1. The Balance between the Action of Cytotoxic Agents and Protective Principles
1.1. Major Classes of Host-Derived Cytotoxic Agents
The contact of cytotoxic agents with living matter worsens cell functions and can induce irreversible changes in cells and tissues including cell death. According to the source of these agents, they can be roughly divided into external and host-derived cytotoxic components. The group of external cytotoxic agents comprises pathogen-derived toxins and manifold external poisons that act on the organism by inhalation, direct contact, or uptake with food. A third group represents environmental cytotoxic agents. An overview about external cytotoxic agents is given in Figure 1. Selected examples of these agents are included.
Figure 1. Classification of external cytotoxic agents. Three selected examples are indicated for each group.
Of course, these external cytotoxic agents can cause sufficient threat to the affected tissues and finally the death of the organism. However, these agents are usually not involved in long-lasting, chronic inflammation.
Host-derived cytotoxic agents result from activated immune cells such as neutrophils, eosinophils, monocytes, macrophages, and T cells, but also from affected tissue cells of non-immunological origin, for instance, muscle cells and red blood cells. Immune cells contain an arsenal of potentially cytotoxic agents that are needed to inactivate and kill pathogens and to remove and digest affected cells and destroyed tissues 
. Usually these agents act within small, bounded compartments, e.g., within the phagosomes of neutrophils and macrophages. However, a certain amount of these cytotoxic agents is released from activated immune cells into the surrounding milieu, where they become dangerous to unperturbed cells. The following classes of immune-cell-derived cytotoxic agents are known: small reactive species, heme peroxidases, free metal ions, serine proteases, matrix metalloproteases, and small pro-inflammatory peptides. These agents are either pre-assembled or generated during cellular immune activation.
Damage to tissue cells of non-immunological origin can result in the uncontrolled release of heme proteins such as hemoglobin and myoglobin and the subsequent formation of free heme. Cellular stress is also associated with enhanced formation of reactive species and deviations in free metal ion metabolism. As a result, the metabolism of mitochondria is disturbed and numerous oxidative processes in biological constituents take place.
According to their mode of action, host-derived cytotoxic agents can be divided into oxidant- and protease-based agents (Figure 2). Products of the first group promote oxidative alterations of biological constituents, whereas members of the second group cause proteolytic cleavage in cell and tissue components.
Figure 2. Major classes of host-derived cytotoxic agents.
1.2. Control of Cytotoxic Agents by Protective Principles
The destructive action of host-derived cytotoxic agents depends not only on the mass of released cytotoxic agents, but also on the current status of host-own protective principles 
. In order to curtail or avoid destruction by these agents, numerous ready-to-use mechanisms exist in cells and tissues to inactivate immediately hazardous components released from activated immune and affected tissue cells. Usually, unperturbed cells and tissues are well equipped with protective principles. In this way, any threat to unperturbed tissue components is minimized.
The balance between host-derived cytotoxic agents and protective principles functions well as long as the activation of immune cells is moderate enough and neighboring tissues are well-equipped with ready-to-use protective mechanisms (Figure 3). Problems can arise with severe and long-lasting immune responses and with the decline, exhaustion, or inactivation of selected antagonizing principles despite an up-regulation of many protective proteins under stress situations. In turn, long-lasting inflammatory processes can result from the permanent release of cytotoxic agents from damaged cells in combination with insufficient inactivation of these agents. In other words, low expression of a few protective principles favors the continuous action of destructive agents and affects still-unperturbed cells. In this vicious circle of permanent cell destruction, not only novel cytotoxic elements but also novel alarmins and antigens are liberated from affected cells. In addition, pro-inflammatory peptides such as angiotensin II and bradykinin are formed by insufficient inactivation of serine proteases. Hence, the inflammation cannot be terminated sufficiently and flares up again and again. In severe cases, a very low level of protection leads to organ failure, sepsis, and septic shock.
Figure 3. The interplay between host-derived cytotoxic agents and antagonizing principles. (Patho)physiological consequences of the release of cytotoxic agents at inflammatory sites highly depend on the status of protective mechanisms.
