Host and viral factors influencing heterosexual HIV transmission.

The World Health Organization estimates that 28-30 million people have been infected with human immunodeficiency virus (HIV). Although many important questions remain, much has been learned regarding the biology of heterosexual HIV transmission. For example, most sexual transmission of HIV is probably mediated by cell-free virus. Langerhans cells in the vaginal epithelium and dendritic cells or macrophages in the lamina propria are the most likely target cells in HIV transmission. Although there is restriction in the genotypes of viruses that are sexually transmitted, the common phenotypic characteristic of these viruses does not appear to be related to in vitro measures of tropism but rather to the relatively high in vivo replicative fitness of the transmitted viral variants. Clearly, host factors such as the amount of HIV-1 co-receptor expression on host cells and the presence of ulcerative sexually transmitted diseases affect the susceptibility of an individual to HIV infection. The growing body of information has increased the likelihood that a vaccine capable of preventing HIV-1 infection after exposure during sexual activity will be developed in the foreseeable future.