Deletion of the ST2 proximal promoter disrupts fibroblast-specific expression but does not reduce the amount of soluble ST2 in circulation.

IL-33 signals through ST2, which is expressed either as a full-length signaling receptor or a truncated soluble receptor that can suppress IL-33 activity. Previous data suggest that soluble ST2 mRNA in fibroblasts is coupled to a serum-inducible proximal promoter, while full-length ST2 expression in immune cells is directed from a distal promoter. In order to better understand the function of the alternative promoters and how they ultimately affect the regulation of IL-33, we generated a mouse in which the ST2 proximal promoter is deleted. Promoter deletion had no impact on ST2 expression in mast cells or their ability to respond to IL-33. In contrast, it resulted in a complete loss of both soluble and full-length ST2 mRNA in fibroblasts, which corresponded with both an inability to secrete soluble ST2 and a defect in IL-33 responsiveness. Importantly, in spite of the fibroblast defect, soluble ST2 concentrations were not reduced in the serum of naïve or allergen-exposed knockout mice. In summary, we found that ST2 promoter usage is largely cell-type dependent but does not dictate splicing. Moreover, the proximal promoter is not a major driver of circulating soluble ST2 under the conditions tested.

CD34+ hemopoietic progenitor cells are potent effectors of allergic inflammation.

BACKGROUND: In steady state, hemopoietic progenitors constantly egress from the bone marrow (BM) into the blood and circulate through the peripheral tissues. In allergic diseases, the BM releases increased numbers of CD34(+) progenitor cells that migrate to the site of allergic inflammation, where they differentiate into tissue-dwelling and classic effector cells of allergy, such as mast cells, eosinophils, and basophils. OBJECTIVE: To examine whether peripheral blood CD34(+) cells in addition to being progenitors may also directly function as inflammatory effector cells. METHODS: Highly purified neonatal or adult blood CD34(+) cells were examined for the expression of thymic stromal lymphopoietin (TSLP) and IL-33 receptors and for their response to these cytokines as well as to supernatants of primary small airway epithelial cells and nasal explants from rhinosinusitis and control subjects. Sputum of patients with asthma was examined before and after allergen inhalation for the presence of IL-5 and IL-13-containing CD34(+) cells. RESULTS: Circulating CD34(+) cells expressed receptors for TSLP and IL-33 and responded to these cytokines by rapidly releasing high levels of proinflammatory T(H)2-like cytokines and chemokines. These cells were activated in a TSLP-dependent manner by the supernatant fluids from activated primary human small airway epithelial cells and from nasal explants of patients with chronic rhinosinusitis. Moreover, activated CD34(+) cells containing IL-5 and IL-13 could be detected in the sputum of individuals with allergic asthma, with numbers increasing in response to specific allergen inhalation challenge. CONCLUSION: Blood CD34(+) cells, in addition to being progenitors, may act as proinflammatory effector cells by themselves and directly contribute to the allergic inflammation.

Inhibition of interleukin-33 signaling attenuates the severity of experimental arthritis.

OBJECTIVE: Interleukin-33 (IL-33; or, IL-1F11) was recently identified as the ligand of the IL-1 family receptor T1/ST2. The aim of this study was to examine IL-33 production in human and mouse joints and to investigate the role of IL-33 and T1/ST2 in experimental arthritis. METHODS: IL-33 expression was examined in human synovial tissue, rheumatoid arthritis (RA) synovial fibroblasts, and arthritic mouse joints. Mice with collagen-induced arthritis (CIA) were treated with blocking anti-ST2 antibody or control antibody beginning at the onset of disease. Arthritis severity was assessed by clinical and histologic scoring. Draining lymph node (LN) cell responses were examined ex vivo, and joint messenger RNA (mRNA) was used for expression profiling. RESULTS: IL-33 was highly expressed in human RA synovium. In cultured synovial fibroblasts, IL-33 expression was strongly induced by IL-1beta and/or tumor necrosis factor alpha. Furthermore, IL-33 mRNA was detected in the joints of mice with CIA and increased during the early phase of the disease. Administration of a blocking anti-ST2 antibody at the onset of disease attenuated the severity of CIA and reduced joint destruction. Anti-ST2 antibody treatment was associated with a marked decrease in interferon-gamma production as well as with a more limited reduction in IL-17 production by ex vivo-stimulated draining LN cells. Finally, RANKL mRNA levels in the joint were reduced by anti-ST2 treatment. CONCLUSION: IL-33 is produced locally in inflamed joints, and neutralization of IL-33 signaling has a therapeutic effect on the course of arthritis. These observations suggest that locally produced IL-33 may contribute to the pathogenesis of joint inflammation and destruction.

Cutting edge: interleukin 17 signals through a heteromeric receptor complex.

IL-17 is an inflammatory cytokine produced primarily by a unique lineage of CD4 T cells that plays critical roles in the pathogenesis of multiple autoimmune diseases. IL-17RA is a ubiquitously expressed receptor that is essential for IL-17 biologic activity. Despite widespread receptor expression, the activity of IL-17 is most classically defined by its ability to induce the expression of inflammatory cytokines, chemokines, and other mediators by stromal cells. The lack of IL-17 responsiveness in mouse stromal cells genetically deficient in IL-17RA is poorly complemented by human IL-17RA, suggesting the presence of an obligate ancillary component whose activity is species specific. This component is IL-17RC, a distinct member of the IL-17R family. Thus, the biologic activity of IL-17 is dependent on a complex composed of IL-17RA and IL-17RC, suggesting a new paradigm for understanding the interactions between the expanded family of IL-17 ligands and their receptors.