Mechanisms of Transplacental Transmission of HIV-1: I. Infection of Fetal Placental Layers in the Intervillous Spaces

Mother-to-child transmission of human immunodeficiency virus type 1 (HIV-1) occurs in 6–50% of children born to HIV-1–seropositive mothers. It remains unclear why some mothers transmit HIV-1 to their infants, whereas the majority do not. Studies to date indicate that multiple factors are associated with maternal–fetal transmission including plasma viremia, maternal immune response, maternal viral phenotypes, use of anti-retroviral agents during pregnancy, and obstetrical factors. Little is known about the mode of transmission or the risk factors associated with transmission in utero. To determine the location of fetal cell layers infected with HIV-1 and origin of virally infected cells, a multiplex in situ polymerase chain reaction (PCR) technique was developed. For this purpose, maternal and fetal peripheral blood mononuclear cells (PMBCs) were first typed for human leukocyte antigen (HLA)-DQa by solution-based PCR method, and then various layers of placenta were examined for the presence of HIV-1–infected cells, DNA-IS-PCR, and the origin of HIV-1 infected cells by reverse-transcriptase-initiated (RT) IS-PCR for the expression of messenger RNA (mRNA) present in the maternal cells but not inherited by the newborn, simultaneously. We analyzed 68 placentas from HIV-1–infected mothers for the presence of HIV-1 proviral sequences, by using DNA-IS-PCR, expression of HIV-1 RNA (by RT-IS-PCR), and presence of maternal cells by multiplex in situ PCR for the maternal-specific HLA-DQa genes, not inherited by the baby, and IS-PCR for HIV-1. Our data revealed that HIV-1–positive provirus sequences were present in the maternal cells in 15 (22%) of 68 placentas. In this group, numerous endothelial cells lining the intervillous spaces of maternal decidua also were found to be infected with HIV-1. In addition, 13 (19.6%) of 15 placentas (which were positive for HIV-1 provirus by DNA-IS-PCR) showed HIV-1–infected cells within the chorioamniotic layers. Our data strongly suggest that several fetal layers of placenta become infected with HIV-1 through penetration of HIV-1–infected maternal cells. These studies are still in progress. Histologic observations combined with IS-PCR and immunohistochemistry findings also are described.