Kerstin Sarter

12/15-Lipoxygenase Orchestrates the Clearance of Apoptotic Cells and Maintains Immunologic Tolerance

Noninflammatory clearance of apoptotic cells (ACs) is crucial to maintain self-tolerance. Here, we have reported a role for the enzyme 12/15-lipoxygenase (12/15-LO) as a central factor governing the sorting of ACs into differentially activated monocyte subpopulations. During inflammation, uptake of ACs was confined to a population of 12/15-LO-expressing, alternatively activated resident macrophages (resMΦ), which blocked uptake of ACs into freshly recruited inflammatory Ly6C(hi) monocytes in a 12/15-LO-dependent manner. ResMΦ exposed 12/15-LO-derived oxidation products of phosphatidylethanolamine (oxPE) on their plasma membranes and thereby generated a sink for distinct soluble receptors for ACs such as milk fat globule-EGF factor 8, which were essential for the uptake of ACs into inflammatory monocytes. Loss of 12/15-LO activity, in turn, resulted in an aberrant phagocytosis of ACs by inflammatory monocytes, subsequent antigen presentation of AC-derived antigens, and a lupus-like autoimmune disease. Our data reveal an unexpected key role for enzymatic lipid oxidation during the maintenance of self-tolerance.

Autoimmunity and chronic inflammation — Two clearance-related steps in the etiopathogenesis of SLE

Systemic lupus erythematosus (SLE) is an autoimmune disease with very prominent chronic inflammatory aspects that render into multiple symptoms and clinical signs. The precise etiology of SLE remains elusive; however, it is known that its etiopathogenesis is of multifactorial nature. The production of autoantibodies (AAb) targeting double stranded DNA (dsDNA) and other nuclear autoantigens is the main characteristic of this disease. These target antigens are often modified and/or translocated when apoptotic cells undergo secondary necrosis as a consequence of the clearance deficiency in patients with SLE. In healthy individuals, dead and dying cells are rapidly removed by macrophages in an anti-inflammatory context; this does not elicit immune responses. In SLE, apoptotic cells are often not properly cleared; autoantigens leak out, and are subsequently presented to B cells by follicular dendritic cells (FDC) in secondary lymphoid tissues. This defect challenges the peripheral self-tolerance. Autoreactive B cell activation and production of anti-nuclear AAb result as the first step in the etiopathogenesis of SLE. The second step is the formation of immune complexes (IC) with apoptotic cell-derived nuclear remnants either in situ or deposited in various tissues. Nucleic acid-containing IC may also be ingested by phagocytes, which subsequently produce pro-inflammatory cytokines. Both processes result in chronic organ and tissue damage, development and maintenance of the systemic autoimmune disease. In conclusion, clearance deficiency may contribute to SLE in two ways: first, in germinal centres it enables the affinity maturation of autoreactive B cells and second, in peripheral tissues it leads to the accumulation of accessible nuclear autoantigens. Chronic inflammation in SLE is consequently promoted by the persistently binding of AAb with their cognate autoantigens forming a binary weapon: the nucleic acid-containing IC.

The cannabinoid receptor CB2 exerts antifibrotic effects in experimental dermal fibrosis

OBJECTIVE The cannabinoid receptor CB2 is predominantly expressed in non-neuronal tissue and exerts potent immunomodulatory effects. This study was undertaken to evaluate the role of CB2 in the pathogenesis of dermal fibrosis. METHODS Mice deficient in CB2 (CB2(-/-) mice) and their wild-type littermates (CB2(+/+) mice) were injected with bleomycin to induce experimental fibrosis. Mice were treated with selective agonists and antagonists of CB2. Lesional skin was evaluated for dermal thickness and numbers of infiltrating leukocytes. Bone marrow transplantation experiments were performed. RESULTS CB2(-/-) mice were more sensitive to bleomycin-induced dermal fibrosis than were CB2(+/+) mice, and showed increased dermal thickness. Leukocyte counts were significantly higher in the lesional skin of CB2(+/+) mice. Increased dermal fibrosis was also observed upon treatment with the CB2 antagonist AM-630. In contrast, the selective CB2 agonist JWH-133 reduced leukocyte infiltration and dermal thickening. The phenotype of CB2(-/-) mice was mimicked by transplantation of CB2(-/-) bone marrow into CB2(+/+) mice, whereas CB2(-/-) mice transplanted with bone marrow from CB2(+/+) mice did not display an increased sensitivity to bleomycin-induced fibrosis, indicating that leukocyte expression of CB2 critically influences experimental fibrosis. CONCLUSION Our findings indicate that CB2 limits leukocyte infiltration and tissue fibrosis in experimental dermal fibrosis. Since selective CB2 agonists are available and well tolerated, CB2 might be an interesting molecular target for the treatment of early inflammatory stages of systemic sclerosis.

