Generation and characterization of recombinant human antibodies specific for native laminin epitopes: potential application in cancer therapy

Immuno-PET Imaging and Pharmacokinetics of an Anti-CEA scFv-based Trimerbody and Its Monomeric Counterpart in Human Gastric Carcinoma-Bearing Mice

Monoclonal antibodies (mAbs) are currently used as therapeutic agents in different types of cancer. However, mAbs and antibody fragments developed so far show suboptimal properties in terms of circulation time and tumor penetration/retention. Here, we report the radiolabeling, pharmacokinetic evaluation, and determination of tumor targeting capacity of the previously validated anti-CEA MFE23-scFv-based N-terminal trimerbody (MFE23^N-trimerbody), and the results are compared to those obtained for the monomeric MFE23-scFv. Dissection and gamma-counting studies performed with the ¹³¹I-labeled protein scaffolds in normal mice showed slower blood clearance for the trimerbody, and accumulation in the kidneys, the spleen, and the liver for both species. These, together with a progressive uptake in the small intestine, confirm a combined elimination scheme with hepatobiliary and urinary excretion. Positron emission tomography studies performed in a xenograft mouse model of human gastric adenocarcinoma, generated by subcutaneous administration of CEA-positive human MKN45 cells, showed higher tumor accumulation and tumor-to-muscle (T/M) ratios for ¹²⁴I-labeled MFE23^N-trimerbody than for MFE23-scFv. Specific uptake was not detected with PET imaging in CEA negative xenografts as indicated by low T/M ratios. Our data suggest that engineered intermediate-sized trivalent antibody fragments could be promising candidates for targeted therapy and imaging of CEA-positive tumors.

Balanced secretion of anti-CEA × anti-CD3 diabody chains using the 2A self-cleaving peptide maximizes diabody assembly and tumor-specific cytotoxicity

Adoptive transfer of genetically engineered human cells secreting bispecific T-cell engagers has shown encouraging therapeutic effects in preclinical models of cancer. However, reducing the toxicity and improving the effectiveness of this emerging immunotherapeutic strategy will be critical to its successful application. We have demonstrated that for gene-based bispecific antibody strategies, two-chain diabodies have a better safety profile than single-chain tandem scFvs (single-chain variable fragments), because their reduced tendency to form aggregates reduces the risk of inducing antigen-independent T-cell activation. Here, we demonstrate that the incorporation of a 2A self-processing peptide derived from foot-and-mouth disease virus conveying co-translational cleavage into a two-chain anti-CD3 × anti-CEA diabody gene enables near-equimolar expression of diabody chains 1 and 2, and thus increases the final amount of assembled diabody. This was found to maximize diabody-mediated T-cell activation and cytotoxicity against carcinoembryonic antigen-positive tumor cells.

Autoimmunity against laminins

Laminins are ubiquitous constituents of the basement membranes with major architectural and functional role as supported by the fact that absence or mutations of laminins lead to either lethal or severely impairing phenotypes. Besides genetic defects, laminins are involved in a wide range of human diseases including cancer, infections, and inflammatory diseases, as well as autoimmune disorders. A growing body of evidence implicates several laminin chains as autoantigens in blistering skin diseases, collagenoses, vasculitis, or post-infectious autoimmunity. The current paper reviews the existing knowledge on autoimmunity against laminins referring to both experimental and clinical data, and on therapeutic implications of anti-laminin antibodies. Further investigation of relevant laminin epitopes in pathogenic autoimmunity would facilitate the development of appropriate diagnostic tools for thorough characterization of patients' antibody specificities and should decisively contribute to designing more specific therapeutic interventions.

