Microbe-specific C3b deposition in the horseshoe crab complement system in a C2/factor B-dependent or -independent manner

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

Tick Immune System: What Is Known, the Interconnections, the Gaps, and the Challenges

Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.

Immune Responses to Gram-Negative Bacteria in Hemolymph of the Chinese Horseshoe Crab, Tachypleus tridentatus

Chinese horseshoe crab, Tachypleus tridentatus, is an ancient marine arthropod with a long evolutionary history. As a kind of living fossil species, the pathogen defenses of horseshoe crabs entirely depend on the innate immune system. Although, there are abundant immune molecules found in the horseshoe crab hemolymph, the biological mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear. In this study, we used high-throughput sequencing at mRNA and protein levels and bioinformatics analysis methods to systematically analyze the innate immune response to Gram-negative bacteria in hemolymph of Chinese horseshoe crab. These results showed that many genes in the complement and coagulation cascades, Toll, NF-κB, C-type lectin receptor, JAK-STAT, and MAPK signaling pathways, and antimicrobial substances were activated at 12 and 24 h post-infection, suggesting that Gram-negative bacteria could activate the hemolymph coagulation cascade and antibacterial substances release via the above pathways. In addition, we conjectured that Toll and NF-κB signaling pathway were most likely to participate in the immune response to Gram-negative bacteria in hemolymph of horseshoe crab through an integral signal cascade. These findings will provide a useful reference for exploring the ancient original innate immune mechanism.

Post-translational protein deimination signatures and extracellular vesicles (EVs) in the Atlantic horseshoe crab (Limulus polyphemus)

The horseshoe crab is a living fossil and a species of marine arthropod with unusual immune system properties which are also exploited commercially. Given its ancient status dating to the Ordovician period (450 million years ago), its standing in phylogeny and unusual immunological characteristics, the horseshoe crab may hold valuable information for comparative immunology studies. Peptidylarginine deiminases (PADs) are calcium dependent enzymes that are phylogenetically conserved and cause protein deimination via conversion of arginine to citrulline. This post-translational modification can lead to structural and functional protein changes contributing to protein moonlighting in health and disease. PAD-mediated regulation of extracellular vesicle (EV) release, a critical component of cellular communication, has furthermore been identified to be a phylogenetically conserved mechanism. PADs, protein deimination and EVs have hitherto not been studied in the horseshoe crab and were assessed in the current study. Horseshoe crab haemolymph serum-EVs were found to be a poly-dispersed population in the 20-400 nm size range, with the majority of EVs falling within 40-123 nm. Key immune proteins were identified to be post-translationally deiminated in horseshoe crab haemolymph serum, providing insights into protein moonlighting function of Limulus and phylogenetically conserved immune proteins. KEGG (Kyoto encyclopaedia of genes and genomes) and GO (gene ontology) enrichment analysis of deiminated proteins identified in Limulus revealed KEGG pathways relating to complement and coagulation pathways, Staphylococcus aureus infection, glycolysis/gluconeogenesis and carbon metabolism, while GO pathways of biological and molecular pathways related to a range of immune and metabolic functions, as well as developmental processes. The characterisation of EVs, and post-translational deimination signatures, revealed here in horseshoe crab, contributes to current understanding of protein moonlighting functions and EV-mediated communication in this ancient arthropod and throughout phylogeny.

A novel complement C3 like gene (Lv-C3L) from Litopenaeus vannamei with bacteriolytic and hemolytic activities and its role in antiviral immune response

