Immune-driven therapeutic combinations in a new mouse model of high-grade glioma

High-grade gliomas (HGGs) are the most common malignant brain tumors; among them, glioblastomas (GBM) represent the most aggressive subtype. The current state-of-the-art treatment is insufficient: after conventional therapies (surgery followed by radio-chemotherapy), the median survival for GBM patients is only 14.6 months and 88% of them die within 36 months from diagnosis.5 In the last years, the development of immunotherapy represented a true revolution in the field of oncology: the therapeutic manipulation of the immunological landscape of tumors made it possible, for the first time, to treat diseases previously characterized by a very poor prognosis. Nevertheless, such revolution still has to come for HGG patients. Despite many efforts, all the immune therapeutic strategies so far proposed failed to substantially modify the prognosis of HGG patients. Many reasons could explain this failure; however, the inadequacy of preclinical models and the poor preclinical optimization of the therapeutic combinations probably played an important role. The present research project was specifically aimed at filling such translational gap. In the first part of this research, we developed a new and more translational HGG mouse model, based on the orthotopic implantation of CT-2A cells previously cultured in neurospheres (NS). Compared to their standard monolayer (ML) counterpart, NS tumors show a higher amount of glioma stem cells (GSCs), a stronger immune suppression and a more abundant tumor vascularization. These features are all related to tumor aggressiveness and therapeutic resistance, in particular to immunotherapies. Therefore, the NS/CT-2A model could represent a suitable and high translational platform for the development of new HGG treatments and therapeutic combinations, aimed at activating a powerful anti-tumor immune reaction and eliminating GSCs. Among conventional therapies, radiotherapy (RT) has been shown to have the highest potential to positively modulate the immune tumor microenvironment (TME) in cancer. However, very limited evidence is currently available concerning how the radiation dose and the fractionation schedule could impact tumor immunology in the context of HGG. The answer to this question has a particular clinical relevance, since some studies (in other types of cancers) have proven that in radio-immunotherapy combinations the effect of the same type of immunotherapy can be potentiated or completely abolished according to which RT schedule is used. By treating mice bearing NS/CT-2A tumors with various RT schemes and extensively analyzing the immune modifications within the TME, we demonstrated that RT can induce a powerful anti-tumor immune activation in experimental HGGs. However, this is strictly dependent on how the treatment is administered. Single-dose RT promotes an immune activation in a dose-dependent manner; however, such effect is drastically reduced when the same cumulative dosage is divided over multiple, smaller fractions. These findings have very important clinical implications, since the current standard of care for GBM patients is based upon a fractionated RT regimen and such schedule is also the most used for radio-immunotherapy combinations. As already mentioned, a possible reason for the failure of immunotherapy in HGG patients could be the lack of synergism between conventional and immune treatments in current therapeutic combinations. To verify this hypothesis, we analyzed the modification of the immune TME in NS/CT-2A tumors following RT, Temozolomide (TMZ) and programmed cell death protein 1 (αPD1) treatment. Despite the contrasting evidence available in the literature, our study revealed that TMZ is clearly immune suppressive. This negative effect is particularly strong, since in RT-TMZ combinations (mimicking the current standard of care for GBM patients) TMZ could also inhibit the positive immune stimulation provided by RT. In the context of this detrimental immune modulation, the administration of αPD1 provided only limited benefits as demonstrated by a slight improvement of tumor immunity and survival. Taken together, the results of this research project demonstrate that all types of therapies (and not only canonical immunotherapies) have an impact on HGG immunity. The schedule-dependent immune modulatory capacity of RT, the strong immune suppressive ability of TMZ when combined with RT and the limited efficacy of checkpoint blockade when aiming at reverting the tumor- and TMZ-induced immune suppression represent our most important findings and could explain the poor results so far obtained with immunotherapy in HGG patients. However, in line with the most recent literature, this study also shows that there is still a potential for the use of immunotherapy in malignant gliomas. Nevertheless, a paradigm shift is necessary. The simple administration of immunotherapies during or after the standard chemo-radiation has been proven inadequate; therefore, synergistic and more effective therapeutic combinations need to be developed. In this view, we believe that future research project should focus on three main pillars: to consider both conventional treatments and canonical immunotherapies as immune treatments, to improve the immune modulatory capacity of conventional therapies by developing new treatment protocols and to extensively test new therapeutic combinations at the preclinical level before clinical translation.

Publication year:2019

Accessibility:Open