MEETING BRIEF
Cross-Talk Among
AIDS Researchers
The Reproductive Tract
and HIV Transmission

By Karen P. Beckerman, M.D.

(Issue 4; posted March 21, 1997; archived April 4, 1997)

Four hundred liters a day is the amount of HIV-1 infected semen ejaculated around the world, according to Anne Kiessling, Ph.D., of Harvard Medical School. Dr. Kiessling's comments were made during the Reproductive Tract and HIV Transmission conference at the NIH on February 11 and 12, 1997. The conference brought together scientists and clinicians from diverse disciplines, ranging from obstetrics/gynecology and urology to infectious diseases, reproductive immunology, endocrinology, venereology, microbiology, virology, gametogenesis, epidemiology, primatology, and mucosal immunology. Its focus was the organ system that is the major infection conduit of HIV transmission worldwide: the reproductive tract. Work has progressed rapidly on many fronts that has enhanced our understanding of the spread of the HIV epidemic on molecular, cellular, tissue, organ, organism, epidemiologic and geographic levels. Never before, however, have representatives of all these scientific perspectives come together to examine the sexual transmission of HIV in such a comprehensive manner.

Speakers on Day 1 dealt with recent advances in the understanding of immune defense mechanisms in the female reproductive tract and mechanisms by which HIV evades those defenses. Global epidemiologic trends were brought into sharp relief by Max Essex, M.D., of the Harvard School of Public Health, who summarized the marked differences in progression of the epidemic in populations of intravenous drug users and homosexual men, among whom the incidence of new infections has leveled off dramatically, versus women and heterosexual men, who continue to be infected at increasing rates. The variation of transmission of different HIV-1 clades among different populations led Dr. Essex to postulate that genotypic changes influence viral phenotype for transmission efficiency, especially pertaining to such characteristics as receptor affinity, diversity and viral transcriptional efficiency.

Shifting from global molecular epidemiology to the female reproductive tract, William Kutteh, M.D., of the University of Tennessee described the immunologic defenses of the female reproductive tract, the production of antibody and cytokines at different sites of the reproductive mucosa and the influence of cyclic hormonal changes on these defenses. Dr. Kutteh stressed the importance of understanding inductive mechanisms of these responses and their relationship to the development of an effective anti-HIV vaccine.

Throughout the meeting, conference participants were confronted with contradictions. Duane Alexander of the NIH opened the conference stressing "a prayer of thanks" that the virus is so specific to the reproductive tract, not spread via the respiratory route, insect vectors or at each act of intercourse. However, the audience was quickly informed of the emergence of seropositivity in more than 40% of pregnant women in Botswana since 1990. Remarks by Sharon Hillier, Ph.D., of the University of Pittsburgh, stressed the hit-or-miss nature of heterosexual transmission of virus to women whose mucosal immune defenses are compromised by perturbations of the "vaginal microbial ecosystem" - a complex ecologic balance between bacterial and fungal species, immune secretions, mucosal defenses and bacterial products. Causes of such disregulation of this environmental balance consist of a variety of infectious and chemical insults including common conditions such as bacterial vaginosis, trichinosis and genital ulcer disease. Sara Mostad, Ph.D., of the University of Washington School of Medicine, emphasized the importance of such ecologic consideration to the infectivity of the female reproductive tract, and reported soon to be published data showing that cervical-vaginal shedding of HIV-1 infected cells was highly correlated with gonococcal cervicitis, vaginal candidiasis and all levels of vitamin A deficiency in a cohort of infected Kenyan women.

The mechanism of viral entry through the female sexual tract occupied the remainder of the first day. David Eschenbach, M.D., of the University of Washington, declared the 1990s "the decade of the vagina." He provided a comprehensive description of host defenses at the vaginal epithelium, their dependency on ovarian function and exogenous hormones and the extraordinary lack of understanding of cell mediated immune responses in the vagina, including the role of dendritic cells, Langerhans cells and organized lymph node structures in this tissue.

