Lung microenvironment contributes to the resistance of alveolar macrophages to develop tolerance to endotoxin*

Sepsis-induced inflammasome impairment facilitates development of secondary A. baumannii pneumonia

BACKGROUND

Acinetobacter baumannii has become one of the most critical pathogens causing nosocomial pneumonia. Existing animal models of A. baumannii pneumonia are not relevant to the majority of critical care patients. We aimed to develop a novel model of secondary A. baumannii pneumonia in post-sepsis mice.

METHODS

A two-hit model of sepsis induced by cecal ligation and puncture followed by A. baumannii pneumonia on day 5 was established. In addition, the two-hit model was established in humanized mice. A period of 2 h of mechanical ventilation followed by observation was used in additional experiments. Lung histopathology, bacterial cultures, and cellular infiltration were analysed as well as markers of the inflammasome activity in vivo and ex vivo.

RESULTS

A. baumannii infection caused mortality and loss of body weight and temperature in post-sepsis mice. Increased lung bacterial burden and dissemination together with signs of enhanced inflammatory injury were observed in post-sepsis mice but not control mice that were challenged with A. baumannii. Post-sepsis mice were unable to mount inflammasome activation in response to secondary pneumonia to the level of control mice. Transfer of wild-type but not capsase-1 KO alveolar macrophages was able to restore the pulmonary protection against A. baumannii. Mechanical ventilation exacerbated the pathological response to pneumonia in post-sepsis mice but enhanced inflammasome signalling in non-sepsis mice with pneumonia.

CONCLUSIONS

We established a novel model of A. baumannii pneumonia that revealed sepsis-induced impairment of inflammasome activation in alveolar macrophages is critical for the control of secondary A. baumannii pneumonia.

Alveolar epithelial cells shape lipopolysaccharide-induced inflammatory responses and reprogramming of alveolar macrophages

Alveolar macrophages (AMs) are sentinels in the airways, where they sense and respond to invading microbes and other stimuli. Unlike macrophages in other locations, AMs can remain responsive to Gram-negative lipopolysaccharides (LPS) after they have responded to LPS in vivo (they do not develop "endotoxin tolerance"), suggesting that the alveolar microenvironment may influence their responses. Although alveolar epithelial cells (AECs) normally limit AMs' innate responses, preventing inflammation induced by harmless antigens in the lung, how AECs influence the innate responses of AMs to infectious agents has been uncertain. Here we report that (1) after exposure to aspirated (intranasal instillation) LPS, AMs increase their responses to TLR agonists and elevate their phagocytic and bactericidal activities in mice; (2) Aspirated LPS pre-exposure increases host resistance to pulmonary infection caused by Gram-negative bacteria and the protection effect lasts for at least 35 days; (3) LPS stimulation of AECs both increases AMs' innate immune responses and prevents AMs from developing tolerance in vitro; (4) Upon LPS stimulation, AMs secreted TNF-α induces AECs to release GM-CSF, which potentiates AMs' response. These experiments have revealed a previously unappreciated role that AECs may play in boosting the innate responses of AMs and promoting resistance to pulmonary infections.

Facing stress and inflammation: From the cell to the planet

As identified in 1936 by Hans Selye, stress is shaping diseases through the induction of inflammation. But inflammation display some yin yang properties. On one hand inflammation is merging with the innate immune response aimed to fight infectious or sterile insults, on the other hand inflammation favors chronic physical or psychological disorders. Nature has equipped the cells, the organs, and the individuals with mediators and mechanisms that allow them to deal with stress, and even a good stress (eustress) has been associated with homeostasis. Likewise, societies and the planet are exposed to stressful settings, but wars and global warming suggest that the regulatory mechanisms are poorly efficient. In this review we list some inducers of the physiological stress, psychologic stress, societal stress, and planetary stress, and mention some of the great number of parameters which affect and modulate the response to stress and render it different from an individual to another, from the cellular level to the societal one. The cell, the organ, the individual, the society, and the planet share many stressors of which the consequences are extremely interconnected ending in the domino effect and the butterfly effect.

