MHC class II engagement inhibits CD99-induced apoptosis and up-regulation of T cell receptor and MHC molecules in human thymocytes and T cell line

Transcriptome Analysis Reveals the Induction of Apoptosis-Related Genes by a Monoclonal Antibody against a New Epitope of CD99 on T-Acute Lymphoblastic Leukemia

CD99 was demonstrated to be a potential target for antibody therapy on T-acute lymphoblastic leukemia (T-ALL). The ligation of CD99 by certain monoclonal antibodies (mAbs) induced T-ALL apoptosis. However, the molecular basis contributing to the apoptosis of T-ALL upon anti-CD99 mAb engagement remains elusive. In this study, using our generated anti-CD99 mAb clone MT99/3 (mAb MT99/3), mAb MT99/3 engagement strongly induced apoptosis of T-ALL cell lines, but not in non-malignant peripheral blood cells. By transcriptome analysis, upon mAb MT99/3 ligation, 13 apoptosis-related genes, including FOS, TNF, FASLG, BCL2A1, JUNB, SOCS1, IL27RA, PTPN6, PDGFA, NR4A1, SGK1, LPAR5 and LTB, were significantly upregulated. The epitope of CD99 recognized by mAb MT99/3 was then identified as the VDGENDDPRPP at residues 60-70 of CD99, which has never been reported. To the best of our knowledge, this is the first transcriptome data conducted in T-ALL with anti-CD99 mAb engagement. These findings provide new insights into CD99 implicated in the apoptosis of T-ALL. The identification of a new epitope and apoptosis-related genes that relate to the induction of apoptosis by mAb MT99/3 may serve as a new therapeutic target for T-ALL. The anti-CD99 mAb clone MT99/3 might be a candidate for further development of a therapeutic antibody for T-ALL therapy.

Cell Adhesion Molecule CD99 in Cancer Immunotherapy

The CD99 antigen is a transmembrane protein expressed in a broad variety of tissues, particularly in hematopoietic cells, thymus, endothelial cells, etc. It participates in several crucial biological processes, including cell adhesion, migration, death, differentiation, and inflammation. CD99 has shown oncogenic or tumor suppressor roles in different types of cancer. Therefore, it has been used as a biomarker and therapeutic target for several types of cancer. Moreover, it has also been reported to be involved in several critical immune processes, such as T cell activation and differentiation, dendritic cell differentiation, and so on. Hence, CD99 may have potential values in cancer immunotherapy. Anti-CD99 antibodies have shown therapeutic effects on certain types of cancer, especially on Ewing sarcoma and T cell acute lymphoblastic leukemia (ALL). This review summarizes the recent progress of CD99 in cancer research and targeting therapies, especially in cancer immunotherapy, which may help researchers understand the crucial roles of CD99 in cancer development and design new therapeutic strategies.

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.

Expression of CD99 in Multiple Myeloma: A Clinicopathologic and Immunohistochemical Study of 170 Cases

Background

Multiple myeloma (MM) is a heterogeneous and ultimately fatal disease. Risk stratification using prognostic biomarkers is crucial to individualize treatments. We sought to investigate the role of CD99, a transmembrane protein highly expressed in many hematopoietic cells including subpopulations of normal and neoplastic plasma cells, for MM risk stratification.

Methods

CD99 expression was measured in paraffin samples of bone marrow and extramedullary biopsies of 170 patients with MM. Patients were divided into those with high score (moderately and strongly positive) and low score (negative and weakly positive), with all staining being cytoplasmic and/or membranous.

Results

High anti-CD99 immunostaining was observed in 72 of 136 (52.9%) bone marrow biopsies and 24 of 87 (27.6%) extramedullary biopsies in MM. High CD99 expression of extramedullary specimens was associated with significantly longer overall survival (OS; p=.016). High CD99 expression of extramedullary specimens was also associated with better prognosis in the nonautologous stem cell transplantation group of MM patients (p=.044). In multivariate analysis, International Staging System stage was an independent prognostic factor, whereas CD99 expression was no longer statistically significant.

Conclusions

Expression of CD99 in extramedullary specimens was correlated with longer OS, suggesting that CD99 may be a helpful immunohistochemical marker for risk stratification.

Frequent expression of CD99 in anaplastic large cell lymphoma: a clinicopathologic and immunohistochemical study of 160 cases

Originally described as a diagnostically useful marker for Ewing sarcoma, CD99 immunoreactivity has also been documented in a variety of other tumors, including hematopoietic neoplasms. By using conventional paraffin immunoperoxidase staining and tissue microarrays, we retrospectively investigated CD99 expression in a series of 160 anaplastic large cell lymphoma (ALCL) cases. Of the 160 cases, 103 (64.4%) were positive for CD99. The distribution of CD99 positivity was similar for nodal (66/103 [64.1%]), extranodal, (21/32 [66%]), and primary cutaneous lesions (16/25 [64%]). CD99 expression was present in 96 (64.4%) of 149 of the common type, 4 (80%) of 5 of the small cell variant, and 3 (50%) of 6 of the lymphohistiocytic variant cases. CD99 expression was slightly more frequent in anaplastic large cell lymphoma kinase (ALK)+ cases compared with ALK- cases (43/54 [80%] vs 44/81 [54%]). With 2 exceptions, ALK+ ALCL was seen only in patients younger than 41 years. We conclude that CD99 is frequently expressed in ALCL, with a slightly increased frequency in the younger age ALK+ cases. Nodal and extranodal ALCL should be considered in the differential diagnosis when a CD99+ neoplasm is encountered.