Cell-cell interactions critical to viral entry and early replication were described by David Phillips, Ph.D., of the Population Council and Melissa Pope, Ph.D., of the Rockefeller University. Dr. Pope's work dealt with dendritic cells and T-cells purified from human skin. She found that alone neither cell type supports HIV-1 replication, while mixtures of the two result in abundant viral replication along with syncytium formation, regardless of type of HIV-1 isolate studied, and regardless of cell type used to infect the culture. She emphasized the impressive effect of even rare contaminating dendritic cells or T-cells to mediate this phenomenon. In addition, using a rhesus macaque model of simian immunodeficiency virus (SIV) infection, she demonstrated that dendritic-T cell mixtures derived from skin, tonsillar and vaginal mucosae also support viral replication and an associated syncytium formation.

Dr. Phillips stressed that host entry is a problem faced by all pathogens, and one that must be solved rapidly by sexually transmitted pathogens in the hostile environment of the healthy vaginal mucosa. Using an experimental model of infection of an epithelial cell line by monocytes and T lymphocytes, he presented evidence of epithelial infection by both cell types by distinctly different mechanisms. While both require an essential epithelial adherence step, he and coworker Xin Tan found that epithelial adherence triggered viral budding and secretion from T-cells into sequestered areas of interdigitation between cells, while adherence by monocytes resulted in rapid epithelial invasion by virus laden pseudopods, allowing infected cells to literally crawl through intact epithelium, secreting virus from the leading edge as it advanced. Adding to this scenario of virus laden cellular invaders, Dr. Phillips ended with recent data showing that murine mononuclear blood cells placed in the vaginas of mice can be detected beneath the vaginal epithelium and in iliac lymph nodes within four hours of inoculation, suggesting that both direct infection of epithelial cells and trafficking of mononuclear blood cells may be involved in the sexual transmission of HIV-1.

Alexander Spira of New York University School of Medicine further elaborated on this exciting hypothesis by presenting data on viral dissemination following intravaginal inoculation of rhesus macaques by SIV. Using in situ PCR analysis, his group has shown that lymphocytes and dendritic cells of the cervico/vaginal lamina propria are the first infected cells after vaginal inoculation by free virus, followed by T-cells of the iliac lymph nodes two days after infection, and that systemic infection is demonstrable by day five of infection. Interestingly, they found no viral expression within vaginal epithelium outside of lymph node structures until considerably later in the course of infection.

Michael Norcross, Ph.D., of the U.S. FDA, emphasized the pivotal role played by proteoglycan, glycolipid and adhesion accessory molecules in the regulation of antiviral host defenses while Michael McChesney, Ph.D., of UC Davis, analyzed data on cell mediated immune responses in reproductive tract mucosal tissues and mechanisms of induction of resistance to herpes simplex virus infection in genital tract tissues of mice. Tom Lehner, Ph.D., of Guys and St. Thomas Hospital, London, completed the program with his comprehensive analysis of induction of mucosal immunity in the macaque, stressing the importance of elicitation of CD8 suppressive factor and chemokines in addition to secretory IgA, IgG and cytotoxic CD8 cells.

Day 2 of the conference began with consideration of transmission to and from the male reproductive tract. Martin Dym, Ph.D., of Georgetown School of Medicine, addressed the question of how virus obtains access to semen, examining potential mechanisms of infection of germ cells, supporting stroma and seminal leukocytes. Anne Kiessling's fascinating analysis of the source of HIV in the ejaculate highlighted the overwhelmingly dominant role that infected semen, which contains both cell free and cell associated HIV, has played in the transmission and propagation of HIV disease throughout the world. By studying paired semen and blood samples at the Deaconess Hospital Research Center in Boston, she has persuasively demonstrated that not only are viral burdens and viral infectivity measures of semen and blood not highly correlated, but that analysis of HIV-1 protease amino acid sequence reveals almost complete genetic divergence between virus collected from these two compartments, indicating separation of genital tract virus from systemic virus long ago, early in the infection of these individuals. These findings also implicate the male genital tract as an important sanctuary site for HIV-1 replication.

