The role of TGF-beta in growth, differentiation, and maturation of B lymphocytes

Immunohistochemical expression of angiogenic cytokines and their receptors in reactive benign lymph nodes and non-Hodgkin lymphoma

Angiogenic cytokines regulate B-cell lymphopoiesis and are related to prognosis in B-cell lymphoproliferative disorders. Transforming growth factor-beta (TGF-beta) inhibits mature B-cell proliferation and immunoglobulin production. Increased levels of serum vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) are associated with poor prognosis in non-Hodgkin lymphoma (NHL). To understand the expression of angiogenic cytokines at different stages of B-cell differentiation in lymph nodes, we examined the immunohistochemical expression of TGF-beta, VEGF, bFGF, and their receptors in five patients with reactive benign lymphadenopathy and 12 patients with B-cell NHL (mantle cell lymphoma, 4; small cleaved cell follicular lymphoma, 5; lymphoplasmacytic lymphoma, 3). In benign lymph nodes, TGF-beta1, TGF-beta2, and TGFbetaRII were positive in prefollicular mantle cells, follicular center cells, and postfollicular plasma cells. Basic FGF, FGF-R1, and FGF-R4 were positive in large follicular center cells and postfollicular plasma cells. Vascular endothelial growth factor was positive in large follicular center cells and postfollicular plasma cells. In NHL, TGF-beta and its receptors were weakly positive in small cleaved cell follicular lymphoma; VEGF was strongly positive in lymphoplasmacytic lymphoma and weakly positive in mantle cell lymphoma. Basic FGF and its receptors were negative in NHL; however, FGF-R4 was positive in some cases of small cleaved cell follicular lymphoma. Our findings suggest that TGF-beta, bFGF, and their receptors have opposite roles in B-cell differentiation and maturation in benign lymph nodes. Transforming growth factor-beta and its receptors have an important role in germinal center development; loss of their activity could be associated with abnormal clonal proliferation of NHL.

Type 1 and type 2 responses in regulation of Ig isotype expression in cattle

Regulation of humoral immune responses is multifactorial involving appropriate activation, costimulation and the presence of specific soluble factors. Polarized type 1 or type 2 humoral responses in the laboratory mouse have been linked to expression of specific cytokines and thus can be used to provide insight into the type of response generated by infection. For example, IFN-gamma has been linked to IgG2a and IgG3 production, IL-4 to IgG1 and IgE production and TGF-beta to IgA production. Unlike the laboratory mouse, generally housed under defined conditions, highly skewed isotype expression patterns generally occur in cattle in chronic infections. A few examples of polarized responses have been noted in chronic experimental or naturally occurring infections including F. hepatica, M. paratuberculosis, C. parvum and B. abortus. In vitro studies using purified bovine B cells and various forms of costimulation and cytokines have demonstrated that isotype responses can be polarized under certain experimental conditions in vitro. That is, IgG1 expression is positively regulated by IL-4 and IgG2 expression is positively regulated by IFN-gamma. Other as yet unidentified factors may play pivotal roles in regulating humoral immune responses in large ruminant species in vivo. This possibility is best exemplified by recent studies using DNA vaccines in cattle that have been demonstrated in the mouse to be generally polarizing to a type 1 response. Surprisingly, studies in cattle using plasmid DNA as vaccination material show an almost exclusive IgG1 response. Based on a number of studies using T cell clones and various biological assays, it is clear that the classical roles of many cytokines in the laboratory mouse do not extrapolate entirely or at all to cattle. Thus, the design of adjuvants and immune modulators should be based on studies done in cattle or using bovine cells. Based on studies to date, several "holes" in the cytokine repertoire exist and these roles may be assumed by unique factors or activities of other known cytokines.

Role of the innate immune system in the development of chronic colitis

Based on Pasteur's work on the microbial nature of fermentation, it was widely believed that the presence of bacteria in the intestine was essential for the life of the host. It has also been known for decades that gut commensal microbes effect the activation and development of the systemic immune system through gut-associated lymphoid tissues (GALT). Recent extensive studies have shown that recognition of microbes is mediated by a set of germline-encoded receptors, Toll-like receptors (TLRs), in mammals. This article reviews the role of the innate immunity system in the development of GALT and the pathogenesis of inflammatory bowel diseases (IBD).

