.2-fold higher D0 values (P = .005) and 3.4-fold lower alpha values (P = .05) than those from patients with a CD19+CD24+CD34+ (stage II BCP) or CD19+CD24+CD34- (stage III BCP) immunophenotype. The relative values for CD24- primary clonogenic blasts signify greater intrinsic radiation resistance. Furthermore, the CD19+CD24-CD34+ immunophenotype had a larger radiation resistant fraction. Whereas only 60% of CD19+CD24+CD34+ and 33% of CD19+CD24+CD34- cases had clonogenic blasts with alpha < or = 0.2, 100% of CD19+CD24-CD34+ cases had clonogenic blasts with alpha < or = 0.2. Furthermore, clonogenic blasts from established CD19+CD24- B-lineage ALL cell lines were significantly more radiation resistant than CD19+CD24+ B-lineage ALL cell lines. Notably, radiation doses sufficient to induce apoptosis of CD24- B-lineage ALL cells were higher than those capable of inducing apoptosis in CD24+ B-lineage ALL cells. Thus, the lack of CD24 surface antigen expression in B-lineage ALL is associated with intrinsic radiation resistance at the level of primary clonogenic blasts.

Antigens, CD↗

The heat stable antigen (CD24) is not required for the generation of CD4+ effector and memory T cells by dendritic cells in vivo.

Previous work has established that CD24 is a costimulatory molecule for T-cell clonal expansion. Studies using CD24 -/- mice demonstrated that CD24 plays a critical role in the CD28-independent immune response against virus and soluble antigens. The role of CD24 on dendritic cells (DCs) has not been reported. Here, we compare the CD24(+/+) and CD24(-/-) DCs in the induction of initial clonal expansion and elicitation of memory CD4(+) T cells in vivo. Our results demonstrate that the CD24 expressed on DCs is essential neither for the induction of initial T-cell clonal expansion nor for elicitation of memory activity of primed T cells in vivo.

Animals↗

Monoclonal antibodies VIB-E3, IB5 and HB9 to the leucocyte/epithelial antigen CD24 resemble BA-1 in recognizing sialic acid-dependent epitope(s). Evidence that VIB-E3 recognizes NeuAc alpha 2-6GalNAc and NeuAc alpha 2-6Gal sequences.

The specificities of seven monoclonal antibodies to the human B cell differentiation marker CD24 have been investigated with respect to sialic acid containing carbohydrates. These are antibodies HB8, HB9, VIB-C5, VIB-E3, AL1a, LC66 and IB5, which are known to bind to polydisperse sialoglycoprotein(s) on Nalm-6 B lymphoblastoid cells. Three of the antibodies, HB9, VIB-E3 and IB5, have been found to resemble the first described antibody in this series, BA-1, in binding also to bovine submaxillary mucin. As with BA-1, the binding of the antibodies is abolished or reduced markedly after desialylation of the epithelial glycoprotein, and the binding to neuraminidase-treated Nalm-6 cells is also reduced. There is evidence for the involvement of of non-O-acetylated sialic acid in the determinants recognized by these antibodies, since there is a substantially enhanced binding following mild-alkali treatment of the epithelial mucin which removes O-acetyl groups. One of the antibodies, VIB-E3, is deduced to recognize the oligosaccharide sequences NeuAc alpha 2-6GalNAc and NeuAc alpha 2-6Gal as part of larger antigenic structures. This conclusion has been reached from the results of inhibition-of-binding experiments using a series of structurally defined sialo-oligosaccharides and direct binding experiments using oligosaccharides chemically linked to lipid (neoglycolipids).

Animals↗

The human B cell-associated antigen CD24 is a single chain sialoglycoprotein.

The CD24 human B cell-associated antigen was originally characterized in this laboratory with the use of monoclonal antibody BA-1 combined with a standard radioimmuneprecipitation technique, and was reported to be a three chain glycoprotein complex of 45, 55, and 65 kilodaltons. We have compared our standard radioimmuneprecipitation technique (BA-1 ascites followed by rabbit anti-mouse IgM-coated protein A-Sepharose) to BA-1 conjugated directly to CNBr-Sepharose (BA-1-Sepharose) and report striking differences in the electrophoretic profile of CD24 immuneprecipitates. The CD24 immuneprecipitate obtained with BA-1-Sepharose showed a single broad band with a relative mobility of 42 kilodaltons. A series of experiments performed with the two immuneprecipitation techniques, reducing or nonreducing electrophoretic conditions, and addition of preformed mock BA-1 immuneprecipitate to BA-1-Sepharose immuneprecipitates convincingly demonstrated that the previously described 55 and 65 kilodalton components were artifacts caused by co-migration of CD24 with IgG and IgM heavy chains, respectively. Because of the consistent association and co-migration of CD24 with IgG heavy chains, we investigated the possibility that CD24 might be related to the 45 kilodalton Fc-gamma receptor expressed on B cells and eosinophils. We found, however, no evidence for such a relationship by cross adsorption analysis with BA-1-Sepharose and IgG-Sepharose.

