Thymic Stromal-derived Lymphopoietin Research Articles

Effect of Chloroquine on Type 2 Inflammatory Response in MC903-Induced Atopic Dermatitis Mice.

Atopic dermatitis (AD) is a chronic, non-infectious inflammatory dermatosis. Chloroquine (CQ) has long been proven to possess anti-inflammatory properties. This paper aims to investigate the impact of CQ on type 2 inflammatory response in MC903-induced AD mice. An AD mouse model was established via MC903 induction. After CQ treatment, AD mice were intraperitoneally injected with polyinosinic: polycyclic acid [poly (I:C)] or Nigericin. Dermatitis severity was scored, and the thickness of the left ear was measured. The pathological changes in mouse skin tissues were observed by H&E staining. The number of mast cells was counted via TB staining. The content of peripheral blood T-helper 2 (Th2) cells and levels of immunoglobulin E (IgE), thymic stromal-derived lymphopoietin (TSLP), interleukin (IL)-4, IL-13, interferon (IFN)-γ, IL-1β, and IL-18 were assessed by flow cytometry and ELISA. The levels of toll-like receptor 3 (TLR3), NLRP3, ASC, and cleaved caspase-1 proteins in skin tissues were determined by Western blot. CQ treatment abated dermatitis severity and left ear thickness in AD mice, alleviated skin damage, reduced mast cell number, diminished IgE, TSLP, IL-4, and IL-13 levels, and peripheral blood Th2 cell content, with no significant changes in IFN-γ level. CQ alleviated type 2 inflammatory response in AD mice by inhibiting the activation of TLR3. CQ suppressed NLRP3 inflammasome activation. Activating TLR3/NLRP3 annulled CQ-mediated alleviation on type 2 inflammatory response in AD mice. CQ alleviated type 2 inflammatory response in AD mice by inhibiting TLR3 activation and NLRP3 inflammasome activation.

Eotaxin-3 as a Plasma Biomarker for Mucosal Eosinophil Infiltration in Chronic Rhinosinusitis.

Objective: Chronic rhinosinusitis with nasal polyps exhibits marked eosinophilic infiltration and its mucosal eosinophilia is associated with more severe symptoms. The Japanese epidemiological survey of refractory eosinophilic chronic rhinosinusitis found that patients with nasal polyps required multiple surgeries when there were higher infiltrating eosinophils in the mucosa. In order to identify plasma biomarkers for local eosinophil infiltration in rhinosinusitis for surgery, we examined the levels of molecules in the plasma of patients and compared the number of infiltrating eosinophils in the nasal mucosa.Materials and Methods: Mucosal tissues from 97 patients with chronic rhinosinusitis (CRS) were obtained from the nasal polyps during surgery. Tissues were immediately fixed and sections were stained with hematoxylin-eosin. The number of eosinophils in the mucosa was counted at HPF (x 400). Blood samples were obtained and the plasma was stored at −80°C. We measured the plasma cytokine and chemokine levels using multiple assay systems according to the manufacturers' protocols. The tissues were divided into high- and low-eosinophil mucosal infiltration group for recurrence after endoscopic sinus surgery (ESS). We also observed chemokine secretion from nasal fibroblasts.Results: The plasma level of eotaxin-3/ CC chemokine ligand 26 (CCL26) was significantly higher in the high-eosinophil mucosal infiltration group (p < 0.005). The number of infiltrating eosinophils in the mucosa was significantly higher in the group with the higher eotaxin-3 level (p < 0.001), but there was no significant difference in the blood eosinophil numbers among two groups. A significant positive correlation was found between the mucosal eosinophil count and the plasma levels of eotaxin-3 (p < 0.005). The levels of interleukin 33 (IL-33) (p < 0.001) and thymic stromal-derived lymphopoietin (TSLP) (p < 0.005) were significantly higher in the high-level eotaxin-3 group. IL-13 strongly induced the secretion of eotaxin-3 from human nasal fibroblasts (p < 0.05).Conclusion: This is the first report suggesting eotaxin-3 as a plasma biomarker for mucosal eosinophil infiltration. Furthermore, the level of eotaxin-3 was found to be closely related to IL-33 and TSLP levels which indicate respiratory diseases.