Intramolecular trimerization, a novel strategy for making multispecific antibodies with controlled orientation of the antigen binding domains

Here, we describe a new strategy that allows the rapid and efficient engineering of mono and multispecific trivalent antibodies. By fusing single-domain antibodies from camelid heavy-chain-only immunoglobulins (V_HHs) to the N-terminus of a human collagen XVIII trimerization domain (TIE^XVIII) we produced monospecific trimerbodies that were efficiently secreted as soluble functional proteins by mammalian cells. The purified V_HH-TIE^XVIII trimerbodies were trimeric in solution and exhibited excellent antigen binding capacity. Furthermore, by connecting with two additional glycine-serine-based linkers three V_HH-TIE^XVIII modules on a single polypeptide chain, we present an approach for the rational design of multispecific tandem trimerbodies with defined stoichiometry and controlled orientation. Using this technology we report here the construction and characterization of a tandem V_HH-based trimerbody capable of simultaneously binding to three different antigens: carcinoembryonic antigen (CEA), epidermal growth factor receptor (EGFR) and green fluorescence protein (GFP). Multispecific tandem V_HH-based trimerbodies were well expressed in mammalian cells, had good biophysical properties and were capable of simultaneously binding their targeted antigens. Importantly, these antibodies were very effective in inhibiting the proliferation of human epidermoid carcinoma A431 cells. Multispecific V_HH-based trimerbodies are therefore ideal candidates for future applications in various therapeutic areas.

Bacterial secretion of soluble and functional trivalent scFv-based N-terminal trimerbodies

Recombinant antibodies are used with great success in many different diagnostic and therapeutic applications. A variety of protein expression systems are available, but nowadays almost all therapeutic antibodies are produced in mammalian cell lines due to their complex structure and glycosylation requirements. However, production of clinical-grade antibodies in mammalian cells is very expensive and time-consuming. On the other hand, Escherichia coli (E. coli) is known to be the simplest, fastest and most cost-effective recombinant expression system, which usually achieves higher protein yields than mammalian cells. Indeed, it is one of the most popular host in the industry for the expression of recombinant proteins. In this work, a trivalent single-chain fragment variable (scFv)-based N-terminal trimerbody, specific for native laminin-111, was expressed in human embryonic kidney 293 cells and in E. coli. Mammalian and bacterially produced anti-laminin trimerbody molecules display comparable functional and structural properties, although importantly the yield of trimerbody expressed in E. coli was considerably higher than in human cells. These results demonstrated that E. coli is a versatile and efficient expression system for multivalent trimerbody-based molecules that is suitable for their industrial production.

CARbodies: Human Antibodies Against Cell Surface Tumor Antigens Selected From Repertoires Displayed on T Cell Chimeric Antigen Receptors

A human single-chain variable fragment (scFv) antibody library was expressed on the surface of human T cells after transduction with lentiviral vectors (LVs). The repertoire was fused to a first-generation T cell receptor ζ (TCRζ)-based chimeric antigen receptor (CAR). We used this library to isolate antibodies termed CARbodies that recognize antigens expressed on the tumor cell surface in a proof-of-principle system. After three rounds of activation-selection there was a clear repertoire restriction, with the emergence dominant clones. The CARbodies were purified from bacterial cultures as soluble and active proteins. Furthermore, to validate its potential application for adoptive cell therapy, human T cells were transduced with a LV encoding a second-generation costimulatory CAR (CAR^v2) bearing the selected CARbodies. Transduced human primary T cells expressed significant levels of the CARbodies-based CAR^v2 fusion protein on the cell surface, and importantly could be specifically activated, after stimulation with tumor cells. This approach is a promising tool for the generation of antibodies fully adapted to the display format (CAR) and the selection context (cell synapse), which could extend the scope of current adoptive cell therapy strategies with CAR-redirected T cells.

Immunotherapeutic organoids: a new approach to cancer treatment

Therapeutic monoclonal antibodies have revolutionized the treatment of cancer and other diseases. However, several limitations of antibody-based treatments, such as the cost of therapy and the achievement of sustained plasma levels, should be still addressed for their widespread use as therapeutics. The use of cell and gene transfer methods offers additional benefits by producing a continuous release of the antibody with syngenic glycosylation patterns, which makes the antibody potentially less immunogenic. In vivo secretion of therapeutic antibodies by viral vector delivery or ex vivo gene modified long-lived autologous or allogeneic human mesenchymal stem cells may advantageously replace repeated injection of clinical-grade antibodies. Gene-modified autologous mesenchymal stem cells can be delivered subcutaneously embedded in a non-immunogenic synthetic extracellular matrix-based scaffold that guarantees the survival of the cell inoculum. The scaffold would keep cells at the implantation site, with the therapeutic protein acting at distance (immunotherapeutic organoid), and could be retrieved once the therapeutic effect is fulfilled. In the present review we highlight the practical importance of living cell factories for in vivo secretion of recombinant antibodies.

