High Salt Diet Exacerbates Intestinal Radiotoxicity by Promoting Intestinal Epithelial Barrier Dysfunction after RT

PURPOSE/OBJECTIVE(S)

Intestinal side effects have a substantial impact on patients' quality of life and constrain radiation therapy (RT) of abdominal and pelvic tumors. We aim at gaining a better understanding of the pathogenesis of intestinal radiotoxicity and identifying contributing factors. A high salt diet is common among countries of the Global North. The effects of an increased sodium-chloride (NaCl) intake on the development of intestinal side effects have not been investigated.

MATERIALS/METHODS

C57BL/6 wild-type (WT) mice were fed a high salt diet (HSD) or normal salt diet (NSD) and irradiated with 12 Gy total body irradiation (TBI) or 13 Gy abdominal irradiation (ABI). Following TBI the mice received a syngeneic bone marrow transplant to reconstitute hematopoiesis. Readouts after RT included weight monitoring, histopathological analysis and assessment of intestinal epithelial barrier function. To corroborate our findings in vitro, murine organoids of the small intestine were cultivated, treated with varying NaCl concentrations (110-200 mM) and irradiated with 2 or 4 Gy. Organoid survival and growth were determined using multiple methods. Additionally, colony formation assays (CFA) of murine MC38 colon carcinoma cells were performed after stimulation with NaCl. Mechanistically, we analyzed the apoptosis rates of MC38 cells and intestinal epithelial organoids.

RESULTS

Irradiated HSD animals showed increased acute weight loss, reduced weight regeneration, a more severe dysfunction of the intestinal barrier (enhanced FITC-dextran permeability) and increased histopathological damage when compared with NSD animals. Murine intestinal organoids and MC38 cells showed decreased survival after RT in hypernatremic conditions. Interestingly, our preliminary data show that there are no increased rates of apoptotic cells in vitro, suggesting a different mechanism affecting the increased radiosensitivity in hypernatremic conditions.

CONCLUSION

Our data show a NaCl induced intensification of intestinal tissue injury following RT. These findings can potentially pave the way for investigating the effect of high salt diet on intestinal radiotoxicity in clinical trials.

Influence of Bacterial Metabolites on the Systemic Antitumor Immune Response after Radiation Therapy in Combination with Immune Checkpoint Inhibitors

PURPOSE/OBJECTIVE(S)

The gut microbiome influences the pathogenesis of many diseases and numerous medical therapies. In addition, it is well known that cancer patients are often treated with antibiotics during the course of their disease, which can lead to intestinal dysbiosis. Importantly, experimental and clinical studies have already shown that this has implications for tumor therapies and that immune-active bacterial metabolites can play a central mechanistic role. However, little is known about the influence of the bacterial microbiota or its metabolites on the outcome of radiotherapy (RT) in combination with immune checkpoint inhibitors (ICIs).

MATERIALS/METHODS

We used a bilateral tumor model (MC38 tumor cells) to investigate the influence of the intestinal microbiota and selected bacterial metabolites on the abscopal effect after RT in combination with ICIs. In brief, RT (1 × 8 Gy) of the right tumor was performed 7 days after tumor induction, followed by application of α-CTLA4 or α-PD1 for three weeks. Antibiotics (ampicillin, neomycin, metronidazole, and vancomycin) or selected immune-activating metabolites were administered daily by gavage. We used bacterial metabolites that are described in the literature as having strong immune-modulating properties.

RESULTS

In mice, combined treatment with different antibiotics had neither a significant effect on tumor growth of the irradiated tumor nor on tumor progression of the unirradiated tumor after RT in combination with α-CTLA4. In contrast, supplementation of specific bacterial metabolites had different effects depending on the type of ICI and the specific type of bacterial metabolite. Specifically, we identified bacterial metabolites that led to enhanced tumor progression of the unirradiated abscopal tumor after RT combined with ICIs, resulting in reduced survival rates.

CONCLUSION

To the best of our knowledge, we are presenting here for the first-time experimental data on the influence of the intestinal microbiome on the abscopal effect after RT combined with ICIs. Our data clearly show that specific immune-active bacterial metabolites can negatively influence the abscopal effect after combined radioimmunotherapy. This implies the design of further experimental studies aimed at using specific strategies (e.g., antibiotics) to decimate certain gut bacteria and reduce the amount of certain bacterial metabolites to improve response rates after radioimmunotherapy.

Experimental Investigation of Lung Toxicity after Radiation Therapy Combined with Immune Checkpoint Inhibitors

PURPOSE/OBJECTIVE(S)

The combination of radiation therapy (RT) with immune checkpoint inhibitors (ICIs) has resulted in prolonged survival in patients with locally advanced lung cancer (NSCLC). However, RT and ICIs carry the risk of inflammatory and irreversible lung damage and might have synergistic effects on these adverse events. Importantly, only little is known about the risk of enhanced side effects of the lung and which factors modulate such toxicity. In addition, there is a need for sensitive diagnostics for the early detection of lung injury after combined radioimmunotherapy. Recently, experimental x-ray dark-field radiography was able to detect radiation-induced lung injury earlier than conventional radiography in mice.

MATERIALS/METHODS

The right thorax of mice was irradiated with unfractionated RT (1x15 Gy or 1x30 Gy) or fractionated RT (5x9 Gy). In addition, indicated experimental groups received ICIs (a-PD1, a-CTLA4 or combination) for one month. After four months, the mice were analyzed using different methods to quantify lung damage: lung coefficient as surrogate marker for lung fibrosis, histopathological staining of fibrosis, conventional x-ray, and dark-field radiography.

RESULTS

We found enhanced signs of lung fibrosis (lung coefficient and histopathological score) after unfractionated RT in combination with ICIs. Surprisingly, combination of ICIs with RT resulted in opposite effects. Specifically, concomitant combination of ICIs with fractionated RT resulted in reduced lung coefficients and lower histopathological signs of lung fibrosis in mice treated in this way. Importantly, in vivo x-ray dark-field radiographs showed the same trend as the ex vivo assessment of the lungs.

CONCLUSION

To the best of our knowledge, this is the first experimental study to find that the combination of RT with ICIs has an impact on the risk of pulmonary fibrosis. Strikingly, ICIs (a-PD1 + a-CTLA4) combined with unfractionated RT resulted in increased lung damage, while concomitant use of ICIs (a-PD1 + a-CTLA4) with fractionated RT resulted in reduced lung toxicity. In the field of radioimmunotherapy, such studies are of great value for the interpretation of clinical studies and of importance for the design of further clinical trials.