Functional heavy-chain antibodies in Camelidae

Identification of a Universal VHH Framework to Graft Non-canonical Antigen-binding Loops of Camel Single-domain Antibodies

Camel single-domain antibody fragments (VHHs) are promising tools in numerous biotechnological and medical applications. However, some conditions under which antibodies are used are so demanding that they can be met by only the most robust VHHs. A universal framework offering the required properties for use in various applications (e.g. as intrabody, as probe in biosensors or on micro-arrays) is highly valuable and might be further implemented when employment of VHHs in human therapy is envisaged. We identified the VHH framework of cAbBCII10 as a potential candidate, useful for the exchange of antigen specificities by complementarity determining region (CDR) grafting. Due to the large number of CDR-H loop structures present on VHHs, this grafting technique was expected to be rather unpredictable. Nonetheless, the plasticity of the cAbBCII10 framework allows successful transfer of antigen specificity from donor VHHs onto its scaffold. The cAbBCII10 was chosen essentially for its high level of stability (47 kJmol(-1)), good expression level (5 mgl(-1) in E.coli) and its ability to be functional in the absence of the conserved disulfide bond. All five chimeras generated by grafting CDR-Hs, from donor VHHs belonging to subfamily 2 that encompass 75% of all antigen-specific VHHs, on the framework of cAbBCII10 were functional and generally had an increased thermodynamic stability. The grafting of CDR-H loops from VHHs belonging to other subfamilies resulted in chimeras of reduced antigen-binding capacity.

Application of monoclonal antibodies in functional and comparative investigations of heavy-chain immunoglobulins in new world camelids

Of the three immunoglobulin G (IgG) isotypes described to occur in camelids, IgG2 and IgG3 are distinct in that they do not incorporate light chains. These heavy-chain antibodies (HCAbs) constitute approximately 50% of the IgG in llama serum and as much as 75% of the IgG in camel serum. We have produced isotype-specific mouse monoclonal antibodies (MAbs) in order to investigate the roles of HCAbs in camelid immunity. Seventeen stable hybridomas were cloned, and three MAbs that were specific for epitopes on the gamma chains of llama IgG1, IgG2, or IgG3 were characterized in detail. Affinity chromatography revealed that each MAb bound its isotype in solution in llama serum. The antibodies bound to the corresponding alpaca IgGs, to guanaco IgG1 and IgG2, and to camel IgG1. Interestingly, anti-IgG2 MAbs bound three heavy-chain species in llama serum, confirming the presence of three IgG2 subisotypes. Two IgG2 subisotypes were detected in alpaca and guanaco sera. The MAbs detected llama serum IgGs when they were bound to antigen in enzyme-linked immunosorbent assays and were used to discern among isotypes induced during infection with a parasitic nematode. Diseased animals, infected with Parelaphostrongylus tenuis, did not produce antigen-specific HCAbs; rather, they produced the conventional isotype, IgG1, exclusively. Our data document the utility of these MAbs in functional and physiologic investigations of the immune systems of New World camelids.

Antigen Binding and Solubility Effects upon the Veneering of a Camel VHH in Framework-2 to Mimic a VH

Heavy chain only antibodies of camelids bind their antigens with a single domain, the VHH, which acquired adaptations relative to classical VHs to function in the absence of a VL partner. Additional CDR loop conformations, outside the canonical loop structures of VHs, broaden the repertoire of the antigen-binding site. The combined effects of part of the CDR3 that folds over the "former" VL binding site and framework-2 mutations to more hydrophilic amino acids, enhance the solubility of VHH domains and prevent VL pairing. cAbAn33, a VHH domain specific for the carbohydrate moiety of the variant surface glycoprotein of trypanosomes, has a short CDR3 loop that does not cover the former VL binding site as well as a VH-specific Trp47 instead of the VHH-specific Gly47. Resurfacing its framework-2 region (mutations Tyr37Val, Glu44Gly and Arg45Leu) to mimic that of a human VH restores the VL binding capacity. In solution, the humanised VHH behaves as a soluble, monomeric entity, albeit with reduced thermodynamic stability and affinity for its antigen. Comparison of the crystal structures of cAbAn33 and its humanised derivative reveals steric hindrance exerted by VHH-specific residues Tyr37 and Arg45 that prevent the VL domain pairing, whereas Glu44 and Arg45 are key elements to avoid insolubility of the domain.

