Characterization of the immunoglobulin repertoire of the spiny dogfish (Squalus acanthias)

Evaluation of variable new antigen receptors (vNARs) as a novel cathepsin S (CTSS) targeting strategy

Aberrant activity of the cysteine protease Cathepsin S (CTSS) has been implicated across a wide range of pathologies. Notably in cancer, CTSS has been shown to promote tumour progression, primarily through facilitating invasion and migration of tumour cells and augmenting angiogenesis. Whilst an attractive therapeutic target, more efficacious CTSS inhibitors are required. Here, we investigated the potential application of Variable New Antigen Receptors (vNARs) as a novel inhibitory strategy. A panel of potential vNAR binders were identified following a phage display panning process against human recombinant proCTSS. These were subsequently expressed, purified and binding affinity confirmed by ELISA and SPR based approaches. Selected lead clones were taken forward and were shown to inhibit CTSS activity in recombinant enzyme activity assays. Further assessment demonstrated that our lead clones functioned by a novel inhibitory mechanism, by preventing the activation of proCTSS to the mature enzyme. Moreover, using an intrabody approach, we exhibited the ability to express these clones intracellularly and inhibit CTSS activity whilst lead clones were also noted to impede cell invasion in a tumour cell invasion assay. Collectively, these findings illustrate a novel mechanistic approach for inhibiting CTSS activity, with anti-CTSS vNAR clones possessing therapeutic potential in combating deleterious CTSS activity. Furthermore, this study exemplifies the potential of vNARs in targeting intracellular proteins, opening a range of previously "undruggable" targets for biologic-based therapy.

IgNAR antibody: Structural features, diversity and applications

In response to the invasion of exogenous microorganisms, one of the defence strategies of the immune system is to produce antibodies. Cartilaginous fish is among those who evolved the earliest humoral immune system that utilizes immunoglobulin-type antibodies. The cartilaginous fish antibodies fall into three categories: IgW, IgM, and IgNAR. The shark Immunoglobulin Novel Antigen Receptor (IgNAR) constitutes disulfide-bonded dimers of two protein chains, similar to the heavy chain of mammalian IgGs. Shark IgNAR is the primary antibody of a shark's adaptive immune system with a serum concentration of 0.1-1.0 mg/mL. Its structure comprises of one variable (V) domain (VNAR) and five constant (C1 -C5) domains in the secretory form. VNARs are classified into several subclasses based on specific properties such as the quantity and position of additional non-canonical cysteine (Cys) residues in the VNAR. The VDJ recombination in IgNAR comprises various fragments; one variable component, three diverse sections, one joining portion, and a solitary arrangement of constant fragments framed in each IgNAR gene cluster. The re-arrangement happens just inside this gene cluster bringing about a VD1D2D3J segment. Therefore, four re-arrangement procedures create the entire VNAR space. IgNAR antibody can serve as an excellent diagnostic, therapeutic, and research tool because it has a smaller size, high specificity for antigen-binding, and perfect stability. The domain characterization, structural features, types, diversity and therapeutic applications of IgNAR molecules are highlighted in this review. It would be helpful for further research on IgNAR antibodies acting as an essential constituent of the adaptive immune system and a potential therapeutic agent.

Generation of VNAR Libraries from Immunized Sharks and Selection of Target-Specific Clones

Cartilaginous fishes (sharks, skates, rays, and chimeras) are the most phylogenetically distant lineage relative to mammals in which somatically rearranging immunoglobulins (Igs or antibodies) have also been found. Alongside their conventional (heavy-light chain) isotypes, IgM and IgW, sharks produce the novel isotype, IgNAR, a heavy-chain homodimer. Naturally lacking light chains, antigen binding is mediated by two highly soluble and independently functioning variable domains, or VNARs, each having a molecular weight of approximately 12 kDa. The small size, high affinity for antigen, and extreme structural stability of single-domain VNARs make them an emerging prospect for use in therapeutic, diagnostic, and research applications. In this chapter, we detail the immunization protocol we use to raise an antigen-specific IgNAR response in the nurse shark (Ginglymostoma cirratum), the subsequent cloning of the variable domains from this isotype, and the selection of antigen-specific VNARs by phage display.

