Signaling mechanisms inducing hyporesponsiveness of phagocytes during systemic inflammation.

The inflammatory responsiveness of phagocytes to exogenous and endogenous stimuli is tightly regulated. This regulation plays an important role in systemic inflammatory response syndromes (SIRSs). In SIRSs, phagocytes initially develop a hyperinflammatory response, followed by a secondary state of hyporesponsiveness, a so-called "tolerance." This hyporesponsiveness can be induced by endotoxin stimulation of Toll-like receptor 4 (TLR4), resulting in an ameliorated response after subsequent restimulation. This modification of inflammatory response patterns has been described as innate immune memory. Interestingly, tolerance can also be triggered by endogenous TLR4 ligands, such as the alarmins myeloid-related protein 8 (MRP8, S100A8) and MRP14 (S100A9), under sterile conditions. However, signaling pathways that trigger hyporesponsiveness of phagocytes in clinically relevant diseases are only barely understood. Through our work, we have now identified 2 main signaling cascades that are activated during MRP-induced tolerance of phagocytes. We demonstrate that the phosphatidylinositol 3-kinase/AKT/GSK-3ß pathway interferes with NF-κB-driven gene expression and that inhibition of GSK-3ß mimics tolerance in vivo. Moreover, we identified interleukin-10-triggered activation of transcription factors STAT3 and BCL-3 as master regulators of MRP-induced tolerance. Accordingly, patients with dominant-negative STAT3 mutations show no tolerance development. In a clinically relevant condition of systemic sterile stress, cardiopulmonary bypass surgery, we confirmed the initial induction of MRP expression and the tolerance induction of monocytes associated with nuclear translocation of STAT3 and BCL-3 as relevant mechanisms. Our data indicate that the use of pharmacological JAK-STAT inhibitors may be promising targets for future therapeutic approaches to prevent complications associated with secondary hyporesponsiveness during SIRS.

The farnesoid-X-receptor in myeloid cells controls CNS autoimmunity in an IL-10-dependent fashion.

Innate immune responses by myeloid cells decisively contribute to perpetuation of central nervous system (CNS) autoimmunity and their pharmacologic modulation represents a promising strategy to prevent disease progression in Multiple Sclerosis (MS). Based on our observation that peripheral immune cells from relapsing-remitting and primary progressive MS patients exhibited strongly decreased levels of the bile acid receptor FXR (farnesoid-X-receptor, NR1H4), we evaluated its potential relevance as therapeutic target for control of established CNS autoimmunity. Pharmacological FXR activation promoted generation of anti-inflammatory macrophages characterized by arginase-1, increased IL-10 production, and suppression of T cell responses. In mice, FXR activation ameliorated CNS autoimmunity in an IL-10-dependent fashion and even suppressed advanced clinical disease upon therapeutic administration. In analogy to rodents, pharmacological FXR activation in human monocytes from healthy controls and MS patients induced an anti-inflammatory phenotype with suppressive properties including control of effector T cell proliferation. We therefore, propose an important role of FXR in control of T cell-mediated autoimmunity by promoting anti-inflammatory macrophage responses.

Sodium chloride promotes pro-inflammatory macrophage polarization thereby aggravating CNS autoimmunity.

The increasing incidence in Multiple Sclerosis (MS) during the last decades in industrialized countries might be linked to a change in dietary habits. Nowadays, enhanced salt content is an important characteristic of Western diet and increased dietary salt (NaCl) intake promotes pathogenic T cell responses contributing to central nervous system (CNS) autoimmunity. Given the importance of macrophage responses for CNS disease propagation, we addressed the influence of salt consumption on macrophage responses in CNS autoimmunity. We observed that EAE-diseased mice receiving a NaCl-high diet showed strongly enhanced macrophage infiltration and activation within the CNS accompanied by disease aggravation during the effector phase of EAE. NaCl treatment of macrophages elicited a strong pro-inflammatory phenotype characterized by enhanced pro-inflammatory cytokine production, increased expression of immune-stimulatory molecules, and an antigen-independent boost of T cell proliferation. This NaCl-induced pro-inflammatory macrophage phenotype was accompanied by increased activation of NF-kB and MAPK signaling pathways. The pathogenic relevance of NaCl-conditioned macrophages is illustrated by the finding that transfer into EAE-diseased animals resulted in significant disease aggravation compared to untreated macrophages. Importantly, also in human monocytes, NaCl promoted a pro-inflammatory phenotype that enhanced human T cell proliferation. Taken together, high dietary salt intake promotes pro-inflammatory macrophages that aggravate CNS autoimmunity. Together with other studies, these results underline the need to further determine the relevance of increased dietary salt intake for MS disease severity.

Transcriptome assessment reveals a dominant role for TLR4 in the activation of human monocytes by the alarmin MRP8.

The alarmins myeloid-related protein (MRP)8 and MRP14 are the most prevalent cytoplasmic proteins in phagocytes. When released from activated or necrotic phagocytes, extracellular MRP8/MRP14 promote inflammation in many diseases, including infections, allergies, autoimmune diseases, rheumatoid arthritis, and inflammatory bowel disease. The involvement of TLR4 and the multiligand receptor for advanced glycation end products as receptors during MRP8-mediated effects on inflammation remains controversial. By comparative bioinformatic analysis of genome-wide response patterns of human monocytes to MRP8, endotoxins, and various cytokines, we have developed a model in which TLR4 is the dominant receptor for MRP8-mediated phagocyte activation. The relevance of the TLR4 signaling pathway was experimentally validated using human and murine models of TLR4- and receptor for advanced glycation end products-dependent signaling. Furthermore, our systems biology approach has uncovered an antiapoptotic role for MRP8 in monocytes, which was corroborated by independent functional experiments. Our data confirm the primary importance of the TLR4/MRP8 axis in the activation of human monocytes, representing a novel and attractive target for modulation of the overwhelming innate immune response.