**3. Promising biomarkers**

#### **3.1 Receptor for advanced glycation end products**

The receptor for advanced glycation end products (RAGE) recipient is a standard recognition receptor that participates in a wide variety of physiological and pathological processes, such as diabetic complications, cancer, atherosclerosis, and inflammation.

The studies that relate soluble RAGE (sRAGE), the extracellular domain of RAGE, to the sepsis are very recent since even the discovery of this receptor's soluble form occurred in 2009 [37]. It has been reported that an increase in the level of sRAGE would be a protective mechanism since its presence in plasma contributes to the removal or neutralization of ligands for RAGE, thus acting as a "false" receptor [38]. However, Wang et al. [39] reported a deletion effect of sRAGE in the inflammatory process, since it would bind to CD11b receptors of leukocytes, thus propagating inflammation.

Based on these contradictory results in scientific literature, in 2014, our group published a study demonstrating a positive correlation between serum levels of sRAGE with IFN-γ in patients with sepsis. We also observed significant correlations between levels of IL-1α, IL-6, IL-8, IL-10, and IP-10 and sRAGE in patients with septic shock. We concluded that sRAGE blood levels may be associated with the mortality of patients with septic shock [35].

Further studies support this assertion, such as the study by Matsumoto et al. [40] demonstrated that the sRAGE serum level of patients with sepsis increases directly proportional to the severity of the disease, suggesting that sRAGE reflects on the RAGE's signaling pathway inducing an excessive inflammatory response involved in endothelial injury and coagulopathy.

In the same year, Wang et al. [41] demonstrated that the decrease in sRAGE levels in mice results in improved sepsis-induced lung damage, thus decreasing mortality in this condition. Another study by Narvaez-Rivera et al. [36] demonstrated in 2015 that sRAGE's level in the plasma is high in patients with communityacquired pneumonia associated with sepsis and is also an independent factor for the likelihood of a fatal outcome.

Further studies are still needed to elucidate the mechanism of action of sRAGE in sepsis and septic shock; however, this receptor appears to be a promising biological marker for sepsis.

#### **3.2 Nitric oxide**

Nitric oxide (NO) is a highly reactive soluble gas that is endogenously synthesized by the three isoforms of the nitric oxide synthase enzyme (NOS), neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2), and endothelial NOS (eNOS or NOS3). These molecules are known to be highly involved in cardiovascular homeostasis, so recent research has focused on its action on sepsis-induced heart disease [42].

In 2014, Nardi et al. [43] showed an increased NOS1 expression in vascular tissues in the sepsis condition, suggesting that this molecule could be a way to justify vascular dysfunction induced by the disease. This study suggests that the inhibition of this isoform may be an alternative to restore the effectiveness of vasopressors in later cases of sepsis.

In the same year, Martin et al. reported an association between NOS2 and NOS3 with sepsis. In the study, the authors suggest that this association could be related to the high level of NO in the blood plasma, which could consequently induce a failure of hemodynamics and increase the mortality of septic patients [44].

More recently, the critical care department of the First People's Hospital in Chun'an (China) demonstrated that monitoring changes in NO serum and amyloid A levels can be an efficient tool for defining patient prognosis and, when compared with CRP, would present better clinical results [45]. Despite the positive results on the use of nitric oxide as a biological marker for sepsis, many studies are still needed to fully understand its role in this condition.

#### **3.3 Haptoglobin**

Haptoglobin (Hp) is a protein whose main biological function is to bind free hemoglobin (Hb) and to prevent the loss of iron and subsequent kidney damage following intravascular hemolysis. When red cells are lysed, Hb binds to circulating Hp forming the Hp-Hb complex, which is then degraded by the reticuloendothelial system [46].

Although it is recognized that Hp is predominantly synthesized in the liver, studies reveal the expression of this protein also in other parts of the body, such as the lung, kidneys, heart, spleen, thymus, and brain [47–49]. There are reports that Hp levels are influenced by the acute inflammatory process and that such protein exerts an important antimicrobial and antioxidant function [50–52].

It is known that some patients with sepsis present deformity of hemoglobins, thus causing lysis of these cells and releasing them into the circulation [53]. In this context, the decrease in blood levels of Hp has already been described as a factor linked to increased mortality in patients diagnosed with sepsis, and, in animal models of sepsis, Hp supplementation has been shown to be able to decrease biomarkers of acute systemic inflammation [54, 55].

Even with this result, prospective and randomized studies are still needed to better elucidate the potential protective effects of endogenous and exogenous haptoglobin against the deleterious effects of free hemoglobin in septic patients.

**17**

**Figure 1.**

*Stages of sepsis and cytokine profile.*

*New Biomarkers of Sepsis with Clinical Relevance DOI: http://dx.doi.org/10.5772/intechopen.82156*

Sepsis is characterized by two phases: a period of hyperinflammation, where the innate immune system is overactivated leading to production of proinflammatory cytokines such as TNF, IL-1b, IL-6, and IL8, and another period of immunosuppression where both adaptive and innate immunity are acting [56]. Clinical trials in septic patients showed an increase in the above-mentioned pro-inflammatory

However, contradictory results have been obtained for TNF and IL-1b. For example, the treatment of septic patients with anti-TNF antibodies did not affect the clinical outcome of patients [59]. In addition, these cytokines are not altered only in sepsis, after surgery or in autoimmune disease they are also altered and

On the other hand, studies have reported that IL-6 shows great promise as a biomarker [60–63]. Like TNF and IL-1b, IL-6 is not altered only in sepsis; nevertheless, several studies have shown its importance in the prognosis of sepsis presenting strong correlations with patient mortality [62, 63]. These results were also shown in an animal model of acute septic peritonitis (CLP) [61]. In this way, IL6 levels can show which patients may develop severe sepsis, and this reflects on possible interventions. Like IL6, IL8 has also been mentioned as a prognostic biomarker in septic

In the immunosuppression stage, IL-10 plays a key role in development of CARS trying to reduce hyperinflammation [65]. Therefore, studies have shown that high levels of IL-10 are correlated with a worse outcome and death [65–67]. In neonatal sepsis, IL-10 also proved to be an accurate biomarker. **Figure 1** illustrates the model proposed by van der Poll and van Deventer [68], emphasizing that the development of CARS still occurs in the pro-inflammatory phase of the disease. Thus, the mortality observed in the early stages of sepsis may be related to the hyper-inflammatory phase, and the late-stage deaths are related to the immunosuppressive phase as well

patients especially in the early stages of the disease [64].

**3.4 Cytokines as biomarkers**

cytokines [57, 58].

therefore are not specific.

as to secondary infections [56].
