HGG-41. CHARACTERIZATION OF THE IMMUNE RESPONSE FOLLOWING VARIOUS RADIOTHERAPY TREATMENTS IN GLIOBLASTOMA

Malignant gliomas represent 6.5% of all childhood brain neoplasms with a 5-years survival rate of less than 20%. Current standard of care for these tumors include radiotherapy; recent data in solid tumors indicate that adequate radiation protocols may synergize with immunotherapy strategies for better outcomes. Nonetheless, a great discrepancy between preclinical studies and clinical trials outcomes persists, the basis of which is not fully understood. One hypothesis may be due to different radiation protocols used. We used the GL261 syngeneic mouse model of glioma to test this hypothesis and characterize the immune response to radiotherapy, with either a single dose of 10Gy, a dose often used in preclinical models, or a fractionated treatment of 2Gy for five consecutive days (2Gyx5), as fractioned radiotherapy is most often used in patients. The immune content of the brain and the blood was assessed by flow cytometry in un-irradiated (control), 10Gyx1 and 2Gyx5 treated mice for three weeks after radiation. In the brain, both radiation regimens drastically reduced the number of CD45(+) cells for the first two weeks after treatment. When compared to controls, 10Gyx1 but not 2Gyx5 treated mice showed a significant increase in tumor infiltrating lymphocytes (CD3(+)) starting from the second week following treatment. This effect persisted until three weeks post treatment. The 10Gyx1 dose was better tolerated by the resident microglia (CD45(low)CD11b(+)) when compared to the 2Gyx5 treatment. Our data describe the dynamics through which the immune microenvironment responds to two radiation regimens over time. Our results show that 10Gyx1 is the most effective regimen to impede tumor growth and to induce lymphocyte infiltration once the system recovers from the treatment. Our work suggests that, in the GL261 model, the fractionated radiation treatment we tested may be less optimal in priming glioma cells to the immune system.