Harmful molecular mechanisms in sepsis

Abstract

Sepsis and sepsis-associated multi-organ failure are major challenges for scientists and clinicians and are a tremendous burden for health-care systems. Despite extensive basic research and clinical studies, the pathophysiology of sepsis is still poorly understood. We are now beginning to understand that sepsis is a heterogeneous, dynamic syndrome caused by imbalances in the 'inflammatory network'. In this Review, we highlight recent insights into the molecular interactions that occur during sepsis and attempt to unravel the nature of the dysregulated immune response during sepsis.

Figure 1. Central hubs of the inflammatory response in sepsis

During sepsis, the complement anaphylatoxin C5a is generated following the activation of the complement system and by the C5-convertase activity of thrombin of the coagulation cascade. C5a triggers the release of pro-inflammatory mediators, including macrophage migration-inhibitory factor (MIF) and high-mobility group box 1 protein (HMGB1), and it activates the coagulation cascade by inducing tissue-factor expression (not shown). HMGB1 is a pleiotropic cytokine that binds to Toll-like receptor 4 (TLR4) and acts as an endogenous alarmin to increase the release of pro-inflammatory mediators. TLR4-mediated responses, in turn, are negatively regulated by C5a. Similar to HMGB1, large amounts of MIF are released during sepsis, which promotes a pro-inflammatory response by amplifying cytokine secretion through the upregulation of TLR4 expression. MIF, which is produced by the pituitary gland as well as by leukocytes, inhibits the anti-inflammatory effects of endogenous glucocorticoids of the endocrine system, which, in turn, induce MIF secretion. HMGB1 links the immune response with the autonomic nervous system, which regulates the release of HMGB1 and other cytokines during sepsis. Interleukin-17A (IL-17A), which is an important regulator of inflammation at the interface between innate and adaptive immunity, orchestrates responses of both innate and adaptive immune cells.

Figure 2. C5a is a central mediator of the inflammatory response in sepsis

During the early stages of sepsis, the complement system is systemically activated, generating large amounts of the anaphylatoxin C5a. C5a, which is a central molecule in the immunopathogenesis of sepsis, exerts its effects through interactions with its two C5a receptor (C5AR) and C5a-like receptor 2 (C5L2). The expression of these receptors is upregulated during sepsis, and their interactions with C5a contribute synergistically to harmful events in sepsis. The numerous effects of C5a include activation of the coagulation cascade by the induction of tissue-factor expression, which can result in disseminated intravascular coagulation (DIC). Furthermore, C5a triggers the release of pro-inflammatory cytokines, including macrophage migration-inhibitory factor (MIF) and high-mobility group box 1 protein (HMGB1), which contribute to the systemic inflammatory response syndrome (SIRS). In the later stages of sepsis, C5a is also responsible for sepsis-induced neutrophil dysfunction, leading to the shut down of intracellular signalling (immune paralysis) and increased susceptibility to secondary infections. C5a-induced apoptosis of thymocytes further aggravates immunosuppression, whereas the apoptosis of adrenal medullary cells results in insufficiency of the adrenergic system, eventually leading to septic shock. Recently, C5a and C5AR were also shown to be directly involved in the development of septic cardiomyopathy.

Figure 3. Cross-talk between the complement, coagulation and fibrinolysis systems

The complement system, the coagulation cascade and the fibrinolysis cascade communicate through many direct and bidirectional interactions (indicated by red arrows). Activated clotting Factor XII (FXIIa) can activate the classical complement pathway through cleavage of the complement component C1. Similarly, thrombin, kallikrein (not shown) and plasmin directly cleave complement component C3, as well as its activation fragments. Moreover, thrombin can cleave C5 into C5a, which occurs independently of C3 and therefore represents a bypass of the three traditional complement-activation pathways (that is, the classical, lectin and alternative pathways). Thrombin-activatable fibrinolysis inhibitor (TAFI) inactivates C3a and C5a in a negative-feedback loop. The complement system also amplifies coagulation through the C5a-mediated induction of expression of tissue factor and plasminogen-activator inhibitor 1 (PAI1) by leukocytes, the latter of which inhibits fibrinolysis. In addition, mannan-binding lectin serine protease 2 (MASP2) of the lectin complement-activation pathway triggers coagulation by converting prothrombin to thrombin. C4b-binding protein (C4BP) of the complement pathway inhibits protein S, which is a co-factor for the activated protein-C pathway of coagulation inhibition, which indicates that the inhibition of anticoagulant mechanisms further augments the pro-coagulant activities of complement. MAC, membrane-attack complex (C5b–C9); TPA, tissue plasminogen activator; UPA, urokinase-like plasminogen activator.

Figure 4. Effects of pathways of the ANS on inflammation during sepsis

The balance between the two branches of the autonomic nervous system (ANS) can direct the inflammatory response towards pro- or anti-inflammatory outcomes. Whereas activation of the cholinergic anti-inflammatory pathway (part of parasympathetic branch of the ANS) dampens inflammation, stimulation of the adrenergic pathways leads to amplification of the inflammatory response. a. In the adrenergic pro-inflammatory pathway, high concentrations of circulating catecholamines amplify the initial inflammatory response, particularly in the early phase of sepsis. Sources for catecholamine production and release are the adrenal medulla, sympathetic neurons and leukocytes (phagocytic cells and lymphocytes). Catecholamines exert their immunomodulatory effects through α- and β-adrenergic receptors that are expressed by various cell types, resulting in the increased release of pro-inflammatory mediators. b. By contrast, the activation of the cholinergic anti-inflammatory pathway in sepsis attenuates the inflammatory response. These effects are mediated through engagement of α7-nicotinic acetylcholine receptors (α7nAChRs). Acetylcholine is released following vagus-nerve stimulation, resulting in inhibition of the synthesis and release of pro-inflammatory mediators such as high-mobility group box 1 protein and tumour-necrosis factor.

Figure 5. The inflammatory network in sepsis

During sepsis, homeostasis between the various biological systems of the inflammatory network is highly imbalanced. In the initiation of sepsis, the release of a large amount of damage-associated molecular patterns (DAMPs) from invading microorganisms and/or damaged host tissue results in the overstimulation of pattern-recognition receptors (PRRs) on immune cells. Activated immune cells release excessive amounts of pro-inflammatory mediators (resulting in a ‘cytokine storm’), free radicals and enzymes, which converts the normally beneficial effects of inflammation into an excessive response that damages the host. Activation of the adrenergic branch of the autonomic nervous system (ANS) and/or decreased activity of the cholinergic anti-inflammatory pathway (of the parasympathetic branch of the ANS) further amplifies the pro-inflammatory responses of neutrophils, macrophages and dendritic cells in sepsis. The presence of invading microorganisms or their products in the blood can cause systemic activation of the complement system, which results in the excessive generation of complement anaphylatoxins, which, at high concentrations, induce numerous harmful effects. Simultaneous activation of the coagulation system and the inhibition of fibrinolysis as a result of the pro-inflammatory environment and/or damaged endothelium can result in disseminated intravascular coagulation (DIC), which is a major complication of sepsis, and in the amplification of the inflammatory response. The complement, coagulation and fibrinolysis systems are tightly connected through direct interactions of serine proteases, and imbalances in each cascade are intensified in a positive-feedback loop (FIG. 4). Finally, the sustained pro-inflammatory environment affects the functional state of immune effector cells, eventually causing the dysfunction of neutrophils and immunoparalysis. Alterations in leukocyte apoptosis in the later stages of sepsis further account for immunosuppression, which increases the susceptibility to secondary infections.