Strain-Dependent Host Transcriptional Responses to <em>Toxoplasma</em> Infection Are Largely Conserved in Mammalian and Avian Hosts

Toxoplasma gondii has a remarkable ability to infect an enormous variety of mammalian and avian species. Given this, it is surprising that three strains (Types I/II/III) account for the majority of isolates from Europe/North America. The selective pressures that have driven the emergence of these particular strains, however, remain enigmatic. We hypothesized that strain selection might be partially driven by adaptation of strains for mammalian versus avian hosts. To test this, we examine in vitro, strain-dependent host responses in fibroblasts of a representative avian host, the chicken (Gallus gallus). Using gene expression profiling of infected chicken embryonic fibroblasts and pathway analysis to assess host response, we show here that chicken cells respond with distinct transcriptional profiles upon infection with Type II versus III strains that are reminiscent of profiles observed in mammalian cells. To identify the parasite drivers of these differences, chicken fibroblasts were infected with individual F1 progeny of a Type II x III cross and host gene expression was assessed for each by microarray. QTL mapping of transcriptional differences suggested, and deletion strains confirmed, that, as in mammalian cells, the polymorphic rhoptry kinase ROP16 is the major driver of strain-specific responses. We originally hypothesized that comparing avian versus mammalian host response might reveal an inversion in parasite strain-dependent phenotypes; specifically, for polymorphic effectors like ROP16, we hypothesized that the allele with most activity in mammalian cells might be less active in avian cells. Instead, we found that activity of ROP16 alleles appears to be conserved across host species; moreover, additional parasite loci that were previously mapped for strain-specific effects on mammalian response showed similar strain-specific effects in chicken cells. These results indicate that if different hosts select for different parasite genotypes, the selection operates downstream of the signaling occurring during the beginning of the host's immune response.刚地弓形虫（Toxoplasma gondii）具备感染极其广泛的哺乳类与鸟类宿主类群的卓越能力。鉴于此，欧洲与北美地区绝大多数弓形虫分离株均归属于I、II、III三种基因型虫株，这一现象颇为出人意料。然而，推动这三类特定虫株演化出现的选择压力仍属未解之谜。 我们提出假说：虫株的定向选择可能部分源于各虫株对哺乳类与鸟类宿主的适应性分化。为验证该假说，我们以代表性鸟类宿主家鸡（Gallus gallus）的成纤维细胞为模型，开展体外虫株依赖性宿主应答实验。 通过对受感染鸡胚成纤维细胞进行基因表达谱分析，并借助通路分析评估宿主应答，本研究发现：家鸡细胞在感染II型与III型虫株后，会呈现出与哺乳类细胞中已报道的转录谱特征高度相似的差异化转录模式。 为鉴定介导上述转录差异的寄生虫效应因子，我们使用II型与III型虫株杂交产生的单株F1子代感染鸡成纤维细胞，并通过基因芯片（microarray）分别检测各组的宿主基因表达情况。 对转录差异进行数量性状位点（QTL）定位分析显示，且经基因缺失菌株验证证实：与哺乳类细胞中的研究结果一致，多态性棒状体激酶ROP16（rhoptry kinase ROP16）是介导虫株特异性宿主应答的核心效应因子。 我们最初曾推测，对比鸟类与哺乳类宿主的应答模式，可能会发现寄生虫虫株依赖性表型的反转；具体而言，对于ROP16这类多态性效应因子，在哺乳类细胞中活性最强的等位基因，在鸟类细胞中的活性或许会更低。 但本研究的实际结果却与之相反：ROP16等位基因的活性在不同宿主物种间呈现保守性；此外，此前在哺乳类宿主中被定位到的、可介导虫株特异性应答的其他寄生虫基因座，在鸡细胞中也表现出了相似的虫株特异性效应。 上述结果表明：若不同宿主确实会对寄生虫基因型产生选择压力，那么这种选择作用将发生于宿主免疫应答初始阶段的信号传导通路下游。