To overcome chronic inflammation, it is highly essential, besides inhibition of selected pathways in the inflammatory cascade, to improve poorly expressed protective systems to better detoxify the damaging agents.
1.3. Disturbed Balance between De Novo Synthesis and Damage of Tissue Components during Resolution of Inflammation
Termination of inflammation is characterized by the down-regulation of pro-inflammatory cells, cytokines, and signaling pathways as well as by the formation of anti-inflammatory mediators and induction of repair processes. During this phase of inflammation, cytokines of the transforming growth factor β (TGF-β) family, which are secreted from M2-type macrophages and some other cells, suppress together with interleukin 10 (IL-10)-activated immune cells 
. These cytokines also promote tissue repair by stimulating fibroblasts to synthesize collagen and other components of the extracellular matrix (ECM) and by the release of tissue inhibitors of metalloproteases (TIMPs) 
. The latter inhibitors down-regulate the activity of matrix metalloproteases (MMPs) and thus prevent degradation of ECM components.
1.4. Selected Environmental Cytotoxic Agents
Although not host-derived, we can also be exposed to external cytotoxic agents (see Figure 1). Of these agents, environmental cytotoxic agents act more or less intensely and permanently on our organism. As this exposure concerns nearly all persons, these agents are usually detoxified by antagonizing principles when the exposure is moderate and does not exceed a critical level.
2. Selected Cytotoxic Agents and Their Counter-Regulating Principles
2.1. Small Reactive Species and Metal Ions
2.1.1. Superoxide Anion Radicals
The stepwise reduction of dioxygen yields the species superoxide anion radical (O2•−
) and hydrogen peroxide (H2
. These species are less dangerous concerning their direct action on tissue components. However, they are involved in the formation of highly reactive and tissue-damaging agents by interaction with radicals, metal ions, and iron-containing proteins.
Activated leukocytes are able to generate large amounts of O2•−
by reducing dioxygen. This reaction is catalyzed by NADPH oxidase, which is assembled from several membranous and cytoplasmic components during the activation of neutrophils, eosinophils, monocytes, and macrophages 
. NADPH oxidases are also distributed in cells of the blood vessel wall, respiratory tract, gastrointestinal tract, and thyroid gland 
. However, these enzymes are less efficient in reducing dioxygen than NADPH oxidase from immune cells. Other sources for superoxide anion radicals are reactions of xanthine oxidase 
, autoxidation of hemoglobin and myoglobin 
, cytochrome P450-driven redox recycling of some xenobiotica 
, and one-electron reduction of dioxygen by different mitochondrial enzymes 
Superoxide anion radicals are unstable. Two superoxide anion radicals dismutate spontaneously to hydrogen peroxide and dioxygen 
. The rate of this dismutation highly depends on pH, with a maximal rate around pH 4.8, the pka
value of O2•−
, and decreasing rates with increasing pH 
. With one unit pH increase, the dismutation rate of O2•−
decreases by one order of magnitude. At pH 7.4 this rate is 2 × 105
Superoxide anion radical reacts in a very rapid reaction with nitrogen monoxide, also a radical species, under the formation of the powerful oxidant peroxynitrite 
. In mitochondria, superoxide anion radical is able to release Fe2+
from molecules containing [4Fe-4S]2+
clusters such as aconitase 
In humans, control over O2•−
is realized with three isoforms of superoxide dismutase (SOD) and cytochrome c. SOD1 is distributed in the cytoplasm, intermembrane space of mitochondria, and nuclei 
. In the mitochondrial matrix, SOD2 dominates 
. SOD3 is mostly found in blood vessel walls and lungs 
. These enzymes catalyze the dismutation of O2•−
with a rate several orders higher than the spontaneous dismutation reaction of O2•−
. In the intermembrane space of mitochondria, oxidized cytochrome c oxidizes O2•−
, thus contributing to the detoxification of O2•− 
Figure 4 depicts the major pathways for the formation of reactive species with a special focus on processes in activated neutrophils and stressed mitochondria. In both systems, the generation of small reactive species starts with the reduction of dioxygen to superoxide anion radicals.
