Arginine is processed by macrophages in response to the cytokines to which these cells are exposed. Th1-type cytokines induce NO synthase 2, which metabolizes arginine into nitrites, while the Th2-type cytokines produce arginase, which converts arginine into polyamines and proline. Activation of bone marrow-derived macrophages by these two types of cytokines increases l-arginine transport only through the y+ system. Analysis of the expression of the genes involved in this system showed that Slc7A1, encoding cationic amino acid transporters (CAT)1, is constitutively expressed and is not modified by activating agents, while Slc7A2, encoding CAT2, is induced during both classical and alternative activation. Macrophages from Slc7A2 knockout mice showed a decrease in l-arginine transport in response to the two kinds of cytokines. However, while NO synthase 2 and arginase expression were unmodified in these cells, the catabolism of arginine was impaired by both pathways, producing smaller amounts of nitrites and also of polyamines and proline. In addition, the induction of Slc7A2 expression was independent of the arginine available and of the enzymes that metabolize it. In conclusion, the increased arginine transport mediated by activators is strongly regulated by CAT2 expression, which could limit the function of macrophages.

After interaction with its receptor, GM-CSF induces phosphorylation of the β-chain in two distinct domains in macrophages. One induces activation of mitogen-activated protein kinases and the PI3K/Akt pathway, and the other induces JAK2-STAT5. In this study we describe how trichostatin A (TSA), which inhibits deacetylase activity, blocks JAK2-STAT5-dependent gene expression but not the expression of genes that depend on the signal transduction induced by the other domain of the receptor. TSA treatment inhibited the GM-CSF-dependent proliferation of macrophages by interfering with c-myc and cyclin D1 expression. However, M-CSF-dependent proliferation, which requires ERK1/2, was unaffected. Protection from apoptosis, which involves Akt phosphorylation and p21waf-1 expression, was not modified by TSA. GM-CSF-dependent expression of MHC class II molecules was inhibited because CIITA was not induced. The generation of dendritic cells was also impaired by TSA treatment because of the inhibition of IRF4, IRF2, and RelB expression. TSA mediates its effects by preventing the recruitment of RNA polymerase II to the promoter of STAT5 target genes and by inhibiting their expression. However, this drug did not affect STAT5A or STAT5B phosphorylation or DNA binding. These results in GM-CSF-treated macrophages reveal a relationship between histone deacetylase complexes and STAT5 in the regulation of gene expression.

Despite idelalisib approval in relapsed follicular lymphoma (FL), a complete characterization of the immunomodulatory consequences of phosphatidylinositol 3-kinase δ (PI3Kδ) inhibition, biomarkers of response, and potential combinatorial therapies in FL remain to be established. Using ex vivo cocultures of FL patient biopsies and follicular dendritic cells (FDCs) to mimic the germinal center (n = 42), we uncovered that PI3Kδ inhibition interferes with FDC-induced genes related to angiogenesis, extracellular matrix formation, and transendothelial migration in a subset of FL samples, defining an 18-gene signature fingerprint of idelalisib sensitivity. A common hallmark of idelalisib found in all FL cases was its interference with the CD40/CD40L pathway and induced proliferation, together with the downregulation of proteins crucial for B–T-cell synapses, leading to an inefficient cross talk between FL cells and the supportive T-follicular helper cells (TFH). Moreover, idelalisib downmodulates the chemokine CCL22, hampering the recruitment of TFH and immunosupressive T-regulatory cells to the FL niche, leading to a less supportive and tolerogenic immune microenvironment. Finally, using BH3 profiling, we uncovered that FL–FDC and FL–macrophage cocultures augment tumor addiction to BCL-XL and MCL-1 or BFL-1, respectively, limiting the cytotoxic activity of the BCL-2 inhibitor venetoclax. Idelalisib restored FL dependence on BCL-2 and venetoclax activity. In summary, idelalisib exhibits a patient-dependent activity toward angiogenesis and lymphoma dissemination. In all FL cases, idelalisib exerts a general reshaping of the FL immune microenvironment and restores dependence on BCL-2, predisposing FL to cell death, providing a mechanistic rationale for investigating the combination of PI3Kδ inhibitors and venetoclax in clinical trials.

