Injury enhances resistance to Escherichia coli infection by boosting innate immune system function

Burn-Induced Local and Systemic Immune Response: Systematic Review and Meta-Analysis of Animal Studies

As burn injuries are often followed by a derailed immune response and excessive inflammation, a thorough understanding of the occurring reactions is key to prevent secondary complications. This systematic review, that includes 247 animal studies, shows the post-burn response of 14 different immune cell types involved in immediate and long-term effects, in both wound tissue and circulation. Peripheral blood neutrophil and monocyte numbers increased directly after burns, whereas thrombocyte numbers increased near the end of the first week. Lymphocyte numbers, however, were decreased for at least two weeks. In burn wound tissue, neutrophil and macrophage numbers accumulated during the first three weeks. Burns also altered cellular functions as we found increased migratory potential of leukocytes, impaired antibacterial activity of neutrophils and enhanced inflammatory mediator production by macrophages. Neutrophil surges were positively associated with burn size and were highest in rats. Altogether, this comprehensive overview of the temporal immune cell dynamics shows that unlike normal wound healing, burn injury induces a long-lasting inflammatory response. It provides a fundamental research basis to improve experimental set-ups, burn care and outcome.

The journey of SARS-CoV-2 in human hosts: a review of immune responses, immunosuppression, and their consequences

Coronavirus disease 2019 (COVID-19) is a highly infectious viral disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Laboratory findings from a significant number of patients with COVID-19 indicate the occurrence of leukocytopenia, specifically lymphocytopenia. Moreover, infected patients can experience contrasting outcomes depending on lymphocytopenia status. Patients with resolved lymphocytopenia are more likely to recover, whereas critically ill patients with signs of unresolved lymphocytopenia develop severe complications, sometimes culminating in death. Why immunodepression manifests in patients with COVID-19 remains unclear. Therefore, the evaluation of clinical symptoms and laboratory findings from infected patients is critical for understanding the disease course and its consequences. In this review, we take a logical approach to unravel the reasons for immunodepression in patients with COVID-19. Following the footprints of the virus within host tissues, from entry to exit, we extrapolate the mechanisms underlying the phenomenon of immunodepression.

The Stress Hormone Cortisol Enhances Interferon-υ-Mediated Proinflammatory Responses of Human Immune Cells

BACKGROUND

Cortisol is a prototypical human stress hormone essential for life, yet the precise role of cortisol in the human stress response to injury or infection is still uncertain. Glucocorticoids (GCs) such as cortisol are widely understood to suppress inflammation and immunity. However, recent research shows that GCs also induce delayed immune effects manifesting as immune stimulation. In this study, we show that cortisol enhances the immune-stimulating effects of a prototypical proinflammatory cytokine, interferon-υ (IFN-υ). We tested the hypothesis that cortisol enhances IFN-υ-mediated proinflammatory responses of human mononuclear phagocytes (monocyte/macrophages [MOs]) stimulated by bacterial endotoxin (lipopolysaccharide [LPS]).

METHODS

Human MOs were cultured for 18 hours with or without IFN-υ and/or cortisol before LPS stimulation. MO differentiation factors granulocyte-macrophage colony stimulating factor (GM-CSF) or M-CSF were added to separate cultures. We also compared the inflammatory response with an acute, 4-hour MO incubation with IFN-υ plus cortisol and LPS to a delayed 18-hour incubation with cortisol before LPS exposure. MO activation was assessed by interleukin-6 (IL-6) release and by multiplex analysis of pro- and anti-inflammatory soluble mediators.

RESULTS

After the 18-hour incubation, we observed that cortisol significantly increased LPS-stimulated IL-6 release from IFN-υ-treated undifferentiated MOs. In GM-CSF-pretreated MOs, cortisol increased IFN-υ-mediated IL-6 release by >4-fold and release of the immune stimulant IFN-α2 (IFN-α2) by >3-fold, while suppressing release of the anti-inflammatory mediator, IL-1 receptor antagonist to 15% of control. These results were reversed by either the GC receptor antagonist RU486 or by an IFN-υ receptor type 1 antibody antagonist. Cortisol alone increased expression of the IFN-υ receptor type 1 on undifferentiated and GM-CSF-treated MOs. In contrast, an acute 4-hour incubation of MOs with IFN-υ and cortisol showed classic suppression of the IL-6 response to LPS.

