Placental trophoblasts resist infection by multiple human immunodeficiency virus (HIV) type 1 variants even with cytomegalovirus coinfection but support HIV replication after provirus transfection.

Whether cell-free human immunodeficiency virus type 1 (HIV-1) can productively infect placental trophoblasts (which in turn could transmit the virus into the fetal circulation) is controversial but essential to know for the evaluation of alternative routes (such as cell-mediated infection or trophoblast damage). We have addressed infection factors such as cell purity, source, culture methods, and activation states as well as virus variant and detection methods to conclusively determine the outcome of trophoblast challenge by free virus. Pure (> 99.98%) populations of trophoblasts from 11 different placentas were challenged at a multiplicity of infection (MOI) as high as 6 with five different HIV-1 variants, three of which are non-syncytium-forming, macrophage-tropic isolates from infected infants, with and without coinfection with cytomegalovirus; these preparations were monitored for productive infection for up to 3 weeks after challenge by five different criteria, the most sensitive of which were cocultivation with target cells that can detect virus at an MOI of 10(-7) and HIV DNA PCR that detects 30 virus copies per 10(5) cells. Infection was never detected. However, molecularly cloned T-cell (pNL4-3)- and macrophage (pNLAD8)-tropic provirus plasmids, when transfected into primary trophoblasts, yielded productive infections, indicating that trophoblasts do not suppress late-stage virus replication and assembly. Because of the purity of the trophoblast preparations, the extended length of the infection culture period, the number of trophoblast preparations and virus types examined, the sensitivity of the bioassays and molecular detection assays, and the observations that trophoblasts can support virus replication from provirus, the results of this study strongly argue that free virus cannot infect primary villous trophoblasts.