Characterization and identification of retrovirus restriction factors in primates

Mammalian cells produce factors, termed restriction factors, that protect against infection by viruses. The research described here involves the investigation of inherent defenses against retroviral infection of primate cells. Cells derived from numerous primate species restrict infection by various retroviruses, including HIV-1. Owl monkey cells are particularly resistant to HIV-1 infection, a block to infection that is completely reversed by interventions that prevent association of the HIV-1 capsid protein with the cellular protein cyclophilin A (CypA), an interaction that, paradoxically, is required for efficient virus replication in human cells.; In experiments described here, HIV-1 restriction in owl monkey cells is characterized and shown to be a general property of primary cells derived from several individual animals. Restriction is dependent on association of the HIV-1 capsid protein with a cyclophilin protein family member in the target cell. Silencing of CypA production in owl monkey cells abolished the block to HIV-1 infection, but reintroduction of CypA did not restore restriction. This observation prompted a search for CypA variants, leading to the identification of a TRIM5-CypA fusion protein (TRIMCyp) that is necessary for HIV-1 restriction in owl monkey cells and sufficient to confer the non-permissive phenotype to cells derived from other species. TRIMCyp is unique to owl monkeys and was formed by the retrotransposition of a CypA cDNA into the TRIM5 gene.; Human cells are also capable of restricting retrovirus infection, specifically by certain forms of murine leukemia virus (N-MLV). The factor responsible for N-MLV restriction has been proposed to also restrict HIV-1 when the association between the viral capsid protein and CypA is disrupted. To study this proposed restriction, a human cell line that efficiently restricts N-MLV was subjected to selection to isolate subclones that had lost restriction activity. HIV-1 infection of a permissive cell line derived in this manner, 17H1, is independent of CypA, a finding that supports the hypothesis that N-MLV and HIV-1 are both subject to restriction by a common pathway in human cells.