Antibody repertoire development in cartilaginous fish

Nature and nurture of catalytic antibodies

Immunoglobulins (antibodies) frequently express constitutive functions. Two such functions are nucleophilic catalysis and the reversible binding to B-cell superantigens. Constitutive or "naturally-occurring" antibodies are produced spontaneously from germline genetic information. The antibody structural elements mediating the constitutive functions have originated over millions of years of phylogenic evolution, contrasting with antigen-driven, somatic sequence diversification of the complementarity determining regions (CDR) that underlies the better-known high affinity antigen binding function of antibodies. Often, the framework regions (FRs) play a dominant role in antibody constitutive functions. Catalytic antibody subsets with promiscuous, autoantigen-directed and microbe-directed specificities have been identified. Mucosal antibodies may be specialized to express high-level catalytic activity against microbes transmitted by the mucosal route, exemplified by constitutive production of IgA class antibodies in mucosal secretions that catalyze the cleavage of HIV gp120. Catalytic specificity can be gained by constitutive noncovalent superantigen binding at the FRs and by adaptive development of noncovalent classical antigen or superantigen binding, respectively, at the CDRs and FRs. Growing evidence suggests important functional roles for catalytic antibodies in homeostasis, autoimmune disease and protection against infection. Adaptive antibody responses to microbial superantigens are proscribed underphysiological circumstances. Covalent electrophilic immunogen binding to constitutively expressed nucleophilic sites in B-cell receptors bypasses the restriction on adaptive antibody production, and simultaneous occupancy of the CDR binding site by a stimulatory antigenic epitope can also overcome the downregulatory effect of superantigen binding at the FRs. These concepts may be useful for developing novel vaccines that capitalize and improve on constitutive antibody functions for protection against microbes.

Overview and discovery of IgNARs and generation of VNARs

Immunoglobulin new antigen receptors (IgNARs) from sharks are a distinct class of immune receptors, consisting of homodimers with no associated light chains. Antigen binding is encapsulated within single VNAR immunoglobulin domains of 13-14 kDa in size. This small size and single domain format means that they exhibit considerable stability and are readily produced in heterologous protein expression systems. In this chapter, I describe the history and discovery of IgNARs, the development of VNAR biotechnology, and highlight important factors in VNAR protein production.

Generation and isolation of target-specific single-domain antibodies from shark immune repertoires

The drive to exploit novel targets and biological pathways has lead to the expansion of classical antibody research into innovative fragment adaptations and novel scaffolds. The hope being that alternative or cryptic epitopes may be targeted, tissue inaccessibility may be overcome, and easier engineering options will facilitate multivalent, multi-targeting approaches. To this end, we have been isolating shark single domains to gain a greater understanding of their potential as therapeutic agents. Their unique shape, small size, inherent stability, and simple molecular architecture make them attractive candidates from a drug discovery perspective. Here we describe protocols to capture the immune repertoire of an immunized shark species and to build and select via phage-display target-specific IgNAR variable domains (VNARs).

Antibody repertoire development in fetal and neonatal piglets XXI. Usage of most VH genes remains constant during fetal and postnatal development

Usage of variable region gene segments during development of the antibody repertoire in mammals is unresolved in part because of the complexity of the locus in mice and humans and the difficulty of distinguishing intrinsic from extrinsic influences in these species. We present the first vertical studies on VH usage that spans the fetal and neonatal period using the piglet model. We tracked VH usage in DNA rearrangements and in VDJ transcripts throughout 75 days of gestation (DG) in outbred fetuses, thereafter in outbred germfree and colonized isolator piglets, isolator piglets infected with swine influenza and in conventionally reared nematode-infected adults. Seven VH genes account for >90% of the pre-immune repertoire which is the same among tissues and in both transcripts and DNA rearrangements. Statistical modeling supports the view that proportional usage of the major genes remains constant during fetal life and that postnatal usage ranking is similar to that during fetal life. Changes in usage ranking are developmental not antigen dependent. In this species exposure to environmental antigens results in diversification of the repertoire by somatic hypermutation of the same small number of VH genes that comprise the pre-immune repertoire, not by using other VH gene available in the germline. Therefore in swine a small number of VH genes shape the antibody repertoire throughout life questioning the need for extensive VH polygeny.

