The alarmin Mrp8/14 as regulator of the adaptive immune response during allergic contact dermatitis.

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 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock.

To identify new components that regulate the inflammatory cascade during sepsis, we characterized the functions of myeloid-related protein-8 (Mrp8, S100A8) and myeloid-related protein-14 (Mrp14, S100A9), two abundant cytoplasmic proteins of phagocytes. We now demonstrate that mice lacking Mrp8-Mrp14 complexes are protected from endotoxin-induced lethal shock and Escherichia coli-induced abdominal sepsis. Both proteins are released during activation of phagocytes, and Mrp8-Mrp14 complexes amplify the endotoxin-triggered inflammatory responses of phagocytes. Mrp8 is the active component that induces intracellular translocation of myeloid differentiation primary response protein 88 and activation of interleukin-1 receptor-associated kinase-1 and nuclear factor-kappaB, resulting in elevated expression of tumor necrosis factor-alpha (TNF-alpha). Using phagocytes expressing a nonfunctional Toll-like receptor 4 (TLR4), HEK293 cells transfected with TLR4, CD14 and MD2, and by surface plasmon resonance studies in vitro, we demonstrate that Mrp8 specifically interacts with the TLR4-MD2 complex, thus representing an endogenous ligand of TLR4. Therefore Mrp8-Mrp14 complexes are new inflammatory components that amplify phagocyte activation during sepsis upstream of TNFalpha-dependent effects.

S100A9 differentially modifies phenotypic states of neutrophils, macrophages, and dendritic cells: implications for atherosclerosis and adipose tissue inflammation.

BACKGROUND: S100A9 is constitutively expressed in neutrophils, dendritic cells, and monocytes; is associated with acute and chronic inflammatory conditions; and is implicated in obesity and cardiovascular disease in humans. Most of the constitutively secreted S100A9 is derived from myeloid cells. A recent report demonstrated that mice deficient in S100A9 exhibit reduced atherosclerosis compared with controls and suggested that this effect was due in large part to loss of S100A9 in bone marrow-derived cells. METHODS AND RESULTS: To directly investigate the role of bone marrow-derived S100A9 in atherosclerosis and insulin resistance in mice, low-density lipoprotein receptor-deficient, S100A9-deficient bone marrow chimeras were generated. Neither atherosclerosis nor insulin resistance was reduced in S100A9-deficient chimeras fed a diet rich in fat and carbohydrates. To investigate the reason for this lack of effect, myeloid cells were isolated from the peritoneal cavity or bone marrow. S100A9-deficient neutrophils exhibited a reduced secretion of cytokines in response to toll-like receptor-4 stimulation. In striking contrast, S100A9-deficient dendritic cells showed an exacerbated release of cytokines after toll-like receptor stimulation. Macrophages rapidly lost S100A9 expression during maturation; hence, S100A9 deficiency did not affect the inflammatory status of macrophages. CONCLUSIONS: S100A9 differentially modifies phenotypic states of neutrophils, macrophages, and dendritic cells. The effect of S100A9 deficiency on atherosclerosis and other inflammatory diseases is therefore predicted to depend on the relative contribution of these cell types at different stages of disease progression. Furthermore, S100A9 expression in nonmyeloid cells is likely to contribute to atherosclerosis.

The absence of cutaneous lymph nodes results in a Th2 response and increased susceptibility to Leishmania major infection in mice.

Lymph nodes (LNs) are important sentinel organs where antigen-presenting cells interact with T cells to induce adaptive immune responses. In cutaneous infection of mice with Leishmania major, resistance depends on the induction of a T-helper-cell-1 (Th1)-mediated cellular immune response in draining, peripheral LNs. We investigated whether draining, peripheral LNs are absolutely required for resistance against L. major infection. We investigated the course of experimental leishmaniasis in wild-type (wt) mice lacking peripheral LNs (pLNs), which we generated by in utero blockade of membrane-bound lymphotoxin, and in mice lacking pLNs or all LNs due to genetic deletion of lymphotoxin ligands or receptors. wt mice of the resistant C57BL/6 strain without local skin-draining LNs were still able to generate specific T-cell responses, but this yielded Th2 cells. This switch to a Th2 response resulted in severe systemic infection. We also confirmed these results with mice lacking pLNs due to genetic depletion of lymphotoxin-beta. The complete absence of LNs due to a genetic depletion of the lymphotoxin-beta receptor also resulted in a marked deterioration of disease and a Th2 response. Thus, in the absence of pLNs, an L. major-specific Th2 response is induced in the remaining secondary lymphoid organs, such as the spleen and non-skin-draining LNs. This indicates a critical requirement for pLNs to induce protective Th1 immunity and suggests that whether Th1 or Th2 priming to the same antigen occurs depends on the site of the primary antigen recognition.

