Use of helper-free replication-defective simian immunodeficiency virus-based vectors to study macrophage and T tropism: Evidence for distinct levels of restriction in primary macrophages and a T-cell line

Cell tropism of human and simian immunodeficiency viruses (HIV and SIV, res pectively) is governed in part by interactions between the viral envelope p rotein and the cellular receptors. However, there is evidence that envelope -host cell interactions also affect postentry steps in viral replication. W e used a helper-free replication-defective SIV macaque (SIVmac)-based retro viral vector carrying the enhanced jellyfish green fluorescent protein inse rted into the nef region (V1EGFP) to examine SIV tropism in a single cycle of infection. Vector stocks containing envelope proteins from three differe nt SIVmac clones, namely, SIVmac239 (T-lymphocyte tropic [T-tropic]), SIVma c316 (macrophage tropic [M-tropic]), and STVmac1A11 (dualtropic), were test ed. SIVmac239 replicates efficiently in many human T-cell lines, but it doe s not efficiently infect primary rhesus macrophages. Conversely, SIVmac316 efficiently infects primary macrophages, but it does not replicate in Molt4 -Clone8 (M4C8)T cells. SIVmac1A11 replicates efficiently in both cell types . When primary macrophages were infected with V1EGFP pseudotyped by SIVmac3 16 or SIVmac1A11 envelopes, the infection was substantially (ca. 200- to 30 0-fold) more efficient than for the SIVmac239 pseudotype. Thus, in primary macrophages, a major component of M versus T tropism involves relatively ea rly events in the infection cycle. Quantitative PCR studies indicated that synthesis and transport of vector DNA into the nucleus were similar for mac rophages infected with the clone 239 and 316 pseudotypes, suggesting that t he restriction for SIVmac239 infection is after reverse transcription and n uclear import of viral DNA. When the same vector pseudotypes were used to i nfect M4C8 cells, they all showed approximately equivalent infectivities, e ven though replication-competent SIVmac316 does not continue to replicate i n these cells. Therefore, in M4C8 cells, restriction involves a late step i n the infection cycle (after proviral integration and expression). Thus, de pending on the cell type infected, envelope-dependent cell interactions tha t govern SIV M and T tropism may involve different steps in infection.