PURPOSE: How best to sequence and integrate immunotherapy into standard of care is currently unknown. Clinical protocols with accelerated non-ablative hypofractionated radiation followed by surgery could provide an opportunity to implement immune checkpoint blockade.

Experimental Design: We therefore assessed the impact of non-ablative hypofractionated radiation on the immune system in combination with surgery in a mouse mesothelioma model. Blunt surgery (R1 resection) was used to analyze the short term impact and radical surgery (R0 resection) was used to analyze the long-term impact of this radiation protocol before surgery.

Results: Non-ablative hypofractionated radiation led to a specific immune activation against the tumor associated with significant upregulation of CD8+ T cells limiting the negative impact of an incomplete resection. The same radiation protocol performed 7 days before radical surgery led to a long term antitumor immune protection that was primarily driven by CD4+ T cells. Radical surgery alone or radical surgery with a short course of non-ablative radiation completed 24 hours before radical surgery did not provide this vaccination effect. Combining this radiation protocol with CTLA-4 blockade provided better results than radiation alone. The effect of PD-1 or PD-L1 blockade with this radiation protocol was less effective than the combination with CTLA-4 blockade.

Conclusions: A specific activation of the immune system against the tumor contributes to the benefit of accelerated, hypofractionated radiation before surgery. Non-ablative hypofractionated radiation combined with surgery provides an opportunity to introduce immune checkpoint blockades in the clinical setting.