Toll-like receptor-induced arginase 1 in macrophages thwarts effective immunity against intracellular pathogens.

Toll-like receptor (TLR) signaling in macrophages is required for antipathogen responses, including the biosynthesis of nitric oxide from arginine, and is essential for immunity to Mycobacterium tuberculosis, Toxoplasma gondii and other intracellular pathogens. Here we report a 'loophole' in the TLR pathway that is advantageous to these pathogens. Intracellular pathogens induced expression of the arginine hydrolytic enzyme arginase 1 (Arg1) in mouse macrophages through the TLR pathway. In contrast to diseases dominated by T helper type 2 responses in which Arg1 expression is greatly increased by interleukin 4 and 13 signaling through the transcription factor STAT6, TLR-mediated Arg1 induction was independent of the STAT6 pathway. Specific elimination of Arg1 in macrophages favored host survival during T. gondii infection and decreased lung bacterial load during tuberculosis infection.

TNF-Mediated Restriction of Arginase 1 Expression in Myeloid Cells Triggers Type 2 NO Synthase Activity at the Site of Infection.

Neutralization or deletion of tumor necrosis factor (TNF) causes loss of control of intracellular pathogens in mice and humans, but the underlying mechanisms are incompletely understood. Here, we found that TNF antagonized alternative activation of macrophages and dendritic cells by IL-4. TNF inhibited IL-4-induced arginase 1 (Arg1) expression by decreasing histone acetylation, without affecting STAT6 phosphorylation and nuclear translocation. In Leishmania major-infected C57BL/6 wild-type mice, type 2 nitric oxide (NO) synthase (NOS2) was detected in inflammatory dendritic cells or macrophages, some of which co-expressed Arg1. In TNF-deficient mice, Arg1 was hyperexpressed, causing an impaired production of NO in situ. A similar phenotype was seen in L. major-infected BALB/c mice. Arg1 deletion in hematopoietic cells protected these mice from an otherwise lethal disease, although their disease-mediating T cell response (Th2, Treg) was maintained. Thus, deletion or TNF-mediated restriction of Arg1 unleashes the production of NO by NOS2, which is critical for pathogen control.

Translational repression of inducible NO synthase in macrophages by l-arginine depletion is not associated with an increased phosphorylation of eIF2alpha.

In mouse inflammatory macrophages the cytokine-mediated expression of inducible nitric oxide synthase (iNOS) is regulated by the availability of the substrate l-arginine. Following arginine starvation the levels of iNOS mRNA remain unimpaired, whereas the translation of iNOS protein is strikingly downregulated. In the present study we addressed the question, whether arginine-deficient macrophages follow the canonical integrated stress response (ISR) that in other cell types depleted of amino acids was characterized by the accumulation of phosphorylated (i.e. inactive) eukaryotic translation initiation factor-2alpha (eIF2alpha), the attenuation of global protein synthesis and the induction of certain stress response target genes. Unexpectedly, resting as well as stimulated inflammatory macrophages constitutively exhibited high levels of phosphorylated eIF2alpha, which was not further increased upon l-arginine starvation. At the same time, macrophages deprived of l-arginine showed a significant upregulation of the mRNA levels of ISR genes. From these data we conclude that l-arginine deficiency blocks the translation of iNOS and elicits a stress response in macrophages, both of which, however, do not result from an enhanced phosphorylation of eIF2alpha. Alternative modes of translational repression of iNOS need to be considered.

Cytokine-mediated control of lipopolysaccharide-induced activation of small intestinal epithelial cells.

Cytokines with anti-inflammatory properties have been implicated in the prevention of inappropriate immune activation by commensal bacteria in the intestinal tract. Here, we analysed receptor expression, cellular signalling, and the inhibitory activity of interleukin (IL)-4, -10, -11, and -13 as well as of transforming growth factor-beta on lipopolysaccharide-mediated small intestinal epithelial cell activation. Only IL-4 and IL-13 had a significant inhibitory effect on chemokine secretion and nitric oxide (NO) production in differentiated and polarized cells. Reverse transcription-polymerase chain reaction of primary intestinal epithelial cells obtained by laser-microdissection confirmed expression of the type II IL-4 receptor consisting of the IL-4 receptor alpha and the IL-13 receptor alpha1. Also, IL-4 or IL-13 led to rapid signal transducer and activator of transcription 6 phosphorylation, diminished inducible NO synthase expression, and enhanced the antagonistic arginase 1 activity. In conclusion, cytokines such as IL-4 and IL-13 affect intestinal epithelial cells and exhibit a modulating activity on Toll-like receptor-4-mediated epithelial cell activation.