Mechanisms for adaptation of simian immunodeficiency virus to replication in alveolar macrophages

In contrast to the simian immunodeficiency virus SIVmac239, which replicate s poorly in rhesus monkey alveolar macrophages, a variant with nine amino a cid changes in envelope (SIVmac239/316E) replicates efficiently and to high titer in these same cells. We examined levels of viral DNA, RNA, antigen, and infectious virus to identify the nature of the block to SIVmac239 repli cation in these cells. Low levels of viral antigen (0.1 to 1.0 ng of p27 pe r ml) and infectious virus (100 to 1,000 infectious units per mi) were prod uced in the supernatant 1 to 4 days after SIVmac239 infection, but these le vels did not increase subsequently. STVmac239 DNA was synthesized in these macrophage cultures during the initial 24 h after infection, but the levels did not increase subsequently. Quantitation of the numbers of infectious c ells in cultures over time and the results of experiments in which cells we re reexposed to SIVmac239 after the initial exposure indicated that only a small proportion of cells were susceptible to SIVmac239 infection in these alveolar macrophage cultures and that the vast majority (> 95%) of cells we re refractory to SIVmac239 infection. In contrast to the results with SIVma c239, the levels of viral antigen, infectious virus, and viral DNA increase d exponentially 2 to 7 days after infection by SIVmac239/316E, reaching lev els greater than 100 ng of p27 per mi and 100,000 infectious units per mi. Since SIVmac239/316E has previously been described as a virus capable of in fecting cells in a relatively CD4-independent fashion, we examined the leve ls of CD4 expression on the surface of fresh and cultured alveolar macropha ges from rhesus monkeys. The levels of CD4 expression were extremely low, b elow the limit of detection by flow cytometry, on greater than 99% of the m acrophages, CCR5(+) cells were profoundly depleted only from alveolar macro phage cultures infected with SIVmac239/316E, High concentrations of an anti body to CD4 delayed but did not block replication of SIVmac239/316E. The re sults suggest that the adaptation of SIVmac316 to efficient replication in alveolar macrophages results from its ability to infect these cells in a CD 4-independent fashion or in a CD4-dependent fashion even at extremely low l evels of surface CD4 expression. Since resident macrophages in brains and l ungs of humans also express little or no CD4, our findings predict the pres ence of human immunodeficiency virus type 1 that is relatively CD4 independ ent in the lung and brain compartments of infected people.