CD4 T cell-intrinsic STING signaling controls the differentiation and effector functions of TH1 and TH9 cells.

BACKGROUND: While stimulator of interferon genes (STING) activation in innate immune cells of the tumor microenvironment can result in CD8 T cell-dependent antitumor immunity, whether STING signaling affects CD4 T-cell responses remains elusive. METHODS: Here, we tested whether STING activation modulated the effector functions of CD4 T cells in vivo by analyzing tumor-infiltrating CD4 T cells and evaluating the contribution of the CD4 T cell-derived cytokines in the antitumor activity of the STING ligand 2'3'-cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) in two mouse tumor models. We performed ex vivo experiments to assess the impact of STING activation on CD4 T-cell differentiation and investigate the underlying molecular mechanisms. Finally, we tested whether STING activation enhances TH9 cell antitumor activity against mouse melanoma upon adoptive transfer. RESULTS: We found that activation of STING signaling cell-intrinsically enhances the differentiation and antitumor functions of TH1 and TH9 cells by increasing their respective production of interferon gamma (IFN-γ) and interleukin-9. IRF3 and type I interferon receptors (IFNARs) are required for the STING-driven enhancement of TH1 cell differentiation. However, STING activation favors TH9 cell differentiation independently of the IFNARs/IRF3 pathway but through mammalian target of rapamycin (mTOR) signaling, underscoring that STING activation differentially affects the fate of distinct CD4 T-cell subsets. The therapeutic effect of STING activation relies on TH1 and TH9-derived cytokines, and STING activation enhances the antitumor activity of TH9 cells upon adoptive transfer. CONCLUSION: Our results reveal the STING signaling pathway as a therapeutic target to boost CD4 T-cell effector functions and antitumor immunity.

Interleukin-9 serves as a key link between systemic inflammation and angiogenesis in psoriasis.

BACKGROUND: Psoriasis is a T helper cell-mediated chronic immune-mediated inflammatory disease affecting mainly the skin, although systemic pathological effects are also observed. Cytokine-mediated interaction between T lymphocytes and keratinocytes lead to excessive proliferation of keratinocytes, which in turn leads to formation of a proinflammatory milieu and finally to psoriatic plaque formation. AIM: To measure interleukin (IL)-9, IL-17 and vascular endothelial growth factor (VEGF) levels in patients with psoriasis compared with controls, and to evaluate the effect of methotrexate (MTX) monotherapy on the aforesaid cytokine levels in psoriasis. METHODS: This cohort study included 54 patients with psoriasis and 54 age- and sex-matched healthy controls (HCs). IL-9, IL-17 and VEGF levels were measured by using commercially available ELISA kits. Patients with psoriasis who were on MTX monotherapy were followed up for a period of 3 months. RESULTS: Patients with psoriasis had increased levels of IL-9, IL-17 and VEGF at baseline, compared with the HC group. After 3 months of MTX monotherapy, Psoriasis Area Severity Index (PASI), Dermatology Life Quality Index (DLQI) and levels of cytokines (IL-9, IL-17 and VEGF) were significantly decreased compared with baseline. PASI and DLQI at baseline also showed a positive correlation with IL-9, IL-17 and VEGF. CONCLUSION: Our results suggest the existence of a proinflammatory milieu in psoriasis, with increased levels of IL-9, IL-17 and the proangiogenic growth factor VEGF, showing an increasing trend with increasing disease severity and impaired quality of life (QoL). MTX treatment helps to reduce levels of IL-9, IL-17 and VEGF, thereby limiting disease progression and improving QoL in psoriasis.

IL-9 exacerbates the development of chronic obstructive pulmonary disease through oxidative stress.

