Interleukin (IL)-33 and its receptor, ST2, centrally control a remarkable variety of inflammatory and fibrotic diseases. The IL-33 – ST2 axis relies mostly on the proteolytically mature form of IL-33 cytokine (MIL33), with less attention usually devoted to the full-length precursor (FLIL33), which is a nuclear factor. The MIL33 – ST2 axis serves diverse functions, among which one of the most prominent is induction of the Th2 phenotype, with an overt elevation of IL-4, IL-5, and IL-13, leading to eosinophilia, mucus production, and clinical manifestation of allergy. It remains unclear how the MIL33 – ST2 pathway, while certainly contributing to inflammation and fibrosis, mediates its proinflammatory and profibrotic effects without always eliciting the Th2 phenotype. Our preliminary data strongly suggest that IL-33, particularly in its FLIL33 form, may promote inflammation and fibrosis in an ST2-independent fashion, and reciprocally, ST2 may control inflammation and fibrosis without requiring IL-33. Thus, we proposed, for the first time, that IL-33 and ST2 may contribute to inflammation and fibrosis in an “off-axis” fashion. FLIL33 behaves distinctly from MIL33. Some earlier studies suggested that as a basally and inducibly expressed nuclear factor, FLIL33 modulates inflammatory responses, wound healing, chromatin stability, and transcriptional regulation in a non-Th2 and receptor-independent manner. Yet, the pathophysiology of the understudied FLIL33 requires more attention in general and is nearly completely unknown in fibrosis specifically. Our published and preliminary data suggest that: FLIL33 but not MIL33 potentiates fibrosis without inducing the Th2 phenotype; such distinct effect is partially mediated by ST2 and also occurs in the absence of ST2; FLIL33 acts so at the levels of both transcriptional and post-transcriptional regulation; ST2 by itself controls fibrosis; and such regulation involves IL-9. Based on these findings, the Hypothesis of this study is that in addition to the commonly acknowledged role of the MIL33 – ST2 axis, FLIL33 contributes to fibrosis both with and without engaging ST2, through both transcriptional and post-transcriptional regulation, and that ST2 itself may regulate fibrosis through a mechanism that in part involves IL-9. To challenge this hypothesis, the following Specific Aims will be pursued. Aim 1 is to define ST2-independent, Th2-independent, post-transcriptional, mechanisms through which full- length IL-33 potentiates fibrosis. Aim 2 is to define ST2-dependent, Th2-independent, transcriptional, mechanisms through which full-length IL-33 potentiates fibrosis. Aim 3 is to determine the role of IL-9 in controlling fibrosis in an ST2-dependent fashion and identify the mechanisms through which both full-length and mature IL-33 forms attenuate IL-9. A successful completion of these studies will form the basis for better therapeutic targeting of IL-33 and ST2 is inflammatory and fibrotic diseases. Inflammation a...