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Cytoplasmic expression of a CD24-related epitope in human PHA activated normal T lymphocytes
Abstract
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.
Heat-stable antigen (HSA)/mouse CD24 (formerly termed Nectadrin) is a membrane glycoprotein with an unusual structure consisting of a small protein core and extensive glycosylation. It is expressed by hematopoietic cells but not by mature T lymphocytes. HSA on accessory cells is an important costimulatory molecule required for the clonal expansion of T lymphocytes. HSA is also involved in cell-cell adhesion events and the isolated antigen has been shown to possess self-binding properties. In the present study we have re-investigated the role of HSA in T cell proliferation. We find that following stimulation of T lymphocytes with concanavalin A or of CD4+ T lymphocytes with a combination of anti-CD3/CD28 monoclonal antibodies (mAb) the HSA antigen is transiently expressed. The expression correlated with the appearance of CD25 and a CD2 activation epitope at the cell surface. Induction of HSA was also seen in vivo on Vβ8+ T lymphocytes in BALB/c mice that were injected with Staphylococcal enterotoxin B. Biosynthetic labeling and analysis of mRNA by reverse transcriptase-polymerase chain reaction showed that HSA was synthesized by activated T lymphocytes. A combination of anti-CD3 and mAb 79 to HSA was incapable of inducing proliferation of purified CD4+ T lymphocytes. However, the antibody strongly enhanced the response obtained with a combination of anti-CD3/CD28 mAb. The augmenting effect of the HSA-specific mAb was dose dependent. Since HSA is bound to the membrane via a glycosyl-phosphatidylinositol (GPI) anchor and GPI-anchored molecules have been implicated in lymphocyte activation, it is conceivable that HSA is not only a costimulatory molecule on accessory cells but is also a signaling molecule in T lymphocytes. The possibility of a homotypic HSA/HSA interaction between T lymphocytes and accessory cells is discussed.
The murine heat stable antigen (HSA, mouse CD24) is a glycosyl phosphatidylinositol-anchored cell surface protein which is primarily expressed in immature but not mature cells of several hematopoietic lineages and in neuronal tissue. The function of HSA is not known but there is evidence in lymphocytes that it is involved in cell adhesion and in cell activation. We examined the effect of constitutive HSA expression on the maturation and on the function of B and T cells in transgenic mice. Transgenic HSA was strongly expressed throughout all stages of T cell maturation without changes in absolute cell number or proportions of subpopulations both in the thymus and in the periphery. The size of the B cell compartment was also unchanged. Thus, we conclude that in the T lineage the loss of HSA expression is not mandatory for maturation. On the functional level, two parameters of immune function were measured. When the ability to activate peripheral T cells expressing transgenic HSA was tested in a mixed lymphocyte reaction, no significant difference in the stimulation index and cytolytic activity was detected between transgenics and control littermates. However, when immunized with a T cell dependent antigen, transgenic mice showed 10-fold higher serum IgG1 titers. This suggests that the expression of transgenic HSA in cells normally negative for HSA (e.g. peripheral T cells or memory B cells) leads to a better stimulation of lymphocytes during a secondary antibody response.
The CD24 surface antigen is a small glycophosphatidylinositol (GPI)-anchored glycoprotein found on human granulocytes and most B lymphocytes. Many CD24 monoclonal antibodies (MoAbs) have been described that identify several epitopes, with the majority of them related to carbohydrate structures associated with the CD24 molecule. Considerable variation has been observed in the apparent tissue distribution of the CD24 antigen depending on the MoAb used, and hence the CD24 epitope studied. In this study, CD24 expression by human cell lines and normal hematopoietic call populations was assessed using a panel of carbohydrate and protein core-specific CD24 MoAbs and reverse transcriptase polymerase chain reaction (RT-PCR) analysis. A number of CD24 carbohydrate epitope-reactive MoAbs bound to both T lymphocytes and several hematopoietic cell lines, despite the absence of concomitant CD24 mRNA or detectable surface CD24 core protein in the same cells. This additional CD24 MoAb reactivity on T lymphocytes was, in common with that observed on granulocytes (CD24 protein+), specifically inhibited by the presence of both sialyllactose and mucin. Similarly, the binding of carbohydrate epitops-reactive CD24 MoAb was reduced on both T lymphocytes and granulocytes by pretreatment with phospholipase C, pronase, or neuraminidase. Together, the data indicate that a number of CD24-associated carbohydrate epitopes have a broader tissue distribution than the CD24 protein and are expressed on additional GPI-linked molecule(s). These findings have immediate implications for both leukemia phenotyping and attempts to examine CD24 function with CD24 MoAb.
