Systemic Infections and Sepsis

Fig. 8.1
Systemic inflammatory response syndrome after polytrauma

Cytokines are polypeptides and act in a para- or autocrine manner. They are capable of exerting many effects on an array of cell types (pleiotropy). In addition to hyperacute proinflammatory cytokines such as TNF-α or IL-1β with an effect after 1–2 h there exist subacute (secondary) cytokines such as IL-6, IL-8 (neutrophil activating peptide), macrophage migratory-inhibitory factor (MIF), HMGB-1, as well as IL-12 and IL-18, two interferon-γ-modulating cytokines. MIF has a pivotal role in regulating systemic and local inflammatory responses by activating macrophages and T-cells. MIF is unique among cytokines in that it links the immune system with the endocrine system. In response to stress, MIF is secreted by the hypothalamus, the anterior pituitary gland and the adrenal glands and antagonizes the anti-inflammatory effect of endogenous steroids. HMGB-1 increases the proinflammatory effect of IL-1β through binding of these mediators. Recently, the IL-17 cytokine family was discovered. IL-17A interplays between innate and adaptive immune responses. This proinflammatory cytokine is mainly produced by TH17 cells, but also by other immune cells. It triggers the production of many other cytokines and provides crosstalk between lymphocytes and phagocytes.

Through the influence of antigens, T-helper lymphocytes (TH cells, CD4+ cells) differentiate two phenotypes, the TH1 and TH2 lymphocytes (adaptive immunity). TH1 cells support the proinflammatory cascade through secretion of IL-2, interferon-γ (IFN-γ) and TNF-β, whereas TH2 cells are important producers of anti-inflammatory mediators. Monocytes/macrophages are involved in the differentiation of TH1 cells through secretion of IL-12. Depressed IL-12 production after trauma correlated with a shift of the TH1/TH2 ratio toward TH2-type pattern with an adverse clinical outcome.

As described, mechanical and hypoxic cellular damages lead to an increase of intracellular Ca2+ levels with an activation of phospholipase A2 (PLA2) and phospholipase C. These enzymes catalyze the release of arachidonic acids from membrane phospholipids. Through the activation of cyclooxygenase and 5-lipooxygenase prostaglandin E2, respectively, leucotriene B4 and thromboxane A2 are produced. These metabolites are involved in the recruitment of inflammatory cells, regulation of vascular permeability and motility, as well as aggregation of thrombocytes. Additionally, PLA2 induces the release of the platelet-activating factor. It supports the activation of macrophages, their interaction with endothelial cells, and the activation and aggregation of thrombocytes.

8.4.3 Hypoinflammation: Compensatory Anti-inflammatory Response Syndrome

TH2-cells and monocytes/macrophages release IL-4, IL-10, IL-13 or transforming growth factor-β. In addition, different cytokines (e.g., IL-6) have shown a dual effect with pro- and anti-inflammatory activities. Serum levels of IL-10 or natural inhibitors of receptors, such as soluble TNF receptors (TNF-RI (55 kD) and TNF-RII (75 kD) or IL-1 receptor antagonist correlate with the incidence of posttraumatic septic complications. Furthermore, the responsiveness of blood monocytes from septic patients to release proin­flammatory cytokines is decreased in in vitro studies after stimulation with gram-negative (endotoxin, lipopolysaccharide [LPS]) or gram-positive bacterial products (e.g., peptioglycan, lipoteichonic acid). This phenomenon is called “endotoxin tolerance” and is explained through IL-10−mediated depression of the activity of intracellular transcription factors (NF-κB). Antigen-presenting cells such as monocytes/marcrophages show a depressed expression of the MHC class II molecule human leucocyte antigen-DR with a correlation to posttraumatic infections.

During the early phase of the posttraumatic course, a lymphocytopenia can be observed and is associated with morbidity after trauma. It can be related to increased apoptosis triggered by stress hormones (steroids) and cell death proteins. Apoptosis is characterized morphologically by cell shrinking with cytoplasmatic condensation (apoptotic bodies), nuclear condensation (pycnosis), and DNA-fragmentation (DNA laddering). The cell membranes stay primarily intact and no surrounding inflammatory signs can be observed in contrast to necrosis.