Michael Coburn, M.D., of Baylor College of Medicine, spoke about the profound urologic pathology associated with HIV infection. He described his group's experience at the Ben Taub Hospital in Houston with the nephrologic, malignant, testicular and neuropathic infectious and other urologic manifestations of HIV infection, noting that 50% of diagnoses of HIV-1 infection in males at his institution are made as part of evaluation of genitourinary conditions. Susan Fiscus, M.D., of the University of North Carolina, examined factors associated with changes of HIV shedding in semen, including anti-retroviral therapy, urethritis and gonococcal infection.

Virological correlates of sexual transmission of lentiretroviruses was examined in detail by four speakers. Roger Pomerantz, M.D., of Jefferson Medical College, reported that reverse transcription can be driven within the HIV-1 viral particle itself, can increase the ability of virus to replicate in stimulated PBMCs, and can be regulated by extracellular factors such as polyamines found inseminal plasma. Christopher Miller, Ph.D., of UC Davis, reported a correlation between in vitro properties of SIV strains and vaginal transmission of these strains. Steven Wolinsky, Ph.D. described his work at Northwestern University studying viral selection during HIV transmission, while C. David Pauza, Ph.D., examined transmission and early virus dissemination in the macaque. Both Drs. Pauza and Wolinsky stressed the finding that small subpopulations of virus are selected during mucosal transmission of both SIV and HIV, whether transmission is effected by free virus or cell associated virus.

Hormonal modulation of the immune system and its effect on transmission of HIV infection was then discussed. Deborah Anderson, Ph.D., of Harvard Medical School described her work on genital tract cytokines as mediators of host resistance, and the potential influence that exogenous and endogenous hormones may have on host mucosal defenses. Charles Wira, Ph.D., of Dartmouth University School of Medicine hypothesized that the entire reproductive tract is immune competent and that HIV may circumvent these defenses at any level, including the uterus. He stressed the fact that sperm can be detected in the fallopian tube within five minutes of intravaginal ejaculation. Preston Marx, Ph.D., of the Aaron Diamond Center, Sheri Hild Petito, Ph.D., of Bioqual and Christine Mauck, M.D., of CONRAD, presented up-to-the-minute data on the effects of Depo-Provera on the integrity of vaginal mucosal defenses and transmission rates in both humans and rhesus macaques. In the discussion that followed, several participants stressed the lack of analysis of estrogen levels in these reports, and the unclear relevance of many of these findings to transmission in humans.

In her discussion of oral transmission of SIV and HIV-1, Ruth Ruprecht, Ph.D., of the Dana-Farber Cancer Research Institute stressed the complexity of human sexual behavior, and examined the significance of her findings that SIV transmits more efficiently via the oral route than the rectal route. A special presentation by Francis Plummer, M.D., of the University of Manitoba, reviewed the mutual facilitation of the sexually transmitted disease epidemic and the HIV epidemic, demonstrating how each has served to fuel the other. His reminder to the group that his original studies of gonococcal infection in sex workers in Nairobi revealed 70% HIV seroprevalence in an era when it was believed that HIV did not infect women was not only sobering, but also effectively demonstrated how far the field has come, and how very far it has to go.

The summary of the conference proceedings provided by Suraiya Rasheed, Ph.D., served to tie together much of the heterogeneous material presented. Unquestionably, the conference displayed the considerable progress made in our understanding of HIV-1 transmission dynamics in the reproductive tract, and the great potential for fruitful collaboration between workers in diverse fields who are attempting to understand and interrupt this vast avenue of HIV transmission. The importance of the massive nature of viral trafficking from and to the genital mucosa was stressed by speakers throughout all sessions of the meeting. Despite the inspiring nature of the progress illustrated in presentation after presentation, the depressing fact remains that this vast traffic has continued to increase all over the world, with no sign of stopping or even slowing down.

Karen P. Beckerman, M.D., is an assistant professor of obstetrics, gynecology and reproductive sciences at University of California, San Francisco, and director of the Bay Area Perinatal AIDS Center at San Francisco General Hospital. As a visiting reseacher at the Gladstone Institute of Virology and Immunology, her current investigative efforts focus on the cellular immunology of maternal to fetal transmission of HIV-1.

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