Activation-induced cytidine deaminase structure and functions: a species comparative view

In the ten years since the discovery of activation-induced cytidine deaminase (AID) there has been considerable effort to understand the mechanisms behind this enzyme's ability to target and modify immunoglobulin genes leading to somatic hypermutation and class switch recombination. While the majority of research has focused on mouse and human models of AID function, work on other species, from lamprey to rabbit and sheep, has taught us much about the scope of functions of the AID mutator. This review takes a species-comparative approach to what has been learned about the AID mutator enzyme and its role in humoral immunity.

Immunoglobulin light (IgL) chains in ectothermic vertebrates

Four major ancesteral IgL isotypes have been identified κ, λ, σ and σ-cart. However, depending on the vertebrate class the genomic representation of these isotypes differs in regards to what is encoded in the germline and how these genes are organized. Also, the relative contribution of each isotype in immune responses varies. This review focuses on the IgL chains of ectothermic vertebrates, specifically the number of different isotypes, their phylogenetic relationship, genomic organizations and expression.

Molecular cloning and characterization of European seabass (Dicentrarchus labrax) and Gilthead seabream (Sparus aurata) complement component C3

We present the complete C3 cDNA sequence of Gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) and its molecular characterization with a descriptive analysis of their structural elements. We obtained one sequence for Gilthead seabream (gsbC3) which encodes a predicted protein of 1656 amino acids, and two sequences for European seabass (esbC3_1 and esbC3_2) which encode two predicted proteins of 1654 and 1587 amino acids respectively. All sequences present the characteristic structural features of C3 but interestingly esbC3_2 lacks the anaphylotoxin domain and the cysteine residue responsible for thiolester bond formation. Moreover, we have detected and quantified (by real-time PCR-based absolute quantification) specific isoform expression in European seabass depending on pathogen and density conditions in vivo. In addition, we have analyzed the tissue distribution pattern of European seabass and Gilthead seabream C3 genes under crowding stress and under pathological challenges in vivo, and we have observed that crowding and infection status provoke changes in expression levels, tissue expression pattern and C3 isoform expression balance.

Description of an elasmobranch TCR coreceptor: CD8α from Rhinobatos productus

Cell-mediated immunity plays an essential role for the control and eradication of intracellular pathogens. To learn more about the evolutionary origins of the first signal (Signal 1) for T-cell activation, we cloned CD8α from an elasmobranch, Rhinobatos productus. Similar to full-length CD8α cDNAs from other vertebrates, Rhpr-CD8α (1800bp) encodes a 219 amino acid open reading frame composed of a signal peptide, an extracellular IgSF V domain and a stalk/hinge region followed by a well-conserved transmembrane domain and cytoplasmic tail. Overall, the mature Rhpr-CD8α protein (201 aa) displays ∼ 30% amino acid identity with mammalian CD8α including absolute conservation of cysteine residues involved in the IgSf V domain fold and dimerization of CD8αα and CD8αβ. One prominent feature is the absence of the LCK association motif (CXC) that is needed for achieving signal 1 in tetrapods. Both elasmobranch and teleost CD8α protein sequences possess a similar but distinctly different motif (CXH) in the cytoplasmic tail. The overall genomic structure of CD8α has been conserved during the course of vertebrate evolution both for the number of exons and phase of splicing. Finally, quantitative RTPCR demonstrated that elasmobranch CD8α is expressed in lymphoid-rich tissues similar to CD8 in other vertebrates. The results from this study indicate the existence of CD8 prior to the emergence of the gnathostomes (>450 MYA) while providing evidence that the canonical LCK association motif in mammals is likely a derived characteristic of tetrapod CD8α, suggesting potential differences for T-cell education and activation in the various gnathostomes.

Targeting the hepatitis B virus precore antigen with a novel IgNAR single variable domain intrabody

The Hepatitis B virus precore protein is processed in the endoplasmic reticulum (ER) into secreted hepatitis B e antigen (HBeAg), which acts as an immune tolerogen to establish chronic infection. Downregulation of secreted HBeAg should improve clinical outcome, as patients who effectively respond to current treatments (IFN-α) have significantly lower serum HBeAg levels. Here, we describe a novel reagent, a single variable domain (V(NAR)) of the shark immunoglobulin new antigen receptor (IgNAR) antibodies. V(NAR)s possess advantages in stability, size (~14 kDa) and cryptic epitope recognition compared to conventional antibodies. The V(NAR) domain displayed biologically useful affinity for recombinant and native HBeAg, and recognised a unique conformational epitope. To assess therapeutic potential in targeting intracellular precore protein to reduce secreted HBeAg, the V(NAR) was engineered for ER-targeted in vitro delivery to function as an intracellular antibody (intrabody). In vitro data from HBV/precore hepatocyte cell lines demonstrated effective intrabody regulation of precore/HBeAg.