An overwhelming anti-inflammatory response (hypoinflammation) seems to be responsible for immunosuppression with a high susceptibility to secondary infections. This immunological status is called compensatory anti-inflammatory response syndrome (CARS). However, it does not resemble a compensatory mechanism in a biphasic model. A few hours after a first hit, anti-inflammatory mediators (e.g., IL-10) are detectable in the serum. It seems that the host defense response tries to find a fine balance between SIRS and CARS to induce reparative mechanisms and limit entry or overload of microorganisms and to avoid autoaggressive inflammation with secondary tissue ­damage and susceptibility to infections. These mixed inflammatory mechanisms are called mixed antagonistic response syndrome.

8.4.4 Complement System

Complement activation is an early event in sepsis. The classical pathway of activation is induced by antigen-antibody (immunoglobulins M or G) complexes or activated coagulation factor XII (FXIIa), whereas bacterial products (e.g., LPS) activate the alternative pathway. Cleavages of C3 by C3 convertase and C5 by C5 convertase lead to the formation of opsonins, anaphylatoxins C3a and C5a, C4a and the membrane-attack complex (also known as C5b-C9). The opsonins C3b and C4b are involved in the phagocytosis of cell detritus and especially bacteria by covalent binding of pathogen surfaces (opsonization). The anaphylatoxins C3a and C5a support different inflammatory mechanisms, the recruitment (chemotaxis) and activation of phagocytic cells (PMNL, monocytes, macrophages), the enhancement of the hepatic acute phase response, the degranulation of mast cells and basophils with release of vasoactive mediators such as histamine as well as the adhesion of leukocytes to endothelial cells leading to increased vascular permeability with edema. In addition, C5a contribute to immunoparalysis, imbalances in the coagulation system, MOF, septic cardiomyopathy and apoptosis of parenchymal cells. In clinical studies of sepsis, increased concentrations of C3a, C4a, and C5a in the plasma linked to poor outcome. In contrast, the C1-Inhibitor, produced by hepatocytes, endothelial cells, monocytes, and macrophages, is decreased during sepsis through degradation by PMNL-elastases. Dual blockade of the two C5a receptors (C5AR and C5L2), rather than blockade of C5a alone, seems to be an encouraging strategy for clinical trials in sepsis.

8.4.5 Coagulation Cascade

Coagulation contributes significantly to the outcome in sepsis with concurrent down-regulation of anticoagulant systems and fibrinolysis. Dysregulation of coagulation during sepsis can range from a moderate coagulopathy to the occurrence of disseminated intravascular coagulation (DIC). In addition, inflammation-induced coagulation in turn contributes to further inflammation.

In primary haemostasis, platelet-derived microparticles (MPs) express functional adhesion receptors including P-selectin on their surface, attach to the site of injury on the vessel wall, and support the rolling of leukocytes in the presence of shear stress. The adhesion of platelets is stabilized by von-Willebrand-factor (VWF) proteins. VWF is synthesized in endothelial cells and released in the plasma as unusually large VWF multimers that are rapidly degraded into smaller VWF multimers by the modulator ADAMTS-13 (a disintegrin-like and metalloproteinase with a thrombospondin type-1 motifs 13). Deficiency of ADAMTS-13, observed during severe sepsis, increases the level of large VWF multimers and leads to platelet aggregation and thrombus, resulting in microvascular failure.

In secondary hemostasis, the complex cascade of complement factors activation results in the formation of fibrin strands, which further strengthen the platelet plug. Two pathways of coagulation cascade are described and converge on the activation of thrombin.

