Medical applications of leukocyte surface molecules--the CD molecules

Leukocytes are the cells of the immune system and are centrally involved in defense against infection, in autoimmune disease, allergy, inflammation, and in organ graft rejection. Lymphomas and leukemias are malignancies of leukocytes, and the immune system is almost certainly involved in most other cancers. Each leukocyte expresses a selection of cell surface glycoproteins and glycolipids which mediate its interaction with antigen, with other components of the immune system, and with other tissues. It is therefore not surprising that the leukocyte surface molecules (CD molecules) have provided targets for diagnosis and therapy. Among the "celebrities" are CD20, a target for lymphoma therapeutic antibodies which earns $2 billion annually (and makes a significant difference to lymphoma patients), and CD4, the molecule used by the human immunodeficiency virus (HIV) as an entry portal into cells of the immune system. This short review provides a background to the CD molecules and antibodies against them, and summarizes research, diagnostic, and therapeutic applications of antibodies against these molecules.     

Multiplex detection of surface molecules on colorectal cancers

A technique of fluorescence multiplexing is described for analysis of the plasma membrane proteome of colorectal cancer cells from surgically resected specimens, enabling detection and immunophenotyping when the cancer cells are in the minority. A single-cell suspension was prepared from a colorectal tumour, and the mixed population of cells was captured on a CD antibody microarray. The cancer cells were detected using a fluorescently tagged antibody for carcinoembryonic antigen (CEA-Alexa647) or epithelial cell adhesion marker (EpCAM-Alexa488). Using this multiplexing procedure, dot patterns from colorectal cancers were distinct from those of adjacent normal tissue. Subtraction of the expression levels for each antigen from normal tissue from those for the cancer shows differential expression in the cancer of CD66c, CD15s, CD55, CD45, CD71, CD45RO, CD11b and CEA, in descending order. Cells captured on the same microarray were also labelled with fluorescent CD3-phycoerythrin antibody revealing the presence of tumour-infiltrating lymphocytes. The immunophenotypes of T lymphocytes from the tumour samples showed differential expression of HLA-DR, TCR alpha/beta, CD49d, CD52, CD49e, CD5, CD95, CD28, CD38 and CD71, in descending order. Fluorescence multiplexing of mixed cell populations captured on a single antibody microarray enables expression profiling of multiple sub-populations of cells within a tumour sample.     

Profiling CD antigens on leukaemias with an antibody microarray

Cluster of differentiation (CD) antigens are defined when a surface molecule found on some members of a standard panel of human cells reacts with at least one novel antibody, and there is good accompanying molecular data. Monoclonal antibodies to surface CD antigens on leukocytes have been used for flow cytometry, and more recently to construct microarrays that capture live cells. These DotScan^TM microarrays enable the rapid and highly parallel characterization of repertoires of CD antigens whose expression patterns may be correlated with discrete leukaemia subtypes, or used to define biomarker ‘signatures’ for non-hematological diseases. DotScan^TM with fluorescence multiplexing enables profiling of CD antigens for minor subsets of cells, such as colorectal cancer cells and tumour-infiltrating lymphocytes from a surgical sample.     

Accurate detection of carcinoma cells by use of a cell microarray chip

Background

Accurate detection and analysis of circulating tumor cells plays an important role in the diagnosis and treatment of metastatic cancer treatment.

Methods and Findings

A cell microarray chip was used to detect spiked carcinoma cells among leukocytes. The chip, with 20,944 microchambers (105 µm width and 50 µm depth), was made from polystyrene; and the formation of monolayers of leukocytes in the microchambers was observed. Cultured human T lymphoblastoid leukemia (CCRF-CEM) cells were used to examine the potential of the cell microarray chip for the detection of spiked carcinoma cells. A T lymphoblastoid leukemia suspension was dispersed on the chip surface, followed by 15 min standing to allow the leukocytes to settle down into the microchambers. Approximately 29 leukocytes were found in each microchamber when about 600,000 leukocytes in total were dispersed onto a cell microarray chip. Similarly, when leukocytes isolated from human whole blood were used, approximately 89 leukocytes entered each microchamber when about 1,800,000 leukocytes in total were placed onto the cell microarray chip. After washing the chip surface, PE-labeled anti-cytokeratin monoclonal antibody and APC-labeled anti-CD326 (EpCAM) monoclonal antibody solution were dispersed onto the chip surface and allowed to react for 15 min; and then a microarray scanner was employed to detect any fluorescence-positive cells within 20 min. In the experiments using spiked carcinoma cells (NCI-H1650, 0.01 to 0.0001%), accurate detection of carcinoma cells was achieved with PE-labeled anti-cytokeratin monoclonal antibody. Furthermore, verification of carcinoma cells in the microchambers was performed by double staining with the above monoclonal antibodies.