Antigens, Differentiation, B-Lymphocyte↗

Antibodies to the protein core of the small cell lung cancer workshop antigen cluster-w4 and to the leucocyte workshop antigen CD24 recognize the same short protein sequence leucine-alanine-proline.

We recently described the identity of the small cell lung cancer (SCLC) cluster-w4 antigen and the human B cell differentiation marker CD24, a glycosylphosphatidylinositol (GPI)-anchored, highly glycosylated surface molecule of only 31-35 amino acids [15]. The specificities of three anti-cluster-w4 and of eleven anti-CD24 MoAbs have been investigated with respect to their binding capacity to the protein core of cluster-w4/CD24 antigen. Four overlapping peptides spanning this protein core were synthesized. MoAbs shown to bind to two overlapping peptides by antibody binding inhibition using the cluster-w4/CD24-positive SCLC cell line SW2 and by direct peptide binding detected in an ELISA were investigated in more detail. To determine the exact epitopes recognized by these MoAbs, an epitope mapping assay using peptides synthesized onto polyethylene pins was established. The three anti-cluster-w4 MoAbs SWA11, SWA21 and SWA22 and the anti-CD24 MoAbs OKB2 and ALB9 recognized the same short leucine-alanine-proline (LAP) sequence in an area without potential glycosylation sites close to the GPI anchor of the protein core of the cluster-w4/CD24 antigen.

Amino Acid Sequence↗

The CD24 antigen discriminates between pre-B and B cells in human bone marrow.

When bone marrow (BM) lymphoid cells from 12 adult healthy donors were labeled by CD24 antibodies and analyzed by flow cytometry, two positive populations of cells were demonstrated in each sample (by a separated bimodal specific immunofluorescence). One population had intermediate CD24-Ag density (termed CD24+ cells) whereas the other had high CD24-Ag density (termed CD24(2+) cells). CD24+ cells represented 5.8 +/- 2.7% of the total lymphoid BM cells and CD24(2+) cells 5.6 +/- 2.5%. Using dual fluorescence analysis on eight samples, all CD24+ cells expressed the CD21 and CD37 mature B cell Ag and also surface IgM (sIgM), but this population lacked CD10 Ag. These cells also expressed CD19 Ag, and at a higher density than CD24(2+) cells. They were also positive for HLA-DR Ag. Conversely, CD24(2+) cells were shown to be early cells of the B cell lineage. While all the CD24(2+) cells were HLA-DR+ and CD19+, 64 +/- 16% of them expressed CD20 Ag (at a lower density than CD24+ cells), 65 +/- 21% CD10 Ag, and 22 +/- 8% were positive for cytoplasmic mu-chains (c mu). None of these cells expressed the CD21 and CD37 mature B cell Ag or sIgM. Additional experiments on four different healthy donors demonstrated that 30 +/- 9% of the CD24(2+) cells expressed the CD34 Ag and that the CD24+ cells did not express it. Thus, the CD24 Ag permits discrimination between two populations of the B cell lineage present in adult BM: 1) A CD24(2+) cell population including "pre" pre-B cells (HLA-DR+, CD19+, CD10+/-, CD20-, CD21-, CD34+, CD37-, c mu-), "intermediate" pre-B cells (HLA-DR+, CD19+, CD10+, CD20+, CD21-, CD34-, CD37-, c mu-), and "true" pre-B cells (HLA-DR+, CD19+, CD10+, CD20+, CD21-, CD34-, CD37-, c mu+). 2) A CD24+ cell population including B cells of the standard phenotype (HLA-DR+, CD19+, CD10-, CD20+, CD21+, CD34-, CD37+, c mu-, sIgM+).

