APOBEC3+ Retrovirus Particles as B-Cell Immunogens
Outside the Box AIDS Vaccine Award
Co-PI - Mario Santiago, PhD
Eliciting potent and broadly-neutralizing antibodies remains one of the most challenging goals in HIV vaccine development. Understanding the basic immunological mechanisms that influence this response is pivotal for the future design of successful immunogens. Accordingly, we pursued studies of a classical resistance trait known as Recovery from Friend Virus 3 or Rfv3. Rfv3 corresponds to a single autosomal dominant gene that stimulates the production of FV-specific neutralizing antibodies. These antibodies allow certain strains of mice to survive FV infection. Although Rfv3 was first detected 30 years ago, the gene encoding this resistance trait remained unidentified. We recently demonstrated that Rfv3 is encoded by the Apobec3 gene. Apobec3 is a potent innate restriction factor that counteracts HIV-1 and other retroviruses (Santiago ML et al. 2008. Science 321: 1343-6). In addition, mouse strains that develop poor neutralizing antibody responses were found to exhibit aberrant splicing of Apobec3 mRNA, leading to its functional impairment. These findings highlight a surprising genetic link between the innate and humoral arms of the immune response involving Apobec3 that may have important implications for HIV-1 vaccine development. During HIV-1 infection, expression of Vif promotes rapid proteasome-mediated degradation of the human Apobec3G and Apobec3F proteins. The targeted destruction of these cytidine deaminases, like aberrant splicing of Apobec3 mRNA in susceptible mice, could contribute to the poor neutralizing antibody responses commonly observed in HIV-1 infected individuals. Based on this discovery, we recently submitted a proposal to explore how the antiviral activity of Apobec3 during the acute phase of infection reduces early immunopathology and facilitates neutralizing antibody development. However, these studies do not explore the exciting concept that Apobec3 incorporation into virus particles not only renders these particles noninfectious in the next target cell, but also imbues these viral particles with the ability to elicit potent neutralizing antibodies. We now propose to test the hypothesis that Apobec3+ noninfectious virions represents a natural virus antigen formulation that can uniquely promote potent neutralizing antibody responses in vivo. Interestingly, Apobec3+ particles resemble inactivated virus-like particles (VLPs), immunogens that are currently being studied for their ability to elicit neutralizing antibody responses against the envelope trimer. However, most of these VLP configurations involve immature rather than mature virions that have failed thus far to yield promising virus neutralization titers in animal model systems.
In this one-year proposal, we propose to investigate whether Apobec3+ Friend retrovirus virions are indeed released in substantial numbers in vivo and drive antigen-specific B cell development (Specific Aim 1). We further propose to compare and contrast the ability of Apobec3+ virions versus VLPs to elicit neutralizing antibodies in mice. VLPs will be produced by various strategies including chemical inactivation of viral particles, pseudotyping of virions and co-transfection of codon-optimized Gag and Env constructs (Specific Aim 2). Immunized mice will then be challenged with wild-type FV and the impact on disease induction will be monitored. These studies will help discern whether Apobec3+ noninfectious virions harbor immunogenic properties that effectively promote neutralizing antibody development. If so, these results could propel the development and testing of related HIV vaccine candidates in monkeys and humans. Should high titer neutralizing antibodies be produced in these systems, the identification of the envelope epitopes recognized by these antibodies could be very instructive for the future design of HIV-1 vaccines.