B7.1 fusion proteins for cancer immunotherapy

The major goal of this thesis is to generate novel reagents for the immunotherapy of cancer using B7.1 costimulation to help initiate an effective anti-tumor response. Optimal T-cell activation depends on both antigen-specific signaling and other costimulatory pathways. A primary costimulatory signal is delivered by B7.1 (CD80) on antigen presenting cells after activation of the Tcell receptor, CD28. In order to increase tumor immunity, two B7.1 fusion proteins were constructed by genetically linking the extracellular region of B7.1 to the N-terminus of the tumor-targeting antibody NHS76 (B7.1/NHS76) or its Fc portion (B7.1/Fc). To demonstrate potency, both fusion proteins were found to costimulate T-cell proliferation, and the B7.1/NHS76 was found to retain its antigen-binding capacity and showed good uptake in tumors.; To assess their antitumor activity, both B7.1 fusion proteins were evaluated in three solid tumor models of the BALB/c mouse. B7.1/NHS76 and B7.1/Fc were administered i.v. daily x5 into mice bearing 0.5 cm s.c. tumors. When administered alone, B7.1/NHS76 produced a 35%-55% reduction in tumor size, while B7.1/Fc led to complete tumor regression in a dose-dependent pattern. The therapeutic effects of both fusion proteins were abrogated by depletion of CD8+ T cells indicating that this lymphoid subset was instrumental. When combined with CD4+ or CD25+ T-cell depletion to deplete naturally occurring CD4 +CD25+ regulatory T cells, however, mice from both B7.1/NHS76 and B7.1/Fc treatment groups underwent complete tumor regression. Dramatic increases in the infiltration and activation of T cells could be seen in tumors and tumor draining lymph nodes (TDLNs) by FACS in the combination therapy groups. Tumor-regressed mice from previous B7.1/Fc and combination therapy treatment groups were protected from tumor rechallenge with parental Colon 26 tumor cells, demonstrating the establishment of immunologic memory by these treatments. The antitumor efficacies of B7.1/Fc and combination therapy are dependent on both IFN-gamma and Perforin as shown by knockout mouse studies. These results suggest that human B7.1 fusion proteins might be promising antitumor reagents in patients especially when used in combination with methods to eliminate T-regulatory cells.