While an undergraduate student I developed an interest in reproductive pathology and placentation, due to a unique opportunity to work at the San Diego Zoo Research Department assisting Dr. K. Benirshke. After receiving a Doctor of Veterinary Medicine degree at UC Davis, I completed my Ph.D. training in virology and comparative pathology at the same institution. My thesis project was to develop an animal model of heterosexual HIV transmission using cell-free simian immunodeficiency virus (SIV) to intravaginally inoculate rhesus monkeys and to then characterize the biology of vaginal SIV transmission. This work resulted in the first paper describing vaginal SIV transmission, "Genital mucosal transmission of simian immunodeficiency virus: animal model for heterosexual transmission of human immunodeficiency virus" (J Virol. 63[10]: 4277-84, October 1989).

“Our interest in carefully defining the host-virus relationship and developing better human vaccines extends beyond AIDS to other human pathogens such as influenza”

The primary focus of our lab is to illuminate the mechanisms of HIV transmission, dissemination, and pathogenesis, and to understand the nature of effective antiviral immune responses in the genital tract so that effective vaccines, or at least barrier methods, can be rationally developed to prevent virus transmission. Our interest in carefully defining the host-virus relationship and developing better human vaccines extends beyond AIDS to other human pathogens such as influenza.

This paper provided the first clear description of productive infection of "resting or minimally activated" CD4+ T cells by HIV and SIV in vivo. However, it is important to note that I was the 10th of 24 authors on this paper and my contribution was limited to the non-human primate studies. Dr. Ashley Haase, the senior author on the paper, established the team of animal model and clinical scientists that was required to establish these findings in both HIV-infected people and SIV-infected macaques.

Previous in-vitro studies had shown that the T-cell activation was necessary to support productive HIV and SIV infection, but in these studies we were able to clearly detect viral infection and RNA production in CD4+ T cells that were not in the cell cycle. This finding has significance for understanding the cell dynamics involved in sexual transmission of HIV; as stated in the paper, "The ability to infect and replicate in resting or perhaps minimally activated T cells could be particularly advantageous at transmission and in the early stages of infection." The results of the in-vivo studies also raise questions regarding the applicability of in-vitro studies of HIV replication.

This paper was the work of the first graduate student in my lab, Dr. Jinjie Hu. The set of experiments reported in this paper convincingly demonstrated that SIV must be in contact with the intact cervicovaginal mucosa for at least 30-60 minutes for transmission to occur. We were also able to show that intraepithelial Langerhans cells, as well as lamina propria CD4+ T cells and macrophages, were productively infected (SIV RNA+) within 18-24 hours of intravaginal inoculation. This finding provides an explanation for how HIV can cross an intact stratified squamous epithelium. We also found that SIV RNA+ cells reach the lymph nodes draining the genital tract as soon as 18 hours after virus exposure. Such rapid virus dissemination may prove a daunting challenge in the effort to produce an effective vaccine.

We have now completed and published the results of a large study that details the spread of SIV from the genital tract to the systemic lymphoid tissues and characterizes the nature and timing of the host immune responses in tissues as the virus disseminates ("Propagation and dissemination of infection after vaginal transmission of simian immunodeficiency virus," J Virol. 79[14]: 9217-27, July 2005). The primary findings are that 1) although SIV rapidly disseminates after vaginal transmission, high-level viral replication is delayed until 5-6 days after transmission, and 2) that the earliest detectable innate and adaptive immune responses to the virus are coincident with the onset of high-level viral replication.

This window between infection and the high-level viral replication characteristic of later stages of infection seems to offer an opportunity for pre-existing vaccine-induced immune responses or even antiretroviral drugs to abort, or at least constrain, viral replication. As viral replication is the driver of HIV transmission and progression to AIDS, these approaches have the potential to blunt the AIDS epidemic.

Our initial attempts in this regard were unsuccessful and we actually increased viral replication in rhesus monkeys after inducing innate immune responses in the genital tract prior to vaginal SIV inoculation ("The Toll-like receptor 7 [TLR7] agonist, imiquimod, and the TLR9 agonist, CpG ODN, induce antiviral cytokines and chemokines but do not prevent vaginal transmission of simian immunodeficiency virus when applied intravaginally to rhesus macaques," J. Virol. 79[22]: 14355-70, November 2005). While these experiments did not lead to the identification of a prevention strategy, the results emphasized the key role of host inflammation in SIV and HIV transmission and replication.

  If you are free to discuss them, please tell us about your current projects.

We have developed a model of vaccine-mediated protection from intravaginal SIV transmission in which about 50% of monkeys immunized with an attenuated lentivirus control virus replication after intravaginal SIV challenge. We are currently assessing immune responses and viral replication in the tissues of these animals to understand the mechanisms of vaccine-mediated protection in this model. Although safety concerns prevent the use of live-attenuated HIV vaccines in people, designing a vaccine to elicit the specific immune responses associated with protection in this model would be a rational approach to HIV vaccine development. In this regard we have already found that the attenuated vaccine virus elicits strong, polyfunctional SIV-specific T-cell responses in the cervicovaginal mucosa that are present at the time of SIV challenge.

  What are your expectations for this particular field in five or ten years?

HIV has proven to be a highly adaptable pathogen that is not vulnerable to conventional strategies of vaccination. Given the recent failed clinical trials of anti-HIV microbicides and vaccines, it seems that empirical approaches are unlikely to be successful and a greater emphasis on understanding basic mechanisms of HIV transmission and effective antiviral immunity is needed to develop effective strategies to limit the AIDS pandemic.

Christopher J. Miller, DVM, Ph.D., Professor
Department of Pathology, Microbiology and Immunology
California National Primate Research Center
School of Veterinary Medicine
University of California
Davis, CA, USA