2.1 Activation of complement in sepsis

The complement system is the humoral part of the innate immune system consisting of approximately 30 serum proteins.^57-59 The system consists of a serine protease cascade reaction, which involves a series of sequential cleavages of complement proteins. This intricate process ultimately leads to the formation of the membrane attack complex (MAC).^{13, 50} The MAC plays a crucial role by creating a pore in the cellular membrane of the target cell where the complement system has been activated. Moreover, the complement system that circulates within the body can be activated through at least three distinct pathways.³² Of these pathways, the classical, lectin, and alternative pathways are the most well established and widely recognized. Each of these pathways contributes to the overall function of the complement system in immune responses, facilitating the elimination of pathogens and damaged cells.^{60, 61} The classical pathway of the complement system is initiated when C1q attaches to complexes formed between antigens and antibodies. This binding event marks the beginning of a complex series of biochemical reactions that ultimately help to eliminate pathogens.²⁷ In contrast, the lectin pathway is activated when mannose-binding lectins (MBLs) or ficolins interact with specific sugars located on the surface of bacterial cell walls. Both pathways work through enzymatic processes that lead to a selective cleavage of complement proteins C3 and C5, performed by enzymes that are referred to as C3 convertases and C5 convertases, respectively. This limited cleavage plays a crucial role in amplifying the immune response and facilitating the destruction of invading microorganisms.^{62, 63} The alternative pathway of complement activation involves the spontaneous hydrolysis of C3, leading to its convergence with two other pathways at the C3 convertase enzyme. This enzyme plays a crucial role in cleaving C3, resulting in the production of the proinflammatory peptide known as C3a, alongside a significant quantity of C3b.⁷⁰ The generation of C3b is particularly important as it acts to condition the pathogen, enhancing the immune response. Furthermore, C3b also plays a vital role in the formation of the C5 convertase, an important complex that catalyzes the release of the potent allergenic toxin C5a. This fragment, C5a, is instrumental in eliciting a robust immune response. In addition to C5a, the process also produces the fragment C5b, which is critical for assembling the MAC known as C5b-9.^{23, 65} This complex forms on the surface of target cells, marking the culmination of the complement activation process.^{66, 67} The release of allergenic toxins, such as C3a and C5a, during complement activation is associated with significant proinflammatory effects.^{60, 68} These include an increase in vascular permeability and the recruitment of leukocytes, both of which contribute to the body's defensive response. However, it is important to note that when complement activation occurs in an uncontrolled manner, it can lead to an overwhelming inflammatory response, resulting in substantial damage to host tissues.

Moreover, an additional mechanism contributing to the production of C5 activation products involves the ligation of leukocyte proteases.⁶⁹ The result of this process is the generation of two allergenic toxins, namely, C3a and C5a, which induce distinct alterations in the acute inflammatory response while also leading to smooth muscle contraction. C5a, in particular, is recognized as a potent agonist for myeloid cells, notably neutrophils.^{19, 70} These cells are characterized by the expression of high levels of C5a receptors (C5aRs), also referred to as C5aR or CD88. Neutrophils not only respond to various chemical stimuli but also exhibit a range of additional biological reactions that are crucial for the immune response.^{4, 71, 72} Nonmyeloid cells, including fine bronchial and alveolar epithelial cells, as well as endothelial cells, have been found to express the C5aR and to have responses to the chemotactic factor C5a.^{74, 81} This indicates that these nonmyeloid cell types are not merely passive participants in immune responses but actively engage with complement components, highlighting their roles in initiating or modulating inflammatory signals.^75-77 Furthermore, C3a has the capacity to interact with a diverse range of receptors across various cell types.^{77, 78} One of the well-documented effects of C3a is its ability to induce deplasticization of basophils and mast cells, which results in the release of vasoactive amines. This release is significant as it contributes to the processes of edema and contraction of smooth muscle, particularly affecting the upper respiratory tract and the intestinal system, which are crucial for maintaining homeostasis during immune responses.⁴⁰ In addition, C3b exhibits a distinct mechanism by binding to the surfaces of bacteria, thereby endowing these pathogens with an “exogenous” characteristic. This modification facilitates their rapid recognition and uptake by phagocytic cells, which possess specific receptors for C3b and its related products.⁸ The binding of C3b not only enhances the efficiency of phagocytosis but also underscores the importance of complement activation in the innate immune response, as it enables the immune system to effectively clear invading microorganisms from the host.

2.2 Coagulation activation of sepsis

DIC is a life-threatening pathophysiological syndrome characterized by increases the systemic overactivation of blood coagulation, leading to microvascular thrombosis and multiple organ dysfunctions, such as sepsis.^{56, 79} During sepsis, there is a rapid increase in the production of pro- and anti-inflammatory cytokines in the blood stream, and their abnormal production will contribute to massive activation of coagulation factors and platelets as well as damage to the vascular endothelium, which result in vascular leakage and DIC.^{14, 80} In addition to ensuring that thrombus production after activation of the coagulation system, advanced DIC may also cause bleeding when platelets and coagulation factors are depleted.^{22, 81} These conditions often lead to extensive crosstalks between inflammation and coagulation, which can eventually lead to multiple organ dysfunction syndrome (MODS) and eventual death.