cDNA cloning of the immunoglobulin heavy chain genes in banded houndshark Triakis scyllium

In this study, cDNAs encoding the secreted forms of the immunoglobulin (Ig) heavy chains of IgM, IgNAR, and IgW were cloned from the banded houndshark Triakis scyllium. Two clones for the IgM heavy chains encoded 569 and 570 amino acids, whose conserved (C) region showed 47-70% amino acid identities to those reported in other cartilaginous fish. Four clones for the IgNAR encoded 673-670 amino acids with conserved Ig-superfamily domains. The IgNAR C region showed 56-69% amino acid identities to those so far reported. High-throughput sequencing revealed that in most of the IgNAR sequences, the two variable regions (CDR1 and CDR3) each possess a cysteine residue. Three types of IgW were identified; one contained Ig-superfamily domains that are in other cartilaginous fish, one lacks the 3rd domain in the constant region, and one lacks the 3rd to 5th domains. Despite these differences, the IgW isoforms clustered with IgWs of other cartilaginous fishes and the C regions showed 47-89% amino acid identities. mRNAs for IgM and IgNAR were detected in various tissues, while IgW mRNA was mainly detected in pancreas. The banded hounded shark also has IgM, IgW and IgNAR as well as the other cartilaginous fish with unique IgW isoform.

Evolution of duplicated IgH loci in Atlantic salmon, Salmo salar

BACKGROUND

The Atlantic salmon (Salmo salar) immunoglobulin heavy chain (IgH) locus possesses two parallel IgH isoloci (IGH-A and IGH-B), that are related to the genomic duplication event in the family Salmonidae. These duplicated IgH loci in Atlantic salmon provide a unique opportunity to examine the mechanisms of genome diversity and genome evolution of the IgH loci in vertebrates. In this study, we defined the structure of these loci in Atlantic salmon, and sequenced 24 bacterial artificial chromosome (BAC) clones that were assembled into the IGH-A (1.1 Mb) and IGH-B (0.9 Mb) loci. In addition, over 7,000 cDNA clones from the IgH variable (VH) region have been sequenced and analyzed.

RESULTS

The present study shows that the genomic organization of the duplicated IgH loci in Atlantic salmon differs from that in other teleosts and other vertebrates. The loci possess multiple Cτ genes upstream of the Cμ region, with three of the Cτ genes being functional. Moreover, the duplicated loci possess over 300 VH segments which could be classified into 18 families. This is the largest number of VH families currently defined in any vertebrate. There were significant structural differences between the two loci, indicating that both IGH-A and -B loci have evolved independently in the short time after the recent genome duplication approximately 60 mya.

CONCLUSIONS

Our results indicate that the duplication of the IgH loci in Atlantic salmon significantly contributes to the increased diversity of the antibody repertoire, as compared with the single IgH locus in other vertebrates.

The cellular context of AID expressing cells in fish lymphoid tissues

It has long been held that the cold-blooded vertebrates lack mammalian-like germinal centers, though they do have affinity maturation and the immunoglobulin mutator activation-induced cytidine deaminase or AID. Using AID as a marker of sites of somatic hypermutation, we have identified discrete cell clusters of up to several thousand cells, in the spleen and kidney of channel catfish (Ictalurus punctatus), which may be primordial germinal centers. In situ hybridization revealed that AID expressing cells are interspersed or surrounded by a population of pigmented CSF1-R expressing cells called melano-macrophages. Significantly, melano-macrophages or associated reticular cells have been previously noted for their ability to retain soluble antigen on or near their surface for several weeks following vaccination. Laser capture microdissection and RT-PCR were used to establish that these cell clusters also contained cells expressing Ig heavy chain transcripts as well as transcripts of TcRbeta and the putative CD4 homologue of fish. These observations, coupled with past work showing that mutations develop in B-cell lineages in fishes, allow us to develop a model for how affinity maturation may have evolved in early gnathostome vertebrates.