Conclusion

The potential application of the cell microarray chip for the detection of CTCs was shown, thus demonstrating accurate detection by double staining for cytokeratin and EpCAM at the single carcinoma cell level.     

Characterization of antibodies submitted to the B cell section of the 8th Human Leukocyte Differentiation Antigens Workshop by flow cytometry and immunohistochemistry

The aim of this study was to characterize the reactivity of monoclonal antibodies (mAbs) that had been submitted to the HLDA8 Workshop. The lineage specificity of target molecules was tested by analyzing their expression patterns on blood cells, leukocytes, and lymphocyte subsets. The expression of target molecules during B cell development, ranging from early precursors to plasma cells, was analyzed using a large panel of B cell lines. Our results have permitted us to characterize the expression of 10 new CD molecules: CD316 (HM1.24, BST2), CD268 (BAFF-R, TNFRSF13C), CD269 (BCMA, TNFRF17), CD267 (TACI, TNFRSF13B), CD275 (ICOSL, B7H2), CD254 (TRANCE, TNFSF11), CD252 (OX40L TNFSF4), CD315 (CD9-P), CD316 (EWI-2, PGRL), and CD307 (IRTA-2 or FcRH5). Three of these new CDs, CD267, CD269, and CD307 presented a B cell-restricted expression pattern. MAbs against these novel cell-surface molecules may offer new tools for research, diagnosis, and therapy.     

Development of quartz-crystal-microbalance-based immunosensor array for clinical immunophenotyping of acute leukemias

An integrated piezoelectric immunosensor array has been developed to immunophenotype acute leukemias in clinic. Each quartz crystal microbalance (QCM) was fabricated with plasma-polymerized film of n-butylamine, nanogold particles, and protein A (PA) to be used to immobilize antibodies in orientation. Leukemic lineage-associated monoclonal antibodies were separately immobilized onto the nanogold-PA-modified surface of the crystals, which were constructed by a 2 x 2 type of probes forming a QCM-based immunosensor array. The main detection conditions were investigated, including the immobilization amount of antibodies, pH, immunoreaction time, sample dilution ratio, etc. The immunophenotyping feasibility of the new technique was investigated through simultaneously analyzing Jurkat cells by the immunosensor array method, immunohistochemistry, and flow cytometry. It was found that the developed technique could readily identify leukemia samples in 5 min and might monitor dynamically the immunoreaction processes. Moreover, comparison studies were carried out for CD antigens expressed on the nucleated cells isolated from 96 acute leukemic patients and 24 normal subjects using the QCM-based immunosensor method and the fluoroimmunoassay. Results obtained by immunophenotyping patients' samples with the immunosensor-based method achieved the rate of 88.93% in 768 groups of numerical data, where no significant statistical difference was observed between the two methods when checked by chi2 analysis (chi2 = 3.4, p > 0.05). This new immunosensor array showed the merits of high sensitivity, high specificity, good reproducibility, easy operation, and low cost. The results of specimen evaluation indicated that it might be clinically suitable for quantifying human differentiated leukocytes and immunophenotyping of acute leukemias.     

Mechanisms of neutrophil transmigration across renal proximal tubular HK-2 cells.

