Immunological abscopal (ab – away from, scopus – target) effect refers to the targeting of a secondary lesion by the immune system following immune stimulation within the primary tumor. Fundamentally, it is synonymous of mounting a persistent and efficient adaptive anti-tumor immune response that is likely to diffuse systemically. Clinically, this a milestone to reach with immunotherapies since it increases the chance of eradicating relapsing tumors and/or distant metastases.
Glioblastoma is the most common, aggressive primary brain tumor in adults and does not respond to any conventional therapy or immunotherapy. T cells rarely infiltrate glioblastoma tumors and pro-tumor macrophages constitute the main intratumoral immune population. Frequent recurrence after radiotherapy results from tumor cells escaping the radiation site and infiltrating healthy parenchyma. In order to address this issue, the Holland lab from the Human Biology Division established a mouse model recapitulating the human disease, in which two different glioblastomas are induced in the two brain hemispheres (RCAS-Tv-a model). Only one of the two tumors express luciferase, which allows the tracking of tumor cells from this tumor only. After radiation of the untraceable tumor and additional systemic immunotherapies, abscopal effects on the luciferase-positive tumor are assessed.
As a recent Phase II clinical trial combining radiation and anti-PD-L1 recently demonstrated a modestly improved survival in the combination arm. The Holland lab sought to understand the mechanism behind this result with their model and recently published their results in Neuro-Oncology.
The researchers led a survival study with animals bearing one tumor in each hemisphere and treated with unilateral radiation alone, systemic anti-PD-L1 antibody, or both. Radiation alone improves survival compared to control animals, anti-PD-L1 alone has no effect, and the combination only modeslty improves survival compared to radiation alone, results similar to the human Phase II clinical trial. To follow the tumor burden following treatments, the authors considered the luciferase signal as a surrogate marker for tumor viability while tumor growth was assessed by magnetic resonance imaging (MRI). Atlhough tumor viability was drastically decreased by the combination therapy, the tumor kept growing as a net result although at a slower pace compared to radiation alone.
Dr. Chibawanye Ene, first author of the study, and his colleagues hypothesized that the limited benefit from PD-L1 blockade may result from the inability of the patient’s immune system to eradicate remaining cancer cells due to a lack of tumor neo-antigen. He explains the strategy to answer this question: “We induced the expression of a tumor neo-antigen, EGFRvIII, present in about 30% of patients with glioblastoma,” instead of luciferase. “We found that following radiation of one lesion and systemic anti-PD-L1 therapy, the presence of EGFRvIII mutation significantly improves the survival.” When assessing the immune contexture after the different treatments, they observed that radiation and PD-L1 blockade mobilized T cells in the non-irradiated lesions only when the tumor neoantigen EGFRvIII was expressed. However, the number of macrophages increased in both luciferase-positive and EGFRvIII-positive tumors after the combination treatment. In agreement with these results, CD8 or CD4 blockade did not prevent the survival benefit of the unilateral radiation combined with anti-PDL-1 demonstrating that T cells do not mediate anti-tumor response in the luciferase-positive tumors.
Following in vitro and in vivo assessment of macrophage functionality in presence of anti-PD-L1 antibodies, Ene and colleagues “discovered that in the absence of T-cells, as in most glioblastoma, the binding of anti-PD-L1 antibody to macrophages intrinsically activates the macrophages to become anti-tumor and pro-inflammatory, resulting in more phagocytosis of tumor cells in both irradiated and un-irradiated tumors,” says Ene.
Dr. Ene shares his perspectives about this work: “Radiation therapy converts ‘cold’ tumors like glioblastoma to ‘hot’ tumors by recruiting immune cells such as T-cells and macrophages, which in the presence of anti-PD-L1 immunotherapy result in more effective eradication of irradiated tumors. Radiation therapy also induces tumor cell death which releases tumor specific antigens that educate an activated immune system resulting in targeting of un-irradiated tumor in the vicinity of radiation (abscopal effect). Our results also indicate that glioblastoma patients will have variable responses to immunotherapy depending on several factors such as the presence of a unique tumor specific antigens and baseline immune cell heterogeneity. We hope that these findings will allow us to not only identify patients with glioblastoma who may be more responsive to immunotherapy but also uncover strategies to overcome resistance in non-responders using the luciferase based bilateral glioblastoma mouse model.”
This work was supported by the National Institutes of Health.
Fred Hutch/UW Cancer Consortium members Drs Holland, Crane, Pierce and Houghton contributed to this research.
Ene CI, Kreuser SA, Jung M, Zhang H, Arora S, White Moyes K, Szulzewsky F, Barber J, Cimino PJ, Wirsching HG, Patel A, Kong P, Woodiwiss TR, Durfy SJ, Houghton AM, Pierce RH, Parney IF, Crane CA, Holland EC. 2019 Anti-PD-L1 antibody direct activation of macrophages contributes to a radiation-induced abscopal response in glioblastoma. Neuro-Oncology, noz226. https://doi.org/10.1093/neuonc/noz226