Étude de la fonction anti-apoptotique de la sous-unité R1 de la ribonucléotide réductase des virus de l’herpès simplex

Elimination of infected cells by apoptosis constitutes an ancestral mechanism of host defense against viral infection. Herpes simplex viruses (HSVs) encode several viral factors to counteract the apoptotic antiviral response. Among them, the R1 subunit of HSV type-2 ribonucleotide reductase (HSV-2 R1, also named ICP10), protects cells by interrupting death receptor-mediated signaling at, or upstream of, caspase-8 activation. Since protection against tumor necrosis factor alpha (TNFα)-induced apoptosis is decreased un cells infected with an HSV type-1 R1 null mutant, it has been proposed that HSV-1 R1 (ICP6) could also possess an antiapoptotic activity. The fundamental goal of this thesis is to better understand the mechanism and the potential of the HSV R1s antiapoptotic activity.   In a first study, we investigated the mechanism of the antiapoptotic activity of HSV R1s by using TNFα and Fas ligand (FasL), two death-receptor inducers involved in anti-HSVs immune response. From this work, we report three main findings on the antiapoptotic activity of HSV R1s. First, HSV-1 R1 like HSV-2 R1 has the ability to protect cells against TNFα- and FasL-induced apoptosis. Second, HSV-1 R1 contributes in protecting infected cells against FasL. Third, HSV R1s and procaspase-8 interact in a way that inhibits the dimerization/activation of caspase-8. These results suggest that in addition to counteracting death receptor-induced apoptosis, HSV R1s could inhibit apoptosis induced by other signals that trigger caspase-8 activation during HSV infection. Double-stranded RNA (dsRNA) are viral intermediates notably produced by HSVs and have been shown to induce apoptosis via caspase-8 activation. We tested in a second study whether HSV R1s have the ability to counteract apoptosis triggered by polyriboinosinic : polyribocytidylic acid (poly(I:C)), a synthetic analog of dsRNA that triggers caspase-8 activation. We showed that HSVs infection protect epithelial cells from apoptosis induced by poly(I:C). We established that HSV-1 R1 is essential for the protection of HSV-1-infected cells against poly(I:C)-induced caspase-8 activation. HSV R1s interact not only with procaspase-8 but also with the receptor interacting protein 1 (RIP1). The interaction between RIP1 and HSV-2 R1 inhibits the binding of RIP1 to the Toll/interleukine-1 receptor-domain-containing adapter-inducing interferon β (TRIF), the adaptor of Toll-like receptor 3 that is an extracellular dsRNA sensor, which is required to activate caspase-8 following extracellular poly(I:C) stimulation and TRIF overexpression. Thus, HSV R1s have the ability to inhibit poly(I:C)-induced apoptosis at several levels by preventing caspase-8 dimerization/activation and TRIF RIP1 interaction. This work sheds light on the ability of HSV R1s to manipulate apoptosis. Early during the lytic cycle, protein levels of the cellular inhibitors of caspase-8 as cFLIP drop but HSV R1s accumulate concomitantly and act as a viral inhibitor of apoptosis by binding to procaspase-8 and RIP1 in a way that impairs caspase-8 activation by death-receptors and dsRNA detectors stimulation.

Keyphrases: HSV, HSV, R1, R1, ICP6, ICP6, ICP10, ICP10, apoptosis, caspase-8, caspase-8, RIP1, RIP1, TNF alpha, TNF alpha, FasL, FasL, poly(I:C), poly(I:C)