Bamboo Shark as a Small Animal Model for Single Domain Antibody Production

The development of shark single domain antibodies (sdAbs) is hindered by the high cost and tediousness of large-sized shark farming. Here, we demonstrated white-spotted bamboo sharks (Chiloscyllium plagiosum) being cultivated commercially as a promising small animal model to produce sdAbs. We found that immunoglobulin new antigen receptor (IgNAR) presented in bamboo shark genome, transcriptome, and plasma. Four complete IgNAR clusters including variable domains (vNARs) were discovered in the germline, and the Variable–Joining pair from IgNAR1 cluster was dominant from immune repertoires in blood. Bamboo sharks developed effective immune responses upon green fluorescent protein (GFP), near-infrared fluorescent protein iRFP713, and Freund’s adjuvant immunization revealed by elevated lymphocyte counts and antigen specific IgNAR. Before and after immunization, the complementarity determining region 3 (CDR3) of IgNAR were the major determinant of IgNAR diversity revealed by 400-bp deep sequencing. To prove that bamboo sharks could produce high-affinity IgNAR, we isolated anti-GFP and anti-iRFP713 vNARs with up to 0.3 and 3.8 nM affinities, respectively, from immunized sharks. Moreover, we constructed biparatopic vNARs with the highest known affinities (20.7 pM) to GFP and validated the functions of anti-GFP vNARs as intrabodies in mammalian cells. Taken together, our study will accelerate the discovery and development of bamboo shark sdAbs for biomedical industry at low cost and easy operation.

Immunoglobulin D and its encoding genes: An updated review

Since the identification of a functional Cδ gene in ostriches, immunoglobulin (Ig) D has been considered to be an extremely evolutionarily conserved Ig isotype besides the IgM found in all classes of jawed vertebrates. However, in contrast to IgM (which remains stable over evolutionary time), IgD shows considerable structural plasticity among vertebrate species and, moreover, its functions are far from elucidated even in humans and mice. Recently, several studies have shown that high expression of the IgD-B-cell receptor (IgD-BCR) may help physiologically autoreactive B cells survive in peripheral lymphoid tissues thanks to unresponsiveness to self-antigens and help their entry into germinal centers to "redeem" autoreactivity via somatic hypermutation. Other studies have demonstrated that secreted IgD may enhance mucosal homeostasis and immunity by linking B cells with basophils to optimize T-helper-2 cell-mediated responses and to constrain IgE-mediated basophil degranulation. Herein, we review the new discoveries on IgD-encoding genes in jawed vertebrates in the past decade. We also highlight advances in the functions of the IgD-BCR and secreted IgD in humans and mice.

The immunoglobulins of cartilaginous fishes

Cartilaginous fishes, comprising the chimeras, sharks, skates, and rays, split from the common ancestor with other jawed vertebrates approx. 450 million years ago. Being the oldest extant taxonomic group to possess an immunoglobulin (Ig)-based adaptive immune system, examination of this group has taught us much about the evolution of adaptive immunity, as well as the conserved and taxon-specific characteristics of Igs. Significant progress has been made analyzing sequences from numerous genomic and transcriptomic data sets. These findings have been supported by additional functional studies characterizing the Igs and humoral response of sharks and their relatives. This review will summarize what we have learned about the genomic organization, protein structure, and in vivo function of these Ig isotypes in cartilaginous fishes and highlight the areas where our knowledge is still lacking.

Sequence structure character of IgNAR Sec in whitespotted bamboo shark (Chiloscyllium plagiosum)

Whitespotted bamboo shark (Chiloscyllium plagiosum) is a demersal cartilaginous fish with an adaptive immune system founded upon immunoglobulins. In this manuscript, we characterize the IgNAR of the whitespotted bamboo shark. A newly discovered alternative splicing form of IgNAR Sec (IgNAR_short (ΔC2-C3) Sec) was identified, in which the C1 domain was spliced directly to the C4 domain, the process resulted in a molecule containing three constant domains. However, a single unpaired cysteine remains in the highly flexible hinge region, contributing in the formation of an interchain disulfide bond. Two types of C1 domain were found, and the one lacking a short α-helix showed lower proportion. This finding suggests that short α-helices might be important to the stability of IgNAR. High-throughput sequencing revealed that the percentage of V_NAR types significantly vary between the diverse species of sharks. The variable region of IgNAR (the V_NAR) with small size and stabilization is a potential candidate for immunotherapeutic agents. The structure and stability analysis in this manuscript may be useful in future biomedical applications.