As a core component of the complement system, complement component 3 (C3) plays a central role in the opsonization of pathogens, immune defense and immune regulation in the mammalian for its activation is required to trigger classical as well as alternative complement pathways. However, the molecular mechanism underlying C3 activation in invertebrates remains unknown. Several C3 genes have been characterized in invertebrates but very few in crustacean. To understand the molecular characterization and immunological functions of shrimp C3, we characterized a novel complement C3 like gene (designated Lv-C3L) with full-length cDNA sequence identified from pacific white shrimp Litopenaeus vannamei in the present study. The full length cDNA of Lv-C3L sequence was 4769 bp (GenBank accession number: MH638255) containing a 4077 bp open reading frame (ORF), which encodes 1358 amino acids contained a putative signal peptide of 17 amino acids. Six model motifs of C3 were found in Lv-C3L including typical A2M domain, a highly conserved thioester region (GCGEQ) and proteolytic cleavage site of ANATO. In addition to typical conservative domains, Lv-C3L also contains a particular GLN-rich region which might be involved in the protein interaction and transcriptional activation. The transcripts of Lv-C3L were mainly detected in hemocytes and gill which might be involved in defense response. At 36 h post V.parahaemolyticus and B.thuringensis infection, the expression level of Lv-C3L gene in hemocytes were significantly upregulated. At 48 h and 72 h post WSSV infection, the expression level of Lv-C3L gene in hemocytes and gill were significantly upregulated. These results indicated that Lv-C3L gene play a pivotal role in innate immune responses to the WSSV and G⁺/G^- bacterial infection. The obvious immune function of Lv-C3L was described as an effective membrane rupture in bacteriolytic and hemolytic activities on V.parahaemolyticus, V.anguillarum and rabbit erythrocytes. Combining with WSSV copy number, WSSV-VP28 gene expression profile and shrimp cumulative mortality analysis, RNAi knockdown of Lv-C3L gene could obviously promote the in vivo propagation of WSSV in shrimp. This is the first report in crustaceans that Lv-C3L, as a key complement like components, is involved in shrimp antiviral immune response. It is speculated that complicated complement response cascade may exist in shrimp. These results collectively indicated that the complement pathway in shrimp might play an important protective role against pathogenic infection and activation of complement pathway including C3 could restrict the propagation of WSSV.

Intermolecular autocatalytic activation of serine protease zymogen factor C through an active transition state responding to lipopolysaccharide

Horseshoe crab hemolymph coagulation is believed to be triggered by the autocatalytic activation of serine protease zymogen factor C to the active form, α-factor C, belonging to the trypsin family, through an active transition state of factor C responding to bacterial lipopolysaccharide (LPS), designated factor C*. However, the existence of factor C* is only speculative, and its proteolytic activity has not been validated. In addition, it remains unclear whether the proteolytic cleavage of the Phe⁷³⁷-Ile⁷³⁸ bond (Phe⁷³⁷ site) of factor C required for the conversion to α-factor C occurs intramolecularly or intermolecularly between the factor C molecules. Here we show that the Phe⁷³⁷ site of a catalytic Ser-deficient mutant of factor C is LPS-dependently hydrolyzed by a Phe⁷³⁷ site-uncleavable mutant, clearly indicating the existence of the active transition state of factor C without cleavage of the Phe⁷³⁷ site. Moreover, we found the following facts using several mutants of factor C: the autocatalytic cleavage of factor C occurs intermolecularly between factor C* molecules on the LPS surface; factor C* does not exhibit intrinsic chymotryptic activity against the Phe⁷³⁷ site, but it may recognize a three-dimensional structure around the cleavage site; and LPS is required not only to complete the substrate-binding site and oxyanion hole of factor C* by interacting with the N-terminal region but also to allow the Phe⁷³⁷ site to be cleaved by inducing a conformational change around the Phe⁷³⁷ site or by acting as a scaffold to induce specific protein-protein interactions between factor C* molecules.

Genetic and molecular basis of the immune system in the brachiopod Lingula anatina

The extension of comparative immunology to non-model systems, such as mollusks and annelids, has revealed an unexpected diversity in the complement of immune receptors and effectors among evolutionary lineages. However, several lophotrochozoan phyla remain unexplored mainly due to the lack of genomic resources. The increasing accessibility of high-throughput sequencing technologies offers unique opportunities for extending genome-wide studies to non-model systems. As a result, the genome-based study of the immune system in brachiopods allows a better understanding of the alternative survival strategies developed by these immunologically neglected phyla. Here we present a detailed overview of the molecular components of the immune system identified in the genome of the brachiopod Lingula anatina. Our findings reveal conserved intracellular signaling pathways as well as unique strategies for pathogen detection and killing in brachiopods.

