Martha A. Wild

CD200 Expression on Tumor Cells Suppresses Antitumor Immunity: New Approaches to Cancer Immunotherapy

Although the immune system is capable of mounting a response against many cancers, that response is insufficient for tumor eradication in most patients due to factors in the tumor microenvironment that defeat tumor immunity. We previously identified the immune-suppressive molecule CD200 as up-regulated on primary B cell chronic lymphocytic leukemia (B-CLL) cells and demonstrated negative immune regulation by B-CLL and other tumor cells overexpressing CD200 in vitro. In this study we developed a novel animal model that incorporates human immune cells and human tumor cells to address the effects of CD200 overexpression on tumor cells in vivo and to assess the effect of targeting Abs in the presence of human immune cells. Although human mononuclear cells prevented tumor growth when tumor cells did not express CD200, tumor-expressed CD200 inhibited the ability of lymphocytes to eradicate tumor cells. Anti-CD200 Ab administration to mice bearing CD200-expressing tumors resulted in nearly complete tumor growth inhibition even in the context of established receptor-ligand interactions. Evaluation of an anti-CD200 Ab with abrogated effector function provided evidence that blocking of the receptor-ligand interaction was sufficient for control of CD200-mediated immune modulation and tumor growth inhibition in this model. Our data indicate that CD200 expression by tumor cells suppresses antitumor responses and suggest that anti-CD200 treatment might be therapeutically beneficial for treating CD200-expressing cancers.

Human antibodies from immunized donors are protective against anthrax toxin in vivo

A panel of Fabs that neutralize anthrax toxin in vitro was selected from libraries generated from human donors vaccinated against anthrax. At least two of these antibodies protect rats from anthrax intoxication in vivo. Fabs 83K7C and 63L1D bind with subnanomolar affinity to protective antigen (PA) 63, and Fab 63L1D neutralizes toxin substoichiometrically, inhibits lethal factor (LF) interaction with PA63 and binds to a conformational epitope formed by PA63.

Internalizing Antibodies to the C-Type Lectins, L-SIGN and DC-SIGN, Inhibit Viral Glycoprotein Binding and Deliver Antigen to Human Dendritic Cells for the Induction of T Cell Responses

The C-type lectin L-SIGN is expressed on liver and lymph node endothelial cells, where it serves as a receptor for a variety of carbohydrate ligands, including ICAM-3, Ebola, and HIV. To consider targeting liver/lymph node-specific ICAM-3-grabbing nonintegrin (L-SIGN) for therapeutic purposes in autoimmunity and infectious disease, we isolated and characterized Fabs that bind strongly to L-SIGN, but to a lesser degree or not at all to dendritic cell-specific ICAM-grabbing nonintegrin (DC-SIGN). Six Fabs with distinct relative affinities and epitope specificities were characterized. The Fabs and those selected for conversion to IgG were tested for their ability to block ligand (HIV gp120, Ebola gp, and ICAM-3) binding. Receptor internalization upon Fab binding was evaluated on primary human liver sinusoidal endothelial cells by flow cytometry and confirmed by confocal microscopy. Although all six Fabs internalized, three Fabs that showed the most complete blocking of HIVgp120 and ICAM-3 binding to L-SIGN also internalized most efficiently. Differences among the Fab panel in the ability to efficiently block Ebola gp compared with HIVgp120 suggested distinct binding sites. As a first step to consider the potential of these Abs for Ab-mediated Ag delivery, we evaluated specific peptide delivery to human dendritic cells. A durable human T cell response was induced when a tetanus toxide epitope embedded into a L-SIGN/DC-SIGN-cross-reactive Ab was targeted to dendritic cells. We believe that the isolated Abs may be useful for selective delivery of Ags to DC-SIGN- or L-SIGN-bearing APCs for the modulation of immune responses and for blocking viral infections.

Targeting CUB Domain-Containing Protein 1 with a Monoclonal Antibody Inhibits Metastasis in a Prostate Cancer Model

Through a whole-cell panning approach, we previously identified a panel of antibodies that bound to prostate cancer cell surface antigens. One such antigen, CUB domain-containing protein 1 (CDCP1), was recognized by monoclonal antibody 25A11 and is a single transmembrane molecule highly expressed in several metastatic cancers as well as on CD34(+)CD133(+) myeloid leukemic blast cells. We show CDCP1 expression on prostate cancer cell lines by real-time quantitative PCR (RT-qPCR), flow cytometry, and immunohistochemistry and on prostate cancer patient samples by RT-qPCR and immunohistochemical staining. In cell-based assays, antibody 25A11 inhibited prostate cancer cell migration and invasion in vitro. Further characterization showed that CDCP1 is internalized on antibody binding. When 25A11 was coupled to the cytotoxin saporin either directly or via a secondary antibody, both resulted in prostate cancer cell killing in vitro. In vivo targeting studies with an anti-CDCP1 immunotoxin showed significant inhibition of primary tumor growth as well as metastasis in a mouse xenograft model. These data provide support for continued evaluation of anti-CDCP1 therapy for potential use in cancer in primary and metastatic disease.

Abstract 1774: Development of potent trimeric DR4 agonist Atrimers with therapeutic potential

TRAIL death receptor DR4 is a promising therapeutic target in oncology with expression in a wide variety of tumors. DR4 agonists, including TRAIL and monoclonal antibodies, can induce p53-independent apoptosis and are currently being evaluated in clinical trials in combination with chemotherapy. We aimed to surpass currently available therapeutics by developing trimeric death receptor agonists with properties expected to significantly exceed that of recombinant human TRAIL. The agonists can mimic the natural trimer-trimer interaction of the native ligand/receptor, but do not cross-react with the decoy receptors. Potent DR4 agonist AtrimersTM were engineered using human tetranectin, a trimeric human serum protein of 60 kDa, as a scaffold. A panel of unique DR4 binders was selected from novel phage libraries displaying the C-type lectin domain (CTLD) of tetranectin containing randomized loop sequences. DR4 Atrimers have sub-nanomolar affinity to recombinant DR4-Fc and showed no detectable binding to recombinant forms of DR5 or the decoy receptors. In vitro, the DR4 Atrimers efficiently killed DR4-positive cancer cell lines with sub-nanomolar EC50, but did not kill DR4-negative cell lines. DR4 Atrimers induced cell death of DR4-expressing tumor cells through the caspase pathway, but did not kill primary human B cells and hepatocytes. Interestingly, DR4 Atrimers have differential activities on various cell lines, and also vary in their degree of internalization. While some Atrimers show potent killing of Colo-205 and are rapidly internalized, other agonist Atrimers did not show measurable internalization. These unique properties open the possibility of developing potent naked Atrimers with prolonged half-lives due to lack of internalization, as well as leveraging rapidly internalizing DR4 Atrimers for design of Atrimer-drug conjugates. Further characterization of DR4 Atrimers is ongoing in Colo-205 xenograft models. DR4 agonist AtrimersTM with their superior potency and expected improvement in tumor penetration (vs. antibodies) represent a novel class of targeted cancer therapeutics for efficient induction of apoptosis and provide a promising approach for the treatment of a broad range of cancer types. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1774. doi:10.1158/1538-7445.AM2011-1774