CONCLUSIONS

These results reveal a surprisingly robust proinflammatory interaction between the human stress response hormone cortisol and the immune activating cytokine IFN-υ. The results support an emerging physiological model with an adaptive role for cortisol, wherein acute release of cortisol suppresses early proinflammatory responses but also primes immune cells for an augmented response to a subsequent immune challenge. These findings have broad clinical implications and provide an experimental framework to examine individual differences, mechanisms, and translational implications of cortisol-enhanced immune responses in humans.

One-hit wonder: Late after burn injury, granulocytes can clear one bacterial infection but cannot control a subsequent infection

OBJECTIVE

Burn injury induces an acute hyperactive immune response followed by a chronic immune dysregulation that leaves those afflicted susceptible to multiple secondary infections. Many murine models are able to recapitulate the acute immune response to burn injury, yet few models are able to recapitulate long-term immune suppression and thus chronic susceptibility to bacterial infections seen in burn patients. This has hindered the field, making evaluation of the mechanisms responsible for these susceptibilities difficult to study. Herein we describe a novel mouse model of burn injury that promotes chronic immune suppression allowing for susceptibility to primary and secondary infections and thus allows for the evaluation of associated mechanisms.

METHODS

C57Bl/6 mice receiving a full-thickness contact burn were infected with Pseudomonas aeruginosa 14 days (primary infection) and/or 17 days (secondary infection) after burn or sham injury. The survival, pulmonary and systemic bacterial load as well as frequency and function of innate immune cells (neutrophils and macrophages) were evaluated.

RESULTS

Following secondary infection, burn mice were less effective in clearance of bacteria compared to sham injured or burn mice following a primary infection. Following secondary infection both neutrophils and macrophages recruited to the airways exhibited reduced production of anti-bacterial reactive oxygen and nitrogen species and the pro-inflammatory cytokineIL-12 while macrophages demonstrated increased expression of the anti-inflammatory cytokine interleukin-10 compared to those from sham burned mice and/or burn mice receiving a primary infection. In addition the BALF from these mice contained significantly higher level so of the anti-inflammatory cytokine IL-4 compared to those from sham burned mice and/or burn mice receiving a primary infection.

CONCLUSIONS

Burn-mediated protection from infection is transient, with a secondary infection inducing immune protection to collapse. Repeated infection leads to increased neutrophil and macrophage numbers in the lungs late after burn injury, with diminished innate immune cell function and an increased anti-inflammatory cytokine environment.

Mild Traumatic Brain Injury in Mice Beneficially Alters Lung NK1R and Structural Protein Expression to Enhance Survival after Pseudomonas aeruginosa Infection

Mild traumatic brain injury (mTBI) in a murine model increases survival to a bacterial pulmonary challenge compared with blunt tail trauma (TT). We hypothesize substance P and its receptor, the neurokinin 1 receptor (NK1R; official name TACR1), play a role in the increased survival of mTBI mice. Mice were subjected to mTBI or TT, and 48 hours after trauma, the levels of NK1R mRNA and protein were significantly up-regulated in mTBI lungs. Examination of the lung 48 hours after injury by microarray showed significant differences in the expression of 433 gene sets between groups, most notably genes related to intercellular proteins. Despite down-regulated gene expression of connective proteins, the presence of an intact pulmonary vasculature was supported by normal histology and bronchoalveolar lavage protein levels. To determine whether these mTBI-induced lung changes benefited in vivo responses, two chemotactic stimuli (a CXCL1 chemokine and a live Pseudomonas aeruginosa infection) were administered 48 hours after trauma. For both stimuli, mTBI mice recruited more neutrophils to the lung 4 hours after instillation (CXCL1: mTBI = 6.3 ± 1.3 versus TT = 3.3 ± 0.7 neutrophils/mL; Pseudomonas aeruginosa: mTBI = 9.4 ± 1.4 versus TT = 5.3 ± 1.1 neutrophils/mL). This study demonstrates that the downstream consequences of mTBI on lung NK1R levels and connective protein expression enhance neutrophil recruitment to a stimulus that may contribute to increased survival.

