Gene expression changes of homologs of human trained immunity markers in the silkworm (<i>Bombyx mori</i>) upon exposure to an innate immunity-enhancing agent

Trained immunity (TI) is a memory-like response of the mammalian innate immune system. However, a convenient assay system for measuring TI is not yet available. Insects possess only innate immunity and therefore have the potential for evaluating innate immunity-enhancing vaccines or chemicals. In this study, larvae of the silkworm (Bombyx mori) were injected with heat-killed (HK) Escherichia coli to induce immune priming, the insect counterpart of mammalian TI, which was confirmed with reduction in the infected Bombyx mori nucleopolyhedrovirus copy numbers, and gene expression was analyzed by RNAseq and reverse transcription quantitative PCR. Among the genes with significant expression changes, B. mori homologs of known human TI marker genes were identified. Changes in the expression of some of these marker genes in response to HK E. coli were also confirmed in mouse macrophage cell lines. Although B. mori responses were results of the insect immune priming and not equivalent to the mammalian TI, these results suggest the potential use of B. mori for detecting innate immunity-enhancing agents for mammals.