Lipolysaccharide as a Pathogen-Associated Molecular Pattern - Literature review Example

?LIPOPOLYSACCHARIDE Lipolysaccharide (LPS) is a pathogen-associated molecular pattern (PAMP) of bacterial origin that bind to pattern recognition receptor (PRRs), toll like receptor 4 (TLR4), of the cells of the infected host (Yadav et al., 2006). It is a glycolipid that makes up the walls of gram-negative bacteria, which are abundant in the digestive tract of humans. Aside from it being the bacteria’s link to the host cells, it also serves to maintain the selectivity of the semi-permeable outer membrane of the bacteria. Structure 1. Components Members of Enterobacteriaceae, such as Klebsiella, Eschericia, and Salmonella, which makes up most of the bacteria in the gut, has LPS covering most of their membrane surface. The LPS of this group of bacteria can be divided into 3 components, 1) lipid A, 2) a core oligosaccharide, which can be further subdivided into an inner and outer core structure, and a glycosyl polymer of repeating units, called O-polysaccharide. The hydrophobic lipid A domain of the molecule contributes to the structural stability of LPS. The core oligosaccharide, on the other hand, maintain the semi-permeable barrier of the bacterial cell‘s membrane by cross-linking with divalent cations or polyamines (Frirdich and Whitfield, 2005). Because of its biological effects, it has been used in experimental research as a prototype endotoxin as well as an adjuvant. Through the elucidation that LPS’ lipid A component is a nontoxic immunostimulatory molecule, it was established that the lipid A domain modulates LPS’ biological activity (Harsoliya et al., 2011). According to Frirdich and Whitfield (2005), this domain of LPS is the most conserved, followed by the inner core oligosaccharide moiety attached to lipid A. 2. Modifications Just like other membrane proteins, LPS undergo structural changes to accommodate environmental changes such as availability of nutrients and balance of ions. In particular, lipid A modifications were found to enable the bacteria to adapt to an environment with low divalent cations, abundant cationic peptides and low temperature (Frirdich and Whitfield, 2005). The TLR4 pathway TLR4 is a membrane protein expressed by hematopoietic-derived, immune cells such as macrophages, neutrophils, lymphocytes and dendritic cells. It is the link of LPS to the innate immune pathway, which allows a faster recruitment of immune cells to the site of infection. It is able to do so by stimulating the transfer of nuclear factor-?? (NF-??) to the nucleus, thus initiating the expression of the genes coding for IL-6, IL-1 and TNF-? (Creely, 2007), whose importance will be discussed in detail later. Because many body surfaces and mucosa are susceptible to bacterial infection, many body parts may be exposed to LPS as well. As such, they are also expressed by non-hematopoietic cells such as epithelia and endothelia. In the respiratory tract, many studies have already recognized the role of TLR4 in inducing defensive mechanisms against pulmonary infection. Since the non-hematopoietic cells do not have immune functions, they release signals that allow the recruitment neutrophils to the site of infection. If uncontrolled, however, this response may lead to a decrease in lung function due to inflammation of the lungs, as well as exacerbation of allergic asthma. (Hollingsworth et al., 2005). However, it was determine by Hollingsworth et al. (2005) that, at least in the respiratory tract, immune response is different in magnitude when LPS is detected by hematopoietic cells than if it is by epithelial and endothelial cells of the airway. In their experiment, they used chimeric mice to limit expression of TLR4 in 1) hematopoietic cells only, or in 2) structural cells only. Number of neutrophils present, cytokine/chemokine production, and airway inflammation, through assessment of tracheal pressure. It was observed that even if the epithelia and endothelia of the respiratory tract do not express TLR4, neutrophil recruitment was still made possible because hematopoietic cells still express the particular PRR. In contrast, there is deficient neutrophil recruitment when TLR4 is expressed in respiratory structural cells instead of hematopoietic cells. The role of cytokines The biological effect of LPS detection lies on its ability to activate cytokines. The LPS-mediated increase in immune response is characterized by their release of interferon-? (IFN-?), a powerful immunogenic molecule whose expression is activated by IFN-?