Molecular and structural basis of cytokine receptor pleiotropy in the interleukin-4/13 system

Interleukin-4 and Interleukin-13 are cytokines critical to the development of T cell-mediated humoral immune responses, which are associated with allergy and asthma, and exert their actions through three different combinations of shared receptors. Here we present the crystal structures of the complete set of type I (IL-4R alpha/gamma(c)/IL-4) and type II (IL-4R alpha/IL-13R alpha1/IL-4, IL-4R alpha/IL-13R alpha1/IL-13) ternary signaling complexes. The type I complex reveals a structural basis for gamma(c)'s ability to recognize six different gamma(c)-cytokines. The two type II complexes utilize an unusual top-mounted Ig-like domain on IL-13R alpha1 for a novel mode of cytokine engagement that contributes to a reversal in the IL-4 versus IL-13 ternary complex assembly sequences, which are mediated through substantially different recognition chemistries. We also show that the type II receptor heterodimer signals with different potencies in response to IL-4 versus IL-13 and suggest that the extracellular cytokine-receptor interactions are modulating intracellular membrane-proximal signaling events.

IL-4 promotes the formation of multinucleated giant cells from macrophage precursors by a STAT6-dependent, homotypic mechanism: contribution of E-cadherin

Multinucleated giant cells (MNG) are central players in the inflammatory response to foreign materials and in adverse responses to implants. IL-4 promotes the formation of MNG from bone marrow-derived precursors in vitro and participates in the development of the foreign body reaction in vivo. Therefore, we investigated the mechanism by which IL-4 promotes formation of MNG and engulfment of foreign bodies. We found that generation of MNG cells by IL-4 was dependent on cell density and expression of STAT6; macrophages derived from STAT6(-/-) mice were unable to form MNG in response to IL-4. No soluble factors including CCL2 or supernatants from IL-4-treated macrophages compensated for the lack of MNG cells in STAT6(-/-) cultures. We found that IL-4 must remain present during the full differentiation process and that STAT6(+/+) macrophage precursors retained their ability to differentiate into MNG over time. These MNG were able to internalize large particles efficiently, and the mononuclear STAT6(-/-) macrophages were unable to do so. Furthermore, we found that IL-4 induced expression of E-cadherin and dendritic cell-specific transmembrane protein in a STAT6-dependent manner. E-cadherin expression was critical for the formation of MNG cells by IL-4; an anti-E-cadherin antibody prevented the formation of large MNG. In addition, we found that STAT6(-/-) progenitors failed to fuse with STAT6(+/+), revealing the need for a homotypic interaction. Thus, IL-4 promotes the formation of MNG in a STAT6-dependent manner by regulating cell surface expression of E-cadherin, leading to homotypic cell fusion and the incorporation of large foreign bodies.

IL-4 PROTECTS THE CH31 B-CELL LINE FROM ANTI-IgM-INDUCED GROWTH ARREST AND APOPTOSIS: CONTRIBUTION OF THE PI-3′ KINASE/AKT PATHWAY (84.11)

Interleukin-4 (IL-4) promotes lymphocyte survival and protects primary lymphomas from apoptosis. Previous studies reported differential requirements for the STAT6 and IRS2/PI-3′ kinase signaling pathways in mediating the IL-4-induced protection from Fas-mediated apoptosis. In this study, we characterized IL-4-activated signals that suppress anti-IgM-mediated apoptosis and growth arrest of CH31, a model B-cell lymphoma line. In these cells, anti-IgM treatment led to mitochondrial dysfunction and the loss of activation-specific phosphorylation of Akt and of CDK2. These were followed by caspase activation, loss of BAD phosphorylation, growth-arrest and then apoptosis. Strikingly, IL-4 treatment prevented or reversed these changes, dramatically rescuing the cells from Go/G1-arrest and driving them back into S- and G2/M- phases of the cell cycle. Furthermore, IL-4 suppressed the activation of caspases 9 and 3 and, in contrast to previous reports, induced the phosphorylation (deactivation) of BAD. IL-4 treatment also induced expression of BclxL, a STAT6-dependent gene. Finally, pharmacologic inhibitors of PI-3′ kinase, of Akt and of JAK/STAT signaling and dominant inhibitory forms of PI-3′ kinase and Akt, abrogated IL-4 anti-apoptotic function. These results suggest that the IL-4 receptor activates several signaling pathways that act to suppress the apoptotic program activated by anti-IgM.