The calcium binding protein S100A9 is essential for pancreatic leukocyte infiltration and induces disruption of cell-cell contacts.

Leukocyte infiltration is an early and critical event in the development of acute pancreatitis. However, the mechanism of leukocyte transmigration into the pancreas and the function of leukocytes in initiating acute pancreatitis are still poorly understood. Here, we studied the role of S100A9 (MRP14), a calcium binding protein specifically released by polymorph nuclear leukocytes (PMN), in the course of acute experimental pancreatitis. Acute pancreatitis was induced by repeated supramaximal caerulein injections in S100A9 deficient or S100A9 wild-type mice. We then determined S100A9 expression, trypsinogen activation peptide (TAP) levels, serum amylase and lipase activities, and tissue myeloperoxidase (MPO) activity. Cell-cell contact dissociation was analyzed in vitro with biovolume measurements of isolated acini after incubation with purified S100A8/A9 heterodimers, and in vivo as measurement of Evans Blue extravasation after intravenous application of S100A8/A9. Pancreatitis induced increased levels of S100A9 in the pancreas. However, infiltration of leukocytes and MPO activity in the lungs and pancreas during acute pancreatitis was decreased in S100A9-deficient mice and associated with significantly lower serum amylase and lipase activities as well as reduced intrapancreatic TAP-levels. Incubation of isolated pancreatic acini with purified S100A8/A9-heterodimers resulted in a rapid dissociation of acinar cell-cell contacts which was highly calcium-dependent. Consistent with these findings, in vivo application of S100A8/A9 in mice was in itself sufficient to induce pancreatic cell-cell contract dissociation as indicated by Evans Blue extravasation. These data show that the degree of intrapancreatic trypsinogen activation is influenced by the extent of leukocyte infiltration into the pancreas which, in turn, depends on the presence of S100A9 that is secreted from PMN. S100A9 directly affects leukocyte tissue invasion and mediates cell contact dissociation via its calcium binding properties.

Vitamin D receptor signaling contributes to susceptibility to infection with Leishmania major.

We have previously reported that 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) can selectively suppress key functions of interferon-gamma (IFN-gamma) activated macrophages. To further explore this mechanism for its relevance in vivo, we investigated an infection model that crucially depends on the function of IFN-gamma activated macrophages, the infection with the intracellular protozoan Leishmania major. 1Alpha,25(OH)2D3 treatment of L. major infected macrophages demonstrated a vitamin D receptor (Vdr) dependent inhibition of macrophage killing activity. Further analysis showed that this was a result of decreased production of nitric oxide by 1alpha,25(OH)2D3-treated macrophages due to Vdr-dependent up-regulation of arginase 1 expression, which overrides NO production by Nos2. When analyzing the course of infection in vivo, we found that Vdr-knockout (Vdr-KO) mice were more resistant to L. major infection than their wild-type littermates. This result is in agreement with an inhibitory influence of 1alpha,25(OH)2D3 on the macrophage mediated host defense. Further investigation showed that Vdr-KO mice developed an unaltered T helper cell type 1 (Th1) response on infection as indicated by normal production of IFN-gamma by CD4+ and CD8+ T cells. Therefore, we propose that the absence of 1alpha,25(OH)2D3-mediated inhibition of macrophage microbicidal activity in Vdr-KO mice results in increased resistance to Leishmania infection.

Calcium-dependent tetramer formation of S100A8 and S100A9 is essential for biological activity.

