An Aged/Autoimmune B-cell Program Defines the Early Transformation of Extranodal Lymphomas

A third of patients with diffuse large B-cell lymphoma (DLBCL) present with extranodal dissemination, which is associated with inferior clinical outcomes. MYD88L265P is a hallmark extranodal DLBCL mutation that supports lymphoma proliferation. Yet extranodal lymphomagenesis and the role of MYD88L265P in transformation remain mostly unknown. Here, we show that B cells expressing Myd88L252P (MYD88L265P murine equivalent) activate, proliferate, and differentiate with minimal T-cell costimulation. Additionally, Myd88L252P skewed B cells toward memory fate. Unexpectedly, the transcriptional and phenotypic profiles of B cells expressing Myd88L252P, or other extranodal lymphoma founder mutations, resembled those of CD11c+T-BET+ aged/autoimmune memory B cells (AiBC). AiBC-like cells progressively accumulated in animals prone to develop lymphomas, and ablation of T-BET, the AiBC master regulator, stripped mouse and human mutant B cells of their competitive fitness. By identifying a phenotypically defined prospective lymphoma precursor population and its dependencies, our findings pave the way for the early detection of premalignant states and targeted prophylactic interventions in high-risk patients.

SIGNIFICANCE

Extranodal lymphomas feature a very poor prognosis. The identification of phenotypically distinguishable prospective precursor cells represents a milestone in the pursuit of earlier diagnosis, patient stratification, and prophylactic interventions. Conceptually, we found that extranodal lymphomas and autoimmune disorders harness overlapping pathogenic trajectories, suggesting these B-cell disorders develop and evolve within a spectrum. See related commentary by Leveille et al. (Blood Cancer Discov 2023;4:8-11). This article is highlighted in the In This Issue feature, p. 1.

Adaptive immune responses in patients requiring revision after total knee arthroplasty

Dissatisfaction occurs in nearly 20% of patients after total knee arthroplasty (TKA); however, there remains only limited understanding of the biologic mechanisms that may contribute to suboptimal postoperative outcomes requiring revision surgery. Expansion of effector T and B cells, could promote an abnormal healing response via local or peripheral immune system mechanisms and contribute to inferior outcomes necessitating revision TKA. In this pilot study, we hypothesized that patients suffering from complications of arthrofibrosis or instability may exhibit differences in adaptive immune function. Patients (n = 31) undergoing revision TKA for an indication of arthrofibrosis or instability were prospectively enrolled. Whole blood and synovial fluid (SF) from the operative knee were collected at time of surgery. Peripheral blood mononuclear cells were isolated and analyzed by flow cytometry. Serum and SF were assessed for immunoglobulin levels by Luminex and antiphospholipid antibodies by enzyme-linked immunoassay. No significant differences were observed in peripheral blood T/B cell populations or serum immunoglobulins levels between groups. SF analysis demonstrated significant differences between the two groups, with higher levels of immunoglobulin G1 (IgG1) (p = 0.0184), IgG3 (p = 0.0084) and antiphosphatidyl serine IgG (p = 0.034) in arthrofibrosis relative to instability patients. Increased levels of both IgG subclasses and antiphospholipid antibodies in the SF suggest that intra-articular T-B cell interactions, potentially triggered by exposure to apoptotic components generated during post-op healing, could be functioning as a source of immune complexes that fuel fibrous tissue growth in arthrofibrotic patients.

Interleukin-13 Receptor α1-Mediated Signaling Regulates Age-Associated/Autoimmune B Cell Expansion and Lupus Pathogenesis

OBJECTIVE

Age-associated/autoimmune B cells (ABCs) are an emerging B cell subset with aberrant expansion in systemic lupus erythematosus. ABC generation and differentiation exhibit marked sexual dimorphism, and Toll-like receptor 7 (TLR-7) engagement is a key contributor to these sex differences. ABC generation is also controlled by interleukin-21 (IL-21) and its interplay with interferon-γ and IL-4. This study was undertaken to investigate whether IL-13 receptor α1 (IL-13Rα1), an X-linked receptor that transmits IL-4/IL-13 signals, regulates ABCs and lupus pathogenesis.

METHODS

Mice lacking DEF-6 and switch-associated protein 70 (double-knockout [DKO]), which preferentially develop lupus in females, were crossed with IL-13Rα1-knockout mice. IL-13Rα1-knockout male mice were also crossed with Y chromosome autoimmune accelerator (Yaa) DKO mice, which overexpress TLR-7 and develop severe disease. ABCs were assessed using flow cytometry and RNA-Seq. Lupus pathogenesis was evaluated using serologic and histologic analyses.

RESULTS

ABCs expressed higher levels of IL-13Rα1 than follicular B cells. The absence of IL-13Rα1 in either DKO female mice or Yaa DKO male mice decreased the accumulation of ABCs, the differentiation of ABCs into plasmablasts, and autoantibody production. Lack of IL-13Rα1 also prolonged survival and delayed the development of tissue inflammation. IL-13Rα1 deficiency diminished in vitro generation of ABCs, an effect that, surprisingly, could be observed in response to IL-21 alone. RNA-Seq revealed that ABCs lacking IL-13Rα1 down-regulated some histologic characteristics of B cells but up-regulated myeloid markers and proinflammatory mediators.

CONCLUSION

Our findings indicate a novel role for IL-13Rα1 in controlling ABC generation and differentiation, suggesting that IL-13Rα1 contributes to these effects by regulating a subset of IL-21-mediated signaling events. These results also suggest that X-linked genes besides TLR7 participate in the regulation of ABCs in lupus.

Molecular mechanisms controlling age-associated B cells in autoimmunity

Age-associated B cells (ABCs) have emerged as critical components of immune responses. Their inappropriate expansion and differentiation have increasingly been linked to the pathogenesis of autoimmune disorders, aging-associated diseases, and infections. ABCs exhibit a distinctive phenotype and, in addition to classical B cell markers, often express the transcription factor T-bet and myeloid markers like CD11c; hence, these cells are also commonly known as CD11c+ T-bet+ B cells. Formation of ABCs is promoted by distinctive combinations of innate and adaptive signals. In addition to producing antibodies, these cells display antigen-presenting and proinflammatory capabilities. It is becoming increasingly appreciated that the ABC compartment exhibits a high degree of heterogeneity, plasticity, and sex-specific regulation and that ABCs can differentiate into effector progeny via several routes particularly in autoimmune settings. In this review, we will discuss the initial insights that have been obtained on the molecular machinery that controls ABCs and we will highlight some of the unique aspects of this control system that may enable ABCs to fulfill their distinctive role in immune responses. Given the expanding array of autoimmune disorders and pathophysiological settings in which ABCs are being implicated, a deeper understanding of this machinery could have important and broad therapeutic implications for the successful, albeit daunting, task of targeting these cells.

