Thromboxane Mobilizes Insect Blood Cells to Infection Foci

Abstract

Innate immune responses are effective for insect survival to defend against entomopathogens including a fungal pathogen, Metarhizium rileyi, that infects a lepidopteran Spodoptera exigua. In particular, the fungal virulence was attenuated by cellular immune responses, in which the conidia were phagocytosed by hemocytes (insect blood cells) and hyphal growth was inhibited by hemocyte encapsulation. However, the chemokine signal to drive hemocytes to the infection foci was little understood. The hemocyte behaviors appeared to be guided by a Ca2+ signal stimulating cell aggregation to the infection foci. The induction of the Ca2+ signal was significantly inhibited by the cyclooxygenase (COX) inhibitor. Under the inhibitory condition, the addition of thromboxane A2 or B2 (TXA2 or TXB2) among COX products was the most effective to recover the Ca2+ signal and hemocyte aggregation. TXB2 alone induced a microaggregation behavior of hemocytes under in vitro conditions. Indeed, TXB2 titer was significantly increased in the plasma of the infected larvae. The elevated TXB2 level was further supported by the induction of phospholipase A2 (PLA2) activity in the hemocytes and subsequent up-regulation of COX-like peroxinectins (SePOX-F and SePOX-H) in response to the fungal infection. Finally, the expression of a thromboxane synthase (Se-TXAS) gene was highly expressed in the hemocytes. RNA interference (RNAi) of Se-TXAS expression inhibited the Ca2+ signal and hemocyte aggregation around fungal hyphae, which were rescued by the addition of TXB2. Without any ortholog to mammalian thromboxane receptors, a prostaglandin receptor was essential to mediate TXB2 signal to elevate the Ca2+ signal and mediate hemocyte aggregation behavior. Specific inhibitor assays suggest that the downstream signal after binding TXB2 to the receptor follows the Ca2+-induced Ca2+ release pathway from the endoplasmic reticulum of the hemocytes. These results suggest that hemocyte aggregation induced by the fungal infection is triggered by TXB2via a Ca2+ signal through a PG receptor.

Immune responses of the Asian onion moth, Acrolepiopsis sapporensis, and their genetic factors from RNA‐Seq analysis

“…Instead of COX, specific peroxidases behave like COX in insects (Park et al, 2014). PGH 2 is then isomerized into different PGs using PGES (Ahmed et al, 2018), PGDS (Sajjadian et al, 2020), PGIS (Ahmed et al, 2021), and TXAS (Roy et al, 2021) in S. exigua . This PG biosynthetic pathway was also found in A. sapporensis by identification of the ortholog genes.…”

“…Different PGs and their biosynthetic pathways have been identified in insects (Ahmed & Kim, 2021; Sajjadian et al, 2020); PGD 2 and PGE 2 induce immune responses, while PGI 2 suppresses these responses (Ahmed et al, 2021). A prostanoid such as thromboxane A 2 mediates hemocyte migration to the infection foci (Roy et al, 2021). All four EETs have been identified in a lepidopteran Spodoptera exigua and known to mediate cellular immune responses (Vatanparast et al, 2020).…”

Parallel pheromone, metabolite, and lipid analyses reveal patterns associated with early life transitions and ovary activation in honey bee (Apis mellifera) queens

“…Similarly, we found that several thromboxanes, which are structurally similar to prostaglandins, were upregulated in mated queens over virgins ( Figure 7 & Figure S7 ). In the Lepidopteran Spodoptera exigua , thromboxanes are immunostimulatory 72,73 , and a thromboxane receptor has been characterized in Aedes and Anopheles mosquitoes 74 , but neither thromboxanes nor their receptors have been investigated in honey bees.…”

Enhancement of an entomopathogenic fungal virulence against the seedcorn maggot, Delia platura, by suppressing immune responses with a bacterial culture broth of Photorhabdus temperata subsp. temperata

“…A fungal pathogen, Beauveria bassiana (Bb), was isolated from a commercial product (LG Farm Hannong, Seoul, Korea). Metarhizium rileyi (Mr) was isolated from dead insects sourced from an onion field and M. guizhouense (Mg) was isolated from Rhinoceros beetle (Roy et al, 2021). The fungi were cultured in potato dextrose broth (Difco, Fisher Scientific Korea, Seoul, Korea) for 14 days with a shaking incubator at 180 rpm.…”

“…Among the numerous immune mediators that have been described to this point, eicosanoids play crucial roles in mediating immune responses against various pathogens (Kim et al, 2018a). Eicosanoids are a group of oxygenated C20 polyunsaturated fatty acids that includes prostaglandins, leukotrienes, and epoxyeicosatrienoic acids (Kim and Stanley, 2021). They are synthesized from a C20 precursor called arachidonic acid (AA), which is released from phospholipids by the catalytic activity of phospholipase A 2 (PLA 2 ) (Mouchlis and Dennis, 2019).…”