Novel Biomarkers for Early Detection of Sepsis in the Emergency Department

Introduction:

Sepsis and septic shock are the dangerous complications of an undetected or uncontrolled infection and tend to result in fatal outcomes if timely intervention is delayed. Septic shock is a further complication of sepsis when the patient undergoes severe hemodynamic instability and needs to be intervened with meticulous fluid resuscitation and vasopressors (given to increase the blood pressure). Sepsis and septic shock remain to be one of the most important reasons requiring ICU (intensive care unit) admission. Emergency care health professionals play a crucial role in early diagnosis and immediate medical intervention by providing appropriate fluid resuscitation and meticulous antibiotic therapy, thereby positively affecting morbidity and mortality rates.

What Is a Sepsis?

Sepsis is a multi-systemic life-threatening complication that is considered to be a consequence of a dysregulated host immune response to an infection, which often makes the diagnosis difficult. The mainstay of the treatment lies in early diagnosis with timely and effective management techniques. Hence, recent studies have suggested the importance of certain diagnostic tools like biomarkers, which are the important biomolecules of the host system, helping in the early detection of the septic condition.

How Does Septic Condition Occur in the Human Body?

• When compared to a local infection, sepsis is a multilevel tissue destruction and organ damage occurring due to an imbalance in the inflammatory and anti-inflammatory mechanism in the host.

• The activation of subsequent inflammatory pathways following an infection causes the widespread systemic release of cytokines, mediators, and pathogen-related molecules, resulting in the coagulation process and activation of the complement components of the immune system, leading to multi-organ damage.

• As soon as pathogen-derived molecular patterns (PAMPs) such as endo and exotoxins of a bacteria invade the host, they are recognized by certain toll-like receptors (TLR) which are present on the surface of antigen-presenting cells (APCs) and monocytes.

• These APCs also recognize host-initiated damage-associated molecular patterns (DAMPs), which evoke an inflammatory response.

• The APCs and monocytes are involved in initiating the transcription of genes responsible for mediators of inflammation such as pro-inflammatory interleukins (IL) like (IL-1, IL-12, IL-18), tumor necrosis factor-alpha (TNF-α), and interferons (IFNs).

• Neutrophils are released, which are the front-line warriors in destroying the pathogen by their phagocytic activity.

• This further causes activation of cytokines (IFN-y, IL-6, IL-8), complement, and coagulation, cascade leading to progressive tissue damage resulting in multi-organ dysfunction.

• In the case of immuno-compromised patients, the cascade of host defense mechanism is dysregulated either because of inactive TLRs or APCs or due to increased immune cell apoptosis (programmed cell death) or T cell exhaustion resulting in immunoparalysis and making the host vulnerable to various opportunistic or nosocomial bacterial and viral infections.

• The progression of sepsis is often concomitant with increased levels of the above-mentioned inflammatory markers, which in turn initiate the release of neutrophil extracellular traps (NETs).

• NET is an extracellular structure with decondensed chromatin containing granular and nuclear proteins that act by immobilizing various pathogens. This includes gram-positive and gram-negative bacteria, viruses, and yeasts. However, Protozoans and parasites cannot be engulfed due to their increased size.

• NETs serve as potential markers of sepsis, which are usually initiated by cytokines, platelet agonists such as thrombin, ADP (adenosine diphosphate), collagen, arachidonic acid, and antibodies.

Which Biomarkers Are Considered for the Early Detection of Sepsis?

These biomarkers play an important role in the early detection of sepsis and also aid in

determining the prognostic value of sepsis. According to the timing of the release of these

biomarkers, they are further classified into:

A. Diagnostic Biomarkers:

1. Presepsin:

• Presepsin (PSEP) is a glycopeptide, a by-product of the CD14 glycoprotein (receptor for bacterial lipopolysaccharide), present on the surface of monocytes and macrophages, that is released into the blood following infection and subsequent pro-inflammatory response.

• The presepsin levels are thought to be elevated even before the mediators of inflammation, such as procalcitonin or IL-6, are released in the blood.

• Hence, presepsin is regarded as an important diagnostic biomarker for early sepsis detection.

2. Pentraxin-3 (PTS-3):

• Pentraxin (PTX-3) is a soluble, pattern recognition molecule (PRMs) that belongs to the long pentraxin subfamily.