-activated STAT1. IFN-? promotes antigen-presenting capabilities of dendritic cells and macrophages secondary to detection of foreign material. In addition, they promote the phagocytosis of pathogens and subsequent denaturation of these injurious cells (Yadav et al., 2006). Aside from initiation of IFN-? expression, increase of IL-1? expression was also implicated in the biological effect LPS. In the assessment of the effects of LPS on the reproductive system of pregnant female mice conducted by Deb, Chaturvedil and Jaiswal (2004), LPS was found to affect pregnancy such that it affects IL-1? expression, which stimulates the production of endometrial leukemia inhibitory factor (LIF) necessary for implantation of the blastocyst. Thus, abortions caused by pelvic infections may have been mediated by making the endometrial, through IL-1? and LIF, unsuitable for implantation. LPS-related tissue and organ injury . Certain conditions expose the tissues to levels of LPS greater than what it is accustomed to. Gastrointestinal inflammatory diseases and alcohol intake are conditions that increases the amount of LPS in blood, probably due to the increased permeability of the blood vessels that provide for and drain the digestive tract. In women, increased LPS is recorded when the patient has bacterial vaginosis. Effect on pregnancy Likewise, many studies have looked into different organs to assess the adverse effects of exposure to LPS during bacterial infection. Miscarriage and preterm labor are highly associated with gram-negative bacterial infections among pregnant females. If infection happens during early gestation, failure of implantation (Deb, Chaturvedil and Jaiswal, 2004), embryonic resorption, or fetal death may occur. LPS exposure at middle gestational stages results to preterm delivery or even fetal death. At late stages, decreased anthropometric parameters indicative of skeletal development retardation, preterm labor and delivery, as well as fetal death may ensue. In addition, the teratogenicity of LPS has also been recorded. In a study by Carey et al. (2003), exencephaly and eye malformation were observed among the fetuses of mothers subcutaneously injected with LPS. In addition, because of its ability to confer DNA mutations, it is also considered as a potential carcinogenic agent (Harsoliya et al., 2011). Effect on patients with obesity and type 2 diabetes mellitus Patients suffering from obesity of type 2 diabetes mellitus have an altered immune system. The Kupffer cells in the liver have decreased functionality during hyperinsulinemia. In addition, the decrease in Kupffer cell function makes the pancreatic cells more sensitive to elevating glucose levels, thus increasing the insulin levels further even at slightest elevation of blood sugar. In such cases, the patient may suffer longer from bacterial infection, particularly the inflammatory response that comes with it, since the means by which LPS should have been cleared is already impaired (Creely et al., 2007). Moreover, the increased LPS exposure due to the immuno-compromised gastrointestinal tract among obese and diabetic patients makes the inflammatory reaction of adipocytes longer to resolve. As a compromise, the fat cells increase the expression of LPS receptor, MyD88 and NF-??, thus making them highly reactive, in terms of inflammation, to the presence of LPS, particularly during infection, in which there is an increase in bacterial population of the gut. Although this is beneficial in efficiently fighting against infectious agents. However, this reaction, in the long run, have deleterious effects on the host, particularly on their blood vessels, which is one of the tissues affected by such problems in metabolism (Creely et al., 2007). Patients with obesity and type 2 diabetes mellitus are at increased risk for developing heart problems, specifically atherosclerosis, or deposition of fat on the arteries supplying oxygen and nutrients to vital organs of the body. The findings of Creely et al. (2007) suggests that even at bacterial levels which should not elicit any inflammatory response from normal patients, the adipose tissue of obese and type 2 diabetic patients are at higher risk for inflammation. This means that even at low levels of bacterial infection the developing clogs in the blood vessels of obese and diabetic patients may suddenly swell up and completely block the flow of blood. In such cases, the organs being supplied would suffer from ischemia, and eventually infarction, which means necrosis or death of certain parts of the organ (Kumar et al., 2011). Targeted effects of LPS Upon binding to antigen-presenting cells (APCs), bacterial LPS enhances antigen presentation, which in turn increases the synthesis and release of proinflammatory mediators, leading to inflammatory reaction that, in the long run, may have adverse effects on the infected host. The recruitment of immune response against bacterial pathogens through LPS rely on the TLR4 pathway. LPS is especially effective on improving the immune