BACKGROUND: Adhesion of intratubular leukocytes to proximal tubules in biopsies of patients with rapidly progressive glomerulonephritis and the appearance of leukocytes in the urine in interstitial nephritis suggest interactions between leukocytes and tubular epithelia in renal diseases. The aim of this study was to investigate the effect of cytokines and endotoxin on leukocyte migration through proximal tubular epithelial cells and also to determine the role of the transmembrane adhesion molecules ICAM-1 and CD47 in this process. METHODS: Experiments determined transepithelial migration (TEM) of PMN (polymorphonuclear) leukocytes through monolayers of HK-2. Expression of ICAM-1 and CD47 was assessed via confocal immunofluorescence, FACS analysis and western blotting. The effect of antibodies against ICAM-1 and CD47 on TEM was examined. Furthermore measurements of cytokine release (IL- 6 and IL-8) were performed. RESULTS: Preincubation of HK-2 cells with either TNFalpha or LPS resulted in stimulation of PMN migration through monolayers of HK-2 cells. There was no preferred direction of transmigration. ICAM-1 was expressed by HK-2 cells and expression was increased after 4 h stimulation with TNFalpha or LPS. Application of ICAM-1 antibodies inhibited TEM. CD47 was expressed in both HK-2 cells and PMN. CD47 antibodies inhibited predominantly basolateral-to-apical TEM. HK-2 cells released IL-8 and IL-6 preferably into the apical compartment. Additionally, we showed that fMLP induced transmigration through monolayers of HK-2 cells was associated with significant increased CD47 expression on PMN cell surfaces. CONCLUSIONS: Inflammatory mediators stimulate TEM of PMN through monolayers of HK-2 cells without a clearly discernible preference of direction. Mechanisms involved in TEM stimulated by cytokines or endotoxin appear to be mainly changes in surface receptor densities of HK-2 cells with ICAM-1 and CD47 playing an essential role.     

Immunological biochips for parallel detection of surface antigens and morphological analysis of cells

A biochip for detecting 26 cluster differentiation (CD), HLA-DR and IgM antigens on lymphocyte surface is described. The biochip, which represents a microarray of antibodies (IgG) against a panel of selected antigens immobilized on transparent plastic surfaces in 1.5-mm spots, was used for the study of normal and neoplastic lymphocytes and can also be used for determining percent of cells expressing definite surface antigens in lymphocyte suspensions. The results are consistent with data obtained by flow cytometry. The novel biochip technology entails a combination of conventional staining of cells immobilized on biochips and morphological analysis.     

C-reactive protein antigenicity on the surface of human peripheral blood lymphocytes. Characterization of lymphocytes reactive with anti-neo-CRP

Previously, we have shown that antibodies specific for C-reactive protein determinants, not present on the native molecule, termed neo-CRP, also react with a significant percentage of PBL. In the present study, cells were evaluated by flow cytometry using alpha-neo-CRP antisera and mAb specific for lymphocyte subsets. With use of either monocyte-depleted PBL or Percoll-enriched large granular lymphocytes, we observed an overlap between cells reactive with alpha-neo-CRP and cells bearing the surface markers CD16, CD11b, Leu-7, and/or Leu-19, which are expressed on NK cells. In addition, we showed co-expression of the neo-CRP antigen with CD19, CD20, and HLA-DR, cell surface markers which are expressed on B lymphocytes. The major proportion of CD3+ cells failed to exhibit co-expression of neo-CRP. Single parameter flow cytometric analyses demonstrated that cells reactive with alpha-neo-CRP exhibited a bimodal staining pattern based on fluorescence intensity: high intensity neo-CRPbright and low intensity neo-CRPdim. Two-color analysis revealed that neo-CRPbright cells co-expressed CD19, CD20, and HLA-DR, whereas neo-CRPdim cells co-expressed CD16, CD11b, Leu-7, and Leu-19. Anti-neo-CRP also reacted with PBL obtained from patients with CD16+ lymphoproliferative disorders and from patients with chronic lymphocytic leukemia of B cell origin, but not with cells from patients with T cell or myeloid leukemias. The alpha-neo-CRP cells from patients with NK cell expansions showed dim fluorescence, whereas patients with B cell expansions showed bright fluorescence, consistent with the staining patterns observed with normal PBL. In addition, cell lines of T cell, B cell, NK cell, myeloid, and erythroid origin were evaluated for reactivity with alpha-neo-CRP. The cloned NK cell line NK 3.3 reacted as neo-CRPdim, but the B cell lines BL41, BL41/95, T1, T2, and CESS all reacted as neo-CRPbright. The cell lines K562, Molt-4, Hut-78, HL-60, U-937, and THP-1-0, which lack characteristic NK and B cell markers, did not react with alpha-neo-CRP. Additional study of the two-color histograms revealed a distinct diagonal staining pattern that was observed only when cells were co-stained with alpha-neo-CRP and either alpha-CD16 (alpha-Fc gamma RIII) or antibody IV3 (alpha-CDw32; alpha-Fc gamma RII). This finding suggests a 1:1 relationship between Fc gamma R on both NK and B cells and determinants recognized by alpha-neo-CRP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Lithographic techniques and surface chemistries for the fabrication of PEG-passivated protein microarrays