Adult↗

B-cell maturation in chimaeric mice deficient for the heat stable antigen (HSA/mouse CD24).

The murine differentiation marker heat stable antigen (HSA) is a GPI-anchored surface glycoprotein showing strong expression on immature B- and T-lymphocytes and gradually reduced expression during maturation. Although HSA has been suggested to be involved in adhesion and/or signalling, its function has not been clearly demonstrated so far. In order to elucidate the function of HSA, we analysed chimaeric mice that were generated by targeted disruption of both HSA alleles in ES cells. These mice contain normal numbers of peripheral B-cells and normal serum IgM and IgG titres of ES cell-derived allotype, demonstrating that HSA expression on B-cells is not an absolute requirement for their maturation. However, a reduction in immature B-cells in the bone marrow and an altered degree of bone marrow and blood chimaerism suggest that HSA expression influences the maturation of B-cells.

Alleles↗

Heat-stable antigen (mouse CD24) in the brain: dual but distinct interaction with P-selectin and L1.

Heat-stable antigen (HSA/mouse CD24) is expressed in both haematopoietic and neural cells. The small core protein of the molecule is extensively glycosylated and anchored to the membrane via glycosylphosphatidylinositol. The role of HSA in the developing brain as well as its functional properties are poorly understood. Here we show that the brain HSA is associated with N- and O-linked oligosaccharide moieties and decorated with the HNK-1 sulfated carbohydrate epitope. It can bind P-selectin but not E-selectin and this interaction requires divalent cations and is sensitive to high salt. Brain derived HSA is also capable of binding to the L1 adhesion molecule. This interaction is distinct from the P-selectin binding as it is resistant to high salt and does not require bivalent cations. Treatment of HSA with OSGE significantly reduced binding of both P-selectin and I.1. Our data suggest that HSA can bind P-selectin and I.1 by distinct mechanism and that the binding epitopes on HSA are in close proximity.

Animals↗

Gene expression of CD24 core polypeptide molecule in normal rat tissues and human tumor cell lines.

CD24 antigen is a glycoprotein expressed on haematopoietic cells, including B cells, T cells and granulocytes and on non-haematopoietic cells, including neural cells, ganglion cells and the cells of the adrenal medulla. The antigen is also expressed on renal cell carcinoma, small cell lung carcinoma and neuroblastoma. We have cloned rat cDNA encoding core polypeptide of CD24 antigen from embryonic brain and shown that the core molecule is highly expressed in embryonic brain and non-neural tissues. Rat tissue and various human neoplastic cell lines were investigated for the gene expression of CD24 core polypeptide by in situ hybridization. The transcript was localized in gastrointestinal epithelia, ductal and acinar epithelia of the salivary gland, the bronchiolar epithelium, renal tubular epithelium, the epithelium of the oviduct, follicular cells of the thyroid, medullary cells of the adrenal gland, Auerbach's plexus, B blastoid cells in lymph nodes, hair follicles, and the sweat glands of the skin. Among the various human neoplastic cell lines investigated, the transcript was detected in squamous cell carcinoma of the lung, gastric carcinoma, colon carcinoma, choriocarcinoma and renal cell carcinoma. The result suggest that the core molecule of CD24 antigen may be expressed in a wider range of epithelial cells and carcinoma cell lines than has been reported. Furthermore, we show that gene expression of CD24 core polypeptide is confined to the proliferative zone of the gastrointestinal mucosa, suggesting that core molecule is transiently expressed on the surface of epithelial cells in the process of cellular maturation. We discuss a possible role for CD24 antigen in the maturation of epithelial cells in the gastrointestinal tract.

Animals↗

The heat stable antigen (mouse CD24) gene is differentially regulated but has a housekeeping promoter.

Expression of the mouse heat stable antigen (HSA or mouse CD24) shows tissue-specific as well as developmental regulation. During the maturation of several hematopoietic lineages, HSA expression is generally high in immature precursor cells and low or absent in terminally differentiated cells. We present evidence suggesting that this regulation of the HSA gene (Cd24a) occurs at the transcriptional level. In addition, sequence and methylation analysis of the Cd24a promoter revealed characteristics of both "housekeeping" and tissue-specific promoters, including a methylation-free, HpaII tiny fragment (HTF) island, multiple putative SP1 and AP-2 consensus binding sites, and a TATA box. Functional analysis of a 0.6-kilobase DNA fragment containing these elements fused to the CAT reporter gene in transient transfection experiments showed activity in both HSA expressing and non-expressing cell lines with a strength similar to that of the herpes-simplex virus-thymidine kinase promoter. Large fragments from the flanking region of the Cd24a promoter did not influence the ubiquitous nature of this promoter. Finally, we mapped the Cd24a, Cd24b, and Cd24c genes to mouse chromosomes 10, 8, and 14 respectively.

