Early effects of different brain radiotherapy modalities on circulating leucocyte subpopulations in rodents

Abstract

Purposes

Lymphopenia is extensively studied, but not circulating leucocyte subpopulations, which however have distinct roles in tumor tolerance. Proton therapy has been shown to have a lesser impact on the immune system than conventional X-ray radiotherapy through lower dose exposure to healthy tissues. We explored the differential effects of brain X-ray and proton irradiation on circulating leucocyte subpopulations.

Materials and methods

Leucocyte subpopulation counts from tumor-free mice were obtained 12 hours after 4 fractions of 2.5 Gy. The relationships between irradiation type (X-rays or protons), irradiated volume (whole-brain/hemi-brain) and dose rate (1 or 2 Gy/min) with circulating leucocyte subpopulations (T-CD4+, T-CD8+, B, and NK-cells, neutrophils, and monocytes) were investigated using linear regression and tree-based modeling approaches. Relationships between dose maps (brain, vessels, lymph nodes (LNs)) and leucocyte subpopulations were analyzed and applied to construct the blood dose model, assessing the hypothesis of a direct lymphocyte-killing effect in radiation-induced lymphopenia.

Results

Radiation-induced lymphopenia occurred after X-ray but not proton brain irradiation in lymphoid subpopulations (T-CD4+, T-CD8+, B, and NK-cells). There was an increase in neutrophil counts following protons but not X-rays. Monocytes remained unchanged under both X-rays and protons. Besides irradiation type, irradiated volume and dose rate had a significant impact on NK-cell, neutrophil and monocyte levels but not T-CD4+, T-CD8+, and B-cells. The dose to the blood had a heterogeneous impact on leucocyte subpopulations: neutrophil counts remained stable with increasing dose to the blood, while lymphocyte counts decreased with increasing dose (T-CD8+-cells > T-CD4+-cells > B-cells > NK-cells). Direct cell-killing effect of the dose to the blood mildly contributed to radiation-induced lymphopenia. LN exposure significantly contributed to lymphopenia and partially explained the distinct impact of irradiation type on circulating lymphocytes.

Conclusions

Leucocyte subpopulations reacted differently to X-ray or proton brain irradiation. This difference could be partly explained by LN exposure to radiation dose. Further researches and analyses on other biological processes and interactions between leucocyte subpopulations are ongoing. The various mechanisms underlying leucocyte subpopulation changes under different irradiation modalities may have implications for the choice of radiotherapy modalities and their combination with immunotherapy in brain cancer treatment.

Keywords:

• Radiotherapy
• proton therapy
• immune system
• modeling
• lymphopenia

Acknowledgement

We would like to thank Alison Johnson for her time reading and editing the language of this manuscript.

Disclosure statement

The authors report there are no competing interests to declare

Biographical note

Thao-Nguyen Pham, MSc, Pharm is a pharmacist and PhD student in cancer biology and biomathematics. Julie Coupey, MSc is a PhD student in cancer biology. Jérôme Toutain, MSc is a specialist in radiation experiment in animals. Serge M. Candéias, PhD is a specialist in immunology. Gaël Simonin, MSc is a PhD in physics and engages in Monte Carlo simulation. Marc Rousseau, PhD is a specialist in radiotherapy and proton therapy. Omar Touzani, PhD is a professor in physiology and cancer biology. Juliette Thariat, MD, PhD is an oncologist and professor in radiotherapy. Samuel Valable, PhD is a specialist in biophysics and radiotherapy

Data availability statement

All other data will be made available upon reasonable request made to the corresponding author.

Additional information

Funding

This project has received financial support from the CNRS through the 80|Prime program, Ligue Contre le Cancer, and French National Agency for Research ‘Investissements d’Avenir’ (n°ANR-10-EQPX1401). SC work is funded in part by the LABEX PRIMES (grant number ANR-11-LABX-0063).