In the early stages of sepsis, proinflammatory cytokines are crucial in influencing the hemostatic system. They exert their effects through several mechanisms.^{82, 83} First, these cytokines can induce dysfunction in endothelial cells, which are vital for maintaining vascular health and proper blood flow. Second, there may be an increase in platelet activation, leading to a heightened risk of abnormal clot formation. Additionally, the activation of coagulation may be mediated by TFs, which are substances that initiate the clotting process in response to vascular injury.^{50, 65, 66} Furthermore, the functioning of physiological anticoagulation pathways is often impaired during this phase, disrupting the natural balance that prevents excessive clotting. Finally, the overall fibrinolytic system, which is responsible for breaking down clots, can also be perturbed, further complicating the hemostatic response. Together, these pathways reflect the complex interplay of inflammatory processes and hemostatic changes that occur in sepsis.^{7, 84} In this period, changes in coagulation profile are dynamic, and once the initial symptoms improve, the function of global coagulation homeostasis would be restored.^{85, 86} Previous studies have suggestion that the hemostatic balance in the organism is always shifted toward the activation of coagulation, either by activation of the coagulation pathway or by downregulation of the anticoagulation pathway.^{87, 88} The interaction between the intrinsic immune system and the hemostatic system, which includes platelets and coagulation factors, plays a critical role in the development of sepsis. This collaboration is significant because it highlights how the body's defensive mechanisms and the blood clotting process are interconnected.^{89, 90} By understanding this relationship, researchers can gain insights into the complex pathophysiology of sepsis, potentially leading to more effective interventions and treatments for this serious condition.^{91, 92} In sepsis patients, activation of coagulation coincides with the release of inflammation-associated mediators, which also is characteristic of systemic inflammatory response syndromes.^93-95

2.3 Coagulation and inflammatory network

The coagulation system is traditionally considered to be an integral part of homeostasis maintenance and a completely independent system that function is to prevent or limit the loss of blood volume and blood components after mechanical injury to the circulatory system, in addition to being the main promoter of thrombosis (pathological formation of intravascular thrombi).^{68, 96} Research has confirmed that the interplay between inflammation and coagulation is deemed pivotal to the pathogenesis of sepsis. Inflammation can induce a coagulation response, and in turn, an activated coagulation response can further promote the occurrence of inflammatory reactions.⁴⁰ During the onset of sepsis, there is a disruption characterized by impaired protein synthesis, continuous consumption of three pathways of coagulation inhibitors, and a relatively low level of protein degradation.^{93, 97} In the context of sepsis, inflammatory mediators such as TNF-α, IL-1, and IL-6 are implicated in the activation of coagulation and the suppression of fibrinolysis by influencing the activation of the coagulation and fibrinolytic systems.⁹⁸ These inflammatory mediators not only facilitate the coagulation response but also, upon activation of coagulation, they can further stimulate the release of inflammatory mediators, establishing a crosstalk between inflammation and coagulation.^{99, 100} Furthermore, during sepsis, the activation of coagulation predominantly occurs via the TF pathway. TF, primarily derived from monocytes or vascular endothelial cells, activates factor VII in the circulation, which in turn activates factor X, culminating in the conversion of prothrombin to thrombin.¹⁹ Conversely, the anticoagulant pathways in sepsis, encompassing antithrombin, the protein C system, and tissue factor pathway inhibitor (TFPI), are severely compromised, leading to a reduction in antithrombin activity and an impairment of the functional capacity of activated protein C.

Moreover, the fibrinolytic system is in a state of inhibition during sepsis, with the fibrinolytic process being dampened at two critical junctures: the swift neutralization of plasmin by alpha-2 antiplasmin and the inactivation of plasminogen activators (PAs) by plasminogen activator inhibitor-1 (PAI-1).¹⁰¹ Collectively, these elements culminate in the excessive production of fibrin and the formation of microvascular thrombi, which in turn precipitate organ dysfunction.⁸⁷ In aggregate, the intricate interplay of the coagulation system within the sepsis paradigm encompasses the liberation of inflammatory mediators, the ignition of coagulative cascades, the undermining of anticoagulant mechanisms, and the subjugation of the fibrinolytic apparatus.¹⁰² These interrelated elements synergize to advance the sepsis trajectory and engender organ injury.