Altered function and differentiation of age-associated B cells contribute to the female bias in lupus mice

Differences in immune responses to viruses and autoimmune diseases such as systemic lupus erythematosus (SLE) can show sexual dimorphism. Age-associated B cells (ABC) are a population of CD11c+T-bet+ B cells critical for antiviral responses and autoimmune disorders. Absence of DEF6 and SWAP-70, two homologous guanine exchange factors, in double-knock-out (DKO) mice leads to a lupus-like syndrome in females marked by accumulation of ABCs. Here we demonstrate that DKO ABCs show sex-specific differences in cell number, upregulation of an ISG signature, and further differentiation. DKO ABCs undergo oligoclonal expansion and differentiate into both CD11c+ and CD11c- effector B cell populations with pathogenic and pro-inflammatory function as demonstrated by BCR sequencing and fate-mapping experiments. Tlr7 duplication in DKO males overrides the sex-bias and further augments the dissemination and pathogenicity of ABCs, resulting in severe pulmonary inflammation and early mortality. Thus, sexual dimorphism shapes the expansion, function and differentiation of ABCs that accompanies TLR7-driven immunopathogenesis.

Selective dysregulation of ROCK2 activity promotes aberrant transcriptional networks in ABC diffuse large B-cell lymphoma

Activated B-cell-like diffuse large B-cell lymphoma (ABC-DLBCL) is an aggressive subtype of lymphoma usually associated with inferior outcomes. ABC-DLBCL exhibits plasmablastic features and is characterized by aberrancies in the molecular networks controlled by IRF4. The signaling pathways that are dysregulated in ABC-DLBCL are, however, not fully understood. ROCK2 is a serine-threonine kinase whose role in lymphomagenesis is unknown. Here we show that ROCK2 activity is constitutively dysregulated in ABC-DLBCL but not in GCB-DLBCL and BL. We furthermore show that ROCK2 phosphorylates IRF4 and that the ROCK2-mediated phosphorylation of IRF4 modulates its ability to regulate a subset of target genes. In addition to its effects on IRF4, ROCK2 also controls the expression of MYC in ABC-DLBCL by regulating MYC protein levels. ROCK inhibition furthermore selectively decreases the proliferation and survival of ABC-DLBCL in vitro and inhibits ABC-DLBCL growth in xenograft models. Thus, dysregulated ROCK2 activity contributes to the aberrant molecular program of ABC-DLBCL via its dual ability to modulate both IRF4- and MYC-controlled gene networks and ROCK inhibition could represent an attractive therapeutic target for the treatment of ABC-DLBCL.

Serine-threonine kinase ROCK2 regulates germinal center B cell positioning and cholesterol biosynthesis

Germinal center (GC) responses require B cells to respond to a dynamic set of intercellular and microenvironmental signals that instruct B cell positioning, differentiation, and metabolic reprogramming. RHO-associated coiled-coil-containing protein kinase 2 (ROCK2), a serine-threonine kinase that can be therapeutically targeted by ROCK inhibitors or statins, is a key downstream effector of RHOA GTPases. Although RHOA-mediated pathways are emerging as critical regulators of GC responses, the role of ROCK2 in B cells is unknown. Here, we found that ROCK2 was activated in response to key T cell signals like CD40 and IL-21 and that it regulated GC formation and maintenance. RNA-Seq analyses revealed that ROCK2 controlled a unique transcriptional program in GC B cells that promoted optimal GC polarization and cholesterol biosynthesis. ROCK2 regulated this program by restraining AKT activation and subsequently enhancing FOXO1 activity. ATAC-Seq (assay for transposase-accessible chromatin with high-throughput sequencing) and biochemical analyses revealed that the effects of ROCK2 on cholesterol biosynthesis were instead mediated via a novel mechanism. ROCK2 directly phosphorylated interferon regulatory factor 8 (IRF8), a crucial mediator of GC responses, and promoted its interaction with sterol regulatory element-binding transcription factor 2 (SREBP2) at key regulatory regions controlling the expression of cholesterol biosynthetic enzymes, resulting in optimal recruitment of SREBP2 at these sites. These findings thus uncover ROCK2 as a multifaceted and therapeutically targetable regulator of GC responses.

Methods for high-dimensional analysis of cells dissociated from cryopreserved synovial tissue

Background

Detailed molecular analyses of cells from rheumatoid arthritis (RA) synovium hold promise in identifying cellular phenotypes that drive tissue pathology and joint damage. The Accelerating Medicines Partnership RA/SLE Network aims to deconstruct autoimmune pathology by examining cells within target tissues through multiple high-dimensional assays. Robust standardized protocols need to be developed before cellular phenotypes at a single cell level can be effectively compared across patient samples.

Methods

Multiple clinical sites collected cryopreserved synovial tissue fragments from arthroplasty and synovial biopsy in a 10% DMSO solution. Mechanical and enzymatic dissociation parameters were optimized for viable cell extraction and surface protein preservation for cell sorting and mass cytometry, as well as for reproducibility in RNA sequencing (RNA-seq). Cryopreserved synovial samples were collectively analyzed at a central processing site by a custom-designed and validated 35-marker mass cytometry panel. In parallel, each sample was flow sorted into fibroblast, T-cell, B-cell, and macrophage suspensions for bulk population RNA-seq and plate-based single-cell CEL-Seq2 RNA-seq.

Results

Upon dissociation, cryopreserved synovial tissue fragments yielded a high frequency of viable cells, comparable to samples undergoing immediate processing. Optimization of synovial tissue dissociation across six clinical collection sites with ~ 30 arthroplasty and ~ 20 biopsy samples yielded a consensus digestion protocol using 100 μg/ml of Liberase™ TL enzyme preparation. This protocol yielded immune and stromal cell lineages with preserved surface markers and minimized variability across replicate RNA-seq transcriptomes. Mass cytometry analysis of cells from cryopreserved synovium distinguished diverse fibroblast phenotypes, distinct populations of memory B cells and antibody-secreting cells, and multiple CD4⁺ and CD8⁺ T-cell activation states. Bulk RNA-seq of sorted cell populations demonstrated robust separation of synovial lymphocytes, fibroblasts, and macrophages. Single-cell RNA-seq produced transcriptomes of over 1000 genes/cell, including transcripts encoding characteristic lineage markers identified.

Conclusions

We have established a robust protocol to acquire viable cells from cryopreserved synovial tissue with intact transcriptomes and cell surface phenotypes. A centralized pipeline to generate multiple high-dimensional analyses of synovial tissue samples collected across a collaborative network was developed. Integrated analysis of such datasets from large patient cohorts may help define molecular heterogeneity within RA pathology and identify new therapeutic targets and biomarkers.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1631-y) contains supplementary material, which is available to authorized users.