High levels of ROS block the differentiation of human and mouse osteoclasts (100.5)

During chronic inflammation at skeletal sites, production of inflammatory cytokines by cells from the joint tissues and recruited activated T cells generate a pro-inflammatory environment with high levels of Reactive Oxygen Species (ROS). A critical cytokine expressed by the joint tissue and by the activated T cells is RANKL, which exerts its activity by inducing the differentiation of osteoclasts (OC) from cells of the monocyte-macrophage lineage. The effect of RANKL on the OC is mediated by a transient generation of ROS which are critical in regulating RANKL signaling. Therefore, we investigated the effect of increased ROS levels on OC differentiation from human and mouse progenitors. We found that addition of the glutathione depleting drug BSO to the cultures blocked OC generation from CD14+ human PBMC and from mouse bone marrow cells. This blockage was reversible; withdrawal of BSO allowed for the recovery of OC formation. The level of recovery diminished with increasing concentrations of BSO. The inhibition was time dependent, and it had a more profound effect if monocytes were the precursor cells rather than macrophages. Thus, while RANKL-induced OC development requires ROS, excess ROS can inhibit OC development.

(Supported by PHS AI59775 (ADK) AI49807 (MSW))

Effect of IL-4 and IL-13 on the chemotaxis of mouse eosinophils (38.2)

Eosinophils are a major pathological component of the inflammatory cell infiltrate found in lungs as a result of allergic disease. They migrate into lungs in response to a variety of chemokines. The development and effector phases of the allergic response are dependent on the actions of interleukin (IL)-4 and IL-13 on a wide variety of cell types. However, how these two cytokines directly affect eosinophil signaling, function and chemotaxis is largely unknown. Using mouse eosinophils purified from IL-5 transgenic mice and a transwell migration assay, eotaxin-1 (30nM) induced eosinophil migration 6–11x over background. IL-4 alone was able to induce eosinophil migration 2x over background. More eosinophils migrated in response to eotaxin when IL-14 or IL-13 was also placed in the bottom chamber. IL-4 appeared to be more potent than IL-13. This effect was dependent upon cytokine concentration. Interestingly, when eosinophils were pretreated with IL-4 for 30 minutes prior to chemotaxis, migration to eotaxin-1 was enhanced 4-fold. However, pretreatment with IL-13 did not enhance chemotaxis to eotaxin-1. These data suggest that IL-4 and IL-13 have direct and rapid effects on the chemotactic function of eosinophils with some differences. We are investigating the mechanisms of these effects, as well as other signaling and functional responses of eosinophils to these cytokines. (AI38985 and HLT32007698)

Complex role of STAT6 in allergic airway inflammation (39.11)

STAT6 plays a critical role in Th2 cell differentiation and in allergic lung inflammation. Using a chimeric mouse model, we observed alternative lung pathology in STAT6 KO mice even when WT bone marrow or Th2 cells were provided. Thus, we hypothesized that STAT6 contributes to inflammation in a complex manner. To detail STAT6 function, WT and STAT6 KO mice were subjected to OVA priming and challenges. Broncho-alveolar lavage (BAL) cell composition, lung histology, and FACS analysis of digested lungs were assessed 48h after the last challenge. As expected, eosinophils composed a majority of BAL cells in WT mice and less than 2% in STAT6 KO mice. The OVA-induced inflammation in STAT6 KO lungs was composed mainly of macrophages with small fractions of neutrophils and lymphocytes. The OVA-treated WT lungs showed strong although divergent expression of F4/80, Mac-2, and CD11b molecules; this was significantly reduced in STAT6 KO mice. However, the numbers of dendritic cells, B cells, CD4+ and CD8+ T cells in the lungs of OVA-treated mice were unaffected by STAT6 deficiency. Interestingly, STAT6 KO mice showed enhanced basal airway reactivity to methacholine and numbers of Mac-2+ cells as compared to WT mice, suggesting that STAT6 deficiency altered lung homeostasis. Taken together, our studies demonstrate STAT6-dependent and –independent features of asthma phenotype which may impact treatments targeting STAT6. (AI38985)

Novel approach to inhibit asthma-mediated lung inflammation using anti-CD147 intervention

