Cyclophilin A protects HIV-1 from restriction by human TRIM5α

Abstract

The capsid (CA) protein lattice of HIV-1 and other retroviruses encases viral genomic RNA and regulates steps that are essential to retroviral invasion of target cells, including reverse transcription, nuclear trafficking, and integration of viral cDNA into host chromosomal DNA1. Cyclophilin A (CypA), the first cellular protein reported to bind HIV-1 CA2, has interacted with invading lentiviruses related to HIV-1 for millions of years3-7. Disruption of the CA-CypA interaction decreases HIV-1 infectivity in human cells8-12, but stimulates infectivity in non-human primate cells13-15. Genetic and biochemical data suggest that CypA interaction with CA protects HIV-1 from a restriction factor in human cells16-20. Discovery of the CA-specific restriction factor TRIM521, and of TRIM5-CypA fusion genes that were independently generated at least four times in phylogeny4,5,15,22-25, pointed to human TRIM5 as the CypA-sensitive restriction factor. However, significant HIV-1 restriction by human TRIM521, let alone inhibition of such activity by CypA26, has not been detected. Here, exploiting reverse genetic tools optimized for primary human CD4+ T cells, macrophages, and dendritic cells, we demonstrate that disruption of the CA-CypA interaction renders HIV-1 susceptible to restriction by human TRIM5, with the block occurring before reverse transcription. Identical findings were obtained with single-cycle vectors or with replication-competent HIV-1, including sexually-transmitted clones from sub-Saharan Africa. Endogenous TRIM5 was observed to associate with virion cores as they entered the macrophage cytoplasm, but only when the CA-CypA interaction was disrupted. These experiments resolve the long-standing mystery of the role of CypA in HIV-1 replication by demonstrating that this ubiquitous cellular protein shields HIV-1 from previously inapparent, but potent inhibition, imposed by human TRIM5. Hopefully this reinvigorates development of CypA-inhibitors for treatment of HIV-1 and other CypA-dependent pathogens27-30.