Rabbits transgenic for human IgG genes recapitulating rabbit B-cell biology to generate human antibodies of high specificity and affinity

Transgenic animals incorporating human antibody genes are extremely attractive for drug development because they obviate subsequent antibody humanization procedures required for therapeutic translation. Transgenic platforms have previously been established using mice, but also more recently rats, chickens, and cows and are now in abundant use for drug development. However, rabbit-based antibody generation, with a strong track record for specificity and affinity, is able to include gene conversion mediated sequence diversification, thereby enhancing binder maturation and improving the variance/selection of output antibodies in a different way than in rodents. Since it additionally frequently permits good binder generation against antigens that are only weakly immunogenic in other organisms, it is a highly interesting species for therapeutic antibody generation. We report here on the generation, utilization, and analysis of the first transgenic rabbit strain for human antibody production. Through the knockout of endogenous IgM genes and the introduction of human immunoglobulin sequences, this rabbit strain has been engineered to generate a highly diverse human IgG antibody repertoire. We further incorporated human CD79a/b and Bcl2 (B-cell lymphoma 2) genes, which enhance B-cell receptor expression and B-cell survival. Following immunization against the angiogenic factor BMP9 (Bone Morphogenetic Proteins 9), we were able to isolate a set of exquisitely affine and specific neutralizing antibodies from these rabbits. Sequence analysis of these binders revealed that both somatic hypermutation and gene conversion are fully operational in this strain, without compromising the very high degree of humanness. This powerful new transgenic strategy will allow further expansion of the use of endogenous immune mechanisms in drug development.

Transcytosis of payloads that are non-covalently complexed to bispecific antibodies across the hCMEC/D3 blood-brain barrier model

A transcellular shuttle system was generated for the delivery of non-covalently linked payloads across blood-brain barrier (BBB) endothelial cells. Transcytosis-enabling shuttles are composed of bispecific antibodies (bsAbs) that simultaneously bind transferrin receptor (TfR) and haptens such as digoxigenin or biocytinamide. Haptenylated payloads are attached to these vehicles via non-covalent hapten-antibody complexation. This enables targeting to and internalization into human BBB-derived microvascular endothelial hCMEC/D3 cells. In contrast to other shuttles, this system does not require special affinities or formats of their TfR-binding moieties for transcytosis and subsequent release. Non-covalent payload complexation to bsAb is flexible and robust, works for a multitude of payloads and enables separation of payloads from shuttles during transcytosis. Released payloads can enter the brain without connected bsAb entities, minimizing potential interference with distribution or functionality. Intracellular separation of shuttle and payload and recycling to cell surfaces may also enable recharging of the cell-bound BBB shuttle with payload for subsequent (merry-go-round) transport cycles.

Hapten-directed spontaneous disulfide shuffling: a universal technology for site-directed covalent coupling of payloads to antibodies

Humanized hapten-binding IgGs were designed with an accessible cysteine close to their binding pockets, for specific covalent payload attachment. Individual analyses of known structures of digoxigenin (Dig)- and fluorescein (Fluo) binding antibodies and a new structure of a biotin (Biot)-binder, revealed a “universal” coupling position (52⁺²) in proximity to binding pockets but without contributing to hapten interactions. Payloads that carry a free thiol are positioned on the antibody and covalently linked to it via disulfides. Covalent coupling is achieved and driven toward complete (95–100%) payload occupancy by spontaneous redox shuffling between antibody and payload. Attachment at the universal position works with different haptens, antibodies, and payloads. Examples are the haptens Fluo, Dig, and Biot combined with various fluorescent or peptidic payloads. Disulfide-bonded covalent antibody-payload complexes do not dissociate in vitro and in vivo. Coupling requires the designed cysteine and matching payload thiol because payload or antibody without the Cys/thiol are not linked (<5% nonspecific coupling). Hapten-mediated positioning is necessary as hapten-thiol-payload is only coupled to antibodies that bind matching haptens. Covalent complexes are more stable in vivo than noncovalent counterparts because digoxigeninylated or biotinylated fluorescent payloads without disulfide-linkage are cleared more rapidly in mice (approximately 50% reduced 48 hour serum levels) compared with their covalently linked counterparts. The coupling technology is applicable to many haptens and hapten binding antibodies (confirmed by automated analyses of the structures of 140 additional hapten binding antibodies) and can be applied to modulate the pharmacokinetics of small compounds or peptides. It is also suitable to link payloads in a reduction-releasable manner to tumor- or tissue-targeting delivery vehicles.—Dengl, S., Hoffmann, E., Grote, M., Wagner, C., Mundigl, O., Georges, G., Thorey, I., Stubenrauch, K.-G., Bujotzek, A., Josel, H.-P., Dziadek, S., Benz, J., Brinkmann, U. Hapten-directed spontaneous disulfide shuffling: a universal technology for site-directed covalent coupling of payloads to antibodies.

