Fes mediates the IL-4 activation of insulin receptor substrate-2 and cellular proliferation

Although Jak kinases are essential for initiating cytokine signaling, the role of other nonreceptor tyrosine kinases in this process remains unclear. We have examined the role of Fes in IL-4 signaling. Examination of Jak1-deficient cell lines demonstrates that Jak1 is required for the activation of Fes by IL-4. Experiments studying signaling molecules activated by IL-4 receptor suggest that IL-4 signaling can be subdivided into Fes-dependent and Fes-independent pathways. Overexpression of kinase-inactive Fes blocks the IL-4 activation of insulin receptor substrate-2, but not STAT6. Fes appears to be a downstream kinase from Jak1/Jak3 in this process. Further examination of downstream signaling demonstrates that kinase-inactive Fes inhibits the recruitment of phosphoinositide 3-kinase to the activated IL-4 receptor complex and decreases the activation of p70(S6k) kinase in response to IL-4. This inhibition correlates with a decrease in IL-4-induced proliferation. In contrast, mutant Fes does not inhibit the activation of Akt by IL-4. These data demonstrate that signaling pathways activated by IL-4 require different tyrosine kinases. This differential requirement predicts that specific kinase inhibitors may permit the disruption of specific IL-4-induced functions.

Positive regulation of interleukin-4-mediated proliferation by the SH2-containing inositol-5'-phosphatase

The SH2-containing inositol 5'-phosphatase (SHIP) is tyrosine-phosphorylated in response to cytokines such as interleukin (IL)-3, granulocyte-macrophage colony-stimulating factor, and macrophage colony-stimulating factor. SHIP has been shown to modulate negatively these cytokine signalings; however, a potential role in IL-4 signaling remains uncharacterized. It has been recently shown that IL-4 induces tyrosine phosphorylation of SHIP, implicating the phosphatase in IL-4 processes. Tyrosine kinases, Jak1 and Jak3, involved in IL-4 signaling can associate with SHIP, yet only Jak1 can tyrosine-phosphorylate SHIP when co-expressed. In functional studies, cells overexpressing wild type SHIP are found to be hyperproliferative in response to IL-4 in comparison to parental cells. In contrast, cells expressing catalytically inactive form, SHIP(D672A), show reduced proliferation in response to IL-4. These changes in IL-4-induced proliferation correlate with alterations in phosphatidylinositol 3,4,5-triphosphate levels. However, no differential activation of STAT6, Akt, IRS-2, or p70(S6k), in response to IL-4, was observed in these cells. These data suggest that the catalytic activity of SHIP acts in a novel manner to influence IL-4 signaling. In addition, these data support recent findings that suggest there are uncharacterized signaling pathways downstream of phosphatidylinositol 3,4,5-triphosphate.

IL-4/IL-13 signaling beyond JAK/STAT

In the past several years, extensive studies on the mechanisms underlying IL-4 and IL-13 signaling have enabled us to gain insight into how these cytokines regulate immune responses. Because both IL-4 and IL-13 use the IL-4Ralpha as a receptor component, these cytokines activate many common signaling pathways. Both of these cytokines use Janus kinases (JAKs) to initiate signaling and activate signal transducer and activator of transcription-6 (STAT6), which is a transcription factor required for many of their biologic functions. In addition to JAK/STAT, these cytokines also activate a variety of other signaling molecules that are important in regulating IL-4-induced proliferation and protection from apoptosis. Suppressor of cytokine signaling-1 (SOCS-1) is a molecule that can inhibit the activation of IL-4 signaling through the inhibition of JAKs. The Fes tyrosine kinase is activated by IL-4 and appears to be important in regulating IL-4-induced proliferation through the phosphorylation of insulin receptor substrate (IRS) molecules. IRS molecules are essential for IL-4-induced proliferation through their ability to recruit phosphoinositol-3 kinase to the activated IL-4 receptor kinase. In addition, IL-4 can activate a number of phosphatases including SH2-containing inositol phosphatase (SHIP), SHP-1, and SHP-2. Finally, B-cell lymphoma gene-6 (BCL-6) appears to regulate a subset of IL-4-induced genes. Thus the biologic responses induced by IL-4/IL-13 require a complex interaction of signaling pathways and regulators.

JAK-STAT signaling activated by Abl oncogenes

The Abl oncoproteins v-Abl and BCR-Abl can activate member of the signal transducers and activators of transcription (STAT) family of signaling proteins. The mechanisms by which these oncoproteins activate STATs appear to differ. In cells transformed by v-Abl, Janus kinase (JAK) tyrosine kinases are constitutively activated. In these cells, the v-Abl oncoprotein and the JAK kinases physically associate. Mapping of the JAK interaction domain in v-Abl demonstrates that amino acids within the carboxyl terminal region of v-Abl bind JAKs through a direct interaction. A mutant of v-Abl lacking this region does not bind or activate JAK 1 in vivo, fails to activate STAT proteins, does not induce cellular proliferation, and is less efficient in cellular transformation. Kinase inactive mutants of JAK 1 inhibit the ability of v-Abl to activate STATs, to induce cytokine-independent proliferation, and to transform bone marrow cells. Interestingly, these effects correlate with defects in the activation of several pathways by v-Abl including Akt, PI3-kinase, STATs, and Ras. These data suggest that Jak kinases may play an important role in v-Abl induced transformation. Oncogene (2000).

Biologic functions of the IFN-gamma receptors

Interferon-gamma (IFN-gamma) is a cytokine that plays an important role in inducing and modulating an array of immune responses. Cellular responses to IFN-gamma are mediated by its heterodimeric cell-surface receptor (IFN-gammaR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. In order to study the role of IFN-gamma in a number of immune responses and pathways, researchers have generated mice with altered patterns of IFN-gammaR gene expression. These studies, together with analyses of naturally occurring mutations of the IFN-gammaR in man, have been instrumental in elucidating the diverse functions of IFN-gamma, and are the subject of this review.

IL-4 receptor mutations

IL-4 plays an important role in regulating immune responses. Distinct signaling pathways, including those for gene activation and cell differentiation and those for cell proliferation and protection from apoptosis, are initiated from the receptor complex for IL-4 following ligand-receptor engagement. Several advances have been made in our understanding of how distinct functions of IL-4 are mediated. Most of these studies employed artificial mutations of the IL-4-receptor alpha chain using site-directed mutagenesis and/or deletional mutation. In addition, naturally occurring mutations of the IL-4-receptor alpha chain have been identified and implicated as a genetic predisposition for allergic disorders. The results of these studies suggest a modular organization of the receptor and an independent regulation of gene activation and cell growth.

