IL-9 is involved in various T cell-dependent inflammatory models including colitis, encepahlitis and asthma. However, the regulation and specificity of IL-9 responsiveness by T cells during immune responses remains poorly understood. Here, we addressed this question using two different models: experimental colitis induced by transfer of naive CD4 CD45RB T cells into immunodeficient mice, and OVA-specific T cell activation. In the colitis model, constitutive IL-9 expression exacerbated inflammation upon transfer of CD4 CD45RB T cells from wild-type but not from Il9r mice, indicating that IL-9 acts directly on T cells. Suprisingly, such naïve CD4 CD45RB T cells failed to express the Il9r or respond to IL-9 in vitro, in contrast with CD4 CD45RB T cells. By using OVA-specific T cells, we observed that T cells acquired the capacity to respond to IL-9 along with CD44 upregulation, after long-lasting (5 to 12 days) in vivo antigenic stimulation. Il9r expression was associated with Th2 and Th17 phenotypes. Interestingly, in contrast to the response IL-2, antigen restimulation downregulated IL-9 responsiveness. Taken together, our results demonstrate that IL-9 does not act on naïve T cells but that IL-9 responsiveness is acquired by CD4 T cells after in vivo activation and acquisition of memory markers such as CD44. This article is protected by copyright. All rights reserved.This article is protected by copyright. All rights reserved.