The biochemical characterization of E2, a T cell surface molecule involved in rosettes

Evaluating the immune effector functions induced by humanized anti-CD99 antibody in eliminating T lymphoblastic leukemia/lymphoma cells

Monoclonal antibodies (mAbs) have emerged as targeted immunotherapies with clinical effectiveness and low adverse effects for various cancers. However, antibody drugs for treating aggressive T cell malignancies, T lymphoblastic leukemia/lymphoma (T-ALL/T-LBL), are still limited. Therefore, a potential mAb for treating T-ALL/T-LBL with minimal toxicity to normal cells needs to be developed. We have previously demonstrated that our in-house produced mouse anti-human CD99 mAb MT99/3 and its humanized version, HuMT99/3, which recognize a newly identified epitope of CD99 can induce apoptosis of T-ALL/T-LBL cells without affecting non-malignant peripheral blood cells. Nevertheless, the immune effector functions activated by HuMT99/3 against T-ALL/T-LBL cells remain unexplored. In this study, we evaluated the anticancer activities of HuMT99/3 against T-ALL/T-LBL cells via immune effector functions. T-ALL/T-LBL cell lines were used as target cells, including Jurkat E6.1, MOLT-4, and SUP-T1. The results demonstrated that HuMT99/3 could mediate potent antibody-dependent cellular cytotoxicity (ADCC) activity to kill all cell lines by activating the Fc receptor CD16 on effector cells. HuMT99/3 significantly enhanced the phagocytosis of monocytes on all three malignant T cell lines through antibody-dependent cellular phagocytosis (ADCP) activity. In addition, HuMT99/3 could activate complement to destroy T-ALL cell lines through complement-dependent cytotoxicity (CDC) activity, without affecting the T-LBL cell line and normal PBMCs. Furthermore, the mAb MT99/3 significantly inhibited tumor growth in a T-ALL xenograft model. These findings provide valuable insights into the development of monoclonal antibodies targeting CD99 as promising therapeutics for T-ALL/T-LBL treatment with minimal toxicity to normal peripheral blood cells.

CD99 tumor associated antigen is a potential target for antibody therapy of T-cell acute lymphoblastic leukemia

Monoclonal antibodies (mAbs) are an effective drug for targeted immunotherapy in several cancer types. However, so far, no antibody has been successfully developed for certain types of cancer, including T-cell acute lymphoblastic leukemia (T-ALL). T-ALL is an aggressive hematologic malignancy. T-ALL patients who are treated with chemotherapeutic drugs frequently relapse and become drug resistant. Therefore, antibody-based therapy is promising for T-ALL treatment. To successfully develop an antibody-based therapy for T-ALL, antibodies that induce death in malignant T cells but not in nonmalignant T cells are required to avoid the induction of secondary T-cell immunodeficiency. In this review, CD99 tumor associated antigen, which is highly expressed on malignant T cells and lowly expressed on nonmalignant T cells, is proposed to be a potential target for antibody therapy of T-ALL. Since certain clones of anti-CD99 mAbs induce apoptosis only in malignant T cells, these anti-CD99 mAbs might be a promising antibody drug for the treatment of T-ALL with high efficiency and low adverse effects. Moreover, over the past 25 years, many clones of anti-CD99 mAbs have been studied for their direct effects on T-ALL. These outcomes are gathered here.

Optimization of culture conditions for stable expression of recombinant fc-fused human extracellular CD99 in HEK293T cells

CD99 has been demonstrated to play a key role in several biological processes, including the regulation of T-cell activation, cell adhesion, and cell migration. We have also demonstrated that CD99 and its ligands regulate proinflammatory cytokines in NK cells, monocytes and activated T cells. These data suggest CD99 as a potential therapeutic target in cancer. However, the molecular mechanisms by which CD99 and CD99 counter receptors participate in such processes are unclear. High-quality CD99 recombinant proteins produced in large amounts are essential for biological studies and clinical research. In this study, we optimized the various culture conditions for increasing amounts of recombinant protein production with good biological activity. Intracellular immunofluorescence staining was performed to identify the highly expressing CD99HIgG cells. We further investigated the culture conditions for recombinant protein production. A double antibody sandwich enzyme-linked immunosorbent assay was employed to determine the level of secreted CD99HIgG proteins in the culture supernatant of various culture conditions. Later, affinity chromatography using protein G was used to purify CD99HIgG proteins from the culture supernatant of three proper culture conditions. According to our previous report, which utilized Western blotting, the purified CD99HIgG obtained from all tested culture conditions is composed of the CD99 extracellular part fused with the human IgG Fc part in dimer form. For biological activity, the obtained CD99HIgG proteins showed the ability to ligate with the CD99 counter receptor, resulting in the induction of cytokine production.

