Trends in Immunology
ReviewEndotoxin tolerance: new mechanisms, molecules and clinical significance
Introduction
Inflammation is a complex pathophysiological state, adopted primarily by innate immune cells in response to infection and/or tissue damage 1, 2. Innate immune cells like monocytes/macrophages detect and respond to “danger signals” (e.g. pathogens, tissue damage) through pattern recognition receptors (PRR) expressed on their surface. Toll-like receptor 4 (TLR4) is the major PRR involved in the detection of Gram-negative bacteria and their associated endotoxins (e.g. Lipopolysaccharide, LPS; Lipid A) 3, 4. While detection of pathogens and/or endotoxins by innate immune cells triggers a robust and essential inflammatory reaction, this process needs to be tightly regulated. Uncontrolled inflammation leads to extensive tissue damage and manifestation of pathological states like sepsis, autoimmune diseases, metabolic diseases and cancer [2].
Pathophysiological adaptations to regulate over-exuberant inflammation serve as an important mechanism for host protection against endotoxin shock. One of the classic examples of such a protective mechanism is endotoxin tolerance (ET) 2, 5, 6, 7, 8, a phenomenon in which cells or organisms exposed to low concentrations of endotoxin (e.g. LPS) enter into a transient unresponsive state and are unable to respond to further challenges with endotoxin; in other words, they develop a kind of “tolerance” to endotoxin. This phenomenon has been observed both in vitro and in vivo in animal models as well as in humans 5, 6, 8, 9, 10, 11, 12, 13. Importantly, the incidence of ET has been reported in several disease settings, including sepsis, trauma, surgery, and pancreatitis, underlining its clinical significance 14, 15. However, the molecular basis of ET remains unclear. Recent progress in the understanding of the multi-level regulation of inflammation 1, 2, Toll-like receptor (TLR) signalling 3, 4, 16 and identification of novel molecules through genetic dissection as well as systems biology 3, 16, 17, necessitates a re-evaluation of ET. This review discusses ET in the light of these new findings and attempts to present an integrated and updated mechanistic view of this phenomenon. In addition, the clinical significance of ET is discussed the with respect to several pathological conditions.
Section snippets
Early observations and evidence of ET in different models
One of the earliest reports of ET came from Paul Beeson in 1946. He showed that repeated injection of typhoid vaccine in rabbits caused a progressive reduction of fever induced by the vaccine [2]. This was observed also in humans who were recovering from typhoid fever or malaria, wherein re-challenge with endotoxin showed reduced fever [8]. Similarly, prior injection of mice with a sublethal dose of LPS protected them from a subsequent and otherwise lethal dose of LPS. These studies with mice
Dysregulated inflammation leads to ET: the sepsis example
One of the clinically relevant examples of ET is observed in the leukocytes of patients suffering from sepsis 8, 15. Sepsis represents an extremely complex pathology resulting from a dysregulated inflammatory response by innate immune cells following a systemic bacterial infection. The complexity of the disease is underlined by its biphasic nature, which is characterized by an initial phase of overt inflammation that leads to a later “immunocompromised” phase wherein the innate immune cells
ET-induced modulation of chemokine receptors on monocytes
Modulation of monocyte chemokine receptors and their ligands might represent an important mechanism in sepsis-induced ET. Both mouse and human studies have shown Fractalkine (CX3CL1) receptor, CX3CR1 to crucially contribute to monocyte migration, activation and host defence against bacteria 51, 52, 53. Indeed, CX3CR1-expressing mouse monocyte subsets are reported to “patrol” the endothelium and the first to migrate to inflammed tissues, where they help in the recruitment of other monocyte
Contribution of immune cell apoptosis to ET