Toxin-specific antibodies for the treatment of Clostridium difficile: current status and future perspectives

Therapeutic agents targeting bacterial virulence factors are gaining interest as non-antibiotic alternatives for the treatment of infectious diseases. Clostridium difficile is a Gram-positive pathogen that produces two primary virulence factors, enterotoxins A and B (TcdA and TcdB), which are responsible for Clostridium difficile-associated disease (CDAD) and are targets for CDAD therapy. Antibodies specific for TcdA and TcdB have been shown to effectively treat CDAD and prevent disease relapse in animal models and in humans. This review summarizes the various toxin-specific antibody formats and strategies under development, and discusses future directions for CDAD immunotherapy, including the use of engineered antibody fragments with robust biophysical properties for systemic and oral delivery.

Phylogeny, genomic organization and expression of lambda and kappa immunoglobulin light chain genes in a reptile, Anolis carolinensis

The reptiles are the last major taxon of jawed vertebrates in which immunoglobulin light chain isotypes have not been well characterized. Using the recently released genome sequencing data, we show in this study that the reptile Anolis carolinensis expresses both lambda and kappa light chain genes. The genomic organization of both gene loci is structurally similar to their respective counterparts in mammals. The identified lambda locus contains three constant region genes each preceded by a joining gene segment, and a total of 37 variable gene segments. In contrast, the kappa locus contains only a single constant region gene, and two joining gene segments with a single family of 14 variable gene segments located upstream. Analysis of junctions of the recombined VJ transcripts reveals a paucity of N and P nucleotides in both expressed lambda and kappa sequences. These results help us to understand the generation of the immunoglobulin repertoire in reptiles and immunoglobulin evolution in vertebrates.

Beneficial catalytic immunity to abeta peptide

We review attempts to treat Alzheimer disease with antibodies that bind amyloid beta peptide (Abeta) and the feasibility of developing catalytic antibodies for this purpose. Naturally occurring immunoglobulin M (IgM) class antibodies that hydrolyze Abeta and inhibit Abeta aggregation were identified. The production of these antibodies increases as a function of age, ostensibly reflecting an attempt by the immune system to protect against the deleterious effect of Abeta accumulation in old age. A search for catalytic antibodies in a library of human immunoglobulins variable (IgV) domains yielded catalysts that hydrolyzed Abeta specifically at exceptionally rapid rates. The catalytic IgVs contained the light-chain variable domains within scaffolds that are structurally reminiscent of phylogenetically ancient antibodies. Inclusion of the heavy-chain variable domain in the IgV constructs resulted in reduced catalysis. We present our view that catalytic antibodies are likely to emerge as more efficacious and safer immunotherapy reagents compared to traditional Abeta-binding antibodies.

How antibodies fold

B cells use unconventional strategies for the production of a seemingly unlimited number of antibodies from a very limited amount of DNA. These methods dramatically increase the likelihood of producing proteins that cannot fold or assemble appropriately. B cells are therefore particularly dependent on 'quality control' mechanisms to oversee antibody production. Recent in vitro experiments demonstrate that Ig domains have evolved diverse folding strategies ranging from robust spontaneous folding to intrinsically disordered domains that require assembly with their partner domains to fold; in vivo experiments reveal that these different folding characteristics form the basis for cellular checkpoints in Ig transport. Taken together, these reports provide a detailed understanding of how B cells monitor and ensure the functional fidelity of Ig proteins.

Going adaptive: the saga of antibodies

Because of their extreme importance to human health, we probably know more about the structure and function of antibodies than practically any other molecule. Despite all the knowledge that has been accrued in the understanding of antibodies, modern approaches, especially comparative genomics, continue to yield novel findings regarding their underlying biology and evolution. In this review, we describe recent research that led to these revelations, and discuss the broad evolutionary implications of these findings. We have restricted our discussion to three vignettes. Considerable attention has been paid to the recent discovery that the teleost IgH locus is highly similar in organization to the Tcra-Tcrd locus, implicating an evolutionary common ancestor and parallels between the functions of B and T cells during development. Second, we discuss how a new type of antibody, recently discovered in jawless vertebrates, composed not of immunoglobulins but leucine-rich repeats, sheds new light on the overall forces driving evolution of all adaptive antigen receptors. Lastly, we discuss how accumulation of genomic sequences of various human subpopulations leads to better understanding of the directionality of antibody evolution. There is always more to learn from the unfolding saga of antibodies.

