Martha S. Hayden

Surface Expression of CD28 Single Chain Fv for Costimulation by Tumor Cells

Single-chain antibody binding fragments (sFv) (Bird et al. 1988) can be directed to distinct cellular compartments where they can cause specific functional alterations based on their binding specificity. For example, intracellular expression of an sFv to the human immunodeficiency virus type-1 (HIV-l) rev regulatory protein prevented viral replication (Duan et al. 1994), and another sFv to the HIV envelope protein reduced infectivity of virus particles (Marasco et al. 1993). This approach has also been used with specific sFv to protect plants from intracellular pathogens (Tavladoraki et al. 1993). To explore the possible uses of sFv as artificial adhesion receptors, we expressed the 2E12 sFv to human CD28 with a transmembrane region or a glycosylphosphatidylinositol (GPI) anchor to generate tumor cells capable of activating the CD28 receptor during adhesion with T cells. In comparison with CD80, a natural adhesion receptor for CD28, the 2E12 sFv showed increased binding affinity for CD28 and was efficient in activating T cells during coculture. These results suggest that cell surface expression of sFv may offer advantages over natural ligands for binding and activation of adhesion receptors. An sFv to the CD28 receptor was chosen for these studies because of the recent experiments showing that tumor cells are not immunogenic when they do not express natural ligands (CD80 or CD86) for CD28 (Baskar et al. 1993). Expression of CD80 or CD86 in experimental murine tumors resulted in the generation of antigen-specific cytotoxic T lymphocytes (CTL) in vivo and allowed acti-