Cyclin D-cdk6 complex is targeted by p21(WAF) in growth-arrested lymphoma cells

Normal human B lymphocytes are sensitive to the growth-inhibitory action of transforming growth factor beta1 (TGFbeta1) whereas malignant B lymphoma cells are mostly resistant to TGFbeta1 effects. We examined the phosphorylation status of retinoblastoma protein and the activity of G(1) cyclin-dependent kinases (cdk) in TGFbeta1-sensitive malignant follicular lymphoma cells during the TGFbeta1 treatment. The kinase activity of cdk2, cdk4, and cdk6 was significantly reduced and hypophosphorylation of pRb on serine 795 (S795) and threonine 373 (T373) was observed. We examined the composition of cdk complexes and the level of cdk inhibitors to explain the inhibitory action of TGFbeta1 toward cdk activity. Both cdk4 and cdk6 were notably dissociated from cyclin D cofactors, while cyclin E-cdk2 complexes remained coupled in TGFbeta1-treated cells. TGFbeta1-induced growth arrest was associated with notably increased binding of p21(WAF1) to cdk4 and cdk6. No induction of cdk-inhibitor molecules of INK family was observed in TGFbeta1-treated DoHH2 cells. As shown, TGFbeta1-induced growth arrest of malignant B cells was associated with the activation of CIP/KIP family members of cdk inhibitors.

Microarray analysis of B-cell stimulation

B-cell development to antibody-producing plasma cells requires the concerted function of a large number of genes and proteins. Genome-level expression profiling during human B-cell maturation was studied in anti-immunoglobulin M-stimulated Ramos cells. cDNA microarrays were used to follow changes in the transcriptome over several days. Close to 1500 genes had significantly altered expression at least at one time point. The genes were organized into clusters based on expression profiles and were further characterized based on the functions of the coded proteins. Several groups of genes important for B cells were analyzed. Here we concentrate on genes involved in signal transduction and cytokines and their receptors. The results provide knowledge on the development of humoral immunity. Several new genes were found to be essential for B-cell development. They can be used as targets for research and possibly for drug development.

The immunology of inflammatory bowel disease

The dogma of systemic immunity can be ignored in relation to mucosal immunity. These distinct immune systems evolved based on specific environmental realities. Given the potentially hostile environment encountered by the mucosal immune system, it is a necessary adaptation that the overall tone is one of suppression. Breaches in this tone may lead to chronic inflammation. Understanding how specific components and processes of the mucosal immune system interact with the environment will elucidate the pathogenesis of IBD and lead to novel therapeutic approaches.

Bone marrow stromal cells mediate androgenic suppression of B lymphocyte development

Castration of normal male mice induces expansion of the bone marrow B cell population, an effect that can be reversed by androgen replacement. We employed in vitro cultures and two in vivo models to investigate whether androgens exert these effects directly on marrow lymphoid precursors or whether actions on marrow stromal elements are required. Immature B cells from normal mouse bone marrow were not responsive to the suppressive effect of androgens unless they were cocultured with marrow stromal cells or with supernatants from androgen-treated stromal cells, suggesting that the androgen effects are exerted through marrow stromal elements by production of a diffusible mediator. Further experiments revealed that bone marrow stromal cells produced TGF-beta in response to dihydrotestosterone (DHT), and neutralization of TGF-beta in the DHT-treated stromal cells reversed the suppressive effects. The stromal cell requirement for androgen-mediated effects was confirmed in vivo by experiments using chimeric animals created by bone marrow transplantation in which androgen receptor expression was restricted to either the stromal or lymphoid cells of the bone marrow. Androgens only affected B cell development in chimeric mice with androgen-sensitive stromal cells. These experiments suggest that effects of androgens on developing B cells are mediated through androgen receptors in bone marrow stromal cells. TGF-beta is a candidate mediator for these hormonal effects.