response of macrophages, as they are the immune cells that possess high levels of TLR4 (Yadav et al., 2006). Non-targeted effects of LPS Bystander mechanism Bystander mechanisms, in which immune cells are activated by signals from other immune cells instead of the pathogen’s antigen, have been shown to be occurring in several types of immune cells. However, the functionality of bystander cells have been found to be less effective against the pathogen compared to the immune cells directly activated by the antigen (Yadav et al., 2006). The mechanisms behind the beneficial effects of LPS on humans and mice may be quite unclear, as TLR4 mRNA and molecules are expressed by the dendritic cells of these organisms at low levels. In the study by Yadav et al. (2006) on the effects of LPS on chimeric mice, the researchers found that LPS conditioning causes dendritic cell activation of both LPS-responsive and -nonresponsive cells, thus increasing the immune response of the body. Activation of LPS nonresponsive dendritic cells was verified by the upregulation of CD86, a cofactor of T- or B-lymphocyte activation, despite the LPS responsive dendritic cells lacking the essential MyD88 gene. However, it must be noted that the activation of LPS-nonresponsive dendritic cells are only possible if LPS-responsive cells are present in the area. Thus, activation of both LPS-responsive and -nonresponsive cells are mediated by a bystander mechanism. Genomic instability Many studies, as may have been obvious in the cited literature above, imply that LPS-exposed cells are more susceptible to DNA damage because of the exposed genetic material resulting from the increased tendency to express cytokines. Moreover, studies, such as that of Kidd et al. (2000), have observed that LPS from Helicobacter pylori induce increased frequency of DNA synthesis in gastric enterochromaffin-like (ECL) cells. Based on the observation on these experiments, it is probable that LPS makes the DNA more exposed so that it can accommodate the proteins needed for the increased transcription and replication observed upon exposure to LPS. Therefore, at greater than usual frequencies, the DNA is unwound to histones, and some parts are on single strand configuration. This is in contrast to the most stable configuration of DNA, which is looped around the protective proteins, compact and heterochromatic. However, as will be discussed in the following parts of this writing, the genomic instability caused by LPS exposure lies on its initiation of inflammatory response, which elevates the levels of reactive oxygen and nitrogen species. There are several consequences of increased replication and transcription to the LPS-exposed cell. First, the unstable configuration of DNA exposes the genetic material to different chemical and physical factors that may break the chain of nucleic acids, or change the nucleotide sequences within the strand. Second, because the replication mechanisms still commit mistakes once in a while, the increased frequency of replication subsequently leads to increased occurrence of genetic mutations. And although corrective mechanisms are also inherently available, there is a certain limit to how much these can repair. If the mutated cell replicates, a neoplasia eventually forms (Campbell and Reece, 2002). Apoptosis Apoptosis, or programmed cell death, is a physiological response of cells to prevent the deleterious effects of eliciting inflammatory reaction. As opposed to necrosis that is characterized by cell swelling and content leakage, apoptotic cells decrease in size and break up into fragments that are taken up by phagocytes of the body (Kumar et al., 2011). 1. Mechanism Vital to the initiation of programmed cell death is the sequential activation of a group of cysteine proteases called caspases. Upon sensing the presence of cell injury, stress, or foreign material, two pathways are set in motion: the intrinsic mitochondrial and the extrinsic death receptor-initiated pathways. To change procaspases into their active form, the former uses cytochrome c from the mitochondria, while the latter uses the cytoplasm domain of a particular receptor embedded in the membrane of the cell. Both of these pathways, despite being independent of each other, lead to the activation of executioner caspases that cleave a DNase inhibitor to allow endonuclease activity, degrade particular components of the nuclear membrane, and promote nucleic RNA and protein denaturation (Kumar et al., 2011). Aside from these caspase-mediated changes, the phosphatidylserine normally located at the inner leaflet of the plasma membrane is flipped towards the outer leaflet, thus signaling to phagocytes the death of the cell (Kumar et al., 2011). 