PM-16 * IMMUNOSUPPRESSIVE FACTORS, MDSC AND ARGINASE, ARE ELEVATED IN DOGS WITH MALIGNANT GLIOMA

Abstract

Immunotherapy for malignant gliomas has striking efficacy in rodent models but very limited effects in clinical trials, which may reflect immunological differences between induced and spontaneous tumors. The monocytes that heavily infiltrate GBM develop myeloid-derived suppressor cell (MDSC) phenotype, and these immunosuppressive cells are increased in GBM patients, but not in murine models. Canine primary brain tumors may offer a more faithful representation of the human disease due to similarities in clinical and pathobiological characteristics. We were recently first to identify MDSC in dogs and found significantly increased in number in dogs with advanced cancers. To add further support to pet dogs with glioma as a translational model, we assessed whether canine glioma patients also have elevated numbers of MDSC relative to healthy dogs. MDSC secrete arginase that removes L-arginine from the microenvironment profoundly impairing T cell proliferation and function. Therefore we also measured serum levels of arginase 1 in all dogs. The mean percentage of monocytic MDSC in peripheral blood of dogs with glioma (n = 26) was 7.51 ± 1.31, which was significantly greater than healthy controls (n = 10) with 0.74 ± 0.34 (P< 0.0001). The median percentages of granulocytic MDSC did not differ between dogs with glioma versus controls, 15.95% and 12.5% respectively. There were also significantly increased levels of arginase in the serum of dogs with gliomas, 8.00 ± 3.32 U/L, relative to that of healthy controls, 2.40 ± 1.40 U/L (P < 0.0001). These data demonstrate that some of the immunosuppressive mechanisms present in human gliomas patients are also found in dogs with malignant gliomas. These findings lend further support that the pet dog with spontaneous brain cancer that shares our environment is an excellent translational model for the human disease. We are currently working to correlate changes in numbers of MDSC and arginase levels in serum with disease burden as possible biomarkers for response to therapy.