Structure of a variable lymphocyte receptor-like protein from the amphioxus Branchiostoma floridae

Alternative splicing derived invertebrate variable lymphocyte receptor displays diversity and specificity in immune system of crab Eriocheir sinensis

Variable lymphocyte receptors (VLRs) play vital roles in adaptive immune system of agnathan vertebrate. In the present study, we first discover a novel VLR gene, VLR2, from an invertebrate, the Chinese mitten crab, Eriocheir sinensis. VLR2 has ten different isoforms formed via alternative splicing, which is different from that in agnathan vertebrate with the assembly of LRR modules. The longest isoform, VLR2-L, responds to Gram-positive bacteria Staphylococcus aureus challenge specifically, while shows no response to Gram-negative bacteria Vibrio parahaemolyticus challenge, confirmed by recombinant expression and bacterial binding experiments. Interestingly, VLR2s with short LRRs regions (VLR2-S8 and VLR2-S9) tend to bind to Gram-negative bacteria rather than Gram-positive bacteria. Antibacterial activity assay proves six isoforms of VLR2 have pluralistic antibacterial effects on bacteria which were never reported in invertebrate. These results suggest that the diversity and specificity of VLR2 resulted from alternative splicing and the length of the LRRs region. This pathogen-binding receptor diversity will lay the foundation for the study of immune priming. Furthermore, studying the immune function of VLR2 will provide a new insight into the disease control strategy of crustacean culture.

Advances in immunological research of amphioxus

Amphioxus, one of the most closely related invertebrates to vertebrates, is an important animal model for studying the origin and evolution of vertebrate immunity, especially the transition from innate immunity to adaptive immunity. The current research progresses of amphioxus in the field of immune organs, immune cells, complement system, cytokines, nuclear factor kappa B, immune-related lectins and enzymes are summarized, and some issues that remain to be understood or are in need of further clarification are highlighted. We hope to provide references for more in-depth study of the amphioxus immune system and lay a solid foundation for the construction of three-dimensional immune network in amphioxus from ontogeny to phylogeny.

Identification, functional characterization and the potential role of variable lymphocyte receptor EsVLRA from Eriocheir sinensis in response to secondary challenge after Vibrio parahaemolyticus vaccine

Variable lymphocyte receptors (VLRs) play an important role via their antigen-special reorganization in jawless vertebrates (agnathans) adaptive immune response. In the present study, the open reading frame (ORF) of Eriocheir sinensis VLRA (designated as EsVLRA) was identified. EsVLRA comprised a 799-amino-acid polypeptide with one LRR_NT domain, thirteen LRR domains and one LRR_CT domain, which showed a high domain consistency of the VLR genes in lamprey (Petromyzon marinus). The transcript of EsVLRA was detected in all examined tissues with the highest level detected in hepatopancreas. Notably, the expression of EsVLRA in hepatopancreas, gonads, gill and intestine of male crabs was significantly higher than that in females. The recombinant EsVLRA exhibited strong bacteria-binding activity rather than antibacterial activity, suggesting its crucial role in immune recognition. Furthermore, 6 h earlier response and a significantly higher peak of EsVLRA mRNA expression was observed after challenge with live Vibrio parahaemolyticus (240.6-fold, P < 0.01, crabs receive secondary challenge after V. parahaemolyticus vaccine to the carbs only receive twice PBS injection, N = 6), compared with those only received first injection with formalin-inactivated V. parahaemolyticus (39.7-fold, P < 0.01, challenge 6 h to vaccination 12 h). The findings of this study together demonstrated that EsVLRA plays an important role in the immune system of E. sinensis, serving as a pattern recognition receptor and involving in the immune priming.

Genome-wide comparison of the protein-coding repertoire reveals fast evolution of immune-related genes in cephalochordates and Osteichthyes superclass

Amphioxus is used to investigate the origin and evolution of vertebrates. To better understand the characteristics of genome evolution from cephalochordates to Osteichthyes, we conducted a genome-wide pairwise comparison of protein-coding genes within amphioxus (a comparable group) and parallel analyses within Osteichthyes (two comparable groups). A batch of fast-evolving genes in each comparable group was identified. Of these genes, the most fast-evolving genes (top 20) were scrutinized, most of which were involved in immune system. An analysis of the fast-evolving genes showed that they were enriched into gene ontology (GO) terms and pathways primarily involved in immune-related functions. Similarly, this phenomenon was detected within Osteichthyes, and more well-known and abundant GO terms and pathways involving innate immunity were found in Osteichthyes than in cephalochordates. Next, we measured the expression responses of four genes belonging to metabolism or energy production-related pathways to lipopolysaccharide challenge in the muscle, intestine or skin of B. belcheri; three of these genes (HMGCL, CYBS and MDH2) showed innate immune responses. Additionally, some genes involved in adaptive immunity showed fast evolution in Osteichthyes, such as those involving "intestinal immune network for IgA production" or "T-cell receptor signaling pathway". In this study, the fast evolution of immune-related genes in amphioxus and Osteichthyes was determined, providing insights into the evolution of immune-related genes in chordates.

Evolution of Alternative Adaptive Immune Systems in Vertebrates

Adaptive immunity in jawless fishes is based on antigen recognition by three types of variable lymphocyte receptors (VLRs) composed of variable leucine-rich repeats, which are differentially expressed by two T-like lymphocyte lineages and one B-like lymphocyte lineage. The T-like cells express either VLRAs or VLRCs of yet undefined antigen specificity, whereas the VLRB antibodies secreted by B-like cells bind proteinaceous and carbohydrate antigens. The incomplete VLR germline genes are assembled into functional units by a gene conversion-like mechanism that employs flanking variable leucine-rich repeat sequences as templates in association with lineage-specific expression of cytidine deaminases. B-like cells develop in the hematopoietic typhlosole and kidneys, whereas T-like cells develop in the thymoid, a thymus-equivalent region at the gill fold tips. Thus, the dichotomy between T-like and B-like cells and the presence of dedicated lymphopoietic tissues emerge as ancestral vertebrate features, whereas the somatic diversification of structurally distinct antigen receptor genes evolved independently in jawless and jawed vertebrates.