Increased Bone Density and Resistance to Ovariectomy-Induced Bone Loss in FoxP3-Transgenic Mice Based on Impaired Osteoclast Differentiation

OBJECTIVE Immune activation triggers bone loss. Activated T cells are the cellular link between immune activation and bone destruction. The aim of this study was to determine whether immune regulatory mechanisms, such as naturally occurring Treg cells, also extend their protective effects to bone homeostasis in vivo. METHODS Bone parameters in FoxP3-transgenic (Tg) mice were compared with those in their wild-type (WT) littermate controls. Ovariectomy was performed in FoxP3-Tg mice as a model of postmenopausal osteoporosis, and the bone parameters were analyzed. The bones of RAG-1(-/-) mice were analyzed following the adoptive transfer of isolated CD4+CD25+ T cells. CD4+CD25+ T cells and CD4+ T cells isolated from FoxP3-Tg mice and WT mice were cocultured with monocytes to determine their ability to suppress osteoclastogenesis in vitro. RESULTS FoxP3-Tg mice developed higher bone mass and were protected from ovariectomy-induced bone loss. The increase in bone mass was found to be the result of impaired osteoclast differentiation and bone resorption in vivo. Bone formation was not affected. Adoptive transfer of CD4+CD25+ T cells into T cell-deficient RAG-1(-/-) mice also increased the bone mass, indicating that Treg cells directly affect bone homeostasis without the need to engage other T cell lineages. CONCLUSION These data demonstrate that Treg cells can control bone resorption in vivo and can preserve bone mass during physiologic and pathologic bone remodeling.

Autoantibodies against galectins are associated with antiphospholipid syndrome in patients with systemic lupus erythematosus

The presence of autoantibodies against immunoregulatory effectors can be relevant for onset and/or the progression of autoimmune disease. Emerging insights into an immunological activity profile including a role as opsonins give reason to systematically monitor sera of patients for immunoglobulin G (IgG) autoantibodies, preferably for several galectins at the same time. Here, we report on a study of chronic inflammatory rheumatic diseases, i.e. systemic lupus erythematosus (SLE; pilot cohort p, n = 40; confirmation cohort c, n = 109), rheumatoid arthritis (RA; p, n = 32; c, n = 25) and primary antiphospholipid syndrome (APS; c, n = 64). Enzyme-linked immunosorbent assay-based series using galectin-1, -2, -3, -4, -7, -8 and -9 and natural processing products, i.e. the truncated version of galectin-3 and the N-terminal domains of galectin-4, -8 and -9, were performed. Normal healthy donors (p, n = 20; c, n = 21) and patients with paraproteins (c, n = 19) served as controls. Highly significant optical density-value readings for IgG autoantibodies were consistently detected for the proto-type galectin-7 (SLE) and the tandem repeat-type galectin-8 and -9 (SLE and RA). Their presence was independent from the autoantibody status against double-stranded DNA (for patients with SLE) or a rheumatoid factor (for patients with RA), respectively. Importantly, anti-galectin-2 autoantibodies highly significantly correlated with the appearance of a secondary APS in patients with SLE so that this parameter may serve as an additional biomarker for APS. Equally of note, the presence of IgG autoantibodies against galectins capable to act as an opsonin may contribute to a sustained immune dysregulation in patients with chronic inflammatory rheumatic diseases.