HIV-1 and influenza antibodies: seeing antigens in new ways

New modes of humoral recognition have been identified by studies of antibodies that neutralize human immunodeficiency virus type 1 and influenza A viruses. Understanding how such modes of antibody-antigen recognition can occur in the context of sophisticated mechanisms of humoral evasion has implications for the development of effective vaccines. Here we describe eight modes of antibody recognition first observed with human immunodeficiency virus type 1. Similarities to four of these modes have been identified with antibodies to a conserved 'stem' epitope on influenza A viruses. We outline how each of these different modes of antibody recognition is particularly suited to overcoming a specific viral evasion tactic and assess potential routes of re-elicitation in vaccine settings.

Immunoglobulins, antibody repertoire and B cell development

Swine share with most placental mammals the same five antibody isotypes and same two light chain types. Loci encoding lambda, kappa and Ig heavy chains appear to be organized as they are in other mammals. Swine differ from rodents and primates, but are similar to rabbits in using a single VH family (VH3) to encode their variable heavy chain domain, but not the family used by cattle, another artiodactyl. Distinct from other hoofed mammals and rodents, Ckappa:Clambda usage resembles the 1:1 ratio seen in primates. Since IgG subclasses diversified after speciation, same name subclass homologs do not exist among swine and other mammals unless very closely related. Swine possess six putative IgG subclasses that appear to have diversified by gene duplication and exon shuffle while retaining motifs that can bind to FcgammaRs, FcRn, C1q, protein A and protein G. The epithelial chorial placenta of swine and the precosial nature of their offspring have made piglets excellent models for studies on fetal antibody repertoire development and on the postnatal role of gut colonization, maternal colostrum and neonatal infection on the development of adaptive immunity during the "critical window" of immunological development. This chapter traces the study of the humoral immune system of this species through its various eras of discovery and compiles the results in tables and figures that should be a useful reference for educators and investigators.

Shark novel antigen receptors--the next generation of biologic therapeutics?

Over recent decades we have witnessed a revolution in health care as new classes of therapeutics based on natural biological molecules have become available to medical practitioners. These promised to target some of the most serious conditions that had previously evaded traditional small molecule drugs, such as cancers and to alleviate many of the concerns of patients and doctors alike regarding adverse side effects and impaired quality of life that are often associated with chemo-therapeutics. Many early 'biologics' were based on antibodies, Nature's answer to invading pathogens and malignancies, derived from rodents and in many ways failed to live up to expectations. Most of these issues were subsequently negated by technological advances that saw the introduction of human or "humanized' antibodies and have resulted in a number of commercial 'block-busters'. Today, most of the large pharmaceutical companies have product pipelines that include an increasing proportion of biologic as opposed to small molecule compounds. The limitations of antibodies or other large protein drugs are now being realized however and ever more inventive solutions are being sought to develop equally efficacious but smaller, more soluble, more stable and less costly alternatives to broaden the range of drug-able targets and therapeutic options. The aim of this chapter is to introduce the reader to one such novel approach that seeks to exploit a unique antibody-like protein evolved by ancestral sharks over 450 M years ago and that may lead to a host of new therapeutic opportunities and help us to tackle some of the pressing clinical demands of the 21 st century.

Selection of non-aggregating VH binders from synthetic VH phage-display libraries

The particular interest in VH antibody fragments stems from the fact that they can rival their "naturally occurring" single-domain antibody (sdAb) counterparts (camelid VHHs and shark VNARs) with regard to such desirable characteristics as stability, solubility, expression, and ability to penetrate cryptic epitopes and outperform them in terms of less immunogenicity, a much valued property in human immunotherapy applications. However, human VHs are typically prone to aggregation. Various approaches for developing non-aggregating human VHs with binding specificities have relied on a combination of recombinant DNA technology and phage-display technology. VH gene libraries are constructed synthetically by randomizing the CDRs of a single VH scaffold fused to a gene encoding a phage coat protein. Recombinant phage expressing the resulting VH libraries in fusion with the pIII protein is propagated in Escherichia coli. Monoclonal phage displaying VHs with specificities for target antigens are isolated from the libraries by a process called panning. The exertion of stability pressure in addition to binding pressure during panning ensures that the isolated VH binders are also non-aggregating. The genes encoding the desired VHs selected from the libraries are packaged within the phage particles, linking genotype and phenotype, hence making possible the identification of the selected VHs through identifying its physically linked genotype. Here, we describe the application of recombinant DNA and phage-display technologies for the construction of a phage-displayed human VH library, the panning of the library against a protein, and the expression, purification, and characterization of non-aggregating VHs isolated by panning.

