To uncover immune mechanisms driving abscopal tumor responses, Wei et al. studied the impact of timing of PD-1 blockade relative to local radiotherapy on primary (irradiated) and secondary (non-irradiated, distal) tumor growth. Administering anti-PD-1 before local tumor irradiation greatly diminished abscopal immune responses and resulted in increased radiosensitivity and death of CD8+ T cells. When given after local tumor irradiation, anti-PD-1 increased polyfunctional CD8+ effector T cells and decreased intratumoral dysfunctional CD8+ T cells in both primary and secondary tumors and resulted in potent abscopal responses.
Contributed by Katherine Turner
ABSTRACT: Although radiotherapy has been used for over a century to locally control tumor growth, alone it rarely induces an abscopal response or systemic antitumor immunity capable of inhibiting distal tumors outside of the irradiation field. Results from recent studies suggest that combining immune checkpoint blockades to radiotherapy may enhance abscopal activity. However, the treatment conditions and underlying immune mechanisms that consistently drive an abscopal response during radiation therapy combinations remain unknown. Here, we analyzed the antitumor responses at primary and distal tumor sites, demonstrating that the timing of αPD-1 antibody administration relative to radiotherapy determined the potency of the induced abscopal response. Blockade of the PD-1 pathway after local tumor irradiation resulted in the expansion of polyfunctional intratumoral CD8+ T cells, a decrease in intratumoral dysfunctional CD8+ T cells, expansion of reprogrammable CD8+ T cells, and induction of potent abscopal responses. However, administration of αPD-1 before irradiation almost completely abrogated systemic immunity, which associated with increased radiosensitivity and death of CD8+ T cells. The subsequent reduction of polyfunctional effector CD8+ T cells at the irradiated tumor site generated a suboptimal systemic antitumor response and the loss of abscopal responses. Therefore, this report maximizes the potential synergy between radiotherapy and αPD-1 immunotherapy, information that will benefit clinical combinations of radiotherapy and immune checkpoint blockade.