Compartmentalization of the inflammatory response during bacterial sepsis and severe COVID-19

Acute infections cause local and systemic disorders which can lead in the most severe forms to multi-organ failure and eventually to death. The host response to infection encompasses a large spectrum of reactions with a concomitant activation of the so-called inflammatory response aimed at fighting the infectious agent and removing damaged tissues or cells, and the anti-inflammatory response aimed at controlling inflammation and initiating the healing process. Fine-tuning at the local and systemic levels is key to preventing local and remote injury due to immune system activation. Thus, during bacterial sepsis and Coronavirus disease 2019 (COVID-19), concomitant systemic and compartmentalized pro-inflammatory and compensatory anti-inflammatory responses are occurring. Immune cells (e.g., macrophages, neutrophils, natural killer cells, and T-lymphocytes), as well as endothelial cells, differ from one compartment to another and contribute to specific organ responses to sterile and microbial insult. Furthermore, tissue-specific microbiota influences the local and systemic response. A better understanding of the tissue-specific immune status, the organ immunity crosstalk, and the role of specific mediators during sepsis and COVID-19 can foster the development of more accurate biomarkers for better diagnosis and prognosis and help to define appropriate host-targeted treatments and vaccines in the context of precision medicine.

Acyloxyacyl hydrolase promotes pulmonary defense by preventing alveolar macrophage tolerance

Although alveolar macrophages (AMs) play important roles in preventing and eliminating pulmonary infections, little is known about their regulation in healthy animals. Since exposure to LPS often renders cells hyporesponsive to subsequent LPS exposures ("tolerant"), we tested the hypothesis that LPS produced in the intestine reaches the lungs and stimulates AMs, rendering them tolerant. We found that resting AMs were more likely to be tolerant in mice lacking acyloxyacyl hydrolase (AOAH), the host lipase that degrades and inactivates LPS; isolated Aoah-/- AMs were less responsive to LPS stimulation and less phagocytic than were Aoah+/+ AMs. Upon innate stimulation in the airways, Aoah-/- mice had reduced epithelium- and macrophage-derived chemokine/cytokine production. Aoah-/- mice also developed greater and more prolonged loss of body weight and higher bacterial burdens after pulmonary challenge with Pseudomonas aeruginosa than did wildtype mice. We also found that bloodborne or intrarectally-administered LPS desensitized ("tolerized") AMs while antimicrobial drug treatment that reduced intestinal commensal Gram-negative bacterial abundance largely restored the innate responsiveness of Aoah-/- AMs. Confirming the role of LPS stimulation, the absence of TLR4 prevented Aoah-/- AM tolerance. We conclude that commensal LPSs may stimulate and desensitize (tolerize) alveolar macrophages in a TLR4-dependent manner and compromise pulmonary immunity. By inactivating LPS in the intestine, AOAH promotes antibacterial host defenses in the lung.

Compartment-Specific Differences in the Activation of Monocyte Subpopulations Are Not Affected by Nitric Oxide and Glucocorticoid Treatment in a Model of Resuscitated Porcine Endotoxemic Shock

Inhaled nitric oxide (iNO) remains one of the treatment modalities in shock, and in addition to its vasoactive properties, iNO exerts immunomodulatory effects. We used a porcine model of endotoxemia with shock resuscitation (control) and additional treatment with iNO and a steroid (treatment group). After 20 h, bone marrow (BM), peripheral blood (PB), and bronchoalveolar lavage fluid (BALF) were collected to analyze the immunophenotype and mitochondrial membrane potential (Δφ) in three subsets of monocytes. In both groups, SLA-DR expression decreased twofold on the circulating CD14+CD163+ and CD14−CD163+ monocytes, while it did not change on the CD14+CD163+. Δφ increased only in the CD14−CD163+ subpopulation (0.8 vs. 2.0, p < 0.001). The analysis of compartment-specific alterations showed that nearly 100% of BALF CD14+CD163+ and CD14−CD163+ monocytes expressed SLA-DR, and it was higher compared to PB (32% and 20%, p < 0.0001) and BM (93% and 67%, p < 0.001, respectively) counterparts. BALF CD14+CD163+ had a threefold higher Δφ than PB and BM monocytes, while the Δφ of the other subsets was highest in PB monocytes. We confirmed the compartmentalization of the monocyte response during endotoxemic shock, which highlights the importance of studying tissue-resident cells in addition to their circulating counterparts. The iNO/steroid treatment did not further impair monocyte fitness.