Cutting Edge: SOCS-1 Is a Potent Inhibitor of IL-4 Signal Transduction

IL-4 is an important regulator of the activation, proliferation, and differentiation of many hematopoetic cells. Many of these biological effects result from the activation of Janus kinases (JAK)1 and JAK3 and the transcription factor Stat6. Recent data suggest that members of the SOCS (suppressor of cytokine signaling) family of proteins can inhibit JAK-STAT signaling. We have examined the ability of SOCS family members to suppress IL-4 signaling, and we have found that SOCS-1 potently inhibits the activation of JAK1 kinase and Stat6 in response to IL-4. Furthermore, SOCS-1 can inhibit the induction of CD23 expression by IL-4. SOCS-2 does not inhibit induction of signaling by IL-4, while inhibition of IL-4 signaling by SOCS-3 can be detected in transient transfection systems, but not in stable cell lines. These studies implicate SOCS-1 in modulation of IL-4 signaling and suggest that SOCS-1 may play a role in regulating the immune response.

Cutting edge: SOCS-1 is a potent inhibitor of IL-4 signal transduction

IL-4 is an important regulator of the activation, proliferation, and differentiation of many hematopoetic cells. Many of these biological effects result from the activation of Janus kinases (JAK)1 and JAK3 and the transcription factor Stat6. Recent data suggest that members of the SOCS (suppressor of cytokine signaling) family of proteins can inhibit JAK-STAT signaling. We have examined the ability of SOCS family members to suppress IL-4 signaling, and we have found that SOCS-1 potently inhibits the activation of JAK1 kinase and Stat6 in response to IL-4. Furthermore, SOCS-1 can inhibit the induction of CD23 expression by IL-4. SOCS-2 does not inhibit induction of signaling by IL-4, while inhibition of IL-4 signaling by SOCS-3 can be detected in transient transfection systems, but not in stable cell lines. These studies implicate SOCS-1 in modulation of IL-4 signaling and suggest that SOCS-1 may play a role in regulating the immune response.

IL-7 reconstitutes multiple aspects of v-Abl-mediated signaling

The mechanism by which early lymphoid cells are selectively transformed by v-Abl is currently unknown. Previous studies have shown constitutive activation of IL-4 and IL-7 signaling pathways, as measured by activation of Janus protein kinase (JAK)1, JAK3, STAT5, and STAT6, in pre-B cells transformed by v-Abl. To determine whether activation of these cytokine signaling pathways by v-Abl is important in the cellular events induced by the Abelson murine leukemia virus, the effects of IL-4 and IL-7 on pre-B cells transformed with a temperature-sensitive v-Abl mutant were examined. Whereas IL-4 had little or no effect, IL-7 delayed both the apoptosis and cell cycle arrest that occur upon v-Abl kinase inactivation. IL-7 also delayed the decreases in the levels of c-Myc, Bcl-2, and Bcl-xL that occur upon loss of v-Abl kinase activity. IL-7 did not maintain v-Abl-mediated differentiation arrest of the pre-B cells, as activation of NF-kappaB and RAG gene transcription was unaffected by IL-7. These results identify a potential role for IL-7 signaling pathways in transformation by v-Abl while demonstrating that a combination of IL-4 and IL-7 signaling cannot substitute for an active v-Abl kinase in transformed pre-B cells.

Analysis of cytokine signaling in patients with extrinsic asthma and hyperimmunoglobulin E

BACKGROUND

Recent data suggest that the regulation of class switching to IgE by cytokines is mediated by STAT transcription factors. The induction of IgE by IL-4 and IL-13 occurs through the activation of the intracellular signal-transducing protein Stat6, whereas the inhibition of IgE class switching by interferon-y (IFN-gamma) occurs through the activation of Statl.

OBJECTIVE

We hypothesized that in extrinsic asthma or in cases of markedly elevated IgE (ie, hyperimmunoglobulin E [HIE]) increased levels of IgE may be associated with alterations in the cytokine levels or the activation of Stat6.

METHODS

PBMCs and sera from 8 patients with extrinsic asthma (mean IgE, 285+/-100 IU/mL), 3 patients with HIE (mean IgE, 7050+/-1122 IU/mL), and 14 nonatopic control subjects (mean IgE, 112+/-28 IU/mL) were analyzed.

RESULTS

The mean IL-4 level detected by ELISA was much greater in patients with HIE than control subjects (88.6+/-11.5 pg/mL vs 11.5+/-7.1 pg/mL, P = .005), and increased IL-4 levels among patients with both asthma and HIE correlated with the increased IgE levels. In contrast, IL-13 levels were not elevated. Levels of Stat6 protein present in PBMCs did not differ in the patients and control subjects. Examination of Stat6 DNA-binding activity demonstrated no activation of IL-4 signaling in patients with either HIE or acute asthma. Interestingly, evidence for the presence of B cells that have already switched to IgE was seen in PBMCs of several patients with asthma or HIE.

CONCLUSION

These results indicate that (1) IgE production in asthma and HIE usually is associated with elevated levels of IL-4, but not IL-13, in the peripheral blood; (2) the increased sera IL-4 levels in asthma and HIE are not sufficient to induce Stat6 activation in PBMCs; and (3) evidence of switch recombination to epsilon may be detected in isolated cases of elevated IgE. This implies that high levels of IgE in these patients either results from B cells that have already undergone class switching, from Ig class switching that is localized to target tissues, or both.

Relationship of cytokines and cytokine signaling to immunodeficiency disorders in the mouse

The contributions of cytokines to the development and progression of disease in a mouse model of retrovirus-induced immunodeficiency (MAIDS) are controversial. Some studies have indicated at etiologic role for type 2 cytokines, while others have emphasized the importance of type 1 cytokines. We have used mice deficient in expression of IL-4, IL-10, IL-4 and IL-10, IFN-gamma, or ICSBP-a transcriptional protein involved in IFN signaling-to examine their contributions to this disorder. Our results demonstrate that expression of type 2 cytokines is an epiphenomenon of infection and that IFN-gamma is a driving force in disease progression. In addition, exogenously administered IL-12 prevents many manifestations of disease while blocking retrovirus expression. Interruption of the IFN signaling pathways in ICSBP-/- mice blocks induction of MAIDS. Predictably, ICSBP-deficient mice exhibit impaired responses to challenge with several other viruses. This immunodeficiency is associated with impaired production of IFN-gamma and IL-12. Unexpectedly, however, the ICSBP-/- mice also develop a syndrome with many similarities to chronic myelogenous leukemia in humans. The chronic phase of this disease is followed by a fatal blast crisis characterized by clonal expansions of undifferentiated cells. ICSBP is thus an important determinant of hematopoietic growth and differentiation as well as a prominent signaling molecule for IFNs.