Evolution of isotype switching

This review discusses evolution of the process of Ig heavy chain class switching, relating it to the first appearance of somatic hypermutation (SHM) of variable region genes. First, we discuss recent findings on the mechanism of class switch recombination (CSR) in mice and humans, and then review the mechanisms of expression of Ig heavy chain isotypes from fishes to mammals. Importantly, activation-induced cytidine deaminase (AID), which is essential for CSR and somatic hypermutation, is found in fishes. Although at least some fishes are likely to undergo SHM, CSR is highly unlikely to occur in this group. We discuss the first appearance of CSR in amphibians and how it differs in birds and mammals.

Affinity maturation of a VHH by mutational hotspot randomization

V(H)Hs from naive libraries have dissociation constants (K(D)s) in the low micromolar range and thus, for most antibody applications, their intrinsic affinities need to be improved significantly. Non-targeted in vitro affinity maturation approaches based on indiscriminate randomization of complementarity-determining region (CDR) residues or random mutagenesis of conventional antibody variable domains have been shown to improve the affinity of recombinant antibodies by 450- to over 6000-fold. A different, targeted approach based on selective randomization of CDR codons containing AGY/RGYW nucleotide mutational hotspots i.e., "hotspot codons", also promises to be very efficient for improving antibody affinities. Here we employed the latter approach for improving the affinity of PTH22, a parathyroid hormone (PTH)-derived peptide-specific V(H)H that was isolated from a naive llama phage display library. A PTH22 mutant ribosome display library was constructed by randomizing nine CDR2 and CDR3 hotspot codons. The affinity improvement of the lead binder was 30-fold, which seems somewhat low in view of the large number of randomized hotspot codons. Nucleotide sequence analyses of PTH22 and 23 naive V(H)Hs suggested that many AGY/RGYW mutational hotspots are not affinity mutational hotspots but play a role in V(H)H solubility, structure, and deletion/insertion events. Our results indicate that the mutagenesis approach described here is beneficial in terms of yielding moderate increases in affinity while fine-tuning physical properties of an antibody.

Strong in vivo maturation compensates for structurally restricted H3 loops in antibody repertoires

A central paradigm in immunology states that successful generation of high affinity antibodies necessitates an immense primary repertoire of antigen-combining sites. Much of the diversity of this repertoire is provided by varying one antigen binding loop, created by inserting randomly a D (diversity) gene out of a small pool between the V and J genes. It is therefore assumed that any particular D-encoded region surrounded by different V and J regions adopts a different conformation. We have solved the structure of two lysozyme-specific variable domains of heavy-chain antibodies isolated from two strictly unrelated dromedaries. These antibodies recombined identical D gene sequences to different V and J precursors with significant variance in their V(D)J junctions. Despite these large differences, the D-encoded loop segments adopt remarkably identical architectures, thus directing the antibodies toward identical epitopes. Furthermore, a striking convergent maturation process occurred in the V region, adapting both binders for their sub-nanomolar affinity association with lysozyme. Hence, on a structural level, humoral immunity may rely more on well developed maturation and selection systems than on the acquisition of large primary repertoires.

Single Domain Antibodies Derived from Dromedary Lymph Node and Peripheral Blood Lymphocytes Sensing Conformational Variants of Prostate-specific Antigen

