Tsetse immune responses and trypanosome transmission: Implications for the development of tsetse-based strategies to reduce trypanosomiasis

Tsetse flies are the medically and agriculturally important vectors of African trypanosomes. Information on the molecular and biochemical nature of the tsetse/trypanosome interaction is lacking. Here we describe three antimicrobial peptide genes, attacin, defensin, and diptericin, from tsetse fat body tissue obtained by subtractive cloning after immune stimulation with Escherichia coli and trypanosomes. Differential regulation of these genes shows the tsetse immune system can discriminate not only between molecular signals specific for bacteria and trypanosome infections but also between different life stages of trypanosomes. The presence of trypanosomes either in the hemolymph or in the gut early in the infection process does not induce transcription of attacin and defensin significantly. After parasite establishment in the gut, however, both antimicrobial genes are expressed at high levels in the fat body, apparently not affecting the viability of parasites in the midgut. Unlike other insect immune systems, the antimicrobial peptide gene diptericin is constitutively expressed in both fat body and gut tissue of normal and immune stimulated flies, possibly reflecting tsetse immune responses to the multiple Gram-negative symbionts it naturally harbors. When flies were immune stimulated with bacteria before receiving a trypanosome containing bloodmeal, their ability to establish infections was severely blocked, indicating that up-regulation of some immune responsive genes early in infection can act to block parasite transmission. The results are discussed in relation to transgenic approaches proposed for modulating vector competence in tsetse.

采采蝇（Tsetse flies）是兼具医学与农业重要性的非洲锥虫（African trypanosomes）传播媒介。目前学界对采采蝇与锥虫互作的分子及生化本质仍缺乏系统认知。本研究通过以大肠杆菌（Escherichia coli）与锥虫实施免疫刺激后进行消减克隆（subtractive cloning），从采采蝇脂肪体组织中分离得到3种抗菌肽基因：攻击素（attacin）、防御素（defensin）与双翅肽（diptericin）。对这些基因的差异调控分析显示，采采蝇免疫系统不仅可区分细菌与锥虫感染的特异性分子信号，还能辨别锥虫的不同生活史阶段。在感染早期，锥虫在血淋巴（hemolymph）或肠道内的存在并不会显著诱导攻击素与防御素的转录。但当寄生虫在肠道定殖后，脂肪体中的这两种抗菌肽基因便会高水平表达，且该过程似乎不会影响中肠（midgut）内寄生虫的存活能力。与其他昆虫免疫系统不同的是，双翅肽（diptericin）在正常及经免疫刺激的采采蝇的脂肪体与肠道组织中均呈组成型表达，这或许反映了采采蝇对其天然携带的多种革兰氏阴性共生菌的免疫应答。当采采蝇在摄入含锥虫的血餐之前先用细菌进行免疫刺激时，其锥虫感染定殖能力会受到严重阻断，这表明感染早期上调部分免疫应答基因可有效阻断寄生虫的传播。本研究结果将结合调控采采蝇媒介传播能力（vector competence）的转基因技术相关提案展开讨论。