Improved stability of multivalent antibodies containing the human collagen XV trimerization domain

We recently described the in vitro and in vivo properties of an engineered homotrimeric antibody made by fusing the N-terminal trimerization region of collagen XVIII NC1 domain to the C-terminus of a scFv fragment [trimerbody (scFv-NC1) 3; 110 kDa]. Here, we demonstrated the utility of the N-terminal trimerization region of collagen XV NC1 domain in the engineering of trivalent antibodies. We constructed several scFv-based trimerbodies containing the human type XV trimerization domain and demonstrated that all the purified trimerbodies were trimeric in solution and exhibited excellent antigen binding capacity. Importantly, type XV trimerbodies demonstrated substantially greater thermal and serum stability and resistance to protease digestion than type XVIII trimerbodies. In summary, the small size, high expression level, solubility and stability of the trimerization domain of type XV collagen make it the ideal choice for engineering homotrimeric antibodies for cancer detection and therapy.

Expressivity tag: a novel tool for increased expression in Escherichia coli

Protein expression in Escherichia coli is rarely trivial as low expression and insolubility are common problems. In this work we define a fusion partner, which increases expression levels similarly to the distinct function of solubility and affinity tags. This type of fusion tag we term an expressivity tag. Our work is based on earlier observations where 3' deletions of the InfB gene displays strongly increased expression levels. We have constructed progressively shortened fragments of the InfB(1-471) gene and fused gene fragments to a gfp reporter gene. A 5-fold increase in GFP expression was seen for an optimal 21 nucleotide InfB(1-21) sequence compared to gfp independently. We defined the InfB(1-21) sequence as an expressivity tag. The tag was tested for improved expression of two biotechnological important proteins streptavidin and a single chain antibody (scFv). Expression of both streptavidin and scFv(L32) was improved as evaluated by SDS-PAGE. Calculation of folding energies in the translation initiation region gave higher free energies for gfp, L32 and streptavidin when linked to InfB(1-21) than independently. InfB(1-21) did however not improve the codon usage or codon adaptation index. The expressivity tag is an important addition to the box of tools available for optimizing heterologous protein expression.

The multicompartmental p32/gClqR as a new target for antibody-based tumor targeting strategies

Tumor-associated cell surface antigens and tumor-associated vascular markers have been used as a target for cancer intervention strategies. However, both types of targets have limitations due to accessibility, low and/or heterogeneous expression, and presence of tumor-associated serum antigen. It has been previously reported that a mitochondrial/cell surface protein, p32/gC1qR, is the receptor for a tumor-homing peptide, LyP-1, which specifically recognizes an epitope in tumor cells, tumor lymphatics, and tumor-associated macrophages/myeloid cells. Using antibody phage technology, we have generated an anti-p32 human monoclonal antibody (2.15). The 2.15 antibody, expressed in single-chain fragment variable and in trimerbody format, was then characterized in vivo using mice grafted subcutaneously with MDA-MB-231 human breast cancers cells, revealing a highly selective tumor uptake. The intratumoral distribution of the antibody was consistent with the expression pattern of p32 in the surface of some clusters of cells. These results demonstrate the potential of p32 for antibody-based tumor targeting strategies and the utility of the 2.15 antibody as targeting moiety for the selective delivery of imaging and therapeutic agents to tumors.