The immunology of sepsis

Sepsis is defined as a life-threatening organ dysfunction caused by a dysregulated host response to an infection. This recently implemented definition does not capture the heterogeneity or the underlying pathophysiology of the syndrome, which is characterized by concurrent unbalanced hyperinflammation and immune suppression. Here, we review current knowledge of aberrant immune responses during sepsis and recent initiatives to stratify patients with sepsis into subgroups that are more alike from a clinical and/or pathobiological perspective, which could be key for identification of patients who are more likely to benefit from specific immune interventions.

Sepsis-Induced Immunosuppression

Sepsis is expected to have a substantial impact on public health and cost as its prevalence increases. Factors contributing to increased prevalence include a progressively aging population, advances in the use of immunomodulatory agents to treat a rising number of diseases, and immune-suppressing therapies in organ transplant recipients and cancer patients. It is now recognized that sepsis is associated with profound and sustained immunosuppression, which has been implicated as a predisposing factor in the increased susceptibility of patients to secondary infections and mortality. In this review, we discuss mechanisms of sepsis-induced immunosuppression and biomarkers that identify a state of impaired immunity. We also highlight immune-enhancing strategies that have been evaluated in patients with sepsis, as well as therapeutics under current investigation. Finally, we describe future challenges and the need for a new treatment paradigm, integrating predictive enrichment with patient factors that may guide the future selection of tailored immunotherapy. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Sepsis therapies: learning from 30 years of failure of translational research to propose new leads

Sepsis has been identified by the World Health Organization (WHO) as a global health priority. There has been a tremendous effort to decipher underlying mechanisms responsible for organ failure and death, and to develop new treatments. Despite saving thousands of animals over the last three decades in multiple preclinical studies, no new effective drug has emerged that has clearly improved patient outcomes. In the present review, we analyze the reasons for this failure, focusing on the inclusion of inappropriate patients and the use of irrelevant animal models. We advocate against repeating the same mistakes and propose changes to the research paradigm. We discuss the long-term consequences of surviving sepsis and, finally, list some putative approaches-both old and new-that could help save lives and improve survivorship.

Immunosuppression is Inappropriately Qualifying the Immune Status of Septic and SIRS Patients

Immunosuppression is the most commonly used concept to qualify the immune status of patients with either sterile systemic inflammatory response syndrome (SIRS) or sepsis. In this review we attempt to demonstrate that the concept of immunosuppression is an oversimplification of the complex anti-inflammatory response that occurs in patients dealing with a severe sterile or infectious insult. Particularly, the immune status of leukocytes varies greatly depending on the compartment from where they are derived from. Furthermore, although certain functions of immune cells present in the blood stream or in the hematopoietic organs can be significantly diminished, other functions are either unchanged or even enhanced. This juxtaposition illustrates that there is no global defect. The mechanisms called reprogramming or trained innate immunity are probably aimed at preventing a generalized deleterious inflammatory reaction, and work to maintain the defense mechanisms at their due levels.

Interleukin-18 serum levels in sepsis: Correlation with disease severity and inflammatory markers

PURPOSE

Sepsis is defined as a life-threatening organ dysfunction caused by a dysregulated host response to infection and a syndrome shaped by pathogen and host factors with characteristic that evolve over time. The study was conducted to evaluate the prognostic and discriminative value of IL-18 assessment in comparison to PCT, CRP, WBC in early stage of sepsis and septic shock.

METHODS

An observational and prospective study was conducted in the group of 40 ICU patients with diagnosis of sepsis or septic shock, serum PCT, IL-18, CRP and WBC measurements were performed on admission, and on the 2nd, 3rd and 5th therapy day. The level of IL-18 was determined with commercially available test according to manufacturer's protocol.

RESULTS

There were no statistically significant differences in IL-18 levels in survivors vs non-survivors and in sepsis vs septic shock subgroups the IL-18 levels were statistically significant in the course of the study except for the 5th day.

