Proton Radiotherapy Offers Immune-Sparing Benefits in Glioblastoma Treatment

Purpose Glioblastoma (GB) and x-ray-based radiotherapy are associated with lymphopenia, which worsens prognosis and impairs anti-tumor 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. Methods and Materials 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. Results 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, while lymphocyte recovery was cell-type dependent. The use of a lateral x-ray beam failed to alter circulating lymphocyte counts. Brain, cervical lymph nodes (CLN) 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 CLNs but did not affect the spleen. Plasma cytokine levels in the IL-1 and MIP families correlated with leucocyte changes following exposure to both particle types. Conclusions 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 incorporation of immune therapy in GB.