Background: The peptidyl-prolyl isomerase cyclophilin A (CypA) increases the kinetics by which HIV-1 spreads in tissue culture. This was conclusively demonstrated by gene targeting in human CD4⁺ T cells but the role of CypA in HIV-1 replication remains unknown. Though CypA binds to mature HIV-1 CA it is also incorporated into nascent HIV-1 virions via interaction with the CA domain of the Gag polyprotein. These findings raised the possibility that CypA might act at multiple steps of the retroviral life cycle. Disruption of the CA-CypA interaction suggested that producer cell CypA was required for full virion infectivity. However, recent studies indicate that CypA within the target cell regulates HIV-1 infectivity by modulating Trim5α-mediated restriction. Here, we re-examine the relative contribution to HIV-1 replication of producer- and target-cell CypA.

Methods:  We exploit tools that disrupt the HIV-1 CA-CypA interaction:  drugs CsA, MeIle⁴-CsA, sanglifehrin, HIV-1 CA mutants with decreased affinity for CypA, or altered CypA-dependence, HeLa cells with CypA knock-down by RNAi, and Jurkat T cells homozygous for a deletion of the gene encoding CypA.

Results:  CsA administration during assembly or entry of virus particles has additive inhibitory effects on HIV-1 infectivity. Inhibition of infectious HIV-1 virion production by CsA is independent of the CA-CypA interaction. MeIle⁴ CsA and sanglifehrin inhibit infectious HIV-1 virion production via a CA-independent mechanism. Producer cell CypA is not required for production of fully infectious HIV-1 virions. The target cell determines cell line–specific effects of CsA on the replication of HIV-1 CA variants.

Conclusions:  Our results clearly demonstrate that target cell CypA, and not producer cell CypA, is important for HIV-1 CA-mediated function. Inhibition of HIV-1 infectivity resulting from virion production in the presence of CsA occurs independently of the CA-CypA interaction. Furthermore, producer cell CypA is not required for virion infectivity, as virions produced from CypA KD cells are as infectious as virions produced from control cells. We also show that cell line–specific effects of CsA on the replication of HIV-1 CA variants are determined by the target cell. These CsA phenotypes of HIV-1 CA variants result from effects of the drug on target cell CypA.

Keywords: HIV-1; CypA; Restriction