Inhaled TLR9 Agonist Renders Lung Tumors Permissive to PD-1 Blockade by Promoting Optimal CD4+ and CD8+ T-cell Interplay

Currently approved inhibitors of the PD-1/PD-L1 pathway represent a major advance for the treatment of lung cancers, yet they are ineffective in a majority of patients due to lack of preexisting T-cell reactivity. Here, we show that a TLR9 agonist delivered by inhalation is able to prime T-cell responses against poorly immunogenic lung tumors and to complement the effects of PD-1 blockade. Inhaled TLR9 agonist causes profound remodeling in tumor-bearing lungs, leading to the formation of tertiary lymphoid structures adjacent to the tumors, CD8 + T-cell infiltration into the tumors, dendritic cell expansion, and antibody production. Inhalation of TLR9 agonist also increased the pool of functional PD-1 low T-bet high effector CD8 + T cells in tumor-bearing lungs. Effector CD8 + T cells generated by inhaled TLR9 agonist treatment were licensed by PD-1 blockade to become highly functional CTLs, leading to a durable rejection of both lung tumors and tumor lesions outside the lungs. CD4 + T cells activated in response to inhaled TLR9 play a critical role in this process by controlling the proliferation, preventing exhaustion, and guiding the differentiation of optimally functional CTLs. This study characterizes a strategy to apply localized TLR9 stimulation to a tumor type not accessible for direct injection, a strategy that may expand the therapeutic potential of PD-1 blockade in non–small cell lung cancer. Significance: These findings demonstrate that local delivery of a toll-like receptor 9 agonist can change the immune content of an entire organ and enhance the efficacy of immune checkpoint inhibition. Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/17/4943/F1.large.jpg. Cancer Res; 78(17); 4943–56. ©2018 AACR .