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Monocytes and macrophages are central players of the innate immune response and play a pivotal role in the regulation of inflammation. Thereby, they actively participate in all phases of the immune response, from initiating inflammation and triggering the adaptive immune response, through to the clearance of cell debris and resolution of inflammation. In this review, we described the mechanisms of monocyte and macrophage adaptation to rapidly changing microenvironmental conditions and discussed different forms of macrophage polarization depending on the environmental cues or pathophysiological condition. Therefore, special focus was placed on the tight regulation of the pro- and anti-inflammatory immune response, and the diverse functions of S100A8/S100A9 proteins and the scavenger receptor CD163 were highlighted, respectively. We paid special attention to the function of pro- and anti-inflammatory macrophages under pathological conditions.

Mrp8 and Mrp14 are endogenous alarmins amplifying inflammation via Toll‐like receptor‐4 (TLR‐4) activation. Due to their pro‐inflammatory properties, alarmins are supposed to enhance adaptive immunity via activation of dendritic cells (DCs). In contrast, analysing a model of allergic contact dermatitis (ACD) we observed a more severe disease outcome in Mrp8/14‐deficient compared to wild‐type mice. This unexpected phenotype was associated with an enhanced T‐cell response due to an accelerated maturation of DCs in Mrp8/14‐deficient mice. Accordingly, Mrp8, the active component of the heterocomplex, inhibits early DC maturation and antigen presentation in a TLR‐4‐dependent manner. Transfer of DCs purified from the local lymph nodes of sensitized Mrp8/14‐deficient to wild‐type mice determined the outcome of ACD. Our results link a pro‐inflammatory role of the endogenous TLR‐4 ligand Mrp8/14 to a regulatory function in adaptive immunity, which shows some similarities with the ‘hygiene hypothesis’ regarding continuous TLR‐4 stimulation and decreased risk of allergy. Mrp8/14, an amplifier of innate immune responses, has distinct roles in the adaptive immune system, preventing its hyper‐activation by regulating dendritic cell maturation and antigen presentation.

Rheumatic diseases are characterized by sterile inflammation that causes severe long-term damage to various organ systems. A growing body of evidence supports a pivotal role for the pro-inflammatory calcium-binding S100 family of proteins in the pathogenesis of rheumatic diseases. Some S100 proteins are released at the site of inflammation and act as danger-associated molecular pattern molecules by activating pattern recognition receptors. Increased concentrations of S100 proteins in serum and synovial fluid closely correlate with disease activity in several rheumatic diseases and serve as useful biomarkers for monitoring disease activity. Some S100 proteins are also valid biomarkers for predicting response to treatment, systemic organ involvement or disease flares in rheumatic diseases. Analyses of knockout mouse models have confirmed a functional role for S100 proteins, particularly S100A8 and S100A9, in rheumatic diseases, indicating that blocking the expression, release or function of these proteins might be an innovative therapeutic strategy. Owing to their local pattern of expression, specific mechanism of release and autoregulatory effects, such therapeutic approaches would primarily target the local inflammatory process and present only minor risks of systemic adverse effects.

Immune reactions are characterized by the rapid immigration of phagocytes into sites of inflammation. Meticulous regulation of these migratory processes is crucial for preventing uncontrolled and harmful phagocyte extravasation. S100A8/S100A9 is the major calcium-binding protein complex expressed in phagocytes. After release, this complex acts as a proinflammatory alarmin in the extracellular space, but the intracellular functions of these highly abundant proteins are less clear. Results of this study reveal an important role of S100A8/S100A9 in coordinated cytoskeleton rearrangement during migration. We found that S100A8/S100A9 was able to cross-link F-actin and microtubules in a calcium- and phosphorylation-dependent manner. Cells deficient in S100A8/S100A9 showed abnormalities in cell adhesion and motility. Missing cytoskeletal interactions of S100A8/S100A9 caused differences in the surface expression and activation of β1-integrins as well as in the regulation of Src/Syk kinase family members. Loss of S100A8/S100A9 led to dysregulated integrin-mediated adhesion and migration, resulting in an overall higher dynamic activity of non-activated S100A8/S100A9-deficient phagocytes. Our data suggest that intracellular S100A8/S100A9 is part of a novel regulatory mechanism that ensures the precise control necessary to facilitate the change between the quiescent and activated state of phagocytes.