S100 proteins comprise the largest family of calcium-binding proteins. Members of this family usually form homo- or heterodimers, which may associate to higher-order oligomers in a calcium-dependent manner. The heterodimers of S100A8 and S100A9 represent the major calcium-binding proteins in phagocytes. Both proteins regulate migration of these cells via modulation of tubulin polymerization. Calcium binding induces formation of (S100A8/S100A9)2 tetramers. The functional relevance of these higher-order oligomers of S100 proteins, however, is not yet clear. To investigate the importance of higher-order oligomerization for S100 proteins, we created a set of mutations within S100A9 (N69A, E78A, N69A+E78A) destroying the high-affinity C-terminal calcium-binding site (EF-hand II). Mutations in EF-hand II did not interfere with formation of the S100A8/S100A9 heterodimer as demonstrated by yeast two-hybrid experiments and pull-down assays. In contrast, mass spectrometric analysis and density gradient centrifugation revealed that calcium-induced association of (S100A8/S100A9)2 tetramers was strictly dependent on a functional EF-hand II in S100A9. Failure of tetramer formation was associated with a lack of functional activity of S100A8/S100A9 complexes in promoting the formation of microtubules. Thus, our data demonstrate that calcium-dependent formation of (S100A8/S100A9)2 tetramers is an essential prerequisite for biological function. This is the first report showing a functional relevance of calcium-induced higher-order oligomerization in the S100 family.

Loss of S100A9 (MRP14) results in reduced interleukin-8-induced CD11b surface expression, a polarized microfilament system, and diminished responsiveness to chemoattractants in vitro.

The S100A9 (MRP14) protein is abundantly expressed in myeloid cells and has been associated with various inflammatory diseases. The S100A9-deficient mice described here were viable, fertile, and generally of healthy appearance. The myelopoietic potential of the S100A9-null bone marrow was normal. S100A8, the heterodimerization partner of S100A9 was not detectable in peripheral blood cells, suggesting that even a deficiency in both S100A8 and S100A9 proteins was compatible with viable and mature neutrophils. Surprisingly, the invasion of S100A9-deficient leukocytes into the peritoneum and into the skin in vivo was indistinguishable from that in wild-type mice. However, stimulation of S100A9-deficient neutrophils with interleukin-8 in vitro failed to provoke an up-regulation of CD11b. Migration upon a chemotactic stimulus through an endothelial monolayer was markedly diminished in S100A9-deficient neutrophils. Attenuated chemokinesis of the S100A9-deficient neutrophils was observed by using a three-dimensional collagen matrix migration assay. The altered migratory behavior was associated with a microfilament system that was highly polarized in unstimulated S100A9-deficient neutrophils. Our data suggest that loss of the calcium-binding S100A9 protein reduces the responsiveness of the neutrophils upon chemoattractant stimuli at least in vitro. Alternative pathways for neutrophil emigration may be responsible for the lack of any effect in the two in vivo models we have investigated so far.

TNF induces distinct gene expression programs in microvascular and macrovascular human endothelial cells.

The relevance of the diversity of endothelial cells (ECs) for the response to inflammatory stimuli is currently not well defined. Using oligonucleotide microarray technique, we systematically analyzed the tumor necrosis factor (TNF)-induced expression profile in human microvascular ECs (HMEC) and macrovascular human umbilical vein ECs (HUVEC), analyzing 13,000 human genes by microarray analysis. Using strict inclusion and exclusion criteria, microarray analysis revealed that about half of the TNF-induced genes were specific for HMEC-1 or HUVEC. The microarray data could widely be confirmed by quantitative reverse transcriptase-polymerase chain reaction and at the protein level. It is interesting that the majority of those genes regulated depending on the cell type encoded for chemokines, cytokines, and cell surface molecules. Our results argue for a more careful consideration of specific effects restricted to distinct subtypes of ECs. The establishment of EC type-specific expression patterns may thus provide the basis for a selective manipulation of specific endothelial subtypes in different inflammatory diseases.

S100A9 deficiency alters adenosine-5'-triphosphate induced calcium signalling but does not generally interfere with calcium and zinc homeostasis in murine neutrophils.

The two calcium- and zinc-binding proteins, S100A9 and S100 A8, abundant in myeloid cells are considered to play important roles in both calcium signalling and zinc homeostasis. Polymorphonuclear neutrophils from S100A9 ko mice are also devoid of S100A8. Therefore, S100A9-deficient neutrophils were used as a model to study the role of the two S100 proteins in the neutrophils's calcium and zinc metabolism. Analysis of the intracellular zinc level upon pyrithione and (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexeneamide (NOR-1) treatment revealed no differences between S100A9-deficient and wildtype neutrophils. Similar, the calcium signals were not distinguishable from S100A9-deficient and wildtype neutrophils upon stimulation with platelet activating factor (PAF), thapsigargin or macrophage inflammatory protein 1 alpha (MIP-1 alpha), indicating despite their massive expression S100A8/A9 do neither serve as calcium nor as zinc buffering proteins in granulocytes. In contrast, stimulation with adenosine-5'-triphosphate (ATP) induces a significant stronger increase of the intracellular free calcium level in S100A9-deficient cells compared to wildtype cells. Moreover, the ATP-induced calcium signal was still different when the cells were incubated in calcium free buffer suggesting that pirinergic receptors of the P(2Y) class could be involved in this signalling pathway.

