Virus-vectoring thrips regulate the excessive multiplication of tomato spotted wilt virus using their antiviral immune responses.

The tomato spotted wilt virus (TSWV) is a member of the Tospoviridae family and has an negative/ambisense single-stranded RNA genome. Frankliniella occidentalis and F. intonsa are known to be dominant pests in Capsicum annuum (hot pepper) and can cause damage to the plant either directly by feeding, or indirectly by transmitting TSWV in a persistent and propagative manner, resulting in serious economic damage. This study compared the immune responses of two different thrips species against TSWV infection by transcriptome analysis, which then allowed the assessment of antiviral responses using RNA interference (RNAi). Both adult thrips shared about 90 % of the transcripts in non-viruliferous conditions. Most signal components of the immune pathways were shared by these two thrips species, and their expression levels fluctuated differentially in response to TSWV infection at early immature stages. The functional assays using RNAi treatments indicated that the Toll and JAK/STAT pathways were associated with the antiviral responses, but the IMD pathway was not. The upregulation of dorsal switch protein one supported its physiological role in recognizing TSWV infection and triggering the eicosanoid biosynthetic pathway, which mediates melanization and apoptosis in thrips. In addition, the signal components of the RNAi pathways fluctuated highly after TSWV infection. Individual RNAi treatments specific to the antiviral signalling and response components led to significant increases in the TSWV amount in the thrips, causing virus-induced mortality. These findings suggest that immune signalling pathways leading to antiviral responses are operating in the thrips to regulate TSWV litres to prevent a fatal viral overload. This study also indicates the differential antiviral responses between the TSWV-transmitting F. occidentalis and F. intonsa.

Enhanced baculoviral virulence by suppressing the degradation of an insect immune resolvin, epoxyoctadecamonoenoic acid, in three lepidopteran insects.

Epoxyoctadecamonoenoic acids (EpOMEs) are produced from linoleic acid by a cytochrome P450 monooxygenase (CYP) and play a crucial role in terminating excessive and unnecessary immune responses during the late infection stage in insects. This suggests that an increase in the EpOME level may enhance the virulence of insect pathogens against pests. This study tested this hypothesis using a specific inhibitor against soluble epoxide hydrolase (sEH) to degrade EpOMEs, which leads to elevated endogenous EpOME levels. A baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), was used to infect three different lepidopteran insects (Spodoptera exigua, Maruca vitrata, and Plutella xylostella) by oral feeding or hemocoelic injection treatments. Within one hour, the viral infection induced the expression of three different phospholipase A2 (PLA2) genes and, after 12 h, up-regulated the expressions of CYP and sEH genes in Spodopera exigua. As expected, AcMNPV virulence was suppressed by the addition of arachidonic acid (a catalytic product of PLA2) but was enhanced by the addition of either of the EpOME regioisomers. In addition, treatment with a specific sEH inhibitor (AUDA) increased AcMNPV virulence against three different lepidopteran insects, presumably by increasing endogenous EpOME levels. This enhanced effect of EpOMEs on virulence was further supported by specific RNA interference (RNAi), in which RNAi specific to CYP expression decreased AcMNPV virulence while a specific RNAi against sEH expression significantly enhanced virulence. In response to AcMNPV infection, TUNEL assay results showed that S. exigua larvae exhibited apoptosis in the midgut, fat body, and epidermis. Inhibition of apoptosis by a pan-caspase inhibitor, Z-VAD-FMK, significantly increased virulence. Similarly, the addition of AUDA to the viral treatment suppressed the gene expression of five inducible caspases and cytochrome C to suppress apoptosis, which led to a significant increase in the tissue viral titers. These results indicate that EpOMEs play a role in terminating excessive and unnecessary immune responses against viral infection during the late stage by down-regulating antiviral apoptosis in lepidopteran insects.

Phospholipase A2 activity is required for immune defense of European (Apis mellifera) and Asian (Apis cerana) honeybees against American foulbrood pathogen, Paenibacillus larvae.

Honeybees require an efficient immune system to defend against microbial pathogens. The American foulbrood pathogen, Paenibacillus larvae, is lethal to honeybees and one of the main causes of colony collapse. This study investigated the immune responses of Apis mellifera and Apis cerana honeybees against the bacterial pathogen P. larvae. Both species of honeybee larvae exhibited significant mortality even at 102 103 cfu/mL of P. larvae by diet-feeding, although A. mellifera appeared to be more tolerant to the bacterial pathogen than A. cerana. Upon bacterial infection, the two honeybee species expressed both cellular and humoral immune responses. Hemocytes of both species exhibited characteristic spreading behaviors, accompanied by cytoskeletal extension along with F-actin growth, and formed nodules. Larvae of both species also expressed an antimicrobial peptide called apolipophorin III (ApoLpIII) in response to bacterial infection. However, these immune responses were significantly suppressed by a specific inhibitor to phospholipase A2 (PLA2). Each honeybee genome encodes four PLA2 genes (PLA2A ~ PLA2D), representing four orthologous combinations between the two species. In response to P. larvae infection, both species significantly up-regulated PLA2 enzyme activities and the expression of all four PLA2 genes. To determine the roles of the four PLA2s in the immune responses, RNA interference (RNAi) was performed by injecting gene-specific double stranded RNAs (dsRNAs). All four RNAi treatments significantly suppressed the immune responses, and specific inhibition of the two secretory PLA2s (PLA2A and PLA2B) potently suppressed nodule formation and ApoLpIII expression. These results demonstrate the cellular and humoral immune responses of A. mellifera and A. cerana against P. larvae. This study suggests that eicosanoids play a crucial role in mediating common immune responses in two closely related honeybees.