Sepsis is one of the leading causes of death globally and is characterised by systemic inflammation in response to an infection (1, 2). According to Płóciennikowska et al. (3), the prevalence of severe sepsis in the European Union has been estimated at 90.4 cases per 100,000 population. Also, approximately 215,000 people die yearly from severe sepsis in the United States. The total mortality rate worldwide is estimated between 30-50% (3). Over the years, although several clinical and experiment studies have been conducted there is still no specific therapy for treating sepsis (4). Several studies have shown that lipopolysaccharide (LPS) a bacteria endotoxin can induce systemic inflammation and sepsis if excessive signals occur (5), LPS can be detected by the innate immune system in low concentrations which can trigger cellular response (6). An increase in the exposure to LPS can result in extreme expression of deregulated inflammatory molecules, which can lead to cardiovascular disorders and organ damage which are characteristics of sepsis (13).
LPS is the major component of the outer membrane of Gram-negative bacteria, which activates the host innate immune system and stimulates excessive release of pro-inflammatory cytokines such as TNF-α and IL-6 etc. from immune cells. It acts as a conserved pathogen associated molecular pattern (PAMP) recognized by innate immune receptors such as TLR4 and CD14 (7, 8). Richard Pfeiffer a German microbiologist first identified LPS in the late 19th century (7, 9). All LPS molecules are similar in terms of their structural architecture, they usually consist of a mainly lipophilic component known as lipid A that is joined covalently to a hydrophilic polysaccharide core, and an oligosaccharide side chain known as O-antigen. There are usually various forms of variations within LPS molecules from one strain of bacteria to another. This is predominantly because the polysaccharide region has diverse chemical compositions. However, there are also considerable variations resulting from the fine structure of the lipid A component (9, 10). The lipid A region is generally known to be responsible for the toxicity of LPS, due to its specific and relatively sensitive recognition of several components of the innate immune system (9, 10).
The interaction of LPS with the host innate immune system is very complex and specific; it usually requires the interaction of several proteins such as LPS binding protein (LBP), CD14, MD-2 and toll-like receptor 4 (TLR4) (5,9). The Beutler’s group demonstrated that TLR4 can be recognised and activated by LPS, a receptor that is expressed on the surface of immune cells such as macrophages, monocytes, dendritic cells, neutrophils (3, 10). TLR4 can also be found in some non-immune cells such as intestinal epithelial cells and endothelial cells (3). CD14 is surface glycoprotein which can either exist in a GPI-anchored membrane form or in a soluble form, it is usually found on the surface of plasma membranes of monocytes, macrophages, dendritic cells and neutrophils in lower levels (6). MD-2 is a soluble protein that is associated with TLR4 non-covalently but can also bind to LPS in the absence of TLR4 (5). LPS cell signalling begins when LBP binds to LPS, which leads to the aggregation of LPS and thereafter facilitates the binding of LPS monomers to CD14 which is done by changing the arrangement of LPS aggregates. CD14 further facilitates the transfer of LPS to TLR4/MD-2 receptor complex. TLR4 forms a complex with MD-2 on the cell surface. This complex serves as the binding component of LPS and thereby regulates the recognition of LPS (3, 14). Upon the recognition of LPS, TLR4 recruits adapter proteins through the intracellular Toll-interleukin-1 receptor domain that activates intracellular pathways, leading to inflammatory gene transcription, these pathways are shown in figure 1 (9). After LPS activation, TLR4 triggers the intracellular mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-kB signalling cascade that results in cellular responses to LPS which is usually the release of pro-inflammatory cytokines (11, 12). Improper cell signalling of LPS may result in recessive inflammation, which can lead to sepsis shock and other chronic inflammatory disorders such as multiple organ failure and acute respiratory distress syndrome etc. (5, 15).
Figure 1: Summary of LPS activation and recognition by the TLR4 receptor pathway. LPS is recognition is facilitated by LBP and CD14, and is mediated by TLR4/MD-2 receptor complex. Afterwards this pathway is separated into MyD88-dependent and MyD88- independent pathways that in turn mediate the activation of pro-inflammatory cytokine and Type I interferon genes (5).
In the lungs, microbial products such as LPS enter through the nasopharynx to the alveolar membrane that can result in acute lung injury (16, 17). The alveolar environment constitutes of different cell types, which include alveolar epithelial cells type, I and II and also alveolar macrophages. Alveolar epithelial type I cells cover about 95% of the alveolar epithelium, they are mainly involved in gas exchange and they form physical barriers to respiratory pathogens while the alveolar epithelial type II cells are involved in the immunomodulatory function of the alveolus (18). Type II alveolar epithelial cells are important because they are involved in the production of cytokines and chemokines such as TNF-α, IL-6, IL-1β, MCP-1, MIP-1α, GM-CSF in response to lung injury from viruses or bacteria. These cells also produce surfactant that reduces surface tension and increases chemotaxis, bacterial uptake and phagocytosis by alveolar macrophages (19). Studies have shown that these type II cells can also express certain immune receptors such as TLR4, CD14 receptor (16). The alveolar macrophages are the first line of defence against respiratory pathogens, they undergo phagocytosis and ingest all particles that enter the alveolar space (16, 20).
In normal physiological condition of the lungs, alveolar macrophages are usually present within the distal air spaces but upon stimulation of LPS alveolar macrophages are activated via the activation of TLR4 receptor pathway or other pathways. The activated alveolar macrophage then releases cytokines such as TNF-α, IL1-β, IL6 etc., chemokines and other mediators for example nitric oxide (NO), reactive oxygen species (ROS) (21). TNF-α is known stimulates the alveolar epithelial type II cells to release several cytokines that are involved in the recruitment and activation of inflammatory cells (22). This may lead to the accumulation of more macrophage and neutrophils. They are also recruited in the alveolar space, which in turn activates an innate immune response. Both alveolar macrophages and alveolar epithelial type II cells express immune receptors e.g. TLR4. As immune response progresses the epithelial barrier is broken which results in vasodilation, permeability and leakage resulting in pulmonary oedema and impaired gas exchange. Pulmonary oedema is characterised by an increase in the release of pro-inflammatory cytokines that can lead to system inflammation depending on the intensity of the immune response (23).