The Evolution and Regulation of the Mucosal Immune Complexity in the Basal Chordate Amphioxus

Abstract

Both amphioxus and the sea urchin encode a complex innate immune gene repertoire in their genomes, but the composition and mechanisms of their innate immune systems, as well as the fundamental differences between two systems, remain largely unexplored. In this study, we dissect the mucosal immune complexity of amphioxus into different evolutionary-functional modes and regulatory patterns by integrating information from phylogenetic inferences, genome-wide digital expression profiles, time course expression dynamics, and functional analyses. With these rich data, we reconstruct several major immune subsystems in amphioxus and analyze their regulation during mucosal infection. These include the TNF/IL-1R network, TLR and NLR networks, complement system, apoptosis network, oxidative pathways, and other effector genes (e.g., peptidoglycan recognition proteins, Gram-negative binding proteins, and chitin-binding proteins). We show that beneath the superficial similarity to that of the sea urchin, the amphioxus innate system, despite preserving critical invertebrate components, is more similar to that of the vertebrates in terms of composition, expression regulation, and functional strategies. For example, major effectors in amphioxus gut mucous tissue are the well-developed complement and oxidative-burst systems, and the signaling network in amphioxus seems to emphasize signal transduction/modulation more than initiation. In conclusion, we suggest that the innate immune systems of amphioxus and the sea urchin are strategically different, possibly representing two successful cases among many expanded immune systems that arose at the age of the Cambrian explosion. We further suggest that the vertebrate innate immune system should be derived from one of these expanded systems, most likely from the same one that was shared by amphioxus.

Characteristic and functional analysis of Toll-like receptors (TLRs) in the lophotrocozoan, Crassostrea gigas, reveals ancient origin of TLR-mediated innate immunity

“…However, there was no evidence of a response when TLR ligands and PAMPs were used to stimulate CgTLR transfected cells. The lack of PAMPs recognition was consistent with previous reports for amphioxus and Drosophila [6,29]. In contrast, Sasaki et al demonstrated that ascidian Cionaintestinalis TLRs show low level responses to high concentrations of PAMPs [30], however, such a response is too weak to distinguish from C. intestinalis TLR's authentic ability and the background response of cell line.…”

Genome-Wide Analyses of Amphioxus MicroRNAs Reveal an Immune Regulation via miR-92d Targeting C3

“…Initially, we obtained the gene-expression profile after bacterial challenge by sequencing differentially expressed genes using the suppression subtractive hybridization method (46) and then by sequencing the cDNA library of amphioxus in response to early antibacterial infection via 454 Sequencing (38). Through these approaches, extensive immune-related genes were identified in amphioxus, but no miRNAs were found to be involved in regulation of the immune system.…”

“…They are representatives of Gram-positive (G+) and Gram-negative (G2) bacteria, respectively. Four amphioxus acute-phase response genes, including Complement component C3 (AB050668), Dermatopontin (EB677005), Peptidoglycan recognition protein S1 precursor (EB676974), and Rhamnose-binding lectin precursor (AY616196), were selected to detect the immune activation based on our pre- vious study (38,46). Under both S. aureus and V. anguillarum challenge, the expression of the four amphioxus genes rose quickly and achieved a steady-state level at ∼24 h (Fig.…”

“…Amphioxus can provide useful insights into the evolution of the immune system based on a series of our previous studies (25,38,46). Initially, we obtained the gene-expression profile after bacterial challenge by sequencing differentially expressed genes using the suppression subtractive hybridization method (46) and then by sequencing the cDNA library of amphioxus in response to early antibacterial infection via 454 Sequencing (38).…”

“…The 39UTR sequences, which were perfect matches to the "seed" sequence of miRNA, were retained (37). These target candidates were compared with the expressed sequence tag library, which was sequenced by 454 Sequencing in our laboratory (38).…”