In vivo tumor targeting and imaging with engineered trivalent antibody fragments containing collagen-derived sequences

There is an urgent need to develop new and effective agents for cancer targeting. In this work, a multivalent antibody is characterized in vivo in living animals. The antibody, termed "trimerbody", comprises a single-chain antibody (scFv) fragment connected to the N-terminal trimerization subdomain of collagen XVIII NC1 by a flexible linker. As indicated by computer graphic modeling, the trimerbody has a tripod-shaped structure with three highly flexible scFv heads radially outward oriented. Trimerbodies are trimeric in solution and exhibited multivalent binding, which provides them with at least a 100-fold increase in functional affinity than the monovalent scFv. Our results also demonstrate the feasibility of producing functional bispecific trimerbodies, which concurrently bind two different ligands. A trimerbody specific for the carcinoembryonic antigen (CEA), a classic tumor-associated antigen, showed efficient tumor targeting after systemic administration in mice bearing CEA-positive tumors. Importantly, a trimerbody that recognizes an angiogenesis-associated laminin epitope, showed excellent tumor localization in several cancer types, including fibrosarcomas and carcinomas. These results illustrate the potential of this new antibody format for imaging and therapeutic applications, and suggest that some laminin epitopes might be universal targets for cancer targeting.

Functionally fused antibodies--a novel adjuvant fusion system

Antibodies capable of recognizing key molecular targets isolated e.g. by phage display technology have been used in the pursuit of new and improved therapies for prevalent human diseases. These approaches often take advantage of non-immunogenic antibody fragments to achieve specific toxin-, radioactivity- or effector-domain delivery. There is now a growing interest in using anti-idiotypic antibodies or other antigen mimics to induce potent immune responses against antigen structures in question. We have earlier reported on the functional rescue of antibodies that are active when fused to the phage, but inactive as soluble protein [Jensen, K.B., Larsen, M., Pedersen, J.S., Christensen, P.A., Alvarez-Vallina, L., Goletz, S., Clark, B.F. and Kristensen, P. (2002) Functional improvement of antibody fragments using a novel phage coat protein III fusion system. Biochem. Biophys. Res. Commun. 298, 566-73.]. The rescue was accomplished by maintaining the fusion between the antibody fragment and portions of the filamentous bacteriophage coat protein 3, as present in the original antibody-displaying phage. In the present study, we have applied this system in an attempt to improve immunogenicity of anti-idiotypic antibodies isolated by phage display. Here we demonstrate that by preserving linkage between phage antibody and the N-terminal domain of phage coat protein 3, we induce multimerization of the antibody fragments, and improve their immunogenicity. This immunization approach allows induction of anti-idiotypic antibodies in mice, and facilitates the use of antibodies that are non-functional as non-fused soluble protein.

Enhanced antiangiogenic therapy with antibody-collagen XVIII NC1 domain fusion proteins engineered to exploit matrix remodeling events

Antiangiogenic therapy is nowadays one of the most active fields in cancer research. The first strategies, aimed at inhibiting tumor vascularization, included upregulation of endogenous inhibitors and blocking of the signals delivered by angiogenic factors. But interaction between endothelial cells and their surrounding extracellular matrix also plays a critical role in the modulation of the angiogenic process. This study introduces a new concept to enhance the efficacy of antibody-based antiangiogenic cancer therapy strategies, taking advantage of a key molecular event occurring in the tumor context: the proteolysis of collagen XVIII, which releases the endogenous angiogenesis inhibitor endostatin. By fusing the collagen XVIII NC1 domain to an antiangiogenic single-chain antibody, a multispecific agent was generated, which was efficiently processed by tumor-associated proteinases to produce monomeric endostatin and fully functional trimeric antibody fragments. It was demonstrated that the combined production in the tumor area of complementary antiangiogenic agents from a single molecular entity secreted by gene-modified cells resulted in enhanced antitumor effects. These results indicate that tailoring recombinant antibodies with extracellular matrix-derived scaffolds is an effective approach to convert tumor progression associated processes into molecular clues for improving antibody-based therapies.