OBJECTIVE: To investigate the role of IL-9 in chronic obstructive pulmonary disease (COPD), and to explore its potential mechanism. MATERIALS AND METHODS: A mouse COPD model was established by exposure to cigarette smoke. COPD mice were then randomly assigned into two groups, including: the PBS group and the IL-9 antibody group. The above two groups were treated with phosphate-buffered saline (PBS) or IL-9 injection, respectively. The histopathological changes in lung tissues of mice were observed by hematoxylin-eosin (H&E) staining. Immunohistochemistry was performed to detect IL-9-positive (IL-9+) cells in lung tissues. Expression levels of IL-9, sIL-9R, STAT3, and p-STAT3 in peripheral blood of mice were determined by quantitative Real time-polymerase chain reaction (qRT-PCR), enzyme-linked immunosorbent assay (ELISA), and Western blot, respectively. In addition, the expression levels of superoxide dismutase (SOD), malondialdehyde (MDA), and reactive oxygen species (ROS) were detected. RESULTS: H&E staining results showed that the airway wall structure of COPD mice in the PBS group was irregular. Ciliated columnar epithelium exhibited marked degeneration, necrosis and shedding. Besides, numerous inflammatory cell infiltration, narrowing and rupture of the alveolar septa, and larger cysts fused by adjacent alveoli were observed. H&E staining also indicated that the structure of alveolar epithelium was severely impaired in COPD mice. However, the pathological changes in lung tissues of mice in the IL-9 antibody group were much milder than those of the PBS group. Immunohistochemistry results showed a significant deposition of IL-9+ cells in the lung tissues of the PBS group. Meanwhile, the mRNA and protein levels of IL-9, sIL-9R, and p-STAT3 in the PBS group were also remarkably higher than those of the IL-9 antibody group. In addition, SOD content in the PBS group was significantly decreased, whereas the levels of MDA and ROS were significantly increased than those of the IL-9 antibody group. CONCLUSIONS: IL-9 activated STAT3 and aggravated lung injury in COPD mice by increasing inflammatory and oxidative stress.

Th9 lymphocytes: A recent history from IL-9 to its potential role in rheumatic diseases.

Various subtypes of effector T cells have been described up to date, and each one has its specific immunological function and a defined cytokine secretion profile. Th9 lymphocytes, recently described, are characterized by a high IL-9 expression. Their differentiation requires the integration of IL-4 and TGF-ß signaling pathways and the coordinated participation of multiple transcription factors. Their role has been mainly found in immunity against parasites and in allergic inflammatory processes. Nevertheless, they have been implicated in processes as autoimmunity, cancer and recently in rheumatic diseases. The objective of this review is to describe the discovery of this cellular subtype, its differentiation, expression regulation and its potential role in rheumatic diseases.

Glucocorticoid-induced tumor necrosis factor receptor-related protein co-stimulation facilitates tumor regression by inducing IL-9-producing helper T cells.

T cell stimulation via glucocorticoid-induced tumor necrosis factor receptor (TNFR)-related protein (GITR) elicits antitumor activity in various tumor models; however, the underlying mechanism of action remains unclear. Here we demonstrate a crucial role for interleukin (IL)-9 in antitumor immunity generated by the GITR agonistic antibody DTA-1. IL-4 receptor knockout (Il4ra(-/-)) mice, which have reduced expression of IL-9, were resistant to tumor growth inhibition by DTA-1. Notably, neutralization of IL-9 considerably impaired tumor rejection induced by DTA-1. In particular, DTA-1-induced IL-9 promoted tumor-specific cytotoxic T lymphocyte (CTL) responses by enhancing the function of dendritic cells in vivo. Furthermore, GITR signaling enhanced the differentiation of IL-9-producing CD4(+) T-helper (TH9) cells in a TNFR-associated factor 6 (TRAF6)- and NF-κB-dependent manner and inhibited the generation of induced regulatory T cells in vitro. Our findings demonstrate that GITR co-stimulation mediates antitumor immunity by promoting TH9 cell differentiation and enhancing CTL responses and thus provide a mechanism of action for GITR agonist-mediated cancer immunotherapies.

Differentiation, regulation and function of Th9 cells.

Naïve CD4(+) T cells are activated and differentiate to distinct lineages of T helper (Th) cells, which are involved in physiological and pathological processes by obtaining the potential to produce different lineage-specific cytokines that mediate adaptive immunity. In the past decade, our knowledge of Th cells has been significantly expanded with the findings of new lineages. Interleukin (IL)-9 producing T cells are recently identified. In consideration of the ability to preferentially secret IL-9, these cells are termed Th9 cells. Given the multiple function of IL-9, Th9 cells participate in the lesion of many diseases, such as allergic inflammation, tumor, and parasitosis. In this chapter, we will focus on the cytokines, co-stimulatory factors, and transcriptional signaling pathways, which regulate Th9 cells development as well as stability, plasticity, and the multiple roles of Th9 cells in vivo.