Constitutive activation of JAKs and STATs in BCR-Abl-expressing cell lines and peripheral blood cells derived from leukemic patients

An important step in the oncogenic transformation of hemopoietic cells and the subsequent development of leukemia is the proliferation of tumor cells in the absence of exogenous growth factors. In most cases of chronic myelocytic leukemia and in some cases of acute myelocytic leukemia and acute lymphocytic leukemia, the bcr-abl oncogene is involved in this process. Although the BCR-Abl oncoprotein demonstrates enhanced tyrosine kinase activity in leukemic cells, the mechanism by which this leads to growth factor independence remains poorly defined. One proposed mechanism is the activation of cytokine signal transduction pathways, possibly by an autocrine loop involving IL-3 and/or granulocyte-macrophage CSF. Examination of several different cell lines expressing BCR-Abl demonstrates that some of these cells have constitutive activation of the JAK/STAT signaling pathway. We have found the constitutive activation of STAT5 in most, but not all, cell lines expressing BCR-Abl. This constitutive activation of STAT5 is variably associated with a corresponding activation of JAK kinases. Ab blocking studies show that the activation of STAT5 in these cell lines cannot be attributed to the activation of an IL-3/granulocyte-macrophage CSF-driven autocrine loop. Interestingly, samples of peripheral blood cells derived from patients with acute myelocytic leukemia and chronic myelocytic leukemia, which express BCR-Abl, demonstrate constitutive activation of STAT family members. These studies suggest that in a variety of leukemic states, BCR-Abl may use a bypass mechanism to activate cytokine signal transduction pathways.

Constitutive activation of JAKs and STATs in BCR-Abl-expressing cell lines and peripheral blood cells derived from leukemic patients

An important step in the oncogenic transformation of hemopoietic cells and the subsequent development of leukemia is the proliferation of tumor cells in the absence of exogenous growth factors. In most cases of chronic myelocytic leukemia and in some cases of acute myelocytic leukemia and acute lymphocytic leukemia, the bcr-abl oncogene is involved in this process. Although the BCR-Abl oncoprotein demonstrates enhanced tyrosine kinase activity in leukemic cells, the mechanism by which this leads to growth factor independence remains poorly defined. One proposed mechanism is the activation of cytokine signal transduction pathways, possibly by an autocrine loop involving IL-3 and/or granulocyte-macrophage CSF. Examination of several different cell lines expressing BCR-Abl demonstrates that some of these cells have constitutive activation of the JAK/STAT signaling pathway. We have found the constitutive activation of STAT5 in most, but not all, cell lines expressing BCR-Abl. This constitutive activation of STAT5 is variably associated with a corresponding activation of JAK kinases. Ab blocking studies show that the activation of STAT5 in these cell lines cannot be attributed to the activation of an IL-3/granulocyte-macrophage CSF-driven autocrine loop. Interestingly, samples of peripheral blood cells derived from patients with acute myelocytic leukemia and chronic myelocytic leukemia, which express BCR-Abl, demonstrate constitutive activation of STAT family members. These studies suggest that in a variety of leukemic states, BCR-Abl may use a bypass mechanism to activate cytokine signal transduction pathways.

Identification of a STAT6 domain required for IL-4-induced activation of transcription

Tyrosine phosphorylation of STAT6 in response to IL-4 results in the formation of STAT6 homodimers that bind specific DNA elements. Although binding sites for STAT6 have been shown to be important for the function of several IL-4-inducible promoters, the role of STAT6 in this activation has not been defined. To determine whether STAT6 is a transcriptional activator, different portions of the carboxyl terminus of STAT6 were fused to the yeast Gal4 protein DNA binding domain. Analysis of these chimeric Gal4-STAT6 proteins demonstrates that a 140-amino-acid proline-rich region of the carboxyl terminus of STAT6 contains a region that activates transcription. Truncation mutants of STAT6 that lack this domain cannot activate transcription and are capable of repressing transcription stimulated by a wild-type STAT6 protein. Strikingly, the ability of IL-4 to induce transcription from the Ig germline epsilon promoter is suppressed by overexpression of a carboxyl-terminal deletion mutant of STAT6. These studies demonstrate that the carboxyl terminus of STAT6 contains an activating domain required for the induction of genes by IL-4.

Identification of a STAT6 domain required for IL-4-induced activation of transcription

Tyrosine phosphorylation of STAT6 in response to IL-4 results in the formation of STAT6 homodimers that bind specific DNA elements. Although binding sites for STAT6 have been shown to be important for the function of several IL-4-inducible promoters, the role of STAT6 in this activation has not been defined. To determine whether STAT6 is a transcriptional activator, different portions of the carboxyl terminus of STAT6 were fused to the yeast Gal4 protein DNA binding domain. Analysis of these chimeric Gal4-STAT6 proteins demonstrates that a 140-amino-acid proline-rich region of the carboxyl terminus of STAT6 contains a region that activates transcription. Truncation mutants of STAT6 that lack this domain cannot activate transcription and are capable of repressing transcription stimulated by a wild-type STAT6 protein. Strikingly, the ability of IL-4 to induce transcription from the Ig germline epsilon promoter is suppressed by overexpression of a carboxyl-terminal deletion mutant of STAT6. These studies demonstrate that the carboxyl terminus of STAT6 contains an activating domain required for the induction of genes by IL-4.

The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation

Structural alterations of the promoter region of the BCL-6 proto-oncogene represent the most frequent genetic alteration associated with non-Hodgkin lymphoma, a malignancy often deriving from germinal-centre B cells. The BCL-6 gene encodes a zinc-finger transcriptional repressor normally expressed in both B cells and CD4+ T cells within germinal centres, but its precise function is unknown. We show that mice deficient in BCL-6 displayed normal B-cell, T-cell and lymphoid-organ development but have a selective defect in T-cell-dependent antibody responses. This defect included a complete lack of affinity maturation and was due to the inability of follicular B cells to proliferate and form germinal centres. In addition, BCL-6-deficient mice developed an inflammatory response in multiple organs characterized by infiltrations of eosinophils and IgE-bearing B lymphocytes typical of a Th2-mediated hyperimmune response. Thus, BCL-6 functions as a transcriptional switch that controls germinal centre formation and may also modulate specific T-cell-mediated responses. Altered expression of BCL-6 in lymphoma represents a deregulation of the pathway normally leading to B cell proliferation and germinal centre formation.

Growth and gene expression are predominantly controlled by distinct regions of the human IL-4 receptor

IL-4 causes hematopoietic cells to proliferate and express a series of genes, including CD23. We examined whether IL-4-mediated growth, as measured by 4PS phosphorylation, and gene induction were similarly controlled. Studies of M12.4.1 cells expressing human IL-4R truncation mutants indicated that the region between amino acids 557-657 is necessary for full gene expression, which correlated with Stat6 DNA binding activity. This region was not required for 4PS phosphorylation. Tyrosine-to-phenylalanine mutations in the interval between amino acids 557-657 revealed that as long as one tyrosine remained unmutated, CD23 was fully induced. When all three tyrosines were mutated, the receptor was unable to induce CD23. The results indicate that growth regulation and gene expression are principally controlled by distinct regions of IL-4R.