High-valency Anti-CD99 Antibodies Toward the Treatment of T Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive form of leukemia that currently requires intensive chemotherapy. While childhood T-ALL is associated with high cure rates, adult T-ALL is not, and both are associated with significant short- and long-term morbidities. Thus, less toxic and effective strategies to treat T-ALL are needed. CD99 is overexpressed on T-ALL blasts at diagnosis and at relapse. Although targeting CD99 with cytotoxic antibodies has been proposed, the molecular features required for their activity are undefined. We identified human antibodies that selectively bound to the extracellular domain of human CD99, and the most potent clone, 10A1, shared an epitope with a previously described cytotoxic IgM antibody. We engineered clone 10A1 in bivalent, trivalent, tetravalent, and dodecavalent formats. Increasing the antibody valency beyond two had no effects on binding to T-ALL cells. In contrast, a valency of ≥3 was required for cytotoxicity, suggesting a mechanism of action in which an antibody clusters ≥3 CD99 molecules to induce cytotoxicity. We developed a human IgG-based tetravalent version of 10A1 that exhibited cytotoxic activity to T-ALL cells but not to healthy peripheral blood cells. The crystal structure of the 10A1 Fab in complex with a CD99 fragment revealed that the antibody primarily recognizes a proline-rich motif (PRM) of CD99 in a manner reminiscent of SH3-PRM interactions. This work further validates CD99 as a promising therapeutic target in T-ALL and defines a pathway toward the development of a selective therapy against T-ALL.

CD99 in malignant hematopoiesis

The CD99 gene encodes a transmembrane protein that is involved in cell differentiation, adhesion, migration, and protein trafficking. CD99 is differentially expressed on the surface of hematopoietic cells both in the myeloid and lymphoid lineages. CD99 has two isoforms, the long and short isoforms that play different roles depending on the cellular context. There has been extensive evidence supporting the role of CD99 in myeloid and lymphoblastic leukemias. Here we review research findings related to the CD99 in malignant hematopoiesis. We also summarize the significance of CD99 as a therapeutic target in hematological malignancies.

Anti-human CD99 antibody exerts potent antitumor effects in mantle cell lymphoma

CD99 is a surface molecule expressed on various cell types including cancer cells. Expression of CD99 on multiple myeloma is associated with CCND1-IGH fusion/t(11;14). This translocation has been reported to be a genetic hallmark of mantle cell lymphoma (MCL). MCL is characterized by overexpression of cyclin D1 and high tumor proliferation. In this study, high expression of CD99 on MCL cell lines was confirmed. Our generated anti-CD99 monoclonal antibody (mAb), termed MT99/3, exerted potent antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) activities against mantle B-cell lymphoma without direct cytotoxic effects. The anti-tumor activities of mAb MT99/3 were more effective in MCL than in other B-cell lymphomas. Moreover, in a mouse xenograft model using Z138 MCL cell line, treatment of mAb MT99/3 reduced tumor development and growth. Our study indicated that mAb MT99/3 is a promising immunotherapeutic candidate for mantle cell lymphoma therapy.

Interaction of CD99 and its ligand upregulates IL-6 and TNF-α upon T cell activation

CD99 has been reported to be involved in T cell regulation. CD99 ligand involvement in the regulation of T cell activation has been postulated. In this study, recombinant CD99 proteins were produced and used as a tool for determining the role of CD99 and its ligand interaction. Recombinant CD99 proteins induced the upregulation of IL-6 and TNF-α expression, but not IFN-γ, in anti-CD3 monoclonal antibody activated T cells. The cytokine alteration was not observed in unstimulated T cells indicating the cytokine upregulation required the signal from T cell activation. The upregulation of IL-6 and TNF-α was, in addition, observed in CD3^- mononuclear cell population including monocytes and NK cells. The recombinant CD99 proteins, however, did not affect either CD25, CD69 or MHC class II expression or T cell proliferation, upon T cell activation. The CD99 ligands were demonstrated to be expressed on monocytes, NK cells and dendritic cells, but not on B and T cells. Our results indicated the presence of CD99 ligands on leukocyte surface. Interaction between CD99 and its ligands involves the regulation of cytokine production.

Targeting Tumor Vascular CD99 Inhibits Tumor Growth

CD99 (MIC2; single-chain type-1 glycoprotein) is a heavily O-glycosylated transmembrane protein (32 kDa) present on leukocytes and activated endothelium. Expression of CD99 on endothelium is important in lymphocyte diapedesis. CD99 is a diagnostic marker for Ewing's Sarcoma (EWS), as it is highly expressed by these tumors. It has been reported that CD99 can affect the migration, invasion and metastasis of tumor cells. Our results show that CD99 is also highly expressed in the tumor vasculature of most solid tumors. Furthermore, we found that in vitro CD99 expression in cultured endothelial cells is induced by starvation. Targeting of murine CD99 by a conjugate vaccine, which induced antibodies against CD99 in mice, resulted in inhibition of tumor growth in both a tumor model with high CD99 (Os-P0109 osteosarcoma) and low CD99 (CT26 colon carcinoma) expression. We demonstrated that vaccination against CD99 is safe, since no toxicity was observed in mice with high antibody titers against CD99 in their sera during a period of almost 11 months. Targeting of CD99 in humans is more complicated due to the fact that the human and mouse CD99 protein are not identical. We are the first to show that growth factor activated endothelial cells express a distinct human CD99 isoform. We conclude that our observations provide an opportunity for specific targeting of CD99 isoforms in human tumor vasculature.