Blood coagulation is primed at the sites of damage to the vascular endothelial cell monolayer.^{103, 104} The coagulation sequence is a complex process that involves a series of transformations where zymogens are converted into their active enzyme forms. This chain of events ultimately leads to the production of thrombin (IIa), a crucial enzyme that plays a key role in hemostasis by converting soluble fibrinogen into insoluble fibrin.^{65, 66} As the thrombin is generated, it triggers the exposure of the subendothelial layer, which is rich in molecules such as collagen and VWF. This exposure allows these molecules to interact with plasma and circulating platelets, which subsequently adhere to the damaged site. The adhered platelets become activated and begin to aggregate, leading to the formation of a primary clot that provides initial closure of the injured vessel.^{105, 106} In addition to forming a physical barrier to blood loss, activated platelets release a variety of vital hemostatic components from their alpha (α) and dense granules. These components include platelet activating factor, platelet factor 4 (PF4), P-selectin, adenosine diphosphate (ADP), and polyphosphate.^107-109 The release of these substances has significant local cellular effects, as they stimulate the recruitment and activation of immune cells such as neutrophils and monocytes. Moreover, these released factors may enhance the accessibility of acute promyelocytic leukemia (aPL), which serves as an essential cofactor for assembling the various coagulation cofactor/enzyme protein complexes necessary for effective blood coagulation.^{98, 110} It is important to note that, with only a few exceptions, these enzymatic reactions predominantly take place on the phospholipid surfaces of activated endothelial cells and platelets, and they require the presence of calcium ions to proceed efficiently. The basic physiological trigger for coagulation is the transbilayer receptor TF, which is normally present in the subendothelial pool throughout the vasculature.^{111, 112} The coagulation process is initiated through two primary pathways: the contact (endogenous) pathway and the TF (exogenous) pathway. These pathways ultimately converge at the activation of factor X, specifically when a vessel sustains injury. The binding of factor VII (FVII), which is a zymogen, to TF instigates a significant conformational change.^{19, 113} This change is critical, as it leads to the hydrolytic activation of FVII into its active form, FVIIa. Once activated, TF enhances the activation of factor X by FVIIa, resulting in the formation of the initial FXa molecule. Importantly, FXa is associated with exogenous tensin cellular lipoproteins that are either released from damaged cells or presented on the surfaces of activated monocytes, endothelial cells, and certain nonvascular cells. This activation cascade additionally triggers the activation of factors IX (FIX) and VIII (FVIII), which are crucial components of the coagulation process.^{114, 115} The coagulation process is carefully regulated and localized to the site of vascular injury by various natural anticoagulants. These anticoagulants can be classified into three distinct categories: the antithrombins, the protein C and protein S system, and the TFPI system. The regulation of blood clot dissolution, termed fibrinolysis, is governed by the plasminogen–plasmin system.^{116, 117} This system is activated by PAs, which facilitate the breakdown of fibrin and modulate the process of fibrin polymerization. Importantly, the activity of this fibrinolytic system can be inhibited by PAI-1, a protein produced by several cell types, including endothelial cells, mast cells, and basophils (Figure 2).^{87, 93} In addition to these coagulation and anticoagulation processes, the components of the coagulation cascade work in close collaboration with platelets, which play a vital role in hemostasis.¹¹⁸ Activated platelets contribute to the coagulation process by providing surfaces that are negatively charged phospholipids, where coagulation reactions can take place.¹¹⁹ Furthermore, these platelets release microparticles (MPs) that carry TF as well as a variety of mediators, thereby enhancing the processes of hemostasis and thrombosis.^{54, 66} This interplay between coagulation factors and platelets underscores the complexity and interdependence of the hemostatic response.^{25, 120} The activation of coagulation and fibrinolysis can be seen as a direct consequence of the acute inflammatory response. When the body experiences inflammation, cytokines—small proteins important in cell signaling—are activated, leading to changes in the endothelium, or the inner lining of blood vessels.^{121, 122} This altered endothelial surface becomes prothrombotic, meaning that it promotes clot formation. As a result of this cytokine activation, TF is produced, triggering the activation of the extrinsic pathway of coagulation.¹²³ This pathway is crucial in the body's response to injury, as it leads to the generation of proteins that activate platelets, which play a significant role in clotting. Although the fibrinolytic system—responsible for breaking down clots, which is initiated during this inflammatory response, it is soon inhibited, disrupting the delicate balance between coagulation and fibrinolysis. This inhibition results in a considerable procoagulant state, characterized by an excess of clot formation compared to breakdown.^{124, 125} Eventually, this dysregulation can culminate in a serious condition known as DIC. The implications of DIC are severe, as it leads to the accumulation of fibrin and the development of microthrombi—tiny clots that can obstruct blood flow. The consequences of this cascade of events can be dire, resulting in multiorgan failure and, in the worst-case scenarios, death. Thus, understanding this interplay between coagulation and fibrinolysis in the context of inflammation is crucial for managing serious clinical outcomes.

2.4 Complement-mediated hyperinflammation

The complement system is an essential part of the innate immune system, which serves as the body's first line of defense against harmful foreign microorganisms.¹²⁶ This system consists of a complex series of proteins that work together to identify, target, and eliminate pathogens.¹²⁷ By facilitating immune responses, the complement system is instrumental in protecting the body from infections caused by various exogenous pathogens, thereby significantly contributing to the overall immune response and the maintenance of health.¹²⁸ In the early pathological processes of sepsis, there is an overactivation of the complement system.¹²⁹ Upon activation, complement fragments such as C3a and C5a are released, which further promote the production of inflammatory cytokines by activating platelets, monocytes, or endothelial cells.¹³⁰ The interactions between the complement system and the coagulation, kinin, and fibrinolytic systems form an extremely complex network within the body, amplifying and exacerbating systemic inflammatory responses.¹⁰ During the middle stages of sepsis, the interplay between complement system activation, inflammatory responses, and disease severity is not fully understood.¹³¹ However, excessive complement activation can lead to a state of heightened inflammation in the host, thereby affecting clinical outcomes.¹³² Complement activation produces C3a and C5a, which possess potent proinflammatory activities, enhancing the chemotactic effects of leukocytes and promoting inflammatory responses.⁶⁵ Additionally, C5a can further promote coagulation by disrupting the function of endothelial glycocalyx. Moreover, the activation of inflammasomes and gasdermin D (GSDMD) is involved in the interplay between inflammation and coagulation, further illustrating the complex connection between inflammation and the complement system.¹³³ Therefore, there is a close interplay between complement system activation and inflammatory responses, and they play a significant role in the development of diseases such as myocardial infarction and sepsis.