Mechanism of apoptosis induced by S100A8/A9 in colon cancer cell lines: the role of ROS and the effect of metal ions.

The protein complex S100A8/A9, abundant in the cytosol of neutrophils, is secreted from the cells upon cellular activation and induces apoptosis in tumor cell lines and normal fibroblasts in a zinc-reversible manner. In the present study, we present evidence that the S100A8/A9 also exerts its apoptotic effect by a zinc-independent mechanism. Treatment of the colon carcinoma cells with different concentrations of human S100A8/A9 or the metal ion chelator diethylenetriaminepentacetic acid (DTPA) resulted in a significant increase of cell death. Annexin V/phosphatidylinositol and Hoechst 33258 staining revealed that cell death was mainly of the apoptotic type. A significant increase in the activity of caspase-3 and -9 was observed in both cell lines after treatment. Caspase-8 activation was negligible in both cell lines. The cytotoxicity/apoptotic effect of human S100A8/A9 and DTPA was inhibited significantly (P<0.05) by Zn(+2) and Cu(+2), more effectively than by Ca(2+) and Mg(2+). The antioxidant N-acetyl-L-cysteine inhibited the cytotoxicity/apoptotic effect of S100A8/A9 and DTPA. However, as a result of the different time-courses of both agents and that the S100A8/A9-induced apoptosis was not completely reversed, we conclude that S100A8/A9 exerts its apoptotic effect on two colon carcinoma cell lines through a dual mechanism: one via zinc exclusion from the target cells and the other through a yet-undefined mechanism, probably relaying on the cell-surface receptor(s).

Monocyte and macrophage functions in M-CSF-deficient op/op mice during experimental leishmaniasis.

Mice with a naturally occurring Csfm(op)/Csfm(op) (op/op) gene mutation lack functional macrophage-colony stimulating factor (M-CSF) and are deficient of M-CSF-derived macrophages. They are severely monocytopenic, and their remaining M-CSF-independent macrophages were shown to differ in differentiation and distinct functions when compared with phenotypically normal mice of the same background. It is not known if osteopetrosis mice (op/op mice) are able to mount a specific immune response against intracellular pathogens, as this would require complex effector functions by macrophages. We therefore investigated the ability of op/op mice and their M-CSF-independent macrophages to combat infection with Leishmania major. op/op mice retained the ability to resist an infection with L. major by mounting a T helper cell type 1 cell response, eliminating parasites and resolving the lesions. Macrophages from op/op mice were able to sufficiently perform effector functions in vitro, such as phagocytosis, production of leishmanicidal nitric oxide (NO), killing of parasites, and release of interleukin (IL)-12. There were quantitative differences, as M-CSF-derived macrophages from hematopoietic organs of control mice showed significantly higher rates of phagocytosis and higher NO release after stimulation with lipopolysaccharides than corresponding macrophages from op/op mice. In contrast, when peritoneally elicited macrophages were used, those from op/op mice revealed a stronger response than those from control mice with regard to release of NO or IL-12. These differences suggest that M-CSF-independent maturation of op/op monocytes subsequent to their release from hematopoietic tissue exerts influence on their effector functions. However, M-CSF or M-CSF-derived macrophages are not necessary for an effective immune response against L. major.

Expression of MRP8 and MRP14 by macrophages is a marker for severe forms of glomerulonephritis.