Abstract 1094: Role of the CRLF2 ligand TSLP in normal human B-cell development

Abstract B cell acute lymphoblastic leukemia (B-ALL) is the most common childhood malignancy. Defects that result in overexpression of CRLF2, a cytokine receptor component, have recently been linked with poor outcome in both high-risk and non-high-risk B-ALL. CRLF2, together with the IL-7R alpha chain, forms the signaling receptor for the cytokine TSLP (Thymic Stromal-derived LymphoPoietin). TSLP-induced CRLF2 signaling results in proliferation of murine B cell progenitors, however, the role of TSLP in murine B cell development remains controversial and little is known of its role in human B lymphopoiesis. Human TSLP and CRLF2, in contrast to other cytokine/receptor pairs that act on B cell progenitors, show low homology to their murine counterparts (&amp;lt;40%), lack species cross reactivity, and activate differing intracellular pathways than in mice. Thus, human model systems are essential for understanding the role of CRLF2 function in normal and malignant B lymphopoiesis. We have developed a novel human-only in vitro culture model that allows the study of human B cell development under highly selective cytokine stimulation. This model is based on co-culturing CD34+ hematopoietic stem cells (HSCs) with primary human bone marrow stroma supplemented with selective exogenous cytokines and/or specific cytokine neutralizing antibodies. We have previously used this model to show that IL-7 increases B cell progenitors by over 60 fold and that IL-7 is essential for normal human B cell production (J. Immunol. 2009 82:4255). Here we use our human-only in vitro model to examine the role of TSLP-induced signals in normal human B cell development. Our data show that TSLP induces an expansion of human B cell progenitor populations generated from HSCs in cord blood that is similar in magnitude to that observed for IL-7. A comparison of TSLP and IL-7 effects shows that TSLP targets early B cell progenitors (CD34+) inducing expansion of this population with limited differentiation. In contrast, IL-7 induces rapid differentiation to the CD34- stage followed by proliferation. RT-PCR and ELISA analysis show that normal human bone marrow stoma express both TSLP and IL-7, thus establishing an endogenous source of these cytokines at the site of normal and malignant hematopoiesis. Our data provide evidence that TSLP and IL-7 exert differential and stage specific effects on proliferation and differentiation of B cell progenitors and suggest that deregulation of the complex interplay between these signals in normal differentiation could set the stage for malignant transformation to B-ALL in the presence of cooperative defects in JAK2 and/or Ikaros. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1094. doi:10.1158/1538-7445.AM2011-1094

Deregulated expression of cytokine receptor gene, CRLF2, is involved in lymphoid transformation in B-cell precursor acute lymphoblastic leukemia

AbstractWe report 2 novel, cryptic chromosomal abnormalities in precursor B-cell acute lymphoblastic leukemia (BCP-ALL): a translocation, either t(X;14)(p22;q32) or t(Y;14)(p11;q32), in 33 patients and an interstitial deletion, either del(X)(p22.33p22.33) or del(Y)(p11.32p11.32), in 64 patients, involving the pseudoautosomal region (PAR1) of the sex chromosomes. The incidence of these abnormalities was 5% in childhood ALL (0.8% with the translocation, 4.2% with the deletion). Patients with the translocation were older (median age, 16 years), whereas the patients with the deletion were younger (median age, 4 years). The 2 abnormalities result in deregulated expression of the cytokine receptor, cytokine receptor-like factor 2, CRLF2 (also known as thymic stromal-derived lymphopoietin receptor, TSLPR). Overexpression of CRLF2 was associated with activation of the JAK-STAT pathway in cell lines and transduced primary B-cell progenitors, sustaining their proliferation and indicating a causal role of CRLF2 overexpression in lymphoid transformation. In Down syndrome (DS) ALL and 2 non-DS BCP-ALL cell lines, CRLF2 deregulation was associated with mutations of the JAK2 pseudokinase domain, suggesting oncogenic cooperation as well as highlighting a link between non-DS ALL and JAK2 mutations.