Robust tumor immunity to melanoma mediated by interleukin-9-producing T cells.

Interleukin-9 (IL-9) is a T cell cytokine that acts through a γC-family receptor on target cells and is associated with inflammation and allergy. We determined that T cells from mice deficient in the T helper type 17 (T(H)17) pathway genes encoding retinoid-related orphan receptor γ (ROR-γ) and IL-23 receptor (IL-23R) produced abundant IL-9, and we found substantial growth inhibition of B16F10 melanoma in these mice. IL-9-blocking antibodies reversed this tumor growth inhibition and enhanced tumor growth in wild-type (WT) mice. Il9r(-/-) mice showed accelerated tumor growth, and administration of recombinant IL-9 (rIL-9) to tumor-bearing WT and Rag1(-/-) mice inhibited melanoma as well as lung carcinoma growth. Adoptive transfer of tumor-antigen-specific T(H)9 cells into both WT and Rag1(-/-) mice suppressed melanoma growth; this effect was abrogated by treatment with neutralizing antibodies to IL-9. Exogenous rIL-9 inhibited tumor growth in Rag1(-/-) mice but not in mast-cell-deficient mice, suggesting that the targets of IL-9 in this setting include mast cells but not T or B cells. In addition, we found higher numbers of T(H)9 cells in normal human skin and blood compared to metastatic lesions of subjects with progressive stage IV melanoma. These results suggest a role for IL-9 in tumor immunity and offer insight into potential therapeutic strategies.

Interleukin (IL)-1beta stimulates migration and survival of first-trimester villous cytotrophoblast cells through endometrial epithelial cell-derived IL-8.

IL-1, secreted by human embryos and trophoblast cells, is important for successful implantation and pregnancy. We previously reported that IL-1beta induced IL-8 production in human endometrial stromal cells (ESCs) and that induction was regulated by substances implicated in implantation. In the present study using human primary cells in culture, we measured IL-1beta-induced production of IL-8 from endometrial epithelial cells (EECs) and ESCs and examined effects of the endometrium-derived IL-8 on migration and number of first-trimester villous cytotrophoblast cells (vCTs). Both basal and IL-1beta-induced IL-8 levels of cell supernatants were much higher in EECs than ESCs. Addition of IL-1beta to EECs increased the chemotactic activity of the supernatants to vCTs, and this effect was suppressed by immunoneutralization with anti-IL-8 antibody. Supernatants of IL-1beta-stimulated EECs yielded significantly higher number of vCTs compared with those of untreated EECs, and the effect was inhibited by IL-8 antibody. These findings suggest that IL-1 promotes implantation by stimulating EECs to produce IL-8, which subsequently induces migration of vCTs and contributes to survival of vCTs.

Induction of the IL-9 gene by HTLV-I Tax stimulates the spontaneous proliferation of primary adult T-cell leukemia cells by a paracrine mechanism.

The etiologic agent of adult T-cell leukemia (ATL) is human T cell lymphotropic virus type I (HTLV-I). The HTLV-I protein Tax alters gene expression, including those of cytokines and their receptors, which plays an important role in early stages of ATL. Here we demonstrate that expression of interleukin-9 (IL-9) is activated by Tax via an NF-kappaB motif in its proximal promoter, whereas IL-9 receptor-alpha (IL-9Ralpha) expression is not induced by Tax. However, supporting a role for IL-9/IL-9Ralpha in ATL, a neutralizing monoclonal antibody directed toward IL-9Ralpha inhibited ex vivo spontaneous proliferation of primary ATL cells from several patients. Fluorescence-activated cell sorter analysis of freshly isolated peripheral blood mononuclear cells from these patients revealed high level expression of IL-9Ralpha on their CD14-expressing monocytes. Furthermore, purified T cells or monocytes alone from these patients did not proliferate ex vivo, whereas mixtures of these cell types manifested significant proliferation through a contact-dependent manner. Taken together, our data suggest that primary ATL cells, via IL-9, support the action of IL-9Ralpha/CD14-expressing monocytes, which subsequently support the ex vivo spontaneous proliferation of malignant T cells. In summary, these data support a role for IL-9 and its receptor in ATL by a paracrine mechanism.

