Peripheral Blood γδ T Cell Response to High-Grade Glioma: Implications for Localized Adoptive Immunotherapy

Abstract 4114

Glioblastoma multiforme (GBM) remains impervious to existing therapy. We have recently established that GBM is vulnerable to recognition and lysis by γδ T cells using in vitro cytotoxicity testing and in a treatment model based on human γδ T cell therapy of intracranial xenografted human glioma cell line tumors in immunodeficient mice. We have also shown that circulating γδ T cell counts and function are significantly reduced in GBM patients at the time of diagnosis. In this study, we sought to resolve the paradox between these findings with a working hypothesis that γδ T cells are depleted by activation-induced cell death following an unsuccessful immune response to GBM. We monitored peripheral and tumor immune responses over time in a C57BL/6 mouse model after intracranial placement of 1 × 10⁵ syngeneic GL261 glioma cells, methylcellulose carrier alone, or no treatment (n=10/group). Tumor-bearing mice uniformly develop aggressive infiltrating gliomas with pseudopalisading morphology and necrotic centers that are uniformly fatal within 30 days. At 11 days following tumor placement, mice were asymptomatic. Brain sections revealed the presence of small infiltrating gliomas in 100% of mice. Peripheral blood expansion of γδ T cells was significantly increased compared to mice that received only the methylcellulose vehicle (p=0.0003). T cell proliferation was specific only to the γδ T cell compartment and did not involve other T cell subtypes, A significant proportion of γδ T cells expressed Annexin-V (20%-45%) in contrast to 2–5% in controls (p<0.0001) indicative of progression to apoptotic death. Mice that received intracranial injection of methylcellulose alone showed no changess relative to untreated mice. Although GL261 gliomas expressed γδ T cell-recognized NKG2D ligands H60, MULT-1 and RAE-1, peripheral γδ T cells did not infiltrate the intracranial tumors nor confer a survival advantage when compared to identical experiments conducted in tumor-bearing γδ T cell-deficient (C57BL/6^δ−/−) mice. Placement of extra-cranial GL261 flank tumors resulted in accumulation of T cells around the periphery of the tumor but no significant T cell invasion into the tumor parenchyma. However, when mice were treated with adoptive transfer of 1.5 × 10⁶ex vivo expanded/activated syngeneic γδ T cells by stereotactic intracranial injection on the same day as GL261 tumor placement, local tumor development was inhibited. Mice that were injected only with saline following tumor placement developed large tumors at the injection site. These data suggest that depletion of γδ T cells previously seen in GBM patients is likely due to activation-induced cell death. Although glioma-reactive γδ T cell effectors are stimulated in the early phase of tumor formation, they are unable to successfully invade and thereby impede tumor progression. Finally, these findings suggest that treatment strategies directed toward both reduction of tumor-derived immunosuppression and localized post-resection adoptive γδ T cell-based therapy may provide an approach to minimal residual GBM.