2. Genomic instability and apoptosis As mentioned awhile ago, if the DNA repair mechanisms of the body cannot cope with the increasing rates of DNA mutation, the next step is the initiation of cell death, with is, in this case, a better alternative to the malignant transformation of the cell with damaged DNA. However, progressive increase of injurious stimuli will result not to apoptosis, but to necrosis, which causes greater tissue damage than the former (Kumar et al., 2011). 3. Non-genomic induction by LPS LPS is an efficient initiator of apoptosis to infected host cells. This is because, aside from the genomic instability it causes, it also stimulates two independent pathways of apoptosis through the initiation of tumor necrosis factor-? (TNF-?) and nitric oxide (NO) release. LPS induces the production of NO through stimulating IFN-? expression, which facilitates the induction of NO synthase isoform (Harsoliya et al., 2011). Thus, even if one pathway is inhibited by a certain bactericidal, another is available to initiate the effects of LPS. However, as will be discussed later in this text, TNF-? has a dual role in promoting apoptosis as well as cell survival. The early stages of LPS-induced apoptosis is stimulated by the presence of TNF-?, while NO is responsible for continuing the process to its later stages until the cell dies. TNF-? is produced by macrophages to elicit effects toward the same type of cells (autocrine). This cytokine promotes DNA fragmentation, and eventually, apoptosis (Xaus et al., 2000). When it binds to a specific receptor, called TNF receptor, it initiates the activation of caspase-8 through its death domain. In turn, caspase 8 activates the execution phase of apoptosis (Xaus et al., 2000; Kumar et al., 2011). NO, on the other hand, increases p53 expression, and subsequently that of the pro-apoptotic protein Bax as well. This channel allows the exit of cytochrome c from the mitochondria. In turn, cytochrome c binds to initiator caspase-9 that eventually lead to the execution of apoptosis. Thus, as opposed to the death receptor-initiated pathway induced by TNF-?, NO initiates the intrinsic mitochondrial pathway of apoptosis (Xaus et al., 2000). TNF-?, aside from the deleterious effect it has on the cell, also promote cell survival by initiating the production of Bcl-2, which is an anti-apoptotic protein that prevents the leakage of mitochondrial proteins out of the organelle, thus preventing the activation of caspases. Although what determines whether it will initiate cell death or survival remains unclear, Kumar et al. (2011) suggests the diversity of the receptors that causes this duality of TNF-? function. Meanwhile, TNF-? is not the only molecule with dual roles. NO is also believed to be an anti-apoptotic factor because it prevents the activation of caspase-3 (Bradford, Barlow and Chazot, 2005). Inflammation Like apoptosis, inflammation serves to protect the organism. In particular, it fights against microbial invasion when the initial defense barriers of the host have been broken. Unlike apoptosis, however, many types of cells are involved in this response. Because of this, it is much harder to regulate. Usually, the morbidity of certain diseases do not lie on its direct injurious effect toward the patient, but on the extent to which it initiates inflammation. For example, the endotoxin shock results from the decrease in blood pressure caused by the increase in the amounts of vasodilatory NO. However, no matter how big the extent of collateral damage is, inflammation is the prerequisite step towards the repair and healing of injured tissue. 1. Mechanism Like apoptosis, there are several steps leading toward inflammation. First, the increase of arteriolar diameter proximal to site of injury allows a greater number of leukocytes and plasma proteins to reach the site of injury. Next, the endothelium of the blood vessels that perfuse the injured tissue changes such that the plasma proteins can extravasate and reach the site of infection. In addition, endothelial cells also express proteins, particularly selectins and integrins, that binds to its complement proteins in leukocytes. Upon recognition of foreign material by these recruited leukocytes, these are taken into the immune cell, which will initiate what is called a respiratory burst, producing reactive oxygen and nitrogen species that degrade and kill the infecting objects (Kumar et al., 2011). In certain cases, however, inflammation makes the injury more serious. In asthma, for example, a harmless environmental agent elicits an inflammatory response, which activates leukocytes that sometimes do not distinguish between self and non-self materials (Kumar et al., 2011). 