Deiminated proteins and extracellular vesicles - Novel serum biomarkers in whales and orca

Peptidylarginine deiminases (PADs) are a family of phylogenetically conserved calcium-dependent enzymes which cause post-translational protein deimination. This can result in neoepitope generation, affect gene regulation and allow for protein moonlighting via functional and structural changes in target proteins. Extracellular vesicles (EVs) carry cargo proteins and genetic material and are released from cells as part of cellular communication. EVs are found in most body fluids where they can be useful biomarkers for assessment of health status. Here, serum-derived EVs were profiled, and post-translationally deiminated proteins and EV-related microRNAs are described in 5 ceataceans: minke whale, fin whale, humpback whale, Cuvier's beaked whale and orca. EV-serum profiles were assessed by transmission electron microscopy and nanoparticle tracking analysis. EV profiles varied between the 5 species and were identified to contain deiminated proteins and selected key inflammatory and metabolic microRNAs. A range of proteins, critical for immune responses and metabolism were identified to be deiminated in cetacean sera, with some shared KEGG pathways of deiminated proteins relating to immunity and physiology, while some KEGG pathways were species-specific. This is the first study to characterise and profile EVs and to report deiminated proteins and putative effects of protein-protein interaction networks via such post-translationald deimination in cetaceans, revealing key immune and metabolic factors to undergo this post-translational modification. Deiminated proteins and EVs profiles may possibly be developed as new biomarkers for assessing health status of sea mammals.

The Immunoglobulins: New Insights, Implications, and Applications

Immunoglobulins (Igs), as one of the hallmarks of adaptive immunity, first arose approximately 500 million years ago with the emergence of jawed vertebrates. Two events stand out in the evolutionary history of Igs from cartilaginous fish to mammals: (a) the diversification of Ig heavy chain (IgH) genes, resulting in Ig isotypes or subclasses associated with novel functions, and (b) the diversification of genetic and structural strategies, leading to the creation of the antibody repertoire we know today. This review first gives an overview of the IgH isotypes identified in jawed vertebrates to date and then highlights the implications or applications of five new recent discoveries arising from comparative studies of Igs derived from different vertebrate species.

Deiminated proteins in extracellular vesicles and serum of llama (Lama glama)-Novel insights into camelid immunity

Peptidylarginine deiminases (PADs) are phylogenetically conserved calcium-dependent enzymes which post-translationally convert arginine into citrulline in target proteins in an irreversible manner, causing functional and structural changes in target proteins. Protein deimination causes generation of neo-epitopes, affects gene regulation and also allows for protein moonlighting. Furthermore, PADs have been found to be a phylogenetically conserved regulator for extracellular vesicle (EVs) release. EVs are found in most body fluids and participate in cellular communication via transfer of cargo proteins and genetic material. In this study, post-translationally deiminated proteins in serum and serum-EVs are described for the first time in camelids, using the llama (Lama glama L. 1758) as a model animal. We report a poly-dispersed population of llama serum EVs, positive for phylogenetically conserved EV-specific markers and characterised by TEM. In serum, 103 deiminated proteins were overall identified, including key immune and metabolic mediators including complement components, immunoglobulin-based nanobodies, adiponectin and heat shock proteins. In serum, 60 deiminated proteins were identified that were not in EVs, and 25 deiminated proteins were found to be unique to EVs, with 43 shared deiminated protein hits between both serum and EVs. Deiminated histone H3, a marker of neutrophil extracellular trap formation, was also detected in llama serum. PAD homologues were identified in llama serum by Western blotting, via cross reaction with human PAD antibodies, and detected at an expected 70 kDa size. This is the first report of deiminated proteins in serum and EVs of a camelid species, highlighting a hitherto unrecognized post-translational modification in key immune and metabolic proteins in camelids, which may be translatable to and inform a range of human metabolic and inflammatory pathologies.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Deiminated proteins in extracellular vesicles and plasma of nurse shark (Ginglymostoma cirratum) - Novel insights into shark immunity