Adding injury to infection: The relationship between injury status and genetic diversity of Theileria infecting plains zebra, Equus quagga

Asymptomatic tick-borne infections are a common feature in wild herbivores. In human-dominated habitats, snare injuries to wild herbivores are common and are likely to co-occur with enzootic infections. The influence of injury on pattern, course and outcome of enzootic infection in wild herbivores is unknown. We identified Theileria species infecting zebra and assessed the relationship between host injury-status and parasitaemia, parasite diversity and selection regimes. We also determined host leucocyte differential as this can reveal mechanisms by which injuries influence infections. Theileria infecting zebra was identified using PCR and sequencing of the V4 region of the 18 s rRNA gene and confirmed with phylogenetic analyses. The influence of injury status on parasite infection patterns, genetic diversity and selection were assessed using population genetic tools. Parasitaemia estimated from prevalence and leucocyte differential were determined from microscopic examination of Giemsa stained thin blood smears. Phylogenetic and sequence analyses revealed that the zebra population studied was infected with three Theileria equi haplotypes. Parasitaemia was lower among injured compared to non-injured animals and lower during dry than wet season. Mean (±SD) genetic diversity was 0.386 (±0.128) in injured and 0.513 (±0.144) in non-injured zebra (P = .549). Neutrality tests indicated that T. equi is under strong purifying selection in injured females (Li & Fu's D* = -2.037) and demographic expansion in all zebra during the wet season (Tajima D = -1.904). Injured zebras had a higher median per cent of neutrophils (64% vs 37%) a lower median per cent of basophils (0% vs 1%) and eosinophils (2% vs 4.5%) than non-injured animals, suggesting a heightened immune response and a shift from a Th2 to Th1 T-Cell response favoring the elimination of intracellular parasites in injured animals. This study demonstrates the utility of population genetics in revealing factors influencing parasite diversity and infection patterns.

Aged Human Stored Red Blood Cell Supernatant Inhibits Macrophage Phagocytosis in an HMGB1 Dependent Manner After Trauma in a Murine Model

Red blood cell transfusions in the setting of trauma are a double-edged sword, as it is a necessary component for life-sustaining treatment in massive hemorrhagic shock, but also associated with increased risk for nosocomial infections and immune suppression. The mechanisms surrounding this immune suppression are unclear. Using supernatant from human packed red blood cell (RBC), we demonstrate that clearance of Escherichia coli by macrophages is inhibited both in vitro and in vivo using a murine model of trauma and hemorrhagic shock. We further explore the mechanism of this inhibition by demonstrating that human-stored RBCs contain soluble high-mobility group box 1 protein (HMGB1) that increases throughout storage. HMGB1 derived from the supernatant of human-stored RBCs was shown to inhibit bacterial clearance, as neutralizing antibodies to HMGB1 restored the ability of macrophages to clear bacteria. These findings demonstrate that extracellular HMGB1 within stored RBCs could be one factor leading to immune suppression following transfusion in the trauma setting.

Glucocorticoids enhance the in vivo migratory response of human monocytes

Glucocorticoids (GCs) are best known for their potent anti-inflammatory effects. However, an emerging model for glucocorticoid (GC) regulation of in vivo inflammation also includes a delayed, preparatory effect that manifests as enhanced inflammation following exposure to an inflammatory stimulus. When GCs are transiently elevated in vivo following exposure to a stressful event, this model proposes that a subsequent period of increased inflammatory responsiveness is adaptive because it enhances resistance to a subsequent stressor. In the present study, we examined the migratory response of human monocytes/macrophages following transient in vivo exposure to stress-associated concentrations of cortisol. Participants were administered cortisol for 6h to elevate in vivo cortisol levels to approximate those observed during major systemic stress. Monocytes in peripheral blood and macrophages in sterile inflammatory tissue (skin blisters) were studied before and after exposure to cortisol or placebo. We found that exposure to cortisol induced transient upregulation of monocyte mRNA for CCR2, the receptor for monocyte chemotactic protein-1 (MCP-1/CCL2) as well as for the chemokine receptor CX3CR1. At the same time, mRNA for the transcription factor IκBα was decreased. Monocyte surface expression of CCR2 but not CX3CR1 increased in the first 24h after cortisol exposure. Transient exposure to cortisol also led to an increased number of macrophages and neutrophils in fluid derived from a sterile inflammatory site in vivo. These findings suggest that the delayed, pro-inflammatory effects of cortisol on the human inflammatory responses may include enhanced localization of effector cells at sites of in vivo inflammation.