Extracellular cyclophilins have been well described as chemotactic factors for various leukocyte subsets. This chemotactic capacity is dependent upon interaction of cyclophilins with the cell surface signaling receptor CD147. Elevated levels of extracellular cyclophilins have been documented in several inflammatory diseases. We propose that extracellular cyclophilins, via interaction with CD147, may contribute to the recruitment of leukocytes from the periphery into tissues during inflammatory responses. In this study, we examined whether extracellular cyclophilin-CD147 interactions might influence leukocyte recruitment in the inflammatory disease allergic asthma. Using a mouse model of asthmatic inflammation, we show that 1) extracellular cyclophilins are elevated in the airways of asthmatic mice; 2) mouse eosinophils and CD4+ T cells express CD147, which is up-regulated on CD4+ T cells upon activation; 3) cyclophilins induce CD147-dependent chemotaxis of activated CD4+ T cells in vitro; 4) in vivo treatment with anti-CD147 mAb significantly reduces (by up to 50%) the accumulation of eosinophils and effector/memory CD4+ T lymphocytes, as well as Ag-specific Th2 cytokine secretion, in lung tissues; and 5) anti-CD147 treatment significantly reduces airway epithelial mucin production and bronchial hyperreactivity to methacholine challenge. These findings provide a novel mechanism whereby asthmatic lung inflammation may be reduced by targeting cyclophilin-CD147 interactions.

Chronic morphine treatment promotes specific Th2 cytokine production by murine T cells in vitro via a Fas/Fas ligand-dependent mechanism

Improper homeostasis of Th1 and Th2 cell differentiation can promote pathological immune responses such as autoimmunity and asthma. A number of factors govern the development of these cells including TCR ligation, costimulation, death effector expression, and activation-induced cell death (AICD). Although chronic morphine administration has been shown to selectively promote Th2 development in unpurified T cell populations, the direct effects of chronic morphine on Th cell skewing and cytokine production by CD4(+) T cells have not been elucidated. We previously showed that morphine enhances Fas death receptor expression in a T cell hybridoma and human PBL. In addition, we have demonstrated a role for Fas, Fas ligand (FasL), and TRAIL in promoting Th2 development via killing of Th1 cells. Therefore, we analyzed whether the ability of morphine to affect Th2 cytokine production was mediated by regulation of Fas, FasL, and TRAIL expression and AICD directly in purified Th cells. We found that morphine significantly promoted IL-4 and IL-13 production but did not alter IL-5 or IFN-gamma. Furthermore, morphine enhanced the mRNA expression of Fas, FasL and TRAIL and promoted Fas-mediated AICD of CD4(+) T cells. Additionally, blockade of Fas/FasL interaction by anti-FasL inhibited the morphine-induced production of IL-4 and IL-13 and AICD of CD4(+) T cells. These results suggest that morphine preferentially enhances Th2 cell differentiation via killing of Th1 cells in a Fas/FasL-dependent manner.

Induction of uteroglobin-related protein 2 (Ugrp2) gene expression by the Th2 cytokines IL-4 and IL-13

Uteroglobin-related proteins 1 and 2 (UGRP1 and -2) are thought to play important roles in inflammation and immunologic responses in the lung. In this study we demonstrate that IL-4 and IL-13 enhance Ugrp2 gene expression in the mouse transformed Clara cell line, mtCC, in a time- and dose-dependent manner. Addition of actinomycin D abrogated the IL-4- and IL-13-induced increase of Ugrp2 expression, demonstrating that this increase occurs at the transcriptional level. When mtCC cells were pretreated with IFN-gamma before the addition of IL-4 or IL-13, IL-4- and 13-induced Ugrp2 mRNA increase was markedly decreased. IL-4 and IL-13 induced phosphorylation of STAT6 in mtCC cells, which binds to the proximal STAT-binding element (SBE) in the Ugrp2 gene promoter, leading to transcriptional activation of this gene. Mutations of the proximal SBE abrogated the binding of activated STAT6 to this site and the IL-4-induced increase in Ugrp2 gene promoter activity. IFN-gamma-activated STAT1 binds to the same SBE in the Ugrp2 gene promoter to which STAT6 binds and decreases the binding of STAT6 to this site. Furthermore, an IL-4-induced increase in Ugrp2 expression was not observed in primary cultures of lung cells derived from STAT6-deficient mice. These results indicate that Ugrp2 expression is enhanced by IL-4 and IL-13 through STAT6 binding to the proximal SBE located in the Ugrp2 gene promoter.