CONCLUSION

The PCT, CRP and WBC levels revealed no significant differences between any analyzed subgroups in all time points during study. According to our results the IL-18 is a biomarker better differentiating sepsis and septic shock status than PCT, CRP and WBC but with no prognostic impact.

Compartment diversity in innate immune reprogramming

Pathogens or endogenous molecules can reprogram innate immunity. This process can take the form of priming or tolerance depending on the activating signal, and favors enhanced resistance to infection and other insults, by modulating inflammation. Similarly to their organ-specific properties, reprogramming of macrophages and NK cells, is also compartmentalized.

Development of Endotoxin Tolerance Does Not Influence the Response to a Challenge with the Mucosal Live-Attenuated Influenza Vaccine in Humans In Vivo

Introduction

The effects of bacterial infections on the response to subsequent viral infections are largely unknown. This is important to elucidate to increase insight into the pathophysiology of bacterial and viral co-infections, and to assess whether bacterial infections may influence the course of viral infections.

Methods

Healthy male subjects received either bacterial endotoxin [Escherichia coli-derived lipopolysaccharide (LPS), 2 ng/kg, n = 15] or placebo (n = 15) intravenously, followed by intranasal Fluenz (live-attenuated influenza vaccine) 1 week later.

Results

LPS administration resulted in increased plasma cytokine levels and development of endotoxin tolerance in vivo and ex vivo, illustrated by attenuated cytokine production upon rechallenge with LPS. Following Fluenz administration, infectivity for the Fluenz A/B strains was similar between the LPS-Fluenz and placebo-Fluenz groups (13/15 subjects in both groups). Also, the Fluenz-induced increase in temperature and IL-6, G-CSF and IP-10 concentrations in nasal wash were similar between both groups.

Conclusion

While endotoxemia profoundly attenuates the immune response upon a second LPS challenge, it does not influence the Fluenz-induced immune response. These results suggest immune suppression after bacterial infection does not alter the response to a subsequent viral infection.

Acyloxyacyl hydrolase promotes the resolution of lipopolysaccharide-induced acute lung injury

Pulmonary infection is the most common risk factor for acute lung injury (ALI). Innate immune responses induced by Microbe-Associated Molecular Pattern (MAMP) molecules are essential for lung defense but can lead to tissue injury. Little is known about how MAMP molecules are degraded in the lung or how MAMP degradation/inactivation helps prevent or ameliorate the harmful inflammation that produces ALI. Acyloxyacyl hydrolase (AOAH) is a host lipase that inactivates Gram-negative bacterial endotoxin (lipopolysaccharide, or LPS). We report here that alveolar macrophages increase AOAH expression upon exposure to LPS and that Aoah+/+ mice recover more rapidly than do Aoah-/- mice from ALI induced by nasally instilled LPS or Klebsiella pneumoniae. Aoah-/- mouse lungs had more prolonged leukocyte infiltration, greater pro- and anti-inflammatory cytokine expression, and longer-lasting alveolar barrier damage. We also describe evidence that the persistently bioactive LPS in Aoah-/- alveoli can stimulate alveolar macrophages directly and epithelial cells indirectly to produce chemoattractants that recruit neutrophils to the lung and may prevent their clearance. Distinct from the prolonged tolerance observed in LPS-exposed Aoah-/- peritoneal macrophages, alveolar macrophages that lacked AOAH maintained or increased their responses to bioactive LPS and sustained inflammation. Inactivation of LPS by AOAH is a previously unappreciated mechanism for promoting resolution of pulmonary inflammation/injury induced by Gram-negative bacterial infection.

