**3. Mechanism of IB dysfunction in sepsis**

All of the aforementioned layers of the IB are the sole protectors of the body, acting as a barrier between 40 trillion luminal microorganisms [49] and the body. Thus they carry a heavy burden in terms of maintaining intestinal homeostasis. Therefore, luminal content should be deliberately compartmentalized, and filtered selectively via preserving intestinal integrity, otherwise increased gut permeability provokes the activation of mucosal immunity via promoting BT along with translocating endotoxins and other pro-inflammatory antigens from lumen to the circulation [3, 5, 9, 14]. This catastrophic cascade can be seen during any infection or any incident triggering local and systemic inflammatory response generating impairment of IB integrity such as sepsis, shock, trauma, abdominothoracic vascular surgery, transplantation, severe burn, and intestinal/mesenteric ischemia and reperfusion [50–52].

#### **3.1 BT**

The concept of BT was first described in 1966 [53] and further expanded in 1979 [54] as the transportation of indigenous bacteria through the intestinal wall into the mesenteric lymph nodes and other sterile organs. Translocation of PAMPs, which are small molecular motifs (i.e., LPS, peptidoglycan, and bacterial DNA) located in the microorganism, are added to this definition later on [12]. The mechanism behind the BT are diverse.

#### *3.1.1 Imbalance of the intestinal flora*

Intestinal microbiota can be affected by diverse factors, including gastrointestinal secretions, antibiotics, secreted IgA (sIgA), bile salts, and peristalsis of the intestines. Overgrowth of the microbiota upsets the protective features of the beneficial intestinal bacteria and disrupts the first line of defense of the IB resulting in BT.

#### *3.1.2 Dysfunction of mucosal immune function*

Under physiological conditions, IgA secreted by the plasma cells has a feature of encapsulation of bacteria to prevent their trespassing and encapsulation of viruses invading the cells. It also has features enhancing the effects of lysozyme and complements. However, pathological processes inhibit the functionality of IgA, including encapsulation ability, as well as their concentration via a declined number of plasma cells secreting the IgA. This will eventually cause BT.

#### *3.1.3 Increased intestinal permeability*

Pathological processes such as conditions causing immunocompromisation or immunosuppression, MODS, severe burn injury generate BT via overgrowth of

*Intestinal Barrier Dysfunction, Bacterial Translocation and Inflammation: Deathly Triad… DOI: http://dx.doi.org/10.5772/intechopen.99554*

intestinal microbiota, immune dysfunction of the body together with the physical intestinal injury, particularly in the mucosal barrier. Oxygen depletion (caused by shock, mesenteric ischemia, cardiovascular surgery, transplant surgery) [50], acidosis, nitric oxide (NO), as exaggerated NO production by inducible NO synthase (iNOS) disturbs mitochondrial functions and cellular respiration, resulting in decreased ATP synthesis and accelerated apoptosis which causes mucosal injury, inflammatory mediators (IFN-γ, IL-4, tumor necrosis factor-α (TNF-α), platelet-activating factor (PAF), reactive oxygen species (ROS)) and endotoxins (causes edema in the submucosa, decreases the intestinal blood flow, villi necrosis) are primary factors in charge of the injured intestinal mucosal barrier and increase the intestinal permeability and BT consequently [38].

#### **3.2 Who is the victim? Who is the criminal?**

Sepsis is a hazardous organ dysfunction with high mortality and morbidity rate. The main characteristic of sepsis is "dysregulated host response to infection" [55], which will eventually bring about microvascular injury, problematic perfusion, cellular hypoxia, and finally, shock [2–5, 15].

Intestines are usually referred to as the "star of the show" during sepsis as they are accused of causing MODS [4, 7, 9, 12], yet sepsis (and septic shock) also disrupts the intestines by impairing the perfusion of the intestinal mucosa, epithelial edema, initiation of excessive apoptosis and necrosis of the gut epithelia [56], coagulationassociated local dysregulation, and cause hyperpermeability, microbiota transformation into pathobiota, BT and loss of absorptive functions [3].

#### *3.2.1 Systemic inflammatory response*

As an exaggerated inflammatory response is implicated in the pathophysiology of sepsis, involvement of inflammatory cells along with pro-inflammatory cytokines in this process is inevitable [2–5, 15, 55].

Upregulation of the adhesion molecules in the endothelial layer of the gut, induced by damage-associated molecular patterns (DAMPs) and PAMPs, leads to migration of the neutrophils, monocytes, and macrophages to the intestinal tissue. Pro-inflammatory cytokines released from these recruited cells initiate local and systemic inflammation [5, 14]. In addition, escalated activated macrophage infiltration into the artery walls inaugurates atherogenesis [14].