• PTX-3 is secreted by monocytes, dendritic cells, macrophages, epithelial cells, vascular endothelial cells, and smooth muscle cells, which are either initiated by toll-like receptor agonists, microbial molecules (lipopolysaccharides), or inflammatory cytokines.

• Circulating PTX-3 molecules are responsible for complement activities, cell extravasation, and initiation of pathogen recognition by myeloid cells.

• PTX-3 also forms an antimicrobial microenvironment, thereby causing resistance to microorganisms.

• PTX-3 plays a crucial role in other systemic conditions like cardiovascular disease, cancer, and wound healing.

• Hence, PTX-3 has been considered a good diagnostic biomarker as well as a prognostic marker of sepsis.

3. Calprotectin:

• Calprotectin is a calcium-binding protein that consists of calgranulin A and calgranulin B subunits.

• These subunits are expressed by various immune cells and are particularly present in the cytosol of myeloid cells.

• Following subsequent binding with surface receptors like toll-like receptor 4 (TLR4), receptors, receptors for advanced glycation end products (RAGE), and the extracellular matrix metalloproteinase inducer (EMMPRIN), calprotectin is released into the blood.

• Recent studies have suggested a link between increased serum calprotectin levels and the bacterial load, especially in critically ill patients suffering from sepsis.

• Also, calprotectin serves as a useful biomarker in differentiating bacterial pneumonia from viral pneumonia.

• Hence, calprotectin levels circulating in a patient suffering from sepsis can be strongly relied on as a diagnostic biomarker.

4. Human Microbiome:

• The human microbiome refers to a complex community of good microflora situated along the gastrointestinal tract, and the intestine is more densely populated than other sites.

• The presence of a surplus amount of intestinal microbiome is crucial for overall well-being as it plays a major role in the following:

• Human energy homeostasis.

• Controls intestinal endocrine activities.

• Serves as a bio-factory for synthesizing vital vitamins and cofactors.

• Development and maturation of the immune system.

• Protective role against pathogens by involving in the competitive inhibition for sharing nutrients, producing antimicrobial substances, and inducing resistance to colonization.

• According to the studies, the intestinal microbiome is thought to be disturbed in sepsis patients when compared to healthy individuals.

• Also, studies revealed the link between the increased mortality of sepsis cases to decreased intestinal microbial diversity.

• The intestinal microbiome serves as an important diagnostic biomarker of sepsis as bacteria like Parabacteroides, Fusobacterium, and Bilophylloma can be isolated in a sepsis patient, which are the mediators of inflammation.

• Enterococcus species isolated from the microbiome of sepsis patients act as important prognostic markers of sepsis, as these species are found in morbid patients or non-survivors of sepsis.

5. Soluble Triggering Receptor Expressed on Myeloid Cell 1 (sTREM-1):

• sTREM-1 is a glycoprotein present on the cell membranes of immune cells and is elevated in cases of sepsis, serving as an important diagnostic biomarker that is initiated by various mediators of inflammation.

B. Prognostic Biomarkers:

1. Angiopoietin:

• Angiopoietins (Angpts) are the angiogenic growth factors that are released by vascular endothelial cells during the inflammatory process.

• Angpt1 and Angpt2 are the subunits of the angiopoietin growth factor, and an increase in Angpt-2 and the Angpt-2/1 ratio is associated with multi-organ damage and predicts a poorer prognosis.

2. Monocyte Chemoattractant Protein 1 (MCP-1):

• MCP-1 is released by monocytes, endothelial cells, fibroblasts, and other immune cells as a part of the pro-inflammatory mechanism.

• MCP-1 is a soluble chemokine, and its release attracts more immune cells to the site of injury.

• Increased levels of MCP-1 indicate progressive organ damage and suggest a poorer prognosis of the case.

Conclusion:

Sepsis is a consequence of a dysregulated host immune response and proves to be fatal in cases of missed diagnosis and delayed intervention. It is one of the most common causes of ICU hospitalization. However, recent diagnostic advances have discovered that several biological molecules act as biomarkers for clinically detecting sepsis. These molecules serve as both diagnostic and prognostic markers of sepsis and aid in timely intervention, which can decrease the mortality rates due to sepsis to a greater extent.