Expression of two S100 proteins, myeloid related protein (MRP)8 and MRP14, as well as their complex formation indicate proinflammatory properties of macrophages. We analyzed if the different forms of glomerulonephritis (GN) are associated with the appearance of certain phenotypes of infiltrating macrophages characterized by expression of MRP8 and MRP14 as well as their complex formation. Immunohistochemical analysis of 89 renal biopsies with different forms of nephritis revealed that expression and complex formation of MRP8 and MRP14 by infiltrating macrophages in the glomeruli correlated with the severity of the inflammatory process. As such, MRP8/MRP14-expressing monocytes prevailed in highly proliferating forms of GN, i.e., systemic lupus erythematosus GN and extracapillary GN. In contrast, a high percentage of macrophages in the renal interstitium expressed MRP8 and MRP14 without concomitant formation of their complex, and they indicated a chronic type of inflammatory reaction in GN. Immunosuppressive drugs had no direct effects on the expression of MRP8 and MRP14 in macrophages in vitro. The correlation of MRP8 and MRP14 expression with disease activity indicates that these calcium-binding proteins are of pathophysiological relevance in GN. In addition, our findings reflect differences in the inflammatory mechanisms underlying the various forms of GN, as they revealed that distinct macrophage subpopulations prevail in the different forms of GN.

Meeting report: molecular mechanisms of inflammation: how leukocytes come, see and seize.

Inflammation has developed in the course of evolution as a process to defend the body against invading microbes and to respond to injuries. Several mechanisms of interaction between endothelial cells and leukocytes have evolved to render inflammation an effective, tightly controlled, and self-limited process. Imperfect executions of this "game plan" lead to pathological abnormalities resulting in diseases. The meeting on Molecular Mechanisms of Inflammation held at Schloss Elmau, Germany in October 2002 has featured activation of endothelial cells, adhesion and migration of leukocytes, as well as receptor pathways for activation and deactivation of leukocytes and, concomitantly, of the inflammatory response. Thus, a review on some of the presented data casts interesting spotlights on different steps of the inflammatory cascade.

L-FABP is exclusively expressed in alveolar macrophages within the myeloid lineage: evidence for a PPARalpha-independent expression.

Peroxisome proliferator-activated receptors (PPARs) play a role in inflammation and, in particular, PPARgamma is involved in monocyte/macrophage differentiation. Members of the fatty acid-binding protein (FABP) family have been reported to function as transactivators for PPARs. Therefore, the expression of PPARs and FABPs in the myeloid lineage was investigated by real-time PCR and immunofluorescence analysis. We found adipocyte-, epidermal-, and heart-type FABP to be ubiquitously expressed within the myeloid lineage. In contrast, liver-type FABP was exclusively detected in murine alveolar macrophages (AM), confirmed on protein level by double fluorescence analysis. The PPAR subtypes also showed a temporally and spatially regulated expression pattern in myeloid cells: the beta-subtype was expressed in bone marrow, peritoneal, and alveolar macrophages, whereas it was not detected in dendritic cells (DCs). The gamma1-isoform was present in all cells, however, at different levels, whereas the gamma2-isoform was expressed in alveolar macrophages and dendritic cells. A low level PPARalpha mRNA could be detected in peritoneal macrophages and immature dendritic cells but not in mature dendritic cells and bone marrow macrophages. Interestingly, PPARalpha mRNA was also absent in the alveolar macrophages although liver-type FABP was expressed, indicating that gene expression of liver-type FABP was independent of PPARalpha. Since liver-type FABP is known as transactivator of PPARgamma the simultaneous expression of both proteins may have general implications for the activation of PPARgamma in alveolar macrophages.

The myeloid expressed EF-hand proteins display a diverse pattern of lipid raft association.

EF-hand proteins are known to translocate to membranes, suggesting that they are involved in signaling events located in the cell membrane. Many proteins involved in signaling events associate cholesterol rich membrane domains, so called lipid rafts, which serve as platforms for controlled protein-protein interaction. Here, we demonstrate that the myeloid expressed EF-hand proteins can be distinguished into three classes with respect to their membrane association. Grancalcin, a myeloid expressed penta EF-hand protein, is constitutively located in lipid rafts. S100A9 (MRP14) and S100A8 (MRP8) are translocated into detergent resistant lipid structures only after calcium activation of the neutrophils. However, the S100A9/A8 membrane association is cholesterol and sphingolipid independent. On the other hand, the association of S100A12 (EN-RAGE) and S100A6 (calcyclin) with membranes is detergent sensitive. These diverse affinities to lipid structures of the myeloid expressed EF-hand proteins most likely reflect their different functions in neutrophils.

Only the soluble form of the scavenger receptor CD163 acts inhibitory on phorbol ester-activated T-lymphocytes, whereas membrane-bound protein has no effect.