Besides, pattern recognition receptors (PRR) of PAMPs (including TLRs and NLRs) recognize the cell wall components of the bacteria via TLRs. TLR4 recognizes gram-negative bacteria, whereas TLR2 recognizes gram-positive bacteria. At this point, LPS-induced TLR4 activates signaling pathways either by activation of mitogen-activated protein kinase (MAPK) and NF-κB via MyD88-dependent signaling pathways or the TIR-domain-containing adapter-inducing IFN-β (TRIF) dependent (MyD88-independent) by TLR4 endocytosis [23]. In the MyD88 dependent pathway, nuclear translocation of NF-κB encourages the transcription of pro-inflammatory genes such as IL-1 α/β, IL-6, IL-18, and TNF-α, whereas, in the TRIF-dependent pathway, nuclear translocation of IFN regulatory factor 3 (IRF3) promotes IFN-inducible genes and the type I IFNs [14, 23, 57].

TLR4 cannot bind LPS per se, needs a cofactor, CD14, which hands the LPS to TLR4 [14], and controls the LPS-induced endocytosis of TLR4 apart from the signaling pathways mentioned above [23, 58].

Although these reactions initially manifest as local inflammation, the process ultimately transforms into a "cytokine storm" [5, 14]. On the contrary, both the MyD88-dependent and TRIF-dependent signaling pathways produce pro- and

anti-inflammatory mediators (IL-10) synchronously [23]. Thus, although it has been presumed that extinguishing the hyperinflammation is beneficial in septic patients, an overbalance of anti-inflammatory activity causes an inadequate response to primary infection and makes the patient prone to secondary infections [5, 59, 60].

Additionally, TLR4 manages intestinal cell turnover. During sepsis, elevated cytokine levels shift the balance between proliferation and apoptosis of the crypt and villus in favor of apoptosis and necrosis of the intestinal mucosa, which increases the intestinal permeability as a consequence of decreased villus height, increased release of DAMPs which feed the inflammatory process and brings about ulcer development along with hemorrhage and acceleration in intestinal impairment [4, 5, 56]. The elevated levels of pro-inflammatory cytokines are also shown to reduce the thickness of the mucus layer, the adherence of the mucus layer, and the luminal coverage [4, 52].

Besides, several studies reported that M cells located in the villi provide antigens a channel to mucosal lymphoid tissue, which is a trap where they encounter antigen-presenting cells (APCs). APCs present them with the help of MHC class II to CD4+T-cells [61]. In addition to that, DCs interact with T cells and B cells and selectively generate pro- and anti-inflammatory immune responses mostly through LPS-induced TLR-associated pathways (for a recent review, *see* reference [23]).

Inflammatory host response during sepsis may alter the layers of the IB and cause intestinal hyperpermeability and BT, and these alterations modulate the changes in the expression of the proteins of the TJs such as transmembrane proteins (occludin, junctional adhesion molecules, claudins) and peripheral proteins like zonula occludens-1 (ZO-1), which is in a relationship with actin-myosin complex of the cytoskeleton [11]. Furthermore, TNF-α, IL-6, and IL-1β levels can be elevated via activation of myosin light chain kinase (MLCK), an enzyme phosphorylating the myosin regulatory light chain and leading to hyperpermeability and create a positive feedback mechanism of MLCK activation through ZO-1 and occludin alterations. Aggravated systemic inflammation because of the increased permeability leads to a futile cycle [4].

#### *3.2.2 Sepsis-induced self-digestion of the gut*

Pancreatic enzymes are shown to cause multiple organ failure via autophagy [62]. Sepsis-induced ischemia of the gut exacerbates self-digestion and causes mucosal barrier damage leading to the release of DAMPs and pro-inflammatory cytokines from intestinal epithelial cells [5, 63].

Proteases, including pancreatic enzymes in the intestines, also activate the pro-metalloproteinases (MMPs) under ischemic conditions. Enzymatic activity of MMPs destructs intercellular junctions via proteolytic cleavage of junctional proteins' ectodomain, therefore increasing the intestinal permeability. Additionally, MMPs can digest the endothelial basal membrane [64].

Besides, LPS can induce expression of MMP7, and Paneth cells' degranulation, which promotes gut hyperpermeability, while MMP7 itself enhances local intestinal inflammation and intestinal damage via activation of α-defensin, subsequently stimulating the release of IL-6 from ileal epithelia and macrophages. Furthermore, it was previously confirmed that MMP7 and MMP13 are correlated with loss of intestinal integrity, inflated BT, and the development of multiple organ dysfunction [5].

