Cyclophilin A protects HIV-1 from restriction by human TRIM5α

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 DNA. Cyclophilin A (CypA), the first cellular protein reported to bind HIV-1 CA, has interacted with invading lentiviruses related to HIV-1 for millions of years. Disruption of the CA-CypA interaction decreases HIV-1 infectivity in human cells, but stimulates infectivity in non-human primate cells. Genetic and biochemical data suggest that CypA interaction with CA protects HIV-1 from a restriction factor in human cells. Discovery of the CA-specific restriction factor TRIM5α, and of TRIM5-CypA fusion genes that were independently generated at least four times in phylogeny, pointed to human TRIM5α as the CypA-sensitive restriction factor. However, significant HIV-1 restriction by human TRIM5α, let alone inhibition of such activity by CypA, 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 TRIM5α, 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 TRIM5α 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 TRIM5α. Hopefully this reinvigorates development of CypA-inhibitors for treatment of HIV-1 and other CypA-dependent pathogens.