Microbiota-Dependent Priming of Antiviral Intestinal Immunity in Drosophila

Abstract

Summary Enteric pathogens must overcome intestinal defenses to establish infection. In Drosophila, the ERK signaling pathway inhibits enteric virus infection. The intestinal microflora also impacts immunity but its role in enteric viral infection is unknown. Here we show that two signals are required to activate antiviral ERK signaling in the intestinal epithelium. One signal depends on recognition of peptidoglycan from the microbiota, particularly from the commensal Acetobacter pomorum, which primes the NF-kB-dependent induction of a secreted factor, Pvf2. However, the microbiota is not sufficient to induce this pathway; a second virus-initiated signaling involving release of transcriptional paused genes mediated by the kinase Cdk9 is also required for Pvf2 production. Pvf2 stimulates antiviral immunity by binding to the receptor tyrosine kinase PVR, which is necessary and sufficient for intestinal ERK responses. These findings demonstrate that sensing of specific commensals primes inflammatory signaling required for epithelial responses that restrict enteric viral infections.

Defense Mechanisms against Viral Infection in <em>Drosophila</em>: RNAi versus Non-RNAi

“…When these studies were extended to the gut epithelium, it was observed that priming by the microbiota was necessary for robust induction of Pvf2 in the intestinal epithelium following viral infection. The mechanism of priming of the microbiota however requires components of the Imd pathway (Imd, Tak1) and the NF-κB factor Relish, but not the Toll pathway [110], which seems in contrast to the previously mentioned study [94]. Both studies also found different effects of antibiotics on the antiviral defense mechanism (no effect: [94]; proviral effect: [110]).…”

“…The necessity for tight control of immune pathways to prevent pathological damage from excessive activation has also been reported for the JAK/STAT pathway and is related to the concept of virus "tolerance" rather than "resistance" ( [79,151,152]; see also section 3.4). As already mentioned in section 3.2.4, RNAi screens identified a signaling circuit involving PVR, its ligand Pvf2 and ERK signaling, that controls DCV and arbovirus (including SINV) infections [110]. When these studies were extended to the gut epithelium, it was observed that priming by the microbiota was necessary for robust induction of Pvf2 in the intestinal epithelium following viral infection.…”

“…Because of their importance to human health, emphasis was placed on arboviruses that are transmitted by mosquitoes but also can establish persistent infections in Drosophila cell lines and adult flies, such as SINV (Alphaviridae), RVFV and La Crosse virus (LACV) (Bunyaviridae), Dengue virus (DENV) and different strains of West Nile virus (WNV), such as the Kunjin strain (KUN) (Flaviviridae) and VSV (Rhabdoviridae) [109]. The Drosophila-specific pathogen DCV was also often included in the screens because it could serve as a model for pathogenic enterovirus infections in mammals [110], while other Drosophila-(Nora virus) or insect-specific viruses (FHV) were rarely included. Besides genome-wide screens, also targeted screens were performed, for instance targeting specific signaling pathways, immune genes, RNA helicases, RNA metabolism and endoplasmic reticulum (ER)-associated proteins [99,[111][112][113][114].…”

“…Further studies demonstrated the involvement of the nucleoporin Nup98, that also can function in transcription, and the transcription factor FoxK in the induction of the antiviral program [134,135], while also an antiviral function could be demonstrated for some of the induced genes such as the splicing factor B52 which was analyzed in more detail [135]. Targeted RNAi screens also identified the receptor tyrosine kinase PVR (Drosophila PDGF/VEGF receptor) and the ERK pathway as a broad-acting antiviral defense mechanism (restricting DCV, VSV, SINV and DENV; Table 1) [110,111]. Interestingly, in the intestinal epithelium this pathway is dependent on the gut microbiota that act by priming the gene encoding Pvf2, a ligand for the PVR receptor kinase, to be able to respond rapidly following viral infection through a mechanism that also involves transcriptional pausing [110].…”

“…Targeted RNAi screens also identified the receptor tyrosine kinase PVR (Drosophila PDGF/VEGF receptor) and the ERK pathway as a broad-acting antiviral defense mechanism (restricting DCV, VSV, SINV and DENV; Table 1) [110,111]. Interestingly, in the intestinal epithelium this pathway is dependent on the gut microbiota that act by priming the gene encoding Pvf2, a ligand for the PVR receptor kinase, to be able to respond rapidly following viral infection through a mechanism that also involves transcriptional pausing [110]. The antiviral immune mechanism in the gut epithelium is also dependent on the Imd pathway and will be discussed in greater detail below.…”