Production, Characterization, and Functional Analysis of Newly Established CD99 Monoclonal Antibodies MT99/1 and MT99/2

The leukocyte surface molecule CD99 is an integral membrane glycoprotein encoded by the E2/MIC2 gene. This molecule is broadly expressed on cells of the hematopoietic system and displays two surface forms, a long 32 kDa form and a short 28 kDa form. While the complete function of the CD99 molecule is unclear, it has been reported to be involved in regulation of cell adhesion, migration, and apoptosis. Thus, several CD99 monoclonal antibodies (MAbs) have been generated for biochemical and functional studies of the CD99 molecule. In the present study two CD99 MAbs, MT99/1 and MT99/2, were produced. The MAbs recognized different epitopes of the CD99 molecule. MAb MT99/1, but not MT99/2, was appropriate for biochemical characterization. Binding of MAb MT99/1 with its epitope led to the induction of cell adhesion and apoptosis. The generated MAbs can be used for future study of the function and mechanism of the CD99 molecule, including its role in the immune system, and may have application in tumor diagnosis and treatment.

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.

Identification and characterization of an Xp22.33;Yp11.2 translocation causing a triplication of several genes of the pseudoautosomal region 1 in an XX male patient with severe systemic lupus erythematosus

The X;Y translocation break point sequence in an XX male patient with prepubertal systemic lupus erythematosus (SLE) was characterized with the intention of identifying a predisposing gene(s) for SLE. Spectral karyotyping of the patient's metaphase chromosomes showed normal autosomes and 2 X chromosomes, one of which displayed a small portion of the Y chromosome. Using a Y chromosome polymerase chain reaction (PCR) walking strategy and inverse PCR, we found that the abnormal recombination occurred between retroviral long terminal repeats located at Xp22.33 (position 0.95 Mb; inside the pseudoautosomal regions) and Yp11.2 (4.20 Mb) downstream of the sex-determining region Y (SRY) gene. The complete DNA sequence of the break point was determined, revealing a partial duplication of the pseudoautosomal region 1 (PAR1) in the derivative X chromosome and causing a partial trisomy of the 12 known genes located between the interleukin-3 receptor alpha (IL3RA; position 1.1 Mb on the X and Y chromosomes) and CD99 (position 2.2 Mb) genes inclusively. All other X chromosome genes were present as 2 copies. Real-time quantitative PCR confirmed the presence of 3 copies of each of the 12 genes in the patient's genomic DNA. We also found that RNA for 1 of the candidate genes was indeed overexpressed in the patient's blood as compared with normal subjects. Taken together, the uniqueness of the translocation, the rarity of severe prepubertal SLE in males, and the presence of SLE in some patients with Klinefelter's syndrome (who have a triplication of the 2 PAR regions) point to a possible relationship between the partial triplication of the PAR1 region and the development of SLE.

Proteomic analysis of malignant lymphocyte membrane microparticles using double ionization coverage optimization

Shed membrane microparticles (MPs) are microvesicles generated from the plasma membrane when cells are submitted to stress conditions. Although MPs reflect the cell state (at least in vitro), little is known on their protein composition. We describe the first set of experiments aiming to characterize the MP proteome. Two ways of triggering MP formation from a T-lymphocytic cell line were analyzed using a 1-D gel approach coupled with LC-MS/MS and the results were compared with those obtained from a classic membrane preparation. In total, 390 proteins were identified in MPs, among which 34% were localized to the plasma membrane. The MPs revealed a broad representation of plasma membrane proteins including 17 hematopoietic clusters of differentiation. This approach was successfully applied to one human chronic B-cell lymphoid malignancy. In all, 413 proteins were identified, including 117 membrane proteins, many of them being pathology associated. The sequence coverage in identified proteins was improved combining both nano-LC-MS/MS and MALDI-MS data. The suppression effect, observed on very complex peptide mixtures, was remediated by chromatographic fractionation. MPs may represent a new tool for studying plasma membrane proteins, displaying the advantages of reproducibility, minimal organelle contamination, and being potentially applicable to most cell types.

Molecular mechanisms of CD99-induced caspase-independent cell death and cell–cell adhesion in Ewing's sarcoma cells: actin and zyxin as key intracellular mediators

CD99 is a unique 32-kDa cell surface molecule with broad cellular expression but still poorly understood biological functions. In cancer cells, CD99 is highly expressed in virtually all Ewing's sarcoma (ES). Engagement of CD99 induces fast homotypic aggregation of ES cells and caspase-independent apoptosis. In this study, we analysed signal transduction after CD99 engagement on ES cells. Findings obtained with selective inhibitors indicated that only actin cytoskeleton integrity was essential for cell-cell adhesion and apoptosis of ES cells. Indeed, CD99 stimulation induced actin repolymerization, further supporting the role of cytoskeleton in CD99 signaling. Gene expression profiling of ES cells after CD99 engagement showed modulation in the expression of 32 genes. Among the pool of upregulated genes reported to be involved in cell adhesion, we chose to analyse the role of zyxin, a cytoplasmic adherens junction protein found to play a role in the regulation of the actin cytoskeleton. Overexpression of zyxin after CD99 ligation was confirmed by real-time PCR and Western blot. Treatment of ES cells with zyxin antisense oligonucleotides inhibited CD99-induced cell aggregation and apoptosis, suggesting a functional role for this protein. Therefore, our findings indicate that CD99 functions occur through reorganization of cytoskeleton and identify actin and zyxin as the early signaling events driven by CD99 engagement.