Identification of Three Rheumatoid Arthritis Disease Subtypes by Machine Learning Integration of Synovial Histologic Features and RNA Sequencing Data

OBJECTIVE

In this study, we sought to refine histologic scoring of rheumatoid arthritis (RA) synovial tissue by training with gene expression data and machine learning.

METHODS

Twenty histologic features were assessed in 129 synovial tissue samples (n = 123 RA patients and n = 6 osteoarthritis [OA] patients). Consensus clustering was performed on gene expression data from a subset of 45 synovial samples. Support vector machine learning was used to predict gene expression subtypes, using histologic data as the input. Corresponding clinical data were compared across subtypes.

RESULTS

Consensus clustering of gene expression data revealed 3 distinct synovial subtypes, including a high inflammatory subtype characterized by extensive infiltration of leukocytes, a low inflammatory subtype characterized by enrichment in pathways including transforming growth factor β, glycoproteins, and neuronal genes, and a mixed subtype. Machine learning applied to histologic features, with gene expression subtypes serving as labels, generated an algorithm for the scoring of histologic features. Patients with the high inflammatory synovial subtype exhibited higher levels of markers of systemic inflammation and autoantibodies. C-reactive protein (CRP) levels were significantly correlated with the severity of pain in the high inflammatory subgroup but not in the others.

CONCLUSION

Gene expression analysis of RA and OA synovial tissue revealed 3 distinct synovial subtypes. These labels were used to generate a histologic scoring algorithm in which the histologic scores were found to be associated with parameters of systemic inflammation, including the erythrocyte sedimentation rate, CRP level, and autoantibody levels. Comparison of gene expression patterns to clinical features revealed a potentially clinically important distinction: mechanisms of pain may differ in patients with different synovial subtypes.

Regulation of systemic autoimmunity and CD11c+ Tbet+ B cells by SWEF proteins

Recent studies have revealed the existence of a T-bet dependent subset of B cells, which expresses unique phenotypic and functional characteristics including high levels of CD11c and CD11b. In the murine system this B cell subset has been termed Age/autoimmune-associated B cells (ABCs) since it expands with age in non-autoimmune mice and it prematurely accumulates in autoimmune-prone strains. The molecular mechanisms that promote the expansion and function of ABCs are largely unknown. This review will focus on the SWEF proteins, a small family of Rho GEFs comprised of SWAP-70 and its homolog DEF6, a newly identified risk variant for human SLE. We will first provide an overview of the SWEF proteins and then discuss the complex array of biological processes that they control and the autoimmune phenotypes that spontaneously develop in their absence, highlighting the emerging involvement of these proteins in regulating ABCs. A better understanding of the pathways controlled by the SWEF proteins could help provide new insights into the mechanisms responsible for the expansion of ABCs in autoimmunity and potentially guide the design of novel therapeutic approaches.

Regulation of Effector Treg Cells in Murine Lupus

OBJECTIVE

Treg cells need to acquire an effector phenotype to function in settings of inflammation. Whether effector Treg cells can limit disease severity in lupus is unknown. Interferon regulatory factor 4 (IRF-4) is an essential controller of effector Treg cells and regulates their ability to express interleukin-10 (IL-10). In non-Treg cells, IRF-4 activity is modulated by interactions with DEF-6 and its homolog switch-associated protein 70 (SWAP-70). Although mice lacking both DEF-6 and SWAP-70 (double-knockout [DKO] mice) develop lupus, they display normal survival, suggesting that in DKO mice, Treg cells can moderate disease development. The purpose of this study was to investigate whether Treg cells from DKO mice have an increased capacity to become effector Treg cells due to the ability of DEF-6 and SWAP-70 to restrain IRF-4 activity.

METHODS

Treg cells were evaluated by fluorescence-activated cell sorting. The B lymphocyte-induced maturation protein 1 (BLIMP-1)/IL-10 axis was assessed by crossing DKO mice with BLIMP-1-YFP-10BiT dual-reporter mice. Deletion of IRF-4 in Treg cells from DKO mice was achieved by generating FoxP3(Cre) IRF-4(fl/fl) DKO mice.

RESULTS

The concomitant absence of DEF-6 and SWAP-70 led to increased numbers of Treg cells, which acquired an effector phenotype in a cell-intrinsic manner. In addition, Treg cells from DKO mice exhibited enhanced expression of the BLIMP-1/IL-10 axis. Notably, DKO effector Treg cells survived and expanded as disease progressed. The accumulation of Treg cells from DKO mice was associated with the up-regulation of genes controlling autophagy. IRF-4 was required for the expansion and function of effector Treg cells from DKO mice.

CONCLUSION

This study revealed the existence of mechanisms that, by acting on IRF-4, can fine-tune the function and survival of effector Treg cells in lupus. These findings suggest that the existence of a powerful effector Treg cell compartment that successfully survives in an unfavorable inflammatory environment could limit disease development.

Targeting the RhoA-ROCK pathway to reverse T-cell dysfunction in SLE

OBJECTIVES

Deregulated production of interleukin (IL)-17 and IL-21 contributes to the pathogenesis of autoimmune disorders such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Production of IL-17 and IL-21 can be regulated by ROCK2, one of the two Rho kinases. Increased ROCK activation was previously observed in an SLE cohort. Here, we evaluated ROCK activity in a new SLE cohort, and an RA cohort, and assessed the ability of distinct inhibitors of the ROCK pathway to suppress production of IL-17 and IL-21 by SLE T cells or human Th17 cells.

METHODS

ROCK activity in peripheral blood mononuclear cells (PBMCs) from 29 patients with SLE, 31 patients with RA and 28 healthy controls was determined by ELISA. SLE T cells or in vitro-differentiated Th17 cells were treated with Y27632 (a pan-ROCK inhibitor), KD025 (a selective ROCK2 inhibitor) or simvastatin (which inhibits RhoA, a major ROCK activator). ROCK activity and IL-17 and IL-21 production were assessed. The transcriptional profile altered by ROCK inhibitors was evaluated by NanoString technology.

RESULTS

ROCK activity levels were significantly higher in patients with SLE and RA than healthy controls. Th17 cells exhibited high ROCK activity that was inhibited by Y27632, KD025 or simvastatin; each also decreased IL-17 and IL-21 production by purified SLE T cells or Th17 cells. Immune profiling revealed both overlapping and distinct effects of the different ROCK inhibitors.

CONCLUSIONS

ROCK activity is elevated in PBMCs from patients with SLE and RA. Production of IL-17 and IL-21 by SLE T cells or Th17 cells can furthermore be inhibited by targeting the RhoA-ROCK pathway via both non-selective and selective approaches.

