Contributed by guest blogger: Jason Adler
An endoparasitoid wasp would disagree with the popular perception of viruses as malevolent. Parasitoids are organisms that spend a substantial portion of their life cycle in the host; unlike a parasite, a parasitoid usually kills or sterilizes the host. Endoparasitoid wasp oviposit into the body cavity of caterpillars. When the wasp larvae emerges, it then consumes the host as it develops.
Polydnaviruses (PDV), a family of double stranded DNA insect viruses, are symbiotic to some endoparasitoid wasps. Of two PDV genera, genus Ichnovirus is specific to ichneumoid wasps and Bracovirus to braconid wasps. The PDV genome is located on host wasp chromosomes in a segmented, proviral form. However, the integrated PDV genome is not fully functional as it cannot replicate independent of the wasp and capsid proteins are non-existent. It is unknown if PDV is derived from wasp genes or if ancestral wasps integrated a beneficial PDV into their genome with resulting loss of the genes responsible for capsid formation and virus replication.
As such, PDV only replicates at specific ovarian cells during the late pupal phase, where it acquires two viral envelopes. PDV integration does not occur in the viral life cycle; instead, the viral genome is vertically transmitted to wasp offspring during meiosis. When the female wasp injects her eggs into the lepidopteran host, virions are co-injected and result in infection. Although, PDV does not replicate in the host caterpillar, it does result in immunosuppression and alters the host development (i.e. prevents metamorphosis) and metabolism to favor the parasitoid larva. The normal response of lepidopteran larvae to small foreign material is phagocytosis, but larger pathogens must be encapsulated. This is accomplished through melanization, where certain hemocytes, invertebrate immune cells found in the hemolymph, secrete melanin, which surrounds the pathogen so that anti-microbial peptides can destroy it. When immune suppressed, host hemocytes do not destroy the wasp egg by forming hemocyte nodules. Thus, PDV and the wasp share a mutualistic relationship.
Cotesia plutellae, a braconid wasp, possesses a PDV – C. plutellae bracovirus (CpBV) – and parasitizes larvae of the diamond-back moth Plutella xylotsella. Recent research has found that CpBV encodes a viral histone H4 that shares high sequence homology with histone H4 on P. xylostella, except for the last 38 residues comprising the N-terminal tail. Additionally, this viral histone H4 N-terminal tail have been observed in other Cotesia-associated PDVs. It has been suggested that the N-terminal tail is altering gene expression regulation as viral H4 histones less easily detach from DNA than host H4 histones, thereby inhibiting transcription. Is the N-terminal tail of CpBV-H4 causing immunosuppression? The researchers hypothesized that the N-terminal tail is causing the suppression of antimicrobial peptide (AMP) genes.
To examine the effects of CpBV-H4, the researchers constructed two viral recombinants: a WT CpBV-H4 and a truncated CpBV-H4 that lacks the N-terminal tail. After injection of the viral vector into the host caterpillar, RT-PCR was used to look at the expression of putative AMP genes. Although basal expression levels were unchanged, when E. coli was introduced to the host to present an immune challenge CpBV-H4 inhibited inducible expression, while truncated CpBV-H4 did not. Additionally, by counting the number of melanized black nodules on the host caterpillar after injection of E. coli and the viral vector, the researchers assessed the immune response. While the larvae show hemocyte nodule formation in response to E. coli infection, transient expression of CpBV-H4 significantly suppressed the immune response by decreasing nodule formation, while truncated CpBV-H4 had no effect. Finally, the researchers examined a possible synergistic effect of CpBV-H4 and the entomopathogenic bacterium X. nematophila. Without CpBV-H4, X. nematophila infection resulted in low mortality; however, with CpBV-H4, there was significantly increased mortality with this synergistic effect lost if CpBV-H4 was truncated.
Based on these results, the researchers concluded that the N-terminal tail appears to be responsible for immunosuppression by inhibiting inducible expression of AMP genes, possibly by altering a normal epigenetic control. CpBV-H4 containing nucleosomes may less easily detach from DNA during transcription due to the increased positive charge resulting from the increased number of lysine residues in the N-terminal tail. By introducing a virus that expresses a viral H4 histone with a N-terminal tail, the parasitoid wasp is able to suppress the host immune system. This is important as without the immune suppression, the host hemocytes would encapsulate and destroy the wasp egg.
With 157 putative genes, CpBV is likely to have more than this one mechanism to suppress host immunity. Are there other mechanisms of CpBV immune suppression? How else is the complex ecological relationship of wasp, virus, and caterpillar host mediated at the molecular level?
Jason Adler is a senior at Vassar College, majoring in biology.