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Repat (=response to pathogen) is proposed for an immune-associated gene family from <i>Spodoptera exigua</i>, a lepidopteran insect. In this gene family, 46 members (<i>Repat1</i>–<i>Repat46</i>) have been identified. They show marked variations in their inducible expression patterns in response to infections by different microbial pathogens. However, their physiological functions in specific immune responses and their interactions with other immune signaling pathways remain unclear. <i>Repat33</i> is a gene highly inducible by bacterial infections. The objective of this study was to analyze the physiological functions of <i>Repat33</i> in mediating cellular and humoral immune responses. Results showed that <i>Repat33</i> was expressed in all developmental stages and induced in immune-associated tissues such as hemocytes and the fat body. RNA interference (RNAi) of <i>Repat33</i> expression inhibited the hemocyte-spreading behavior which impaired nodule formation of hemocytes against bacterial infections. Such RNAi treatment also down-regulated expression levels of some antimicrobial genes. Interestingly, <i>Repat33</i> expression was controlled by eicosanoids. Inhibition of eicosanoid biosynthesis by RNAi against a phospholipase A₂ (PLA₂) gene suppressed <i>Repat33</i> expression while an addition of arachidonic acid (a catalytic product of PLA₂) to RNAi treatment recovered such suppression of <i>Repat33</i> expression. These results suggest that Repat33 is a downstream component of eicosanoids in mediating immune responses of <i>S. exigua</i>.