Sexual transmission of human immunodeficiency virus: virus entry into the male and female genital tract.
This review of the risks of heterosexual transmission of HIV examines what is known about routes of viral entry into the ejaculate, and into female reproductive tract secretions, routes of viral passage from the ejaculate into the female, and from female reproductive tract secretions. The risk of transmission of HIV during heterosexual exposure is much lower than that of gonorrhea or hepatitis B (about 25%), estimated at about 0.3% per event. Probably certain factors, such as a male IVDU partner, greatly increase this risk. Most of the HIV in semen resides in the CD4-positive lymphocytes and macrophages, of which there are about 1 million per ejaculate. Greater risk of transmission occurs in men with STDs because there are more white blood cells, disrupted genital epithelium with activated white cells at the site, immunosuppression in some STDs, and facilitated HIV disease process with some STDs, notably cytomegalovirus. The site of uptake of HIV into the female reproductive tract is not known, whether the vagina, cervix or uterus. Exposure to semen causes local immunosuppression, especially in the rectum. Whether use of oral contraceptives, IUDs, diaphragms, or sponges affects HIV binding to the female tract remains to be studied. No data exist on the effects of vaginal spermicides, either the commonly used nonoxynol-9 or other substances known to be potent virucides, on HIV survival in vivo. One study in rhesus monkeys showed that the 100% killing of HIV seen in vitro is reduced to 50% in vivo. Mucosal cells are known to stimulate budding of mature HIV virions from CD4- lymphocytes during mutual contact. The tissue source of HIV in the sexual secretions of HIV-positive women, whether blood, cervical or vaginal secretions, is unknown. How HIV enters the male tract during exposure is unknown, although lack of circumcision is a risk factor. Possible animal models for research on these issues are discussed. Research is needed to determine the site of entry of HIV into reproductive tissues, risks throughout the menstrual cycle, and protective effects of contraceptives as result of this knowledge.