The ability of human immunodeficiency virus strain MN (HIV MN ), a T-cell line-adapted strain of HIV, and X4 and R5 primary isolates to bind to various cell types was investigated. In general, HIV MN bound to cells at higher levels than did the primary isolates. Virus bound to both CD4-positive (CD4
is an important route for infection of T cells in vivo.Human immunodeficiency virus type 1 (HIV-1) is known to infect T cells by a sequence of events including binding of gp120 to CD4 and chemokine receptors, membrane fusion, reverse transcription, and integration. Four forms of infectious virus particles have been shown to be present in vivo, and all could be important for infection of CD4 ϩ target cells. These forms include cell-associated virus, cell-free virus, immunecomplexed virus, and cell-bound virus. During HIV replication, progeny virions assemble and bud from the surface of infected cells. The assembling and budding virus on the surface of infected cells is generally referred to as cell-associated virus and has been shown to be highly infectious to neighboring target cells (2, 33). Transmission of cell-associated virus to target cells can be Ͼ100 times more efficient than that of cell-free virus (2, 4). Virus released from infected cells is considered cell free and can reach high levels (Ͼ10 6 RNA copies/ ml) in blood (6). The cell-free virus half-life in plasma is less than 110 min, but the exact turnover mechanism(s) remains poorly understood (31). Several studies have shown that a portion of the cell-free virus exists as immune complexes (HIV IC) resulting from binding of specific antibody and/or complement deposition on the virion surface (7,22,24,36,37).HIV may also bind to CD4-negative (CD4 Ϫ ) cells in vivo, which we refer to as cell-bound virus. Alternatively, several studies have demonstrated that virus bound to the surface of cells remains infectious for T cells. Thus, HIV IC bound to FDC can infect T cells (11) even in the presence of neutralizing antibody (13). A non-syncytium-inducing strain of HIV bound to erythrocytes through the Duffy antigen receptor for chemokines was shown to infect peripheral blood mononuclear cells (PBMC) (19). Infection of T cells with HIV IC bound to B cells was 10-to 100-fold more efficient than cell-free virus infection of T cells (15,16). The mechanism of infection of T cells by virus bound to CD4 Ϫ cells may vary depending on the cell type but could represent an important pathway of HIV infection in vivo.The goal of the current study was to determine if HIV binds to CD4Ϫ primary cells and cell lines. Furthermore, we determined if virus bound to CD4Ϫ cells can infect CD4 ϩ T lymphocytes and investigated the mechanism of infection.
MATERIALS AND METHODSCell lines and isolation of primary cells. The T-lymphocytic H9 (HTB-176) and B-lymphocytic Raji (CCL-86) cell lines used were obtained from the American Type Culture Collection (ATCC; Manassas, Va.). Cells were grown in RPMI 1640 medium supplemented with 10% heat-inactivated fetal bovine serum (Whittaker M. A. Biop...