Figure 4. Major pathways in formation of reactive species in activated neutrophils (upper panel) and stressed mitochondria (lower panel). Antagonizing principles against these species are displayed on grey backgrounds. In deactivation of transition metal ions, the term chelators stands for numerous proteins that scavenge, transport, and store iron and copper ions. Further explanations are given in the text. Abbreviations: MPO—myeloperoxidase, SOD—superoxide dismutase.
2.1.2. Hydrogen Peroxide
Spontaneous and SOD-catalyzed dismutation of O2•−
represent the main route of formation of H2
. Thus, all processes generating O2•−
also yield H2
. Otherwise, different peroxisomal enzymes are able to reduce O2
directly to H2
Due to its electronic structure, reactions of H2
are restricted to transition metal ions, complexes of these ions, and some proteins with selenocysteine (or cysteine) residues at the active site 
. Hydrogen peroxide is freely permeable through biological membranes, unlike O2•−
. The interaction of transition metal ions such as Fe2+
yields very reactive hydroxyl radicals and metal-based reactive species that can cause manifold damaging reactions on biological material 
Heme peroxidases, different cytochromes, hemoglobin, and myoglobin are activated by H2
leading to reactive states of the heme in these proteins. During immune response, H2
activates the heme peroxidases myeloperoxidase (MPO), eosinophil peroxidase (EPO), and lactoperoxidase (LPO), which are involved in both pro- and anti-inflammatory activities 
Several enzymes are known to catalyze the reduction of H2
O (Figure 4
). Glutathione peroxidase (GPX) utilizes glutathione (GSH) to reduce H2
. The highly distributed isoforms GPX1 and especially GPX4 also detoxify peroxynitrite, lipid hydroperoxides, and other organic hydroperoxides 
. GSH is recovered from the resulting oxidized glutathione (GSSG) by glutathione reductase 
. Peroxiredoxins, which are closely coupled to the thioredoxin system, also efficiently reduce H2
. Catalase removes H2
by both reduction to H2
O and oxidation to O2 
2.1.3. Transition Metal Ions and Hydroxyl Radicals
In the reaction between H2
, which is known as the Fenton reaction, the highly reactive hydroxyl radical is formed. Alternatively, iron–oxygen complexes such as ferryl or perferryl compounds are discussed as products of this reaction 
. Similarly, the reaction of H2
also yields hydroxyl radicals 
. Organic hydroperoxides are also oxidized by Fe2+
under the formation of reactive radical species that are involved in subsequent destructive reactions. Beyond Fenton chemistry, further mechanisms apparently contribute to metal-ion-induced tissue damage such as the interaction of Fe2+
with biological buffer components or the formation of Fe2+
Hydroxyl radicals react in a nearly diffusion-controlled manner with many substrates by abstraction of an H-atom or by addition to an unsaturated system under formation of a hydroxylated product 
. In both reaction types, substrate radicals are formed that can undergo manifold further reactions.
