CD4 and inflammatory bowel disease: IL-9-producing CD4 T helper cells [Th9] were identified as a new subset of CD4 T helper cells with potent pro-inflammatory functions.16–18 In these cells, IL-9 gene expression is regulated by the cytokines TGFβ, IL-4, and IL-33 and transcription factors like PU.1, STAT6, GATA3, and IRF4.19–22 Th9 cells were identified in a variety of different chronic inflammatory and autoimmune diseases.23–25 In IBD, IL-9 was mainly produced by T cells in the lamina propria.