Drosophilosophical: Re-thinking Adaptive Immunity in the Fly.

For decades, flies have been a model for innate immunity. In this issue of Cell, Tassetto et al. describe a mechanism for antiviral RNAi spreading that parallels mammalian adaptive immunity through reverse-transcribed vDNA circles and the systemic dissemination of small-RNA-containing exosomes.

p38b and JAK-STAT signaling protect against Invertebrate iridescent virus 6 infection in Drosophila.

The fruit fly Drosophila melanogaster is a powerful model system for the study of innate immunity in vector insects as well as mammals. For vector insects, it is particularly important to understand all aspects of their antiviral immune defenses, which could eventually be harnessed to control the transmission of human pathogenic viruses. The immune responses controlling RNA viruses in insects have been extensively studied, but the response to DNA virus infections is poorly characterized. Here, we report that infection of Drosophila with the DNA virus Invertebrate iridescent Virus 6 (IIV-6) triggers JAK-STAT signaling and the robust expression of the Turandots, a gene family encoding small secreted proteins. To drive JAK-STAT signaling, IIV-6 infection more immediately induced expression of the unpaireds, a family of IL-6-related cytokine genes, via a pathway that required one of the three Drosophila p38 homologs, p38b. In fact, both Stat92E and p38b were required for the survival of IIV-6 infected flies. In addition, in vitro induction of the unpaireds required an NADPH-oxidase, and in vivo studies demonstrated Nox was required for induction of TotA. These results argue that ROS production, triggered by IIV-6 infection, leads to p38b activation and unpaired expression, and subsequent JAK-STAT signaling, which ultimately protects the fly from IIV-6 infection.

Complement Activation by Adeno-Associated Virus-Neutralizing Antibody Complexes.

Treatment of monogenetic disorders using vectors based on adeno-associated virus (AAV) is an area of intense interest. AAV is non-pathogenic human virus, and preexisting capsid antibodies are prevalent in the population posing a challenge to the safety and efficacy of AAV-mediated gene therapies. In this study, we investigated the risk of AAV-mediated complement activation when sera from a cohort of human donors were exposed to AAV9 capsid. Seropositive donor sera carrying neutralizing antibodies from a previous environmental exposure activated complement when admixed with AAV9 capsids and complement activation was associated with donors who had higher levels of anti-AAV IgG1 antibodies. These findings were consistent with mass spectrometry analysis that identified increased binding of immunoglobulins and complement factors when AAV9 capsids were admixed with seropositive sera. Finally, complement activation was abrogated after IgG-depletion using affinity columns or serum pretreatment with an IgG degrading enzyme. Overall, these results demonstrate an important role of preexisting neutralizing antibodies in activating complement; a risk that can be mitigated by using adequate immunosuppression strategies when dosing seropositive patients with vector.

IIV-6 Inhibits NF-κB Responses in Drosophila.

The host immune response and virus-encoded immune evasion proteins pose constant, mutual selective pressure on each other. Virally encoded immune evasion proteins also indicate which host pathways must be inhibited to allow for viral replication. Here, we show that IIV-6 is capable of inhibiting the two Drosophila NF-κB signaling pathways, Imd and Toll. Antimicrobial peptide (AMP) gene induction downstream of either pathway is suppressed when cells infected with IIV-6 are also stimulated with Toll or Imd ligands. We find that cleavage of both Imd and Relish, as well as Relish nuclear translocation, three key points in Imd signal transduction, occur in IIV-6 infected cells, indicating that the mechanism of viral inhibition is farther downstream, at the level of Relish promoter binding or transcriptional activation. Additionally, flies co-infected with both IIV-6 and the Gram-negative bacterium, Erwinia carotovora carotovora, succumb to infection more rapidly than flies singly infected with either the virus or the bacterium. These findings demonstrate how pre-existing infections can have a dramatic and negative effect on secondary infections, and establish a Drosophila model to study confection susceptibility.

A single vertebrate DNA virus protein disarms invertebrate immunity to RNA virus infection.

Virus-host interactions drive a remarkable diversity of immune responses and countermeasures. We found that two RNA viruses with broad host ranges, vesicular stomatitis virus (VSV) and Sindbis virus (SINV), are completely restricted in their replication after entry into Lepidopteran cells. This restriction is overcome when cells are co-infected with vaccinia virus (VACV), a vertebrate DNA virus. Using RNAi screening, we show that Lepidopteran RNAi, Nuclear Factor-κB, and ubiquitin-proteasome pathways restrict RNA virus infection. Surprisingly, a highly conserved, uncharacterized VACV protein, A51R, can partially overcome this virus restriction. We show that A51R is also critical for VACV replication in vertebrate cells and for pathogenesis in mice. Interestingly, A51R colocalizes with, and stabilizes, host microtubules and also associates with ubiquitin. We show that A51R promotes viral protein stability, possibly by preventing ubiquitin-dependent targeting of viral proteins for destruction. Importantly, our studies reveal exciting new opportunities to study virus-host interactions in experimentally-tractable Lepidopteran systems.