PGG-Betafectin, a soluble β(1,3: 1,6) glucan, is processed by macrophages into an active moiety capable of priming neutrophil CR3 (CD11b) for cytotoxicity against iC3b- and monoclonal antibody-opsonized tumors

4267 The use of antibodies for the treatment of cancer within humans may be an effective method of therapy due to the fact that antibodies have the ability to target specific antigens within the body. Many tumor antigens that are targeted by immunization strategies are self-antigens. The mechanism by which the body discriminates between self and non-self antigens is through tolerance. Some of the established mechanisms of tolerance include clonal deletion, anergy, and immunological ignorance (1). In addition to these well characterized methods of immune self-tolerance, T-regulatory (Treg) cells have been shown to actively downregulate the activation and expansion of self-reactive lymphocytes (1). One major question in tumor immunology is whether the various self-tolerance mechanisms hamper the body s ability to destroy nacient tumor lesions. Studies performed by us and others have demonstrated that Treg cells appear to play an important role in impeding the effectiveness of tumor-effector T cells, because direct elimination of Treg cells using a depleting antibody (PC61) results in an increase in immunity against cancer (2). The depleting antibody used in mice, PC61, is a monoclonal rat anti-CD25 antibody that was created by Lowenthal et al. using hybridoma technology and has been shown to accelerate the dissociation of IL-2 from its high and low affinity receptor (3). Yeast display is a new technology developed by Feldhaus et al. in which human scFv are expressed as an a-agglutinin fusion protein on the surface of yeast (4). The expression of scFv is under the control of a galactose promoter which can be monitered using N and C terminal tags adjacent to the scFv (4). Therefore, this technology provides a rapid screening tool, which allows one to isolate antigen-specific single chains in real-time using flow cytometry. The purpose of our study is to construct fully human anti-CD25 antibodies using a novel yeast surface display library, which have the ability to deplete human Treg cells in vivo. The extracellular portion of human CD25 was cloned and used as a source of antigen for the selection of single chain variable fragments from a yeast display library. Our data demonstrate that after five rounds of selection we have isolated a population of scFv, which are specific for CD25. Once this population was isolated, ten scFv clones were randomly chosen and their affinity for CD25 was evaluated by flow cytometry. Our results indicate that we have isolated at least two high affinity clones, which will be used to construct fully human anti-CD25 antibodies. Future studies will involve the assessment of these antibodies to deplete human Treg cells. Lastly, these antibodies will be used in combinatorial studies with other forms of treatment devised within our laboratory, which may lead to a more effective mode of therapy for the treatment of cancer.