A robust high throughput platform to generate functional recombinant monoclonal antibodies using rabbit B cells from peripheral blood

We have developed a robust platform to generate and functionally characterize rabbit-derived antibodies using B cells from peripheral blood. The rapid high throughput procedure generates a diverse set of antibodies, yet requires only few animals to be immunized without the need to sacrifice them. The workflow includes (i) the identification and isolation of single B cells from rabbit blood expressing IgG antibodies, (ii) an elaborate short term B-cell cultivation to produce sufficient monoclonal antigen specific IgG for comprehensive phenotype screens, (iii) the isolation of VH and VL coding regions via PCR from B-cell clones producing antigen specific and functional antibodies followed by the sequence determination, and (iv) the recombinant expression and purification of IgG antibodies. The fully integrated and to a large degree automated platform (demonstrated in this paper using IL1RL1 immunized rabbits) yielded clonal and very diverse IL1RL1-specific and functional IL1RL1-inhibiting rabbit antibodies. These functional IgGs from individual animals were obtained at a short time range after immunization and could be identified already during primary screening, thus substantially lowering the workload for the subsequent B-cell PCR workflow. Early availability of sequence information permits one to select early-on function- and sequence-diverse antibodies for further characterization. In summary, this powerful technology platform has proven to be an efficient and robust method for the rapid generation of antigen specific and functional monoclonal rabbit antibodies without sacrificing the immunized animal.

PK modulation of haptenylated peptides via non-covalent antibody complexation

We applied noncovalent complexes of digoxigenin (Dig) binding antibodies with digoxigeninylated peptide derivatives to modulate their pharmacokinetic properties. A peptide derivative which activates the Y2R receptor was selectively mono-digoxigeninylated by reacting a NHS-Dig derivative with an ε-amino group of lysine 2. This position tolerates modifications without destroying receptor binding and functionality of the peptide. Dig-peptide derivatives can be loaded onto Dig-binding IgGs in a simple and robust reaction, thereby generating peptide-IgG complexes in a defined two to one molar ratio. This indicates that each antibody arm becomes occupied by one haptenylated peptide. In vitro receptor binding and signaling assays showed that Dig-peptides as well as the peptide-antibody complexes retain better potency than the corresponding pegylated peptides. In vivo analyses revealed prolonged serum half-life of antibody-complexed peptides compared to unmodified peptides. Thus, complexes are of sufficient stability for PK modulation. We observed more prolonged weight reduction in a murine diet-induced obesity (DIO) model with antibody-complexed peptides compared to unmodified peptides. We conclude that antibody-hapten complexation can be applied to modulate the PK of haptenylated peptides and in consequence improve the therapeutic efficacy of therapeutic peptides.

Endostatin down-regulates soluble guanylate cyclase (sGC) in endothelial cells in vivo: influence of endostatin on vascular endothelial growth factor (VEGF) signaling

Endostatin was suggested to be an antiangiogenic agent with the potential for clinical use in cancer therapy. Unfortunately, up to now no antiangiogenic effect was seen in clinical trials using this substance. The lack of response might be caused by an incomplete understanding of endostatin signaling. Endostatin is known to influence the vascular endothelial growth factor (VEGF) signaling pathway. It has been reported to bind to the VEGF receptor KDR directly and to decrease the phosphorylation of endothelial nitric oxide synthase (eNOS) at Ser1177 via the protein phosphatase 2A (PP2A). But so far no details of endostatin signaling with regard to NO downstream effectors have been revealed. In the present work the authors demonstrate that endostatin down-regulates the protein level of soluble guanylate cyclase (sGC) in endothelial cells of newly formed blood vessels in 5 day-old wounds (control: 62.5 +/- 33 vessels/mm2, endostatin: 9.2 +/- 3.2 vessels/mm2). This was confirmed in experiments with endothelial tubes of embryoid bodies and endothelial cells derived from embryonic stem cells (eESCs; control: 126 +/- 20, endostatin: 58 +/- 10). The decrease of sGC protein levels in response to endostatin was abolished after preincubation with the PP2A inhibitor okadaic acid. No alterations of sGC mRNA levels could be found under endostatin treatment in eESC. The authors conclude that endostatin affects VEGF signaling in endothelial cells by a post-transcriptional PP2A-dependent down-regulation of sGC protein levels.