Interleukin-4 (IL-4) pathway

Interleukin-4 (IL-4) is a cytokine produced by T(H)2 type helper T cells and by mast cells, basophils, and eosinophils. This cytokine can elicit many responses, some of which are associated with allergy and asthma. Studies with long-term cell lines and primary cells have revealed differences in the signaling between these two experimental systems. Understanding these differences is important because therapeutic strategies targeting IL-4 and its signaling pathways are currently being tested to treat allergy and asthma.

Interleukin-13 (IL-13) pathway

Interleukin-13 (IL-13), like IL-4, is a cytokine produced by T(H)2 type helper T cells in response to signaling through the T cell antigen receptor and by mast cells and basophils upon cross-linkage of the high-affinity receptor for immunoglobulin E (IgE). It is also produced by activated eosinophils. IL-13 induces many of the same responses as IL-4 and shares a receptor subunit with IL-4. IL-13 has been implicated in airway hypersensitivity and mucus hypersecretion, inflammatory bowel disease, and parasitic nematode expulsion.

Abnormal Th1 cell differentiation and IFN-gamma production in T lymphocytes from motheaten viable mice mutant for Src homology 2 domain-containing protein tyrosine phosphatase-1

Src homology 2 domain-containing protein tyrosine phosphatase-1 (SHP-1) plays an important role in T and B lymphocyte signaling; however, the function of SHP-1 in Th cell differentiation, in particular, the Th1 response, has not been defined. In this study, we provide evidence that SHP-1 phosphatase negatively regulates Th1 cell development and IFN-gamma production. Compared with the wild-type control, anti-CD3-activated mouse T lymphocytes carrying the motheaten viable mutation in the SHP-1 gene produced a significantly increased amount of IFN-gamma in the presence of IL-12. This increase was also seen at the basal level without IL-12 addition. Similarly, Th1 cell differentiation and proliferation of anti-CD3-activated SHP-1 mutant lymph node cells in the presence or absence of IL-12 were markedly enhanced, indicating a negative role for SHP-1 phosphatase in such lymphocyte activities. Interestingly, IL-12-induced activation of Jak2 and STAT4, critical components for IL-12-mediated cellular responses, was shortened or attenuated in mutant T cells. Together these results suggest that SHP-1 negatively regulates Th1 cell development and functions through a mechanism that is not directly related to IL-12 signaling.

The opioid antagonist naltrexone blocks acute endotoxic shock by inhibiting tumor necrosis factor-alpha production

Septic shock is believed to be a consequence of excessive stimulation of the immune system by bacterial toxins that results in systemic overproduction of proinflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha), IL-1, and IL-6. Various studies have shown that TNF-alpha, a major mediator of septic shock, induces tissue injury, loss of blood pressure, organ failure, and ultimately death. Administration of the opioid antagonist naloxone has been reported to reverse opiate-mediated hypotension, promote organ perfusion and increase patient survival. In this study, we examined the mechanism by which the opioid receptor antagonist, naltrexone, modulates the septic shock response in BALB/c mice after injection with lipopolysaccharide (LPS) or staphylococcal enterotoxin B (SEB) in combination with d-galactosamine (d-gal), or with agonistic anti-Fas antibody (Jo2) alone. Each of these treatments induced rapid-onset, acute shock, and ultimately mortality (6-9h after injection), although different mechanisms are involved. Administration of the opioid antagonist naltrexone protected mice from shock induced by LPS+d-gal, but not SEB+d-gal or Jo2 antibody, a protective effect that was reversed by morphine. Naltrexone significantly inhibited the production of TNF-alpha induced by LPS, but not SEB in vivo. When bone marrow-derived, splenic or peritoneal macrophages were treated with LPS in vitro, administration of naltrexone had no direct effect on TNF-alpha production. These results suggest that naltrexone is capable of preventing LPS-induced septic shock mortality by indirect inhibition of TNF-alpha production in vivo.

