Proton Radiotherapy Offers Immune-Sparing Benefits in Glioblastoma Treatment

Glioblastoma (GB) and x-ray-based radiation therapy are associated with lymphopenia, which worsens prognosis and impairs antitumor immunity. Proton therapy, with its dosimetric properties that spare healthy brain tissue, is an alternative to reduce radiotoxicity. This study examined the effects of brain irradiation with x-rays or protons on lymphopenia.

We used GL261-tumor-bearing and tumor-free mice models to evaluate various lymphoid and myeloid cell populations after brain irradiation, assessing volume of brain irradiation, beam orientation, and lymphopenia onset timing via flow cytometry.

In the GB mouse model, both tumor- and radiation-induced lymphopenia were evidenced. Severe lymphopenia, with a 50% depletion of CD45⁺CD3⁺ lymphocytes, CD3⁺CD4⁺ and CD3⁺CD8⁺ T-cell, CD3^-B220⁺ B cells, and CD45⁺CD11b⁺ myeloids, CD11b⁺Ly6C⁺ monocytes, and CD11b⁺Ly6G⁺ neutrophils, occurred as early as 12 hours after x-ray exposure in tumor-free mice. Myeloid subtypes recovered by day 2, whereas lymphocyte recovery was cell type dependent. The use of a lateral x-ray beam failed to alter circulating lymphocyte counts. Brain, cervical lymph nodes, and spleen immune cell quantification revealed a reduction in CD3⁺CD4⁺, CD3⁺CD8⁺ T and CD3^-B220⁺ B cells post-x-ray exposure. Protons caused a 15% decrease in circulating lymphocytes, with no change in myeloid cells. Proton exposure also reduced CD3⁺CD4⁺ T and CD3^-B220⁺ B cells in the cervical lymph nodes but did not affect the spleen. Plasma cytokine levels in the interleukin 1 and macrophage inflammatory protein families correlated with leucocyte changes following exposure to both particle types.

We describe that proton therapy resulted in less lymphopenia than proton radiation, and propose that the mechanism may be related to biological processes rather than solely decreased exposure of circulating lymphocytes. This may be used to guide the incorporation of immune therapy in GB.

Understanding how brain irradiation affects circulating immunity can inform strategies to improve brain tumor treatment. This involves combining irradiation with immunotherapy and adjusting clinical protocols to restore lymphocytes/boost antitumor T cells.