Syncytium formation results from the conversation of the gp120 subunit of envelope glycoprotein expressed on infected cells with CD4 and a coreceptor, typically CXCR4, on the surface of target cells (3,11,28,32,35,50,51,62,67)

Syncytium formation results from the conversation of the gp120 subunit of envelope glycoprotein expressed on infected cells with CD4 and a coreceptor, typically CXCR4, on the surface of target cells (3,11,28,32,35,50,51,62,67). and lacked the novel properties of A80. These results suggest a new role for CXCR4 in homologous lymphocyte adhesion that is ligand impartial and in HIV-1 contamination. Human immunodeficiency computer virus type 1 (HIV-1) infects target cells through sequential binding of the gp120 subunit of envelope glycoprotein with cellular receptors. Binding to the primary receptor, CD4 (26,47,50,51), induces a gp120 conformation that is permissive for conversation with a coreceptor, which is required for envelope-mediated fusion (3,7,21,28,30,32,35). CCR5 is the front collection coreceptor for generally transmitted forms of HIV-1 and CXCR4 serves this role for T-cell-tropic (T-tropic) strains that evolve late in the course of contamination (22,24,28,29,60,70). CCR5 and CXCR4 belong to the chemokine receptor family, which transmit signals through heterotrimeric G proteins (3,8,7,35). T-tropic HIV-1, designated X4 strains based on the functional relationship with CXCR4, has been suggested to be more virulent than R5 or macrophage-tropic strains (7,9,23), possibly due to the wider spectrum of target cells that express CXCR4 (13). The unique ligand of CXCR4 is usually stromal cell-derived factor 1 (SDF-1), a member of the family of chemo-attractant cytokines (54,56). This chemokine has been demonstrated to play a critical role during embryologic development in the homing of hepatic hematopoietic precursors Aurantio-obtusin to bone marrow, the arborization of small blood vessels, the formation of the cerebellum, and B-cell lymphopoiesis (54,71). SDF-1 regulates homing and directed the migration of lymphocytes and modulates the expression Aurantio-obtusin of cell surface adhesion molecules (18,66). SDF-1 can interfere with contamination by X4 strains of HIV-1 by receptor blockade and downmodulation from Aurantio-obtusin your cell surface (54,56,68). Activation of CXCR4 by SDF-1 or gp120 may induce cell activation and apoptosis of neurons and CD4+cells (10,12,27,39,42,55,69). The structural basis for the conversation of CXCR4 with SDF-1 and HIV-1 envelope glycoproteins has not yet been elucidated. Structure-function studies with chimeras, point mutants, or domain-specific monoclonal antibodies (MAbs) show that these functions involve multiple domains of the receptor and are not coincident (14,16,19,20,31,33,35,41). Whereas the membrane-proximal region of the N-terminal (NT) extracellular domain name and the third extracellular loop (ECL3) appear to be critical for SDF-1 binding and signaling, regions contiguous to the second ECL have been implicated in coreceptor activity (14,15,16,31). Studies with CXCR4 mutants that are not coupled to G proteins have revealed that coreceptor activity is usually independent of transmission transduction (31,52). In contrast, it has been shown that signaling through CCR5 is required for fusion of R5 viruses with primary CD4+T lymphocytes (2), although signal transduction is not necessary for contamination of cell lines (4,5,34,38). Cell fusion with syncytium formation represents an important cytopathic effect of HIV-1 contamination that may be a crucial mechanism for depletion of CD4+T lymphocytes (49,50,51,62,67). Syncytium formation results from the conversation of the gp120 subunit of envelope glycoprotein expressed on infected cells with CD4 and a coreceptor, typically CXCR4, on the surface of target cells (3,11,28,32,35,50,51,62,67). The involvement of cytoadhesion molecules in syncytium formation has been exhibited by inhibition with MAbs to LFA-1 and ICAM-1 (17,37,40,65) and the observation that LFA-1-deficient CD4+T lymphocytes exhibit decreased syncytium formation (57). Moreover, this process can be enhanced by the modulation of LFA-1 Aurantio-obtusin conformation using the NKI-IL-16 MAb (6). In the physiologic response to SDF-1 signaling through CXCR4, rolling of T lymphocytes and tight adhesion to endothelial cells is dependent upon LFA-1 activation (18,25,45). Similarly, SDF-1 activates integrins (VLA-4 and VLA-5) in CD34+cells (57,66). These findings link CXCR4 signaling to integrin activation in physiologic responses and implicate this mechanism in HIV-1 contamination as well. Here we demonstrate that an GLUR3 MAb to the ECL3 of CXCR4, A80, has the unique properties of inducing cell agglutination and enhancing syncytium formation by HIV-1, providing additional evidence for the association between CXCR4 signaling and cell adhesion. This unique activity of the A80 MAb provides important insights into the mechanism for CXCR4 function in physiologic responses and HIV-1 envelope-mediated membrane fusion. == MATERIALS AND METHODS == == Reagents. == RPMI.