Expression of IL-9 receptor alpha chain on human germinal center B cells modulates IgE secretion.

BACKGROUND: IL-9 has been shown to affect the differentiation pathway of different cell types. However, its potential role in the maturation pathway of antigen-driven B-cell differentiation and its functional effects remain unknown. OBJECTIVE: To characterize IL-9 receptor alpha chain (IL-9R alpha) expression on human tonsillar B cells at different maturational stages, and to assess its effect on IgE production. METHODS: Freshly purified human tonsillar B cells were fractionated into 3 populations: low-density (LD), medium-density, and high-density cells. Expression levels of IL-9R alpha were determined by using immunohistochemistry and flow cytometry. IL-9R alpha(high)-expressing cells were stimulated with IL-9 in the presence or absence of IL-4, and IgE release was measured by ELISA. RESULTS: IL-9R alpha was expressed on human LD tonsillar B cells, with an ability to transduce signals through activation of signal transducer and activator of transcription 3 and 5. Although IL-9 was unable to induce IgE secretion by itself, it potentiated IL-4-mediated IgE production from LD cells. Moreover, increased IgE was paralleled by an upregulation of IL-9R alpha and CD27, with the latter a memory B-cell marker implicated in increased IgE secretion. CONCLUSION: These results highlight a crucial role for IL-9 in modulating T-cell-dependent B-cell differentiation and establish a new paradigm for understanding the synergistic role of T(H)2 cytokines and their modulatory effect on B-cell maturation and IgE production. CLINICAL IMPLICATIONS: IL-9 appears to be involved in memory B-cell differentiation and T(H)2-mediated allergic diseases such as asthma.

Autocrine release of interleukin-9 promotes Jak3-dependent survival of ALK+ anaplastic large-cell lymphoma cells.

The aberrant fusion protein NPM-ALK plays an important pathogenetic role in ALK+ anaplastic large-cell lymphoma (ALCL). We previously demonstrated that Jak3 potentiates the activity of NPM-ALK. Jak3 activation is restricted to interleukins that recruit the common gamma chain (gammac) receptor, including IL-9. NPM-ALK was previously shown to promote widespread lymphomas in IL-9 transgenic mice by unknown mechanisms. We hypothesized that IL-9 plays an important role in ALK+ ALCL via Jak3 activation. Our studies demonstrate the expression of IL-9Ralpha and IL-9 in 3 ALK+ ALCL-cell lines and 75% and 83% of primary tumors, respectively. IL-9 was detected in serum-free culture medium harvested from ALK+ ALCL-cell lines, supporting autocrine release of IL-9. Treatment of these cells with an anti-IL-9-neutralizing antibody decreased pJak3 and its kinase activity, along with pStat3 and ALK kinase activity. These effects were associated with decreased cell proliferation and colony formation in soft agar and cell-cycle arrest. Evidence suggests that cell-cycle arrest can be attributed to up-regulation of p21 and down-regulation of Pim-1. Our results illustrate that IL-9/Jak3 signaling plays a significant role in the pathogenesis of ALK+ ALCL and that it represents a potential therapeutic target for treating patients with ALK+ ALCL.

IL-9 protects against bleomycin-induced lung injury: involvement of prostaglandins.

IL-9 is a Th2 cytokine that exerts pleiotropic activities, and might be involved in the regulation of lung inflammatory processes. To characterize the activity of IL-9 on lung injury, we compared the pulmonary responses to bleomycin (blm) in IL-9 transgenic (Tg5) and wild-type (FVB) mice. Following intratracheal instillation of lethal doses of blm, the mortality rate was markedly reduced in Tg5 mice compared to their wild-type counterparts (ie, 25% mortality for Tg5 versus 85% for FVB mice, 21 days after instillation of 0.05U blm/mouse). Histological and biochemical analyses showed that blm induced less lung injury and less epithelial damage in Tg5 as compared to FVB animals. This protection of Tg5 mice was accompanied by an expansion of eosinophils and B cells in the lungs. In addition, TGF-beta and prostaglandin-E2 (PGE2) levels in broncho-alveolar lavage fluid were also increased in transgenic mice. The contribution of B cells and eosinophils to the protective mechanism did not appear essential since eosinophil-deficient (IL-5 KO) and B-deficient (muMT) mice overexpressing IL-9 were also resistant to high doses of blm. We could rule out that TGF-beta was a key factor in the protective effect of IL-9 by blocking this mediator with neutralizing antibodies. Indomethacin treatment, which inhibited PGE2 production in both strains, suppressed the protection in Tg5 mice, supporting the idea that IL-9 controls blm-induced lung injury through a prostaglandin-dependent mechanism.

