Center for Comparative Medicine
SVM: Pathology, Microbiology & Immunology
California National Primate Research Center
Core Scientist: Infectious Diseases Unit
University of California, Davis
Research Interests: Viral Pathogenesis, Anti-viral Immunity, Mucosal Immunology
Christopher J. Miller, D.V.M., Ph.D. is a Professor of Pathology, Microbiology and Immunology in the School of Veterinary Medicine and an Adjunct Professor of Medicine in the School of Medicine. Dr Miller is a veterinarian and virologist, a core faculty member of the Center for Comparative Medicine and a Staff Scientist at the California National Primate Research Center. His laboratory utilizes non-human primate models of AIDS and influenza A virus infection to define the pathogenesis of these viral infections, study the nature of protective antiviral immunity, and test vaccines and immunotherapeutic strategies to prevent AIDS and influenza.
Biology of HIV TransmissionHIV target cells in the female genital tract. Dendritic cells are key cells in initiating both adaptive and innate antiviral immunity. There has been considerable controversy regarding the role of productively infected dendritic cells (DC) in HIV transmission and we set out to determine the full range of SIV target cells in the female genital tract. These studies determined that all CD4+ cells in the genital mucosa; T cells, macrophages and dendritic cells (including intraepithelial Langerhans cells) become infected within 18 hours of intravaginal SIV inoculation and that virus disseminates rapidly from the genital tract to systemic lymphoid tissues. We have also shown that the genotype of the virus circulating in the blood maybe a recombinant variant between the virus that a person was exposed to and may not necessarily reflect the genotype of a transmitted virus.
AIDS PathogenesisIn early simian immunodeficiency virus (SIV) and human immunodeficiency virus-1 (HIV-1) infections, gut-associated lymphatic tissue (GALT), the largest component of the lymphoid organ system, is a principal site of both virus production and depletion of primarily lamina propria memory CD4+ T cells; that is, CD4-expressing T cells that previously encountered antigens and microbes and homed to the lamina propria of GALT. In a recent paper we showed that peak virus production in gut tissues of SIV-infected rhesus macaques coincides with peak numbers of infected memory CD4+ T cells. Surprisingly, most of the initially infected memory cells were not, as expected activated but were instead immunophenotypically 'resting' cells that, unlike truly resting cells, but like the first cells mainly infected at other mucosal sites and peripheral lymph nodes are capable of supporting virus production. In addition to inducing immune activation and thereby providing activated CD4+ T-cell targets to sustain infection, virus production also triggered an immunopathologically limiting Fas-Fas-ligand-mediated apoptotic pathway in lamina propria CD4+ T cells, resulting in their preferential ablation. Thus, SIV exploits a large, resident population of resting memory CD4+ T cells in GALT to produce peak levels of virus that directly (through lytic infection) and indirectly (through apoptosis of infected and uninfected cells) deplete CD4+ T cells in the effector arm of GALT. The scale of this CD4+ T-cell depletion has adverse effects on the immune system of the host, underscoring the importance of developing countermeasures to SIV that are effective before infection of GALT.
Although a number of preclinical studies in macaques have been published demonstrating vaccine-mediated protection from intravenous SIV or SHIV challenge, there are very few studies that have achieved reproducible protection against intravaginal viral challenge. In non-human primate models of AIDS, live attenuated lentiviruses provide the most reliable protection from systemic and mucosal challenge with pathogenic SIV. Although live attenuated lentiviruses may never be used in humans due to safety concerns, understanding the nature of the protective immune mechanisms induced by live attenuated vaccines in primate models is thought to be critical for developing other vaccine approaches. Approximately 60% of rhesus macaques immunized with nonpathogenic SHIV89.6 are protected from the infection or clinical disease after intravaginal (IVAG) challenge with pathogenic SIVmac239 and that the route of immunization does not affect the efficacy of this vaccine approach. This is the highest level of protection ever reproducibly achieved in an intravaginal SIV challenge system. The protection in this model was associated with anti-viral CD8+ T cell responses and for the first time we showed that innate antiviral immune responses (IFN-alpha) were also associated with the vaccine-mediated protection. In fact, in chronic stages post-infection unregulated expression of IFN-gamma in lymphoid tissues is associated with uncontrolled viral replication due to the continuous recruitment of viral target cells to the sites of replication. In a further study of the same animals, we demonstrated that lymphoid tissues vaccinated-unprotected monkeys had significantly higher beta-chemokine mRNA expression levels and increased number of beta-chemokine positive cells compared to vaccinated-protected animals. Thus, despite considerable in-vitro evidence to the contrary, we found no evidence that beta-chemokines contributed to vaccine-mediated control of viral replication after intravaginal challenge with SIVmac239. Thus a successful HIV vaccine must generate robust specific and innate antiviral immunity and allow the recipient to make adaptive immune responses without the immune activation and inflammation that contributes to viral replication.
Depo-Provera is an injectable synthetic progestin that is widely used by women for hormonal contraception and its use has been associated with increased susceptibility to HIV transmission. To determine if Depo-Provera could affect the efficacy of an HIV vaccine designed to prevent HIV sexual transmission, we designed a preclinical study using our live-attenuated vaccine model. We found that the rate of protection following intravaginalSIVmac239 challenge was significantly lower and the acute post-challenge plasma vRNA levels significantly higher in Depo-Provera-treated/SHIV89.6 immunized monkeys compared to Depo-Provera-naive/SHIV89.6 immunized (SHIV) monkeys. Clearly progesterone treatment prior to intravaginal challenge with pathogenic virus can decrease the efficacy of a model "vaccine" in the primate model of HIV sexual transmission. This study indicates that care should be taken in designing and interpreting the results of preclinical primate vaccine studies that utilize progesterone pretreatment prior to intravaginal challenge with pathogenic virus. Further, Phase III HIV vaccine trial design should consider whether the use of exogenous progestins for contraception can decrease the efficacy of an HIV vaccine in women.