To avoid the disastrous formation of reactive species such as hydroxyl radicals and others, the main strategy of living matter is the tight control of transport, storage, and utilization of free metal ions (Figure 4
) as both iron and copper ions are necessary constituents of many proteins 
. Major components controlling iron metabolism are hepcidin (intestinal absorption), transferrin (blood transport), transferrin receptor (uptake by cells), and ferritin (intracellular storage) 
. Similarly, different import and export transporters and chaperones are involved in copper metabolism 
. Ceruloplasmin is able to oxidize both Fe2+
and Cu+ 
. Lactoferrin released from activated neutrophils binds Fe3+
and promotes its transfer to transferrin 
As already mentioned, peroxynitrite is formed in a very rapid reaction between O2•−
and NO 
. Peroxynitrite is involved in the formation of thiyl radicals and nitration of tyrosine residues, and is able to induce lipid-peroxidation processes 
. In reaction with CO2
, it yields nitrosoperoxycarbonate, which can decompose into radical species 
At inflammatory sites where heme peroxidases are present, peroxynitrite is decomposed in its reaction with resting MPO 
. Other redox-active heme proteins scavenge peroxynitrite and inactivate this powerful oxidant 
2.1.5. Heme Peroxidases and Hypohalous Acids
At an inflammatory site, the heme protein MPO can be released from activated neutrophils (Figure 4
. Eosinophils contain a similar peroxidase, the eosinophil peroxidase (EPO) 
. A third immunologically relevant heme peroxidase is LPO, which is distributed in mucous surfaces 
. All three heme peroxidases are able to oxidize SCN−
OSCN. MPO and EPO also oxidize Br−
to HOBr, whereas only MPO is able to yield HOCl from Cl−
The MPO product HOCl reacts efficiently with methionine and cysteine residues of proteins. Further major protein targets for HOCl are residues of cystine, histidine, tryptophan, lysine, and α-amino groups 
. HOBr , like HOCl, also oxidizes many residues in proteins, especially cysteine and methionine ones. HOBr induces ring halogenation in tyrosine residues more efficiently than HOCl 
Both HOCl and HOBr are inactivated at a high rate by thiocyanate (SCN−
. HOCl is additionally inactivated by Br−
. Further antagonizing principles against both hypohalous acids are ascorbate, GSH, taurine, and, additionally for HOBr, urate 
In blood, MPO and EPO are inactivated by ceruloplasmin through the formation of a tight inhibitory complex between heme peroxidase and ceruloplasmin 
2.2. Hemoglobin and Myoglobin Metabolites
There is always a release of intact hemoglobin from red blood cells and myoglobin from muscles at low levels. Intravascular hemolysis and rhabdomyolysis can be markedly enhanced under stress and disease situations (Figure 5
). Once released from red blood cells, tetrameric ferric hemoglobin dissociates into dimers and is easily oxidized to methemoglobin. This oxidation is usually caused by nitric monoxide. Excessive intravascular hemolysis can affect the bioavailability of NO 
. The serum protein haptoglobin is able to scavenge free methemoglobin. The resulting haptoglobin–methemoglobin complex is eliminated from circulating blood by spleen and liver macrophages in a CD163-dependent process 
. In a similar way, haptoglobin also scavenges metmyoglobin formed after the release of myoglobin from muscle cells.
Figure 5. Formation of methemoglobin, metmyoglobin, and free heme as a result of excessive intravascular hemolysis and rhabdomyolysis. Protective mechanism are presented on grey backgrounds. Further explanations are given in the text.
Although it is an acute-phase protein, the capacity of haptoglobin is limited when severe intravascular hemolysis or rhabdomyolysis occur. Both methemoglobin and metmyoglobin spontaneously liberate ferric protoporphyrin IX, briefly known as free heme, a very dangerous molecule 
. Free heme easily intercalates into the lipid phases of membranes and lipoproteins and the hydrophobic areas of proteins. At these loci, it catalyzes oxidative processes 
. In intact red blood cells, free heme induces hemolytic processes, thus enhancing existing intravascular hemolysis 
. Free heme is highly cytotoxic to kidney and liver 
. It is also a ligand to toll-like receptor 4 and thus contributes to the intensification of inflammatory processes 
. In the nucleus, free heme interacts with parallel guanine-rich quadruplex DNA and RNA structural elements, known as G4 structures 
In order to avoid the disastrous activities of free heme, different serum proteins are able to complex and inactivate free heme. Hemopexin binds free heme with high affinity. This free-heme–hemopexin complex is liberated from circulating blood via CD91-mediated internalization by hepatocytes 
. In humans, unlike mice, hemopexin is not an acute-phase protein 
Inside cells, free heme is detoxified by an interaction with heme oxygenase 
2.3. Oxidation of Cell and Tissue Components
In addition to proteolytic cleavage, lipids, proteins, nucleic acids, and carbohydrates are subjected under stress conditions to numerous chemical processes, whereby oxidative modifications predominate 
. The major initiating agents of these oxidative processes are highly reactive species, free transition metal ions, free heme, and aldehydes. Besides the open chain form of glucose 
, aldehydes result mostly from oxidative modifications of lipids 
Oxidative alterations of biological substrates are counterbalanced by lipid- and water-based antioxidant mechanisms. In lipid phases, major natural antioxidants are α-tocopherol, β-carotene, ubiquinol, and dehydrolipoic acid. They are mainly involved in the scavenging of lipid peroxyl radicals 
. Inactivation of lipid hydroperoxides is a further strategy to prevent oxidative processes. This is achieved most of all by the action of glutathione peroxidase 4 (GPX4). A high intracellular level of GSH is essential for the proper action of GPX4 and other glutathione peroxidases 
. In addition, perturbed acyl residues in phospholipids are cleaved by phospholipases 
. A thorough control over transition free metal ions also contributes to the prevention of oxidative processes in membranes and lipoproteins.
