Spätzle1 is a constituent of the extracellular signaling pathway that promotes nodule formation, a cell‐mediated immune response, in caterpillar hemolymph

Abstract

The existence of an extracellular signaling pathway that mediates nodule formation, a cell‐mediated immune response, has been reported in Bombyx mori larvae. In this pathway, C‐type lectins and the hemolymph serine proteinase BmHP‐8 function in pathogen associated molecular pattern (PAMPs) recognition and signaling transduction. However, which molecule elicits the cellular response at the end of the pathway is unknown. In this study, the Toll ligand Bombyx mori Spätzel1 was shown to be involved in the pathway by applying anit‐Spätzel1 antiserum in an in vitro nodule‐like aggregate formation assay and an in vivo nodule formation assay.

Mechanisms and roles of the first stage of nodule formation in lepidopteran insects

“…2 ). In addition, antiserum raised against B. mori Spätzle1 (BmSPZ1) suppressed both nodule and in vitro nodule-like aggregation ( Tang et al 2022 ), which indicates involvement of this cytokine in the signaling pathway during nodule formation ( Fig. 2 ).…”

“…2 – 4 ). However, western blot analysis identified BmSPZ1 in hemocytes isolated from hemolymph pretreated with S. cerevisiae cells ( Tang et al 2022 ), but not consistently in S. cerevisiae cells pretreated with plasma (unpublished data). In contrast, activated BmHP8 was detected in microorganism cells that were pretreated with B. mori larval plasma ( Tokunaga et al 2021 ).…”

“…A) Schematic diagram of the signaling pathways of the first stage of nodule formation in B. mori . The diagram includes results from inhibitory experiments that used antisera ( Koizumi et al 1997 , 1999a , Watanabe et al 2006 , Tokunaga et al 2021 , Tang et al 2022 ) and knockout experiments ( Suzuki et al 2022 ). The results of inhibitory and knockout experiments were superimposed on the cascading induction scheme for AMP production in M. sexta ( Kanost and Jiang 2015 ).…”

“…The model is based on the reports of Kim et al (2009) , Park and Kim (2012) , Shestha and Kim (2009) , and Shafeeq et al (2018) , which suggested that 5-HT acts upstream in conjunction with eicosanoids to induce nodule formation. The model also takes account of the findings of Tokunaga et al (2021) , Tang et al (2022) , Suzuki et al (2022) , and Otuka and Sato (2023) , which imply that BmToll10-3 pathway activation via BmHP8 and BmSPZ1 contributes to aggregation formation in the first stage of nodule formation. Activated BmHP8 on microbial cell(s) is proposed to activate the precursor of BmSPZ1, which in turn activates the BmToll10-3 signaling pathway of the surrounding granulocytes.…”

“…Figure 2 shows a schematic diagram of the serine proteinase cascade that induces the first stage of nodule formation. The results of inhibitory experiments using antisera and B. mori larvae were superimposed on the cascading induction scheme for AMP production in M. sexta ( Kanost and Jiang 2015 , Tokunaga et al 2021 , Tang et al 2022 ).…”