Objectives To determine whether trophoblasts are selectively infected by specific SIV env variants. A critical function of the trophoblast is to protect the fetus from infection by maternal pathogens, but in a subset of instances where the mother is infected with HIV, this layer of protection fails and HIV is transmitted by an intrauterine route which remains poorly understood. Under in vitro conditions, human and rhesus syncytiotrophoblasts are permissive only for limited infection by HIV or SIV, thus it seems that syncytiotrophoblasts constitute a primary barrier to maternal-fetal HIV transmission. To determine whether specific variants are selected by trophoblasts, we have cloned SIV env regions from extravillous trophoblasts, syncytiotrophoblasts, and transformed human T lymphocyte cell lines infected with SIVmac239 and SIVmac251 and have sequenced the V1 through V5 region, the transmembrane region, and the gp41 coding regions. Although there were significant differences in the deduced amino acid sequences between the two virus stocks, particularly in the V1 and V4 regions, there was only minor variation within a particular SIV stock even between different cell types. Thus, we did not find any evidence for selection of specific viral variants from the input stock by trophoblasts. We have developed cell lines from cultured rhesus monkey blastocysts which demonstrate morphology, hormone gene expression, and growth characteristics consnstent with a extravillous trophoblast origin. While cultured syncytiotrophoblasts are equally permissive for three distinct strains of SIV (Sivmac239, SIVmac251 and SIVdeltaB670), SIVmac251 has a much greater capacity to infect extravillous trophoblast cell lines. We hypothesize that the differences seen between the two viral stocks in the env gene may underly the strain differences seen in infection of extravillous trophoblasts, and we will prepare chimeric viruses with SIVmac251 and SIVmac239 in the SIVmac239 backbone to localize the domain responsible for selective trophoblast infection. Interestingly, we also sequenced the entire LTR cloned from infected trophoblasts and T lymphocyte cell lines as described above, and we detected the presence of large (~300 bp) deletions 5' from the previously described Sp1 and NF-KB transcriptional control elements, as well as smaller deletions between the Sp1 elements. This deletion predominates in the human T cells, but appears to represent a relatively minor component in infected trophoblasts. We will examine the transcriptional activation potential of both the deleted and wild type LTR sequences with transient transfection of trophoblast cultures to determine whether there may be a transcriptional component to selective infection by SIVmac251 in rhesus extravillous trophoblast cell lines. Key words placenta, SIV, HIV, envelope gene, transcription, polymerase chain reaction