Construction and Preclinical Characterization of Fc-mGITRL for the Immunotherapy of Cancer

Purpose: To provide proper costimulation required for effective cancer T-cell immunity, Fc-GITRL fusion proteins were generated for use in immunotherapy protocols. Experimental Design: Soluble fusion proteins consisting of the Fc fragment of immunoglobulin and the murine glucocorticoid-induced tumor necrosis factor–related receptor ligand (mGITRL) connected with different linkers were genetically engineered and tested for their potency in two BALB/c solid tumor models. Results: In vivo , construct #178-14 (−5aa, −linker) showed the best activity (>90% tumor reduction) at doses ranging from 5 to 25 μg and was found to be intact by gel electrophoresis. Similar doses used with construct #175-2 (-linker) produced good but not as high tumor regression. Construct #5-1 (+linker), which was found to be relatively unstable by SDS gel electrophoresis, produced 3 months compared with controls that died by day 40. T-cell depletion studies showed that CD8 + T cells play a major role in Fc-mGITRL immunotherapy, and tumors removed from Fc-mGITRL– and DTA-1–treated mice showed a significant influx of granzyme B + lymphocytes compared with controls. Finally, T regulatory (Treg) cell assays showed that, unlike other Fc fusion proteins, all three Fc-mGITRL constructs profoundly suppressed Treg activity. Conclusions: These studies suggest that a stable, intact Fc-mGITRL fusion protein can provide missing costimulation for the immunotherapy of solid tumors. In addition, Fc-mGITRL may alter Treg activity to enhance its effectiveness for tumor immunotherapy.