Complex role of the IL-4 receptor alpha in a murine model of airway inflammation: expression of the IL-4 receptor alpha on nonlymphoid cells of bone marrow origin contributes to severity of inflammation

Recent studies have suggested the IL-4Ralpha expressed on lung epithelium is necessary for TH2-mediated goblet cell differentiation and mucus hypersecretion in a murine model of allergic lung disease. However, the IL-4Ralpha is expressed on numerous cell types that could contribute to the overall pathology and severity of asthma. The relative role of the receptor on these cells has not yet been conclusively delineated. To dissect the contribution of IL-4Ralpha in the development of pulmonary allergic responses, we generated murine radiation bone marrow (BM) chimeras. BM from IL-4Ralpha(+) or IL-4Ralpha(-) mice was transferred into recipient mice that expressed or lacked IL-4Ralpha. In the absence of IL-4Ralpha in recipient mice, there was no goblet cell metaplasia or mucus hypersecretion in response to OVA, even in the presence of TH2 cells and substantial eosinophilic infiltration. More importantly, we found that expression of the IL-4Ralpha on a nonlymphoid, MHC class II(+), BM-derived cell type contributes to the severity of inflammation and mucus production. These results suggest that IL-4 and IL-13 contribute to the development of allergic inflammation by stimulating a complex interaction between IL-4Ralpha(+) cell types of both bone marrow and non-bone marrow origin.

Increased numbers of committed myeloid progenitors but not primitive hematopoietic stem/progenitors in mice lacking STAT6 expression

Signal transducer and activator of transcription-6 (STAT6) plays important roles in cytokine signaling via interleukin-4 and -13 receptors (IL-4R and IL-13R). Mice in which STAT6 has been disrupted by homologous recombination show defects in T helper cell type 2 (Th2) lymphocyte production, resulting in an accumulation of Th1 cells. In addition to defects in differentiation and proliferation of T lymphocytes, STAT6-deficient mice show increased cell-cycle activation and frequency of myeloid progenitors. Although this has been shown to be mediated through Oncostatin M production by T cells, IL-4Ralpha and STAT6 have also recently been found to be enriched for expression in primitive hematopoietic stem cells (HSCs) in gene expression-profiling studies. Therefore, we have investigated whether defects in hematopoietic function in mice lacking STAT6 expression extended into the primitive hematopoietic compartments of the bone marrow. Here, we report that STAT6 deficiency increased bone marrow-committed myeloid progenitors but did not alter the number of cells enriched for HSC/multipotent progenitors, primitive cobblestone area-forming cells assayed in vitro, or bone marrow short-term or long-term repopulating cells assayed in vivo. Therefore, the requirement for STAT6 activation during hematopoiesis is limited, and primitive hematopoietic cell types are insulated against possible effects of cytokine stimulation by Th1 cells.

Transgenic Expression of Insulin Receptor Substrate 2 in Murine B Cells Alters the Cell Density-Dependence of IgE Production In Vitro and Enhances IgE Production In Vivo

Previous studies have shown that insulin receptor substrate (IRS)1 and IRS2 mediate proliferative and antiapoptotic signaling through the IL-4R in 32D cells; however their role in regulating normal B cell responses is not clear. To investigate the role of IRS2 in normal B cell function, we developed IRS2 transgenic (Tg) mice on the C57BL/6 background. Western blot analysis revealed a 2-fold elevation in IRS2 protein levels in Tg(+) mice compared with littermate controls and a 3-fold increase in basal tyrosine phosphorylated IRS2 in the absence of IL-4 stimulation. IL-4-induced tyrosine phosphorylation of IRS2 was elevated in Tg(+) B cells, whereas IL-4-induced phosphorylation of STAT6 was similar between Tg(+) and Tg(-) B cells. Tg expression of IRS2 had little effect on IL-4-mediated proliferation and no effect on protection from apoptosis. However, production of IgE and IgG1 by Tg(+) B cells using standard in vitro conditions was diminished 50-60%. Because Ig production in vitro is known to be highly cell concentration-dependent, we performed experiments at different cell concentrations. Interestingly, at very low B cell concentrations (1000-5000 B cells/well), IgE and IgG1 production by Tg(+) B cells was greater than that of controls, whereas at higher cell concentrations (10,000-20,000 cells/well) Ig production by Tg(+) B cells was less than controls. Furthermore, in vivo immunization with OVA-alum or goat anti-IgD resulted in elevated serum IgE levels in the Tg(+) mice. These results indicate that overexpression of IRS2 alters the B cell intrinsic density-dependence of IgE and IgG1 production in vitro and enhances IgE responses in vivo.