Peptidylarginine deiminases (PADs) are phylogenetically conserved calcium-dependent enzymes which post-translationally convert arginine into citrulline in target proteins in an irreversible manner, causing functional and structural changes in target proteins. Protein deimination causes generation of neo-epitopes, affects gene regulation and also allows for protein moonlighting. Extracellular vesicles are found in most body fluids and participate in cellular communication via transfer of cargo proteins and genetic material. In this study, post-translationally deiminated proteins and extracellular vesicles (EVs) are described for the first time in shark plasma. We report a poly-dispersed population of shark plasma EVs, positive for phylogenetically conserved EV-specific markers and characterised by TEM. In plasma, 6 deiminated proteins, including complement and immunoglobulin, were identified, whereof 3 proteins were found to be exported in plasma-derived EVs. A PAD homologue was identified in shark plasma by Western blotting and detected an expected 70 kDa size. Deiminated histone H3, a marker of neutrophil extracellular trap formation, was also detected in nurse shark plasma. This is the first report of deiminated proteins in plasma and EVs, highlighting a hitherto unrecognized post-translational modification in key immune proteins of innate and adaptive immunity in shark.

Transcriptomic analysis of immunoglobulin novel antigen receptor (IgNAR) heavy chain constant domains of brownbanded bamboo shark (Chiloscyllium punctatum)

Cartilaginous fish are the evolutionarily oldest group of animals which possess antibodies, T cell receptors and major histocompatibility complex (MHC). The immunoglobulin novel antigen receptor (IgNAR) found in cartilaginous fish is a heavy chain homodimer which lacks light chain. The presence of non-canonical cysteine molecules and lack of CDR2 region make it more significant. To synthesize active binding domains based on variable region of IgNAR (VNAR), knowledge on the constant region dynamics play a significant role. The IgNAR exhibit species variations in its primary sequence features; hence, this study was conducted to determine the IgNAR heavy chain constant domain of the brownbanded bamboo shark (Chiloscyllium punctatum). Peripheral blood leukocytes (PBL) isolated from adult bamboo sharks were used to synthesize a cDNA library. A total of four billion residues of two million sequences (average length 218.41 bp) were obtained. Assembled sequences were aligned with published cartilaginous fish IgNAR constant region sequences. Transcriptome analysis revealed two distinct types of IgNAR in the brownbanded bamboo shark. Also, constant-1 domain sequences displayed 13 unique sequences which may reflect the least number of IgNAR gene clusters. The phylogenetic analysis revealed the closest relationship with the nurse shark (Ginglymostoma cirratum) followed by the wobbegong shark (Orectolobus maculatus) which belong to the same order Orectolobiformes. Analysis of the constant domains of the brownbanded bamboo shark IgNAR revealed an evolutionarily conserved nature and this knowledge can be used to design primers for VNAR cloning. Furthermore, knowledge on the structural features in IgNAR constant domains that increase the stability could be useful in the process of stabilizing human immunoglobulins.

Novel, Anti-hTNF-α Variable New Antigen Receptor Formats with Enhanced Neutralizing Potency and Multifunctionality, Generated for Therapeutic Development

The management of chronic inflammatory diseases, such as inflammatory bowel disease, psoriasis, and rheumatoid arthritis has significantly improved over the last decade with the clinical availability of anti-TNF-α biologics. Despite this undoubted treatment success, a combination of acquired resistance together with an increased risk of systemic complications, means that a significant number of patients either fail to find a suitable targeted therapy or frustratingly discover that an approach that did work is no longer efficacious. Here, we report the isolation and characterization of a new class of super-neutralizing anti-TNF-α biologics formats, the building blocks of which were originally derived as variable new antigen receptor (VNAR) domains from an immunized nurse shark. These parental small, stable VNAR monomers recognize and neutralize tumor necrosis factor (TNF)-α, in cell-based assays, at nanomolar concentrations. However, the simple, single-chain molecular architecture of VNARs allows for easy and multiple reformatting options. Through reformatting, we achieved a 50,000-fold enhancement in in vitro efficacy with super-neutralizing fusion proteins able to block TNF-α induced cytotoxicity in the 2–5 pM range while retaining other functionality through the addition of fusion proteins known to extend serum half-life in vivo. In an in vitro intestinal epithelial barrier dysfunction efficacy model, the lead VNAR domains, restored barrier function and prevented paracellular flux with comparable efficacy to adalimumab (Humira^®). In addition, all multivalent VNAR constructs restored trans-epithelial electrical resistance (TEER) to approximately 94% of the untreated control. Reformatted VNAR domains should be considered as a new class of biologic agents for the treatment of hTNF-α driven diseases; either used systemically with appropriate half-life extension or alternatively where site-specific delivery of small and stable neutralizers may provide improvements to current therapy options.