Mono and bivalent binding of a scFv and covalent diabody to murine laminin-1 using radioiodinated proteins and SPR measurements: effects on tissue retention in vivo

Phage display techniques identified a scFv, 15-9, which binds to murine laminin-1 and accumulated selectively in tumors. In this study, a covalent diabody was constructed by changing the amino acid residues at positions VH44 and VL100 to cysteine residues so that the diabody form could be stabilized via a disulfide bond. The covalent diabody was expressed in Pichia pastoris and purified by affinity chromatography. The binding properties were measured by surface plasmon resonance and solid phase binding of (125)I diabody and scFv. Data from the plasmon resonance method yielded calculated K(D)s of 4.4 x 10(-10) M for the covalent diabody and 9.9 x 10(-8) M for the scFv. K(D)s calculated from solid phase binding of radioiodinated proteins were 1.7-2.1 x 10(-10) M and 2.1-2.4 x 10(-8) M respectively. The rate of dissociation of (125)I scFv from solid phase laminin was independent of laminin concentration; however, the dissociation of the (125)I diabody was dependent both on the concentration of laminin and on the concentration of the diabody. Specifically, high concentrations of laminin yielded very slow rates of diabody dissociation indicating that bivalent attachments had formed. When higher amounts of diabody were used that essentially saturated the laminin sites with univalent binding, the dissociation rate was similar to that for the scFv indicating univalent binding. Biodistribution studies in tumor-bearing SCID mice showed that the covalent diabody improved the ratio of tumor/muscle 2 fold over that obtained with the scFv, although the absolute amount of protein bound to the tumor site was not significantly different for the two forms. The data also showed that retention of the diabody in the tumor and kidney, sites where laminin is present in high concentration, was much longer compared to that of scFv. These data are consistent with the hypothesis that both scFv and diabody forms bind to available laminin in vivo with similar association kinetics, but that in situations of high target concentration, the diabody can bind bivalently and is thus retained at the binding site much longer than the scFv.

Identification of an antilaminin-1 scFv that preferentially homes to vascular solid tumors

The tumor vasculature and extracellular matrix make attractive targets for distinguishing solid tumors from normal cells. In solid tumors, the processes of angiogenesis and metastasis potentially give rise to unique epitopes not usually accessible in homeostatic organs. Specific targeting of solid tumors for radioimmunotherapy requires that the targeting agent accumulate rapidly and at high levels at the tumor site. This study involved the selection of scFvs that recognize laminin-1 in vitro from the Tomlinson I and J phage display libraries. Selected, purified scFvs were radioiodinated and injected in tumor-bearing mice. One of these, scFv 15-9, exhibited preferential accumulation at subcutaneous tumors when compared to other antilaminin scFvs or to a control scFv. Autoradiographic analysis indicated that scFv15- 9 also displayed a higher vessel:parenchyma ratio than did two other antilaminin scFvs, scFv 15-6 and scFv 15-1, indicating a preferential accumulation of scFv 15-9 around vessel structures. Immunohistochemistry confirmed that scFv 15-9 accumulated at sites of endothelial cells lining vessel structures where significant levels of laminin were present. These data demonstrate that scFv 15-9 binds to a specific epitope on laminin and has potential for tumor endoradiotherapy in subcutaneous tumors.

Antibody engineering: facing new challenges in cancer therapy

Antibody-based therapeutics are beginning to realize the promise enclosed in their early denomination as magic bullets. Initial disappointment has turned into clinical and commercial success, and engineered antibodies currently represent over 30% of biopharmaceuticals in clinical trials. Recent structural and functional data have allowed the design of a new generation of therapeutic antibodies, with strategies ranging from complement-mediated and antibody-dependant cellular cytotoxicity enhancement to improved cytotoxic payloads using toxins, drugs, radionucleids and viral delivery. This review considers the structure of different types of recombinant antibodies, their mechanism of action and how their efficacy has been increased using a broad array of approaches. We will also focus on the additional benefits offered by the use of gene therapy methods for the in vivo production of therapeutic antibodies.