Urate and ascorbate are the main water-soluble antioxidants in our organism 
. Different polyphenols are important dietary antioxidants 
. They exert their protective action by radical scavenging, sequestration of free metal ions, and interaction with activated complexes of heme proteins 
2.4. Serine Proteases
2.4.1. Release of Serine Proteases from Immune Cells
At inflammatory sites, activated neutrophils can release the serine proteases elastase, cathepsin G, proteinase 3, and neutrophil serine protease 4. These proteases are primarily involved in the deactivation, killing, and digestion of phagocytosed microorganisms in neutrophils. Their pH optimum is around 8–9, a condition that predominates in early phagosomes of neutrophils 
. Elastase exhibits a killing activity against Gram-negative bacteria 
and a variety of cancer cells 
. In cancer cells, unlike non-cancer cells, elastase cleaves CD95 to liberate a death domain fragment that acts cytotoxically together with histone H1 
Serine proteases participate in the recruitment of neutrophils to a destination site by digestion of the surrounding tissue components and the induction and regulation of immune signaling. Elastase and proteinase 3 are able to cleave a broad range of chemokines and cytokines 
. The substrate specificity of cathepsin G is also relatively broad but more restricted than that observed for elastase and proteinase 3 
. In these experiments, only a few cytokines and chemokines, such as TNF-α, interleukin 5 (IL-5), interleukin 8 (IL-8), macrophage colony-stimulating factor (M-CSF), monocyte chemoattractant protein 1 (MCP-1), IL-1α, and Rantes, were resistant to neutrophil serine proteases.
Elastase and other serine proteases are attached together with other neutrophil proteins to a DNA network in neutrophil extracellular traps. These traps can kill external microbes independent of phagocytosis 
Activated mast cells release the serine proteases chymase, tryptase, and cathepsin G 
. These proteases are involved in matrix destruction, tissue remodeling, and regulation of inflammation. Mast cell tryptase and chymase are more restrictive than neutrophil serine proteases in the cleavage of chemokines and cytokines 
2.4.2. Activities of Neutrophil Serine Proteases
Although all serine proteases contribute to damaging reactions, the focus is mostly directed on elastase. An overview about multiple activities of neutrophil elastase during immune response is given in Figure 6
. Once released from activated neutrophils, elastase can affect healthy tissues. Elastase is involved in the destruction of extracellular matrix components such as elastin, collagens, proteoglycans, and laminin 
Figure 6. Activities of neutrophil elastase at inflammatory sites.
Like cathepsin G, proteinase 3, and cathepsin B, neutrophil elastase is able to convert angiotensinogen and angiotensin I into angiotensin II 
. This pro-inflammatory peptide can further foment inflammatory processes.
