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IL-9 contributes to lung inflammatory processes such as asthma, by promoting mast cell differentiation, B cell activation, eosinophilia, and mucus production by lung epithelial cells. The observation that IL-9 overexpressing mice show increased mast cell numbers in the intestinal mucosa suggests that this cytokine might also play a role in intestinal inflammation. In colons from IL-9 transgenic mice, the expression of Muc2, a major intestinal mucin gene, was up-regulated, together with that of CLCA3 chloride channel and resistin like alpha, which are goblet cell-associated genes. Additional IL-9 up-regulated genes were identified and included innate immunity genes such as angiogenin 4 and the PLA2g2a phospholipase A(2), which are typical Paneth cell markers. Histochemical staining of Paneth cells by phloxine/tartrazine showed that IL-9 induces Paneth cell hyperplasia in Lieberkühn glands of the small intestine, and in the colonic mucosa, where this cell type is normally absent. Expression of Paneth cell markers, including angiogenin 4, PLA2g2a, and cryptdins, was induced in the colon of wild-type mice after two to four daily administrations of IL-9. By crossing IL-9 transgenic mice with IL-13(-/-) mice, or by injecting IL-9 into IL-4R(-/-) mice, we showed that IL-13 was required for the up-regulation of these Paneth cell-specific genes by IL-9. Taken together, our data indicate that Paneth cell hyperplasia and expression of their various antimicrobial products contribute to the immune response driven by TH2 cytokines, such as IL-9 and IL-13 in the intestinal mucosa.

Increased IL-9 expression, either systemically or under the control of lung-specific promoter, induces an asthma-like phenotype, including mucus overproduction, mastocytosis, lung eosinophilia, and airway hyperresponsiveness. These activities correlate with increased production of other Th2 cytokines such as IL-4, IL-5, and IL-13 in IL-9 Tg mice. To determine the exact role of IL-13 in this phenotype, mice overexpressing IL-9 were crossed with IL-13-deficient mice. In these animals, IL-9 could still induce mastocytosis and B lymphocyte infiltration of the lungs. Although IL-9-induced eosinophilia in the peritoneal cavity was not diminished in the absence of IL-13, IL-13 was required for IL-9 to increase eotaxin expression and lung eosinophilia. Mucus production and up-regulation of lung epithelial genes upon IL-9 overexpression were completely abolished in the absence of IL-13. Using hemopoietic cell transfer experiments with recipients that overexpressed IL-9 but were deficient in the IL-9 receptor (IL-9R), we could demonstrate that the effect of IL-9 on lung epithelial cells is indirect and could be fully restored by transfer of hemopoietic cells expressing IL-9R. Mucus production by lung epithelial cells was only up-regulated when hemopoietic cells simultaneously expressed functional IL-9R and IL-13 genes, indicating that IL-13 is not a cofactor but a direct mediator of the effect of IL-9 on lung epithelial cells. Taken together, these data indicate that IL-9 can promote asthma through IL-13-independent pathways via expansion of mast cells, eosinophils, and B cells, and through induction of IL-13 production by hemopoietic cells for mucus production and recruitment of eosinophils by lung epithelial cells.

Anaphylaxis represents an extreme form of allergic reaction, consisting of a sensitization phase during which allergen-specific IgE are produced and an acute effector phase triggered by allergen-induced degranulation of mast cells. We studied the role of IL-9, a Th2 cytokine implicated in asthma, in different models of murine anaphylaxis. Using a passive model of systemic anaphylaxis, in which anti-DNP IgE Abs were administered before challenge with DNP-BSA, we found that IL-9-transgenic mice or wild-type mice treated with IL-9 for 5 days were highly sensitive to fatal anaphylaxis. This effect was reproduced in both anaphylaxis-susceptible and -resistant backgrounds (FVB/N or [FVB/N x BALB/c] F(1) mice, respectively) and correlated with increased serum concentrations of mouse mast cell protease-1 level, a protein released upon mast cells degranulation. By contrast, IL-9 did not increase the susceptibility to passive cutaneous anaphylaxis. IL-9 expression also increased the susceptibility to fatal anaphylaxis when mice were sensitized by immunization against OVA before challenge with the same Ag. In this model, serum from sensitized, IL-9-transgenic mice was more potent in transferring susceptibility to OVA challenge into naive mice, indicating that IL-9 also promotes the sensitization stage. Finally, using IL-9R-deficient mice, we found that despite its anaphylaxis-promoting activity, IL-9 is dispensable for development of both passive and active anaphylaxis, at least in the C57BL/6 mouse background. Taken together, the data reported in this study indicate that IL-9 promotes systemic anaphylaxis reactions, acting at both the sensitization and effector stages, but is not absolutely required for this process.