The RhoA-ROCK pathway in the regulation of T and B cell responses

Effective immune responses require the precise regulation of dynamic interactions between hematopoietic and non-hematopoietic cells. The Rho subfamily of GTPases, which includes RhoA, is rapidly activated downstream of a diverse array of biochemical and biomechanical signals, and is emerging as an important mediator of this cross-talk. Key downstream effectors of RhoA are the Rho kinases, or ROCKs. The ROCKs are two serine-threonine kinases that can act as global coordinators of a tissue’s response to stress and injury because of their ability to regulate a wide range of biological processes. Although the RhoA-ROCK pathway has been extensively investigated in the non-hematopoietic compartment, its role in the immune system is just now becoming appreciated. In this commentary, we provide a brief overview of recent findings that highlight the contribution of this pathway to lymphocyte development and activation, and the impact that dysregulation in the activation of RhoA and/or the ROCKs may exert on a growing list of autoimmune and lymphoproliferative disorders.

Cigarette smoke inhibits ROCK2 activation in T cells and modulates IL-22 production

Gene-environment interactions are known to play a key role in the development of rheumatoid arthritis (RA). Exposure to cigarette smoke (CS) is one of the strongest environmental risk factors associated with RA and has been shown to mediate a range of complex immunomodulatory effects from decreased T and B cell activation to depressed phagocytic function. The effects of CS on the function of TH17 cells, one of the key TH effector subsets implicated in RA pathogenesis, are not fully understood. IRF4 is one of the crucial transcription factors involved in TH-17 differentiation and is absolutely required for the production of IL-17 and IL-21 but, interestingly, inhibits the synthesis of IL-22. The production of IL-17 and IL-21 by IRF4 can be augmented by its phosphorylation by the serine-threonine kinase ROCK2. Given that CS has been reported to increase ROCK activity in endothelial cells, here we investigated the effects of CS on the ROCK2-IRF4 axis in T cells. Surprisingly, we found that CS leads to decreased ROCK2 activation and IRF4 phosphorylation in T cells. This effect was associated with increased IL-22 production. Using a GEF pull-down assay we furthermore identify ARHGEF1 as a key upstream regulator of ROCK2 whose activity in T cells is inhibited by CS. Thus CS can inhibit the ROCK2-IRF4 axis and modulate T cell production of IL-22.

IRF4-Dependent and IRF4-Independent Pathways Contribute to DC Dysfunction in Lupus

Interferon Regulatory Factors (IRFs) play fundamental roles in dendritic cell (DC) differentiation and function. In particular, IRFs are critical transducers of TLR signaling and dysregulation in this family of factors is associated with the development of autoimmune disorders such as Systemic Lupus Erythematosus (SLE). While several IRFs are expressed in DCs their relative contribution to the aberrant phenotypic and functional characteristics that DCs acquire in autoimmune disease has not been fully delineated. Mice deficient in both DEF6 and SWAP-70 (= Double-knock-out or DKO mice), two members of a unique family of molecules that restrain IRF4 function, spontaneously develop a lupus-like disease. Although autoimmunity in DKO mice is accompanied by dysregulated IRF4 activity in both T and B cells, SWAP-70 is also known to regulate multiple aspects of DC biology leading us to directly evaluate DC development and function in these mice. By monitoring Blimp1 expression and IL-10 competency in DKO mice we demonstrate that DCs in these mice exhibit dysregulated IL-10 production, which is accompanied by aberrant Blimp1 expression in the spleen but not in the peripheral lymph nodes. We furthermore show that DCs from these mice are hyper-responsive to multiple TLR ligands and that IRF4 plays a differential role in in these responses by being required for the TLR4-mediated but not the TLR9-mediated upregulation of IL-10 expression. Thus, DC dysfunction in lupus-prone mice relies on both IRF4-dependent and IRF4-independent pathways.

CD95 Rapidly Clusters in Cells of Diverse Origins

We have shown that CD95-mediated cell death requires a clustering of the receptor in distinct sphingolipid-rich domains of the cell membrane (Grassme et al., 2000, Cremesti et al., 2000). These domains form in response to acid sphingomyelinase (ASM)-induced ceramide generation. However, recent studies challenged the finding of early CD95 clustering (Algeciras-Schimnich et al., 2002). Here, six independent groups tested clustering of CD95 in diverse cell type including primary cells ex vivo and established cell lines. The studies show clustering of CD95 within seconds to minutes in all cell types tested by the different groups. In addition, clustering of CD95 was detected after stimulation of cells using three agonistic anti-CD95 antibodies (CH11, APO-1-3 and JO2), CD95 ligand and stimuli that induce an upregulation and activation of the endogenous CD95/CD95 ligand system. The data confirm our previous studies and suggest rapid, i.e., within seconds to minutes, CD95 clustering as a general phenomenon occurring in many cell types.

Enhanced rho-associated protein kinase activation in patients with systemic lupus erythematosus

OBJECTIVE

Rho-associated protein kinases (ROCKs) have been implicated in the pathogenesis of cardiovascular and renal disorders. We recently showed that ROCKs could regulate the differentiation of murine Th17 cells and the production of interleukin-17 (IL-17) and IL-21, two cytokines associated with systemic lupus erythematosus (SLE). The goal of this study was to assess ROCK activation in human Th17 cells and to evaluate ROCK activity in SLE patients.

METHODS

An enzyme-linked immunosorbent assay (ELISA)-based ROCK activity assay was used to evaluate ROCK activity in human cord blood CD4+ T cells differentiated under Th0 or Th17 conditions. We then performed a cross-sectional analysis of 28 SLE patients and 25 healthy matched controls. ROCK activity in peripheral blood mononuclear cell (PBMC) lysates was determined by ELISA. Cytokine and chemokine profiles were analyzed by ELISA.

RESULTS

Human cord blood CD4+ T cells differentiated under Th17 conditions expressed higher levels of ROCK activity than did CD4+ T cells stimulated under Th0 conditions. Production of IL-17 and IL-21 was inhibited by the addition of a ROCK inhibitor. SLE PBMCs expressed significantly higher levels of ROCK activity than did healthy control PBMCs (1.25 versus 0.56; P = 0.0015). Sixteen SLE patients (57%) expressed high levels of ROCK (optical density at 450 nm >1). Disease duration, lymphocyte count, and azathioprine use were shown to be significant independent predictors of ROCK activity in multivariable analyses.

CONCLUSION

Consistent with previous results in the murine system, increased ROCK activation was associated with Th17 cell differentiation. Moreover, enhanced ROCK activity was observed in a subgroup of SLE patients. These data support the concept that the ROCK pathway could represent an important therapeutic target for SLE.