Interleukin 4 signals through two related pathways

The interleukin 4 (IL-4) signaling pathway involves activation, by tyrosine phosphorylation, of two distinct substrates, a signal-transducing factor (STF-IL4) and the IL-4-induced phosphotyrosine substrate (4PS). It is not known whether the IL-4-mediated activation of these substrates occurs via related or distinct signaling pathways. We report that 32D cells, an IL-3-dependent myeloid progenitor cell line in which no phosphorylated 4PS is found, activate high levels of STF-IL4 in response to IL-4. Consistent with the known requirement for 4PS or insulin receptor substrate 1 (IRS-1) in IL-4-mediated mitogenesis, activation of STF-IL4 in 32D cells is not sufficient for IL-4-inducible c-myc expression. In addition, we have examined the ability of 32D cells transfected with different truncation mutants of the human IL-4 receptor to activate Jak-3 kinase and STF-IL4 in response to human IL-4. As in the case of 4PS/IRS-1, we have found that activation of both Jak-3 and STF-IL4 requires the presence of the IL-4 receptor region comprising aa 437-557. The finding that the same region of the IL-4 receptor is required for the induction of both 4PS/IRS-1 and STF-IL4 suggests that the IL-4-stimulated activation of these two substrates might involve common factors.

Lack of interferon gamma receptor beta chain and the prevention of interferon gamma signaling in TH1 cells

The ability of interferon gamma (IFN-gamma) to inhibit the proliferation of type 2 T helper cells (TH2), but not that of type 1 (TH1) cells, suggests that helper cell subsets might differ in their activation of the IFN-gamma signaling pathway. The IFN-gamma-inducible signal transducing factor (STF-IFN gamma) was activated in murine TH2 but not in TH1 cell clones, because in the latter the second chain of the IFN-gamma receptor (accessory factor 1 or IFN-gamma R beta) was absent. Thus, TH1 cells use receptor modification to prevent the activation of STF-IFN gamma and achieve an IFN-gamma-resistant state.

Gamma chain-associated cytokine receptors signal through distinct transducing factors

The IL-2, IL-4, and IL-7 signaling pathways have been shown to utilize shared components. The receptors for these cytokines are composed of ligand-specific binding chains that associate with a shared signaling subunit, the common gamma (gamma c) chain. In addition, IL-2, IL-4, and IL-7 induce activation of a common set of nonreceptor tyrosine kinases, Jak-1 and Jak-3. We have further investigated the signaling events induced by these cytokines and find that the gamma c-associated receptors activate distinct signal transducing factors (STFs). In addition, we show that a 94-kDa STAT-related protein (p94) is activated in response to IL-2 and IL-7, but not IL-4. These data indicate that IL-2, IL-4, and IL-7 activate distinct signaling molecules which might be differentially recruited to the receptor complex by the ligand-specific units of the IL-2, IL-4, and IL-7 receptors.

The non-histone chromosomal protein HMG-I(Y) contributes to repression of the immunoglobulin heavy chain germ-line epsilon RNA promoter

The rate of germ-line RNA transcription correlates with the rate of immunoglobulin heavy chain isotype switching. A promoter element for the transcription of RNA from the germ-line mouse immunoglobulin epsilon heavy chain constant region gene is induced by interleukin(IL)-4 and lipopolysaccharide, and is bound at its transcription initiation sites by an IL-4-inducible nuclear protein, NF-BRE. To examine the function of the binding site for this IL-4-inducible complex, substitution mutations were introduced in the promoter. These binding site mutations increased promoter activity and decreased binding of NF-BRE. To investigate the paradox of an IL-4-inducible protein binding to a repressor site in an IL-4-inducible promoter, we determined that the non-histone chromosomal protein HMG-I(Y) binds at the transcription initiation sites of the germ-line epsilon promoter. Assays with antisera against HMG-I(Y) revealed monomeric HMG-I(Y) in nuclear extracts. Cotransfection of an expression construct directing the synthesis of anti-sense HMG-I(Y) RNA also increased promoter activity, consistent with a repressor function of HMG-I(Y). Thus, the data are most consistent with a model in which HMG-I(Y) participates in repression of promoter activity. The effects of IL-4 may include derepression at this site.

Cytokines and growth factors signal through tyrosine phosphorylation of a family of related transcription factors

The ability of cytokines to activate distinct but overlapping sets of genes defines their characteristic biological response. We now show that IFN gamma, IL-3, IL-4, IL-6, erythropoietin, EGF, and CSF-1 activate differing members of a family of latent cytoplasmic transcription factors. Although these factors have distinct physical and functional properties and exhibit different patterns of expression, they share many important features, including recognition of a related set of enhancer elements, rapid activation, tyrosine phosphorylation, and cross-reactivity to antibodies against p91, a cytoplasmic signaling protein activated by IFN alpha, IFN gamma, and IL-6. These shared features point to either parallel or common patterns of signal transduction. A general model of cytokine signal transduction is presented, in which receptor-associated tyrosine kinases activate ligand-specific members of a family of signal-transducing factors. Once activated, these factors carry their signals to the nucleus, where they bind a family of related enhancer elements.

IFN-gamma represses epsilon germline transcription and subsequently down-regulates switch recombination to epsilon

Cytokines have the ability to regulate the isotypes of antibodies produced during an immune response. For instance, IL-4 has been shown to induce the production of IgE by B cells, while IFN-gamma has been shown to inhibit this induction. Recent work has revealed that IL-4 appears to induce class switching to epsilon through its ability to specifically induce germline epsilon transcripts. Germline epsilon transcription appears to target class-switch recombination to the epsilon locus. However, the mechanism by which IFN-gamma inhibits the IL-4 induction of IgE is unknown. We hypothesized that IFN-gamma and IL-4 may have antagonistic effects on the same stage of B cell differentiation. Northern blotting analyses show that IFN-gamma suppresses the IL-4 induction of germline epsilon transcripts. In transient transfection assays, the IL-4 induction of transcription imparted by the minimal 179 bp germline epsilon promoter is repressed by IFN-gamma. Utilizing a digestion circularization-polymerase chain reaction assay we show that IL-4 induces switch recombination to epsilon, while IFN-gamma suppresses switch recombination to epsilon. These studies support a model that, through their differential effects on a cis-controlling element that regulates germline epsilon transcription, IL-4 and IFN-gamma are able to modulate B cell switch recombination to epsilon in a coordinated manner.

