In order to counteract the spread of clotting, antithrombin, alongside its anticoagulation properties, provides downregulation of the cytokine receptors by linking to inflammatory cells
[32]. Antithrombin interacts with endothelial cells, monocytes, neutrophils, and lymphocytes, and enhances prostacyclin release. The latter inhibits the interaction between endothelial cells and inflammatory cells, and also reduces the production of various cytokines and chemokines by endothelial cells
[33]. In addition, activation of tissue factor induces an increase in the levels of the thrombin/antithrombin III (ATIII) complex (TAT), the plasminogen activator inhibitor (PAI), and the plasmin-α2–antiplasmin complex (PAP). Elevated levels of TAT and PAP in septic VLBW infants at 26–32 weeks’ gestation have been recorded, but no clear trend towards either thrombosis or hemorrhage has been shown
[34]. In a prospective cohort study of full-term neonates with sepsis, severe infection was associated with activation of the contact system and consumption of anticoagulant proteins, in parallel with increased levels of the proteins of the complement system. Moreover, protein S was inactivated and anticoagulant proteins, including the TAT complex, increased, while fibrinolysis was inhibited, establishing a hypercoagulable state which resolved after antibiotic therapy among survivors
[35]. Similarly, in a prospective case–control study, a significant simultaneous reduction in the TAT complex and protein C with elevated levels of inactive protein S was observed in full-term neonates with confirmed sepsis, while protein C levels were most markedly reduced in those who fatally developed DIC
[36]. It has also been proven that neonates in the early stages of sepsis are prone to a prothrombotic state due to the consumption of coagulation inhibitors and activation of the coagulation cascade through cytokine release. This hypercoagulable state could mostly be resolved after administration of appropriate therapy. As there is ample evidence to show that activated protein C induces a reduction in TNF-α, IL-1β, IL-6, and IL-8 by blocking monocytes/macrophages, protein C has been suggested as potential therapeutic agent in neonatal sepsis
[37]. In children with sepsis, fibrinolysis is profoundly inhibited, which is mostly attributed to an increase in plasma activity of the fibrinolysis inhibitor plasminogen activator inhibitor-1 (PAI-1) during sepsis, leading to severe sepsis and septic shock. Plasminogen activator inhibitor-1 has also been proposed as a promising treatment in pediatric sepsis, but relevant studies in neonates are still lacking
[38].
Sepsis-induced endothelial dysfunction expressed as the disruption of antithrombotic properties results in the accumulation of fibrinogen
[39]. During sepsis, disturbance in the endothelial glycocalyx structure modulates endothelium–neutrophil–platelet interactions, leading to thrombus formation and also to exacerbated fibrin formation and circulatory disorders. Glycocalyx impairment, along with inflammation during sepsis, leads to capillary leakage and vascular damage, which enhances inflammation and hypercoagulation. These aberrations result in increased vascular permeability, altered blood flow, impaired oxygen delivery, and ultimately to organ dysfunction
[40]. Disturbances in the endothelial glycocalyx function induces disorders in tissue factor activation; thus disturbing the production of the tissue-type plasminogen activator and the plasminogen activator inhibitor-1. Moreover, the expression of glycosaminoglycans (such as heparan sulfates) of the injured glycocalyx is also diminished. Recently, reduced levels of endothelial glycocalyx components have been highlighted for reducing the hemostatic response of the endothelium. Furthermore, in adults, syndecan-1 levels are associated with the severity of sepsis and the development of DIC
[41]. The attenuation of the anticoagulant properties of the glycosaminoglycans will directly impair the anticoagulant effect of the endothelium. Consequently, the fine line between thrombosis and bleeding becomes apparent
[42]. Compared with adults, a decrease in TNF-a production from endothelial cells after inflammatory stimulation was shown in neonatal mouse models with
Pneumocystis carinii infection of the respiratory tract. The diminished TNF-a production failed to enhance the expression of adhesion molecules in the surface of endothelial cells and finally attenuated the T cells’ migration capacity and the host’s defense response to infection
[43]. This point underlines the importance of molecules deriving from endothelial cells and the glycocalyx during neonatal sepsis. In this context, TNFα has been proposed as a potential immunomodulator in neonatal sepsis
[43]. Certainly, strong evidence regarding the contribution of the glycocalyx and endothelial cells to managing neonatal sepsis is still lacking, but this is a fairly promising research field.
4. Neonatal Platelets in Sepsis
Platelets are major players in sepsis-induced coagulopathy. During systemic inflammation, P-selectin is expressed on the platelet surface, facilitating the platelets’ adhesion to leukocytes and platelet aggregation, in parallel with tissue factor expression on monocytes
[44]. Recently, the enhanced expression of GPIIb/IIIa receptors on activated platelet surfaces has been recognized in association with infection by
Staphylococcus aureus and
Escherichia coli, thereby demonstrating direct platelet activation in response to bacterial invasion and simultaneously introducing thromboinflammation and immunothrombosis
[45][46][47]. The etiopathology and management of immunothrombosis in infancy and early childhood still lack sufficient evidence, as little research has been conducted in the pediatric population
[48].
Thrombocytopenia noted in septic patients is mainly attributed to platelet consumption during clot propagation and thrombus formation through the activated endothelium
[49]. In neonates, thrombocytopenia driven by LPS in Gram-negative sepsis is thought to be related to diminished expression of platelet Toll-like receptor 4 (TLR4) and is linked to elevated mortality rates
[50]. Platelet activation was associated with high expression of platelet CD40L following endothelium inflammation, and higher platelet aggregation was observed after LPS stimulation (mostly in Gram-negative sepsis)
[51]. Higher CD40L expression levels in platelets from cord blood samples were observed in premature neonates with histologically proven chorioamnionitis
[52].
Moreover, research on neonatal thrombopoiesis during sepsis strongly suggested that neonates respond to sepsis by upregulating thrombopoietin (Tpo) production; although the degree of upregulation is modest, neonates present with a hypercoagulant profile at the onset of infection. In septic neonates, elevated levels of circulating megakaryocyte progenitors (CMPs) have also been observed. Simultaneous measurements of serum TPO levels and reticulated platelets (RP%) are helpful for discriminating hyperdestructive from hypoplastic thrombocytopenia in septic neonates
[53][54].
As thrombocytopenia is linked to increased morbidity and mortality in ICU admissions, the delineation of platelet functionality may potentially alter the threshold levels of platelet transfusions
[55]. Septic preterm neonates, when compared with healthy individuals, present a lower platelet adhesion capacity, which is mostly attributed to deficiencies in the intrinsic platelet properties rather than to an impairment in the concentrations or function of vWf
[50]. These studies demonstrated that platelet activation and degranulation may follow thrombocytopenia, and that this phenomenon should be further investigated by means of accurate qualitative modalities for mapping distinct platelet phenotypes in patients with sepsis.
The missing part of this complex interplay between inflammation and coagulation is quantification of this model of cell-based coagulation triggered by an agent, such as sepsis, by means of a validated and practical tool for use in everyday clinical settings.