IrC2/Bf - A yeast and Borrelia responsive component of the complement system from the hard tick Ixodes ricinus

Ticks possess components of a primordial complement system that presumably play a role in the interaction of the tick immune system with tick-borne pathogens and affect their transmission. Here we characterized a novel complement component, tagged as IrC2/Bf, from the hard tick Ixodes ricinus, the principal vector of Lyme disease in Europe. IrC2/Bf is a multi-domain molecule composed of 5-7 CCP modules, varied by alternative splicing, followed by a von Willebrand factor A domain and a C-terminal trypsin-like domain. The primary structure and molecular architecture of IrC2/Bf displays the closest homology to the C3-complement component convertases described in horseshoe crabs. The irc2/bf gene is mainly expressed in the tick fat body associated with the trachea and, as determined by western blotting, the protein is present in low amounts in tick hemolymph. Expression of irc2/bf mRNA was significantly up-regulated in response to the intra-hemocoelic injection of the yeast Candida albicans and all tested Borrelia sp. strains (B. burgdorferi NE5264, B. burgdorferi CB26, B. garinii MSLB, B. afzelii CB43), but was not affected by injection of model Gram-negative and Gram-positive bacteria or the aseptic injection control. In-line with these results, RNAi-mediated silencing of irc2/bf inhibited phagocytosis of B. afzelii and C. albicans but not the other bacteria. Tissue expression profiles, specific responses to microbial challenges, and patterns of phagocytic phenotypes upon RNAi silencing observed for IrC2/Bf match well with the previously reported characteristics of I. ricinus C3-related molecule 1 (IrC3-1). Therefore we presume that IrC2/Bf functions as a convertase in the same complement activation pathway protecting ticks against yeast and Borrelia infection.

Characterization of Ixodes ricinus Fibrinogen-Related Proteins (Ixoderins) Discloses Their Function in the Tick Innate Immunity

Ticks are important vectors of serious human and animal disease-causing organisms, but their innate immunity can fight invading pathogens and may have the ability to reduce or block transmission to mammalian hosts. Lectins, sugar-binding proteins, can distinguish between self and non-self-oligosaccharide motifs on pathogen surfaces. Although tick hemolymph possesses strong lectin activity, and several lectins have already been isolated and characterized, little is known about the implementation of these molecules in tick immunity. Here, we have described and functionally characterized fibrinogen-related protein (FReP) lectins in Ixodes ticks. We have shown that the FReP family contains at least 27 genes (ixoderins, ixo) that could, based on phylogenetic and expression analyses, be divided into three groups with differing degrees of expansion. By using RNA interference-mediated gene silencing (RNAi) we demonstrated that IXO-A was the main lectin in tick hemolymph. Further, we found that ixoderins were important for phagocytosis of Gram-negative bacteria and yeasts by tick hemocytes and that their expression was upregulated upon injection of microbes, wounding, or after blood feeding. However, although the tick hemocytes could swiftly phagocytose Borrelia afzelii spirochetes, their transmission and burst of infection in mice was not altered. Our results demonstrate that tick ixoderins are crucial immune proteins that work as opsonins in the tick hemolymph, targeting microbes for phagocytosis or lysis.

The Role of Complement in Cnidarian-Dinoflagellate Symbiosis and Immune Challenge in the Sea Anemone Aiptasia pallida