S100A8/A9 and S100A9 reduce acute lung injury

S100A8 and S100A9 are myeloid cell-derived proteins that are elevated in several types of inflammatory lung disorders. Pro- and anti-inflammatory properties are reported and these proteins are proposed to activate TLR4. S100A8 and S100A9 can function separately, likely through distinct receptors but a systematic comparison of their effects in vivo are limited. Here we assess inflammation in murine lung following S100A9 and S100A8/A9 inhalation. Unlike S100A8, S100A9 promoted mild neutrophil and lymphocyte influx, possibly mediated in part, by increased mast cell degranulation and selective upregulation of some chemokine genes, particularly CXCL-10. S100 proteins did not significantly induce proinflammatory mediators including TNF-α, interleukin-1β (IL-1β), IL-6 or serum amyloid A3 (SAA3). In contrast to S100A8, neither preparation induced S100A8 or IL-10 mRNA/protein in airway epithelial cells, or in tracheal epithelial cells in vitro. Like S100A8, S100A9 and S100A8/A9 reduced neutrophil influx in acute lung injury provoked by lipopolysaccharide (LPS) challenge but were somewhat less inhibitory, possibly because of differential effects on expression of some chemokines, IL-1β, SAA3 and IL-10. Novel common pathways including increased induction of an NAD⁺-dependent protein deacetylase sirtuin-1 that may reduce NF-κB signalling, and increased STAT3 activation may reduce LPS activation. Results suggest a role for these proteins in normal homeostasis and protective mechanisms in the lung.

Local Microenvironment Controls the Compartmentalization of NK Cell Responses during Systemic Inflammation in Mice

Systemic inflammatory response syndrome is a whole-body reaction to a triggering insult that often results in life-threatening illness. Contributing to the development of this inflammatory cascade are numerous cellular partners, among which NK cells were shown to play a key role. Accumulating evidence points to organ-specific properties of systemic inflammation and NK cells. However, little is known about compartment-specific activation of NK cells during systemic inflammatory response syndrome or the relative contribution of NK cell-intrinsic properties and microenvironmental cues. In this study, we undertook a sequential characterization of NK responses in the spleen, lungs, bone marrow, peritoneum, and blood using a mouse model of endotoxemia. We report that, despite similar systemic dynamics of NK cell responses, expression of activation markers (CD69 and CD25) and effector molecules (IFN-γ, granzyme B, and IL-10) display organ-specific thresholds of maximum activation. Using adoptive transfers of spleen and lung NK cells, we found that these cells have the capacity to quickly adapt to a new environment and adjust their response levels to that of resident NK cells. This functional adaptation occurs without significant alterations in phenotype and independently of subpopulation-specific trafficking. Thus, using a dynamic in vivo-transfer system, to our knowledge our study is the first to report the compartmentalization of NK cells responses during systemic inflammation and to show that NK cell-intrinsic properties and microenvironmental cues are involved in this process, in a sequential manner.

The Early Expression of HLA-DR and CD64 Myeloid Markers Is Specifically Compartmentalized in the Blood and Lungs of Patients with Septic Shock

Identification of reliable biomarkers is key to guide targeted therapies in septic patients. Expression monitoring of monocyte HLA-DR and neutrophil CD64 could fulfill the above need. However, it is unknown whether their expression on circulating cells reflects the status of tissue resident cells. We compared expressions of HLA-DR and CD64 markers in the circulation and airways of septic shock patients and evaluated their outcome prognostic value. The expression of CD64 on neutrophils and HLA-DR on monocytes was analyzed in the peripheral blood and mini-bronchoalveolar lavage fluid cells by flow cytometry. Twenty-seven patients with septic shock were enrolled into the study. The fluorescence intensity of HLA-DR on circulating monocytes was 3.5-fold lower than on the pulmonary monocytes (p = 0.01). The expression of CD64 on circulating and airway neutrophils was similar (p = 0.47). Only the expression of CD64 on circulating neutrophils was higher in nonsurvivors versus survivors (2.8-fold; p = 0.031). Pulmonary monocytes display a higher level of HLA-DR activation compared to peripheral blood monocytes but the expression of neutrophil CD64 is similar on lung and circulating cells. Death in septic patients was effectively predicted by neutrophil CD64 but not monocytic HLA-DR. Prognostic value of cellular activation markers in septic shock appears to strongly depend on their level of compartmentalization.