Molecular classification of renal tumors by gene expression profiling

Renal tumor classification is important because histopathological subtypes are associated with distinct clinical behavior. However, diagnosis is difficult because tumor subtypes have overlapping microscopic characteristics. Therefore, ancillary methods are needed to optimize classification. We used oligonucleotide microarrays to analyze 31 adult renal tumors, including clear cell renal cell carcinoma (RCC), papillary RCC, chromophobe RCC, oncocytoma, and angiomyolipoma. Expression profiles correlated with histopathology; unsupervised algorithms clustered 30 of 31 tumors according to appropriate diagnostic subtypes while supervised analyses identified significant, subtype-specific expression markers. Clear cell RCC overexpressed proximal nephron, angiogenic, and immune response genes, chromophobe RCC oncocytoma overexpressed distal nephron and oxidative phosphorylation genes, papillary RCC overexpressed serine protease inhibitors, and extracellular matrix products, and angiomyolipoma overexpressed muscle developmental, lipid biosynthetic, melanocytic, and distinct angiogenic factors. Quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry of formalin-fixed renal tumors confirmed overexpression of proximal nephron markers (megalin/low-density lipoprotein-related protein 2, alpha-methylacyl CoA racemase) in clear cell and papillary RCC and distal nephron markers (beta-defensin 1, claudin 7) in chromophobe RCC/oncocytoma. In summary, renal tumor subtypes were classified by distinct gene expression profiles, illustrating tumor pathobiology and translating into novel molecular bioassays using fixed tissue.

Selection of functional human antibodies from retroviral display libraries

Antibody library technology represents a powerful tool for the discovery and design of antibodies with high affinity and specificity for their targets. To extend the technique to the expression and selection of antibody libraries in an eukaryotic environment, we provide here a proof of concept that retroviruses can be engineered for the display and selection of variable single-chain fragment (scFv) libraries. A retroviral library displaying the repertoire obtained after a single round of selection of a human synthetic scFv phage display library on laminin was generated. For selection, antigen-bound virus was efficiently recovered by an overlay with cells permissive for infection. This approach allowed more than 10(3)-fold enrichment of antigen binders in a single selection cycle. After three selection cycles, several scFvs were recovered showing similar laminin-binding activities but improved expression levels in mammalian cells as compared with a laminin-specific scFv selected by the conventional phage display approach. Thus, translational problems that occur when phage-selected antibodies have to be transferred onto mammalian expression systems to exert their therapeutic potential can be avoided by the use of retroviral display libraries.

Replicating retroviral vectors mediating continuous production and secretion of therapeutic gene products from cancer cells

The successful application of cancer gene therapy has been hampered by the low efficiency of in vivo gene delivery by currently used replication-defective vectors. Accordingly, considerable efforts are now being directed toward development and use of vectors capable of replicating in cancer cells. However, for replicating retroviruses, insertion of additional reading frames into the viral genome often resulted in the generation of unstable viruses. Here, we report a novel concept for the generation of replication-competent murine leukemia virus (MLV) vectors capable of mediating the secretion of soluble therapeutic proteins from infected cells. As a proof of principle, we inserted transgene regions encoding either a single-chain variable region fragment (scFv), here, the laminin-specific L36-scFv, or the T-cell-specific 7A5-scFv, or the cytokine GM-CSF into the MLV envelope (env) gene after +1 codon of the envelope (Env) protein, followed by a sequence specifying a furin protease cleavage site. The resulting viruses, termed L36-furin-A, 7A5-furin-A and GMCSF-furin-Mo, respectively, infected a variety of human cell lines, including HMEC-1 (endothelial), A301 (lymphoid), MDA-MB231 and MDA-MB468 (breast cancer) and HT1080 (fibrosarcoma) cells. Western blot analysis of conditioned culture medium from HT1080 cells infected by replicating L36-furin A, as an example, revealed that more than 90% of the Env fusion protein molecules were indeed intracellularly cleaved. After 5 days of infection, up to 3-4 mug/ml of soluble L36-scFv accumulated in the supernatant of HT1080 cells. The eukaryotically produced L36-scFv and 7A5-scFv were able to recognize their native antigens with high avidity, as assessed by ELISA and flow cytometry. Furthermore, the replicating viruses were genetically stable for more than 12 cell passages. In conclusion, a new generation of replication-competent retroviral vectors capable of mediating long-term and efficient secretion of therapeutic proteins suitable for cancer therapy was generated.