  • The intrinsic coagulation system is linked to the contact activation system (contact factor pathway). The plasma proteins FXII, prekallikrein, kininogen, and the factor XI (FXI) represent this system. They are characterized by the fact that they can be activated by negative charged cellular surfaces (contact activation). FXII and prekallikrein activate mutually and form FXIIa and kallikrein. FXIIa stimulates the complement cascade on the classical pathway and amplifies the prothrombotic events during sepsis through the formation of FIXa by FXIa. Kallikrein induces fibrinolysis through conversion of plasminogen to plasmin or activation of the urokinase-like plasminogen activator. The tissue plasminogen activator works as a cofactor. In addition, kallikrein stimulates the formation of bradykinin from kininogen. Kinins are vasodilators, increase the vascular permeability and inhibit the functions of thrombocytes. A consumption of FXII and FXI is observed during sepsis, whereas plasma levels of enzyme-inhibitor-complexes such as FXIIa-C1-inhibitor are increased. C1-inhibitor and α1-protease-inhibitor represent the inhibitors of the intrinsic coagulation system.

  • However, the coagulation system is primarily activated over the extrinsic pathway (tissue factor pathway) with an increased expression of the tissue factor (TF) on endothelial and subendothelial cells, fibroblasts, and monocytes induced by bacterial cell wall ­fragments and proinflammatory cytokines (TNF-α, IL-1β). The FVII-TF-complex stimulates the coagulation cascade with formation of aFX and finally thrombin (FIIa) from prothrombin (FII). Thrombin activates FV, FVIII, and FXI leading to an enhanced formation of thrombin. After cleavage of fibrinogen by thrombin, fibrin monomers polymerate to stable fibrin clots through support of aFXIII. The consumption of coagulation factors is further enhanced through the proteolysis of fibrin clots to fibrin fragments by the protease plasmin (fibrinolysis). To control the consumption of coagulation factors antithrombin (ATIII) produced by hepatocytes inhibits thrombin and FXa through formation of a thrombin-antithrombin complex. This effect can be enhanced by heparin. Furthermore thrombin inhibits the factors IXa, XIa and XIIa. Other inhibitors are the tissue factor pathway inhibitor and the activated protein C (APC). Protein C is synthesized in the liver, keratinocytes and the endothelium and is activated by thrombin bound to the thrombomodulin complex and by endothelial protein C receptor (EPCR). After dissociation from EPCR, APC binds to its cofactor protein S, resulting in the inactivation of clotting factors Va and VIIIa. Levels of APC, protein C and protein S are depleted in sepsis. APC is a central endogenous anticoagulant protein with antithrombotic, antiinflammatory, antiapoptotic, and profibrinolytic (through inhibition of plasminogen activator inhibitor 1) activities. APC can cleave and activate protease activated receptor-1 (PAR-1)-dependent cellular pathways in endothelial cells in competition with thrombin.

The DIC represents the most serious dysfunction of coagulation cascade during sepsis. In the initial phase of DIC, thrombin activation in combination with a reduced fibrinolytic cascade results in intra- and extravascular (e.g., intraalveolar in adult respiratory distress syndrome [ARDS]) fibrin clots (hypercoagulability) and an increased interaction of endothelial cells and leukocytes is observed. The consumption of the coagulation factors (hypocoagulability) and dysfunctions of thrombocytes are responsible for diffuse bleedings (hemorrhagic diathesis). In the late phase of DIC, intravascular fibrin clots lead finally to microcirculatory disturbances with hypoxia-induced cellular damage and multiple organ failure. Another ­consequence of DIC is the inhibition of fibrinolysis. As DIC develops, inflammation and coagulation interact in a bidirectional manner. Thrombin, FXa, and FVII-TF-complex interact with PAR1-4 system on endothelial cells, platelets and leukocytes and promote proinflammatory cytokine release generation of C5a, and expression of adhesion molecules by endothelial cells, platelets and leukocytes.