Clofarabine inhibits Ewing sarcoma growth through a novel molecular mechanism involving direct binding to CD99

Ewing sarcoma (ES) is an aggressive bone and soft tissue malignancy that predominantly affects children and adolescents. CD99 is a cell surface protein that is highly expressed on ES cells and is required to maintain their malignancy. We screened small molecule libraries for binding to extracellular domain of recombinant CD99 and subsequent inhibition of ES cell growth. We identified two structurally similar FDA-approved compounds, clofarabine and cladribine that selectively inhibited the growth of ES cells in a panel of 14 ES vs. 28 non-ES cell lines. Both drugs inhibited CD99 dimerization and its interaction with downstream signaling components. A membrane-impermeable analog of clofarabine showed similar cytotoxicity in culture, suggesting that it can function through inhibiting CD99 independent of DNA metabolism. Both drugs drastically inhibited anchorage-independent growth of ES cells, but clofarabine was more effective in inhibiting growth of three different ES xenografts. Our findings provide a novel molecular mechanism for clofarabine that involves direct binding to a cell surface receptor CD99 and inhibiting its biological activities.

CD99 at the crossroads of physiology and pathology

CD99 is a cell surface protein with unique features and only partly defined mechanisms of action. This molecule is involved in crucial biological processes, including cell adhesion, migration, death, differentiation and diapedesis, and it influences processes associated with inflammation, immune responses and cancer. CD99 is frequently overexpressed in many types of tumors, particularly pediatric tumors including Ewing sarcoma and specific subtypes of leukemia. Engagement of CD99 induces the death of malignant cells through non-conventional mechanisms. In Ewing sarcoma, triggering of CD99 by specific monoclonal antibodies activates hyperstimulation of micropinocytosis and leads to cancer cells killing through a caspase-independent, non-apoptotic pathway resembling methuosis. This process is characterized by extreme accumulation of vacuoles in the cytoplasmic space, which compromises cell viability, requires the activation of RAS-Rac1 downstream signaling and appears to be rather specific for tumor cells. In addition, anti-CD99 monoclonal antibodies exhibit antitumor activities in xenografts in the absence of immune effector cells or complement proteins. Overall, these data establish CD99 as a new opportunity to treat patients with high expression of CD99, particularly those that are resistant to canonical apoptosis-inducing agents.

4D intravital microscopy uncovers critical strain differences for the roles of PECAM and CD99 in leukocyte diapedesis

Leukocyte transendothelial migration (TEM) is an essential component of the inflammatory response. In vitro studies with human cells have demonstrated that platelet/endothelial cell adhesion molecule (PECAM) functions upstream of CD99 during TEM; however, results in vivo with mice have been apparently contradictory. In this study we use four-dimensional (4D) intravital microscopy to demonstrate that the site and order of function of PECAM and CD99 in vivo are dependent on the strain of mice. In FVB/n mice, PECAM functions upstream of CD99, as in human cells in vitro, and blocking antibodies against either molecule arrest neutrophils before they traverse the endothelium. However, in C57BL/6 mice, PECAM and CD99 appear to function at a different step, as the same antibodies arrest leukocyte migration through the endothelial basement membrane. These results are the first direct comparison of PECAM and CD99 function in different murine strains as well as the first demonstration of the sequential function of PECAM and CD99 in vivo.

CD99 isoforms regulate CD1a expression in human monocyte-derived DCs through ATF-2/CREB-1 phosphorylation

CD1a expression is considered one of the major characteristics qualifying in vitro human dendritic cells (DCs) during their generation process. Here, we report that CD1A transcription is regulated by a mechanism involving the long and short isoforms of CD99. Using a lentiviral construct encoding for a CD99 short hairpin RNA, we were able to inhibit CD99 expression in human primary DCs. In such cells, CD1a membrane expression increased and CD1A transcripts were much higher in abundance compared to cells expressing CD99 long form (CD99LF). We also show that CD1A transcription is accompanied by a switch in expression from CD99LF to expression at comparable levels of both CD99 isoforms during immature DCs generation in vitro. We demonstrate that CD99LF maintains a lower level of CD1A transcription by up-regulating the phosphorylated form of the ATF-2 transcription factor and that CD99 short form (SF) is required to counteract this regulatory mechanism. Elucidation of the molecular mechanisms related to CD99 alternative splicing will be very helpful to better understand the transcriptional regulatory mechanism of CD1a molecules during DCs differentiation and its involvement in the immune response.

