Purpose: The underlying mechanism for radiation as a potentiator of immune checkpoint inhibition (ICI) is unclear. We developed a novel murine model to investigate the effects of post-irradiation intratumoral heterogeneity (ITH) on response to ICI.

Experimental Design: Parental mouse melanoma B16F10 cells were irradiated in vitro (5Gy x 3 fractions), then an a priori determined number of resulting colonies were implanted in C57BL/6J immunocompetent mice creating syngeneic models of unirradiated (parental) and irradiated tumors with low (irradiated-L) and high (irradiated-H) ITH. Mice were treated with placebo, α-PD-L1, α-CTLA-4 or dual ICI. Murine tumors underwent whole exome sequencing (WES). Clinically correlated paired pre- and post-irradiation patient rectal adenocarcinoma samples underwent WES.

Results: Irradiated-L tumors showed increased tumor mutational burden (TMB) and a sustained decrease in ITH. Irradiated-L tumors were predicted to express five neoantigens with high variant allele frequency/clonal distribution. Mice with irradiated-L and irradiated-H versus parental B16F10 tumors demonstrated longer overall survival with dual ICI. Only mice with irradiated-L tumors experienced an overall survival benefit with single agent ICI. Clinically correlated rectal adenocarcinoma samples showed similarly increased TMB and decreased ITH following irradiation.

Conclusions: Post-irradiation ITH modulates ICI response in a murine melanoma model. Irradiation may offer a mechanism to widen the therapeutic window of ICI.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9800194PMC
http://dx.doi.org/10.1016/j.neo.2022.100864DOI Listing

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