Exceptional amyloid beta peptide hydrolyzing activity of nonphysiological immunoglobulin variable domain scaffolds

Nucleophilic sites in the paired variable domains of the light and heavy chains (VL and VH domains) of Ig can catalyze peptide bond hydrolysis. Amyloid beta (Abeta)-binding Igs are under consideration for immunotherapy of Alzheimer disease. We searched for Abeta-hydrolyzing human IgV domains (IgVs) in a library containing a majority of single chain Fv clones mimicking physiological VL-VH-combining sites and minority IgV populations with nonphysiological structures generated by cloning errors. Random screening and covalent selection of phage-displayed IgVs with an electrophilic Abeta analog identified rare IgVs that hydrolyzed Abeta mainly at His14-Gln15. Inhibition of IgV catalysis and irreversible binding by an electrophilic hapten suggested a nucleophilic catalytic mechanism. Structural analysis indicated that the catalytic IgVs are nonphysiological structures, a two domain heterodimeric VL (IgVL2-t) and single domain VL clones with aberrant polypeptide tags (IgVL-t'). The IgVs hydrolyzed Abeta at rates superior to naturally occurring Igs by 3-4 orders of magnitude. Forced pairing of the single domain VL with VH or VL domains resulted in reduced Abeta hydrolysis, suggesting catalysis by the unpaired VL domain.Angstrom level amino acid displacements evident in molecular models of the two domain and unpaired VL domain clones explain alterations of catalytic activity. In view of their superior catalytic activity, the VL domain IgVs may help attain clearance of medically important antigens more efficiently than natural Igs.

Immunoglobulin light chain (IgL) genes in zebrafish: Genomic configurations and inversional rearrangements between (V(L)-J(L)-C(L)) gene clusters

In mammals, Immunoglobulin light chain (IgL) are localized to two chromosomal regions (designated kappa and lambda). Here we report a genome-wide survey of IgL genes in the zebrafish revealing (V(L)-J(L)-C(L)) clusters spanning 5 separate chromosomes. To elucidate IgL loci present in the zebrafish genome assembly (Zv6), conventional sequence similarity searches and a novel scanning approach based on recombination signal sequence (RSS) motifs were applied. RT-PCR with zebrafish cDNA was used to confirm annotations, evaluate VJ-rearrangement possibilities and show that each chromosomal locus is expressed. In contrast to other vertebrates in which IgL exon usage has been studied, inversional rearrangement between (V(L)-J(L)-C(L)) clusters were found. Inter-cluster rearrangements may convey a selective advantage for editing self-reactive receptors and poise zebrafish by virtue of their extensive numbers of V(L), J(L) and C(L) to have greater potential for immunoglobulin gene shuffling than traditionally studied mice and human models.

Four primordial immunoglobulin light chain isotypes, including lambda and kappa, identified in the most primitive living jawed vertebrates

The discovery of a fourth immunoglobulin (Ig) light (L) chain isotype in sharks has revealed the origins and natural history of all vertebrate L chains. Phylogenetic comparisons have established orthology between this new shark L chain and the unique Xenopus L chain isotype sigma. More importantly, inclusion of this new L chain family in phylogenetic analyses showed that all vertebrate L chains can be categorized into four ancestral clans originating prior to the emergence of cartilaginous fish: one restricted to elasmobranchs (sigma-cart/type I), one found in all cold-blooded vertebrates (sigma/teleost type 2/elasmobranch type IV), one in all groups except bony fish (lambda/elasmobranch type II), and one in all groups except birds (kappa/elasmobranch type III/teleost type 1 and 3). All four of these primordial L chain isotypes (sigma, sigma-cart, lambda and kappa) have maintained separate V region identities since their emergence at least 450 million years ago, suggestive of an ancient physiological distinction of the L chains. We suggest that, based upon unique, discrete sizes of complementarity determining regions 1 and 2 and other features of the V region sequences, the different L chain isotypes arose to provide different functional conformations in the Ig binding site when they pair with heavy chains.