Substance P mediates reduced pneumonia rates after traumatic brain injury

OBJECTIVES

Traumatic brain injury results in significant morbidity and mortality and is associated with infectious complications, particularly pneumonia. However, whether traumatic brain injury directly impacts the host response to pneumonia is unknown. The objective of this study was to determine the nature of the relationship between traumatic brain injury and the prevalence of pneumonia in trauma patients and investigate the mechanism of this relationship using a murine model of traumatic brain injury with pneumonia.

DESIGN

Data from the National Trauma Data Bank and a murine model of traumatic brain injury with postinjury pneumonia.

SETTING

Academic medical centers in Cincinnati, OH, and Boston, MA.

PATIENTS/SUBJECTS

Trauma patients in the National Trauma Data Bank with a hospital length of stay greater than 2 days, age of at least 18 years at admission, and a blunt mechanism of injury. Subjects were female ICR mice 8-10 weeks old.

INTERVENTIONS

Administration of a substance P receptor antagonist in mice.

MEASUREMENTS AND MAIN RESULTS

Pneumonia rates were measured in trauma patients before and after risk adjustment using propensity scoring. In addition, survival and pulmonary inflammation were measured in mice undergoing traumatic brain injury with or without pneumonia. After risk adjustment, we found that traumatic brain injury patients had significantly lower rates of pneumonia compared to blunt trauma patients without traumatic brain injury. A murine model of traumatic brain injury reproduced these clinical findings with mice subjected to traumatic brain injury demonstrating increased bacterial clearance and survival after induction of pneumonia. To determine the mechanisms responsible for this improvement, the substance P receptor was blocked in mice after traumatic brain injury. This treatment abrogated the traumatic brain injury-associated increases in bacterial clearance and survival.

CONCLUSIONS

The data demonstrate that patients with traumatic brain injury have lower rates of pneumonia compared to non-head-injured trauma patients and suggest that the mechanism of this effect occurs through traumatic brain injury-induced release of substance P, which improves innate immunity to decrease pneumonia.

Immune response to traumatic injury: harmony and discordance of immune system homeostasis

Trauma remains one of the leading causes of death worldwide. Traumatic injury disrupts immune system homeostasis and may predispose patients to opportunistic infections and inflammatory complications. Prevention of multiple organ dysfunction syndrome due to septic complications following severe trauma is a challenging problem. Following severe injury, the immune system usually tends toward a pro-inflammatory phenotype and then changes to a counter-inflammatory phenotype. This immune system homeostasis is believed to be a protective response based on the balance between the innate and adaptive immune systems. We reported that injury activates inflammasomes and primes Toll-like receptors. The primed innate immune system is prepared for a rapid and strong antimicrobial immune defense. However, trauma can also develop the "two-hit" response phenotype. We also reported that injury augments regulatory T cell activity, which can control the "two-hit" response phenotype in trauma. We discuss the current idea that traumatic injury induces a unique type of innate and adaptive immune response that may be triggered by damage-associated molecular pattern molecules, which are a combination of endogenous danger signal molecules that include alarmins and pathogen-associated molecular pattern molecules.

G-CSF drives a posttraumatic immune program that protects the host from infection

Traumatic injury is generally considered to have a suppressive effect on the immune system, resulting in increased susceptibility to infection. Paradoxically, we found that thermal injury to the skin induced a robust time-dependent protection of mice from a lethal Klebsiella pneumoniae pulmonary challenge. The protective response was neutrophil dependent and temporally associated with a systemic increase in neutrophils resulting from a reprioritization of hematopoiesis toward myeloid lineages. A prominent and specific activation of STAT3 in the bone marrow preceded the myeloid shift in that compartment, in association with durable increases in STAT3 activating serum cytokines G-CSF and IL-6. Neutralization of the postburn increase in serum G-CSF largely blocked STAT3 activation in marrow cells, reversing the hematopoietic changes and systemic neutrophilia. Daily administration of rG-CSF was sufficient to recapitulate the changes induced by injury including hematopoietic reprioritization and protection from pulmonary challenge with K. pneumoniae. Analysis of posttraumatic gene expression patterns in humans reveals that they are also consistent with a role for G-CSF as a switch that activates innate immune responses and suppresses adaptive immune responses. Our findings suggest that the G-CSF STAT3 axis constitutes a key protective mechanism induced by injury to reduce the risk for posttraumatic infection.