Differential induction of inflammatory cytokines and reactive oxygen species in murine peritoneal macrophages and resident fresh bone marrow cells by acute staphylococcus aureus infection: contribution of toll-like receptor 2 (TLR2)

Among the known Toll-like receptors (TLRs), Toll-like receptor 2 (TLR2) is a key sensor for detecting Staphylococcus aureus invasion. But the function of TLR2 during S. aureus infection in different cell populations is unclear. Two different cell subtypes were chosen to study the interaction of S. aureus with TLR2 because macrophages are extremely different from one compartment to another and their capacity to respond to live bacteria or bacterial products differs from one site to another. The contribution of TLR2 to the host innate response against acute live S. aureus infection and heat-killed S. aureus (HKSA) using anti-TLR2 antibody in murine peritoneal macrophages and resident fresh bone marrow cells has been investigated here. TLR2 blocking before infection induces the release of interleukin (IL)-10 by macrophages thereby inhibiting excessive production of oxidants by activating antioxidant enzymes. TLR2-blocked peritoneal macrophages showed impaired release of tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and IL-6 in response to both live and heat-killed S. aureus infection except bone marrow cells. TLR2-mediated free radical production and killing of S. aureus were modulated by TLR2 blocking in peritoneal macrophages and resident bone marrow cells. This study supported that S. aureus persists in resident bone marrow cells in a state of quiescence.

SP-R210 (Myo18A) Isoforms as Intrinsic Modulators of Macrophage Priming and Activation

The surfactant protein (SP-A) receptor SP-R210 has been shown to increase phagocytosis of SP-A-bound pathogens and to modulate cytokine secretion by immune cells. SP-A plays an important role in pulmonary immunity by enhancing opsonization and clearance of pathogens and by modulating macrophage inflammatory responses. Alternative splicing of the Myo18A gene results in two isoforms: SP-R210S and SP-R210L, with the latter predominantly expressed in alveolar macrophages. In this study we show that SP-A is required for optimal expression of SP-R210L on alveolar macrophages. Interestingly, pre-treatment with SP-A prepared by different methods either enhances or suppresses responsiveness to LPS, possibly due to differential co-isolation of SP-B or other proteins. We also report that dominant negative disruption of SP-R210L augments expression of receptors including SR-A, CD14, and CD36, and enhances macrophages' inflammatory response to TLR stimulation. Finally, because SP-A is known to modulate CD14, we used a variety of techniques to investigate how SP-R210 mediates the effect of SP-A on CD14. These studies revealed a novel physical association between SP-R210S, CD14, and SR-A leading to an enhanced response to LPS, and found that SP-R210L and SP-R210S regulate internalization of CD14 via distinct macropinocytosis-like mechanisms. Together, our findings support a model in which SP-R210 isoforms differentially regulate trafficking, expression, and activation of innate immune receptors on macrophages.

Ascites bacterial burden and immune cell profile are associated with poor clinical outcomes in the absence of overt infection

Bacterial infections, most commonly spontaneous bacterial peritonitis in patients with ascites, occur in one third of admitted patients with cirrhosis, and account for a 4-fold increase in mortality. Bacteria are isolated from less than 40% of ascites infections by culture, necessitating empirical antibiotic treatment, but culture-independent studies suggest bacteria are commonly present, even in the absence of overt infection. Widespread detection of low levels of bacteria in ascites, in the absence of peritonitis, suggests immune impairment may contribute to higher susceptibility to infection in cirrhotic patients. However, little is known about the role of ascites leukocyte composition and function in this context. We determined ascites bacterial composition by quantitative PCR and 16S rRNA gene sequencing in 25 patients with culture-negative, non-neutrocytic ascites, and compared microbiological data with ascites and peripheral blood leukocyte composition and phenotype. Bacterial DNA was detected in ascitic fluid from 23 of 25 patients, with significant positive correlations between bacterial DNA levels and poor 6-month clinical outcomes (death, readmission). Ascites leukocyte composition was variable, but dominated by macrophages or T lymphocytes, with lower numbers of B lymphocytes and natural killer cells. Consistent with the hypothesis that impaired innate immunity contributes to susceptibility to infection, high bacterial DNA burden was associated with reduced major histocompatibility complex class II expression on ascites (but not peripheral blood) monocytes/macrophages. These data indicate an association between the presence of ascites bacterial DNA and early death and readmission in patients with decompensated cirrhosis. They further suggest that impairment of innate immunity contributes to increased bacterial translocation, risk of peritonitis, or both.