The importance of the lymphocyte source to generate hybridomas or to construct antibody gene libraries from which to identify potent monoclonal antibodies is understudied. However, the few comparative studies that exist seem to favor the lymph node tissue as a B-cell source. Here the peripheral blood and lymph node lymphocytes of a dromedary immunized with prostate-specific antigen (PSA) have been employed to clone two independent gene banks of the variable domains of heavy-chain antibodies (i.e. the VHHs). Several PSA-specific VHHs were retrieved after panning of these phage-displayed VHH libraries. Some of them were derived from the same B-cell lineage, possibly reflecting the restricted primary repertoire of heavy-chain antibodies. Other binders originated from different B-cell lineages and apparently converged toward a striking homologous amino acid sequence motif in their CDR3. This illustrates the strong somatic hypermutation and stringent antigen-driven selection ongoing in these animals. Although the various antigen binders exhibit a broad range of kinetic rate constants for their interaction with the PSA, leading to equilibrium constants from 70 pM to 100 nM, no significant difference existed between the binders from the two B-cell sources. The VHHs of both libraries were categorized in three groups based on nonoverlapping epitopes. Some of these VHHs could inhibit and others could enhance the proteolytic activity of the antigen. Remarkably, VHHs seem to sense or induce conformational changes on different PSA isoforms, a feature that might be exploited to study the PSA conformational flexibility and to discriminate the stages of prostate cancer.

Silencing of the immunoglobulin heavy chain locus by removal of all eight constant-region genes in a 200-kb region

Silencing or removal of individual C (constant)-region genes and/or adjacent control sequences did not generate fully deficient Ig (immunoglobulin)- mice. A reason is that different C genes share many functional tasks and most importantly are individually capable of ensuring lymphocyte differentiation. Nevertheless, incomplete arrests in B-cell development were found, most pronounced at the onset of H-chain expression. Here we show that removal of 200 kb accommodating all C genes, Cmu-Cdelta-Cgamma3-Cgamma1-Cgamma2b-Cgamma2a-Cepsilon-Calpha, stops antibody production. For this two loxP targeting constructs were introduced into the most 5' C gene and the distal alpha 3' enhancer. Cre-loxP-mediated in vivo deletion was accompanied by extensive germ-line mosaicism, which could be separated by breeding. Homozygous C-gene deletion mice did not express Ig H or L chains and flow cytometry revealed a complete block in B-cell development. However, C-gene removal did not affect DNA rearrangement processes following locus activation, as recombination efficacy appears to be similar to what is found in normal mice.

Production of Novel Recombinant Single-Domain Antibodies against Tandem Repeat Region of MUC1 Mucin

Recently, the existence of "heavy-chain" antibody in Camelidae has been described. However, as yet there is no data on the binding of this type of antibody to peptides. In addition, there was not any report of production of single-domain antibodies in two-humped camels (Camelus bactrianus). In the present study, these questions are addressed. We showed the feasibility of immunizing old world camels, cloning the repertoire of the variable domain of their heavy-chain antibodies, panning and selection, leading to the successful identification of minimum-sized antigen binders. Antigen-specific fragments of the heavy-chain IgGs (V(HH)) are of great interest in biotechnology because they are very stable, highly soluble, and react specifically and with high affinity to the antigens. In this study, we immunized two camels (Camelus dromedarius and Camelus bactrianus) with homogenized cancerous tissues, synthetic peptide, and human milk fat globule membrane (HMFG), and generated two V(HH) libraries displayed on phage particles. Some single-domain antibody fragments have been isolated that specifically recognize the tandem repeat region of MUC1. The camels' single-domain V(HH) harbor the original, intact antigen binding site and reacted specifically and with high affinity to the tandem repeat region of MUC1. Indeed soluble, specific antigen binders and good affinities (in the range of 0.2 x 10(9) M(-1) to 0.6 x 10(9) M(-1)) were identified from these libraries. This is the first example of the isolation of camel anti-peptide V(HH) domains.

Efficient Cancer Therapy with a Nanobody-Based Conjugate

Nanobodies are the smallest fragments of naturally occurring single-domain antibodies that have evolved to be fully functional in the absence of a light chain. Nanobodies are strictly monomeric, very stable, and highly soluble entities. We identified a nanobody with subnanomolar affinity for the human tumor-associated carcinoembryonic antigen. This nanobody was conjugated to Enterobacter cloacae beta-lactamase, and its site-selective anticancer prodrug activation capacity was evaluated. The conjugate was readily purified in high yields without aggregation or loss of functionality of the constituents. In vitro experiments showed that the nanobody-enzyme conjugate effectively activated the release of phenylenediamine mustard from the cephalosporin nitrogen mustard prodrug 7-(4-carboxybutanamido) cephalosporin mustard at the surface of carcinoembryonic antigen-expressing LS174T cancer cells. In vivo studies demonstrated that the conjugate had an excellent biodistribution profile and induced regressions and cures of established tumor xenografts. The easy generation and manufacturing yield of nanobody-based conjugates together with their potent antitumor activity make nanobodies promising vehicles for new generation cancer therapeutics.