After all, it has been reported in several studies that the inhibition of pancreatic enzymes protects the sepsis-induced intestinal autophagy and improves the overall progress [64, 65].

*Intestinal Barrier Dysfunction, Bacterial Translocation and Inflammation: Deathly Triad… DOI: http://dx.doi.org/10.5772/intechopen.99554*

#### *3.2.3 Intestinal circulatory problems during sepsis*

25% of the total cardiac output (up to 35% during digestion) is normally diverted to splanchnic vasculature [5]. Intestinal hypoperfusion can be caused by various reasons like mesenteric ischemia, abdominothoracic vascular surgery, shock, severe burns, transplantation surgery, necrotizing enterocolitis, sepsis, and septic shock [50], because of the redistribution of blood to protect the vital organs [38]. Intestines are highly sensitive to hypoperfusion as the enterocytes have the highest turnover rate among other fixed-cell populations in the body, with a lifespan of 2–6 days [3, 66]. Thus, hypoperfusion causes damage to the intestinal mucosal barrier [5]; moreover, the inflammation as a response to hypoperfusion caused by the ischemia/reperfusion further injures the intestines [3], resulting in loss of IB integrity, BT accompanied by systemic inflammatory response via release of pro-inflammatory cytokines [5, 50, 67].

In addition to that, vasodilation emerges as the pro-inflammatory cytokines (IFN-γ, TNF-α, IL-4, platelet-activating factor) affect vascular smooth musculature and endothelium, as well as capillary leakage, venous stasis, and ultimately diminished cardiac output, and hypoperfusion is seen [38, 68, 69]. Furthermore, compensatory mechanisms, such as the renin-angiotensin-aldosterone system, provokes the release of vasoconstriction and contribute to hypoperfusion. Also, regulatory features of the microvasculature (arterioles, venules, and capillaries) related to perfusion and oxygen distribution are lost due to sepsis-induced hyperinflammation and its inevitable outcome, ROS [38, 69–71].

#### *3.2.4 Coagulopathy*

Disseminated intravascular coagulation (DIC) is still an up-to-date challenge as of the significant lethal problems of sepsis. It is suggested that both intravascular and extravascular fibrin formation are seen in sepsis-associated DIC due to coagulation activation combined with fibrinolysis inhibition [72].

In sepsis, activation of coagulation is driven mainly by the tissue factor (TF) pathway, in which TF is derived from endothelial cells, monocytes, neutrophils, and liver, whereas suppression of fibrinolysis is coordinated by plasminogen activator inhibitor-1 (PAI-1) [5]. "Activation of coagulation": Neutrophil activation, caused either by direct contact with the pathogen or damaged cell- or bacteria-originated small molecules cause neutrophils initially to release a significant amount of TF [73, 74]. Later on, elastase is released from the neutrophils as well. Elastase has the ability to inhibit plasminogen together with the TF pathway, which is a crucial coagulation suppressor system [75, 76]. Neutrophil extracellular traps (NETs), which are released from neutrophils in order to trap and eliminate the microbes, also contribute to alterations in the coagulation via activation of Factor XII due to their negatively charged surfaces, fibrin formation, and competitively blocking the binding sites of tissue plasminogen activator (tPA) for clot degradation by fibrin cleavage [77–80] This explains the persevere on-going of the post-sepsis micro- and macrothrombosis events [81]. "Inhibition of fibrinolysis": Sepsis-associated DIC differs from malignancy-associated one, since it is characterized by intensive suppression of fibrinolysis via overproduction of PAI-1, instead of the consumption of coagulation factors, od and differs from fibrinolytic phenotype DIC, as the consequential effects of this suppression in sepsis-associated DIC lead to tissue hypoperfusion resulted in organ dysfunction. Hence, the fibrin-related markers are not safe to use to assess the severity of sepsis as hypofibrinogenemia is uncommon [81, 82].

Besides, antibody-mediated action in the presence of pathogens can activate the complement system during sepsis. Coagulation system is also affected by the activated complement factors via generating an epithelial surface to facilitate clot formation. Activated C3 also activates platelets through the alternative pathway for stimulating aggregation, while activated C5 stimulates endothelial and inflammatory cells, inducing TF expression [83, 84]. In reverse, activated coagulation system can also impact the complement system. TCC production via activation of C5 is achieved by thrombin. Additionally, plasmin can also activate C3 and C5, forby other proteases of the coagulation cascade activate several complement factors like IXa, Xa, XIa, and XIIA [85]. These factors collectively disturb intestinal microcirculation and intestinal physiology and recruit the immune cells throughout the incidence of sepsis-associated DIC [5].