Microenvironment contributes to follicular lymphoma (FL) pathogenesis and impacts survival with macrophages playing a controversial role. In the present study, using FL primary samples and HK follicular dendritic cells (FDC) to mimic the germinal center, together with mouse models, we have analyzed the three-way crosstalk of FL-FDC-macrophages and derived therapeutic opportunities. Ex vivo primary FL-FDC co-cultures (n = 19) and in vivo mouse co-xenografts demonstrated that FL-FDC crosstalk favors tumor growth and, via the secretion of CCL2 and CSF-1, promotes monocyte recruitment, differentiation, and polarization towards an M2-like protumoral phenotype. Moreover, FL-M2 co-cultures displayed enhanced angiogenesis, dissemination, and immunosuppression. Analysis of the CSF-1/CSF-1R pathway uncovered that CSF-1 was significantly higher in serum from grade 3A FL patients, and that high CSF-1R expression in FL biopsies correlated with grade 3A, reduced overall survival and risk of transformation. Furthermore, CSF-1R inhibition with pexidartinib (PLX3397) preferentially affected M2-macrophage viability and polarization program disrupting FL-M2 positive crosstalk. In vivo CSF1-R inhibition caused M2 reduction and repolarization towards M1 macrophages and antitumor effect cooperating with anti-CD20 rituximab. In summary, these results support the role of macrophages in FL pathogenesis and indicate that CSF-1R may be a relevant prognostic factor and a novel therapeutic target cooperating with anti-CD20 immunotherapy.

LPS induces the expression of NO synthase 2 (nos2) in macrophages. The expression of this molecule is one of the hallmarks of classical activation. In this paper, we describe that trichostatin A (TSA), which inhibits deacetylase activity, blocks LPS-dependent nos2 expression. TSA specifically inhibits LPS-dependent genes of secondary response, which require new protein synthesis for their induction but not those belonging to the primary response, which do not depend on this process. Deacetylase activity acts at the transcriptional level because RNA polymerase II was not bound after LPS stimulus when we added TSA. A link between the global acetylation caused by HDAC inhibitor and gene promoter recruitment of CDK8 was found. This Mediator complex subunit associates with Med 12, Med13, and cyclin C to form a submodule that is a transcriptional negative regulator. We also found that TSA reduces C/EBPβ phosphorylation without affecting its binding to DNA. Taken together, these results shed light on the molecular mechanisms involved in the transcriptional regulation of LPS-treated macrophages and on how TSA targets critical LPS-induced genes, such as nos2 and tnf-α, in inflammatory macrophage response.

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The amount of arginine available at inflammatory loci is a limiting factor for the growth of several cells of the immune system. IL-4-induced activation of macrophages produced arginase-1, which converts arginine into ornithine, a precursor of polyamines and proline. Trichostatin A (TSA), a pan-inhibitor of histone deacetylases (HDACs), inhibited IL-4-induced arginase-1 expression. TSA showed promoter-specific effects on the IL-4-responsive genes. While TSA inhibited the expression of arginase-1, fizz1, and mrc1, other genes, such as ym,1 mgl1, and mgl2, were not affected. The inhibition of arginase-1 occurred at the transcriptional level with the inhibition of polymerase II binding to the promoter. IL-4 induced STAT6 phosphorylation and binding to DNA. These activities were not affected by TSA treatment. However, TSA inhibited C/EBPβ DNA binding. This inhibitor induced acetylation on lysine residues 215-216, which are critical for DNA binding. Finally, using macrophages from STAT6 KO mice we showed that STAT6 is required for the DNA binding of C/EBPβ. These results demonstrate that the acetylation/deacetylation balance strongly influences the expression of arginase-1, a gene of alternative activation of macrophages. These findings also provide a molecular mechanism to explain the control of gene expression through deacetylase activity. Keywords:Alternative activation r Arginase-1 r Gene regulation r Histone deacetylases (HDACs) r Macrophages r STAT6 r Trichostatin A (TSA) IntroductionIn processes, such as wound healing, angiogenesis, and host defense against parasites, the phenotype of macrophages is related to alternative activation [1,2], which includes arginase-1 expression. This enzyme converts arginine to ornithine, a precursor of polyamines and proline. This conversion is critical to induce the growth and proliferation of several cell types in damaged tissues, the production of the extracellular matrix, and tissue remodeling [3]. An excess of this type of activation contributes to fibrosis in Correspondence: Dr. Antonio Celada e-mail: acelada@ub.edu some pathological conditions [4]. In fact, the catabolism of arginine by macrophages has emerged as a critical mechanism for the regulation of the immune response in several parasitic diseases [5][6][7]. Moreover, high arginase-1 expression has been associated with a variety of conditions, such as cancer, asthma, psoriasis, cardiovascular disease, and also pregnancy [8]. The consumption of arginine by macrophages in the inflammatory loci may impair the growth of other immune cells and thus result in potent immune suppressive activity.Recently, considerable research effort has been devoted in macrophages to the regulation of genes in their natural setting, namely the chromatin substrate [9]. The recruitment of histone acetyl-transferases (HATs) and histone deacetylases (HDACs) to the transcriptional machinery is a key element in the dynamic regulation of genes. HAT activity promotes the acetylation of histonewww.eji-journal.eu Eur. J. Immunol. 2012. 42: 3028-30...