Expression Efficiency of Multiple Il9 Reporter Alleles Is Determined by Cell Lineage

Activin A Promotes Differentiation of a Pathogenic Multicytokine IL-9-secreting CD4+ T Cell Population

The development of Th subsets results from cellular and cytokine cues that are present in the inflammatory environment. The developing T cell integrates multiple signals from the environment that sculpt the cytokine-producing capacity of the effector T cell. Importantly, T cells can discriminate similar cytokine signals to generate distinct outcomes, and that discrimination is critical in Th subset development. IL-9-secreting Th9 cells regulate multiple immune responses, including immunity to pathogens and tumors, allergic inflammation, and autoimmunity. In combination with IL-4, TGF-β or activin A promotes IL-9 production; yet, it is not clear if both TGF-β family members generate Th9 cells with identical phenotype and function. We observed that in contrast to TGF-β that efficiently represses Th2 cytokines in murine Th9 cultures, differentiation with activin A produced a multicytokine T cell phenotype with secretion of IL-4, IL-5, IL-13, and IL-10 in addition to IL-9. Moreover, multicytokine secreting cells are more effective at promoting allergic inflammation. These observations suggest that although TGF-β and IL-4 were identified as cytokines that stimulate optimal IL-9 production, they might not be the only cytokines that generate optimal function from IL-9-producing T cells in immunity and disease.

Allergic airway recall responses require IL-9 from resident memory CD4+ T cells

Asthma is a chronic inflammatory lung disease with intermittent flares predominately mediated through memory T cells. Yet, the identity of long-term memory cells that mediate allergic recall responses is not well defined. In this report, using a mouse model of chronic allergen exposure followed by an allergen-free rest period, we characterized a subpopulation of CD4+ T cells that secreted IL-9 as an obligate effector cytokine. IL-9-secreting cells had a resident memory T cell phenotype, and blocking IL-9 during a recall challenge or deleting IL-9 from T cells significantly diminished airway inflammation and airway hyperreactivity. T cells secreted IL-9 in an allergen recall-specific manner, and secretion was amplified by IL-33. Using scRNA-seq and scATAC-seq, we defined the cellular identity of a distinct population of T cells with a proallergic cytokine pattern. Thus, in a recall model of allergic airway inflammation, IL-9 secretion from a multicytokine-producing CD4+ T cell population was required for an allergen recall response.

IL-33, an Alarmin of the IL-1 Family Involved in Allergic and Non Allergic Inflammation: Focus on the Mechanisms of Regulation of Its Activity

Interleukin-33 (IL-33) is a member of the interleukin-1 (IL-1) family that is expressed in the nuclei of endothelial and epithelial cells of barrier tissues, among others. It functions as an alarm signal that is released upon tissue or cellular injury. IL-33 plays a central role in the initiation and amplification of type 2 innate immune responses and allergic inflammation by activating various target cells expressing its ST2 receptor, including mast cells and type 2 innate lymphoid cells. Depending on the tissue environment, IL-33 plays a wide variety of roles in parasitic and viral host defense, tissue repair and homeostasis. IL-33 has evolved a variety of sophisticated regulatory mechanisms to control its activity, including nuclear sequestration and proteolytic processing. It is involved in many diseases, including allergic, inflammatory and infectious diseases, and is a promising therapeutic target for the treatment of severe asthma. In this review, I will summarize the literature around this fascinating pleiotropic cytokine. In the first part, I will describe the basics of IL-33, from the discovery of interleukin-33 to its function, including its expression, release and signaling pathway. The second part will be devoted to the regulation of IL-33 protein leading to its activation or inactivation.