Interleukin 9 (IL-9), the signature Th9 cytokine, is associated with T regulatory cell function, parasite expulsion and allergic airway inflammation. Because IL-9 can promote a tolerogenic/immunosuppressive environment, we used IL-9 knock-out (IL-9ko) mice to estimate the role of endogenous IL-9 in the biology of two aggressive breast cancers, TUBO and 4T1, and CT26 a colon carcinoma. Merely eliminating endogenous IL-9 enabled efficient sensitization of host T cells leading to early rejection of growing tumors. Duplicate tumor challenges simply grew progressively in wildtype mice. Interestingly, tumor rejection occurred without additional vaccines or immunomodulatory therapies. The IL-9ko mice acquired immunologic memory, which actively protected from residual disease and tumor rechallenges. The ability of a normally lethal tumor challenge to stimulate an enduring therapeutic response in IL-9ko mice was linked to the activation of CD8+ T cells, as IL-9ko mice failed to be cured if either CD8+ or CD4+ T cells were depleted. Importantly, T cells from tumor-rejecting IL-9ko mice retained effector competence in wildtype mice, and treatments with neutralizing anti-IL-9 significantly slowed tumor progression in wildtype mice. IL-9 inhibits the immune system's ability to prime effective immunological memory after exposure to growing tumor; it therefore functions as a checkpoint inhibitor of adaptive immunity. Optimization of IL-9 neutralization may play a unique role in immunotherapy.

Citation Format: Dominique B. Hoelzinger, Ana Lucia Dominguez, Peter A. Cohen, Sandra J. Gendler. Checkpoint inhibition of adaptive immunity by IL-9 can be disrupted to achieve rapid T cell sensitization and rejection of progressive tumor challenges. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr A44.