Chitin Stimulates Expression of Acidic Mammalian Chitinase and Eotaxin-3 by Human Sinonasal Epithelial Cells in Vitro

Sinonasal epithelial cells participate in host defense by initiating innate immune mechanisms against potential pathogens. Antimicrobial innate mechanisms have been shown to involve Th1-like inflammatory responses. Although epithelial cells can also be induced by Th2 cytokines to express proeosinophilic mediators, no environmental agents have been identified that promote this effect. Human sinonasal epithelial cells from patients with chronic rhinosinusitis with nasal polyps (CRSwNPs) and controls were harvested and grown in primary culture. Cell cultures were exposed to a range of concentrations of chitin for 24 hours, and mRNA for acidic mammalian chitinase (AMCase), eotaxin-3, and thymic stromal-derived lymphopoietin (TSLP) were assessed. Other cultures were exposed to interleukin 4 (IL- 4) alone and in combination with dust-mite antigen (DMA) for 36 hours. Extracted mRNA and cell culture supernatant were analyzed for expression of AMCase and eotaxin-3. Chitin induced a dose-dependent expression of AMCase and eotaxin-3 mRNA but not TSLP. Patients with recalcitrant CRSwNPs showed lower baseline expression of AMCase when compared with treatment-responsive CRSwNP and less induction of AMCase expression by chitin. DMA did not directly induce expression of AMCase or eotaxin-3. Expression of eotaxin-3 was stimulated by IL-4 and further enhanced with the addition of DMA. Levels of AMCase were not significantly affected by either IL-4 or DMA exposure. In some cases, the combination of IL-4 and DMA was able to induce AMCase expression in cell cultures not producing AMCase at baseline. The abundant biopolymer chitin appears to be recognized by a yet uncharacterized receptor on sinonasal epithelial cells. Chitin stimulates production of AMCase and eotaxin-3, two pro-Th2 effector proteins. This finding suggests the existence of a novel innate immune pathway for local defense against chitin-containing organisms in the sinonasal tract. Dysregulation of this function could precipitate or exacerbate Th2 inflammation, potentially acting as an underlying factor in recalcitrant CRSwNP.

IGH@ Translocations Involving the Pseudoautosomal Region 1 (PAR1) of Both Sex Chromosomes Deregulate the Cytokine Receptor-Like Factor 2 (CRLF2) Gene in B Cell Precursor Acute Lymphoblastic Leukemia (BCP-ALL)

We have recently described recurrent IGH@ translocations in BCP-ALL, which result in deregulated expression of the translocated oncogene. We carried out screening for novel IGH@ translocations in BCP-ALL by fluorescence in situ hybridization (FISH) and identified two cryptic translocations involving PAR1 of either sex chromosomes in 29 patients: t(X;14)(p22;q32) (n=18) and t(Y;14)(p11;q32) (n=7) (X or Y unknown, n= 4) and 2 BCP-ALL cell lines with t(Y;14)(p11;q32). The median age of the patients was 18 years (range 4–76 years) and median WBC was 70 ×109/l (range 1–342). Follow-up data was available for 16 patients. Among 5 children diagnosed 1996–2001, 1 failed to remit, 1 suffered an early death and 3 relapsed (16m, 54m, and 64m post diagnosis). To date, all 5 children diagnosed 2004–2005 are in first continuing complete remission (CCR) after 33–46 months, suggesting an improved outcome on the current childhood ALL treatment trial. All 6 adults were diagnosed 2004–2006: 2 died within a year (1 in remission and 1 post transplant); 2 relapsed and died within 1 year and 2 are in 1st CCR after 20 and 36 months. These observations indicate a variable outcome among adults. Translocation breakpoint cloning from IGHJ by long distance inverse PCR and subsequent FISH mapping identified CRLF2, also known as thymic stromal-derived lymphopoietin receptor, at Xp22 and Yp11, to be juxtaposed to IGH@ in these translocations. Breakpoints were cloned in 11 patients and clustered between 2–27kb centromeric of the 3′ UTR of CRLF2. Expression levels of CRLF2, measured by qRT-PCR in 4 patients and the cell lines, was 500–6,500 fold higher than the control cohort of BCP-ALL patients without the translocation. This overexpression confirmed CRLF2 to be the target gene. CRLF2 and IL2RGC bind to IL7RA to form functional receptors for TSLP and IL7, respectively. However, neither IL2RGC nor IL7RA were aberrantly expressed. The presence of associated genomic copy number alterations was shown by array-based comparative genomic hybridization and FISH. As described for BCP-ALL in general, the significant changes included deletions of the B cell differentiation genes: PAX5 and IKAROS, as well as the cell cycle control gene, CDKN2A. The cell lines revealed some biochemical insights into the functional consequences of this translocation involving PAR1. Cell surface expression of CRLF2 was demonstrated, together with constitutive tyrosine phosphorylation of JAK2 and STAT5, the signaling intermediates of the classical JAK-STAT pathway. Further evidence of increased STAT5 activation came from retroviral infection of normal mouse fetal liver cells with human CRLF2, which sustained the proliferation of B-cell precursors in vitro. Conversely, initial studies of CRLF2 knockdown in the cell lines using shRNA demonstrated decreased proliferation. Together, these results show for the first time, the involvement of both sex chromosomes in an IGH@ translocation in leukemia, resulting in deregulated expression of CRLF2 and constitutive JAK-STAT activation. These data suggest CRLF2 as a potential therapeutic target in this subset of patients.Laboratories of MJSD, RS and CJH contributed equally to this work.