Isolation of cDNAs encoding T-BAM, a surface glycoprotein on CD4+ T cells mediating contact-dependent helper function for B cells: identity with the CD40-ligand

"T-cell B-cell Activating Molecule" (T-BAM) is an activation-induced surface protein on CD4+ T cells that mediates a contact-dependent signal for B cell differentiation and immunoglobulin (Ig) secretion. The T-BAM protein on a helper clone of Jurkat (D1.1) was affinity purified using the anti-T-BAM mAb, 5c8. The NH2-terminal amino acid sequence of purified T-BAM was determined and found to be highly homologous to the predicted NH2-terminal sequence of a T cell ligand to the B cell CD40 molecule (CD40-L). From a D1.1 cDNA library, a clone was isolated that encodes CD40-L by sequence and drives expression of T-BAM protein on transfected cells, demonstrating that the T-BAM and CD40-L genes and proteins are identical. Moreover, transfection of T-BAM was shown to confer to non-lymphoid cells, the ability to induce B cells to upregulate the expression of surface CD23 molecules. In previous studies we showed that T-BAM was expressed predominantly on activated CD4+ and on few if any CD8+ cells. Although the current work confirms that T-BAM is largely restricted to activated CD4+ T cells, we now provide definitive evidence that T-BAM can be expressed by a small population of CD8+ T cells after activation. Importantly, a subset of CD8+ T cells do not express T-BAM after activation and this T-BAM- phenotype is maintained on certain CD8+ T cell clones. Taken together, these data unify the biology and structure of T-BAM and CD40-L and this synthesis has implications for understanding the T cell regulation of the humoral immune response.

The transcription factor BSAP (NF-HB) is essential for immunoglobulin germ-line epsilon transcription

Treatment of murine splenic B lymphocytes and certain B-lineage cell lines with mitogen (LPS) and the lymphokine IL-4 has been shown to induce expression of germ-line epsilon transcripts (l epsilon transcripts) and class switching to the C epsilon gene. Three protein complexes, one of which (complex 3) is constitutively expressed, have been shown to bind to a 179-base pair LPS/IL-4-responsive l epsilon promoter (Rothman, P., S. C. Li, B. Gorham, L. Glimcher, F. W. Alt, and M. Boothby. 1991. Mol. Cell. Biol. 11:5551). Complex 3 is indispensable for this inducible promoter activity. In this report, we have used electrophoretic mobility shift assays (EMSA) to demonstrate that the early B cell-specific transcription factor (BSAP) is involved in the formation of complex 3. In addition, BSAP is implicated functionally in l epsilon transcription because a BSAP binding site either from a sea urchin histone promoter (H2A-2.2) or from 5' of murine immunoglobulin S gamma 2a can substitute for the epsilon-associated site (epsilon(foot), as assayed by transient transfection assays of the l epsilon:CAT reporter constructs into the M12.4.1 B cell line. Like the sea urchin histone BSAP site, the complex 3 binding site (epsilon(foot)) functions as an upstream promoter element when assayed in the OVEC vector. These results indicate that BSAP is an essential protein required for LPS/IL-4 induction of the l epsilon promoter. In addition, experiments showing that a BSAP binding site from 5' of S gamma 2a also functions as an upstream promoter element in OVEC suggest a potential role for BSAP in regulation of the IgG2a isotype.

STF-IL-4: a novel IL-4-induced signal transducing factor

The mechanism by which interleukin-4 (IL-4) regulates the expression of particular genes is unknown. We have determined that IL-4 induces a DNA binding factor (termed STF-IL-4) which has a strong affinity for an IFN-gamma activation site (GAS). Interestingly, STF-IL-4 also binds to the IL-4 responsive promoter for the Ig heavy chain germline epsilon transcript. The IL-4 dependent activation of STF-IL-4 is rapid, does not require protein synthesis and results in the sequential appearance of binding activity first in the cytoplasm and then later in the nucleus. Activation of STF-IL-4 is sensitive to tyrosine kinase inhibitors and the active factor is tyrosine phosphorylated. This pattern of activation is similar to the activation of interferon-induced transcription factors. STF-IL-4 appears to be a new member of a growing family of cytokine-induced transcriptional regulators.

The transcription factor BSAP (NF-HB) is essential for immunoglobulin germ-line epsilon transcription

Treatment of murine splenic B lymphocytes and certain B-lineage cell lines with mitogen (LPS) and the lymphokine IL-4 has been shown to induce expression of germ-line epsilon transcripts (l epsilon transcripts) and class switching to the C epsilon gene. Three protein complexes, one of which (complex 3) is constitutively expressed, have been shown to bind to a 179-base pair LPS/IL-4-responsive l epsilon promoter (Rothman, P., S. C. Li, B. Gorham, L. Glimcher, F. W. Alt, and M. Boothby. 1991. Mol. Cell. Biol. 11:5551). Complex 3 is indispensable for this inducible promoter activity. In this report, we have used electrophoretic mobility shift assays (EMSA) to demonstrate that the early B cell-specific transcription factor (BSAP) is involved in the formation of complex 3. In addition, BSAP is implicated functionally in l epsilon transcription because a BSAP binding site either from a sea urchin histone promoter (H2A-2.2) or from 5' of murine immunoglobulin S gamma 2a can substitute for the epsilon-associated site (epsilon(foot), as assayed by transient transfection assays of the l epsilon:CAT reporter constructs into the M12.4.1 B cell line. Like the sea urchin histone BSAP site, the complex 3 binding site (epsilon(foot)) functions as an upstream promoter element when assayed in the OVEC vector. These results indicate that BSAP is an essential protein required for LPS/IL-4 induction of the l epsilon promoter. In addition, experiments showing that a BSAP binding site from 5' of S gamma 2a also functions as an upstream promoter element in OVEC suggest a potential role for BSAP in regulation of the IgG2a isotype.

T-BAM/CD40-L on helper T lymphocytes augments lymphokine-induced B cell Ig isotype switch recombination and rescues B cells from programmed cell death

An important component of T cell help for B lymphocyte differentiation is the contact-dependent signaling mediated by the T cell-B cell activating molecule (T-BAM/CD40-L), an activation-induced surface membrane protein on CD4+ T helper cells in lymphoid follicles that interacts with the B cell surface molecule, CD40. The present study dissects the roles of T-BAM/CD40-L in helper function by means of a neutralizing anti-T-BAM/CD40-L mAb (5c8), a T-BAM/CD40-L-expressing T cell tumor subclone (Jurkat D1.1), and a T-BAM/CD40-L-responsive IgM+ B cell tumor of germinal center origin (RAMOS 266). Like activated T cells, D1.1 cells induce B cells to synthesize IgG, IgA, and IgE in a process that is specifically inhibited by the mAb 5c8. Although rIL-4 alone, but not Jurkat D1.1, induces IgH C gamma mRNA transcripts in RAMOS 266, the T-BAM/CD40-L molecule on D1.1 acts on rIL-4-primed RAMOS B cells to augment expression of C gamma transcripts. In addition, IgG+ RAMOS 266 clones were expanded from D1.1- and rIL-4-stimulated cultures that had undergone deletional IgH isotype switch recombination events. Furthermore, T-BAM/CD40-L signals delivered by the D1.1 clone dramatically rescue RAMOS 266 from mAb anti-IgM-induced apoptosis. Taken together, these data support the idea that T-BAM/CD40-L plays important roles in inducing Ig isotype switch recombination and the clonal selection of isotype-switched B cells.