Mice expressing the X-linked immunodeficiency (xid) mutation lack functional Bruton's tyrosine kinase and were shown to be specifically deficient in peritoneal B-1 lymphocytes. We have previously shown that IL-9, a cytokine produced by TH2 lymphocytes, promotes B-1 cell expansion in vivo. To determine whether IL-9 overexpression might compensate the xid mutation for B-1 lymphocyte development, we crossed xid mice with IL-9-transgenic mice. In this model, IL-9 restored normal numbers of mature peritoneal B-1 cells that all belonged to the CD5(-) B-1b subset. Despite this normal B-1 lymphocyte number, IL-9 failed to restore classical functions of B-1 cells, namely, the production of natural IgM Abs, the T15 Id Ab response to phosphorylcholine immunization, and the antipolysaccharide humoral response against Streptococcus pneumoniae. By using bromelain-treated RBC, we showed that the antigenic repertoire of these IL-9-induced B-1b lymphocytes was different from the repertoire of classical CD5(+) B-1a cells, indicating that the lack of B-1 function by B-1b cells is associated with distinct Ag specificities. Taken together, our data show that B-1b cell development can restore the peritoneal B-1 population in xid mice but that these B-1b cells are functionally distinct from CD5(+) B-1a lymphocytes.

IL-9 is a Th2 cytokine active on various cell types such as T and B lymphocytes, mast cells, and eosinophils, and potentially involved in allergy and asthma. To understand better the molecular mechanisms underlying the activity of this cytokine, we used a cDNA subtraction method to identify genes specifically induced by IL-9 in mouse T cells. One of the IL-9-regulated genes isolated by this approach turned out to encode a 180-amino acid long protein, including a potential signal peptide, and showing 22% amino acid identity with IL-10. This protein, designated IL-10-related T cell-derived inducible factor (IL-TIF), is induced by IL-9 in thymic lymphomas, T cells, and mast cells, and by lectins in freshly isolated splenocytes. Experiments concerning the mechanism regulating IL-TIF expression in T cells indicate that IL-9 induction is rapid (within 1 h), does not require protein synthesis, and depends on the activation of the Janus kinase (JAK)-STAT pathway. In vivo, constitutive expression of IL-TIF was detected by RT-PCR in thymus and brain, suggesting that the role of this new factor is not restricted to the immune system. Transfection of HEK293 cells with the IL-TIF cDNA resulted in the production of a glycosylated protein of about 25 kDa that was found to induce STAT activation in mesangial and neuronal cell lines. Further studies will have to address the possibility that some of the IL-9 activities may be mediated by IL-TIF.

IL-9 transgenic mice were analyzed for the presence of mast cells in different tissues. In these mice, increased mast cell infiltration was found in the gastric and intestinal epithelium as well as in the upper airways and kidney epithelium, but not in other organs, such as skin. IL-9 transgenic mast cells do not show signs of massive degranulation such as that found in IL-4 transgenic mice and are not involved in spontaneous pathologic changes. Gastric mast cells showed a phenotype related to connective-type mast cells, since they were stained by safranin, and strong expression of mouse mast cell protease-4 and -5 was found in this organ. However, they also expressed proteases related to the mucosal cell type, such as mouse mast cell protease-1 and -2. In vitro, although IL-9 by itself did not induce mast cell development from bone marrow progenitors, it strongly synergized with stem cell factor for the growth and differentiation of mast cells expressing the same protease pattern as that observed in IL-9 transgenic mice. Since constitutive stem cell factor expression was observed in vivo, and anti-c-Kit Abs inhibited IL-9 transgenic mastocytosis in the gut, this synergistic combination of factors is likely to be responsible for the mastocytosis observed in IL-9 transgenic mice. Taken together, these data demonstrate that IL-9 induces the in vivo amplification of a nonclassical mast cell subset with a mucosal localization but expressing proteases characteristic of both connective tissue-type and mucosal mast cells.

Mouse helper T cell lines were developed that proliferate permanently without Ag and APCs in response to either IL-2, IL-4, or IL-9, three cytokines whose receptors interact with the IL-2R gamma-chain for signal transduction. Depending on the growth factor, a marked difference was observed regarding the ability of dexamethasone (DEX) to inhibit cell proliferation. In three different cell lines, proliferation induced by IL-2 was completely arrested, while that supported by IL-9 was hardly affected. With IL-4, proliferation was also maintained but less markedly than with IL-9. Although DEX was able to induce apoptosis in these cells, the inhibition of IL-2-induced proliferation was not the result of apoptosis, as this process was equally antagonized by all three factors. Moreover, addition of IL-4 or IL-9 to cultures previously incubated with IL-2 and DEX for several days restored cell proliferation. Finally, autonomous cell variants derived from the factor-dependent cell lines were still protected by IL-4 and IL-9 against growth inhibition by DEX. Together, these results indicate that growth stimulation in the presence of glucocorticoids and inhibition of apoptosis involve distinct aspects of cytokine activities.

Interleukin 9 (IL-9) is a TH2 cytokine that has been shown to promote the antigen-independent growth of some mouse T helper clones. To characterize the specificity of IL-9-mediated T cell activation, we used a murine T cell clone that could grow with either IL-9 or IL-2. After differential hybridization of a cDNA library, we isolated three genes that were expressed preferentially in the presence of IL-9. Two of them correspond respectively to granzyme A and granzyme B, two proteases expressed by activated T cells. By Northern blot hybridization and functional assays, we found that IL-9 induced the expression of granzyme B in several T cell clones as well as in mast cell lines. In addition, other proteases such as the mouse mast cell proteases were also found to be expressed by IL-9-activated T cell clones. The third IL-9-induced cDNA corresponds to the alpha-chain of the high-affinity receptor for IgE. Several T cell clones expressed this IgE receptor mRNA and were able to bind IgE with high affinity. Taken together, our results indicate that IL-9 induces a mast cell-like phenotype in T cell clones.