Pulmonary contusion is associated with toll-like receptor 4 upregulation and decreased susceptibility to pseudomonas pneumonia in a mouse model

Pulmonary contusion is a major cause of respiratory failure in trauma patients. This injury frequently leads to immune suppression and infectious complications such as pneumonia. The mechanism whereby trauma leads to an immune-suppressed state is poorly understood. To further study this phenomenon, we developed an animal model of pulmonary contusion (PC) complicated by pneumonia and assessed the effect of PC and pneumonia on toll-like receptor expression in alveolar macrophages. Using a mouse model, PC was induced on the right lung, and pneumonia was induced with Pseudomonas aeruginosa (Pa) injected intratracheally 48 h after injury. Susceptibility to pneumonia was assessed by mortality at 7 days. Uninjured animals were used as controls. Bronchoalveolar lavage fluid and blood were assayed 48 h after injury and 24 h after Pa instillation to look at markers of systemic inflammation. Toll-like receptor expression in the initial inflammatory response was analyzed by flow cytometry. Unexpectedly, injured animals subjected to intratracheal injection of Pa at 48 h after PC demonstrated increased survival compared with uninjured animals. Bronchoalveolar lavage cytokine expression was increased significantly after Pa administration but not after PC alone. Toll-like receptor 4 expression on alveolar macrophages was significantly elevated in the injured group compared with sham but not in neutrophils. Animals subjected to PC are more resistant to mortality from infection with Pa and display an enhanced cytokine response when subsequently subjected to Pa. Increased expression of toll-like receptor 4 on alveolar macrophages and enhanced innate immunity are a possible mechanism of increased cytokine production and decreased susceptibility to pneumonia.

The burn wound inflammatory response is influenced by midazolam

Burn patients requiring hospitalization are often treated for anxiety with benzodiazepines (BDZs). Benzodiazepines are reported to influence immune system function. Immune system alterations are a major cause of burn-induced mortality. We wanted to determine whether the BDZ, midazolam given daily at an anxiolytic dose, had any influence on the burn injury-induced inflammatory response in the blood and wound. Mice received a 15% total body surface area flame burn and received either midazolam 1 mg/kg i.p. or saline 0.1 ml daily. Blood and skin wounds were harvested 24 h after injection on post-burn day 2, 3, 7, or 8. Mice treated with midazolam had significantly lower serum IL-1β (p=0.002), TNF-α (p=0.002), IL-6 (p=0.016), IL-10 (p=0.009), and TGF-β (p=0.004) than saline-treated mice, with little impact on serum chemokine levels. In the wound, TNF-α and IL-10 were the only cytokines significantly influenced by the drug, being lower (p=0.018) and higher (p=0.006), respectively. The chemokines in the wound influenced significantly by midazolam were MIP-1α, MIP-1β, and MIP-2 while MCP-1 and KC were not. There were more inflammatory cells at the burn wound margin in midazolam-treated mice on post-burn day 3. Although serum nitrate/nitrite was significantly increased by midazolam (p=0.03), both eNOS and iNOS mRNA expression in the wound were similar to the saline group. We found that midazolam given daily after burn injury significantly influenced the inflammatory response. The clinical implications of these findings on wound healing and shock following burn injury, especially larger burns, deserve further investigation.

Prior thermal injury accelerates endotoxin-induced inflammatory cytokine production and intestinal nuclear factor-κB activation in mice