Exploring gene expression changes in the amphioxus gill after poly(I:C) challenge using digital expression profiling

Amphioxus, a cephalochordate, is a key model animal for studying the evolution of vertebrate immunity. Recently, studies have revealed that microRNA (miRNA) expression profiles change significantly in the amphioxus gill after immune stimulation, but it remains largely unknown how gene expression responds to immune stress. Elucidating gene expression changes in the amphioxus gill will provide a deeper understanding of the evolution of gill immunity in vertebrates. Here, we used high-throughput RNA sequencing technology (RNA-seq) to conduct tag-based digital gene expression profiling (DGE) analyses of the gills of control Branchiostoma belcheri and of those exposed to the viral mimic, poly(I:C) (pIC). Six libraries were created for the control and treatment groups including three biological replicates per group. A total of 1999 differently expressed genes (DEGs) were obtained, with 571 and 1428 DEGs showing up- or down-regulation, respectively, in the treatment group. Enrichment analysis of gene ontology (GO) terms and pathways revealed that the DEGs were primarily related to immune and defense response, apoptosis, human disease, cancer, protein metabolism, enzyme activity, and regulatory processes. In addition, eight DEGs were randomly selected to validate the RNA-seq data using real-time quantitative PCR (qRT-PCR), and the results confirmed the accuracy of the RNA-seq approach. Next, we screened eight key responding genes to examine the dynamic changes in expression levels at different time points in more detail. The results indicated that expressions of TRADD, MARCH, RNF31, NF-κb, CYP450, TNFRSF6B, IFI and LECT1 were induced to participate in the antiviral response against pIC. This study provides a valuable resource for understanding the role of the amphioxus gill in antiviral immunity and the evolution of gill immunity in vertebrates.

Convergent evolution of the adaptive immune response in jawed vertebrates and cyclostomes: An evolutionary biology approach based study

Two different adaptive immune systems (AIS) are present in the two phyla of vertebrates (jawed vertebrates and cyclostomes). The jawed vertebrate system is based on IG/TCR/RAG/MHC while the cyclostome system is based on VLRCs and AID-like enzymes both systems using homologous Cell types (B-cell and B-cell Like, T-cell and T-cell like). We will present our current view of the evolution of these two AISs and present alternative hypotheses that could explain the apparent convergent evolution of the two systems. We will also discuss why comparative immunology analyses should be based on evolutionary biology approaches and not on the scale of progress one.

Molecular mechanisms underlying the evolution of the slp76 signalosome

The well-defined mammalian slp76-signalosome is crucial for T-cell immune response, yet whether slp76-signalosome exists in invertebrates and how it evolved remain unknown. Here we investigated slp76-signalosome from an evolutionary perspective in amphioxus Branchiostoma belcheri (bb). We proved slp76-signalosome components bbslp76, bbGADS and bbItk are present in amphioxus and bbslp76 interacts with bbGADS and bbItk, but differences exist between the interaction manners within slp76-signalosome components of amphioxus and human (h). Specifically, bbslp76 has a unique WW-domain that blocked its association with hItk and decreased TCR-induced tyrosine-phosphorylation and NFAT-activation. Deletion of WW-domain shifted the constitutive association between bbslp76 and hPLCγ1 to a TCR-enhanced association. Among slp76-signalosome, the interaction between slp76 and PLCγ1 is the most conserved and the binding between Itk and slp76 evolved from constitutive to stimulation-regulated. Sequence alignment and 3D structural analysis of slp76-signalosome molecules from keystone species indicated slp76 evolved into a more unfolded and flexible adaptor due to lack of WW-domain and several low-complexity-regions (LCRs) while GADS turned into a larger protein by a LCR gain, thus preparing more space for nucleating the coevolving slp76-signalosome. Altogether, through deletion of WW-domain and manipulation of LCRs, slp76-signalosome evolves from a rigid and stimulation-insensitive to a more flexible and stimulation-responding complex.

microRNA expression changes after lipopolysaccharide treatment in gills of amphioxus Branchiostoma belcheri

Recently, amphioxus has served as a model for studying the origin and evolution of vertebrate immunity. However, little is known about how microRNAs (miRNAs) are involved in the immune defense in amphioxus. In this article, we identified the amphioxus miRNAs in the acute-phase response to lipopolysaccharide (LPS). We determined the time point for the peak of immune response in amphioxus after LPS challenge by evaluating the expression of Branchiostoma belcheri toll-like receptor 1, NF-κb (c-Rel), and big defensin which react with pathogen-associated molecular patterns(PAMPs). Then we chose 12 h as the point to perform miRNA microarray analysis to select the differentially expressed miRNAs. Furthermore, we used quantitative real-time PCR to detect the expression patterns of selected amphioxus miRNAs under effective LPS challenge during the time course. The microarray data revealed that the miRNA expression file was significantly changed after LPS stimulation. The changes of the 10 most upregulated and 7 most downregualted miRNAs in gills of the amphioxus following challenge with LPS revealed a temporal induction kinetic. Our current study will provide valuable information to take an insight into molecular mechanism of innate immune and the evolution of the miRNA family.