This article presents a new technique to fabricate patterns of functional molecules surrounded by a coating of the inert poly(ethylene glycol) (PEG) on glass slides for applications in protein microarray technology. The chief advantages of this technique are that it is based entirely on standard lithography processes, makes use of glass slides employing surface chemistries that are standard in the microarray community, and has the potential to massively scale up the density of microarray spots. It is shown that proteins and antibodies can be made to self-assemble on the functional patterns in a microarray format, with the PEG coating acting as an effective passivating agent to prevent non-specific protein adsorption. Various standard surface chemistries such as aldehyde, epoxy and amine are explored for the functional layer, and it is conclusively demonstrated that only an amine-terminated surface satisfies all the process constraints imposed by the lithography process sequence. The effectiveness of this microarray technology is demonstrated by patterning fluorescent streptavidin and a fluorescent secondary antibody using the well-known and highly specific interaction between biotin and streptavidin.     

Role of gene expression profiling for diagnosing acute leukemias

Cytomorphology and cytochemistry in combination with multiparameter immunophenotyping today are the standard methods for establishing the diagnosis of acute leukemias. In addition, cytogenetics, fluorescence in situ hybridization, and polymerase chain reaction based assays provide important information regarding biologically defined and prognostically relevant subgroups and allow a comprehensive diagnosis of well defined subentities. With regard to the clinical setting a better understanding of the clinical course of distinct biologically defined disease subtypes is needed to select disease-specific therapeutic approaches. Paralleling the increase in knowledge on deregulated pathways in leukemia the development of new therapeutics is accelerated and therefore requires a detailed and comprehensive diagnostic tool. Revealing and quantifying the expression status of many ten thousands of genes in a single analysis the microarray technology holds this potential to become an essential tool for the molecular classification of leukemias. It may therefore be used as a routine method for diagnostic purposes in the near future. Furthermore, it is anticipated that new biologically defined and clinically relevant subtypes of leukemia will be identified based on gene expression profiling. This method may therefore guide therapeutic decisions.     

Dot immunodetection for sphingomyelinase with monoclonal antibody

An assay for the detection of sphingomyelinase with monoclonal antibodies is described. The assay takes advantage of nitrocellulose membranes as antigen adsorbent on which a dilute sample can be concentrated as a spot, using a specially designed 96-well filtration device which is commercially available. The method requires only 6 micrograms of the extracts from leukocytes and liver, and it is 10 times more sensitive than the colorimetric assay. This reduced amount of sample material also has the merit of requiring only a 0.5-ml blood sample from patients. The combination of this dot immunoassay with the monoclonal antibody allows a sensitive and a specific assay, and is also applicable as a screening test on a large number of samples. Furthermore, the possibility of differential diagnosis of Niemann-Pick disease types by detecting isoenzymes by this method was examined after isoelectric focusing of placental sphingomyelinase.     

Characterization of murine and humanized anti-CD33, gelonin immunotoxins reactive against myeloid leukemias

M195 antibodies recognize CD33, an antigen present on acute myeloid leukemia blasts as well as some myeloid progenitor cells, but not on the ultimate hematopoietic progenitor stem cell. Immunotoxins (IT) reactive with human myeloid leukemias were constructed by conjugating gelonin, a single-chain ribosome-inactivating protein, to murine and genetically engineered, humanized M195 antibodies via anN-succinimidyl-3-(2-pyridyldithio)-propionate linkage. No losses of gelonin cytotoxic activity or M195 binding activity were observed after conjugation of up to two toxin molecules per antibody. Toxin conjugates displayed specific, potent toxicity for CD33⁺ cells. The murine and humanized IT were not toxic to CD33^– cells and were 600 and 4500 times more potent, respectively, than free gelonin in inhibiting CD33⁺ HL60 cells. Treatment of HL60 cells with 1 g/ml HuM195-gelonin resulted in more than 1000 times lower colony formation; normal bone marrow mononuclear cell colonyforming units treated with HuM195-IT were reduced by a factor of 10. HL60 leukemia cells could be effectively purged from an excess of normal bone marrow cells. Exposure of target cells to IT for as little as 30 min was as effective as continuous exposure of IT for up to 6 days. However, measures of the efficacy of the immunotoxin were directly related to the length of time of observation after IT exposure and were inversely related to cell concentration. M195-gelonin immunoconjugates are potential candidates for therapeutic use in in vivo or ex vivo bone marrow purging of myeloid leukemias.These studies were supported in part by the Lucille P. Markey Charitable Trust, ACS Grant No. IM551, NIH PO1CA33049, NIH RO1CA55349. Research conducted, in part, by the Clayton Foundation for Research. David A Scheinberg is a Lucille P. Markey Scholar     