Multivalent scFv Display of Phagemid Repertoires for the Selection of Carbohydrate-specific Antibodies and its Application to the Thomsen–Friedenreich Antigen

The Thomsen-Friedenreich disaccharide (TF) is a promising target antigen for tumor immunotherapy, since it is almost exclusively expressed in carcinoma tissues. The TF-specific antibodies generated so far are IgMs of mouse origin with limited therapeutic potential. Phage-displayed scFv repertoires are an established source for recombinant antibodies; however, we were unable to identify scFvs binding to TF when applying libraries in the standard monovalent display format of phagemid systems. Here, we report on the successful selection of TF-specific antibody fragments using a multivalent scFv phagemid library format based on shortened linkers (one amino acid residue). The libraries were constructed from mice immunized with asialoglycophorin and selected using TF displayed on two different carrier molecules in combination with the proteolytically cleavable helper phage KM13. All isolated clones encoded the same framework genes and the same complementarity-determining regions. After affinity maturation only scFv with the founder sequence were selected from secondary repertoires. This indicates a very narrow sequence window for TF-specific antibodies. Investigating other linker-length formats revealed a clear inverse correlation between linker length and binding activity both as soluble proteins and displayed on phages. The highest affinity was obtained with the tetrameric format. The selected scFv was specific for TF on various carrier molecules and tumor cells and performed well in ELISA and immunohistochemistry. We postulate that scFv phagemid library formats with short linkers (i.e. multimeric scFvs) may, in general, be advantageous in selections for the generation of scFvs against carbohydrate epitopes or other epitopes associated with low intrinsic affinity per binding site), and expect that they will be superior in applications for diagnosis or therapy.

Generation of non-permissive basement membranes by anti-laminin antibody fragments produced by matrix-embedded gene-modified cells

Tumor-induced blood vessel formation is a key process for the growth and spread of solid tumors, traditionally attributed to activated host endothelial cells (angiogenesis). Recently, highly aggressive cancer cells have been shown to form vascular channels in the absence of endothelial cells (vasculogenic mimicry). In this work, we have focused on the common dependence of both processes in their interactions with the surrounding extracellular matrix. We had previously described a human recombinant anti-laminin antibody that blocked the capillary morphogenesis of human endothelial cells. Here, we demonstrate that the purified antibody is capable of inhibiting channel formation by human cancer cells, suggesting a common morphogenic pathway in both processes. Moreover, matrix-embedded cells producing antibody fragments may render the surrounding matrix non-permissive for aggressive tumor cells. These results open the way for the development of new therapeutic strategies for cancer.

The extracellular matrix: a new turn-of-the-screw for anti-angiogenic strategies

Anti-angiogenic therapy is currently one of most active fields in cancer research. The initial strategies, which were aimed at inhibiting tumor vascularization, included upregulation of endogenous inhibitors and blocking of the signals delivered by angiogenic factors. However, interactions between endothelial cells and their surrounding extracellular matrix also play a crucial role in modulation of the angiogenic process. Compounds that target either the integrins implicated in these interactions or the proteases responsible for matrix remodeling have been shown to halt tumor growth in murine models and are now in clinical trials. However, little attention has been paid to integrin ligands, the extracellular matrix components that support endothelial cell survival, movement and reorganization. Here, we summarize the current knowledge about these angiogenesis inhibitors and propose a novel therapeutic approach based on the blocking of crucial binding sites present in the extracellular matrix.

A novel cell binding site in the coiled-coil domain of laminin involved in capillary morphogenesis

Recently, we reported the isolation and characterization of an anti-laminin antibody that modulates the extracellular matrix-dependent morphogenesis of endothelial cells. Here we use this antibody to precisely map the binding site responsible for mediating this biologically important interaction. By using a phage display-assisted mapping strategy to preserve protein structure, we demonstrate for the first time that the coiled-coil region of laminin contains a cell binding site. The adhesion motif is formed by residues contributed by both alpha and gamma chains, and is located in the middle part of the rod-like portion in a highly flexible area, which corresponds to a protease-susceptible site. Based on this information, a peptide mimotope was used to characterize the cognate receptor. Although we can not rule out the implication of other receptors, our results demonstrate that the laminin helical rod active site interacts with alpha2beta1 integrin on the surface of endothelial cells. These findings provide new insight into the complex mechanisms regulating capillary morphogenesis.

