IL-9 stimulates an anti-tumor immune response and facilitates immune checkpoint blockade in the CMT167 mouse model

Abstract There is mounting evidence that interleukin-9 (IL-9) is associated with cancer although its function in lung cancer remains elusive. This study aimed to elucidate the role of IL-9 in lung cancer and the mechanisms involved. We report the detection of interleukin-9 receptor (IL-9R) expression in CMT167 cells, but not in Lewis lung carcinoma (LLC) cells. LLC or CMT167 cells were inoculated in wildtype C57BL/6J immunocompetent mice. Tumor-bearing mice were randomized on day 3 post-inoculation (8 mice per group) to receive intraperitoneal treatment with murine recombinant IL-9 (50 ng/mouse on alternate days) ± anti-programmed cell death protein 1 (anti-PD-1, 10 mg/kg, every three days) or control until reaching humane endpoints whereby tumors were harvested. In LLC tumor-bearing mice, neither tumor growth nor intratumoral T cells were affected by IL-9 treatment. Nonetheless IL-9 decreased CMT167 tumor growth and enhanced anti-tumor T cell responses, both of which were absent in IL-9R knockdown CMT167 tumors. CD8+ T cells were identified as the key effector driving IL-9-induced tumor suppression in the CMT167 model. Increased dendritic cell population and MHC-I expression were observed in IL-9-treated CMT167 tumors. Simultaneously, PD-1 and programmed death ligand 1 (PD-L1) expression by CD8+ T cells and CMT167 cells was upregulated. The combination of IL-9 and anti-PD-1 antibody synergistically suppressed CMT167 tumor and enhanced tumor-infiltrating proportion of CD8+ T cells. Taken together, our study determined the role of IL-9 in anti-tumor immunity and proposes IL-9 as a promising adjuvant to immune checkpoint blockade in lung cancer.