Kinetics analysis of binding between melanoma cells and neutrophils

It has been determined previously that polymorphonuclear leukocytes, or PMNs, can facilitate melanoma cell extravasation through the endothelium under shear conditions. The interactions between melanoma cells and PMNs are mediated by the beta2-integrins expressed by PMNs and intercellular adhesion molecules (ICAM-1) expressed on melanoma cells. In this study, the kinetics of these interactions was studied using a parallel plate flow chamber. The dissociation rates were calculated under low force conditions for ICAM-1 interactions with both beta2-integrins, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18), together and separately by using functional blocking antibodies on PMNs. The kinetics of PMNs stimulated with IL-8 was also determined. It was concluded that the small number of constitutively expressed active beta2-integrins on PMNs are sufficient to bind to ICAM-1 expressed on melanoma cells and that the intrinsic dissociation rate for these adhesion molecules appear to be more dependent on what method is used to determine them than on what cells express them.     

Tissue distribution of the C3d/EBV-receptor: CD21 monoclonal antibodies reactive with a variety of epithelial cells,medullary thymocytes,and peripheral T-cells

Summary The CD21 antigen has been described to represent CR2, the receptor for the complement fragment C3d and also the receptor for the Epstein-Barr virus (EBV). Monoclonal antibodies B2, HB5, and B-ly4 belong to the CD21 cluster, recognizing different epitopes of the CD21-molecule. Immunohistology of lymphoid tissues employing these antibodies showed the known staining of B cells and dendritic reticulum cells. Surprisingly, B2, but not HB5 or B-ly4, stained a distinct spot in the cytoplasm of a major proportion of medullary thymocytes, in almost all peripheral blood lymphocytes, and in a substantial amount of cells in T-cell areas of peripheral lymphoid tissues. This distinct cytoplasmic B2 staining was confirmed by immuno-electronmicroscopy. A similar B2+ cytoplasmic dot was observed in B-lymphoblastic lymphomas. Staining of non-lymphoid tissues showed reactivity with all three CD21 mAb with epithelial cells of skin, lung, esophagus, jejunum, colon, pancreas, tonsil, adrenal cortex, renal tubuli, and parotid glands, and with hepatocytes and tongue muscle. In addition, endothelial cells of small vessels showed B2 staining. One possible explanation for our results is, that apart from the presence of B cells and follicular dendritic cells, a CD21-molecule may be expressed by other cell types. However, a maybe more likely explanation may be that the recognized epitopes are not exclusively associated with the C3d/EBV-receptor, but also with other structures. In particular should the possibility be recognized of cross-reactivity with CR2-related proteins, encoded by the large gene family, to which CR2 belongs.     

Sensitive gene expression profiling of human T cell subsets reveals parallel post-thymic differentiation for CD4+ and CD8+ lineages

The differentiation of CD4(+) or CD8(+) T cells following priming of naive cells is central in the establishment of the immune response against pathogens or tumors. However, our understanding of this complex process and the significance of the multiple subsets of differentiation remains controversial. Gene expression profiling has opened new directions of investigation in immunobiology. Nonetheless, the need for substantial amount of biological material often limits its application range. In this study, we have developed procedures to perform microarray analysis on amplified cDNA from low numbers of cells, including primary T lymphocytes, and applied this technology to the study of CD4 and CD8 lineage differentiation. Gene expression profiling was performed on samples of 1000 cells from 10 different subpopulations, defining the major stages of post-thymic CD4(+) or CD8(+) T cell differentiation. Surprisingly, our data revealed that while CD4(+) and CD8(+) T cell gene expression programs diverge at early stages of differentiation, they become increasingly similar as cells reach a late differentiation stage. This suggests that functional heterogeneity between Ag experienced CD4(+) and CD8(+) T cells is more likely to be located early during post-thymic differentiation, and that late stages of differentiation may represent a common end in the development of T-lymphocytes.     