Phosphatidylcholine-Specific Phospholipase C Activity Is Necessary for the Activation of STAT6

It is well established that Janus kinase (JAK) tyrosine kinases play a key role in the activation of STAT6 by IL-4. In this study, we investigated additional molecules involved in this process. We previously found that IL-4 and TNF-alpha cooperate in the activation of STAT6 and NF-kappaB, suggesting that these transcription factors are regulated by common intracellular signaling pathways. To test this hypothesis, we analyzed the effect of known inhibitors of NF-kappaB on the activation of STAT6. We discovered that inhibitors of phosphatidylcholine-specific phospholipase C (PC-PLC), but not other lipases, blocked the activation of STAT6 by IL-4. The activation of PC-PLC seems to be an early event in IL-4 signaling, because its inhibition abrogated JAK activation and STAT6 tyrosine phosphorylation. Interestingly, we found that the effects of pervanadate and sodium orthovanadate on STAT6 activation correspond to their effect on PC-PLC. Thus, pervanadate by itself activated PC-PLC, JAK, and STAT6, whereas sodium orthovanadate suppressed PC-PLC, JAK, and STAT6 activation by IL-4. We further found that PC-PLC activation is necessary but not sufficient to promote STAT6 activation, and therefore, additional intracellular pathways regulated by IL-4 and pervanadate may collaborate with PC-PLC to signal STAT6 activation. It has been reported that IL-4 signals PC-PLC activation; in this study, we provide evidence that this phospholipase plays a key role in IL-4 signaling.

New programming of IL-4 receptor signal transduction in activated T cells: Stat6 induction and Th2 differentiation mediated by IL-4Ralpha lacking cytoplasmic tyrosines

Signaling by the IL-4 receptor alpha-chain (IL-4Ralpha) is a key determinant of the development of the Th2 lineage of effector T cells. Studies performed in tissue culture cell lines have indicated that tyrosines of the IL-4Ralpha cytoplasmic tail are necessary for the induction of Stat6, a transcription factor required for Th2 differentiation. Surprisingly, we have found that in activated T cells, IL-4Ralpha chains lacking all cytoplasmic tyrosines promote induction of this IL-4-specific transcription factor and efficient commitment to the Th2 lineage. Mutagenesis of a tyrosine-free cytoplasmic tail identifies a requirement for the serine-rich ID-1 region in this new program of IL-4R signal transduction observed in activated T cells. Additional findings suggest that an extracellular signal-regulated kinase pathway can be necessary and sufficient for the ability of such tyrosine-free IL-4Ralpha chains to mediate Stat6 induction. These results provide novel evidence that the molecular mechanisms by which a cytokine specifically induces a Stat transcription factor can depend on the activation state of T lymphoid cells. Furthermore, the data suggest that one pathway by which such new programming may be achieved is mediated by extracellular signal-regulated mitogen-activated protein kinases.

IL-4 suppresses osteoclast development and mature osteoclast function by a STAT6-dependent mechanism: irreversible inhibition of the differentiation program activated by RANKL

Numerous reports have described the effects of interleukin-4 (IL-4) on bone biology. Previous studies, performed using complex coculture systems, demonstrated the effects of IL-4 on osteoblasts and osteoclasts. To directly test the effect of IL-4 on osteoclasts, we took advantage of a simplified system using recombinant receptor activator of nuclear factor kappaB ligand (RANKL) as the osteoclast differentiation factor. We analyzed the ability of IL-4 to directly regulate osteoclast differentiation and mature osteoclast function. We found that IL-4 inhibited the differentiation of osteoclasts from bone marrow precursors in an irreversible manner and also inhibited the resorptive capacity of mature osteoclasts. In the presence of IL-4, we detected the appearance of tartrate-resistant acid phosphatase (TRAP)-negative multinucleated giant (MNG) cells. Both IL-4 effects were dependent on signal transducer and activator of transcription 6 (STAT6). We found that IL-4 suppresses RANK mRNA expression in the developing precursor cells. When RANK was ectopically expressed under the cytomegalovirus (CMV) promoter in RAW264.7 macrophages, IL-4 treatment did not inhibit osteoclast development. Furthermore, when osteoclastogenesis was induced independently of RANKL by using tumor necrosis factor-alpha (TNF-alpha), IL-4 inhibited osteoclast differentiation through a STAT6-dependent mechanism. These results suggest that IL-4 regulates osteoclast development by regulating gene expression, including RANK. We propose that IL-4 irreversibly regulates the lineage commitment of precursor cells by regulating gene expression, resulting in the suppression of osteoclast development and the generation of MNG cells as an alternative pathway of differentiation.