At inflammatory sites, neutrophil elastase activates MMP2, MMP3, and MMP9 from inactive precursors by cleaving an inhibitory protein residue 
. Cathepsin G is also able to activate MMP3 
. Cathepsin G and proteinase 3 are involved in MMP2 activation 
. Elastase might additionally degrade TIMP-1 
2.4.3. Mast Cell Serine Proteases
Human mast cells contain several types of tryptases and two members of chymase-like proteins, namely α-chymase and cathepsin G, which are secreted in response to allergens and pathogens 
. Mast cell proteases are known to stimulate the production of pro-inflammatory mediators such as IL-6 and IL-8 from bronchial epithelial cells and promote procollagen cleavage. With these activities they contribute to the recruitment of neutrophils and eosinophils at inflamed epithelium 
Other inflammation-promoting activities of chymase are the cleavage of angiotensin I into angiotensin II, activation of MMPs, and release of selected extracellular matrix elements 
. Tryptase is involved in the degradation of fibronectin and chemokines 
. Both tryptases and chymases contribute to the activation of different MMPs 
. MMPs are implicated in the pathogenesis of atherosclerosis and abdominal aortic aneurysms 
. Mast cell proteases are implied in airway epithelial remodeling and alterations in epithelium functions 
. They also contribute to angiogenesis induction during tumor growth 
. Chymase promotes the formation of active TGF-β from its precursor 
Tetrameric tryptase is stabilized by heparin and some other glycosoaminoglycans 
. In this complex, tryptase is not accessible to anti-proteases such as A1AT, SPLI, and α2
. Lactoferrin, myeloperoxidase, and antithrombin III, which are known to have heparin-binding domains, can inhibit tryptase activity 
. Spontaneous dissociation of the tryptase tetramer is a further mechanism to control tryptase activity 
Several antagonizing proteins against elastase and other serine proteases exist in blood and tissues (Figure 7
). The most abundant anti-protease is the serpin α1
-antitrypsin (A1AT). This serum protein is synthesized in the liver and represents an acute-phase protein. A1AT inhibits elastase and cathepsin G but not in the presence of heparin 
. The activity of proteinase 3 is affected by A1AT to a lesser degree. Heparin, however, enhanced the inactivation of proteinase 3 
Figure 7. The interplay between neutrophil-derived serine proteases and antiproteases.
Several factors contribute to the failure of A1AT to inhibit elastase. The inactivation of elastase requires two unperturbed methionine residues (Met-351 and Met-358) at the active site of A1AT. By oxidation of these residues A1AT loses its ability to inhibit elastase 
. Under stress conditions methionine oxidation in A1AT can be initiated by highly reactive species such as hydroxyl radicals, peroxynitrite, HOCl, HOBr, and others 
. Tobacco smoke and activated phagocytes are under discussion to contribute to methionine oxidation in A1AT and thus cause an acquired A1AT deficiency 
. Furthermore, neutrophil elastase can bind to negatively charged surfaces and polymers. Surface-bound elastase cannot be inhibited by endogenous antiproteases 
Serpin A3, also known as α1
-antichymotrypsin, is, like A1AT, an acute-phase protein. This antiprotease efficiently inactivates cathepsin G and mast cell chymase 
Secretory leukocyte protease inhibitor (SLPI) is able to inactivate several serine proteases such as neutrophil elastase, cathepsin G, tryptase, and chymase 
. SLPI is constitutively expressed in mucous secretions 
and also secreted from activated immune cells. It is assumed that SLPI exhibits an anti-apoptotic effect on immune cells and thus contributes to a better removal of dying cells and microbes at inflammatory sites 
Elafin, which is also known as proteinase inhibitor 3, is able to inactivate neutrophil elastase and proteinase 3 
. It exerts anti-inflammatory, anti-microbial, and wound-healing effects 
. Contradictory results were reported about the action of elafin on tumorigenesis. These results range from promotion of cell proliferation and induction of resistance against chemotherapy to tumor-suppressive effects 
. In early-stage hepatocellular carcinoma, elafin promotes metastasis formation via activation of EGFR/AKT signaling 
The antiprotease serpin B1 efficiently inactivates elastase, cathepsin G, and proteinase 3 
. Under oxidative stress, the cysteine residue at the active site in serpin B1 is oxidized with the loss of the antiprotease activity.
