PD-1 or CTLA-4 blockade promotes CD86-driven Treg responses upon radiotherapy of lymphocyte-depleted cancer in mice

Radiotherapy (RT) is considered immunogenic, but clinical data demonstrating RT-induced T-cell priming are scarce. Here, we show in a mouse tumor model representative of human lymphocyte-depleted cancer that RT enhances spontaneous priming of thymus-derived (FOXP3+ Helios+) regulatory T-cells (Tregs) by the tumor. These Tregs acquire an effector phenotype, populate the tumor and impede tumor control by a simultaneous, RT-induced CD8+ cytotoxic T-cell (CTL) response. Combination of RT with CTLA-4 or PD-1 blockade, which enables CD28 costimulation, further increased this Treg response and failed to improve tumor control. We discovered that upon RT, CD28-ligands CD86 and CD80 differentially affected the Treg response. CD86, but not CD80, blockade prevented the effector (e)Treg response, enriched the tumor-draining lymph node for PD-L1+CD80+ migratory, conventional dendritic cells (cDCs) and promoted CTL priming. Blockade of CD86 alone or in combination with PD-1, enhanced intra-tumoral CTL accumulation and the combination significantly increased RT-induced tumor regression and overall survival. We advise that combining RT with PD-1 and/or CTLA-4 blockade may be counterproductive in lymphocyte-depleted cancers, since they drive Treg responses in this context. However, combining RT with CD86 blockade may promote control of such tumors by enabling a CTL response.