175 Cyclophilin and Innate Resistance to HIV-1 Jeremy Luban Columbia Univ, New York, NY, USA

Capsid (CA)-specific restriction factors are determinants of retroviral tropism in mammalian cells. These dominant factors act after viral entry but before integration into the host genome. The archetype of such factors is murine Fv1, which restricts infection by MLV-strains bearing particular CA residues. As part of a strategy intended to identify Fv1 from MLV CA binding proteins, it was fortuitously discovered that HIV-1 CA binds cyclophilin A (CypA). Among retroviral CAs, interaction with CypA is unique to HIV-1. Genetic determinants in CA that promote interaction with CypA correlate with determinants that preclude HIV-1 replication in non-human primate cells. Failure of HIV-1 to replicate in macaque, baboon, or vervet cells, though, was not explained by species-specific differences in CypA sequence. Disruption of CypA in human target cells decreased HIV-1 infectivity but had no effect on the titer of HIV-2, SIV, or MLV. Paradoxically, disruption of CypA partially rescued HIV-1 from restriction in macaque and vervet cells, and completely released HIV-1 from restriction in owl monkey cells. Reintroduction of CypA protein to macaque cells in which CypA mRNA had been knocked down by RNAi restored HIV-1 restriction. Owl monkey restriction was not restored by reintroduction of CypA prompting a search for additional owl monkey mRNAs that respond to CypA-specific RNAi. A TRIM5–CypA fusion was cloned that accounts for all HIV-1 restriction activity in owl monkey cells and is sufficient to transfer restriction to otherwise infectable human, rat, or cat cells. TRIMCyp is unique to owl monkeys and arose from LINE-1 retrotransposon-mediated insertion of a CypA cDNA into the TRIM5 locus. Simultaneous with the identification of TRIMCyp, the Sodroski lab reported the discovery of macaque TRIM5a in an expression screen for HIV-1 restriction factors. In macaque and vervet cells, HIV-1 replication is inhibited by both TRIM5a and by CypA, though these two factors function via independent mechanisms. Human TRIM5a also inhibits HIV-1 but CypA seems to promote HIV-1 replication in human cells by protecting the virus from an unknown restriction factor. Many labs are currently attempting to elucidate the mechanisms of retroviral restriction.