2. The effects of TNF-? and NO in inflammation As opposed to the autocrine production suggested by Xaus et al., recent studies suggest that TNF-? and NO actually affect other cells aside from the macrophages exposed to LPS. In addition, because of the extent by which they induce changes to cells, probably most of the indirect effects of LPS is attributed to its induction of increased secretion of TNF-? and NO by tissue macrophages exposed to LPS. Aside from its direct effect of inducing apoptosis to macrophages, they also have important roles in inflammation. If in apoptosis, the roles of TNF-? and NO are limited at specific steps, these molecules have multiple effects that reverberate at different stages of inflammation. TNF-? promotes various steps of inflammation, and subsequently, repair. It is a cytokine that induces cellular changes in preparation for the migration of immune cells to the site of injury. Specifically, once TNF-? is released, endothelial cells lining the arteriole in close proximity to the site of injury express selectins and integrins, which bind to leukocytes, slowing them down so that these immune cells will reach their intended destination. It also increases the levels of another cytokine, IL-1, which also aids in the migration of leukocytes, thus amplifying inflammation. In addition, these two cytokines work in tandem to promote the release of other cytokines, as well as NO, whose role in inflammation will be discussed shortly. They also have an important role in repair, by inducing fibroblasts to proliferate and produce collagen, which is an important component of extracellular matrix. Systemically, TNF-? promotes lipid and protein mobilization, and decreases appetite, leading to decrease in weight, and at severe cases, muscle wasting, observed during chronic infections (Kumar et al., 2011). NO, on the other hand, is an interesting molecule involved in inflammation. Its dual role in promoting as well as inhibiting inflammation makes it an endogenous mechanism to regulate inflammatory responses. For its pro-inflammatory effects, it is involved in different stages of apoptosis. First, NO is a known vasodilator, which increases the diameter of blood vessels, making the blood flow, including plasma proteins and immune cells, greater as well. Second, it also aids in the destruction of the microbes that occur within phagocytic cells after microbe ingestion. Coupled with reactive oxygen species, reactive nitrogen species derived from NO produce a highly reactive free radical peroxynitrite that attack and damage the bacterial membrane, as well as the proteins and nucleic acids of the microbes. However, NO also has an anti-inflammatory role. Its vasodilatory effect reduces platelet aggregation and adhesion. In addition, it also inhibits leukocyte recruitment (Kumar et al., 2011). INFRARED RADIATION Light from the sun is polychromatic, that is, it can be divided into several ranges of wavelength, each with their own set of characteristics. Broadly, light spectrum can be divided into X-ray (0.1 - 10 nm), ultraviolet (10 - 400 nm), visible light (380 - 760 nm), infrared (IR), whose wavelength ranges from 760 nm - 1 mm, microwave (0.01 mm - 1 mm), and radio, which belong to the range of 1 - 100 mm (Cho et al., 2009). The familiarity of these names to a layman is a testament to the extent of their use in ordinary setting. Infrared radiation is used in therapeutics as treatment to musculoskeletal disorders and indolent wounds (Bradford, Barlow and Chazot, 2005). According to Bradford, Barlow and Chazot (2005), light in the 1050 - 1100 nm (named in this study as IR1072) range improves cell viability. Particularly, leukocytes undergo apoptosis upon exposure to IR1072 because of the light’s stimulation of iNOS activity. Chen et al. (2010) also noted that mitochondria, particularly the cytochrome c oxidase, are targets of the protective effects of infrared light. By increasing cytochrome c oxidase activity, the production of ATP through electron transport chain is increased as well. Aside from the improvement in energetics, increased cytochrome c oxidase activity also increased the ability of cells to produce reactive oxygen species needed in the microbe degradation step of inflammation. Effects on skin No matter how useful infrared may be, infrared can cause adverse effects towards its user. Almost half of the solar energy hitting the Earth’s surface is composed of IR light. And because all energy is ultimately transformed to heat, most of the adverse effects of chronic and prolonged sun exposure may be due to the activity of IR. IR light hitting the skin raises its temperature to more than 40?C. IR within 760 - 1400 nm can penetrate up to the dermal layer of the skin without increasing the body’s temperature. The rest, on the other hand, can only penetrate the epidermis. In addition, it increases the skin’s temperature significantly (Cho et al., 2009). Thus, IR at 1400 nm - 1 mm is