New visions for basic research and primary prevention of pediatric allergy: An iPAC summary and future trends

Hydrolyzed formula feeding, delayed introduction of solid food, indoor allergen avoidance, smoke and pollutants avoidance have been applied for several decades as primary preventive measures for allergic diseases. Unfortunately, some of these strategies have had no or modest success. Therefore, resources need to be focused on better understanding of the early allergic events and on interventional studies to investigate new strategies of primary and secondary prevention. Accordingly, this review summarizes the state-of-the-art of genetic, immunological and clinical aspects of primary prevention of allergic diseases. Studies investigating gene-by-gene and gene-by-environment interactions suggest that prevention of allergic diseases must be tailored to the individual genetic susceptibilities ('gene profiling') and environmental exposures. The expanding knowledge on new T cell populations (Th17, TSLP (thymic stromal derived lymphopoietin)-dependent 'inflammatory Th2 cells') is also inspiring new concepts on the origins of allergic diseases. The old concept of 'blocking immunoglobulin G antibodies' has been re-appraised and it is likely to generate novel preventive and therapeutic strategies. The major task for future clinical research is to clearly define the timing of optimal exposure to potential allergens. In addition, the role of microbial products such as certain bacteria, or their components, and of helminths or their larvae at different times in early life, alone or with potential allergens, definitely need to be further investigated. The benefit of efficient allergy prevention, based on focusing resources on novel and promising research lines, will be of prime importance to both affluent countries and other parts of the world where allergy is only currently emerging.

Thymic Stromal-Derived Lymphopoietin Induces Proliferation of Pre-B Leukemia and Antagonizes mTOR Inhibitors, Suggesting a Role for Interleukin-7Rα Signaling

Understanding the pathogenesis of leukemia in the context of lymphopoiesis may reveal novel therapeutic targets. Previously, we have shown that mTOR inhibitors (MTI) show activity in vitro and in preclinical models of both human and murine precursor B acute lymphoblastic leukemia (pre-B ALL), inhibiting cell proliferation and inducing apoptosis. These MTI-mediated effects can be reversed by interleukin-7 (IL-7), an important regulator of early B-cell development. This observation led us to examine the contribution of signaling via the IL-7Ralpha chain, which is shared by the receptor complexes of IL-7 and thymic stromal-derived lymphopoietin (TSLP). TSLP is closely related to IL-7 and active in lymphopoiesis, but an effect of TSLP on leukemia cells has not been described. We examined the effect of TSLP on pre-B ALL cells and their response to MTIs. Here, we show that TSLP stimulates proliferation of pre-B ALL cell lines. TSLP also partially reverses the effects of MTI on proliferation, apoptosis, and ribosomal protein S6 and 4E-BP1 phosphorylation in cell lines, with similar biological effects seen in some primary human lymphoblast samples. These data show that TSLP can promote survival of pre-B ALL cells and antagonize the effects of MTIs. These findings suggest that IL-7Ralpha chain is responsible for transducing the survival signal that overcomes MTI-mediated growth inhibition in pre-B ALL. Thus, further exploration of the IL-7Ralpha pathway may identify potential therapeutic targets in the treatment of ALL. Our data illustrate that growth-factor-mediated signaling may provide one mechanism of MTI resistance.

Association between genetic polymorphisms in the human interleukin‐7 receptor α‐chain and inhalation allergy

Thymic stromal-derived lymphopoietin (TSLP) and interleukin-7 share a common receptor chain, IL-7Ralpha. IL-7 is involved in T-cell homeostasis, and TSLP induces production of pro-allergic cytokines. The gene encoding the IL-7Ralpha chain is polymorphic, and investigation of inhalation allergic patients compared with controls showed significant association with two alleles at position +1237 and +2087.