IL-9 and its receptor: from signal transduction to tumorigenesis.

IL-9 is a multifunctional cytokine secreted by TH2 lymphocytes. Besides its role during immune responses, its growth factor and antiapoptotic activities on multiple transformed cells suggest a potential role in tumorigenesis. Indeed, IL-9 overexpression induces thymic lymphomas in mice, and IL-9 production is associated with Hodgkin disease and HTLV-I transformed T cells in humans. IL-9 activities are mediated by a specific receptor chain that forms a heterodimeric receptor with the common gamma chain also involved in IL-2,4,7,15 and 21 signaling. The IL-9 receptor and common gamma chains associate with JAK1 and JAK3, respectively and trigger the STAT-1, -3 and -5, IRS and RAS-MAPK pathways. Moreover, in vitro, dysregulated IL-9 response can lead to autonomous cell growth and malignant transformation of lymphoid cells associated with constitutive activation of the Jak/STAT pathway.

[Role of interleukin-9 in the pathogenesis of chronic obstructive pulmonary disease].

OBJECTIVE: To study the role of interleukin-9 in the airway inflammation from patients with COPD. METHODS: Induced sputum was obtained from 30 COPD patients with stable disease(group A),31 asthmatics patients with stable disease(group B) and 15 healthy individuals(group C). IL-9,IL-5 and IL-8 in sputum supernatants were measured by sandwich enzyme-linked immunosorbent assay(ELISA) and IL-9 positive expression and quantitative analysis were conducted by Streptavidin peroxidase method and image analysis technology. RESULTS: The levels of IL-9 in group A and B were all significantly higher than those in groups C. IL-9 positive expression mainly located in the cytoplasm of macrophages. The positive rates of IL-9 in group A and B were all significantly higher than that of group C (chi(2)=20.821, 19.908, P<0.001 . The percentage of neutrophil and the level of IL-8 in group A was significantly higher than those of groups B and C. The level of IL-9 was positively correlated with the number of macrophages in group A(r=0.407,P=0.039). CONCLUSION: The level of IL-9 from the patients with COPD was significantly higher than that of healthy individuals and correlated with the number of macrophages and the level of IL-8. This suggests that IL-9 may be used as a new diagnostic marker in airway inflammation of COPD.

Lymphomagenesis, hydronephrosis, and autoantibodies result from dysregulation of IL-9 and are differentially dependent on Th2 cytokines.

Interleukin-9 is an immunoregulatory cytokine implicated in the development of asthma and allergy. To investigate the role of IL-9 in vivo, we have generated transgenic mice in which IL-9 is expressed from its own promoter. Strikingly, overexpression of IL-9 resulted in premature mortality associated with a complex phenotype characterized by the development of autoantibodies, hydronephrosis, and T cell lymphoma. By intercrossing IL-9 transgenic mice with a panel of Th2 cytokine-deficient mice, we demonstrate that these disorders represent distinct phenotypes that can be dissociated by their differential dependence on Th2 cytokines. Autoantibody production was ablated in IL-9 transgenic animals with a combined absence of IL-4, IL-5, and IL-13, coincident with a reduction in peritoneal B-1 cells. Hydronephrosis arose in 75% of IL-9 transgenic animals and was dependent on the presence of IL-4 and IL-13. In contrast, T cell lymphomas developed independently of the other Th2 cytokines, with the generation of rapidly proliferating CD8(+) or CD4(+)CD8(+) T cell clones that arose in the thymus before infiltrating both lymphoid and nonlymphoid tissues. Our data highlight potentially important new roles for IL-9, through its regulation of downstream Th2 effector cytokines, in autoantibody production and in hydronephrosis.