Dendritic cells (DC) are characterized as leucocytes that lack mature lineage specific markers and stimulate naive T-lymphocyte proliferation in vitro and in vivo. The mouse heat stable antigen (HSA) participates in T lymphocyte co-stimulation and is expressed by DC isolated from thymus, skin and spleen. The human HSA homologue, CD24, is predominantly expressed by B lymphocytes and granulocytes, but its expression on DC has not been studied in detail. CD24 clearly participates in B-lymphocyte signalling but co-stimulatory activity for T lymphocytes has not yet been described. We have examined the expression of CD24 on human peripheral blood DC populations isolated directly or following in vitro culture. The CD24 antigen was absent from blood DC however, cross-reactive sialylated carbohydrate epitopes were detected on DC with some CD24 monoclonal antibodies (mAb). These CD24 mAb define a protein surface antigen, which is expressed by an immature or resting subpopulation of peripheral blood DC and is down-regulated following activation differentiation in vitro.
Membrane expression of the CD24 molecule on activated T lymphocytes is not elucidated fully. We previously described the intracellular and cell-surface expression of the CD24 sialic acid-dependent epitope(s) on phytohemagglutinin-activated peripheral blood mononuclear cells. However, the CD24 core protein was not detected previously on human T cells. This study reinvestigated the expression and role of CD24 in T cell subsets. We analyzed binding of anti-CD24 monoclonal antibodies (mAbs) to sialic and leucine-alanine-proline (LAP) epitopes in resting and activated, normal T lymphocytes. CD24 LAP and CD24 sialic epitopes were detected on activated CD4- and CD8-positive cells. Although expression of CD24 sialic epitopes remained stably expressed in interleukin (IL)-2-dependent cultures, T cell expression of the LAP epitope was transient. Anti-LAP antibodies strongly enhanced the response of T cells to a combination of anti-CD3/CD28 mAbs and enhanced proliferative response induced by recombinant IL-2. We found similarities in the tissue distribution and function of the human CD24 LAP molecule and the murine, heat-stable antigen, which suggests that CD24 might function as a signaling molecule on human T cells.
In the hematopoietic system, the B-cell associated antigen CD24 is expressed at high density on B cells, B-cell precursors, and B-cell malignancies as well as at low density on peripheral blood polymorphonuclear leukocytes. The 42-Kd sialoglycoprotein has not been previously demonstrated to be expressed on T cells, thymocytes, or T-cell malignancies. We identified three patients with mycosis fungoides/Sézary syndrome that showed low density expression of the CD24-related epitope recognized by antibody BA-1 on circulating T cells. All three patients had Sézary cells by morphologic assessment and clonal T-cell populations in the peripheral blood by gene rearrangement studies. In two of these patients, indirect immunofluorescence with a panel of six anti-CD24 monoclonal antibodies demonstrated reactivity for two of six antibodies in one case and only one of six antibodies in the other. The biologic significance of CD24-related epitope expression on circulating T cells in mycosis fungoides/Sézary syndrome is unclear. However, these findings suggest that differential, low density expression of CD24-related epitopes (BA-1+, OKB2-) may be a useful phenotypic marker for identifying circulating Sézary cells.