Whole body low dose irradiation improves the course of beginning polyarthritis in human TNF-transgenic mice

Rheumatoid arthritis (RA) displays a chronic inflammatory joint disease, accompanied by symmetric polyarthritis (PA) which evokes synovial inflammation, cartilage damage, and bone erosion. Patients with RA are routinely treated by immunosuppressive drugs. The therapy of inflammatory diseases and degenerative disorders with Low-dose radiotherapy (LD-RT) (single doses from 0.3 to 1.0 Gy) represents a low cost therapy with low toxicity, and is able to substitute at least in part treatment with drugs. The efficiency of LD-RT has already been proven in several animal models of inducible arthritis. In the present study we used a human TNF transgenic mouse model to examine the effects of LD-RT on PA. We observed a significant temporal improvement of the clinical progression of disease when mice were irradiated at the beginning of the disease. These data emphasize the role of LD-RT in clinical settings to treat patients with chronic and degenerative disorders and diseases.

Detection and chromatographic removal of lipopolysaccharide in preparations of multifunctional galectins

The functional spectrum of human galectins is currently explored, with a wide range of activities being described. The role of galectin-3 as adhesin for bacteria is based on its strong binding to lipopolysaccharides (LPSs), which brings the possibility of such a contamination in galectin preparations to awareness. This assumption was verified in three independent functional assay systems using polymyxin B as inhibitor of LPS-dependent effects. Moreover, a commercial LPS quantification kit also revealed LPS in galectin preparations. Chromatography was effective in removing LPS, suggesting that such a technique needs to be applied to prevent assigning cellular responses to galectins rather than LPS.

Sweet clearance: Involvement of cell surface glycans in the recognition of apoptotic cells

Glycans cover the surfaces of all mammalian cells. Their structural variety provides enormous potential for information storage and transfer. According to the concept of the sugar code, they act as biochemical signals decoded by a large number of lectins which are defined as sugar binding proteins. The importance of glycan–lectin interaction in diverse immune system functions becomes increasingly apparent. Here, we review apoptotic cell clearance and especially focus on modifications of glycans on apoptotic cells.

Treatment with DNAse I fosters binding to nec PBMC of CRP

CRP is an important inflammatory marker, however, CRP levels are relatively low in patients with SLE. In addition patients with SLE often display low activities and serum levels for DNase I and complement, respectively. Here we show that DNase I treatment of nec PBMC increased their binding of CRP. Consequently, reduced DNase I activity in patients with SLE may contribute to the impaired opsonisation by CRP of dead cells, exacerbating the clearance defect in SLE of apo and nec cells.

Short-chain fatty acids regulate systemic bone mass and protect from pathological bone loss

Microbial metabolites are known to modulate immune responses of the host. The main metabolites derived from microbial fermentation of dietary fibers in the intestine, short-chain fatty acids (SCFA), affect local and systemic immune functions. Here we show that SCFA are regulators of osteoclast metabolism and bone mass in vivo. Treatment of mice with SCFA as well as feeding with a high-fiber diet significantly increases bone mass and prevents postmenopausal and inflammation-induced bone loss. The protective effects of SCFA on bone mass are associated with inhibition of osteoclast differentiation and bone resorption in vitro and in vivo, while bone formation is not affected. Mechanistically, propionate (C3) and butyrate (C4) induce metabolic reprogramming of osteoclasts resulting in enhanced glycolysis at the expense of oxidative phosphorylation, thereby downregulating essential osteoclast genes such as TRAF6 and NFATc1. In summary, these data identify SCFA as potent regulators of osteoclast metabolism and bone homeostasis.Short-chain fatty acids (SCFA) are a main class of metabolites derived from fermentation of dietary fibre in the intestine. Here, the authors show that dietary administration of SCFA is associated with inhibition of osteoclast differentiation, increased bone mass, and reduced pathological bone loss in mice.