IL-9-induced expansion of B-1b cells restores numbers but not function of B-1 lymphocytes in xid mice.

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.

Interleukin-9 induces mucous cell metaplasia independent of inflammation.

Interleukin-9 (IL-9) has been strongly implicated in the pathogenesis of asthma, including the overproduction of mucus, in humans and in animal models. We evaluated the inflammatory changes associated with the upregulation of mucus production by examining the time course of inflammation after daily intratracheal IL-9 administration to naive C57Bl6 mice for 9 d. IL-9 induced an asthmatic phenotype, which in general took several days to develop, as assessed by the measurement of airway hyperresponsiveness, pulmonary inflammation, and serum immunoglobulin E. However, within 24 h of a single dose of IL-9, muc5ac mRNA upregulation occurred, and increased numbers of periodic acid Schiff/Alcian blue-positive mucous cells appeared. This response occurred before the development of an inflammatory cell influx and was the result of epithelial metaplasia. It seemed that IL-9 evoked mucous cell metaplasia independent of IL-13 because mRNA tissue evaluation indicated that muc5ac upregulation preceded any increase in IL-13 mRNA expression or detectable levels of IL-13 in the brochoalveolar lavage fluid. Therefore, the upregulation of IL-13 by IL-9 may be responsible for the amplification of mucus production but is not required for its initiation. IL-9 seems to directly stimulate mucous cell metaplasia without the requirement of inflammatory cell influx.

MAP kinase activation by interleukin-9 in lymphoid and mast cell lines.

Interleukin-9 (IL-9) stimulates the proliferation of mast cells and lymphocytes. In the present study, we showed that IL-9 induced a transient phosphorylation of MEK, ERK2 and p90/RSK in murine lymphoid and mast cell lines. ERK2 in vitro kinase activity was also increased upon IL-9 stimulation. Similar results were obtained with IL-4, which had not been previously reported to activate these kinases in hematopoietic cells. Analysis of IL-9 receptor mutants showed that activation of the pathway was correlated with proliferation and with phosphorylation of the adaptor protein SHC, but not IRS2 or GAB2. The MEK inhibitor PD98059 reduced the mitogenic response to IL-4 and IL-9. In addition, expression of a dominant-negative RAS variant blocked ERK phosphorylation and significantly decreased Ba/F3 cell growth in the presence of IL-9, but did not affect expression of pim-1, a STAT target gene. In summary, these results indicate that IL-9 can transiently activate the mitogen-activated protein kinase pathway, which contributes to growth stimulation of hematopoietic cell lines.

IL-9 enhances the growth of human mast cell progenitors under stimulation with stem cell factor.

We examined the effects of IL-9 on human mast cell development from CD34(+) cord blood (CB) and peripheral blood cells in serum-deprived cultures. IL-9 apparently enhanced cell production under stimulation with stem cell factor (SCF) from CD34(+) CB cells. A great majority of the cultured cells grown with SCF + IL-9 became positive for tryptase at 4 wk. In methylcellulose cultures of CD34(+) CB cells, IL-9 increased both the number and size of mast cell colonies grown with SCF. Furthermore, SCF + IL-9 caused an exclusive expansion of mast cell colony-forming cells in a 2-wk liquid culture of CD34(+) CB cells, at a level markedly greater than for SCF alone. Clonal cell cultures and RT-PCR analysis showed that the targets of SCF + IL-9 were the CD34(+)CD38(+) CB cells rather than the CD34(+)CD38(-) CB cells. IL-9 neither augmented the SCF-dependent generation of progeny nor supported the survival of 6-wk-cultured mast cells. Moreover, there was no difference in the appearance of tryptase(+) cells and histamine content in the cultured cells between SCF and SCF + IL-9. The addition of IL-9 increased numbers of mast cell colonies grown with SCF from CD34(+) peripheral blood cells in children with or without asthma. It is of interest that mast cell progenitors of asthmatic patients responded to SCF + IL-9 to a greater extent than those of normal controls. Taken together, IL-9 appears to act as a potent enhancer for the SCF-dependent growth of mast cell progenitors in humans, particularly asthmatic patients.