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) and CD99 are critical in lymphatic transmigration of human dendritic cells

The reverse transmigration (RT) of tissue-resident dendritic cells (DCs) across lymphatic endothelia is prerequisite for the initiation of adaptive immune responses and might be regulated in a manner similar to diapedesis. Specifically, CD31 and CD99, which act as gatekeepers during diapedesis, might have a role in RT of DCs. We found that human lymphatic endothelial cells (LECs) and DCs in vitro and in human skin explants express CD31 and CD99. In human skin, CD31 was enriched along intercellular surfaces of LECs, whereas CD99 was preferentially confined to luminal surfaces as evidenced by immunoelectron microscopy. Confocal microscopy analysis revealed that tumor necrosis factor-alpha (TNF-α) and CXCL12 acted as inducers of RT in vitro, but only CXCL12 stimulation resulted in a significant increase in migration rate of DCs. Upon TNF-α stimulation, CXCL12 mRNA levels transiently increased in human fibroblasts and LECs, whereas CXCL12 protein expression levels did not significantly change. Blocking mAbs to CD31 and CD99 significantly reduced RT of DCs across cultured human LEC monolayers and blocked CXCL12-induced migration of DCs in whole-skin explants. In sum, this study shows that CD31 and CD99 are involved in the RT of DCs across LECs and that similar mechanisms promote both diapedesis and RT.

Association of CD99 short and long forms with MHC class I, MHC class II and tetraspanin CD81 and recruitment into immunological synapses

Background

CD99, a leukocyte surface glycoprotein, is broadly expressed in many cell types. On the cell surface, CD99 is expressed as two distinct isoforms, a long form and a short form. CD99 has been demonstrated to play a key role in several biological processes, including the regulation of T cell activation. However, the molecular mechanisms by which CD99 participates in such processes are unclear. As CD99 contains a short cytoplasmic tail, it is unlikely that CD99 itself takes part in its multi-functions. Association of CD99 with other membrane proteins has been suggested to be necessary for exerting its functions.

Results

In this study, we analyzed the association of CD99 with other cell surface molecules involved in T cell activation. We demonstrate the association of MHC class I, MHC class II and tetraspanin CD81 with CD99 molecules on the cell surface. Association of CD99 with its partners was observed for both isoforms. In addition, we determined that CD99 is a lipid raft-associated membrane protein and is recruited into the immunologic synapse during T cell activation. The implication of CD99 on T cell activation was investigated. Inhibition of anti-CD3 induced T cell proliferation by an anti-CD99 monoclonal antibody was observed.

Conclusions

We provide evidence that CD99 directly interact and form the complex with the MHC class I and II, and tetraspanin CD81, and is functionally linked to the formation of the immunologic synapse. Upon T cell activation, CD99 engagement can inhibit T cell proliferation. We speculate that the CD99-MHC-CD81 complex is a tetraspanin web that plays an important role in T cell activation.

Regulation of HLA class I surface expression requires CD99 and p230/golgin-245 interaction

By presenting antigenic peptides on the cell surface, human leukocyte antigen (HLA) class I molecules are critical for immune defense. Their surface density determines, to a large extent, the level of CD8(+) T cell-dependent immune reactions; their loss is a major mechanism of immune escape. Therefore, powerful processes should regulate their surface expression. Here we document the mechanisms used by CD99 to mediate HLA class I modulation. Up-regulation of HLA class I by IFN-gamma requires CD99. In the trans Golgi network (TGN), and up to the cell surface, CD99 and HLA class I are physically associated via their transmembrane domain. CD99 also binds p230/golgin-245, a coiled-coil protein that recycles between the cytosol and buds/vesicles of the TGN and which plays a fundamental role in trafficking transport vesicles. p230/golgin-245 is anchored within TGN membranes via its Golgin-97, RanBP1, IMh1p, P230 (GRIP) domain and the overexpression of which leads to surface and intracellular down-modulation of HLA class I molecules.

CD99 expressed on human mobilized peripheral blood CD34+ cells is involved in transendothelial migration

Hematopoietic progenitor cell trafficking is an important phenomenon throughout life. It is thought to occur in sequential steps, similar to what has been described for mature leukocytes. Molecular actors have been identified for each step of leukocyte migration; recently, CD99 was shown to play a part during transendothelial migration. We explored the expression and role of CD99 on human hematopoietic progenitors. We demonstrate that (1) CD34+ cells express CD99, albeit with various intensities; (2) subsets of CD34+ cells with high or low levels of CD99 expression produce different numbers of erythroid, natural killer (NK), or dendritic cells in the in vitro differentiation assays; (3) the level of CD99 expression is related to the ability to differentiate toward B cells; (4) CD34+ cells that migrate through an endothelial monolayer in response to SDF-1α and SCF display the highest level of CD99 expression; (5) binding of a neutralizing antibody to CD99 partially inhibits transendothelial migration of CD34+ progenitors in an in vitro assay; and (6) binding of a neutralizing antibody to CD99 reduces homing of CD34+ progenitors xenotransplanted in NOD-SCID mice. We conclude that expression of CD99 on human CD34+ progenitors has functional significance and that CD99 may be involved in transendothelial migration of progenitors.