Neutrophils and monocytes are sentinels of inflammatory signals. To reach the sites of action, both cell types attach to and then transmigrate the endothelial cell layer that lines the luminal side of blood vessels. While it has been reported that neutrophils and monocytes actively migrate along the surface of the vasculature, it remains elusive whether and how these motion patterns augment the efficiency of the immune system. Here, we conducted co-culture experiments of primary human monocytes and neutrophils, respectively, with primary human umbilical vein endothelial cells (HUVECs). Combining classical biomedical approaches with quantitative image analysis and numerical models, we find that immune cells simultaneously increase the number of sampled cells versus traveled distance and sensitivity to chemokines by migrating along endothelial cell–cell boundaries. Collectively, these findings establish search optimization of neutrophils and monocytes through limitation of motion pattern to cell–cell boundaries.

Objectives To assess the sensitivity of the phagocyte-specific molecules myeloid-related protein (MRP) 8 and MRP14 (calprotectin) for monitoring disease activity during anti-interleukin (IL)-1 therapies in patients with cryopyrin-associated periodic syndromes (CAPS), including familial cold autoinflammatory syndrome (FCAS), Muckle–Wells syndrome (MWS) and chronic infantile neurological, cutaneous and articular (CINCA) syndrome. Methods A total of 39 patients with CAPS, including 5 FCAS, 16 MWS and 18 CINCA syndrome, received anti-IL-1 therapy. All patients with CINCA and 12 with MWS were treated with IL-1Ra (anakinra), 14 patients with MWS with a monoclonal anti-IL-1β antibody (canakinumab) and patients with FCAS received IL-1 Trap (rilonacept). During serial clinical visits serum amyloid A, C-reactive protein, erythrocyte sedimentation rate and MRP8/14 serum levels were analysed. Results Untreated patients with CAPS had significantly elevated MRP8/14 values. In response to treatment there was a significant reduction of MRP8/14 levels in CINCA (2,830 (range 690 – 8,480) ng/ml to 670 ng/ml, p < 0.001) and MWS patients (anakinra-treated: 4,390 (1790 – 9780) ng/ml to 1,315 ng/ml (p = 0.003); canakinumab-treated: 3,000 (500 – 13060) ng/ml to 630 ng/ml (p=0.001)). However, in many patients with CAPS, MRP8/14 levels were still elevated compared with healthy individuals, reflecting residual disease activity. However, canakinumab-treated patients with CAPS showed normalised MRP8/14 levels, suggesting control of phagocyte activation. Conclusions Monitoring of cellular systems involved in inflammatory cascades of the innate immunity was successfully applied to the IL-1-driven CAPS diseases. This is the first study illustrating different states of subclinical disease activity in all types of CAPS depending on the type of anti-IL-1 therapy. MRP8/14 is a sensitive biomarker for monitoring disease activity, status of inflammation and response to IL-1 blockade in patients with CAPS.

Neutrophils and monocytes/macrophages are sentinels of inflammatory signals. To reach the sites of action, both cell types attach to and then transmigrate the endothelial cell layer that lines the luminal side of blood vessels. While it has been reported that neutrophils and monocytes/macrophages actively migrate along the surface of the vasculature, it remains elusive if and how these motion pattern augment the efficiency of the immune system. Here, we conducted co-culture experiments of primary human monocytes and neutrophils, respectively, with human umbilical vein endothelial cells (HUVECs). Combining classical biomedical approaches with quantitative image analysis and numerical models, we find that immune cells simultaneously increase the number of sampled cells vs. traveled distance and sensitivity to chemokines by migrating along endothelial cell-cell boundaries. Collectively, these findings establish search optimization of neutrophils and monocytes/macrophages through limitation of motion pattern to cell-cell boundaries.