Analysis of 5518 unique, productively rearranged human VH3-23*01 gene sequences reveals CDR-H3 length-dependent usage of the IGHD2 gene family

Clear and accurate understanding of diversity in antibody complementarity-determining regions (CDRs) is critical for antibody discovery and engineering. Previous observations of antibody CDR-H3 diversity were based on analyzing available antibody sequences in the public databases. The results may not accurately reflect that of natural antibody repertoire due to erroneous species annotation and the presence of man-made CDR loop diversity in public antibody sequence databases. In this study, in a precisely controlled germline context, we explored the relationship between amino acid composition and CDR-H3 length using 5518 unique productively rearranged human VH3-23*01 gene sequences. CDR-H3 length-dependent usage of the Cys-Xn-Cys motif is reported here.

Assembly and Expression of Shark Ig Genes

Sharks are modern descendants of the earliest vertebrates possessing Ig superfamily receptor-based adaptive immunity. They respond to immunogen with Abs that, upon boosting, appear more rapidly and show affinity maturation. Specific Abs and immunological memory imply that Ab diversification and clonal selection exist in cartilaginous fish. Shark Ag receptors are generated through V(D)J recombination, and because it is a mechanism known to generate autoreactive receptors, this implies that shark lymphocytes undergo selection. In the mouse, the ∼2.8-Mb IgH and IgL loci require long-range, differential activation of component parts for V(D)J recombination, allelic exclusion, and receptor editing. These processes, including class switching, evolved with and appear inseparable from the complex locus organization. In contrast, shark Igs are encoded by 100-200 autonomously rearranging miniloci. This review describes how the shark primary Ab repertoire is generated in the absence of structural features considered essential in mammalian Ig gene assembly and expression.

Fish Immunoglobulins

The B cell receptor and secreted antibody are at the nexus of humoral adaptive immunity. In this review, we summarize what is known of the immunoglobulin genes of jawed cartilaginous and bony fishes. We focus on what has been learned from genomic or cDNA sequence data, but where appropriate draw upon protein, immunization, affinity and structural studies. Work from major aquatic model organisms and less studied comparative species are both included to define what is the rule for an immunoglobulin isotype or taxonomic group and what exemplifies an exception.

Analysis of the binding loops configuration and surface adaptation of different crystallized single-domain antibodies in response to various antigens

Monoclonal antibodies have revolutionized the biomedical field through their ubiquitous utilization in different diagnostics and therapeutic applications. Despite this widespread use, their large size and structural complexity have limited their versatility in specific applications. The antibody variable region that is responsible for binding antigen is embodied within domains that can be rescued individually as single-domain antibody (sdAb) fragments. Because of the unique characteristics of sdAbs, such as low molecular weight, high physicochemical stability, and the ability to bind antigens inaccessible to conventional antibodies, they represent a viable alternative to full-length antibodies. Consequently, 149 crystal structures of sdAbs, originating from human (VH), camelids (VHH), or sharks (VNAR), were retrieved from the Protein Data Bank, and their structures were compared. The 3 types of sdAbs displayed complementarity determining regions (CDRs) with different lengths and configurations. CDR3 of the VHH and VNAR domains were dominated by pleated and extended orientations, respectively. Although VNAR showed the smallest average molecular weight and molecular surface area compared with VHH and VH antibodies. However, the solvent accessible surface area measurements of the 3 tested sdAbs types were very similar. All the antihapten VHH antibodies showed pleated CDR3, which were sufficient to create a binding pocket to accommodate haptens (methotrexate and azo dyes) in terms of shape and electrostatic potential. The sdAbs that recognized lysozyme showed more diversity in their CDR3 orientation to enable them to recognize various topographies of lysozyme. Subsequently, the three sdAb classes were different in size and surface area and have shown distinguishable ability to optimize their CDR length and orientation to recognize different antigen classes.