A splice variant of CD99 increases motility and MMP-9 expression of human breast cancer cells through the AKT-, ERK-, and JNK-dependent AP-1 activation signaling pathways

The CD99 gene encodes two distinct transmembrane proteins by alternative splicing of its transcript. To examine the effects of two CD99 isoforms on the invasive phenotypes of breast cancer cells, MDA-MB-231 and MCF-7 human breast cancer cell lines were stably transfected with CD99 cDNAs encoding the major wild-type form (type I) or a minor splice variant (type II). As a result, expression of CD99 type II, but not type I, markedly elevated the motility, binding to fibronectin, MMP-9 expression, and invasiveness of MDA-MB-231 and MCF-7 breast cancer cells. In MDA-MB-435 breast cancer cells expressing both CD99 type I and type II, invasion-related cellular activities were inhibited by the transfection of small interfering RNA (siRNA) targeted to CD99 type II. Meanwhile, CD99 type II-induced MMP-9 expression in MDA-MB-231 cells was shown to be mediated by the binding of AP-1 factors to the MMP-9 gene promoter. Gel shift assay revealed that ligation of CD99 type II with antibody resulted in the binding of JunD to the AP-1 site of the MMP-9 promoter region. Initiation of CD99 type II signaling by antibody ligation increased expression of JunD and FosB AP-1 factors, along with phosphorylation of Src, Akt, p38 MAPK, ERK, and JNK. Knockdown of JunD and FosB by siRNA transfection abolished the positive effects of CD99 type II on the motility and MMP-9 expression of MDA-MB-231 cells. Increased expression of JunD and FosB as well as elevated cell motility and MMP-9 expression by CD99 type II ligation were also abrogated by inhibitors, dominant-negative forms, and siRNAs for Akt1, ERK1/2, and JNK1 but not for p38 MAPK. These results suggest that expression of a splice variant of CD99 contributes to the invasive ability of human breast cancer cells by up-regulating AP-1-mediated gene expression through the Akt-dependent ERK and JNK signaling pathways.

Gingipains of Porphyromonas gingivalis modulate leukocyte adhesion molecule expression induced in human endothelial cells by ligation of CD99

Porphyromonas gingivalis has been implicated as a key etiologic agent in the pathogenesis of destructive chronic periodontitis. Among virulence factors of this organism are cysteine proteinases, or gingipains, that have the capacity to modulate host inflammatory defenses. Intercellular adhesion molecule expression by vascular endothelium represents a crucial process for leukocyte transendothelial migration into inflamed tissue. Ligation of CD99 on endothelial cells was shown to induce expression of endothelial leukocyte adhesion molecule 1, vascular cell adhesion molecule 1, intercellular adhesion molecule 1, and major histocompatibility complex class II molecules and to increase adhesion of leukocytes. CD99 ligation was also found to induce nuclear translocation of NF-kappaB. These results indicate that endothelial cell activation by CD99 ligation may lead to the up-regulation of adhesion molecule expression via NF-kappaB activation. However, pretreatment of endothelial cells with gingipains caused a dose-dependent reduction of adhesion molecule expression and leukocyte adhesion induced by ligation of CD99 on endothelial cells. The data provide evidence that the gingipains can reduce the functional expression of CD99 on endothelial cells, leading indirectly to the disruption of adhesion molecule expression and of leukocyte recruitment to inflammatory foci.

Mouse CD99 participates in T-cell recruitment into inflamed skin

Human CD99 is a small highly O-glycosylated cell-surface protein expressed on most leukocytes. It was recently found to be expressed at endothelial cell contacts and to participate in the transendothelial migration (TEM) of monocytes in vitro. In order to analyze the physiologic relevance of CD99 in vivo we searched for the mouse homolog. We cloned a mouse cDNA coding for a protein 45% identical in its sequence with human CD99. Based on the cDNA, we generated antibodies against this mouse homolog of CD99, which detected the antigen on most leukocytes, on endothelia of various tissues, and at cell contacts of cultured endothelial cells. Cell aggregation of CD99-transfected Chinese hamster ovary (CHO) cells was completely blocked by anti-CD99 antibodies. The same antibodies inhibited TEM of lymphocytes in vitro, independent of whether T cells or endothelial cells were preincubated with antibodies. In a cutaneous delayed-type hypersensitivity (DTH) reaction, anti-CD99 antibodies inhibited the recruitment of in vivo-activated T cells into inflamed skin as well as edema formation. We conclude that mouse CD99 participates in the TEM of lymphocytes and in their recruitment to inflamed skin in vivo. This establishes CD99 as a valid target for interference with cutaneous inflammatory processes.

Leukocyte-endothelial-cell interactions in leukocyte transmigration and the inflammatory response

Both the innate and adaptive immune responses are dependent on the migration of leukocytes across endothelial cells. The process of diapedesis, in which the leukocyte crawls between tightly apposed endothelial cells, is a unique and complex process. Several molecules concentrated at the junctions of endothelial cells, originally described as having a role in holding the endothelial monolayer together, have also been shown to have a role in the emigration of leukocytes. Several mechanisms have been proposed for 'loosening' the junctions between endothelial cells to enable leukocyte passage. These leukocyte-endothelial-cell adhesion molecules are probably involved in regulating the signaling as well as the adhesion events of diapedesis. In addition, this Review introduces a new and unified nomenclature for the junctional adhesion molecule (JAM) family.