Characterization of serum and mucosal antibody responses in white sturgeon (Acipenser transmontanus Richardson) following immunization with WSIV and a protein hapten antigen

Serum and cutaneous mucus antibodies were monitored in white sturgeon for 15 weeks following intraperitoneal immunization. Ten fish were immunized (50 microg) with white sturgeon iridovirus (WSIV) or white sturgeon gonad (WSGO) tissue culture cells emulsified with or without FCA. An additional group was immunized with FITC:KLH+FCA. Fish were booster immunized at 6 weeks. Fish immunized with FITC:KLH+FCA produced significant serum antibodies to FITC by 6 weeks and this response peaked at 12 weeks (average titer 31,000). Mucosal antibodies to FITC were first detected at 12 weeks and significantly elevated by 15 weeks (average titer 18). Anti-WSIV antibody titers were detected in the serum by 9 weeks in fish immunized with WSIV and WSIV+FCA, but only a small number responded to immunization. At 15 weeks, four fish immunized with WSIV produced serum antibodies (average titer 838) and one fish immunized with WSIV+FCA had a serum titer of 1600. Mucosal anti-WSIV antibody titers of 8 and 16 were observed in two fish from the WSIV group at 12 weeks while four different fish from this group responded at 15 weeks (average titer 4). Western Blot using a monoclonal antibody confirmed immunoglobulin in mucus, and specificity to WSIV was further demonstrated by immunocytochemistry using serum from fish immunized with WSIV. Specific antibody was not detected in mucus of fish immunized with WSIV+FCA, WSGO, or WSGO+FCA. Collectively, these experiments demonstrate that white sturgeon can generate a specific antibody response following immunization, and is the first report showing mucosal immunoglobulin is present in this species.

Bispecific Antibodies: Molecules That Enable Novel Therapeutic Strategies

Bispecific antibodies are unique in the sense that they can bind simultaneously two different antigens. This property enables the development of therapeutic strategies that are not possible with conventional monoclonal antibodies. The large panel of imaginative bispecific antibody formats that has been developed reflects the strong interest for these molecules. Although in many cases the manufacturing of clinical grade material remains challenging, several bispecific antibody formats are currently in clinical trials.

The plasticity of immunoglobulin gene systems in evolution

The mechanism of recombination-activating gene (RAG)-mediated rearrangement exists in all jawed vertebrates, but the organization and structure of immunoglobulin (Ig) genes, as they differ in fish and among fish species, reveal their capability for rapid evolution. In systems where there can exist 100 Ig loci, exon restructuring and sequence changes of the constant regions led to divergence of effector functions. Recombination among these loci created hybrid genes, the strangest of which encode variable (V) regions that function as part of secreted molecules and, as the result of an ancient translocation, are also grafted onto the T-cell receptor. Genomic changes in V-gene structure, created by RAG recombinase acting on germline recombination signal sequences, led variously to the generation of fixed receptor specificities, pseudogene templates for gene conversion, and ultimately to Ig sequences that evolved away from Ig function. The presence of so many Ig loci in fishes raises interesting questions not only as to how their regulation is achieved but also how successive whole-locus duplications are accommodated by a system whose function in other vertebrates is based on clonal antigen receptor expression.

The antibody repertoire in evolution: chance, selection, and continuity

All jawed vertebrates contain the genetic elements essential for the function of the adaptive/combinatorial immune response, have diverse sets of natural antibodies resulting from segmental gene recombination, express comparable functional repertoires and can produce specific antibodies following appropriate immunization. Profound variability occurs in the third hypervariable (CDR3) segments of light and heavy chains even within antibodies of the same ostensible specificity. Germline VH and VL elements, as well as the joining (J) segments are highly conserved among the distinct vertebrate species. Conservation is particularly noted among the VH3-like sequences of all jawed vertebrates in the FR2 and FR3 segments, as well as in the FGXGT(R or K)L J-segment characteristic of light chains and TCRs and the WGXGT(uncharged)VT JH segments. Human VH3-53 and Vlambda6 family orthologs may be present over the entire range of vertebrates. Models of the three-dimensional structures of shark VH/VL combining sites indicate similarity in framework structure and comparable CDR usage to those of man. Although carcharhine shark VH regions show greater than 50% identity to the human VH germline prototype, searches of lower deuterostome and invertebrate databases fail to detect molecules with significant relatedness. Overall, antibodies of jawed vertebrates show tremendous individual diversity, but are constructed incorporating design features that arose with the evolutionary emergence of the jawed vertebrates and have been conserved through at least 450 million years of evolutionary time.