Animals↗

Heat stable antigen (mouse CD24) supports myeloid cell binding to endothelial and platelet P-selectin.

P-selectin is a Ca(2+)-dependent lectin that participates in leukocyte adhesion to vascular endothelium and platelets. Myeloid cells and a subset of T lymphocytes express carbohydrate ligands at the cell surface. Previously, we suggested that heat stable antigen (HSA/mouse CD24), an extensively glycosylated cell surface molecule on many mouse cells, is a ligand for P-selectin. Here we show that HSA mediates the binding of monocytic cells and neutrophils to P-selectin. The monocytic cell lines ESb-MP and J774, peritoneal exudate cells, and bone marrow neutrophils could bind to lipopolysaccharide-activated bend3 endothelioma cells under rotation-induced shear forces and this binding was inhibited by mAb to P-selectin and HSA. Blocking was weak at room temperature but more efficient at 4 degrees C when integrin-mediated binding was decreased. Also the adhesion of neutrophils to stimulated platelets expressing P-selectin was blocked by HSA- and P-selectin-specific mAb. Latex beads coated with purified HSA from myeloid cells bound to activated endothelioma cells or platelets, and the binding was similarly blocked by mAb to P-selectin and HSA respectively. The HSA-coated beads were stained with P-selectin-IgG, very weakly with L-selectin-IgG but not with E-selectin-IgG. The staining was dependent on divalent cations and treatment with endoglycosidase F or neuraminidase indicated that sialylated N-linked glycans were recognized. The presence of these glycans was confirmed by biosynthetic labeling studies. Our data suggest that HSA, in addition to the recently identified 160 kDa glycoprotein ligand on mouse neutrophils, belongs to a group of monospecific P-selectin ligands on myeloid cells.

Animals↗

Selection of retrovirally transduced hematopoietic cells using CD24 as a marker of gene transfer.

We have investigated the use of a cell surface antigen as a dominant selectable marker to facilitate the detection and selection of retrovirally infected target cells. The small coding region of the human cell surface antigen CD24 (approximately 240 bp) was introduced into a myeloproliferative sarcoma virus (MPSV)-based retroviral vector, which was then used to infect day 4 5-fluorouracil (5-FU)-treated murine bone marrow cells. Within 48 hours of termination of the infection procedure CD24-expressing cells were selected by fluorescent-activated cell sorting (FACS) with an antibody directed against the CD24 antigen. Functional analysis of these cells showed that they included not only in vitro clonogenic progenitors and day 12 colony-forming unit-spleen but also cells capable of competitive long-term hematopoietic repopulation. Double-antibody labeling studies performed on recipients of retrovirally transduced marrow cells showed that some granulocytes, macrophages, erythrocytes, and, to a lesser extent, B and T lymphocytes still expressed the transduced CD24 gene at high levels 4 months later. No gross abnormalities in hematopoiesis were detected in mice repopulated with CD24-expressing cells. Our results show that the use of the CD24 cell surface antigen as a retrovirally encoded marker enables the rapid, efficient, and nontoxic selection in vitro of infected primary cells, facilitates tracking and phenotyping of their progeny, and should provide a unique tool to identify elements that regulate the expression of transduced genes in the most primitive hematopoietic cells.

Animals↗

Selection of transduced CD34+ progenitors and enzymatic correction of cells from Gaucher patients, with bicistronic vectors.