The complement system circulating in plasma is primarily produced and secreted by the liver; however, it is important to note that complement components can also be generated by various immune and nonimmune cells.¹³⁴ A substantial body of evidence supports the notion that hyperactivation of the complement system plays a critical role in the pathophysiological processes associated with sepsis.¹³⁵ The mechanisms through which sepsis influences the complement system are multifaceted. Sepsis can activate the complement system directly through established pathways, including the lectin, classical, and alternative pathways (Figure 3). Additionally, sepsis can lead to endotheliopathy, characterized by endothelial cell injury and dysfunction, as well as thromboinflammation, which combines inflammation with coagulation and thrombosis.¹³⁶ These pathological conditions can further stimulate the activation of the complement system, emphasizing a complex interplay between sepsis and immune responses.⁶⁶ By contrast, in response to high levels of immune stimulation, complement can become uncontrolled and cause hyperinflammation.⁷⁶ Regulation of these processes is crucial, as imbalances can have significant consequences. Under conditions of controlled inflammation, low concentrations of complement can actually inhibit the production of cytokines, leading to a situation where the concentration of C5a in serum or plasma remains extremely low.¹³⁷ In stark contrast, when there is a high level of immune stimulation, the complement system can become dysregulated, resulting in uncontrolled complement activation and hyperinflammation.¹⁶ In this maladaptive state, the complement system can inflict considerable collateral damage. Neutrophils, for instance, may become unresponsive to anaphylatoxins, which severely impairs their antimicrobial capabilities (Figure 3).

Meanwhile, macrophages tend to produce excessive levels of cytokines, paving the way for cytokine storms, while endothelial cells generate TF that promotes DIC.^{9, 138} Furthermore, inflammation has the capacity to induce the transcription of various complement and coagulation genes, including C1r, C1s, factor B, C3, and C5, as well as fibrinogen. The presence of C5 and other products resulting from complement activation in the bloodstream, commonly observed in sepsis, signifies a loss of regulatory control over complement activation.^{51, 53} This deregulation gives rise to a cascade of detrimental events that compromise innate immune defenses, disrupt the balance of the clotting and fibrinolytic systems, and trigger the apoptosis of lymphocytes.⁵³ This apoptotic process is believed to contribute to the profound immunosuppression frequently observed in patients suffering from sepsis. Genetic mutations affecting complement regulator genes, particularly CD55 and factor H, have been associated with an increased susceptibility to SARS-CoV-2 infection. This suggests that the complement system, which plays a crucial role in the immune response, may become overly active and poorly regulated in the context of sepsis. As a result, this dysregulation can contribute to a heightened inflammatory response, exacerbating the condition and potentially leading to more severe outcomes for affected individuals.

2.5 Complement and coagulation

The hyperactivation of both the complement and coagulation systems has been increasingly recognized as a critical aspect of the clinical syndrome associated with sepsis.^{6, 46} This review aims to explore the intricate mechanisms of complement activation and coagulation response triggered by the severe acute sepsis-causing viruses. Additionally, it focuses on understanding the established connections between these immune responses, hyperinflammation, and thrombosis, which are common complications in septic patients.³ Recent clinical findings and emerging evidence from clinical trials suggest that targeting the complement and coagulation pathways may offer promising therapeutic benefits in the management of sepsis.⁵⁰ Upon activation, neutrophils release a wide array of bioactive molecules, such as neutrophil elastase, cathepsin G, myeloperoxidase, and reactive oxygen species.⁷ These substances may modulate the production of coagulation factors and complement proteins, leading to complex cellular interactions that can either enhance or inhibit these pathways.⁷ One significant aspect of neutrophil activity is the release of neutrophil extracellular traps (NETs), composed of DNA and proteins that form a scaffold facilitating interactions with activated platelets (Figure 4). This mechanism creates an environment conducive to the localization of TF activity, potentially by trapping extracellular vesicles that further influence coagulation processes.⁵¹ Both coagulation and complement pathways can be amplified through direct interactions with NET structures, which serve as a platform for the activation of the contact and alternative pathways, respectively.¹⁰² Such multistep crosstalk between these systems contributes to the increased production of key components such as thrombin, C3a, and C5a, all of which have significant biological implications in the inflammatory response during sepsis. Furthermore, histones H3 and H4, which are prominent proteins found within NETs, play a crucial role in enhancing coagulation and complement activation.^{15, 66} They do this by inhibiting regulatory proteins like antithrombin and thrombomodulin, thereby stabilizing clots and impairing fibrinolysis initiated by tissue plasminogen activator (tPA). The surface of activated platelets is generally understood to promote coagulation due to its provision of anionic phospholipids.^{50, 68} However, the presence of proteins such as P-selectin and properdin on platelet surfaces also contributes to the stabilization of C3 convertase complexes through their interactions with C3b and the water-soluble form of C₃ (C₃(H₂O)). Additionally, platelets may directly participate in complement activation by associating with C₃(H₂O), thereby further integrating the coagulation and complement systems in the context of sepsis.