8.4.6 Acute Phase Reaction

The local (Kupffer-cells) and systemic release of proinflammatory cytokines induce the acute phase reaction in the liver to enhance tissue protective and antimicrobial mechanisms. The synthesis of positive APPs in hepatocytes such as C-reactive proteins (CRP), α1-antitrypsin, α2-makroglobulin, caeruloplasmin, LPS-binding protein (LBP), fibrinogen, prothrombin, and C4BP is increased, whereas the production of negative APPs such as albumin, high-density lipoproteins, protein C, protein S and ATIII are reduced. CRP increases the expression of TF on PMNLs and monocytes/macrophages and enhances therefore the activation of the extrinsic coagulation cascade. α1-antitrypsin inactivates proteases, secreted by PMNLs or macrophages, whereas α2-macroglobulin and caeruloplasmin neutralize free oxygen radicals and proinflammatory cytokines. LBP suppress the effects of LPS in high concentrations, whereas in small quantities an enhancement of the LPS-effects can be observed. Serum levels of LBP are increased during sepsis. The elevated ratio of positive to negative APPs accelerates the development of DIC.

Procalcitonin (PCT), a new representer of positive APPs, is a precursor of calcitonin, which is normally produced in the C-cells of the thyroid. Hepatocytes as well as immune cells are also capable of secreting PCT. The biological function of this APP is still unclear. However, increased levels of PCT can be observed after trauma and especially with a complicated course with sepsis or MODS. Recently, pancreatic stone protein, known from the pathogenetic cascade of pancreatits, was detected as positive APP in sepsis with an influence on adhesion molecules on PMNLs.

8.4.7 Leukocytes Recruitment and Oxidative Stress

The infiltration and accumulation of PMNLs and macrophages at the local side of infection with large stores of proteolytic enzymes (elastase, metalloproteinase) and with the capacity to rapidly generate reactive oxygen (ROS), the so-called respiratory burst or oxidative stress, and reactive nitrogen (RNS) species represent a crucial event for degradation of internalized pathogens.

  • Leukocyte/endothelial cell interaction (adherence) involves two sets of adhesion molecules. During the initial phase of adherence, selectins on leukocytes (L-selectin, leukocyte adhesion molecule-1) and endothelial cells (E-selectin, endothelial leukocyte adhesion molecule-1) are responsible for the “rolling” of PMNLs. In the second step, an up-regulation of integrins on PMNLs (CD11a-c/CD18 complexes) and intercellular adhesion molecules (ICAM-1) or vascular cell adhesion molecules (VCAM-1) on endothelial cells are involved. The interaction of these adhesion molecules leads to a stable cell-to- cell contact with a PMNL-attachment, the so-called “sticking” of PMNLs. Through shedding increased levels of adhesion receptors (selectins, soluble ICAM-1 (sICAM-1) or sVCAM-1) are detectable in serum of injured patients with a ­predictive value for the development of septic complications. Finally, the migration (diapedesis), accumulation, and activation of leukocytes in tissue are mediated by chemoattractant factors, such as chemokines (IL-8, macrophage inflammatory protein-1α) and complement anaphylatoxins (C3a, C5a) after binding to their ­corresponding receptors.

  • Neutrophil elastase has the capacity to degrade most proteins in the extracellular matrix and important plasma proteins. Its proteolytic activity is regulated by endogenous protease inhibitors (PI) such as α1-antitrypsin, α2-macroglobulin or α1-PI. In addition, neutrophil elastase induces the release of proinflammatory cytokines.

  • Stimulated PMNLs produce ROS and RNS through the membrane associated nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase complex, myeloperoxidase and xanthine oxidorductase and represent a defense mechanism against invading microorganisms. Superoxide anions (O2 ) are generated by NADPH-oxidase, which is activated by proinflammatory cytokines, arachidonic acid metabolites, complement factors and bacterial products. Thereafter, O2 are reduced in the Haber-Weiss reaction to hydrogen peroxide (H2O2) by superoxide dismutase in cytosol (SOD 1), mitochondrium (SOD 2) or cell membrane (SOD 3). H2O2 is the substrate for the myeloperoxidase, which forms the high toxic and bacterizid hypocholorous acid (HOCL) and chloride anion (Cl). In addition, accumulated H2O2 is transformed to hydroxyl ions (OH) in the Fenton reaction. The free ROS induce lipid peroxidation, cell membrane disintegration and DNA-damage of endothelial and parenchymal cells. Furthermore, oxygen radicals and HOCL activate PMNL to release proteases and collagenase and inactivate PIs. In addition, the capacity of non-enzymatic anti­oxidants such as vitamins E or C (scavenger) or enzymatic antioxidants such as SOD, katalase, or glutathione peroxidase are reduced during sepsis.