Structural insights and biomedical potential of IgNAR scaffolds from sharks

In addition to antibodies with the classical composition of heavy and light chains, the adaptive immune repertoire of sharks also includes a heavy-chain only isotype, where antigen binding is mediated exclusively by a small and highly stable domain, referred to as vNAR. In recent years, due to their high affinity and specificity combined with their small size, high physicochemical stability and low-cost of production, vNAR fragments have evolved as promising target-binding scaffolds that can be tailor-made for applications in medicine and biotechnology. This review highlights the structural features of vNAR molecules, addresses aspects of their generation using immunization or in vitro high throughput screening methods and provides examples of therapeutic, diagnostic and other biotechnological applications.

Structural and genetic diversity in antibody repertoires from diverse species

The antibody repertoire is the fundamental unit that enables development of antigen specific adaptive immune responses against pathogens. Different species have developed diverse genetic and structural strategies to create their respective antibody repertoires. Here we review the shark, chicken, camel, and cow repertoires as unique examples of structural and genetic diversity. Given the enormous importance of antibodies in medicine and biological research, the novel properties of these antibody repertoires may enable discovery or engineering of antibodies from these non-human species against difficult or important epitopes.

Antibody Repertoires in Fish

As in mammals, cartilaginous and teleost fishes possess adaptive immune systems based on antigen recognition by immunoglobulins (Ig), T cell receptors (TCR), and major histocompatibility complex molecules (MHC) I and MHC II molecules. Also it is well established that fish B cells and mammalian B cells share many similarities, including Ig gene rearrangements, and production of membrane Ig and secreted Ig forms. This chapter provides an overview of the IgH and IgL chains in cartilaginous and bony fish, including their gene organizations, expression, diversity of their isotypes, and development of the primary repertoire. Furthermore, when possible, we have included summaries of key studies on immune mechanisms such as allelic exclusion, somatic hypermutation, affinity maturation, class switching, and mucosal immune responses.

The immunoglobulins of cold-blooded vertebrates

Although lymphocyte-like cells secreting somatically-recombining receptors have been identified in the jawless fishes (hagfish and lamprey), the cartilaginous fishes (sharks, skates, rays and chimaera) are the most phylogenetically distant group relative to mammals in which bona fide immunoglobulins (Igs) have been found. Studies of the antibodies and humoral immune responses of cartilaginous fishes and other cold-blooded vertebrates (bony fishes, amphibians and reptiles) are not only revealing information about the emergence and roles of the different Ig heavy and light chain isotypes, but also the evolution of specialised adaptive features such as isotype switching, somatic hypermutation and affinity maturation. It is becoming increasingly apparent that while the adaptive immune response in these vertebrate lineages arose a long time ago, it is most definitely not primitive and has evolved to become complex and sophisticated. This review will summarise what is currently known about the immunoglobulins of cold-blooded vertebrates and highlight the differences, and commonalities, between these and more “conventional” mammalian species.

What the shark immune system can and cannot provide for the expanding design landscape of immunotherapy

INTRODUCTION

Sharks have successfully lived in marine ecosystems, often atop food chains as apex predators, for nearly one and a half billion years. Throughout this period they have benefitted from an immune system with the same fundamental components found in terrestrial vertebrates like man. Additionally, sharks have some rather extraordinary immune mechanisms which mammals lack.

AREAS COVERED

In this review the author briefly orients the reader to sharks, their adaptive immunity, and their important phylogenetic position in comparative immunology. The author also differentiates some of the myths from facts concerning these animals, their cartilage, and cancer. From thereon, the author explores some of the more remarkable capabilities and products of shark lymphocytes. Sharks have an isotype of light chain-less antibodies that are useful tools in molecular biology and are moving towards translational use in the clinic. These special antibodies are just one of the several tricks of shark lymphocyte antigen receptor systems.

EXPERT OPINION

While shark cartilage has not helped oncology patients, shark immunoglobulins and T cell receptors do offer exciting novel possibilities for immunotherapeutics. Much of the clinical immunology developmental pipeline has turned from traditional vaccines to passively delivered monoclonal antibody-based drugs for targeted depletion, activation, blocking and immunomodulation. The immunogenetic tools of shark lymphocytes, battle-tested since the dawn of our adaptive immune system, are well poised to expand the design landscape for the next generation of immunotherapy products.