The extracellular moiety of the hemoglobin/haptoglobin scavenger receptor CD163 (RM3/1 antigen) can be shed from monocytes and is a normal plasma component. We found that in a dose-dependent manner soluble CD163 induces a decrease in CD69 expression, a reduced [(3)H]thymidine uptake and a down-regulated matrix metalloproteinase-9 RNA expression in phorbol myristate acetate-stimulated T-cells. Co-culturing T-cells on transgenic fibroblasts, expressing membrane-bound CD163, yielded no differences compared to culture on non-transfected cells. We conclude that CD163 has at least two distinct functions: the clearance of hemoglobin in its cell-bound form and participation in anti-inflammation as a soluble factor, exhibiting cytokine-like functions.

Computational searches for missing orthologs: the case of S100A12 in mice.

The interaction of the Ca2+-binding protein S100A12 with RAGE (receptor of advanced glycation endproducts) has been considered as a novel proinflammatory axis, since blockage of RAGE/S100A12 ligation suppresses chronic cellular activation and tissue injury in mouse models. However, the existence of a murine S100A12 ortholog is unknown. Because experimental approaches failed to identify it, we started an analysis of gene locus evolution. Human S100A12 is localized in the S100 gene cluster between S100A8 and S100A9, which are neighbors in both mouse and human. Confirming identical gene order, we found a DNA region between the murine S100A8 and S100A9 genes that is 60.9% identical to a region of the human S100A12 gene, including the first exon. Instead of the second and third exon, we found homology to a region close to the human S100A9 locus. To exclude a murine S100A12 ortholog elsewhere in the genome, we used human S100A12 as query for TBlastN homology searches. The matches were either too short, or identity was too low, or they could clearly be identified as distinct S100 genes. Obviously, an S100A12 ortholog is neither present in mouse nor rat, indicating that S100A12 has been lost during rodent evolution, probably due to a deletion.

S100A9/S100A8: Myeloid representatives of the S100 protein family as prominent players in innate immunity.

Neutrophils are rapidly recruited to sites of inflammation and are thereby at the forefront of the organism's defense against numerous attacks. As unspecific phagocytes, they belong to the so-called innate immunity. Two S100 proteins, namely S100A9 (MRP14) and S100A8 (MRP8), constitute roughly 40% of the cytosolic protein in these cells, implying by their pure abundance an important role in the effector functions of neutrophils. However, despite intense research in the past 15 years, the puzzle that may embed both molecules into the neutrophil/monocyte physiology is still incomplete. One reason might be the conformational variability the S100A9 and S100A8 molecules can adopt. They readily form hetero- and homodimeric, trimeric as well as tetrameric complexes, but they evidently do also exert specific functions as monomers. An ever-increasing body of information suggests that S100A9 plays a prominent role in leukocyte trafficking and arachidonic acid metabolism. In addition, elevated levels of S100A9 and S100A8 in body fluids of inflamed tissues strengthen the view that these molecules are important players in fighting inflammation. The aim of this review is to give an update on the current developments concerning the S100A9/S100A8 molecule in biology and medicine.

Phagocyte-specific calcium-binding S100 proteins as clinical laboratory markers of inflammation.

The EF-hand homolog family of S100 proteins comprises the largest group of calcium-binding proteins. Within this S100 family, the phagocyte-specific calcium-binding proteins are pro-inflammatory molecules expressed and secreted by phagocytes, which play a pivotal role within the innate immune system. Although the exact biological functions of these proteins still remain to be defined in greater detail, there is evidence that they are involved in a pro-inflammatory axis associated with various inflammatory conditions. The three members of this group, S100A8, S100A9 and S100A12 are overexpressed at local sites of inflammation. High concentrations are found in synovial fluid, sputum, stool and blood plasma/serum during inflammation. Both the S100A8/S100A9 complex and S100A12 have been proven to be useful as diagnostic markers of inflammation especially in non-infectious inflammatory diseases such as arthritis, chronic inflammatory lung and bowel disease. They indicate phagocyte activation more sensitively than conventional parameters of inflammation. As a consequence, there is a strong correlation to the inflammation of various acute and chronic disorders, making these proteins sensitive parameters for the monitoring of disease activity and response to treatment in individual patients. The phagocyte-specific S100 proteins are able to indicate minimal residual inflammation, which is not detected by other diagnostic tests, and they may even be prospective markers for the outcome of patients. In this review, pro-inflammatory functions of S100 proteins and their usefulness as biomarkers of inflammation are presented.