The complement system is an innate immune pathway that in vertebrates, is responsible for initial recognition and ultimately phagocytosis and destruction of microbes. Several complement molecules including C3, Factor B, and mannose binding lectin associated serine proteases (MASP) have been characterized in invertebrates and while most studies have focused on their conserved role in defense against pathogens, little is known about their role in managing beneficial microbes. The purpose of this study was to (1) characterize complement pathway genes in the symbiotic sea anemone Aiptasia pallida, (2) investigate the evolution of complement genes in invertebrates, and (3) examine the potential dual role of complement genes Factor B and MASP in the onset and maintenance of cnidarian-dinoflagellate symbiosis and immune challenge using qPCR based studies. The results demonstrate that A. pallida has multiple Factor B genes (Ap_Bf-1, Ap_Bf-2a, and Ap_Bf-2b) and one MASP gene (Ap_MASP). Phylogenetic analysis indicates that the evolutionary history of complement genes is complex, and there have been many gene duplications or gene loss events, even within members of the same phylum. Gene expression analyses revealed a potential role for complement in both onset and maintenance of cnidarian-dinoflagellate symbiosis and immune challenge. Specifically, Ap_Bf-1 and Ap_MASP are significantly upregulated in the light at the onset of symbiosis and in response to challenge with the pathogen Serratia marcescens suggesting that they play a role in the initial recognition of both beneficial and harmful microbes. Ap_Bf-2b in contrast, was generally downregulated during the onset and maintenance of symbiosis and in response to challenge with S. marcescens. Therefore, the exact role of Ap_Bf-2b in response to microbes remains unclear, but the results suggest that the presence of microbes leads to repressed expression. Together, these results indicate functional divergence between Ap_Bf-1 and Ap_Bf-2b, and that Ap_Bf-1 and Ap_MASP may be functioning together in an ancestral hybrid of the lectin and alternative complement pathways. Overall, this study provides information on the role of the complement system in a basal metazoan and its role in host-microbe interactions.

Characterization of a Gene Coding for the Complement System Component FB from Loxosceles laeta Spider Venom Glands

The human complement system is composed of more than 30 proteins and many of these have conserved domains that allow tracing the phylogenetic evolution. The complement system seems to be initiated with the appearance of C3 and factor B (FB), the only components found in some protostomes and cnidarians, suggesting that the alternative pathway is the most ancient. Here, we present the characterization of an arachnid homologue of the human complement component FB from the spider Loxosceles laeta. This homologue, named Lox-FB, was identified from a total RNA L. laeta spider venom gland library and was amplified using RACE-PCR techniques and specific primers. Analysis of the deduced amino acid sequence and the domain structure showed significant similarity to the vertebrate and invertebrate FB/C2 family proteins. Lox-FB has a classical domain organization composed of a control complement protein domain (CCP), a von Willebrand Factor domain (vWFA), and a serine protease domain (SP). The amino acids involved in Mg2+ metal ion dependent adhesion site (MIDAS) found in the vWFA domain in the vertebrate C2/FB proteins are well conserved; however, the classic catalytic triad present in the serine protease domain is not conserved in Lox-FB. Similarity and phylogenetic analyses indicated that Lox-FB shares a major identity (43%) and has a close evolutionary relationship with the third isoform of FB-like protein (FB-3) from the jumping spider Hasarius adansoni belonging to the Family Salcitidae.

Evolution of the complement system in protostomes revealed by de novo transcriptome analysis of six species of Arthropoda

To elucidate the evolutionary history of the complement system in Arthropoda, de novo transcriptome analysis was performed with six species among the Chelicerata, Myriapoda, and Crustacea, and complement genes were identified based on their characteristic domain structures. Complement C3 and factor B (FB) were identified from a sea spider, a jumping spider, and a centipede, but not from a sea firefly or two millipede species. No additional complement components identifiable by their characteristic domain structures were found from any of these six species. These results together with genome sequence information for several species of the Hexapoda suggest that the common ancestor of the Arthropoda possessed a simple complement system comprising C3 and FB, and thus resembled the alternative pathway of the mammalian complement system. It was lost at least twice independently during the evolution of Arthropoda in the millipede lineage and in the common ancestor of Crustacea and Hexapoda.

Evolution of the complement system

The mammalian complement system constitutes a highly sophisticated body defense machinery comprising more than 30 components. Research into the evolutionary origin of the complement system has identified a primitive version composed of the central component C3 and two activation proteases Bf and MASP in cnidaria. This suggests that the complement system was established in the common ancestor of eumetazoa more than 500 million years ago. The original activation mechanism of the original complement system is believed to be close to the mammalian lectin and alternative activation pathways, and its main role seems to be opsonization and induction of inflammation. This primitive complement system has been retained by most deuterostomes without major change until the appearance of jawed vertebrates. At this stage, duplication of the C3, Bf and MASP genes as well as recruitment of membrane attack components added the classical and lytic pathways to the primitive complement system, converting it to the modern complement system. In contrast, the complement system was lost multiple times independently in the protostome lineage.