IMMU-05. IMMUNOTHERAPY WITH RESIQUIMOD REPOLARIZES TUMOR-ASSOCIATED MYELOID CELLS AND IMPROVES EVENT-FREE SURVIVAL IN A TRANSGENIC MOUSE MODEL OF SONIC-HEDGEHOG (SHH) MEDULLOBLASTOMA

Resiquimod is a synthetic small molecule agonist of Toll-like receptors 7 and 8 (TLR-7/8) that modulates innate immune cells. We found TLR-7/8 are expressed in medulloblastoma exclusively by tumor-associated myeloid cells (TAMs). We tested whether systemically administered resiquimod modulated TAMs in a genetic Sonic hedgehog (SHH) medulloblastoma model, and whether this modulation would be therapeutically beneficial. We generated mice with medulloblastoma by crossing hGFAP-Cre and SmoM2 mouse lines. The resulting hGFAP-Cre/SmoM2 (G-Smo) mice developed medulloblastoma with 100% frequency and showed a median survival of 14.5 days (n=12). Treatment with 3 doses of resiquimod at postnatal days 10, 12 and 14 reduced tumor size and increased median survival to 37 days (n=10) (p=0.003508). Cellular studies showed that resiquimod altered TAM phenotype, rapidly inducing expression of the inflammatory marker VCAM1, and more slowly increasing TAM populations. Responses to the 3-dose regimen were ultimately limited by recurrence and all mice eventually died of tumor progression. Continued resiquimod therapy with every other day dosing was less effective than the 3-dose regimen, suggesting that TAM responses to resiquimod are dynamic and change with prolonged exposure. Our data show that innate immunity, mediated by TAMs and stimulated by TLR-7/8 agonist therapy, can produce a significant anti-tumor effect in medulloblastoma. The common expression of TLR-7/8 on TAMs in patient-derived medulloblastoma samples and in the mouse model suggests that resiquimod may produce similar anti-medulloblastoma effects in humans. Further studies are needed to define the mechanism of the anti-tumor effect in detail, to determine the optimal dose regimen, and to determine if resiquimod can combine effectively with additional adjuvant therapies to produce curative effects.