Endostatin influences endothelial morphology via the activated ERK1/2-kinase endothelial morphology and signal transduction

Endostatin, the proteolytic fragment of collagen XVIII, is known to be a potent inhibitor of angiogenesis. However, to date, only limited knowledge exists with regard to the effects of endostatin on vessel morphology and the underlying signaling pathway. The aim of the present work was therefore to determine the impact of endostatin and its collagen XV analogue restin on vessel development during wound healing and embryonic angio- and vasculogenesis. Time lapse experiments and electron microscopy demonstrate similar morphological changes evoked by endostatin and the ERK1/2-kinase inhibitor PD98059. Furthermore, we show that ERK1/2 phosphorylation, a crucial signaling event in vascular morphogenesis, is regulated by endostatin via the protein phosphatase 2A PP2A. These findings provide new insight into a key signaling pathway of vascular remodeling evoked by a matrix-derived factor.

A crucial role of β1 integrins for keratinocyte migration in vitro and during cutaneous wound repair

Integrins are ubiquitous transmembrane receptors that play crucial roles in cell-cell and cell-matrix interactions. In this study, we have determined the effects of the loss of β1 integrins in keratinocytes in vitro and during cutaneous wound repair. Flow cytometry of cultured β1-deficient keratinocytes confirmed the absence of β1 integrins and showed downregulation of α6β4 but not of αv integrins. β1-null keratinocytes were characterised by poor adhesion to various substrates, by a reduced proliferation rate and by a strongly impaired migratory capacity. In vivo, the loss of β1 integrins in keratinocytes caused a severe defect in wound healing. β1-null keratinocytes showed impaired migration and were more densely packed in the hyperproliferative epithelium. Surprisingly, their proliferation rate was not reduced in early wounds and even increased in late wounds. The failure in re-epithelialisation resulted in a prolonged inflammatory response, leading to dramatic alterations in the expression of important wound-regulated genes. Ultimately, β1-deficient epidermis did cover the wound bed, but the epithelial architecture was abnormal. These findings demonstrate a crucial role of β1 integrins in keratinocyte migration and wound re-epithelialisation.

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Different forms of 130 kD connective tissue protein are specific for boundaries in the nervous system and basement membrane of muscle cells in leech

The nervous system and muscle tissue of the leech express two different organ-specific forms of connective tissue protein. The nervous system-specific form appears in regional boundaries separating cell bodies, axonal tracts and areas of the neuropile during late embryogenesis. In contrast, the muscle-specific form appears earlier during development in the basement membrane of muscle cells. In extraction experiments both forms behave like extracellular matrix proteins and because of their molecular weight, are considered members of a group of cell type-specific 130 kD proteins (leech gp130s). However, the two forms differ in their posttranslational modification. As determined by Con A and lentil lectin affinity chromatography, only the nervous system-specific, but not the muscle-specific form, has fucosylated and high mannose N-linked carbohydrates. These differences in the developmental onset and glycosylation suggest that nervous system-specific and muscle-specific connective tissue proteins are regulated differently and participate in different molecular interactions.

Developmentally regulated epitopes on a neurofilament protein visualized by monoclonal antibodies

Two monoclonal antibodies (mabs) which recognized the 68 kDa subunit of the rat neurofilament triplet were isolated. In immunoblots with SDS-solubilized and reduced proteins, these mabs recognized their epitopes equally well in embryonic and in adult tissue. However, these epitopes were developmentally regulated in paraformaldehyde-fixed rat brain sections. They were abundant in all compartments of differentiating neurons, whereas in mature neurons their presence was markedly attenuated, with a moderate abundance in perikarya and larger dendrites and low concentrations in axons. Thus, a differential developmental modification, possibly involving the masking of an epitope, is demonstrated for the small neurofilament polypeptide in rat and monkey brain tissue.