T-BAM/CD40-L on helper T lymphocytes augments lymphokine-induced B cell Ig isotype switch recombination and rescues B cells from programmed cell death

An important component of T cell help for B lymphocyte differentiation is the contact-dependent signaling mediated by the T cell-B cell activating molecule (T-BAM/CD40-L), an activation-induced surface membrane protein on CD4+ T helper cells in lymphoid follicles that interacts with the B cell surface molecule, CD40. The present study dissects the roles of T-BAM/CD40-L in helper function by means of a neutralizing anti-T-BAM/CD40-L mAb (5c8), a T-BAM/CD40-L-expressing T cell tumor subclone (Jurkat D1.1), and a T-BAM/CD40-L-responsive IgM+ B cell tumor of germinal center origin (RAMOS 266). Like activated T cells, D1.1 cells induce B cells to synthesize IgG, IgA, and IgE in a process that is specifically inhibited by the mAb 5c8. Although rIL-4 alone, but not Jurkat D1.1, induces IgH C gamma mRNA transcripts in RAMOS 266, the T-BAM/CD40-L molecule on D1.1 acts on rIL-4-primed RAMOS B cells to augment expression of C gamma transcripts. In addition, IgG+ RAMOS 266 clones were expanded from D1.1- and rIL-4-stimulated cultures that had undergone deletional IgH isotype switch recombination events. Furthermore, T-BAM/CD40-L signals delivered by the D1.1 clone dramatically rescue RAMOS 266 from mAb anti-IgM-induced apoptosis. Taken together, these data support the idea that T-BAM/CD40-L plays important roles in inducing Ig isotype switch recombination and the clonal selection of isotype-switched B cells.

S region transcription per se promotes basal IgE class switch recombination but additional factors regulate the efficiency of the process

Stimulation of B lymphocytes with a combination of lipopolysaccharide (LPS) and interleukin-4 (IL-4) induces germline transcription of and subsequent switching to the epsilon heavy chain constant region (C epsilon) gene. Mature germline C epsilon transcripts contain a non-coding exon (I epsilon exon) spliced to the C epsilon exons. To distinguish between the potential roles of germline transcription and those of germline transcripts in regulating the class switch process, we replaced the LPS- and IL-4-inducible I epsilon promoter and exon in ES cells with an LPS-inducible E mu enhancer/VH promoter expression cassette. Wildtype, heterozygous or homozygous mutant ES cells were injected into RAG-2 deficient blastocysts to generate somatic chimeras in which all B cells derived from ES cells. In contrast to normal B cells, heterozygous and homozygous mutant B cells had substantial transcription through the epsilon switch recombination region (S epsilon) following treatment with LPS alone and, under these conditions, both underwent low level switching (10- to 100-fold less than wildtype cells stimulated with LPS + IL-4) to IgE production. Heterozygous mutant cells underwent switching to IgE at essentially wildtype levels when stimulated with LPS and IL-4. However, homozygous mutant cells still showed extremely low levels of switching to IgE upon LPS and IL-4 stimulation. Analyses of hybridomas from heterozygous mutants indicated that the mutation is cis-acting and normal switching to other isotypes indicated that it is specific for IgE. Thus transcription per se generates low levels of class switch recombination in the absence of I region sequences. However, we demonstrate for the first time that, for optimal efficiency, the process requires the presence of the intact I region and/or I region promoter in cis, implicating factors beyond transcription through the S region in the regulation of class switching.

Replacement of germ-line epsilon promoter by gene targeting alters control of immunoglobulin heavy chain class switching

Recent work has shown that the ability of cytokines to direct immunoglobulin heavy chain class-switch recombination to particular heavy chain constant (C) region (CH) genes correlates with the induction of specific germ-line CH transcripts. To test the role of germ-line transcripts in class switching, we have used homologous recombination to mutate the immunoglobulin heavy chain locus of the 18.81A20 murine pre-B-cell line. In the parent cell line, the combination of interleukin-4 (IL-4) and lipopolysaccharide (LPS) induces germ-line epsilon locus transcription prior to class switching to epsilon. The heavy chain locus of the mutated cell line contains the immunoglobulin heavy chain enhancer and variable region gene promoter in place of the LPS/IL-4-responsive germ-line epsilon promoter. The mutant cell line constitutively transcribes the epsilon locus in the absence of IL-4. Strikingly, the mutant cell line also switches to epsilon in the absence of IL-4. This result demonstrates that, at least in the 18.81A20 cell line, germ-line epsilon transcription plays a direct role in class switching to the epsilon locus. In addition, the ability to change the pattern of class switching by altering transcriptional activity indicates that transcription of germ-line CH is mechanistically important in regulation of class switching.

Transforming growth factor beta 1 selectivity stimulates immunoglobulin G2b secretion by lipopolysaccharide-activated murine B cells

Bacterial lipopolysaccharide (LPS) has been reported to induce immunoglobulin (Ig)G2b class switching, yet we observed strain differences in IgG2b secretion in response to this mitogen. Specifically, BALB/c B cells, unlike those from DBA/2, synthesized relatively low amounts of IgG2b relative to IgG3, IgG1, or IgM. This report demonstrates that transforming growth factor (TGF) beta 1, previously shown to induce IgA class switching, selectively stimulates IgG2b secretion by BALB/c resting B cells activated with LPS. This activity was specifically reversed with a neutralizing anti-TGF-beta 1 antibody. The ability of TGF-beta 1 to act directly on highly purified membrane (m)IgM+ mIgG2b- cells to stimulate IgG2b production, stimulate an increase in IgG2b-secreting cells, and selectively increase the steady-state levels of germline gamma 2b RNA, suggests that it promotes IgG2b class switching. In this regard, addition of anti-TGF-beta antibody to cultures of DBA/2-derived resting B cells activated by LPS, alone, led to selective reduction in IgG2b secretion, indicating that endogenous TGF-beta 1 accounts for the high IgG2b secretory response observed in that strain. Finally, TGF-beta 1 failed to stimulate IgG2b secretion by B cells activated with dextran-conjugated anti-IgD antibody. We propose that TGF-beta 1 is a switch factor for the murine IgG2b subclass for appropriately activated B cells. In combination with other data, this would show that all six non-IgM, non-IgD isotypes in the mouse can be selectively induced by specific cytokines.