Drastically decreased circulating lymphocyte counts have been noted in both sepsis and trauma patients which, in many cases, is correlated to their susceptibility to nocosomial infections [43]. Autopsy studies by Hotchkiss's group have demonstrated profound apoptosis of CD4+ T-cells, B-cells and follicular DCs in the spleens of sepsis patients 15, 43. However, apoptosis was not observed for CD8+ T-cells, monocytes, macrophages or NK cells. Loss of DCs has been reported in sepsis patients as
Signalling mechanisms in ET
Signalling through the TLR4 pathway is one of the principal molecular mechanisms for the detection of Gram-negative pathogens and their endotoxins by host immune cells 4, 16. The TLR4 pathway employs signalling through two distinct adaptors, MyD88 and TRIF 4, 9, 16. The MyD88 pathway leads to activation of the transcription factor NF-κB and transcription of inflammatory genes like TNFA, IL1B, IL6 and IL12B (Figure 2). The TRIF pathway triggers activation of the transcription factors IRF3 and
Chromatin modification and gene reprogramming in ET
Transcriptome studies in monocytes/macrophages have shown these cells to undergo a major gene reprogramming during ET. Explaining the basis of this gene reprogramming, Foster et al. [11] described two subsets of genes with distinct regulatory and functional properties in LPS-tolerant mouse macrophages. Inflammatory genes were shown to be inhibited (referred to as tolerant genes), while antimicrobial genes were either upregulated or remained inducible (referred to as non-tolerant genes), upon
MicroRNA-mediated regulation of endotoxin response
MicroRNA has emerged as an important post-transcriptional regulatory mechanism for gene expression. The influence of microRNA on innate immune signalling arises from the finding that proinflammatory stimuli like LPS, TNFα, and IL-1β could induce the expression of specific microRNAs that, in turn, affect the TLR4 and IL-1 receptor (IL-1R) signalling pathways in monocytes/macrophages 86, 87, 88. David Baltimore's group initially demonstrated that LPS treatment augments the expression of two
Mechanism of heterotolerance
The various ET mechanisms discussed above have been largely examined with respect to LPS-TLR4-mediated ET or Gram-negative sepsis. However, ET is encountered also upon pre-exposure to other TLR ligands and in Gram-positive (i.e. LPS negative) sepsis. Tolerance to an endotoxin induced by earlier exposure to a different endotoxin is referred to as heterotolerance or crosstolerance [12]. For example, earlier exposure to TLR2 ligands, such as lipoteichoic acid (LTA), Pam3Cysk4 or MALP2, renders
Conclusion
Recent progress in our understanding of inflammation, TLR signalling and gene expression profiling of monocytes/macrophages has called for a review of ET in the light of these findings. Several salient points have emerged from the above discussion. First, ET can be viewed as a negative feedback response arising as a result of dysregulated inflammation (e.g. sepsis). Second, ET is a case of gene reprogramming and immunomodulation rather than a global downregulation of gene expression and
Acknowledgments
S.K.B. is grateful for funding from the Biomedical Research Council, Agency for Science, Technology and Research (A*STAR). E.L.C. is grateful for funding from the Fondos Investigación Sanitaria, Ministerio de Ciencia e Innovación and Fundación Mutua Madrileña de Automovilística. S.K.B. and E.L.C. acknowledge contributions from Dr Irina Shalova, Dr Carlos del Fresno, Dr Vanesa Gomez-Piña, Irene Fernández Ruiz, Teresa Jurado and Tasneem Kajiji. We thank Dr Saki Paul for her help in editing and
Glossary
- AOAH
- Acyloxyacyl hydrolase
- ATF3
- Activating transcription factor 3
- BCL3
- B-cell CLL/lymphoma 3
- CEBPD
- CCAAT/enhancer-binding protein Delta
- CIITA
- Class II transactivator
- CLEC4a
- C-type Lectin domain family 4, member A
- COX-2
- Cyclooxygenase 2
- DC
- Dendritic cells
- ET
- Endotoxin tolerance
- FLN29
- TRAF-type zinc finger domain containing 1(TRAFD1)
- FPR1
- Formyl peptide receptor 1
- HA
- Hyaluronic acid
- HMGB1
- High mobility group box 1 proteins
- IRF3
- Interferon regulatory factor 3
- JNK
- Jun N-terminal kinase
- LPS
- Lipopolysaccharide
- MALP-2
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