Developmental switch of mouse hematopoietic stem cells from fetal to adult type occurs in bone marrow after birth

Hematopoiesis originated by hematopoietic stem cells (HSCs) is distinguishable between fetal and adult mice. However, it is not clear whether the altered mode of differentiation is due to the change of properties of HSCs or different microenvironments in fetuses and adults. Here we show that fetal HSCs are fully capable of giving rise to all classes of B cells in the adult microenvironment. HSCs that are derived from fetal liver but not adult bone marrow (BM) of IL-7 receptor alpha chain (IL-7Ralpha)-deficient mice can also differentiate into B cells, suggesting that both IL-7 and thymic stromal-derived lymphopoietin (TSLP) are dispensable for fetal B cell development, because IL-7Ralpha is commonly used as a subunit of functional receptor complexes for IL-7 and TSLP. Similar IL-7/TSLP independent B cell potential is maintained by BM HSCs until 1 week after birth. In contrast, BM HSCs in mice older than 2 weeks of age absolutely requires IL-7Ralpha for B lymphopoiesis. These results demonstrate that fetal HSCs acquired adult characteristics between 1 and 2 weeks after birth in mouse BM.

Delayed functional maturation of natural regulatory T cells in the medulla of postnatal thymus: role of TSLP.

BackgroundGeneration of functional CD4+CD8-CD25+ regulatory T cells (Treg) in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice has been shown to result in a multiple organ autoimmune disease phenotype that can be prevented by introducing the FoxP3+ Treg population to the animal. It has therefore, been proposed that functional FoxP3+ Treg cells are not made in the neonatal thymus; however, it remains unclear when and where functional FoxP3+CD4+CD8-CD25+ thymocytes are generated in postnatal thymus.ResultsWe report that neither FoxP3 mRNA nor protein is expressed in CD4+CD8-CD25+, or CD4+CD8-CD25- thymocytes until 3–4 days post birth, despite the presence of mature CD4+CD8-CD25+/- thymocytes in the thymus by 1–2 days after birth. FoxP3-CD4+CD8-CD25+ thymocytes from day 2 newborn mice show no Treg activity. Interestingly, we are able to detect low numbers of FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as early as 3–4 days post birth. Expression of FoxP3 is induced in embryonic day 17 fetal thymus organ culture (FTOC) after 4–6 days of in vitro culture. Treatment of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. Furthermore, TSLP stimulates FoxP3 expression in purified CD4+CD8- thymocytes, but not in CD4+CD8+, CD4-CD8+ and CD4-CD8- thymocytes.ConclusionExpression of FoxP3 or Treg maturation is ontogenically distinct and kinetically delayed from the generation of CD4+CD8-CD25+ or CD4+CD8-CD25- thymocytes in the postnatal thymus. TSLP produced from medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 and the maturation of natural regulatory T cells. Overall, these results suggest that the development of Treg cells requires paracrine signaling during late stages of thymocyte maturation that is distinct from signaling during positive or negative selection.

Thymic stromal-derived lymphopoietin distinguishes fetal from adult B cell development.

Deletions of interleukin 7 (IL-7) or its receptor components permit fetal but not adult B cell development in mice. Mice deficient in IL-7 receptor alpha (IL-7R alpha) had 1% the number of B cells of controls and 10% that of mice deficient in the common gamma chain. As IL-7R alpha is also a receptor for thymic stromal-derived lymphopoietin (TSLP), we assayed the ability of TSLP to support proliferation of fetal or adult precursor B cells. Only fetal-derived pro-B cells were able to respond to TSLP, although pre-B cells from both origins were TSLP-responsive. Fetal but not adult precursors generated a measurable B cell compartment in the absence of IL-7. The residual B cells found in IL-7R alpha-deficient mice required fetal liver kinase 2 (Flk-2) for their development. Thus, IL-7R alpha- and Flk-2-mediated signals account for the generation of almost all mouse B lymphocytes.