The gene transfer efficiency of human hematopoietic stem cells is still inadequate for efficient gene therapy of most disorders. To overcome this problem, a selectable retroviral vector system for gene therapy has been developed for gene therapy of Gaucher disease. We constructed a bicistronic retroviral vector containing the human glucocerebrosidase (GC) cDNA and the human small cell surface antigen CD24 (243 bp). Expression of both cDNAs was controlled by the long terminal repeat enhancer/promoter of the Molony murine leukemia virus. The CD24 selectable marker was placed downstream of the GC cDNA and its translation was enhanced by inclusion of the long 5' untranslated region of encephalomyocarditis virus internal ribosomal entry site. Virus-producing GP+envAM12 cells were created by multiple supernatant transductions to create vector producer cells. The vector LGEC has a high titer and can drive expression of GC and the cell surface antigen CD24 simultaneously in transduced NIH 3T3 cells and Gaucher skin fibroblasts. These transduced cells have been successfully separated from untransduced cells by fluorescence-activated cell sorting, based on cell surface expression of CD24. Transduced and sorted NIH 3T3 cells showed higher GC enzyme activity than the unsorted population, demonstrating coordinated expression of both genes. Fibroblasts from Gaucher patients were transduced and sorted for CD24 expression, and GC enzyme activity was measured. The transduced sorted Gaucher fibroblasts had a marked increase in enzyme activity (149%) compared with virgin Gaucher fibroblasts (17% of normal GC enzyme activity). Efficient transduction of CD34+ hematopoietic progenitors (20-40%) was accomplished and fluorescence-activated cell sorted CD24(+)-expressing progenitors generated colonies, all of which (100%) were vector positive. The sorted, CD24-expressing progenitors generated erythroid burst-forming units, colony-forming units (CFU)-granulocyte, CFU-macrophage, CFU-granulocyte/macrophage, and CFU-mix hematopoietic colonies, demonstrating their ability to differentiate into these myeloid lineages in vitro. The transduced, sorted progenitors raised the GC enzyme levels in their progeny cells manyfold compared with untransduced CD34+ progenitors. Collectively, this demonstrates the development of high titer, selectable bicistronic vectors that allow isolation of transduced hematopoietic progenitors and cells that have been metabolically corrected.

3T3 Cells↗

[Histogenesis of renal cell carcinoma].

Immunomorphological characteristics of 27 renal cell carcinomas--18 clear cell, 6 granular, 2 chromophobic, 1 sarcomatoid--as well as 1 oncocytoma were analyzed. The investigation was performed on cryostat sections with indirect immunoperoxidase technique using monoclonal antibodies to intermediate filaments-cytokeratin and vimentin--and renal differentiation antigens--CD10 and CD24. All carcinomas, with the exception of chromophobic type, showed cytokeratin/vimentin coexpression together with strong CD24 and weak (or absent) CD10 staining indicating at primitive cells with initial differentiation toward proximal tubule epithelium as most probable site of origin. In chromophobic cells only cytokeratin and CD24 antigen presence was observed, pattern similar to that seen in oncocytoma. It could be supposed that those two tumors have closely related histogenesis, originating from more differentiated cells with tendency to develop toward distal tubule epithelium.

Carcinoma, Renal Cell↗

CD24 (heat stable antigen, nectadrin), a novel keratinocyte differentiation marker, is preferentially expressed in areas of the hair follicle containing the colony-forming cells.

We have isolated, by subtractive and differential hybridization from a library constructed from keratinocyte colony-forming cells (K-CFCs), a cDNA coding for the rat CD24 (nectadrin, heat stable antigen). CD24, a glycoprotein thought to be involved in cell-cell adhesion and signalling, is highly expressed in keratinocytes located in the bulge area of the rat vibrissa which contains the most K-CFCs. CD24 is also expressed in the outer epithelial sheath of human hair follicles and in glabrous epidermis. However, its expression is not restricted to K-CFCs as demonstrated by cell sorting experiments, and it is thus not a specific marker of clonogenic keratinocytes. Rather, its preferential distribution in keratinocytes located in the most innervated area of the rat vibrissal follicle, i.e., the bulge, suggests that is function could be related to the tactile role of the hair follicle.

3T3 Cells↗

Cytoplasmic expression of a CD24-related epitope in human PHA activated normal T lymphocytes.

In this study we have analysed by immunoperoxidase (IPx) and indirect immunofluorescence (IIF) the intracellular and cell surface reactivity of VIB-E3 mAb, previously clustered as anti-CD24 antigen, on resting and activated normal human T lymphocytes. By IPx assay VIB-E3 mAb did not show reactivity with normal resting T cells. In contrast, the analysis of 11 different samples of PHA activated normal mononuclear cells, showed an intracytoplasmic expression of CD24. Kinetic studies showed that CD24 appears 24 to 48 h after PHA stimulation. To our knowledge, this is the first evidence that a CD24-related epitope is expressed in normal activated T lymphocytes.

Antigens, CD↗