Control of Innate Antiviral Immunity by HIV-1
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Human immunodeficiency virus 1 (HIV-1) infection continues to be a major public health problem, with 34 million people infected worldwide. Cell-intrinsic innate immune defenses are essential for the control of HIV-1 infection but are subverted by the virus to establish successful infection. Interferon regulatory factor 3 (IRF3) is a central transcription factor of innate immune signaling that is activated by cellular pattern recognition receptors in response to the presence of non-self molecules (e.g. viral RNA or DNA). Activation of IRF3 induces the expression of antiviral and immunomodulatory genes whose products can suppress HIV-1 infection within target cells and regulate the adaptive immune response to infection. We have found that during acute infection HIV-1 evades innate antiviral immunity through the actions of HIV-1 viral protein u (Vpu), which interacts with IRF3 and inhibits its activity. While HIV infection eventually results in proteolytic destruction of IRF3 at later time points of acute infection, we found that inhibition of IRF3-dependent IFN-β transcription by Vpu occurs at early time points. In addition, Vpu blocked both IRF3- and NFκB-dependent activities at the IFN-β promoter. These findings led us to hypothesize that Vpu blocks IRF3 activation to prevent IRF3 from carrying out the necessary biochemical steps to drive antiviral gene expression. We investigated the process of Vpu regulation of IRF3, and found that IRF3 and Vpu form a stable complex during infection of CD4+ T cells with HIV-1. Using truncation and deletion mutants of recombinant IRF3, we mapped the binding epitope for Vpu on IRF3 to a region of IRF3 protein called the IRF association domain. This domain is the site necessary for homodimerization of IRF3 molecules after activation and interaction with transcriptional cofactors. Thus, we hypothesized that Vpu alters IRF3 dimerization and cofactor interaction. Indeed, when we examined the IRF3 activation pathway in the presence of Vpu to identify the site of the Vpu-induced block in IRF3 activity, we found that Vpu inhibited IRF3 dimerization and CBP binding. We predict that Vpu antagonism of IRF3-directed innate immunity is a key step in HIV-1 pathogenesis during acute infection. Further, IRF3 depletion and control of innate antiviral immunity by HIV-1 may correlate with disease progression in HIV-infected patients. To test these predictions, we have developed two novel monoclonal antibodies to human IRF3 to support the study of IRF3 activation and HIV-mediated IRF3 depletion among patient samples in a high-throughput manner. One of these antibodies, AR-1, is specific for activated IRF3. The other, AR-2, detects total IRF3 levels in a flow cytometric assay of blood leukocytes. Use of these new antibodies to study IRF-3 levels during HIV infection could reveal an innate immune correlate of HIV-1 disease progression, while studies to fully define the interaction between Vpu and IRF3 may reveal novel targets for the development of drugs that preserve IRF3 activity during HIV-1 infection.
- Pathobiology