Tissue distribution of the C3d/EBV-receptor: CD21 monoclonal antibodies reactive with a variety of epithelial cells, medullary thymocytes, and peripheral T-cells

The CD21 antigen has been described to represent CR2, the receptor for the complement fragment C3d and also the receptor for the Epstein-Barr virus (EBV). Monoclonal antibodies B2, HB5, and B-ly4 belong to the CD21 cluster, recognizing different epitopes of the CD21-molecule. Immunohistology of lymphoid tissues employing these antibodies showed the known staining of B cells and dendritic reticulum cells. Surprisingly, B2, but not HB5 or B-ly4, stained a distinct spot in the cytoplasm of a major proportion of medullary thymocytes, in almost all peripheral blood lymphocytes, and in a substantial amount of cells in T-cell areas of peripheral lymphoid tissues. This distinct cytoplasmic B2 staining was confirmed by immuno-electronmicroscopy. A similar B2+ cytoplasmic dot was observed in B-lymphoblastic lymphomas. Staining of non-lymphoid tissues showed reactivity with all three CD21 mAb with epithelial cells of skin, lung, esophagus, jejunum, colon, pancreas, tonsil, adrenal cortex, renal tubuli, and parotid glands, and with hepatocytes and tongue muscle. In addition, endothelial cells of small vessels showed B2 staining. One possible explanation for our results is, that apart from the presence of B cells and follicular dendritic cells, a CD21-molecule may be expressed by other cell types. However, a maybe more likely explanation may be that the recognized epitopes are not exclusively associated with the C3d/EBV-receptor, but also with other structures. In particular should the possibility be recognized of cross-reactivity with CR2-related proteins, encoded by the large gene family, to which CR2 belongs.     

The murine CD99-related molecule CD99-like 2 (CD99L2) is an adhesion molecule involved in the inflammatory response

CD99, a glycoprotein found on the surfaces of leukocytes and concentrated at the borders of endothelial cells, plays a major role in the migration of leukocytes across endothelial cells into sites of inflammation, and has other roles in thymocyte development. The human and mouse genomes encode only two proteins related to CD99. One of these, XGA, is a red blood cell surface antigen. The function of the other, CD99-like 2 (CD99L2), is not known. We cloned mouse CD99L2 and used CD99L2 isolated from transfected cells to raise specific antibodies. Similar to human CD99, CD99L2 was expressed at the borders between transfected cells as well as on mouse leukocytes and vascular endothelial cells in situ. Transfection of L cell fibroblasts with CD99L2 imparted to them the ability to adhere to each other in a divalent cation-dependent, homophilic manner. Anti-CD99L2 antibody blocked influx of neutrophils and monocytes into a site of inflammation in vivo.     

CD31/PECAM-1 is a ligand for alpha v beta 3 integrin involved in adhesion of leukocytes to endothelium

To protect the body efficiently from infectious organisms, leukocytes circulate as nonadherent cells in the blood and lymph, and migrate as adherent cells into tissues. Circulating leukocytes in the blood have first to adhere to and then to cross the endothelial lining. CD31/PECAM- 1 is an adhesion molecule expressed by vascular endothelial cells, platelets, monocytes, neutrophils, and naive T lymphocytes. It is a transmembrane glycoprotein of the immunoglobulin gene superfamily (IgSF), with six Ig-like homology units mediating leukocyte-endothelial interactions. The adhesive interactions mediated by CD31 are complex and include homophilic (CD31-CD31) or heterophilic (CD31-X) contacts. Soluble, recombinant forms of CD31 allowed us to study the heterophilic interactions in leukocyte adhesion assays. We show that the adhesion molecule alpha v beta 3 integrin is a ligand for CD31. The leukocytes revealed adhesion mediated by the second Ig-like domain of CD31, and this binding was inhibited by alpha v beta 3 integrin-specific antibodies. Moreover alpha v beta 3 was precipitated by recombinant CD31 from cell lysates. These data establish a third IgSF-integrin pair of adhesion molecules, CD31-alpha v beta 3 in addition to VCAM-1, MadCAM-1/alpha 4 integrins, and ICAM/beta 2 integrins, which are major components mediating leukocyte-endothelial adhesion. Identification of a further versatile adhesion pair broadens our current understanding of leukocyte-endothelial interactions and may provide the basis for the treatment of inflammatory disorders and metastasis formation.