the one mostly contributing to the amount of heat an infrared brings. Since slight changes in temperature affects the structure and function of the biomolecules in the body’s cells, it is expected that most of the effects of IR involve the changes in the configuration of biomolecules, cells, and tissues in the body. Even if single dose of IR irradiation increased procollagen production in skin, multiple doses were found to decrease the protein’s production. In particular, the light wave decreases the expression of TGF-?, which stimulates procollagen-producing fibroblasts in the dermis of the skin. In addition, the heat produced from IR induces the expression of metalloproteinase (MMP) that degrades extracellular matrix proteins such as collagen and elastin. Together with the disintegration of elastic fibers, the heat-induced abnormality in tropoelasin mRNA, fibrillin-1 mRNA and protein expression causes the exposed skin to be inelastic. On the other hand, the production of new blood vessels through angiogenesis is also promoted by IR (Cho et al., 2009). Effects on the host’s protective response Because heat is one of the stress factors that the cells of the body act against, the body produces cytokines in response to IR exposure. In Seo’s and Chung’s study (2006), the transcription of IL-6 and IL-12 genes increased significantly upon exposure to IR radiation. In addition, heat shock, which can be caused by IR exposure, can increase the amounts of ROS, such as hydrogen peroxide, which is also one of the free radicals that break down microbe components inside the phagolysosome. As mentioned earlier, the increase in reactive oxygen species also increases the DNA mutations that may occur within a cell (Cho et al., 2009). When Chen et al. (2010) studied how murine, bone marrow- derived dendritic cells respond to low IR (810 nm), they noted that, similar to LPS, light at 810 nm requires the activation of TLRs to elicit response. References Andrea Bradford, Amelia Barlow, Paul L. Chazot. 2005. Probing the differential effects of infrared light sources IR1072 and IR880 on human lymphocytes: Evidence of selective cytoprotection by IR1072. Journal of Photochemistry and Photobiology B: Biology. 81. 9-14. Neil A. Campbell and Jane B. Reece. 2002. Biology. 6th ed. Benjamin Cummings: San Francisco. Aaron C-H Chena, Ying-Ying Huang, Sulbha K Sharmaa , Michael R Hamblin. 2010. Can Dendritic Cells See Light?. Proceedings of Society of Photo-optical Instrumentation Engineers. 7565. 1-7. Soyun Cho, Mi Hee Shin, Yeon Kyung Kim, Jo-Eun Seo, Young Mee Lee, Chi-Hyun Park, and Jin Ho Chung. 2009. Effects of Infrared Radiation and Heat on Human Skin Aging in vivo. Journal of Investigative Dermatology Symposium Proceedings. 14. 15-19. S. J. Creely, P. G. McTernan, C. M. Kusminski, M. Fisher, N. F. Da Silva, M. Khanolkar, M. Evans, A. L. Harte, and S. Kumar. 2007. Lipopolysaccharide activates an innate immune system response in human adipose tissue in obesity and type 2 diabetes. Am J Physiol Endocrinol Metab. 292. E740-E747. Kaushik Deb, Madan M Chaturvedi1 and Yogesh K Jaiswal. 2004. A ‘minimum dose’ of lipopolysaccharide required for implantation failure: assessment of its effect on the maternal reproductive organs and interleukin-1a expression in the mouse. Reproduction. 128. 87-97. Emilisa Frirdich and Chris Whitfield. 2005. Review: Lipopolysaccharide inner core oligosaccharide structure and outer membrane stability in human pathogens belonging to the Enterobacteriaceae. Journal of Endotoxin Research. 11. 133-144. Mohmed S Harsoliya, Javed K Pathan, Neelam Khan, Vishnu M. Patel, Dr. Savita Vyas. 2011. Toxicity of Lps and Opa Exposure on Blood with Different Methods. unpublished. 1-13. John W. Hollingsworth, Benny J. Chen, David M. Brass, Katie Berman, M. Dee Gunn, Donald N. Cook, and David A. Schwartz. 2005. The Critical Role of Hematopoietic Cells in Lipopolysaccharide-induced Airway Inflammation. An J Resp Crit Care Med. 171. 806-813. M. Kidd, L.H. Tang, S. Schmid, J. Lauffer, J.A. Louw, I.M. Modlin. 2000. Helicobacter pylori Lipopolysaccharide Alters ECL Cell DNA Synthesis via a CD14 Receptor and Polyamine Pathway in Mastomys. Digestion. 62. 217-224. Vinay Kumar, Abul K. Abbas, Nelson Fausto, Jon C. Aster. 2011. Robbins and Cotran Pathologic Basis of Disease. 8th ed. Saunders: Philadelphia. Jordi Xaus, Monica Comalada, Annabel F. Valledor, Jorge Lloberas, Francisco Lo?pez-Soriano, Josep M. Argiles, Christian Bogdan, and Antonio Celada. 2000. LPS induces apoptosis in macrophages mostly through the autocrine production of TNF-a. Blood. 95(12). 3823-3831. Rajwardhan Yadav, David J. Zammit, Leo Lefrancois, and Anthony T. Vella. 2006. Effects of LPS-mediated bystander activation in the innate immune system. Journal of Leukocyte Biology. 80. 1251-1261. Read More