CD99 isoforms expression dictates T cell functional outcomes

CD99, a unique integral membrane protein present on the surface of all human T cells, has previously been shown to regulate cell function and fate. In peripheral T cells, it triggers immediate activation of alpha4b1 integrin and cell arrest on inflamed vascular endothelium, whereas it mediates an apoptotic signal in double-positive thymocytes undergoing the selection process. Two isoforms of CD99 exist, a long form corresponding to the full-length protein and a short form harboring a deletion in the intracytoplasmic segment. Here, we show that while peripheral T cells display exclusive expression of the long form, double-positive thymocytes express both isoforms. Moreover, differential expression of these two CD99 molecules can lead to distinct functional outcomes. Expression of the long form in a CD99-deficient Jurkat T cell line is sufficient to promote CD99-induced cell adhesion, whereas coexpression of the two isoforms is required to trigger T-cell death. When coexpressed, the two proteins form covalent heterodimers, which locate within glycosphingolipidic rafts and induce sphingomyelin degradation. Cholesterol depletion experiments show that this localization is required for the induction of apoptosis. Thus, the surface expression pattern of CD99 isoforms determines T-cell functional outcomes.

CD99 plays a major role in the migration of monocytes through endothelial junctions

CD99 is a heavily O-glycosylated 32-kD type I transmembrane protein that is expressed on most hematopoietic cells. We show here that CD99 is expressed on endothelial cells and is concentrated at the borders between confluent cells. We found that a monoclonal antibody to CD99, hec2, selectively inhibited diapedesis of monocytes across endothelial cells by >90%. Diapedesis involved the homophilic interaction of CD99 on monocytes with CD99 on endothelial junctions. CD99 functioned distally to the point at which platelet-endothelial cell adhesion molecule 1 (PECAM-1, also known as CD31), another adhesion molecule involved in transmigration, played its critical role. Confocal microscopy showed that anti-PECAM-1 arrested leukocytes on the apical surface of endothelium, whereas blocking CD99 arrested monocytes at a point where they were partially through the junction. Therefore, diapedesis, the forward migration of leukocytes through endothelial junctions, is regulated sequentially by two distinct molecules, PECAM-1 and CD99.

A study of the coregulation and tissue specificity of XGand MIC2 gene expression in eukaryotic cells

CD99, the product of the MIC2 gene, exhibits an erythroid-specific quantitative polymorphism coregulated with the polymorphism of the XG blood group gene. As a preliminary study of this phenomenon, human XG and CD99 recombinant proteins were expressed in murine RAG cells and analyzed by flow cytometry. Both proteins were expressed independently and at a similar level in single and double transfectants. Immunoprecipitation and Western blot analysis, using the murine monoclonal antibodies NBL-1 and 12E7, revealed species of 26 kd (XG) and 32 kd (CD99), respectively. A putative 28-kd intracellular precursor of CD99 was also detected, as was a 26-kd species after neuraminidase treatment of CD99-expressing cells. No evidence of association or complex formation between XG and CD99 proteins could be proven, either on transfected RAG cells or on human erythrocytes. These results were confirmed using somatic hybrids between single transfectants. These findings suggest that the phenotypic relationship between XG and CD99 is mostly regulated at the transcriptional level, but they do not formally exclude some posttranscriptional effect. Studies on the tissue specificity of XG expression showed that surface expression of the XG protein could not be restored in somatic hybrids between B-lymphoblastoid cell lines from Xg(a+) persons and fibroblasts (RAG) or erythroid (MEL) cells. RT-PCR analysis of the transcripts revealed the existence of an XG mRNA in each cell line, suggesting that the tissue-specific regulation of cell surface XG expression occurs either at a quantitative transcriptional level or is a posttranscriptional event. By Northern blot analysis,XG transcripts were detected in erythroid tissues and several nonerythroid tissues.

CD99 monoclonal antibody induce homotypic adhesion of Jurkat cells through protein tyrosine kinase and protein kinase C-dependent pathway

CD99 is a 32 kDa cell surface glycoprotein which is involved in cell adhesion. Engagement of the CD99 molecule by CD99 monoclonal antibodies has been shown to induce homotypic aggregation of various cell types. By using a newly established CD99 monoclonal antibody, MT99/3, we show here that LFA-1/ICAM-1 independent cell adhesion pathways are activated via CD99. Engagement of the CD99 molecule by MT99/3 induced homotypic cell aggregation of Jurkat T-cells within 30 min reaching its maximal level within 4 h. The Jurkat cell aggregation was not blocked by addition of CD11a (LFA-1) and CD54 (ICAM-1) mAbs. Furthermore, MT99/3 treatment did not alter the expression of LFA-1 and ICAM-1 molecules. Induction of Jurkat homotypic aggregation by MT99/3 was, however blocked by the protein kinase C inhibitor, sphingosine, the protein tyrosine kinase inhibitor, genistein, and by actin filament polymerization blocking agent, cytochalasin B. Thus, these observations suggest that CD99 can mediate beta2-integrin independent cell adhesion that depends on activation of protein kinases and reorganization of the cytoskeleton.

