Primate lentiviruses use at least three alternative strategies to suppress NF-κB-mediated immune activation

Abstract

Primate lentiviruses have evolved sophisticated strategies to suppress the immune response of their host species. For example, HIV-2 and most simian immunodeficiency viruses (SIVs) use their accessory protein Nef to prevent T cell activation and antiviral gene expression by downmodulating the T cell receptor CD3. This Nef function was lost in HIV-1 and other vpu-encoding viruses suggesting that the acquisition of Vpu-mediated NF-κB inhibition reduced the selection pressure for inhibition of T cell activation by Nef. To obtain further insights into the modulation of NF-κB activity by primate lentiviral accessory factors, we analyzed 32 Vpr proteins from a large panel of divergent primate lentiviruses. We found that those of SIVcol and SIVolc infecting Colobinae monkeys showed the highest efficacy in suppressing NF-κB activation. Vpr-mediated inhibition of NF-κB resulted in decreased IFNβ promoter activity and suppressed type I IFN induction in virally infected primary cells. Interestingly, SIVcol and SIVolc differ from all other primate lentiviruses investigated by the lack of both, a vpu gene and efficient Nef-mediated downmodulation of CD3. Thus, primate lentiviruses have evolved at least three alternative strategies to inhibit NF-κB-dependent immune activation. Functional analyses showed that the inhibitory activity of SIVolc and SIVcol Vprs is independent of DCAF1 and the induction of cell cycle arrest. While both Vprs target the IKK complex or a factor further downstream in the NF-κB signaling cascade, only SIVolc Vpr stabilizes IκBα and inhibits p65 phosphorylation. Notably, only de-novo synthesized but not virion-associated Vpr suppressed the activation of NF-κB, thus enabling NF-κB-dependent initiation of viral gene transcription during early stages of the replication cycle, while minimizing antiviral gene expression at later stages. Our findings highlight the key role of NF-κB in antiviral immunity and demonstrate that primate lentiviruses follow distinct evolutionary paths to modulate NF-κB-dependent expression of viral and antiviral genes.

Loss of Nef-mediated CD3 down-regulation in the HIV-1 lineage increases viral infectivity and spread

“…This has led to the notion that loss of Nefmediated CD3 down-regulation was facilitated by the acquisition of a vpu gene in a subset of primate lentiviruses, including HIV-1 (41,42). Vpu suppresses T cell activation by interfering with NF-κB to limit interferon-stimulated gene induction and innate immune responses (41,(43)(44)(45), in addition to antagonizing the restriction factor tetherin (46) and mediating CD4 degradation. By contrast, Nef stimulates IKKβ-mediated NF-κB activation to boost viral LTR activity and increase proviral transcription (43).…”

Species-specific host factors rather than virus-intrinsic virulence determine primate lentiviral pathogenicity

“…In fact, HIV-1 Nef boosts the activation of NF-κB early during the viral life cycle to initiate efficient proviral transcription, while Vpu suppresses NF-κB-dependent antiviral gene expression at later stages 15 . In contrast, SIVagm and most other lentiviruses lacking vpu use Nef-mediated CD3 downmodulation as an alternative strategy to suppress T lymphocyte activation and to minimize antiviral gene expression during later stages of their replication cycle 55 . It is tempting to speculate that the SIVagm GU construct failed to replicate efficiently in vivo because the combination of both suppression of NF-κB activity and CD3 downmodulation may reduce the state of T cell activation below the threshold required for effective viral gene expression.…”

“…HEK293T cells were cotransfected with a firefly luciferase reporter construct under the control of three NF-κB binding sites 55 , a Gaussia luciferase construct 55 for normalization, and expression vectors for a constitutively active mutant of IKKβ 56 and the functional vpu alleles as described 15 . Two days post-transfection, Gaussia luciferase activity in the supernatant was measured using the Gaussia -Juice Kit (p.j.k.)…”

SIVcol Nef counteracts SERINC5 by promoting its proteasomal degradation but does not efficiently enhance HIV-1 replication in human CD4+ T cells and lymphoid tissue

“…HIV-1, its direct simian precursors SIVcpz and SIVgor from chimpanzees and gorillas, respectively, and some closely related SIVs infecting several Cercopithecus species [ 6 , 18 ]. These primate lentiviruses are unable to block TCR-CD3-mediated T cell activation and instead use Vpu to suppress antiviral gene expression by inhibiting activation of the transcription factor NF-κB [ 19 , 20 ]. Most primate lentiviruses lacking Vpu as well as SIVcpz, SIVgor and HIV-1 group O also use Nef to antagonize the restriction factor tetherin to allow efficient release of viral particles from infected cells [ 21 – 24 ].…”