Coagulation and complement are distinct biological systems, each playing a unique role in pathophysiological processes.^{60, 67} However, they exhibit several functional similarities that are often overlooked, highlighting a fascinating interplay between the two systems.^{3, 64} First, both systems act as essential innate defenses against external threats, such as microbial invasions. This innate defense mechanism is crucial for maintaining the body's integrity against various pathogens. Second, both coagulation and complement pathways require exogenous stimuli and changes in cell surface receptors for their activation. This criterion is vital, as it ensures that the initiation of these pathways is not only accurate but also tightly coordinated. For instance, when these systems are in proximity to the vascular endothelium, there is a reduction in the kinetic requirements for the reactions to take place. This proximity leads to a localized increase in the concentration of the necessary triggers, facilitating a more efficient response. Moreover, the reactions within both systems are organized into three distinct phases: initiation, amplification, and propagationg.³ This structured cascade is not merely coincidental but often operates in a self-reinforcing manner (Figure 4). Such organization allows for multiple activation points and the capacity for both amplification and regulation—either positively or negatively. Additionally, the interactions between these cascades and other systems help to fine-tune the body's response to various stimuli. Another critical aspect is the presence of regulatory molecules, including natural inhibitors or cofactors, that are found within the same environments as the coagulation and complement systems.¹³⁹ These molecules play an essential role in the physiological regulation of both systems, providing a mechanism for their restraint at two levels: either through the inhibition of enzymatic activities or by limiting the binding capacity of components within the cascade. Finally, certain elements from each cascade are capable of interacting with receptors on the cell surface, which mediates various downstream biological effects.³⁹ The shared characteristics of the coagulation and complement systems facilitate numerous instances of crosstalk between them, leading to their involvement in a range of clinical inflammatory and thrombotic conditions. This crosstalk emphasizes the importance of understanding how these systems interact within the broader context of human health and disease.

The complement and coagulation pathways play critical and interconnected roles in the context of sepsis.⁵⁰ The byproducts of complement activation have the potential to enhance the clot-forming ability of blood.¹⁴⁰ This occurs through the simultaneous promotion of procoagulant proteins, which facilitate clot formation, and antifibrinolytic proteins, which deter the breakdown of clots. Concurrently, this process involves the suppression of natural anticoagulants that typically help regulate clot formation.⁶⁰ Notably, the terminal complement complex C5b-9 has the ability to activate TF, while the C5b-7 initiation complex can also trigger the expression of TF through mechanisms that rely on protein disulfide isomerase.⁶⁶ Furthermore, C5b-9 plays a critical role in exposing phosphatidylserine on the surface of platelets, which in turn creates a conducive environment for the assembly of prothrombinase, a key component in the clotting process.¹⁴¹ Recent research has demonstrated that peptidoglycan (PGN) can elicit complement activation via the classical pathway.¹³⁶ This activation leads to the deposition of C5b-9 on the surface of platelets, which promotes platelet aggregation and the exposure of procoagulant activity characterized by a high concentration of phosphatidylserine.¹⁴² The underlying mechanism involves PGN–anti-PGN immune complexes that facilitate the consumption of complement components. In experiments involving baboons challenged with Escherichia coli, the inhibition of the complement pathway using compstatin (COM), a selective C3 convertase inhibitor, was found to significantly reduce both thrombocytopenia and microvascular thrombosis.⁵⁴ Additionally, this treatment helped to preserve the endothelial properties that prevent clotting, improve the barrier function of blood vessels, and mitigate organ injury. These findings underscore the intricate relationship between complement activation and coagulation, which together contribute to the deterioration observed in severe sepsis.⁸ Therefore, targeting the adverse effects of complement activation products, particularly during the critical phase of organ failure associated with sepsis, emerges as a promising therapeutic approach. The interaction between the coagulation and complement systems is bidirectional; complement proteins can activate the coagulation cascade, while certain enzymes involved in coagulation—such as thrombin and factor Xa—can independently activate elements within the complement cascade.^{89, 143} Additionally, there exists a pathway involving the anticoagulant thrombomodulin-protein C that serves to inhibit the activation of the complement system, further illustrating the complex and reciprocal nature of these two critical biological processes.⁹⁰ Moreover, there are various shared elements between the complement and coagulation pathways, enhancing the understanding of their interdependent roles in maintaining hemostasis and pathogen responses during sepsis. Factor XIIa plays a significant role in two critical processes within the body: the contact activation of the coagulation cascade and the activation of the complement system through the classical pathway.¹⁴⁴ Its involvement in coagulation is essential for the proper formation of blood clots, while its role in the complement activation is crucial for the immune response. Furthermore, the C1 inhibitor serves as a powerful neutralizing agent. It effectively inhibits both factor XIa, which is a key player in the coagulation pathway, and the C1 component of the classical complement pathways.⁹⁹ This dual action of the C1 inhibitor highlights its importance in regulating these interconnected systems, ensuring a balanced response to physiological challenges.

