COSTIMULATION BY CD28 SFV EXPRESSED ON THE TUMOR-CELL SURFACE OR AS ASOLUBLE BISPECIFIC MOLECULE TARGETED TO THE L6 CARCINOMA ANTIGEN

Interaction of the CD80 (B7-1) and CD86 (B7-2) molecules on antigen pr esenting cells with the receptors CD28 and CTLA-4 on T cells generates signals important in the regulation of immune responses. Because this receptor system involves multiple receptor-ligand interactions, deter mining the function for individual receptors has been difficult. One. approach is the use of antibodies and their derivatives with singular specificity as substitute ligands to explore the activities of these m olecules. We have constructed recombinant mono-and bi-specific sFv mol ecules specific for the CD28 receptor that are capable of binding: and generating costimulatory signals to activate T cells. We demonstrate that these soluble molecules are capable of higher levels of costimula tion than soluble CD80Ig at equivalent concentrations. We also constru cted artificial adhesion receptors on the cell surface using two diffe rent CD28-specific sFvIgs Fused to the CD80 cytoplasmic and transmembr ane domains. In this report, we compared costimulation by a soluble bi specific (alpha CDS-alpha L6) single chain sFvIg fusion protein to tha t generated by L6 antigen positive (L6(+)) H3347 tumor cells transduce d with cell surface expressed forms of alpha CD28 sFv's, We show that the bispecific protein can target potent CD28 costimulatory activity t o L6(+) tumor cells in vitro. We also show that transfection of the ce ll surface forms of the two different CD28 sFvIgs into H3347 tumor cel ls allows them to generate significant costimulatory signals to activa ted T cells. Finally, we demonstrate that tumor cell presentation of e ither the soluble bispecific or transduced cell surface sFv generate s imilar costimulatory effects resulting in T cell activation.