Ariel E. Marciscano, Ali Ghasemzadeh, Thomas R. Nirschl, Debebe Theodros, Christina M. Kochel, Brian J. Francica, Yuki Muroyama, Robert A. Anders, Andrew B. Sharabi, Esteban Velarde, Wendy Mao, Kunal R. Chaudhary, Matthew G. Chaimowitz, John Wong, Mark J. Selby, Kent B. Thudium, Alan J. Korman, David Ulmert, Daniel L.J. Thorek, Theodore L. DeWeese, Charles G. Drake

In the proper context, radiation therapy (RT) can promote anti-tumor immunity. It is unknown if elective nodal irradiation (ENI), a strategy that irradiates tumor-associated draining lymph nodes (DLN), impacts adaptive immune responses and combinatorial efficacy of RT with immune checkpoint blockade (ICB). We developed a preclinical model to compare stereotactic RT (Tumor RT) with or without ENI to examine immunological differences between RT techniques that spare or irradiate the DLN. Tumor RT was associated with up-regulation of an intratumoral T-cell chemoattractant chemokine signature (CXCR3, CCR5-related) that resulted in robust infiltration of antigen-specific CD8+ effector T-cells as well as FoxP3+ regulatory T-cells (Tregs). The addition of ENI attenuated chemokine expression, restrained immune infiltration and adversely impacted survival when combined with ICB, especially with anti-CLTA4 therapy. The combination of stereotactic RT and ICB led to long-term survival in a subset of mice and was associated with favorable CD8 effector-to-Treg ratios and increased intratumoral density of antigen-specific CD8+ T-cells. While RT technique (Tumor RT vs. ENI) impacted initial tumor control and survival, the ability to reject tumor upon re-challenge was partially dependent upon the mechanism of action of ICB; as RT/anti-CTLA4 was superior to RT/anti-PD-1. Our results highlight that irradiation of the DLN restrains adaptive immune responses through altered chemokine expression and CD8+ T-cell trafficking. These data have implications for combining RT and ICB, long-term survival and induction of immunological memory. Clinically, the immunomodulatory effect of the RT strategy should be considered when combining stereotactic RT with immunotherapy.