Kinetics of HIV infection of human placental syncytiotrophoblast cultures: An ultrastructural and immunocytochemical study

We previously demonstrated that syncytiotrophoblast (ST) cells from term human placentas could be infected when cocultured with HIV-infected lymphocytic cells. Here, we have used fluorescence microscopy and transmission electron microscopy to examine the kinetics of this infection process. Molt-4 clone 8 cells infected with HIV-1(Lai) or filtered supernatant from these cultures were incubated with ST cells for different times. In cell-associated infection, immunofluorescence microscopy revealed that some ST colonies were positive for HIV core proteins (p24,p55) after 1 hr. The number of positive colonies and the intensity of the ST-associated fluorescence increased with time. Transmission electron microscopy showed viral particles with HIV morphology associated with the ST cell surface at 1 hr. Immature virions with budding morphology were observed at 2 hr. In cell- free infection, positive p24,p55 staining was first detected in a few ST colonies at 4 hr. The number of positive colonies increased with time. At 24 hr, the fluorescence pattern and intensity resembled that seen with cell- mediated infection at 4 hr. Transmission electron microscopy revealed an increasing number of viral particles associated with the ST cell plasma membrane with respect to time, and budding virions first appeared at 8 hr. These results demonstrate that HIV infection of placental ST cells proceeds very rapidly in culture and that, furthermore, cell-associated infection of ST is much more efficient than the infection with cell-free virus.