Dual regulation of IRF4 function in T and B cells is required for the coordination of T-B cell interactions and the prevention of autoimmunity

Effective humoral responses to protein antigens require the precise execution of carefully timed differentiation programs in both T and B cell compartments. Disturbances in this process underlie the pathogenesis of many autoimmune disorders, including systemic lupus erythematosus (SLE). Interferon regulatory factor 4 (IRF4) is induced upon the activation of T and B cells and serves critical functions. In CD4(+) T helper cells, IRF4 plays an essential role in the regulation of IL-21 production, whereas in B cells it controls class switch recombination and plasma cell differentiation. IRF4 function in T helper cells can be modulated by its interaction with regulatory protein DEF6, a molecule that shares a high degree of homology with only one other protein, SWAP-70. Here, we demonstrate that on a C57BL/6 background the absence of both DEF6 and SWAP-70 leads to the development of a lupus-like disease in female mice, marked by simultaneous deregulation of CD4(+) T cell IL-21 production and increased IL-21 B cell responsiveness. We furthermore show that DEF6 and SWAP-70 are differentially used at distinct stages of B cell differentiation to selectively control the ability of IRF4 to regulate IL-21 responsiveness in a stage-specific manner. Collectively, these data provide novel insights into the mechanisms that normally couple and coordinately regulate T and B cell responses to ensure tight control of productive T-B cell interactions.

Administration of fasudil, a ROCK inhibitor, attenuates disease in lupus-prone NZB/W F1 female mice

Accumulating evidence from murine studies suggests that the RhoA/ROCK pathway plays an important role in the development of autoimmune disorders. We previously demonstrated that ROCK inhibition ameliorates disease in MRL/lpr mice, a spontaneous model of lupus. This study aimed to explore the protective effects of the ROCK inhibitor fasudil in a distinct model of lupus, NZB/W F1 female mice, to assess the broad applicability of ROCK inhibition for the treatment of lupus. NZB/W F1 female mice were administered fasudil continuously in their drinking water starting at 18 or 24 weeks of age up until 44 weeks of age, or remained untreated. Fasudil treatment significantly improved survival and decreased proteinuria, particularly when treatment was started at 18 weeks. There was also a significant decrease in serum anti-dsDNA autoantibody production, glomerular IgG and C3 deposition, and glomerulonephritis. Analysis of the splenic lymphocyte compartment revealed reduced effector/memory CD4(+) T cell and plasma cell numbers in fasudil treated mice while the frequency of other B cell and T cell subsets was unchanged. These results thus indicate that fasudil can ameliorate disease in NZB/W F1 female mice, suggesting that ROCK inhibition might be broadly effective for the treatment of lupus.

A murine autoimmune model of rheumatoid arthritis and systemic lupus erythematosus associated with deregulated production of IL-17 and IL-21

T-helper cell 17 (Th17) cells play an important role in the pathogenesis of many autoimmune disorders including Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE). In this chapter we describe a murine model where deregulated production of IL-17 and IL-21 can lead to either lupus-like disease or RA-like symptoms depending on the genetic background. We delineate the key techniques that can be used to dissect the mechanisms responsible for the pathogenesis of these diseases at both a cellular and molecular level including in vitro Th17 cell differentiation, chromatin immunoprecipitation assays, and retroviral transduction experiments. We also describe the methodologies that can be utilized to monitor the classic clinical findings of RA and SLE in murine models. Given the broad involvement of deregulated production of IL-17 and IL-21 in autoimmunity, many of these techniques could also be valuable for the investigation of these pathways in murine models of other autoimmune diseases.

Aberrant ROCK activation promotes the development of type I diabetes in NOD mice

Aberrant production of IL-21 by T cells is critical for the development of type 1 diabetes (T1D) in NOD mice. The pathogenic effects of IL-21 are partly due to its ability to promote the generation of T(H)-17 cells. Interferon Regulatory Factor (IRF4) is a crucial regulator of IL-17 and IL-21 production. We recently found that the serine-threonine kinase ROCK2 phosphorylates IRF4 and regulates its ability to control IL-17 and IL-21 production. Here we show that NOD T cells aberrantly activate ROCK2. We furthermore demonstrate that ROCK inhibition corrects the abnormal IRF4 function in NOD T cells and diminishes their production of IL-17 and IL-21. Importantly, administration of a ROCK inhibitor to NOD mice protects against diabetes development. These studies thus support the idea that ROCK2 is inappropriately activated in NOD T cells and that ROCK kinases could represent important therapeutic targets for the treatment of T1D.

Phosphorylation of IRF4 by ROCK2 regulates IL-17 and IL-21 production and the development of autoimmunity in mice

Deregulated production of IL-17 and IL-21 plays a key pathogenic role in many autoimmune disorders. A delineation of the mechanisms that underlie the inappropriate synthesis of IL-17 and IL-21 in autoimmune diseases can thus provide important insights into potential therapies for these disorders. Here we have shown that the serine-threonine kinase Rho-associated, coiled-coil-containing protein kinase 2 (ROCK2) becomes activated in mouse T cells under Th17 skewing conditions and phosphorylates interferon regulatory factor 4 (IRF4), a transcription factor that is absolutely required for the production of IL-17 and IL-21. We furthermore demonstrated that ROCK2-mediated phosphorylation of IRF4 regulated the synthesis of IL-17 and IL-21 and the differentiation of Th17 cells. Whereas CD4+ T cells from WT mice activated ROCK2 physiologically under Th17 conditions, CD4+ T cells from 2 different mouse models of spontaneous autoimmunity aberrantly activated ROCK2 under neutral conditions. Moreover, administration of ROCK inhibitors ameliorated the deregulated production of IL-17 and IL-21 and the inflammatory and autoantibody responses observed in these autoimmune mice. Our findings thus uncover a crucial link among ROCK2, IRF4, and the production of IL-17 and IL-21 and support the idea that selective inhibition of ROCK2 could represent an important therapeutic regimen for the treatment of autoimmune disorders.

IRF4 and its regulators: evolving insights into the pathogenesis of inflammatory arthritis?

Accumulating evidence from murine and human studies supports a key role for interleukin-17 (IL-17) and IL-21 in the pathogenesis of inflammatory arthritis. The pathways and molecular mechanisms that underlie the production of IL-17 and IL-21 are being rapidly elucidated. This review focuses on interferon regulatory factor 4 (IRF4), a member of the IRF family of transcription factors, which has emerged as a crucial controller of both IL-17 and IL-21 production. We first outline the complex role of IRF4 in the function of CD4(+) T cells and then discuss recent studies from our laboratory that have revealed a surprising role for components of Rho GTPase-mediated pathways in controlling the activity of IRF4. A better understanding of these novel pathways will hopefully provide new insights into mechanisms responsible for the development of inflammatory arthritis and potentially guide the design of novel therapeutic approaches.