The complement and coagulation systems play a crucial role in the genesis, progression, and exacerbation of sepsis, with intricate interplay between them.⁴⁶ Initially, the activation of the complement system is a key component of the early pathological process in sepsis.⁶ The production of complement components such as C3a and C5a possesses potent proinflammatory activity, capable of promoting the activation of leukocytes, endothelial cells, and platelets, leading to the onset of inflammatory responses.⁶³ The activation of the complement system can exacerbate the coagulation dysfunction caused by sepsis through its interaction with the coagulation system, potentially leading to DIC.¹⁸ The activation of the coagulation system can be considered part of the initial immune response to invading pathogens, aiding in the triggering of innate defense mechanisms.³² However, in sepsis, the activation of the coagulation system is imbalanced, leading to a propensity for thrombosis within the microvasculature, which can progress to DIC in severe cases.²⁰ TF, as the primary initiator of coagulation, increases its expression in sepsis, activating the coagulation process by forming a complex with coagulation factor FVIIa.⁷⁶ Additionally, NETs released by neutrophils in sepsis can promote platelet adhesion, activation, and aggregation, forming thrombi, which further illustrates the interplay between the complement and coagulation systems in sepsis.^{145, 146} The complement system interacts with the coagulation system; while complement activation can induce inflammatory responses, excessive activation can induce dysregulation of inflammation, exacerbating inflammatory injury and worsening the condition of sepsis.^{147, 148} Levels of complement C3 and C4 may decrease in patients with sepsis, and their reduction is associated with the development of acute kidney injury (AKI) in sepsis.¹⁴⁹ It is noteworthy that if the coagulation dysfunction in sepsis is persistent and uncorrected, it often evolves into DIC, which is closely related to the occurrence of multiple organ failure and increased mortality rates. Therefore, interventions targeting the complement and coagulation systems may offer new avenues for the treatment of sepsis-associated coagulopathy. In summary, the interplay between the complement and coagulation systems significantly impacts the severity of the disease and patient prognosis, and inhibiting complement activation may help control coagulopathy associated with sepsis, improving patient outcomes.

3.1 Tissue factor

TF, also referred to as coagulation factor 3 (F3), plays a crucial role in the process of blood coagulation.⁶⁶ Its involvement in the pathological mechanisms of sepsis has been substantiated by preclinical research focused on bacterial sepsis.^{150, 151} Recent investigations have illuminated the role of TF in modulating pyroptosis of macrophages associated with caspase-11, which is activated via type I interferon (IFN).^{128, 129} During this pyroptotic process, macrophages can release TF through extracellular vesicles that are expelled from pores formed during cell lysis.²⁶ This release contributes to a rapid activation of pathological coagulation, which can exacerbate the condition.^{102, 149} Further insights into this area of study have revealed that certain compounds, such as dimethyl fumarate (DMF) and 4-octyl itaconate (4-OI), are capable of inhibiting TF-dependent thrombin generation.²² This inhibition occurs through the blockade of type I IFN signaling and caspase-11-mediated macrophage pyroptosis, thereby preventing the subsequent release of TF in the setting of lipopolysaccharide (LPS)-induced sepsis.¹⁴⁷

Additionally, the activation of inflammasomes is critical, as they not only facilitate the processing and release of IL-1 family cytokines but also induce the secretion of TF from activated macrophages and monocytes.¹⁰¹ The interplay between inflammasomes and innate immunity contributes to coagulation processes that can worsen the state of sepsis.¹⁵² Moreover, the propensity for thrombus formation during sepsis can be traced back to the simultaneous dysfunction of three significant anticoagulant pathways: antithrombin, TFPI, and the protein C system.¹⁰⁴ This highlights the complexity of coagulation responses in sepsis. Consequently, strategies aimed at inhibiting the release of TFs, enhancing levels of antithrombin, and activating protein systems hold promise for ameliorating the inflammatory response observed in both in vivo and in vitro sepsis models. Collectively, these findings enhance our comprehension of the intricate immune mechanisms underlying coagulopathy and pave the way for future research endeavors targeting infection-induced sepsis in preclinical contexts.

3.2 Platelets

Platelets, which are small non-nucleated cells within the bloodstream, have a critical role in sepsis.¹⁴⁸ They not only contribute to the homeostasis of the coagulation system but also actively participate in immune regulation.¹⁵³ Upon activation, platelets release proteases and bactericidal peptides.¹⁵⁴ Additionally, they collaborate with white blood cells (neutrophils and monocytes) to modulate immune responses.¹⁴⁵ Remarkably, scientific investigations have highlighted the significant impact of deleting the STING gene in platelets on platelet activity levels.¹⁵⁵ This genetic manipulation leads to a decreased production of sepsis-induced NETs and thrombosis formation, ultimately enhancing the survival rate of mice.¹⁵⁶ Another intriguing study, utilizing mice lacking the GSDMD gene specific to platelets, demonstrated that platelet pyroptosis triggered by GSDMD results in the release of oxidized mitochondrial DNA (ox-mtDNA), consequently promoting the formation of NETs.¹⁴⁶ Succinctly, this creates a positive feedback loop that leads to an excessive release of inflammatory cytokines. However, when platelet pyroptosis was inhibited, the survival rate of mice suffering from cecal ligation and puncture (CLP) induced septicemia significantly improved.¹⁵⁷ Furthermore, it has been discovered that PF4 effectively activates the clotting contact pathway by preventing the fragmentation of cfDNA and NETs into shorter fragments and single-stranded cfDNA.¹⁵⁸ As a result, the formation of cfDNA and NETs clots is hindered, thus offering valuable insights for the development of clinical interventions for sepsis.

