Cyclophilin A protects HIV-1 from restriction by human TRIM5α

The HIV-1 capsid (CA) protein lattice encases viral genomic RNA and regulates steps essential to target-cell invasion 1 . Cyclophilin A (CypA) has interacted with the CA of lentiviruses related to HIV-1 for millions of years 2 – 7 . Disruption of the CA−CypA interaction decreases HIV-1 infectivity in human cells 8 – 12 but stimulates infectivity in non-human primate cells 13 – 15 . Genetic and biochemical data suggest that CypA protects HIV-1 from a CA-specific restriction factor in human cells 16 – 20 . Discovery of the CA-specific restriction factor tripartite-containing motif 5α (TRIM5α) 21 and multiple, independently derived, TRIM5−CypA fusion genes 4 , 5 , 15 , 22 – 26 pointed to human TRIM5α being the CypA-sensitive restriction factor. However, HIV-1 restriction by human TRIM5α in tumour cell lines is minimal 21 and inhibition of such activity by CypA has not been detected 27 . Here, by exploiting reverse genetic tools optimized for primary human blood cells, we demonstrate that disruption of the CA−CypA interaction renders HIV-1 susceptible to potent restriction by human TRIM5α, with the block occurring before reverse transcription. Endogenous TRIM5α associated with virion cores as they entered the 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 restriction by human TRIM5α. This study answers the long-standing question of why the interaction between cyclophilin A (CypA) and HIV-1 capsid (CA) protein stimulates HIV-1 infectivity in human cells. Disruption of the CA−CypA interaction renders HIV-1 susceptible to potent restriction by human TRIM5α in primary blood cells, which occurs before reverse transcription.