Th17 cells in rheumatoid arthritis and systemic lupus erythematosus

Recent work has implicated a novel Th effector cell subset, the Th17 cell subset, in the development of both rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) because of the ability of Th17 cells to produce cytokines like IL-17 and IL-21 that can drive both inflammatory and humoral responses. In this review, we will discuss recent studies that have begun elucidating the factors that regulate the development of Th17 cells and provide a brief overview of the role of Th17 cells in RA and SLE.

Rho GTPase-mediated pathways in mature CD4+ T cells

Effective immune responses require the appropriate activation and differentiation of peripheral CD4(+) T cells. These processes need to be followed by the timely elimination of the responding T cells in order to restore T cell homeostasis. Defects in the appropriate regulation of T cell activation, expansion, and survival underlie the pathogenesis of many autoimmune disorders including SLE. The molecular machinery employed by T cells to properly control these processes and prevent the onset of autoimmunity has not been fully elucidated. Rho GTPases (which include the Rac, Cdc42, and Rho subfamilies) are molecular switches that control a wide range of cellular processes. Their fundamental role in biology is due to their ability to regulate both cytoskeletal dynamics and a large number of signal transduction pathways. Activation of Rho GTPases is now recognized as a key event in the coordination of immune responses and, particularly, in the activation of T cells. In this review, we will first provide an overview of the role of Rho GTPase-mediated pathways in mature CD4(+) T cells and then we will discuss recent studies, which suggest that deregulation of these pathways may play a role in the pathogenesis of SLE.

Regulation of TLR4-mediated signaling by IBP/Def6, a novel activator of Rho GTPases

TLRs play a fundamental role in innate immune responses. Although Rho GTPases have been implicated in TLR-mediated signaling pathways, the molecules that control their activation in response to TLR engagement are largely unknown. IFN regulatory factor-4-binding protein (IBP; which is encoded by the gene Def6) is a unique type of activator for Rac that plays a crucial role in TCR-mediated signaling and adaptive immune responses. Here, we demonstrate that IBP/Def6 also controls innate immune responses by modulating TLR-induced signaling events. Mice deficient in IBP/Def6 are protected from LPS-induced septic shock. This protection is associated with a decrease in the production of proinflammatory cytokines and is accompanied by diminished activation of MAPKs and NF-kappaB. Our results thus identify IBP/Def6 as a novel component of the TLR4-induced signaling cascade that controls the production of proinflammatory cytokines.

IRF-4-binding protein inhibits interleukin-17 and interleukin-21 production by controlling the activity of IRF-4 transcription factor

The T helper 17 (Th17) cell lineage is important in inflammatory and autoimmune responses, via its ability to produce interleukin-17 (IL-17) and IL-21. Given the potentially deleterious effects of Th17 cells, their generation needs to be strictly controlled. IRF-4 is a transcription factor that has recently emerged as a key regulator of Th17 cell differentiation. Here, we showed that mice deficient in a previously isolated protein, IBP (IRF-4-binding protein), rapidly developed rheumatoid arthritis-like joint disease and large-vessel vasculitis. The pathology was associated with an enhanced responsiveness of T cells to low levels of stimulation and with the inappropriate synthesis of IL-17 and IL-21. IBP sequestered IRF-4 and prevented it from targeting the transcriptional regulatory regions of the genes that encode IL-17 and IL-21. Thus, IBP appears to be important in preventing T cell-mediated autoimmunity by ensuring that the production of IL-17 and IL-21 does not occur in response to self-antigens.

Estrogen and CD4+ T cells

PURPOSE OF REVIEW

Many autoimmune rheumatic autoimmune disorders predominantly affect women. Sex hormones, in particular estrogen, can influence CD4 T-helper development and function. We highlight recent studies that begin to provide insights into the mechanisms by which estrogen modulates CD4 T-cell development and function, and thus potentially contribute to disease pathogenesis.

RECENT FINDINGS

High levels of estrogen can lead to thymic atrophy. Recent studies showed that this phenomenon results from effects of estrogen at multiple stages in early T-cell development. Estrogen is also known to affect mature CD4 T-cell function, and, in particular, their ability to produce selected cytokine profiles. The mechanisms by which estrogen can exert these effects were also recently explored and shown to include effects on expression of critical molecules known to be involved in these processes.

SUMMARY

Dissecting the molecular pathways employed by estrogen to modulate CD4 T cells will be critical in elucidating the manner by which estrogen exerts its effects on this compartment. Given that cell type specific differences underlie the ability of many hormonal therapies to exert tissue-specific estrogenic or antiestrogenic activities, this knowledge will be crucial to further exploitation of hormonal therapies in rheumatic autoimmune diseases.

Loss of IRF-4-binding protein leads to the spontaneous development of systemic autoimmunity

IFN regulatory factor 4-binding (IRF-4-binding) protein (IBP) is a novel type of activator of Rho GTPases that is recruited to the immunological synapse upon TCR stimulation. Here we demonstrate that loss of IBP leads to the spontaneous development of a systemic autoimmune disorder characterized by the accumulation of effector/memory T cells and IgG+ B cells, profound hypergammaglobulinemia, and autoantibody production. Similar to human SLE, this syndrome primarily affects females. T cells from IBP-deficient mice are resistant to death in vitro as well as in vivo and exhibit selective defects in effector function. In the absence of IBP, T cells respond suboptimally to TCR engagement, as demonstrated by diminished ERK1/2 activation, decreased c-Fos induction, impaired immunological synapse formation, and defective actin polymerization. Transduction of IBP-deficient T cells with a WT IBP protein, but not with an IBP mutant lacking the Dbl-like domain required for Rho GTPase activation, rescues the cytoskeletal defects exhibited by these cells. Collectively, these findings indicate that IBP, a novel regulator of Rho GTPases, is required for optimal T cell effector function, lymphocyte homeostasis, and the prevention of systemic autoimmunity.