3.3 GSDMD

Coagulation not only plays a vital function in the physiological hemostasis process, but also plays an important role in the body's anti-infection immune response.⁹⁸ GSDMD plays a pivotal role in the coagulation process during sepsis.¹²³ In sepsis, the activation of GSDMD facilitates the externalization of phosphatidylserine, thereby augmenting the procoagulant activity of TF on the surface of the cell membrane. This initiates the coagulation cascade, leading to the occurrence of DIC.^{133, 150} The type I IFN pathway, activated during bacterial infection, induces the activation of caspase-11 and its downstream target GSDMD, thereby precipitating the formation of DIC.^{19, 50, 93} The activation of GSDMD can indirectly affect the complement system by modulating the inflammatory environment.¹⁵⁹ The release of IL-1β and other cytokines can amplify the complement activation cascade, leading to a more robust inflammatory response.¹⁶⁰ The interaction between GSDMD and the complement system offers potential therapeutic targets for the treatment of sepsis. Modulating the activity of GSDMD or the complement system could help control the inflammatory response and reduce the severity of sepsis.¹⁶¹ Consequently, interventions targeting the functionality of GSDMD offer a potential opportunity for the prevention and treatment of sepsis-associated DIC, rendering it an important therapeutic target for pharmacological intervention in the management of sepsis.

3.4 HMGB1

Clinical studies have shown that the serum high mobility group protein-1 (HMGB1) level of normal people is very low, while the expression level of HMGB1 in the serum of patients with sepsis and hemorrhagic shock is significantly increased, and the level of HMGB1 in the death group is higher than that in the survival group.¹⁶² In sepsis, the release of HMGB1 into the bloodstream by immune cells mediates the activation of caspase-11 and GSDMD, leading to a hypercoagulable state, the induction of DIC, and resulting in multiple organ failure.^{143, 163} HMGB1 is a typical risk warning molecule. Previous studies have found that platelets can release a large amount of endogenous HMGB1 during the development of sepsis, which further leads to platelet aggregation and activation and plays a protective role in the body.^164-166 Furthermore, HMGB1 facilitates the formation of NETs, which are capable of capturing bacteria and activating platelets, thus promoting thrombus formation.^{163, 167} Given HMGB1's role in the coagulopathy associated with sepsis, it may serve as a potential therapeutic target for anticoagulant treatment, with interventions aimed at HMGB1 potentially improving the coagulation function in septic patients.

In sepsis, the effect of HMGB1 in the complement system is particularly relevant due to its ability to exacerbate the inflammatory response and contribute to the development of a cytokine storm, a hallmark of severe sepsis and septic shock.^{164, 168-170} HMGB1 is an important factor mediating infection, tissue injury, and inflammation. Normally, HMGB1 is primarily located in the nucleus and acts as a chaperone for DNA to bind to chromatin, but it can also shuttle from the nucleus to the cytoplasm under various stress conditions.^{143, 171} HMGB1 is actively secreted by macrophages and other cells or after infection and release from damaged/necrotic cells.^{163, 172, 173} Its function is to recruit inflammatory cells and mediate signaling among macrophages, dendritic cells, and natural killer cells. Extracellular HMGB1 can activate endothelial cells, promote angiogenesis, enhance hematopoietic stem cell migration, and trigger local or systemic inflammation.^{174, 175} Given these multifaceted roles, HMGB1 represents an important therapeutic target for interventions aimed at modulating the complement system and treating sepsis. Strategies to inhibit HMGB1, such as the use of neutralizing antibodies or small molecules that interfere with its release or activity, could potentially improve outcomes in patients with sepsis by reducing the hypercoagulable state and associated organ damage.

3.5 STING1

T platelets play a pivotal role in the abnormal coagulation function in sepsis. Beyond their traditional involvement in the coagulation process, platelets also possess immune regulatory functions.^{39, 176} Platelets play a pivotal role in the abnormal coagulation function in sepsis. Beyond their traditional involvement in the coagulation process, platelets also possess immune regulatory functions.^{54, 89, 144, 177} This release is essential for platelet activation and highly dependent on the assembly of the SNARE complex.^{90, 152} STING interacts with the secretory protein STXBP2 to maintain the effective assembly of the SNARE complex, while the STING agonist cGAMP can enhance this binding, further promoting the secretion and functional activity of platelet granules.^{16, 104, 178} In addition, research has identified potential binding sites between STING and STXBP2,^179-181 and the design of corresponding peptides can inhibit their binding, thereby suppressing the level of platelet activity and the formation capacity of NETs in vitro,^{182, 183} as well as alleviating the formation of septic thrombi.¹⁸⁴ The absence of platelet STING has been found to alleviate the formation of microthrombi and NETs structures in the livers and lungs of septic mice induced by sepsis, reduce liver and lung damage and coagulation abnormality-related indicators, and enhance the survival rate of mice.¹⁸⁵ These findings reveal the mechanism by which platelet STING regulates thrombus formation in sepsis infection and provide potential intervention strategies for the treatment of sepsis.

Research has demonstrated that myeloid STING plays a crucial role in modulating coagulation during bacterial infections, functioning through a mechanism that does not rely on the type I IFN response.¹⁸⁵ In terms of its mechanism of action, STING interacts with ITPR1, which regulates the release of calcium from the endoplasmic reticulum (ER) within macrophages and monocytes.^{186, 187} The increase of cytosolic Ca²⁺ that is dependent on STING subsequently promotes the cleavage and activation of GSDMD.^{139, 180, 182} This activation is significant as it initiates the release of TF F3, which is a key promoter of coagulation processes. Moreover, the pathways associated with STING have profound implications on systemic coagulation. Specifically, either genetic alterations or pharmacological interventions that inhibit the STING-GSDMD-F3 pathway can impede systemic coagulation, leading to improved survival outcomes in animal models of sepsis. This suggests that targeting this pathway may provide a therapeutic strategy in critically ill patients.^{188, 189} Furthermore, clinical observations indicate that the upregulation of the STING pathway correlates with increased severity and mortality rates associated with DIC in septic patients, highlighting its potential as a biomarker for disease prognosis.¹⁰⁶ In short, STING-mediated blood coagulation activation, the blood coagulation activation depends on the ER calcium release, and independent of the type I IFN response, inhibit lethal infection of the blood coagulation activation. Direct targets for STING, ATP2A2, it relies on ER calcium intake limit STING-mediated F3 release, and STING-mediated GSDMD cracking decided to release and death F3 clotting.^{181, 190, 191} Finally, STING-GSDMD pathways associated with sepsis patients with DIC severity. Thus, STING is a key regulator of coagulation during lethal bacterial infections. Platelet STING thrombosis in the regulation of sepsis infection play a major role, and further reveal the pathogenesis of sepsis, offers the potential for sepsis treatment intervention strategy.