In contrast to the aforementioned antiproteases, which directly interact with the active site of proteases, α2
-macroglobulin forms a tetrameric cage around active proteases, thus inhibiting the direct contact between protease and substrate molecules. In this way, large substrate molecules such as collagen are excluded from direct contact, whereas small peptide substrates can be digested 
. Although α2
-macroglobulin inhibits the activities of elastase, cathepsin G, proteinase 3, and MMP9 released from neutrophils 
, the complex between elastase and α2
-macroglobulin is still active against small substrates 
. Moreover, neutrophil-derived reactive species such as HOCl can hinder α2
-macroglobulin to form tetramers and promote stabilization of dimers with the loss of the antiprotease activity 
High-affinity complexes are also known between ceruloplasmin and serine proteases of neutrophils 
. In this way, a destructive action of serine proteases on tissue components is minimized.
2.5. Small Pro-Inflammatory Peptides
2.5.1. Angiotensin II
The peptide hormone angiotensin II is an essential part of the renin–angiotensin–aldosteron system. It is involved in the regulation of blood pressure and water metabolism. During this activity, angiotensin II is formed from angiotensin I by the angiotensin-converting enzyme (ACE).
At inflammatory sites, angiotensin II can also be produced from cleavage of both angiotensinogen and angiotensin I by serine proteases released from immune cells such as elastase, cathepsin G, proteinase 3, and mast cell chymase (Figure 8
. Increased angiotensin II contributes via docking to AT1 and AT2 receptors to proteolysis, actin cleavage, apoptosis induction, and activation of the ubiquitin-mediated protein degradation 
. It also promotes superoxide anion radical production via activation of NADH/NADPH oxidases 
. Generally, these pro-inflammatory activities of angiotensin II mediate the prolonged existence of inflammatory states 
Figure 8. Effects of serine proteases on the renin–angiotensin–aldosteron system. Further explanations are given in the text.
Angiotensin II is under the control of ACE2, which converts this octapeptide to angiotensin 1–7 
. This limits the devastating activity of angiotensin II. Moreover, angiotensin 1–7 exerts an anti-inflammatory activity 
As an essential member of the contact system, the nonapeptide bradykinin is responsible for increased vascular permeability, vasodilation, hypotension, and other effects via interaction with its constitutively expressed B2
. A further pro-inflammatory metabolite is des-Arg9-bradykinin, which is formed from bradykinin by carboxypeptidase N. At inflammatory sites, des-Arg9-bradykinin acts selectively via bradykinin B1
receptors, which are only expressed in inflamed and injured tissue 
Bradykinin is a short-lived mediator of inflammation. It is inactivated by aminopeptidase P and the angiotensin-converting enzyme (ACE). Inhibition of ACE enhances bradykinin’s effects 
2.6. Inhibition of Matrix Metalloproteases
In human tissues, 23 MMPs and four TIMPs are found. Most MMPs are normally not expressed in healthy tissue. The activity of MMPs is essential in tissue remodeling, such as angiogenesis, bone growth, wound healing, and repair processes during the resolution of inflammation 
MMPs are secreted as inactive enzymes bearing an inhibitory prodomain that must be cleaved. In addition to neutrophil serine proteases, plasmin, chymases, and other MMPs are involved in MMP activation 
. At low concentrations, highly reactive species such as HOCl, •
OH, and ONOO−
can activate MMPs. However, higher concentrations of these species inactivate active MMPs 
During the exudation and infiltration phase of inflammation, MMP2 and MMP9 are mainly secreted from invading immune cells, smooth muscle cells, and fibroblasts 
. These and other MMPs contribute to cleaving the matrix components collagen and elastin.
The activity of MMPs is tightly controlled by TIMPs and α2
-macroglobulin. The latter inhibitor, which has a very broad activity range against proteases, acts in blood and other biological fluids 
. Generally, TIMPs have a broad spectrum of inhibition of MMPs. The constitutively expressed TIMP-2, like TIMP-3 and TIMP-4, is able to inhibit nearly all MMPs. TIMP-1 has a low activity against membrane-bound MMPs 
. TIMP-3 additionally inhibits members of disintegrin metalloproteinases. Moreover, it is the only TIMP that binds to the ECM