Endocrine effects and pharmacokinetic characteristics of a potent new gonadotropin-releasing hormone antagonist (Ganirelix) with minimal histamine-releasing properties: studies in postmenopausal women

A potent and safe GnRH antagonist has been sought unsuccessfully for the last 2 decades. The recently developed GnRH antagonist RS-26306 or Ganirelix ([N-Ac-D-Nal(2)1,D-pClPhe2,D-Pal(3)3,D-hArg(Et2)6,L-++ +hArg(Et2)8,D-Ala10]GnRH ; Syntex Research, Palo Alto, CA), exhibited high antiovulatory potency and low histamine-releasing properties in preclinical studies. Therefore, we determined the extent to which single sc injections of three doses of RS-26306 (1, 3, and 6 mg) decreased serum concentrations of LH and FSH, the free alpha-subunit of LH/FSH/TSH, PRL, and testosterone in five healthy postmenopausal women. We also examined the pharmacokinetic characteristics of RS-26306 by quantifying serum levels of the drug by RIA. RS-26306 rapidly suppressed serum concentrations of LH, FSH, and free alpha-subunit. RS-26306 (6 mg) maximally decreased serum concentrations (mean +/- SEM) of LH, FSH, and free alpha-subunit by 70.1 +/- 3.6%, 42.3 +/- 2.5%, and 74.6 +/- 3.5%, respectively. RS-26306 also decreased serum testosterone, but not serum PRL, concentrations. RS-26306 concentrations reached peak serum levels at 1.2 +/- 0.3, 1.9 +/- 0.4, and 1.8 +/- 0.5 h, respectively, after 1-, 3-, and 6-mg sc injections. The mean serum half-life values based on the terminal portion of the disappearance curves were 22.8 +/- 2.5 and 26.9 +/- 1.0 h, respectively, after 3- and 6-mg s.c. doses. No systemic side-effects were noted after the administration of RS-26306. Our results demonstrate that the GnRH antagonist RS-26306 has favorable pharmacokinetic characteristics and is a potent suppressor of pituitary gonadotropin secretion in postmenopausal women. These attributes and the lack of systemic side-effects make RS-26306 a promising candidate for future clinical applications.

Acute onset of celiac disease in the puerperium

An acute, severe onset of celiac disease is very unusual in adults. We describe its occurrence postpartum in two patients. These, together with two similar cases previously reported, suggest a causal association with the puerperium. Possible causes include potentiation of a pathogenetic immune reaction and hormonal mechanisms.

T helper cell membranes promote IL-4-independent expression of germ-line C gamma 1 transcripts in B cells

Studies using plasma membranes from activated Th cell clones (Th membranes) to stimulate B cells have shown that both a contact-mediated activation signal plus Th-derived cytokines are required for antibody production. In order to clearly separate and define the role of these two signals in isotype switching, B cells were stimulated with Th membranes in the presence or absence of cytokines, and the transcriptional activity of the unrearranged H chain loci was determined. In the presence of Th membranes, two known switch factors were shown to specifically induce germ-line transcription of the same H chain loci as in LPS-stimulated B cells (IL-4 induced C gamma 1 and C epsilon transcription, transforming growth factor-beta induced C alpha transcription). The contact-mediated activation signal provided by the Th membranes, in the absence of any added cytokines, resulted in the specific induction of C gamma 1 germ-line transcription, and thus functioned as a switch signal for IgG1. These findings provide a mechanism for previously observed IL-4-independent isotype switching to IgG1.

T helper cell membranes promote IL-4-independent expression of germ-line C gamma 1 transcripts in B cells

Studies using plasma membranes from activated Th cell clones (Th membranes) to stimulate B cells have shown that both a contact-mediated activation signal plus Th-derived cytokines are required for antibody production. In order to clearly separate and define the role of these two signals in isotype switching, B cells were stimulated with Th membranes in the presence or absence of cytokines, and the transcriptional activity of the unrearranged H chain loci was determined. In the presence of Th membranes, two known switch factors were shown to specifically induce germ-line transcription of the same H chain loci as in LPS-stimulated B cells (IL-4 induced C gamma 1 and C epsilon transcription, transforming growth factor-beta induced C alpha transcription). The contact-mediated activation signal provided by the Th membranes, in the absence of any added cytokines, resulted in the specific induction of C gamma 1 germ-line transcription, and thus functioned as a switch signal for IgG1. These findings provide a mechanism for previously observed IL-4-independent isotype switching to IgG1.

Identification of a conserved lipopolysaccharide-plus-interleukin-4-responsive element located at the promoter of germ line epsilon transcripts

Treatment of splenic B lymphocytes and certain B-lineage cell lines with the mitogen lipopolysaccharide (LPS) and the lymphokine interleukin-4 (IL-4) induces expression of germ line immunoglobulin C epsilon transcripts and class switching to the C epsilon gene. We show that LPS-plus-IL-4 induction of germ line epsilon transcripts (termed I epsilon transcripts) occurs at the transcriptional level in an Abelson murine leukemia virus-transformed pre-B-cell line. A 1.1-kb region of DNA surrounding the I epsilon promoter endows inducible transcription to a heterologous reporter gene stably transfected into these cells; such inducible expression depends on combined treatment with LPS and IL-4. Analyses of constructs transiently introduced into a B-cell lymphoma line demonstrated that LPS-plus-IL-4-inducible expression can be conferred by a 179-bp segment of DNA spanning the I epsilon transcriptional initiation site. Mutational analyses demonstrated that this expression depended on DNA sequences within a conserved region directly upstream from the I epsilon transcriptional initiation region. One nuclear protein that is constitutively expressed in normal B cells binds to the downstream end of the conserved sequence; its binding specificity correlates with the functional effect of several mutations. Two additional proteins, which are induced by IL-4 treatment of splenic B cells, bind to the transcription initiation sites of I epsilon. These proteins are indistinguishable in binding assays from proteins previously shown to bind an enhancer region of the class II major histocompatibility complex gene A alpha.

A placebo-controlled trial of maintenance therapy with fluconazole after treatment of cryptococcal meningitis in the acquired immunodeficiency syndrome. California Collaborative Treatment Group

BACKGROUND AND METHODS

In patients with the acquired immunodeficiency syndrome (AIDS), the rate of relapse after primary treatment for cryptococcal meningitis remains high. We conducted a controlled, double-blind trial to evaluate the efficacy of maintenance therapy with fluconazole. At entry into the study, all participants had sterile cultures of cerebrospinal fluid, blood, and urine after following a standardized course of therapy for culture-proved cryptococcal meningitis. The patients were randomly assigned to take either fluconazole or placebo as maintenance therapy. The dose of fluconazole was 100 mg daily in the first phase of study and 200 mg daily in the second phase.

