Mechanisms of intracellular resistance to HIV-1

Mammalian cells resist viruses through a collection of mechanisms termed innate or intrinsic immunity. These mechanisms differ from the classical adaptive immune response in which specialized B, T helper and cytolytic T cells recognize foreign antigens and clonally expand upon engagement of their antigen receptor. Intrinsic and innate mechanisms are more generalized. They are present in many different cell types and some are constitutively active in the cell. In some cases, innate immune mechanisms are activated by signals from cytokines such as interferons or from toll-like receptors that warn the cell of the presence of a foreign invader. Viruses have responded over the course of evolution by developing remarkable and diverse ways to escape the adaptive and the innate response.

We are focused on understanding the innate immune mechanisms against retroviruses, in particular, HIV-1 and the related simian immunodeficiency virus, SIV. One focus is the APOBEC3 gene family. APOBEC3 genes encode a family of related proteins (APOBEC3A-H) that have in common that they are cytidine deaminases. The APOBEC3 proteins act as DNA mutators that target the genome of viruses, changing their cytosines to uracil in a chemical reaction termed deamination. One family member, APOBEC3G, is of particular importance because it specifically inhibits HIV-1. APOBEC3G expressed in cells that are targets of HIV-1 infection. When a cell becomes infected with HIV-1 and the virus tries to generate new copies of itself, APOBEC3G molecules can be packaged into new virus particles as they assemble. The packaged APOBEC3G molecules later attack the virus when it copies its genome from RNA into DNA. In a form of evolutionary biological warfare, the virus has developed a protein of its own termed virion infectivity factor (Vif) that binds to APOBEC3G before it has a chance to become packaged into the virus and then shunts it to the proteasome for degradation. We are interested to understand how Vif binds to APOBEC3G, how APOBEC3G is degraded, how the APOBEC3 genes are regulated, why the APOBEC3 genes evolved, why there are so many of them and importantly, how we can target this system for development of a new class of antiretroviral drugs.

We are also trying to uncover new mechanisms of innate immunity. Intriguingly, HIV-1 encodes a another protein that may play a role in neutralizing an innate immune mechanism. Viral protein R (Vpr) is a protein that is conserved in all HIV-1 isolates. Its role in HIV-1 replication and pathogenesis is still not understood. We recently found that in the cell, Vpr, like Vif, is associated with an E3 ubiquitin ligase. These multi-subunit structures act like molecular machines that target specific host proteins for degradation. Interestingly, Vpr is present in HIV-1 particles. We think that after the virus enters a target cell, Vpr staves off attack from a host antiviral protein. We are trying identify the antiviral protein and to understand how Vpr works. Understanding this mechanism may, uncover yet a new target for antiretroviral drug development.