Interleukin-4 and interleukin-13 signaling connections maps

Cytokines are inflammatory mediators important in responding to pathogens and other foreign challenges. Interleukin-4 (IL-4) and IL-13 are two cytokines produced by T helper type 2 cells, mast cells, and basophils. In addition to their physiological roles, these cytokines are also implicated in pathological conditions such as asthma and allergy. IL-4 can stimulate two receptors, type I and type II, whereas IL-13 signaling is mediated only by the type II receptor (see the STKE Connections Maps). These cytokines activate the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling cascades, which may contribute to allergic responses. In addition, stimulation of the phosphatidylinositol 3-kinase (PI3K) pathway through recruitment of members of the insulin receptor substrate family may contribute to survival and proliferation.

Regulation of the dephosphorylation of Stat6. Participation of Tyr-713 in the interleukin-4 receptor alpha, the tyrosine phosphatase SHP-1, and the proteasome

Signal transducer and activator of transcription 6 (Stat6) plays an important role in interleukin (IL)-4-induced responses. To analyze the regulation of Stat6 phosphorylation, cells were cultured in the continuous presence of IL-4 or after a pulse and washout. In the continual presence of IL-4, Stat6 remained phosphorylated for an extended period. After IL-4 removal and inhibition of the Janus family kinase, tyrosine-phosphorylated Stat6 decayed at a rate dependent upon the length of IL-4 stimulation. The decay of tyrosine-phosphorylated Stat6 was similar in the presence or absence of either cycloheximide or actinomycin D. In the absence of functional Src homology-containing phosphatase-1 (SHP-1), the early loss of tyrosine-phosphorylated Stat6 was substantially reduced. Furthermore, the rate of loss of tyrosine-phosphorylated Stat6 in cells expressing a mutation of the human IL-4 receptor alpha in the immunoreceptor tyrosine-based inhibitory motif sequence (Y5F) was dramatically decreased compared with wild-type cells. The early rate of decay was similar in the presence or absence of MG132, an inhibitor of the proteasome, but the later rate of decay was decreased 5-fold. These results suggest that the loss of tyrosine phosphorylation of Stat6 is regulated by the action of SHP-1 and the proteasome but is not dependent on new protein synthesis.

Reciprocal expression of TRAIL and CD95L in Th1 and Th2 cells: role of apoptosis in T helper subset differentiation

Upon activation, naïve T helper cells can differentiate into two major distinct subsets, T helper 1 (Th1) and T helper 2 (Th2), as defined by their effector functions and cytokine secretion patterns. Cytokine milieu and costimulatory molecules have been shown to play an essential role in determining T helper differentiation. However, it is still unclear how the effects of signals of costimulatory molecules and cytokines are exerted during T helper differentiation. We show evidence suggesting that while cytokine signals initiate the differentiation program, the selective action of death effectors determines the end point balance of differentiating T helper subsets. We examined the expression of TNF-related apoptosis-inducing ligand (TRAIL) and CD95L in cloned and in vitro differentiated Th1 and Th2 cells. We found that activation-induced expression of TRAIL is exclusively observed in Th2 clones and primary T helper cells differentiated under the Th2 condition, while the expression of CD95L is mainly in Th1 cells. Furthermore, these two subsets exhibit distinct susceptibilities to TRAIL- and CD95L-mediated apoptosis. Th2 cells are more resistant to either TRAIL- or CD95L-induced apoptosis than Th1 cells. More importantly, both Th1 and Th2 cells could induce apoptosis in labeled Th1 but not Th2 cells. Blocking TRAIL and CD95L significantly enhance IFN-gamma production in vitro. Likewise, young MRL/MpJ-lpr/lpr mice also showed more Th1 response to ovalbumin immunization as compared to MRL/MpJ+/+. Therefore, apoptosis mediated by CD95L and TRAIL is critical in determining the fate of differentiating T helper cells.