RESULTS

Of 84 patients initially enrolled, 16 (19 percent) were found to have silent, persistent infection on the basis of cultures that became positive after entry into the study; 7 other patients were lost to follow-up shortly after entry. Of the remaining 61 patients, 10 of 27 assigned to placebo (37 percent) and 1 of 34 assigned to fluconazole (3 percent) had a recurrence of cryptococcal infection at any site (difference in risk, 34 percent; 95 percent confidence interval, 15 to 53). Of the 11 recurrent infections, 7 were detected in urine obtained after prostatic massage. There were four recurrent meningeal infections in the patients taking placebo, but none in those taking fluconazole (mean duration of follow-up, 164 days) (P = 0.03). In multivariate analyses, the best predictors of recurrence-free survival were fluconazole treatment (P = 0.02; relative hazard, 13.2), a lower serum cryptococcal-antigen titer (P = 0.05; relative hazard, 1.2), and more prolonged primary therapy with flucytosine (P = 0.09; relative hazard, 1.1). Survival and toxicity were similar in the two maintenance-treatment groups.

CONCLUSIONS

In patients with AIDS, silent persistent infection is common after clinically successful treatment for cryptococcal meningitis. Maintenance therapy with fluconazole is highly effective in preventing recurrent cryptococcal infection.

Isolation of coordinately regulated genes that are expressed in discrete stages of B-cell development

We have utilized subtractive hybridization to isolate 16 distinct cDNA sequences representing genes expressed in pre-B-cell lines but not myeloma cell or fibroblast lines. These sequences represent RNA transcripts that vary in abundance in pre-B-cell lines from 0.001% to 0.05%. Five of these sequences were not related to any known genes. One was related to but distinct from known myosin regulatory light chain genes and another encoded a protein with lectin domains. Three represented previously identified genes encoding carbonic anhydrase type II, thymosin, and CD2; these genes were not previously known to be specifically expressed in early stages of B-cell development. Other isolated genes corresponded to pre-B-cell-specific or pre-B-cell/B cell-specific genes recently described by others. The isolated cDNA sequences may be divided into two general categories--those representing genes expressed only in the pre-B-cell stage of B-cell development and those expressed in both the pre-B-cell and B-cell stages. The in vivo expression patterns of the identified genes suggest that some function specifically in lymphocytes while others may have roles in additional lineages.

Structure and expression of germ line immunoglobulin heavy-chain epsilon transcripts: interleukin-4 plus lipopolysaccharide-directed switching to C epsilon

We have isolated cDNA clones complementary to a truncated immunoglobulin heavy-chain C epsilon RNA transcript previously found to be induced in B lymphoid cells by treatment with lipopolysaccharide (LPS) combined with interleukin-4 (IL-4). We demonstrate that this transcript initiates from a promoter upstream of the germ line epsilon class-switch recombination region (S epsilon region). The major germ line C epsilon transcript contains a small 5' exon contributed by sequences upstream of the S epsilon region spliced to the normal C epsilon exons. Treatment of splenic B lymphoid cells with LPS plus IL-4 induces the expression of transcripts from the germ line epsilon transcription unit followed by expression of normal immunoglobulin epsilon heavy-chain mRNA. Furthermore, we demonstrate that similar treatment of transformed precursor B cell lines induces the expression of germ line epsilon transcripts followed by class switching to epsilon expression in these lines. This is the first demonstration of switching to epsilon in cells of the pre-B stage. The general structure of the germ line epsilon transcript and transcription unit is similar to that previously characterized for germ line gamma 2b transcripts. However, expression of these two germ line transcription units in B-lineage cells is inversely regulated by IL-4 (plus LPS) treatment, correlating with the effects of these treatments on switching to these loci.

Structure and expression of germline immunoglobulin gamma 3 heavy chain gene transcripts: implications for mitogen and lymphokine directed class-switching

We have characterized the structure and expression of transcripts synthesized from the murine germline immunoglobulin gamma 3 heavy chain gene in certain B-lineage cells. The transcripts initiate upstream of the switch gamma 3 region, generating a 5' exon that is spliced to C gamma 3 exons. Expression of this germline transcript is induced when splenic B cells or A-MuLV-transformed pre-B cell lines are cultured in the presence of lipopolysaccharide (LPS). Addition of interleukin-4 (IL-4) to these lipopolysaccharide (LPS) cultures dramatically inhibits induction of the germline gamma 3 transcript. Induction of germline gamma 3 transcripts occurs before the increased accumulation of gamma 3-producing cells and VDJ-gamma 3 mRNA in cultures of splenic B cells. These data provide further evidence that germline CH transcriptional units are important components in the regulation of heavy chain class-switching. In addition, the pre-B cell lines that we describe represent the first example of permanent cell lines that regulate expression of the germline gamma locus in response to LPS plus IL-4 treatment in a manner analogous to normal B cells; therefore these lines should represent an excellent model system to further study the molecular mechanisms by which germline expression is regulated by these agents.

The molecular events in heavy chain class-switching

Heavy chain class-switching is the process by which B lymphoid cells change the constant region of the immunoglobulin heavy chain they produce. Class-switching is most commonly accomplished by recombinationldeletion between switch recombination regions that lie upstream of each germline heavy chain constant region gene. Recent studies support a model that recombination to specific switch regions is directed by modulation of the accessibility of these regions to a common class-switch recombination system. T cell lymphokines seem to be able to alter the accessibility of different heavy chain constant region loci, and thereby direct the specificity of class-switch recombination in B cells.

Control of recombination events during lymphocyte differentiation. Heavy chain variable region gene assembly and heavy chain class switching

Our recent studies have focused on the organization of immunoglobulin genes in mice and humans and the mechanism and control of the recombination events that are involved in their assembly and expression. This report describes our progress in this area with particular focus on elucidating factors that influence the generation of the antibody repertoire in normal and diseased states. We present a detailed analysis of the organization of the human VH locus, studies that help to elucidate the nature of the recombination defect in mice with severe combined immunodeficiency, and studies of transgenic mice that focus on the mechanism that regulates tissue-specific variable region gene assembly. In addition, we also characterize mechanisms that control the heavy chain class-switch process. Although the latter process apparently involve a recombination system distinct from that involved in variable region assembly, we find that the two recombination events appear to be controlled by similar mechanisms.

Mitogen plus interleukin 4 induction of C epsilon transcripts in B lymphoid cells

To elucidate the mechanism of IL-4-induced enhancement of IgE and IgG1 production, murine splenic B cells and A-MuLV-transformed cells were cultured with LPS and IL-4 and assayed for epsilon and gamma 1 transcripts. Concomitant treatment with IL-4 and LPS induced expression of C epsilon transcripts in both normal and transformed cells. Expression of these truncated C epsilon transcripts preceded accumulation of normal epsilon mRNA in treated cells. Consistent data were obtained with respect to gamma 1 RNA expression. These results suggest that IL-4 can direct class switching in the context of a mechanism associated with differential expression of germline constant region genes.