T cell receptor engagement leads to the recruitment of IBP, a novel guanine nucleotide exchange factor, to the immunological synapse

Reorganization of the actin cytoskeleton is crucial to the formation and function of the immunological synapse. Rho GTPases are critical mediators of cytoskeletal reorganization, and their activity at the synapse is likely to be stringently regulated. Guanine nucleotide exchange factors (GEFs) belonging to the Dbl family of proteins represent one major class of proteins that regulate the activity of Rho GTPases. Here we demonstrate that IBP, a homologue of SWAP-70, is a novel GEF for Rac1 and Cdc42 in T lymphocytes, which is recruited to the immunological synapse upon engagement of the antigen receptor. Mutational analysis supports a model whereby IBP is inactive in unstimulated cells. Upon engagement of the T cell receptor, its GEF activity is enhanced by tyrosine phosphorylation, as well as by binding newly generated phosphatidylinositol 3,4,5-trisphosphate. Although it is known that T cell receptor engagement leads to the recruitment of Vav to the immunological synapse, these findings indicate that other GEFs, such as IBP, also relocalize to this intercellular region. The recruitment and activation of distinct classes of GEFs may allow for precise control of Rho GTPase function at the crucial interface between T cells and antigen presenting cells.

Molecular cloning of IBP, a SWAP-70 homologous GEF, which is highly expressed in the immune system

Rho GTPases play a fundamental role in a variety of biological processes ranging from the reorganization of the actin cytoskeleton to the regulation of cell proliferation. The activation of Rho GTPases is regulated by guanine nucleotide exchange factors (GEFs) belonging to the Dbl family of proteins. The hallmark of this large family of GEFs is the presence of a tandem DH-PH module in which a pleckstrin-homology (PH) domain is located at the C-terminus of a Dbl-homology (DH) domain. Recent studies have demonstrated that SWAP-70 constitutes a novel class of Rac-GEF, in which the PH domain is located at the N-terminus, rather than the C terminus, of the DH domain. Here we report the molecular cloning of human IBP (IRF-4 binding protein), a new member of this novel family of GEFs. The IBP gene maps to human chromosome 6p21.31 centromeric to the MHC locus. Isolation of the murine IBP cDNA reveals a very high degree of homology with the human IBP cDNA suggesting that IBP is evolutionarily conserved. The 5' portion of the murine IBP cDNA is furthermore identical to the Def-6 cDNA fragment, which was identified in the course of a search for genes differentially expressed in the murine hematopoietic system. IBP is broadly expressed in the immune system and can be detected in both T and B cell compartments in contrast to SWAP-70 whose expression is primarily restricted to B cells. Taken together these findings indicate that IBP is a novel type of GEF, which participates in the activation of Rho GTPases in lymphoid tissues.

Regulation of lymphocyte apoptosis by interferon regulatory factor 4 (IRF-4)

To ensure that homeostasis of the immune system is maintained, the sensitivity of lymphocytes to Fas-mediated apoptosis is differentially regulated during their activation. The molecular mechanisms that link the activation program of lymphocytes to changes in sensitivity to Fas-mediated apoptosis have, however, not been fully characterized. In these studies, we have investigated whether Fas-mediated apoptosis can be regulated by interferon regulatory factor 4 (IRF-4), a lymphoid-restricted member of the IRF family of transcription factors. IRF-4 expression is upregulated during lymphocyte activation and IRF-4-deficient mice have defects in both lymphocyte activation and homeostasis. Here, we show that stable expression of IRF-4 in a human lymphoid cell line that normally lacks IRF-4 leads to a significantly enhanced apoptotic response on Fas receptor engagement. A systematic examination of the downstream effectors of Fas signaling in IRF-4-transfected cells demonstrates an increased activation of caspase-8, as well as an increase in Fas receptor polarization. We demonstrate that IRF-4-deficient mice display defects in activation-induced cell death, as well as superantigen-induced deletion, and that these defects are accompanied by impairments in Fas receptor polarization. These data suggest that IRF-4, by modulating the efficiency of the Fas-mediated death signal, is a novel participant in the regulation of lymphoid cell apoptosis.

Modulation of T cell cytokine production by interferon regulatory factor-4

Production of cytokines is one of the major mechanisms employed by CD4(+) T cells to coordinate immune responses. Although the molecular mechanisms controlling T cell cytokine production have been extensively studied, the factors that endow T cells with their ability to produce unique sets of cytokines have not been fully characterized. Interferon regulatory factor (IRF)-4 is a lymphoid-restricted member of the interferon regulatory factor family of transcriptional regulators, whose deficiency leads to a profound impairment in the ability of mature CD4(+) T cells to produce cytokines. In these studies, we have investigated the mechanisms employed by IRF-4 to control cytokine synthesis. We demonstrate that stable expression of IRF-4 in Jurkat T cells not only leads to a strong enhancement in the synthesis of interleukin (IL)-2, but also enables these cells to start producing considerable amounts of IL-4, IL-10, and IL-13. Transient transfection assays indicate that IRF-4 can transactivate luciferase reporter constructs driven by either the human IL-2 or the human IL-4 promoter. A detailed analysis of the effects of IRF-4 on the IL-4 promoter reveals that IRF-4 binds to a site adjacent to a functionally important NFAT binding element and that IRF-4 cooperates with NFATc1. These studies thus support the notion that IRF-4 represents one of the lymphoid-specific components that control the ability of T lymphocytes to produce a distinctive array of cytokines.

The role of IRF-4 in B and T cell activation and differentiation

Appropriate activation and differentiation of lymphocytes are critical for effective immune responses. These processes are normally guided by exposure of lymphocytes to different stimuli, which need to be appropriately integrated in order for lymphocytes to proceed along their activation and differentiation pathways. Although the early steps in lymphocyte activation have been studied extensively, the downstream effectors of these activation pathways and the basic mechanisms employed by lymphocytes to integrate the information provided by different activation stimuli are not fully characterized. Interferon (IFN) regulatory factor-4 (IRF-4) is a recently described member of the IRF family of transcription factors whose expression is largely restricted to lymphocytes. Genetic studies have indicated that IRF-4 is critical for the function of mature T and B cells. Here we review the role of IRF-4 as a downstream effector and potentially an integrator of lymphocyte responses.

Stage-specific modulation of IFN-regulatory factor 4 function by Krüppel-type zinc finger proteins

Optimal humoral responses depend on the activation of Ag-specific B cells, followed by their progression toward a fully differentiated phenotype. Acquisition of stage-appropriate patterns of gene expression is crucial to this differentiation program. However, the molecular mechanisms used by B cells to modulate gene expression as they complete their maturation program are poorly understood. IFN-regulatory factor 4 (IRF-4) plays a critical role in mature B cell function. Using the transcriptional regulation of the human B cell activation marker CD23 as a model system, we have previously demonstrated that IRF-4 is induced in response to B cell-activating stimuli and that it acts as a transactivator of CD23 gene expression. We have furthermore found that IRF-4 function can be blocked by B cell lymphomas 6 (BCL-6) protein, a Krüppel-type zinc finger repressor normally expressed in germinal center B cells. However, CD23 expression is known to be down-regulated in plasma cells despite high level expression of IRF-4 and the lack of BCL-6, suggesting that in plasma cells the IRF-4-mediated induction of CD23 is prevented by its interaction with a distinct repressor. In this set of studies, we demonstrate that IRF-4 interacts with B lymphocyte-induced maturation protein/positive regulatory domain I-binding factor 1 (Blimp1/PRD1-BF1), a Krüppel-type zinc finger protein whose expression correlates with terminal B cell differentiation. Functional studies indicate that Blimp1, like BCL-6, can block IRF-4-transactivating ability. These findings thus support a model whereby IRF-4 function is modulated in a stage-specific manner by its interaction with developmentally restricted sets of Krüppel-type zinc finger proteins.