3.6 C3/C5

Sepsis is a condition that arises from a dysregulated immune response to infection, where the body's defenses become impaired. The complement system is a vital component of the host's defense mechanism against various pathogens, operating through three main pathways: the classical, lectin, and alternative pathways.^{10, 68} Each of these pathways plays a crucial role in activating the complement system, leading to the proteolytic cleavage of key proteins such as C3 and C5.^{8, 50, 66} This process ultimately results in the formation of terminal complement complexes, which are necessary for combating infections. However, it is important to note that when there is excessive activation of the complement system, it can trigger a hyperinflammatory response in the host, which may exacerbate the condition.^{109, 192} The complement system serves as an essential part of the innate immune system, contributing significantly to immune regulation and the maintenance of immune homeostasis. Within this complex system, complement component C3 stands out as a central element, as it is crucial for both the classical and alternative pathways of complement activation.¹¹⁰ The activities of C3 highlight its importance in effectively managing and orchestrating the immune response to ensure that the body can adequately respond to infections while avoiding an overactive inflammatory state that could be detrimental to the host.⁴⁸ The chemical component is glycoprotein, composed of two peptide chains, α and β, connected by disulfide bonds. It is the highest content of all complement components in serum. C3 convertase ACTS on the C3 cracking of C3a and C3b.^{9, 65} C3a, which is released into body fluids, has anaphylatoxin and chemotactic activity. C3b combines with C4b2a to become C4b2a3b, the C5 convertase in the classical activation pathway, and C5 is the substrate of this enzyme. C3b also has adhesion and regulate immune phagocytosis.^{11, 12} C3 defects of the body prone to suppurative infection, closely associated with many diseases, especially the blood coagulation disorder caused by sepsis.

In the early stages of sepsis, the activation of the complement system triggers the release of potent proinflammatory mediators, specifically C3a and C5a. These molecules play critical roles in enhancing the immune response by facilitating the recruitment and activation of various immune cells, including leukocytes, as well as endothelial cells and platelets.^{16, 17} While the activation of the complement system is a vital element of the body's initial defense mechanisms, it can become detrimental if left unregulated. Excessive or uncontrolled complement activation can result in tissue damage, potentially leading to severe complications such as organ failure.^{51, 76} Furthermore, sepsis is characterized by a dysregulation in the activation of the coagulation system, which creates a tendency for thrombosis within the microvasculature. One of the most severe consequences of this sepsis-induced coagulopathy (SIC) is DIC. This condition is marked not only by thrombosis but can also involve bleeding, attributed to the depletion of coagulation factors, anticoagulant proteins, and platelets during the coagulation process.¹⁹³ A major contributor to the initiation of coagulation is TF, which promotes coagulation by activating coagulation factors FX and FIX in conjunction with the factor FVIIa. TF is predominantly expressed by perivascular cells, including fibroblasts, pericytes, and epithelial cells, highlighting its crucial role in maintaining hemostasis and vascular integrity.^{52, 193} In response to various microbial agents and a range of inflammatory stimuli, including cytokines and complement components, TF can be expressed on endothelial cells, monocytes, and macrophages. Under pathological conditions, an abundance of biologically active TFs is found within macrovesicles derived from multiple cell types.^{53, 194} These macrovesicles can interact with other cells, such as activated platelets, neutrophils, and endothelial cells, thereby strengthening the coagulation cascade. In addition to their role in coagulation, TF, and various coagulation factors such as FVIIa, FXa, thrombin, and fibrin can generate proinflammatory signals by activating members of the G-protein-coupled protease-activated receptor family.^{9, 66} Additionally, the relationship between complement factors and thromboplastin is reciprocal, as each can activate the other, further contributing to the complex interplay of hemostasis and inflammation in sepsis. For instance, the proteins FIXa, FXa, FXIa, and thrombin are capable of converting complement components C3 and C5 into their active peptides, C3a and C5a, respectively.¹⁷ This process underscores the intricate relationship between the complement system and the coagulation cascade. Notably, C5a, alongside the MAC (C5b-9), has a significant role in upregulating TF expression in endothelial cells.¹¹² This expression is crucial as it contributes to the procoagulant state observed during inflammatory responses. Moreover, C5a can exacerbate coagulation by disrupting the function of the endothelial glycocalyx, which serves as a protective barrier for the endothelium. In summation, complement proteins C3 and C5 are pivotal in the onset and progression of sepsis, highlighting the critical connection that inflammasome activation represents between the complement system and coagulation pathways.