Efficient targeting of conserved cryptic epitopes of infectious agents by single domain antibodies. African trypanosomes as paradigm

Antigen variation is a successful defense system adopted by several infectious agents to evade the host immune response. The principle of this defense strategy in the African trypanosome paradigm involves a dense packing of variant surface glycoproteins (VSG) exposing only highly variable and immuno-dominant epitopes to the immune system, whereas conserved epitopes become inaccessible for large molecules. Reducing the size of binders that target the conserved, less-immunogenic, cryptic VSG epitopes forms an obvious solution to combat these parasites. This goal was achieved by introducing dromedary Heavy-chain antibodies. We found that only these unique antibodies recognize epitopes common to multiple VSG classes. After phage display of their antigen-binding repertoire, we isolated a single domain antibody fragment with high specificity for the conserved Asn-linked carbohydrate of VSG. In sharp contrast to labeled concanavalin-A that stains only the flagellar pocket where carbohydrates are accessible because of less dense VSG packing, the single domain binder stains the entire surface of viable parasites, irrespective of the VSG type expressed. This corroborates the idea that small antibody fragments, but not larger lectins or conventional antibody fragments, are able to penetrate the dense VSG coat to target their epitope. The diagnostic potential of this fluorescently labeled binder was proven by the direct, selective, and sensitive detection of parasites in blood smears. The employment of this binder as a molecular recognition unit in immuno-toxins designed for trypanosomosis therapy becomes feasible as well. This was illustrated by the specific trypanolysis induced by an antibody::beta-lactamase fusion activating a prodrug.

Heavy-chain only antibodies derived from dromedary are secreted and displayed by mouse B cells

Whereas functional heavy (H)-chain antibodies devoid of light (L)- chains account for about half of the circulating immunoglobulins in Camelidae, H-chain only antibodies (HCAbs) are not produced in other healthy mammals including rodents and humans. To test the feasibility of expressing single chain antibodies in the mouse, which on account of their small size and antigen-recognition properties would have a major impact on antibody engineering strategies, we constructed a rearranged dromedary H-chain gene encoding the immunoglobulin G2a (IgG2a) isotype with specificity for hen-egg lysozyme (HEL). This IgG2a H-chain gene was introduced into mouse myeloma cells not expressing endogenous immunoglobulin H- or L-chains. Unexpectedly the mouse cells processed and expressed the introduced H-chain as naturally occurring dromedary antibody. For this the first constant (C) region exon was proficiently removed from the recombinant H-chain transcript. This resulted in specific H-chain antibodies of the correct molecular weight (2 x 50 000 MW) secreted as disulfide-linked homodimers and displayed on the mouse cell surface as glycosyl-phosphatidyl-inositol-linked B-cell receptor. The results indicate that antibody expression and maturation without immunoglobulin L-chain is feasible and paves the way for the generation of transgenic single chain antibody repertoires.

Emergence and evolution of functional heavy-chain antibodies in Camelidae

Antibodies of jawed-vertebrates are composed of paired heavy (H) and light (L) polypeptide chains. Surprisingly, the sera of camelids, nurse shark and wobbegong shark, and possibly ratfish contain antibodies that lack L-chains. In camelids, these Heavy-chain antibodies (HCAbs) are gamma-isotypes, and are functional in antigen binding. In this review we focus on the dedicated immunoglobulin (Ig) genes that encode the HCAb in Camelidae (camels, dromedaries and llamas), about their origin, and how these camel immunoglobulins evolved and acquire a large and diverse repertoire of antigen binding sites in absence of the H-L combinatorial diversity.

The production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi

In this review we will focus on the current status and views concerning the production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi. We will focus on single-chain antibody fragment production (scFv and VHH) by these lower eukaryotes and the possible applications of these proteins. Also the coupling of fragments to relevant enzymes or other components will be discussed. As an example of the fusion protein strategy, the 'magic bullet' approach for industrial applications, will be highlighted.