Fractionated low-dose radiotherapy primes the tumor microenvironment for immunotherapy in a murine mesothelioma model

Our plain-English summary of Fractionated low-dose radiotherapy primes the tumor microenvironment for immunotherapy in a murine mesothelioma model, by R.A. D'Alonzo, S. Keam, S. Gill, et al (Cancer Immunology, Immunotherapy, 2025).

The study

• Authors: R.A. D'Alonzo, S. Keam, S. Gill, P. Rowshanfarzad, A.K. Nowak, M.A. Ebert, A.M. Cook
• Journal: Cancer Immunology, Immunotherapy
• Published: 2025;74(2):44
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Combination immune checkpoint inhibitors (nivolumab and ipilimumab) are currently a first-line treatment for mesothelioma; however, not all patients respond. The efficacy of treatment is influenced by the tumor microenvironment. Murine mesothelioma tumors were irritated with various radiotherapy doses. Radiotherapy induced vasculature changes were monitored by power Doppler and photoacoustic ultrasound and analyzed via mixed-effects models. Tissue staining was used to investigate the immune cell infiltrate of tumors. The optimal radiotherapy schedule was combined with immune checkpoint inhibitors, and the survival of mice was analyzed. Using low-dose, low-fraction radiotherapy allowed favorable modification of the murine mesothelioma tumor microenvironment. Irradiating tumors with 2 Gy × 5 fractions significantly improved blood flow and reduced hypoxia, consequently increasing the presence of CD8+ and regulatory T cells in the tumor. Understanding the transient nature of these changes is crucial for optimizing the timing of therapeutic delivery. The combination of radiotherapy with dual immunotherapy (anti-PD-1 plus anti-CTLA-4) proved highly curative when administered concurrently. A diminishing rate of cures was noted with an increasing delay between radiotherapy and subsequent immunotherapy. Concurrent low-dose, low-fraction radiotherapy emerges as a translatable approach for improving the efficacy of immune checkpoint inhibitors in patients.

© 2024. The Author(s).

In plain English

This was a laboratory experiment in mice, not a study of people or of asbestos exposure. Female mice were injected under the flank skin with AB1-HA mouse mesothelioma cells. The parent AB1 line was made by injecting crocidolite asbestos into mice, then modified with an influenza protein to create AB1-HA. The authors compared several radiotherapy schedules, then combined their chosen one (2 Gy daily for 5 days) with immune checkpoint drugs (anti-PD-1 and anti-CTLA-4). Tumors given 2 Gy × 5 had on average 10.5 times more blood flow than unirradiated tumors. They also had less low-oxygen (hypoxic) tissue, but by 8 days after radiotherapy hypoxia was back to unirradiated levels. Irradiated tumors held more CD8+ T cells and regulatory T cells. Complete responses occurred in 100% of mice given both drugs alongside radiotherapy, 87.5% when the drugs began one day after, and 43% with the drugs alone. Only one mouse responded completely when the drugs began 7 days after radiotherapy. The authors concluded that this schedule can create a tumor environment more conducive to the success of these drugs. Investigators were not blinded.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment (laboratory study in mice with tumors grown under the skin)
• Who was studied: Female BALB/cJAusBP mice aged 6 to 8 weeks, in Perth, Australia, injected under the skin of the right flank with AB1-HA mouse mesothelioma cells. The parent AB1 cell lines were originally generated by injecting crocidolite asbestos into the abdominal cavity of BALB/c mice. They were then transfected with influenza hemagglutinin to make the AB1-HA line. Groups were mostly 4 mice for the imaging and tissue studies, and at least 7 mice per group for the combination treatment experiments.
• Exposure: Radiotherapy to the tumor in 2 Gy fractions of varying number, compared with 6 Gy fractions and with sham irradiation. The chosen schedule, 2 Gy on 5 consecutive days, was then given with immune checkpoint antibodies (anti-PD-1 with or without anti-CTLA-4). The antibodies were started at the same time as radiotherapy, 1 day after, or 7 days after. The asbestos fiber (crocidolite) relates only to how the parent cell line was originally created.
• Main results:
  • Compared with unirradiated control tumors, tumors given 2 Gy × 5 had a mean blood flow 10.5 times higher, and tumors given 2 Gy × 4 had a mean blood flow 7.5 times higher.
  • Oxygen saturation of the tumor tissue was 4 times that of unirradiated tumors after 2 Gy × 5, and 3.5 times after 6 Gy × 2.
  • After 2 Gy × 5, hypoxic areas were smaller than in sham-irradiated tumors at 0.1, 1 and 4 days after irradiation. By day 8 they had returned to unirradiated levels, which the authors described as a transient effect.
  • CD8+ T cell numbers were significantly higher in irradiated tumors than in sham-irradiated tumors from 1 to 4 days after irradiation. Regulatory T cells were also more numerous. The ratio of CD8+ T cells to regulatory T cells did not differ significantly between groups.
  • Tumors collected 1 day after irradiation showed the most differentially expressed genes: 6378 in total, with 3532 upregulated and 2827 downregulated, as printed.
  • With both checkpoint drugs, complete responses occurred in 100% of mice when the drugs were given at the same time as radiotherapy and in 87.5% when they started 1 day after the final fraction. The rate was 43% in mice given the drugs without radiotherapy.
  • Only one irradiated mouse had a complete response when both drugs were started 7 days after irradiation.
  • Median survival rose in all groups given radiotherapy plus both drugs, compared with the drugs alone. The increase was statistically significant only when the drugs were given at the same time as radiotherapy (p = 0.0217).
  • With radiotherapy plus anti-PD-1 alone, 55% of mice treated at the same time as radiotherapy had a complete response. Cure rates fell the later after radiotherapy anti-PD-1 began.
  • Cured mice that were injected again with tumor cells on the opposite flank did not develop second tumors. One primary tumor later relapsed in one group.
• The authors' conclusion: The authors concluded that radiotherapy given as 2 Gy × 5 fractions can create a tumor environment more conducive to the success of immune checkpoint inhibitors in this mouse model. They wrote that the combination should soon be possible to assess in patients.
• Limitations the authors state:
  • Mice were randomized, but the investigators were not blinded to which group each mouse was in.
  • Some imaging time points have fewer than 4 data points because of equipment error.
  • Mice whose tumors were too small at the start were excluded, which reduced some combination-treatment groups to as few as seven mice.
  • The authors stated that these experiments could not conclusively determine why some unirradiated tumors responded to the drugs alone.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Tumor Microenvironment, Immunotherapy, Mesothelioma, checkpoint inhibitors, Nivolumab (Opdivo), Ipilimumab (Yervoy), Ultrasound, Hypoxia.

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