Regulation of thymic epithelium by keratinocyte growth factor

AbstractHere we demonstrate that keratinocyte growth factor (KGF) and FGFR2IIIb signaling can affect development and function of thymic epithelium (TE) and that αβ-lineage thymocytes contribute to intrathymic levels of KGF. Thymocyte expression of KGF is developmentally regulated, being undetectable in CD3−4−8− thymocytes and expressed at highest levels by mature CD4 or CD8 thymocytes. Exposure of thymocyte-depleted fetal thymic lobes to KGF resulted in reduced thymic epithelial expression of class II major histocompatibility complex (MHC), invariant chain (Ii), and cathepsin L (CatL) molecules involved in thymocyte-positive selection and also stimulated expression of the cytokines interleukin 6 (IL-6) and thymic stromal-derived lymphopoietin (TSLP), while having little effect on IL-7 or stem cell factor expression. Within intact fetal thymic organ culture (FTOC), exogenous KGF impairs the generation of CD4 thymocytes. Two lines of evidence point to responsiveness of the medullary TE compartment to KGF and FGFR2IIIb signaling. First, the medullary compartment is expanded in intact FTOC exposed to KGF in vitro. Second, in the RAG-deficient thymus, where the thymocytes do not express detectable levels of KGF message, the hypoplastic medullary TE compartment can be expanded by administration of recombinant KGF in vivo. This expansion is accompanied by restoration of the normal profile of medullary TE–associated chemokine expression in the RAG2−/−thymus. Collectively, these findings point to a role for KGF and FGFR signaling in the development and function of thymic epithelium.

Identification of an IL-7-Associated Pre-Pro-B Cell Growth-Stimulating Factor (PPBSF). II. PPBSF Is a Covalently Linked Heterodimer of IL-7 and aMr 30,000 Cofactor

AbstractEvidence is provided in a companion paper for an IL-7-associated molecular complex that selectively stimulates the proliferation and presumptive differentiation of pre-pro-B cells in our long-term bone marrow culture system and “primes” them to proliferate in response to monomeric IL-7. Here, Western immunoblot analysis reveals that this pre-pro-B cell growth-stimulating factor (PPBSF) is a self-assembling heterodimer of IL-7 and a cofactor with a Mr of 30,000. Thus, when developed with anti-IL-7 mAb, PPBSF migrates electrophoretically as a covalently bound ∼55-kDa molecule under nonreducing conditions but dissociates under reducing conditions. Furthermore, the addition of rIL-7 or native IL-7 to medium conditioned by stromal cells from IL-7 gene-deleted (−/−) mice results in the formation of active 45-kDa and 55-kDa molecular complexes, respectively. Antiserum prepared in IL-7(−/−) mice against affinity-purified PPBSF contained separable reactivities for IL-7 and the non-IL-7 component of PPBSF. The PPBSF cofactor detected by this antiserum migrates as an ∼30-kDa molecule and is able to maintain the viability, but not the proliferation, of pre-pro-B cells. Furthermore, the cofactor is produced constitutively by IL-7(−/−) and IL-7(+/+) bone marrow stromal cells under pro-B- but not pre-B-type culture conditions. Consequently, IL-7 appears to exist almost entirely as a heterodimer (i.e., PPBSF) in pro-B-type cultures, whereas it exists almost entirely as a monomer in pre-B-type cultures. Although the identity of the PPBSF cofactor remains to be determined, it does not appear to be stem cell factor, insulin-like growth factor-1, thymic stromal-derived lymphopoietin, flt3, stromal cell-derived factor-1, or IL-7R.

Characterization of thymic stromal-derived lymphopoietin (TSLP) in murine B cell development in vitro.

B cell development is dependent on both direct interactions with stromal cells and their secreted cytokines. The precise mechanisms by which these interactions regulate B cell differentiation are currently unknown. We report here that a novel growth factor thymic stromal-derived lymphopoietin (TSLP) can replace the activity of interleukin-7 (IL-7) in supporting B cell development in vitro. TSLP was found to promote the proliferation and differentiation of committed B220+ B cell progenitors from day 15 fetal liver. Phenotypic analysis of these cells revealed that they are at the pro-B cell stage of differentiation and express cell surface markers characteristic of pro-B cells cultured in IL-7. TSLP can replace the activity of IL-7 in supporting the progression of B lymphocytes from uncommitted bipotential precursors. In the absence of either TSLP or IL-7, the progeny of cells that give rise to mature B lymphocytes fail to develop from these bipotential precursors. Moreover, TSLP can substitute for IL-7 in supporting the sustained proliferative response exhibited by B cell progenitors from CBA/N mice. Together these results show that TSLP can replace the requirement for IL-7 during in vitro B cell development.