URPP Fellow: Nikiforos Kapetanakis
Radiotherapy is a standard therapy for cancer and site-directed ionizing radiation is used to specifically target solid cancer masses. Induction of irreversible DNA-damage to tumor cells resulting in cell death is the main goal of radiotherapy. Recent data from us and others, however, show that the inflammatory and immune response elicited by radiation is an integral part of the therapeutic response. We recently identified radiotherapy-induced, local production of anaphylatoxins as an essential response with respect to boosting of tumor-specific immunity and to efficacy. Localized radiotherapy of solid cancers induces a plethora of changes in the tumor, which collectively support intratumoral tumor-specific immune effector functions. We have seen this in both animal models and sarcoma patients. It is currently not known which mechanisms and pathways are involved in local and systemic radiation-induced immune stimulation. In addition, virtually nothing is known about the tissue response to radiation and specifically, the differences between conventional and hypo-fractionated radiotherapy have not been investigated in this context.
The therapeutic efficacy of radiotherapy crucially depends on the concomitant activation of tumor-associated dendritic cells, local production of type I interferon and the presence of CD8+ T cells. Production of type I interferon is downstream of the cGAS/STING pathway that is activated upon sensing of cytoplasmic dsDNA. Upon binding of dsDNA, cyclic GMP-AMP (cGAMP) synthase (cGAS) catalyzes the formation of the second messenger cGAMP. Subsequently, cGAMP binds to stimulator of interferon genes (STING), resulting in phosphorylation of interferon regulatory factor 3 (IRF3) and production of type I IFN. Cancer cells often constitutively contain a high concentration of cytoplasmic dsDNA, which further increases upon DNA-damaging therapies such as radio- or chemotherapy. Given the important role of type I IFN in priming of protective T-cell immunity, the presence of cytoplasmic dsDNA in cancer cells may contribute to their immunogenicity. We found that cancer cells produce cGAMP that is transferred via gap junctions to tumor-associated DCs, which respond by producing type I IFN in situ. Cancer cell-intrinsic expression of cGAS – but not STING – promotes infiltration by effector CD8+ T-cells and consequently, results in prolonged survival (Schadt et al., submitted). Our preliminary experiments suggested that cGAS-expressing cancers respond better to genotoxic treatments.
To investigate the tissue response to radiation, we analyzed inflammatory and immune parameters by high-dimensional flow cytometry (FACSymphony). We observed the emergence of CD39+ PD-1+ Tim-3+ CD8+ T-cells in mouse tumors one week after radiotherapy (1x 10 Gy). We know that the therapeutic response to radiotherapy depends on CD8+ T cells, but preliminary experiments showed that recruitment of CD8+ T cells after radiotherapy was not required. This suggests T cells associated with the tumor at the time point of radiotherapy may change their function as a result of radiation-induced inflammation.
- We will investigate the question why cGAS-expression cancers respond better to genotoxic therapies. On the one hand, it is possible that increased concentrations of cytoplasmic dsDNA result in more cGAMP and better T cell responses. On the other hand, cGAS-expressing tumors are hotter and may for that reason respond better to any therapy. We will address this question by generating cancer cell lines in which we conditionally can express or delete cGAS. Furthermore, we will investigate whether cGAS-expressing tumors respond better to immunotherapy (e.g. immune checkpoint blockade) and whether the synergy between genotoxic and immune therapies depend on cancer cell-intrinsic cGAS. We will validate our findings using human samples with a known response to genotoxic or immune therapy and address the question whether expression of cancer cell-intrinsic cGAS can be used as a biomarker to stratify patients with respect to therapy.
- We will further characterize the CD39+ CD8+ T cell population that emerges as a result of radiotherapy with respect to phenotype and function and whether this population is essential to therapeutic efficacy of radiotherapy. Furthermore, we will investigate the origin (i.e. tumor-associated or recruited after radiotherapy) of the CD39+ CD8+ T cells.
We expect that this project will improve our understanding of immunological processes that take place within the tumor and how these are influenced by cancer cell-intrinsic and -extrinsic features or events. This knowledge may allow prediction of disease progression or therapy response. In addition, this knowledge may be used to improve the clinical response to standard or immune therapy.