Lineage-specific modulation of interleukin 4 signaling by interferon regulatory factor 4

Interleukin (IL)-4 is an immunoregulatory cytokine that exerts distinct biological activities on different cell types. Our studies indicate that interferon regulatory factor (IRF)-4 is both a target and a modulator of the IL-4 signaling cascade. IRF-4 expression is strongly upregulated upon costimulation of B cells with CD40 and IL-4. Furthermore, we find that IRF-4 can interact with signal transducer and activator of transcription (Stat)6 and drive the expression of IL-4-inducible genes. The transactivating ability of IRF-4 is blocked by the repressor factor BCL-6. Since expression of IRF-4 is mostly confined to lymphoid cells, these data provide a potential mechanism by which IL-4-inducible genes can be regulated in a lineage-specific manner.

IFN-alpha activates Stat6 and leads to the formation of Stat2:Stat6 complexes in B cells

IFN-alpha consists of a family of highly homologous proteins, which exert pleiotropic effects on a wide variety of cell types. The biologic activities of IFN-alpha are mediated by its binding to a multicomponent receptor complex resulting in the activation of the Janus kinase-STAT signaling pathway. In most cell types, activation of Stat1 and Stat2 by IFN-alpha leads to the formation of either STAT homo-/heterodimers or of the IFN-stimulated gene factor 3 complex composed of Stat1, Stat2, and p48, a non-STAT protein. These distinct transcriptional complexes then target two different sets of cis-elements, gamma-activated sites and IFN-stimulated response elements. Here, we report that IFN-alpha can activate complexes containing Stat6, which, until now, has been primarily associated with signaling by two cytokines with biologic overlap, IL-4 and IL-13. Induction of Stat6 complexes by IFN-alpha appears to be cell type specific, given that tyrosine phosphorylation of Stat6 in response to IFN-alpha is predominantly detected in B cells. Activation of Stat6 by IFN-alpha in B cells is accompanied by the formation of novel Stat2:Stat6 complexes, including an IFN-stimulated gene factor 3-like complex containing Stat2, Stat6, and p48. B cell lines resistant to the antiproliferative effects of IFN-alpha display a decrease in the IFN-alpha-mediated activation of Stat6. Activation of Stat6 as well as of Stat2:Stat6 complexes by IFN-alpha in B cells may allow modulation of target genes in a cell type-specific manner.

IFN-α Activates Stat6 and Leads to the Formation of Stat2:Stat6 Complexes in B Cells

IFN-α consists of a family of highly homologous proteins, which exert pleiotropic effects on a wide variety of cell types. The biologic activities of IFN-α are mediated by its binding to a multicomponent receptor complex resulting in the activation of the Janus kinase-STAT signaling pathway. In most cell types, activation of Stat1 and Stat2 by IFN-α leads to the formation of either STAT homo-/heterodimers or of the IFN-stimulated gene factor 3 complex composed of Stat1, Stat2, and p48, a non-STAT protein. These distinct transcriptional complexes then target two different sets of cis-elements, γ-activated sites and IFN-stimulated response elements. Here, we report that IFN-α can activate complexes containing Stat6, which, until now, has been primarily associated with signaling by two cytokines with biologic overlap, IL-4 and IL-13. Induction of Stat6 complexes by IFN-α appears to be cell type specific, given that tyrosine phosphorylation of Stat6 in response to IFN-α is predominantly detected in B cells. Activation of Stat6 by IFN-α in B cells is accompanied by the formation of novel Stat2:Stat6 complexes, including an IFN-stimulated gene factor 3-like complex containing Stat2, Stat6, and p48. B cell lines resistant to the antiproliferative effects of IFN-α display a decrease in the IFN-α-mediated activation of Stat6. Activation of Stat6 as well as of Stat2:Stat6 complexes by IFN-α in B cells may allow modulation of target genes in a cell type-specific manner.

Signaling pathways mediated by the TNF- and cytokine-receptor families target a common cis-element of the IFN regulatory factor 1 promoter

CD40 activation of B cells is strongly influenced by the presence of cytokines. However, the molecular basis for the interplay between these distinct stimuli is not clearly delineated. IFN regulatory factor 1 (IRF-1) is a transcription factor activated by either CD40 or cytokines. We have found that these different sets of signals target a common cis-acting element in the promoter of this gene, the IRF-1 gamma-activated site (GAS). Targeting of the IRF-1 GAS is not confined to activation via CD40 but extends to other stimuli that mimic the CD40 signaling cascade, like TNF-alpha and EBV. In contrast to induction of STATs by cytokines, the IRF-1 GAS-binding complex activated by CD40, TNF-alpha, or EBV contains Rel proteins, specifically p50 and p65. In this system, simultaneous exposure to CD40L together with either IL-4 or IFN-gamma does not lead to the activation of novel Rel/STAT complexes. Given the importance of IRF-1 in a variety of biologic functions from proliferation to apoptosis, our findings support the notion that modulation of IRF-1 levels may be a critical control point in B cell activation.

Signaling Pathways Mediated by the TNF- and Cytokine-Receptor Families Target a Common cis-Element of the IFN Regulatory Factor 1 Promoter

CD40 activation of B cells is strongly influenced by the presence of cytokines. However, the molecular basis for the interplay between these distinct stimuli is not clearly delineated. IFN regulatory factor 1 (IRF-1) is a transcription factor activated by either CD40 or cytokines. We have found that these different sets of signals target a common cis-acting element in the promoter of this gene, the IRF-1 gamma-activated site (GAS). Targeting of the IRF-1 GAS is not confined to activation via CD40 but extends to other stimuli that mimic the CD40 signaling cascade, like TNF-α and EBV. In contrast to induction of STATs by cytokines, the IRF-1 GAS-binding complex activated by CD40, TNF-α, or EBV contains Rel proteins, specifically p50 and p65. In this system, simultaneous exposure to CD40L together with either IL-4 or IFN-γ does not lead to the activation of novel Rel/STAT complexes. Given the importance of IRF-1 in a variety of biologic functions from proliferation to apoptosis, our findings support the notion that modulation of IRF-1 levels may be a critical control point in B cell activation.