A role for CD99 in T cell activation

The function of the T cell surface protein CD99 was investigated in human CD4(+) peripheral T cells. Crosslinking of the CD99 molecule using anti-CD99 mAbs in the presence of anti-CD3 Ab resulted in a marked enhancement of proliferation. CD99 coligation also enhanced CD25 expression and early markers of T cell activation, CD69 and CD40L. Ligation of CD99 resulted in the pronounced tyrosine phosphorylation of an approximately 29-kDa protein suggesting that a specific CD99-induced signal transduction pathway may exist. Simultaneous costimulation with anti-CD99 and anti-CD28 Abs appeared to have additive effects on CD40L expression while CD99 ligation had no effect on CD2-mediated T cell induction of CD40L expression. These results demonstrate that CD99 signal transduction can deliver effective costimulatory signals to T cells.

CD99 Engagement on Human Peripheral Blood T Cells Results in TCR/CD3-Dependent Cellular Activation and Allows for Th1-Restricted Cytokine Production

We have assessed the functional effect of CD99 engagement on resting human peripheral blood (PB) T cells. CD99, as detected by the mAb 3B2/TA8, is constitutively expressed on all PB T cells and becomes further up-regulated upon cellular activation. In this study we demonstrate that cross-linking of the CD99 molecule with the agonistic mAb 3B2/TA8 cooperates with suboptimal TCR/CD3 signals, but not with phorbol ester, ionomycin, or CD28 mAb stimulation, to induce proliferation of resting PB T cells. Comparable stimulatory effects were observed with the CD99 mAb 12E7. Characterization of the signaling pathways involved revealed that CD99 engagement leads to the elevation of intracellular Ca2+, which is dependent on the cell surface expression of the TCR/CD3 complex. No CD99 mAb-induced calcium mobilization was observed on TCR/CD3-modulated or TCR/CD3-negative T cells. To examine the impact of CD99 stimulation on subsequent cytokine production by T cells, we cross-linked CD99 molecules in the presence of a suboptimal TCR/CD3 trigger followed by determination of intracellular cytokine levels. Significantly, T cell lines as well as Th1 and Th0 clones synthesized TNF-α and IFN-γ after this treatment. In contrast, Th2 clones were unable to produce IL-4 or IFN-γ when stimulated in a similar fashion. We conclude that CD99 is a receptor that mediates TCR/CD3-dependent activation of resting PB T cells and specifically induces Th1-type cytokine production in polyclonally activated T cell lines, Th1 and Th0 clones.

The 21-kDa polypeptide (VAP21) in the rabies virion is a CD99-related host cell protein

In our monoclonal antibody (MAb) stocks prepared against the BHK-21 cell antigens, two (#11875 and 28276) recognized a 21-kDa polypeptide (referred to as VAP21) which is efficiently incorporated into the rabies virion. By using these MAbs, we isolated the cDNA clones that encoded a polypeptide of 144 amino acids from our BHK-21 cell cDNA library. Based on the following evidence, the cDNA was assumed to encode a full-length sequence of VAP21 antigen: i) expression of the cDNA in animal cells resulted in the production of a polypeptide recognized by the two MAbs, and its electrophoretic mobility was the same as that of authentic VAP21 antigen; and ii) immunization with the products from the cDNA-transformed E. coli cells raised specific antibodies in rabbits that recognized a 21-kDa polypeptide in the virion. From the deduced amino acid sequence, it is suggested that the VAP21 antigen has a molecular structure of type-I transmembrane protein containing characteristic proline-rich and glycine-rich regions in its ectodomain. Homology searches resulted in finding homologous sequences (totally about 40% homology) in the human MIC2 gene product (CD99; 32-kDa) of T lymphocytes. These results suggest that the VAP21 antigen in the rabies virion is a cellular CD99-related transmembrane protein.

[Blood groups and structure-activity relations]

In the recent years, advances in biochemistry and molecular genetics have contributed to establish the structure of the genes and proteins from most of the 23 blood group systems presently known. From these findings, five functional classes of molecules can be schematically distinguished: (i) transporters and channels, (ii) receptors for ligands, viruses, bacteria and parasites, (iii) adhesion molecules, (iv) enzymes, and (v) structural proteins. Recent advances on these molecules will be reviewed, particularly by illustrating available structure-function relationships.

Enzymatic release of oligosaccharides from glycoproteins for chromatographic and electrophoretic analysis

For most separations-based analyses of glycoprotein oligosaccharides, the first step is release of the oligosaccharides from the polypeptide. Historically, O-linked and N-linked oligosaccharides have been released from glycoproteins using chemical means, such as alkaline degradation (beta-elimination) or hydrazinolysis. In the last two decades, a growing repertoire of enzymes, including endoglycosidases and glycoamidases, able to release glycoprotein oligosaccharides under mild conditions, have become available. This review traces the discovery, characterization and use of these glycoprotein oligosaccharide releasing enzymes. Emphasis is placed on providing information of practical value for the researcher wishing to incorporate enzymatic oligosaccharide release into their study of glycoprotein oligosaccharide structure and function.

