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To examine the effects of ligand engagement and accessory molecule juxtaposition on T cell receptor (TCR) signaling, we prepared LFA-3/ICAM-1 Rg and LFA-3/VCAM-1 Rg bispecific immunoglobulin fusion proteins (Rg, recombinant globulin). These novel fusion proteins allowed us to examine the effects of ligand driven co-engagement of T cell proteins CD2 and LFA-1 or CD2 and VLA-4 on TCR-dependent mobilization of intracellular Ca2+. We observed that preincubation of resting T cells with LFA-3/ICAM-1 Rg or LFA-3/VCAM-1 Rg fusion proteins resulted in significantly enhanced mobilization of intracellular Ca2+ following TCR-accessory molecule cross-linking relative to T cells preincubated with each of the monospecific Rgs alone or with combinations of the monospecific Rg fusion proteins. In addition, such coengagement stimulated TCR-dependent activation and tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1). These results suggest that when T cells interact with antigen presenting cells the engagement of multiple cell adhesion molecules such as CD2, LFA-1, and VLA-4 primes the T cell to respond more effectively to signals delivered through the TCR.

In recent years the functional consequences of receptor/ligand interactions have been studied in vitro and in vivo using monospecific recombinant immunoglobulin fusion proteins (recombinant/receptor globulins, Rg). These proteins are encoded by chimeric genes composed of a DNA fragment encoding the extracellular domain of a cell surface protein grafted onto a DNA fragment encoding immunoglobulin constant domains. In order to extend the range of applications of Rgs we investigated the possibility of preparing bispecific Rgs. These bispecific fusion proteins contain the extracellular domains of two cell surface proteins held together in close proximity by the constant domains of an immunoglobulin. Here we describe the preparation and characterization of a bispecific Rg which contains the extracellular domains of two adhesion molecules expressed by activated vascular endothelial cells, E-selectin and P-selectin. These two proteins play an important role in initiating leukocyte adhesion to the vascular cell wall at sites of inflammation. Binding studies showed that the E-selectin/P-selectin bispecific immunoglobulin fusion protein (ELAM-1/GMP140 Rg) has an enhanced ability to bind to the myeloid cell line HL60 when compared to the monospecific Rg fusion proteins from which it was derived.

GMP-140 (P-selectin), a 140-kDa granular membrane glycoprotein localized to the alpha granules of platelets and the Weibel-Palade bodies of endothelial cells, is thought to play an important role in adhesive interactions predominantly between granulocytes, platelets and vascular endothelial cells during inflammation. Although GMP-140 binds to granulocytes, its binding to lymphocytes has not been demonstrated. Using genetically engineered IgG C gamma 1 fusion protein of the extracellular domains of GMP-140, we demonstrate that GMP-140 binds to chronically antigen (Ag)-stimulated CD4+ T cells. Freshly isolated CD4+ T cells did not bind GMP-140, but priming and subsequent stimulation with alloantigen induced and gradually increased expression of GMP-140-reactive structures on their surface. T cells isolated from rheumatoid synovial fluids also exhibited strong binding to GMP-140. The binding of GMP-140 to primed T cells is not influenced by preactivation with phorbol 12-myristate 13-acetate, is almost completely abolished by pretreatment of T cells with neuraminidase or trypsin, and is also strongly inhibited by EDTA, the soluble sulfated glycans dextran sulfate, fucoidan, and heparin, but not by chondroitin sulfates. In spite of its strong binding to Ag-primed T cells, GMP-140 did not modulate the proliferative responses of these cells to various stimuli. However, GMP-140 in conjunction with anti-T cell receptor alpha beta monoclonal antibodies augmented the production of granulocyte-macrophage colony-stimulating factor GM-CSF and inhibited the production of interleukin-8 by Ag-primed T cells without influencing their tumor necrosis factor-alpha production. These results suggest that GMP-140 binds to chronically stimulated CD4+ T cells and differentially modulates their production of proinflammatory cytokines. The ability of Ag-primed T cells to bind GMP-140 may facilitate interactions with activated platelets and endothelial cells affecting the course of inflammation.

The glycoproteins granule membrane protein 140 (GMP140), endothelial-leukocyte adhesion molecule 1 (ELAM-1), and Leu-8 are members of a family of glycoprotein receptors (selectins or LEC-CAMs) that play an important role in adhesive interactions between circulating leukocytes and vascular endothelium. Recently it has been reported that ELAM-1 is able to mediate the binding of the colon carcinoma cell line HT-29 to cytokine-activated vascular endothelium, suggesting that tumor cell adhesion to vascular endothelium, a prerequisite for tumor extravasation and metastasis, is in part the result of adhesive interactions between blood-borne tumor cells and cell surface proteins expressed by vascular endothelium. Here, using an approach in which soluble immunoglobulin chimeras of the GMP140 and ELAM-1 receptors were prepared and used to carry out immunohistological studies, we establish that GMP140 binds to tumor cells in a variety of human carcinoma tissue sections (colon, lung, and breast), whereas ELAM-1 binds exclusively to tumor cells in colon carcinoma tissue sections. In addition, GMP140 was found to bind to the cell surface of a number of cell lines derived from various carcinomas but not from melanomas, whereas ELAM-1 bound only colon carcinoma cell lines. We further investigated the nature of the ligands of GMP140 and ELAM-1 on the surface of the carcinoma cells and found that the GMP140 ligand on the surface of tumor cells appears to be distinct from that expressed on the myeloid cell line HL-60. Neuraminidase treatment of a breast carcinoma cell line does not affect, or in some instances increases, GMP140 binding, whereas it completely abolishes GMP140 binding to HL-60 cells. On the other hand, the ligand of ELAM-1 on both the colon carcinoma and HL-60 cells is neuraminidase sensitive in accord with its identification as sialyl-CD15. Parallel results were obtained with neuraminidase-treated frozen carcinoma tissue sections. The present findings form the basis for investigating the role of GMP140 in tumor invasiveness and metastasis.