Cross-talk between the p42/p44 MAP kinase and Smad pathways in transforming growth factor beta 1-induced furin gene transactivation

Furin, a predominant convertase of the cellular constitutive secretory pathway, is known to be involved in the maturation of a number of growth/differentiation factors, but the mechanisms governing its expression remain elusive. We have previously demonstrated that transforming growth factor (TGF) beta 1, through the activation of Smad transducers, regulates its own converting enzyme, furin, creating a unique activation/regulation loop of potential importance in a variety of cell fate and functions. Here we studied the involvement of the p42/p44 MAPK pathway in such regulation. Using HepG2 cells transfected with fur P1 LUC (luciferase) promoter construct, we observed that forced expression of a dominant negative mutant form of the small G protein p21(ras) (RasN17) inhibited TGF beta 1-induced fur gene transcription, suggesting the involvement of the p42/p44 MAPK cascade. In addition, TGF beta induced sustained activation/phosphorylation of endogenous p42/p44 MAPK. Further-more, the role of MAPK cascade in fur gene transcription was highlighted by the use of the MEK1/2 inhibitors, PD98059 or U0126, or co-expression of a p44 antisense construct that repressed the induction of fur promoter transactivation. Conversely, overexpression of a constitutively active form of MEK1 increased unstimulated, TGF beta 1-stimulated, and Smad2-stimulated promoter P1 transactivation, and the universal Smad inhibitor, Smad7, inhibited this effect. Activation of Smad2 by MEK1 or TGF beta 1 resulted in an enhanced nuclear localization of Smad2, which was inhibited upon blocking MEK1 activity. Our findings clearly show that the activation of the p42/p44 MAPK pathway is involved in fur gene expression and led us to propose a co-operative model whereby TGF beta 1-induced receptor activation stimulates not only a Smad pathway but also a parallel p42/p44 MAPK pathway that targets Smad2 for an increased nuclear translocation and enhanced fur gene transactivation. Such an uncovered mechanism may be a key determinant for the regulation of furin in embryogenesis and growth-related physiopathological conditions.

Calmodulin differentially modulates Smad1 and Smad2 signaling

The members of the Smad protein family are intracellular mediators of transforming growth factor beta (TGF-beta) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/TGF-beta signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to down-regulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem. 273, 677-680). Here, we extend those data and demonstrate that calmodulin alters Smad signaling in living embryos, increasing Smad1 activity while inhibiting Smad2 function. To characterize this regulation, we undertook a structure-function analysis and found that calmodulin binds to two distinct and conserved regions in both Smad1 and Smad2. Receptor tyrosine kinase signaling also modifies Smad activity (Kretzschmar, M., Doody, J., and Massagué, J. (1997) Nature 389, 618-622; Kretzschmar, M., Doody, J., Timokhina, I., and Massagué, J. (1999) Genes Dev. 13, 804-816; de Caestecker, M. P., Parks, W. T., Frank, C. J., Castagnino, P., Bottaro, D. P., Roberts, A. B., and Lechleider, R. J. (1998) Genes Dev. 12, 1587-1592). We show that calmodulin binding to Smads inhibits subsequent Erk2-dependent phosphorylation of Smads and vice versa. These observations suggest the presence of a cross-talk between three major signaling cascades as follows: Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.

Smad7 is induced by CD40 and protects WEHI 231 B-lymphocytes from transforming growth factor-beta -induced growth inhibition and apoptosis

Transforming growth factor-beta (TGF-beta) is a potent inducer of apoptosis in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance. Here we show that concomitant signaling through CD40 sustains proliferation and rescues the premature B cell line WEHI 231 from both TGF-beta-induced and anti-IgM-induced apoptosis. The anti-apoptotic effect of CD40 is associated with the transcriptional activation of the inhibitory Smad7 protein. The transactivation of Smad7 by CD40 is NFkappaB-dependent in that pharmacological inhibitors of this pathway, N-tosyl-l-phenylalanine chloromethyl ketone and pyrrolidine dithiocarbamate, abrogate CD40-induced Smad7 expression. Ectopic overexpression of Smad7 inhibited Smad2 activation, TGF-beta-mediated growth inhibition, and apoptosis in WEHI 231 cells. Consistent with this result, dominant negative interference with Smad2 and Smad3 function also inhibited TGF-beta-induced apoptosis. The inhibitory effects of Smad7 overexpression were specific to TGF-beta-induced apoptosis and were without effect on anti-IgM-induced cell death. These results suggest a mechanism of suppression of TGF-beta-induced apoptosis by CD40, mediated through activation of NF-kappaB and, consequently, induction of Smad7 expression.