Generation and tumor recognition properties of two human monoclonal antibodies specific to cell surface anionic phospholipids.

Phosphatidylserine (PS) and other anionic phospholipids, which become exposed on the surface of proliferating endothelial cells, tumor cells and certain leukocytes, have been used as targets for the development of clinical-stage biopharmaceuticals. One of these products (bavituximab) is currently being investigated in Phase 3 clinical trials. There are conflicting reports on the ability of bavituximab and other antibodies to recognize PS directly or through beta-2 glycoprotein 1, a serum protein that is not highly conserved across species. Here, we report on the generation and characterization of two fully human antibodies directed against phosphatidylserine. One of these antibodies (PS72) bound specifically to phosphatidylserine and to phosphatidic acid, but did not recognize other closely related phospholipids, while the other antibody (PS41) also bound to cardiolipin. Both PS72 and PS41 stained 8/9 experimental tumor models in vitro, but both antibodies failed to exhibit a preferential tumor accumulation in vivo, as revealed by quantitative biodistribution analysis. Our findings indicate that anionic phospholipids are exposed and accessible in most tumor types, but cast doubts about the possibility of efficiently targeting tumors in vivo with PS-specific reagents.

A highly functional synthetic phage display library containing over 40 billion human antibody clones.

Several synthetic antibody phage display libraries have been created and used for the isolation of human monoclonal antibodies. The performance of antibody libraries, which is usually measured in terms of their ability to yield high-affinity binding specificities against target proteins of interest, depends both on technical aspects (such as library size and quality of cloning) and on design features (which influence the percentage of functional clones in the library and their ability to be used for practical applications). Here, we describe the design, construction and characterization of a combinatorial phage display library, comprising over 40 billion human antibody clones in single-chain fragment variable (scFv) format. The library was designed with the aim to obtain highly stable antibody clones, which can be affinity-purified on protein A supports, even when used in scFv format. The library was found to be highly functional, as >90% of randomly selected clones expressed the corresponding antibody. When selected against more than 15 antigens from various sources, the library always yielded specific and potent binders, at a higher frequency compared to previous antibody libraries. To demonstrate library performance in practical biomedical research projects, we isolated the human antibody G5, which reacts both against human and murine forms of the alternatively spliced BCD segment of tenascin-C, an extracellular matrix component frequently over-expressed in cancer and in chronic inflammation. The new library represents a useful source of binding specificities, both for academic research and for the development of antibody-based therapeutics.

Monoclonal antibodies to murine thrombospondin-1 and thrombospondin-2 reveal differential expression patterns in cancer and low antigen expression in normal tissues.

There is a considerable interest for the discovery and characterization of tumor-associated antigens, which may facilitate antibody-based pharmacodelivery strategies. Thrombospondin-1 and thrombospondin-2 are homologous secreted proteins, which have previously been reported to be overexpressed during remodeling typical for wound healing and tumor progression and to possibly play a functional role in cell proliferation, migration and apoptosis. To our knowledge, a complete immunohistochemical characterization of thrombospondins levels in normal rodent tissues has not been reported so far. Using antibody phage technology, we have generated and characterized monoclonal antibodies specific to murine thrombospondin-1 and thrombospondin-2, two antigens which share 62% aminoacid identity. An immunofluorescence analysis revealed that both antigens are virtually undetectable in normal mouse tissues, except for a weak staining of heart tissue by antibodies specific to thrombospondin-1. The analysis also showed that thrombospondin-1 was strongly expressed in 5/7 human tumors xenografted in nude mice, while it was only barely detectable in 3/8 murine tumors grafted in immunocompetent mice. By contrast, a high-affinity antibody to thrombospondin-2 revealed a much lower level of expression of this antigen in cancer specimens. Our analysis resolves ambiguities related to conflicting reports on thrombosponding expression in health and disease. Based on our findings, thrombospondin-1 (and not thrombospondin-2) may be considered as a target for antibody-based pharmacodelivery strategies, in consideration of its low expression in normal tissues and its upregulation in cancer.

Reformatting of scFv antibodies into the scFv-Fc format and their downstream purification.

The scFv-Fc format allows for rapid characterization of candidate scFvs isolated from phage display libraries before conversion into a full-length IgG. This format offers several advantages over the phage display-derived scFv, including bivalent binding, longer half-life, and Fc-mediated effector functions. Here, a detailed method is presented, which describes the cloning, expression, and purification of an scFv-Fc fragment, starting from scFv fragments obtained from a phage display library. This method facilitates the rapid screening of candidate antibodies, prior to a more time-consuming conversion into a full IgG format. Alternatively, the scFv-Fc format may be used in the clinic for therapeutic applications.

A novel synthetic naïve human antibody library allows the isolation of antibodies against a new epitope of oncofetal fibronectin.

Human monoclonal antibodies (mAbs) can routinely be isolated from phage display libraries against virtually any protein available in sufficient purity and quantity, but library design can influence epitope coverage on the target antigen. Here we describe the construction of a novel synthetic human antibody phage display library that incorporates hydrophilic or charged residues at position 52 of the CDR2 loop of the variable heavy chain domain, instead of the serine residue found in the corresponding germline gene. The novel library was used to isolate human mAbs to various antigens, including the alternatively-spliced EDA domain of fibronectin, a marker of tumor angiogenesis. In particular, the mAb 2H7 was proven to bind to a novel epitope on EDA, which does not overlap with the one recognized by the clinical-stage F8 antibody. F8 and 2H7 were used for the construction of chelating recombinant antibodies (CRAbs), whose tumor-targeting properties were assessed in vivo in biodistribution studies in mice bearing F9 teratocarcinoma, revealing a preferential accumulation at the tumor site.