Elsevier

Immunobiology

Volume 222, Issues 8–9, August 2017, Pages 913-917
Immunobiology

Short communication
The absence of TNF permits myeloid Arginase 1 expression in experimental L. monocytogenes infection

https://doi.org/10.1016/j.imbio.2017.05.012Get rights and content

Abstract

During an immune response inflammatory macrophages with their wide variety of effector mechanisms including the expression of inducible nitric oxide synthase play an important part in the defense against invading pathogens. The inflammatory phenotype requires the presence of TNF which suppresses alternative activation. In the bacterial Listeria monocytogenes infection model inflammatory macrophages are crucial for protection. After infection, TNF-deficient hosts have a similar number of splenic macrophages but die rapidly. A more detailed analysis of these cells showed that while inducible nitric oxide synthase is expressed at a comparable level TNF-deficient macrophages show an increased expression of Arginase 1.

Introduction

Failure to resolve inflammation is intrinsic in many human pathologies. Mechanisms that influence and modulate the inflammatory response are therefore, of general interest for our understanding of the underlying disease processes. One of the classical protagonists of inflammation is the proinflammatory cytokine tumor necrosis factor (TNF). This cytokine is present in high concentrations in inflammatory sites and has a variety of functions, such as activating cells and inducing the expression of functional cell surface molecules (Sedgwick et al., 2000). TNF is expressed by macrophages and T cells early after an immunological challenge and has been identified as a target for therapeutic intervention in a variety of chronic and autoimmune inflammatory diseases, including rheumatoid arthritis (RA) and inflammatory bowel disease (IBD) (Efimov et al., 2009, Udalova et al., 2016a). Anti TNF-therapy that blocks TNF activity using antagonists based on antibodies or TNFR fusion proteins is now a proven method of treatment for these pathologies (Udalova et al., 2016b). However, TNF has also been identified to be essential in the establishment of protective immunity to infection. Experiments that block TNF and the use of gene-deficient animals have resulted in a list of bacterial and parasitic pathogens that are controlled by a functioning TNFR1-TNF signaling axis (Pfeffer et al., 1993, Rothe et al., 1993, Wilhelm et al., 2001). The implicit mechanisms that contribute to the protective role of TNF have been difficult to define due to the ubiquitous expression of this pleiotropic cytokine. In the murine model of experimental cutaneous leishmaniasis, TNF-deficiency induced a progressive visceral infection and was ultimately fatal in the normally resistant mouse strain C57BL/6 (Wilhelm et al., 2001). The underlying cause of this lack of protection proved to be elusive since both innate and adaptive immune responses seem largely unchanged. However, it has recently been shown that TNF is essential for the suppression of Arginase 1 (Arg1) and other genes of the pro-homeostatic alternatively activated macrophage signature due to a restriction of accessibility of their promoters and enhancers (Schleicher et al., 2016). Thus, TNF was deemed irreplaceable for an effective differentiation of monocytes to classically activated macrophages (Schleicher et al., 2016). Indeed, a large percentage of skin and lymph node-resident macrophages and inflammatory dendritic cells (DC) from Leishmania (L.) major infected B6.TNF−/− mice co-expressed classically and alternatively activated macrophage signature molecules such as Arg1 and iNOS, respectively (Schleicher et al., 2016). This co-expression resulted in a lack of the central effector molecule nitric oxide (NO) in the lesion and the draining lymph node in infected tissues presumably due to a depletion of L-arginine, the substrate of both enzymes. This biological function of TNF to restrict alternative macrophage differentiation during the inflammatory response to L. major has also been demonstrated in tumor models and we hypothesized that it would be applicable generally. Therefore, we used Listeria (L.) monocytogenes infection as a bacterial model with a well-established history of myeloid differentiation to analyze the consequences of TNF-deficiency in more detail (Serbina et al., 2003). The TNF-deficient mouse strain (B6.TNF−/−) was highly susceptible to the pathogen. Inflammatory splenic macrophages from B6. TNF−/− mice could be identified using Ly6 C and CD11b. These cells exhibited a high bacterial burden, normal iNOS and an elevated Arg1 and TGM2 expression.

Section snippets

Animals and infection

Eight to 16-week-old C57BL/6 (B6. WT) and B6.TNF−/− mice were used in all experiments. All mice were housed and bred in a certified SPF environment. To infect the mice we cultured a single colony of L. monocytogenes (obtained from Prof. Dirk Busch, Institute for Microbiology, Technical University Munich, Germany) in brain-heart infusion broth (BD Biosciences, North Ryde, Australia) overnight at 37 °C with shaking. The overnight culture was added to fresh medium at volume ratio 1: 50 and cultured

Results

The experimental infection of mice with L. monocytogenes is a well-validated model for a bacterial infection that relies on macrophages as predominant host cells. Spleen and liver are the two major target organs of systemic L. monocytogenes infection. In B6.WT mice the infection reached a peak of 104 bacteria per organ at day 3 post-infection (p.i.) and started to subside thereafter (Fig. 1A and B). At the same time post infection, in spleens and livers of immunocompromised B6.TNF−/− mice the

Discussion

A deficiency in the TNF-TNFR1 signaling pathway causes a significant aggravation of the pathology of C57BL/6 mice infected with L. monocytogenes (Grivennikov et al., 2005, Pfeffer et al., 1993, Rothe et al., 1993). Investigations using a tissue-specific TNF-deficiency have been able to localize the source of the protective production of TNF in macrophages (Grivennikov et al., 2005). The TNF-deficient strain B6.TNF−/− displayed a rapid increase of the bacterial burden in spleen and liver within 4

Conflict of interest

The authors state that they had no conflict of interest.

Acknowledgments

We wish to thank Amanda Patchett for qPCR advice, and Jocelyn Darby and Terry Pinfold for technical help. We would also like to thank the staff of the animal facility of the University of Tasmania. The work was supported by the Menzies Institute of Medical Research Tasmania and an IPGR scholarship to X. Li.

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