Identification of keratinocyte-specific markers using phage display and mass spectrometry

Specific molecular markers for various normal and pathogenic cell states and cell types provide knowledge of basic biological systems and have a direct application in targeted therapy. We describe a proteomic method based on the combination of new and improved phage display antibody technologies and mass spectrometry that allows identification of cell type-specific protein markers. The most important features of the method are (i) reduction of experimental noise originating from background binding of phage particles and (ii) isolation of affinity binders after a single round of selection, which assures a high diversity of binders. The method demonstrates, for the first time, the ability to detect, identify, and analyze both secreted and membrane-associated extracellular proteins as well as a variety of different cellular structures including proteins and carbohydrates. The optimized phage display method was applied to analysis of human skin keratinocytes resulting in the isolation of a panel of antibodies. Fourteen of these antibodies were further characterized, half of which predominantly recognized keratinocytes in a screen of a range of different cell types. Three cognate keratinocyte antigens were subsequently identified by mass spectrometry as laminin-5, plectin, and fibronectin. The combination of phage display technology with mass spectrometry methods for protein identification is a general and promising approach for proteomic analysis of cell surface complexity.

Functional improvement of antibody fragments using a novel phage coat protein III fusion system

Functional expressions of proteins often depend on the presence of host specific factors. Frequently recombinant expression strategies of proteins in foreign hosts, such as bacteria, have been associated with poor yields or significant loss of functionality. Improvements in the performance of heterologous expression systems will benefit present-day quests in structural and functional genomics where high amounts of active protein are required. One example, which has been the subject of considerable interest, is recombinant antibodies or fragments thereof as expressions of these in bacteria constitute an easy and inexpensive method compared to hybridoma cultures. Such approaches have, however, often suffered from low yields and poor functionality. A general method is described here which enables expressions of functional antibody fragments when fused to the amino-terminal domain(s) of the filamentous phage coat protein III. Furthermore, it will be shown that the observed effect is neither due to improved stability nor increased avidity.

Single-chain antibody-based gene therapy: inhibition of tumor growth by in situ production of phage-derived human antibody fragments blocking functionally active sites of cell-associated matrices

Experimental evidence suggests that blocking the interactions between endothelial cells and extracellular matrix (ECM) components may provide a potent and general strategy to inhibit tumor neovascularization. Based on these considerations, we have focused our efforts on laminin, component of the vascular basement membrane of every tumor-associated vessel, which serves an essential role in tube formation. We screened anti-laminin single-chain antibody fragments (scFv) derived from a human phage-display library and identified one that blocks the formation of capillary-like structures in vitro. This scFv inhibits angiogenesis in vivo in the chick embryo chorioallantoic membrane assay and prevents the establishment and growth of subcutaneous tumors in mice, either when administered as bolus protein therapy or when produced locally by gene-modified tumor cells. Our work represents the first demonstration of a direct in vivo therapeutic effect of a single-chain antibody secreted by gene-modified mammalian cells. These results open the way for a new antibody-based gene therapy strategy of cancer.

Development of a computer-assisted high-throughput screening platform for anti-angiogenic testing

Angiogenesis is widely accepted to play a central role in tumor growth and spreading. Therefore, anti-angiogenic agents appear as promising tools in cancer therapy, and great efforts have been devoted to their identification in recent years. Despite fast progress in this area, a quantitative and objective system for rapid anti-angiogenic testing is not currently available. In this work, we describe a new platform for the large-scale screening of agents with potential anti-angiogenic activity. This involved the miniaturization of an in vitro two-dimensional assay model of vascular morphogenesis to run in a high-density format (384- or 1536-well microplates) and the development of a specific digital image-analysis system for the automated and quantitative evaluation of biological activity. To test the efficacy and reproducibility of the platform, we used a wide variety of well-characterized angiogenesis inhibitors, including antibodies against components of the extracellular matrix, peptides, and chemicals. The system proved to be a reproducible, polyvalent, and powerful tool for massive screening of agents with potential application in cancer therapy. Ongoing developmental work, directed toward optimizing this system for use with a fully automated high-throughput screening platform, should accelerate research into the mechanisms of tumor angiogenesis and the development of drugs that target the essential tumor vasculature.