In the hematopoietic system, the B-cell associated antigen CD24 is expressed at high density on B cells, B-cell precursors, and B-cell malignancies as well as at low density on peripheral blood polymorphonuclear leukocytes. The 42-Kd sialoglycoprotein has not been previously demonstrated to be expressed on T cells, thymocytes, or T- cell malignancies. We identified three patients with mycosis fungoides/Sezary syndrome that showed low density expression of the CD24-related epitope recognized by antibody BA-1 on circulating T cells. All three patients had Sezary cells by morphologic assessment and clonal T-cell populations in the peripheral blood by gene rearrangement studies. In two of these patients, indirect immunofluorescence with a panel of six anti-CD24 monoclonal antibodies demonstrated reactivity for two of six antibodies in one case and only one of six antibodies in the other. The biologic significance of CD24- related epitope expression on circulating T cells in mycosis fungoides/Sezary syndrome is unclear. However, these findings suggest that differential, low density expression of CD24-related epitopes (BA- 1+, OKB2-) may be a useful phenotypic marker for identifying circulating Sezary cells.
CD24 is a signal-transducing molecule on the surfaces of most human B cells that can modulate their response to activation signals by antagonizing IL-induced differentiation into antibody-forming cells and inducing proliferation in combination with signals generated by Ag receptors. A cDNA that directs the expression of CD24 on the surfaces of transfected COS cells was cloned by its homology to a cDNA encoding the murine M1/69-J11d heat stable Ag. The CD24 cDNA encodes a mature peptide of only 31 to 35 amino acids that is extensively glycosylated and is attached to the outer surface of the plasma membrane by a glycosyl phosphatidylinositol lipid anchor. Although CD24 is structurally similar to M1/69-J11d, and the two Ag appear to have a common genetic ancestry, the homology of CD24 to the M1/69-J11d Ag is confined to a small cluster of amino acids comprising potential N-linked glycosylation sites. Combined with the differences in expression patterns of the human and murine Ag, this indicates that CD24 and M1/69-J11d may not have equivalent functional roles in lymphoid development. The novel structure of CD24 suggests that signaling could be triggered by the binding of a lectin-like ligand to the carbohydrates projecting from the CD24 peptide, and transduced through the release of second messengers derived from the glycosyl phosphatidylinositol membrane anchor of CD24.
The surface phenotype of neoplastic plasma cells from peripheral blood of plasma cell leukaemia patients and bone marrow of patients with myelomatosis was investigated with two monoclonal antibody panels including 50 selected from the B cell panel of the IVth International Workshop on Leucocyte Differentiation Antigens. The majority of myelomas expressed CD24 (HB8 epitope only), CD38, CD44, CD54, and the antigen recognized by the monoclonal antibody 8A. A range of other antigens may also be expressed including CD10, CD32 (FcR II), CD19, CD20 and MHC Class II. Antigens expressed by myeloma plasma cells can be considered in three groups: (a) antigens associated with lymphocyte and plasma cell differentiation: (b) antigens which are not lineage specific: and (c) molecules concerned with lymphocyte recirculation and intercellular adhesion (CD44 and CD54). The significance of CD44 and CD54 expression by plasma cells and the potential interaction of plasma cells with T lymphocytes and monocytes is discussed.
NALM-6-M1 is an acute lymphoblastic leukemia cell line previously shown in our laboratory to express the pre-B lymphocyte phenotype, i.e., cytoplasmic IgM+, surface immunoglobulin-. Hybridomas were produced against this cell line by fusing spleen cells from hyperimmunized mice with NS-1 mouse myeloma cells. One monoclonal antibody derived from this fusion, designated BA-1, reacted with peripheral blood B lymphocytes, chronic lymphocytic leukemias, pre-B-ALL, most non-Hodgkin's lymphomas, and most non-T, non-B-ALL. BA-1 showed weak reactivity with B lymphoblastoid cell lines and failed to react with multiple myeloma and pokeweed mitogen-induced plasma cells. BA-1 also reacted with peripheral blood granulocytes and the erythroleukemia cell line K-562. No reactivity was seen with cells of T lymphocyte origin, platelets, red cells, monocytes, or acute myelocytic leukemias. Evidence is presented indicating that the determinant recognized by BA-1 is not surface immunoglobulin, HLA-DR, or receptors for C3 or Fc. We conclude that monoclonal antibody BA-1 may be useful in the study of early stages of human B lymphocyte development.