Unique monoclonal antibody recognizing the third extracellular loop of CXCR4 induces lymphocyte agglutination and enhances human immunodeficiency virus type 1-mediated syncytium formation and productive infection

To increase insight into the structural basis of CXCR4 utilization in human immunodeficiency virus type 1 (HIV-1) infection, a new generation of three monoclonal antibodies (MAbs) was developed in WKA rats. The A80 MAb, which binds an epitope in the third extracellular loop (ECL3) of CXCR4, has uniq ue biologic properties that provide novel insights into CXCR4 function. Thi s agent enhanced syncytium formation in activated human peripheral blood mo nonuclear cells (PBMC) infected with X4 or R5 and CEM cells infected with X 4 HIV-1 strains. Exposure to ASO increased the productive infection of acti vated CD4(+) T cells and CEM cells with R5 and X4 viruses, respectively. Th is antibody uniquely induced agglutination of PBMC and CEM cells but did no t activate calcium mobilization. Agglutination induced by A80 was Inhibited by stromal cell-derived factor 1, T22, and phorbol 12-myristate 13-acetate but was not significantly altered by pretreatment of cells with pertussis toxin, wortmannin, or MAbs to LFA-1, ICAM-1, ICAM-2, and ICAM-3. The bindin g of the A145 and A120 MAbs was mapped to the N-terminal extracellular doma in and a conformational epitope involving ECL1. and ECL2, respectively. Bot h of these MAbs inhibited HIV-1 infection and lacked the novel properties o f ASO. These results suggest a new role for CXCR4 in homologous lymphocyte adhesion that is ligand independent and in HIV-1 infection.