Activation of lung toll-like receptors does not exacerbate sickness responses to lipopolysaccharide in mice

Pneumonia represents a leading cause of death. Recently, a novel treatment strategy for pneumonia has involved enhancing the host pulmonary innate immune response by pre-exposure to aerosolized toll-like receptor (TLR)9 and TLR2/6 agonists, known as O/P. O/P inhalation in mice has been demonstrated to stimulate innate lung immunity, and thus increase survival against subsequent pneumonia infection while producing barely detectable increases in systemic cytokines. Here, we examined the safety of O/P treatment when used in mice that are inflamed systemically. Swiss-Webster mice were treated with two doses of aerosolized O/P (1× or 8×) vs phosphate buffered saline (PBS) either immediately before intraperitoneal injection of 0.1mg/kg lipopolysaccharide (LPS) or PBS (equivolume) or 2h after. Sickness responses (reduced body weight, food intake, activity and social interaction) were examined at 2 and 5.5h post-treatment. Immediately following behavioral testing, mice were euthanized, perfused with PBS, and brains, spleens, livers and lungs snap frozen for assessment of pro-inflammatory cytokine mRNAs. While O/P treatment alone increased lung IL-1β, IFNγ and TNF-α, no such effects were observed in the brain, spleen or liver. Furthermore, there was no evidence that O/P treatment administered before or after LPS had any synergizing effect to potentiate the cytokine response to LPS in any compartment measured. Supportive of these findings were the measures of sickness behaviors that did not show any increased sickness response in O/P-treated mice exposed to LPS, suggestive that the cytokine signal produced in the lungs from O/P inhalation did not propagate to the brain and synergize with LPS-induced neuroinflammation. These findings support the safety of the use of O/P inhalation as a preventative measure against pneumonia and demonstrate a unique ability of the lungs to compartmentalize pulmonary inflammation and limit propagation of the cytokine signal to the brain.

Is boosting the immune system in sepsis appropriate?

A relative immunosuppression is observed in patients after sepsis, trauma, burns, or any severe insults. It is currently proposed that selected patients will benefit from treatment aimed at boosting their immune systems. However, the host immune response needs to be considered in context with pathogen-type, timing,and mainly tissue specificity. Indeed, the immune status of leukocytes is not universally decreased and their activated status in tissues contributes to organ failure. Accordingly, any new immune-stimulatory therapeutic intervention should take into consideration potentially deleterious effects in some situations.

Protective or deleterious role of scavenger receptors SR-A and CD36 on host resistance to Staphylococcus aureus depends on the site of infection

Staphylococcus aureus is a major human opportunistic pathogen responsible for a broad spectrum of infections ranging from benign skin infection to more severe life threatening disorders (e.g. pneumonia, sepsis), particularly in intensive care patients. Scavenger receptors (SR-A and CD36) are known to be involved in S. aureus recognition by immune cells in addition to MARCO, TLR2, NOD2 and α5β1 integrin. In the present study, we further deciphered the contribution of SR-A and CD36 scavenger receptors in the control of infection of mice by S. aureus. Using double SR-A/CD36 knockout mice (S/C-KO) and S. aureus strain HG001, a clinically relevant non-mutagenized strain, we showed that the absence of these two scavenger receptors was protective in peritoneal infection. In contrast, the deletion of these two receptors was detrimental in pulmonary infection following intranasal instillation. For pulmonary infection, susceptible mice (S/C-KO) had more colony-forming units (CFU) in their broncho-alveolar lavages fluids, associated with increased recruitment of macrophages and neutrophils. For peritoneal infection, susceptible mice (wild-type) had more CFU in their blood, but recruited less macrophages and neutrophils in the peritoneal cavity than resistant mice. Exacerbated cytokine levels were often observed in the susceptible mice in the infected compartment as well as in the plasma. The exception was the enhanced compartmentalized expression of IL-1β for the resistant mice (S/C-KO) after peritoneal infection. A similar mirrored susceptibility to S. aureus infection was also observed for MARCO and TLR2. Marco and tlr2 -/- mice were more resistant to peritoneal infection but more susceptible to pulmonary infection than wild type mice. In conclusion, our results show that innate immune receptors can play distinct and opposite roles depending on the site of infection. Their presence is protective for local pulmonary infection, whereas it becomes detrimental in the peritoneal infection.