  • RNS are also involved in the pathogenesis of secondary tissue damages. Nitric oxide (NO) is generated from the amino acid L-arginine by inducible nitric oxide synthase (iNOS) in PMNLs or vascular muscle cells and by endothelial nitric oxide synthase (eNOS) in endothelial cells. NO induces vasodilatation through increase of guanosine 3′,5′-cyclic monophosphate by activation of the guanylate cyclase. The activity of iNOS is stimulated by cytokines and toxins, and eNOS by mechanical shearing forces or acetycholin. O2 in the presence of NO generates peroxynitrite (ONOO), a key player in the pathogenesis of sepsis-induced organ dysfunction. The results for the vascular dysfunctions by oxygen radicals and NO are a generalized edema, clinically manifested as capillary leakage syndrome with a disturbance of nutritional and metabolic exchange, cell swelling and cellular dysfunctions.

8.4.8 Leukocytes Apoptosis

The accumulation of PMNLs and macrophages is accelerated by colony-stimulating factors such as granulocyte-colony stimulating factor and granulocyte macrophage-colony stimulating factor enhancing monocyte- or granulocytopoiesis and reducing the apoptosis of PMNLs during sepsis. As neutrophils kill pathogens using ROS and RNS and a mixture of lytic enzymes, delayed clearance of PMNLs in sepsis can potentially contribute to cell and organ injury (“Janus face” of PMNLs). However, the highly proapoptotic nature of PMNLs is designed to maintain a balance between antimicrobial effectiveness and the potential for neutrophil-mediated tissue damage. PMNLs can undergo apoptosis via intrinsic (mitochondrial) and extrinsic (activation of death receptors) pathways. Extrinsic signals (TNF-α, Fas ligand [CD95 ligand]) bind to their receptors (TNF-R, Fas antigen [CD95 antigen]) and trigger intracellular signaling, leading to caspase-8 and finally caspase-3 activation. Phagocytosis of apoptotic PMNLs by macrophages inhibits the release of proinflammatory cytokines and promotes the secretion of anti-inflammatory mediators. The recognition of apoptotic cells is dependent on the cell ­surface appearance of an anionic phospholipid, phosphatidyl-serine (eat-me sign), which is normally confined to the inner leaflet of the plasma membrane.

8.4.9 Microcirculatory Dysfunction

Microcirculation as functional unity, consisting of terminal arterioles, capillaries, and venules, regulates nutritional and metabolic exchange in organs and tissues. Microcirculatory dysfunction during systemic inflammation is primarily determined through the ­sympathetic-adrenal reaction leading to a vasoconstriction of arterioles and venules. However, through a decrease of the catecholamine effect on arterioles a reduced capillary flow with an increased hydrostatic pressure can be observed. This microcirculatory alteration in combination with the cytokine and NO mediated capillary leakage, is responsible for a secondary hypovolemia and hemoconcentration with agglutinations of erythrocytes (red sludge) and thrombocytes (white sludge). The sludge phenomenon evokes an obstruction of the microcirculation with a failure of the transcapillar exchange. The cellular oxygen deficiency and the accumulation of metabolic products (hidden acidosis) are finally responsible for tissue and cell damages. In addition, NO (vasodilatation) and endothelin (vasoconstriction) induce a shock specific microcirculatory change with a shunting of some organ or tissue areas enhancing the damage.

8.4.10 Autonomic Nervous System and Neuroendocrine Reaction

Advances in neuroimmunology have shown that the nervous system and the immune system communicate during inflammation. The main pathways involved in this crosstalk are the hypothalamic-pituitary-adrenal axis and the autonomic nervous system (ANS).

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Apr 6, 2017 | Posted by in CRITICAL CARE | Comments Off on Systemic Infections and Sepsis
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