背景:先天免疫在动物中提供了预防感染的核心识别系统,而且在管理动物宿主与其共生体之间的关系方面也起着重要作用。我们关于先天免疫的大部分知识来自一些动物模型系统,但是比较基因组研究已经揭示了后生动物门之间的实质性差异。为了更好地理解免疫进化的相关机制,还需要探索更多的分类单元。胎盘动物在形态上是最简单的有组织的后生动物,最近已经阐明了这些神秘动物与其立克次体内共生体之间的联系。我们对Trichoplaxsp。的新型胎盘动物核基因组的分析。H2及其相关的立克次体内共生体基因组清楚地指出了一种相互进化和共同进化的关系。这一发现提出了一个问题,即placozoanholobiont如何管理共生关系,相反,它是如何抵御有害微生物的。在这项研究中,我们检查了带注释的Trichoplaxsp。H2表示存在参与先天免疫识别和下游信号传导的基因。
结果:属于Toll样和NOD样受体途径的丰富基因库,在Trichoplaxsp。的基因组中鉴定出清道夫受体和分泌的纤维蛋白原相关域基因。H2.然而,在几种情况下,胎盘动物的先天免疫相关途径与经过充分研究的脊椎动物和无脊椎动物有所不同。虽然没有真正的Toll和NOD样受体,下游信号级联的许多基因的存在表明胎盘动物中至少存在原始的Toll样受体信号。大量的清道夫受体,纤维蛋白原相关结构域基因和Apaf-1基因显然构成了胎盘动物特异性免疫相关基因库的扩展。
结论:鉴于极其简单的胎盘体计划及其稀疏的细胞类型组成,胎盘动物中存在的大量免疫相关基因令人惊讶且相当惊人。有必要进行研究以揭示胎盘动物如何利用这种免疫库来管理和维持其相关的微生物群以及抵御病原体。
BACKGROUND: Innate immunity provides the core recognition system in animals for preventing infection, but also plays an important role in managing the relationship between an animal host and its symbiont. Most of our knowledge about innate immunity stems from a few animal model systems, but substantial variation between metazoan phyla has been revealed by comparative genomic studies. The exploration of more taxa is still needed to better understand the evolution of immunity related mechanisms. Placozoans are morphologically the simplest organized metazoans and the association between these enigmatic animals and their rickettsial endosymbionts has recently been elucidated. Our analyses of the novel placozoan nuclear genome of
Trichoplax sp. H2 and its associated rickettsial endosymbiont genome clearly pointed to a mutualistic and co-evolutionary relationship. This discovery raises the question of how the placozoan holobiont manages symbiosis and, conversely, how it defends against harmful microorganisms. In this study, we examined the annotated genome of
Trichoplax sp. H2 for the presence of genes involved in innate immune recognition and downstream signaling.
RESULTS: A rich repertoire of genes belonging to the Toll-like and NOD-like receptor pathways, to scavenger receptors and to secreted fibrinogen-related domain genes was identified in the genome of
Trichoplax sp. H2. Nevertheless, the innate immunity related pathways in placozoans deviate in several instances from well investigated vertebrates and invertebrates. While true Toll- and NOD-like receptors are absent, the presence of many genes of the downstream signaling cascade suggests at least primordial Toll-like receptor signaling in Placozoa. An abundance of scavenger receptors, fibrinogen-related domain genes and Apaf-1 genes clearly constitutes an expansion of the immunity related gene repertoire specific to Placozoa.
CONCLUSIONS: The found wealth of immunity related genes present in Placozoa is surprising and quite striking in light of the extremely simple placozoan body plan and their sparse cell type makeup. Research is warranted to reveal how Placozoa utilize this immune repertoire to manage and maintain their associated microbiota as well as to fend-off pathogens.