Improving the pharmacokinetic properties of biologics by fusion to an anti-HSA shark VNAR domain

Advances in recombinant antibody technology and protein engineering have provided the opportunity to reduce antibodies to their smallest binding domain components and have concomitantly driven the requirement for devising strategies to increase serum half-life to optimise drug exposure, thereby increasing therapeutic efficacy. In this study, we adopted an immunization route to raise picomolar affinity shark immunoglobulin new antigen receptors (IgNARs) to target human serum albumin (HSA). From our model shark species, Squalus acanthias, a phage display library encompassing the variable binding domain of IgNAR (VNAR) was constructed, screened against target, and positive clones were characterized for affinity and specificity. N-terminal and C-terminal molecular fusions of our lead hit in complex with a naïve VNAR domain were expressed, purified and exhibited the retention of high affinity binding to HSA, but also cross-selectivity to mouse, rat and monkey serum albumin both in vitro and in vivo. Furthermore, the naïve VNAR had enhanced pharmacokinetic (PK) characteristics in both N- and C-terminal orientations and when tested as a three domain construct with naïve VNAR flanking the HSA binding domain at both the N and C termini. Molecules derived from this platform technology also demonstrated the potential for clinical utility by being available via the subcutaneous route of delivery. This study thus demonstrates the first in vivo functional efficacy of a VNAR binding domain with the ability to enhance PK properties and support delivery of multifunctional therapies.

Shark IgW C region diversification through RNA processing and isotype switching

Sharks and skates represent the earliest vertebrates with an adaptive immune system based on lymphocyte Ag receptors generated by V(D)J recombination. Shark B cells express two classical Igs, IgM and IgW, encoded by an early, alternative gene organization consisting of numerous autonomous miniloci, where the individual gene cluster carries a few rearranging gene segments and one C region, μ or ω. We have characterized eight distinct Ig miniloci encoding the nurse shark ω H chain. Each cluster consists of VH, D, and JH segments and six to eight C domain exons. Two interspersed secretory exons, in addition to the 3'-most C exon with tailpiece, provide the gene cluster with the ability to generate at least six secreted isoforms that differ as to polypeptide length and C domain combination. All clusters appear to be functional, as judged by the capability for rearrangement and absence of defects in the deduced amino acid sequence. We previously showed that IgW VDJ can perform isotype switching to μ C regions; in this study, we found that switching also occurs between ω clusters. Thus, C region diversification for any IgW VDJ can take place at the DNA level by switching to other ω or μ C regions, as well as by RNA processing to generate different C isoforms. The wide array of pathogens recognized by Abs requires different disposal pathways, and our findings demonstrate complex and unique pathways for C effector function diversity that evolved independently in cartilaginous fishes.

Humoral immune response of the small-spotted catshark, Scyliorhinus canicula

Cartilaginous fishes are the oldest group in which an adaptive immune system based on immunoglobulin-superfamily members is found. This manuscript compares humoral immune function in small-spotted catshark (Scyliorhinus canicula) with that described for spiny dogfish (Squalus acanthias), another member of the Squalomorphi superorder, and nurse shark, the model for humoral immunity in elasmobranchs and a member of the Galeomorphi superorder. Although small-spotted catshark and nurse shark are separated by over 200 million years we found that immunoglobulin isoforms are well conserved between the two species. However, the plasma protein profile of small-spotted catshark was most similar to that of spiny dogfish, with low levels of pentameric IgM, and IgNAR present as a multimer in plasma rather than a monomer. We show that an antigen-specific monomeric IgM response, with a profile similar to that described previously for nurse sharks, can be raised in small-spotted catshark. Lacking polyclonal or monoclonal antibody reagents for detecting catshark IgNAR we investigated phage-display and recombinant Fc-fusion protein expression as alternative methods to look for an antigen-specific response for this isotype. However, we could find no evidence of an antigen-specific IgNAR in the animals tested using either of these techniques. Thus, unlike nurse sharks where antigen-specific monomeric IgM and IgNAR appear together, it seems there may be a temporal or complete 'uncoupling' of these isotypes during a humoral response in the small-spotted catshark.