PBDX is the XG blood group gene

We have identified the Xga antigen, encoded by the XG blood group gene, by employing rabbit polyclonal and mouse monoclonal antibodies raised against a peptide derived from the N-terminal domain of a candidate gene, referred to earlier as PBDX. In indirect haemagglutination assays, these anti-peptide antibodies react with Xg(a+) but not Xg(a-) erythrocytes. In antibody-specific immobilization of antigen (ASIA) and immunoblot assays, the anti-peptide antibodies react with the same molecule as does human anti-Xga. Therefore, by its identity with PBDX, Xga is identified as a cell-surface protein that is 48% homologous to CD99 (previously designated the 12E7 antigen), the product of MIC2 which is tightly linked to XG. PBDX is renamed here XG.

Monoclonal antibodies directed against the E2 protein (MIC2 gene product) induce exposure of phosphatidylserine at the thymocyte cell surface

Monoclonal antibodies (mAbs) directed against E2, a 32-kDa transmembrane protein encoded by the MIC2 gene located in the pseudoautosomal region, induce a transbilayer movement of phosphatidylserine and, to a lesser extent, phosphatidylethanolamine in human thymocytes and a Jurkat T lymphocytes. The translocation of phosphatidylserine has been evidenced by using either derivatization of anionic phospholipids with trinitrobenzenesulfonate (TNBS) or cytofluorimetry after labeling of cells with antiphosphatidylserine antibodies. The perturbation of membrane phospholipids induced by anti-E2 mAbs was further evidenced by labeling the cells with merocyanine 540. The specificity of anti-E2-induced perturbations of membrane asymmetry was tested by using a number of mAbs able to activate T cells, including CD3 and CD2. The results strongly suggest that anti-E2-induced changes in PtdSer are related to cell aggregation since the same mAbs specifically induce the aggregation of both thymocytes and Jurkat cells and since the E2 molecule has been previously implicated in the adhesive properties of human T cells with erythrocytes.

Isoforms of the E2 molecule: D44 monoclonal antibody defines an epitope on E2 and reacts differentially with T cell subsets

Human T cell rosetting with erythrocytes is clearly dependent on the CD2-CD58 interaction. We have previously demonstrated that other T cell molecules are involved in the rosette phenomenon, including the E2 molecule, a 32-kDa transmembrane glycoprotein. In this report we show that the D44 monoclonal antibody (mAb), previously shown to subdivide T cells into subpopulations with distinct functional repertoires and to identify 70% of lymphocytes from bronchoalveolar lavage from HIV+ patients, defined a new epitope on the E2 molecule. This was illustrated by the reactivity of the D44 mAb in Western blot experiments performed with the immunoaffinity purified E2 molecule. Moreover, double-labeling experiments showed that the E2 molecule exhibited varying epitopes when expressed in different cell types. The D44 and 12E7 epitopes were restricted to distinct subpopulations of T cells and, more importantly, the D44 expression was limited to the CD29+ population including the memory subset. The O662 and L129 epitopes are present on all T cells. Thus, the E2 molecule has both common and variable epitopes in its extracellular domain, and only the common epitopes seem to be involved in T cell adhesion processes.

Erythrocyte receptor (CD2)-bearing T lymphocytes are affected by diet in experimental pulmonary tuberculosis

Outbred (Hartley) and inbred (strain 2 and 13) guinea pigs were vaccinated with Mycobacterium bovis BCG and placed on isocaloric, purified diets containing either 10 or 30% ovalbumin or on commercial guinea pig chow. Six weeks later, the animals were challenged by the respiratory route with virulent M. tuberculosis H37Rv. At intervals postchallenge, groups were tuberculin tested and sacrificed. Thymus-dependent (T) lymphocytes were enumerated either by rosette formation with rabbit erythrocytes or by the indirect immunofluorescence assay (IFA) with a guinea pig pan-T-cell monoclonal antibody, 8BE6. Protein-deficient guinea pigs of all three strains had significantly fewer erythrocyte rosette-forming (CD2+) T cells in the peripheral blood, and malnourished strain 2 and Hartley guinea pigs exhibited reduced levels of CD2+ T cells in the thymus. In contrast, animals of all three strains fed the low-protein diet harbored more CD2+ T cells in the bronchotracheal lymph nodes than did their control-fed counterparts. A larger proportion of lymphocytes from the blood and lymph nodes of all three strains were IFA positive than formed erythrocyte rosettes regardless of diet treatment. Diet had no effect on IFA-positive lymphocytes in those organs. Protein deficiency is associated with significant alterations in the number and/or distribution of T lymphocytes expressing functional CD2-receptors in BCG-vaccinated animals exposed to virulent mycobacteria by the pulmonary route. These alterations may contribute to the reduction in BCG vaccine efficacy observed in this model.