The objective of this study was to increase the understanding of the "second-hit" response in thermal injury. The authors hypothesized that prior thermal injury increases the endotoxin-induced inflammatory response of intestinal mucosa. Mice underwent sham or 25% TBSA scald injury. Seven days after injury, mice were injected with lipopolysaccharide. Blood, jejunum, and colon specimens were obtained at intervals. Serum, jejunal, and colon inflammatory cytokine levels were measured by enzyme-linked immunosorbent assay. Jejunal and colon nuclear factor (NF)-κB activation was measured by electrophoretic mobility shift assay. After remote thermal injury, lipopolysaccharide exposure led to an acute increase in serum interleukin (IL)-6, IL-10, and chemokine keratinocyte-derived chemokine (KC) levels. This correlated with lipopolysaccharide-induced increased IL-6 in colon and chemokine KC in the jejunum and colon in burned mice when compared with sham-injured mice. Lipopolysaccharide-induced NF-κB activation occurred more rapidly in jejunum and colon from burned mice compared with sham-injured mice. Prior thermal injury accelerates lipopolysaccharide-induced inflammatory cytokine production systemically in jejunum and colon. The "second hit" of lipopolysaccharide led to earlier intestinal NF-κB activation in burned mice compared with sham-injured mice. These results indicate that there is a heightened inflammatory response by jejunum and colon in response to a "second hit" of lipopolysaccharide after burn injury.

Trauma equals danger--damage control by the immune system

Traumatic injuries induce a complex host response that disrupts immune system homeostasis and predisposes patients to opportunistic infections and inflammatory complications. The response to injuries varies considerably by type and severity, as well as by individual variables, such as age, sex, and genetics. These variables make studying the impact of trauma on the immune system challenging. Nevertheless, advances have been made in understanding how injuries influence immune system function as well as the immune cells and pathways involved in regulating the response to injuries. This review provides an overview of current knowledge about how traumatic injuries affect immune system phenotype and function. We discuss the current ideas that traumatic injuries induce a unique type of a response that may be triggered by a combination of endogenous danger signals, including alarmins, DAMPs, self-antigens, and cytokines. Additionally, we review and propose strategies for redirecting injury responses to help restore immune system homeostasis.

Cecal ligation and puncture followed by methicillin-resistant Staphylococcus aureus pneumonia increases mortality in mice and blunts production of local and systemic cytokines

Mortality in the intensive care unit frequently results from the synergistic effect of two temporally distinct infections. This study examined the pathophysiology of a new model of intra-abdominal sepsis followed by methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. Mice underwent cecal ligation and puncture (CLP) or sham laparotomy followed 3 days later by an intratracheal injection of MRSA or saline. Both CLP/saline and sham/MRSA mice had 100% survival, whereas animals with CLP followed by MRSA pneumonia had 67% 7-day survival. Animals subjected to CLP/MRSA had increased bronchoalveolar lavage concentrations of MRSA compared with sham/MRSA animals. Animals subjected to sham/MRSA pneumonia had increased bronchoalveolar lavage levels of interleukin 6 (IL-6), tumor necrosis factor α, and granulocyte colony-stimulating factor compared with those given intratracheal saline, whereas CLP/MRSA mice had a blunted local inflammatory response with markedly decreased cytokine levels. Similarly, animals subjected to CLP/saline had increased peritoneal lavage levels of IL-6 and IL-1β compared with those subjected to sham laparotomy, whereas this response was blunted in CLP/MRSA mice. Systemic cytokines were upregulated in both CLP/saline and sham/MRSA mice, and this was blunted by the combination of CLP/MRSA. In contrast, no synergistic effect on pneumonia severity, white blood cell count, or lymphocyte apoptosis was identified in CLP/MRSA mice compared with animals with either insult in isolation. These results indicate that a clinically relevant model of CLP followed by MRSA pneumonia causes higher mortality than could have been predicted from studying either infection in isolation, and this was associated with a blunted local (pulmonary and peritoneal) and systemic inflammatory response and decreased ability to clear infection.

A protective role for inflammasome activation following injury

The inflammasome is activated in response to pathogen or endogenous danger signals and acts as an initiator and mediator of inflammatory reactions. In this study, we wished to identify whether the inflammasome is activated in vivo by injury. And if so, we wanted to characterize the kinetics, the immune cell distribution, and the functional impact of inflammasome activation on the injury response. Because caspase-1 activation is the final product of the inflammasome pathway, we used cleaved caspase-1 p10 and p20 as a measure for inflammasome activation in cells. We first developed a procedure to stain for caspase-1 p10 and p20 by flow cytometry (FACS) in lipopolysaccharide + adenosine triphosphate-stimulated spleen cells. This method for measuring caspase-1 activation was validated using FLICA (fluorochrome inhibitor of caspase), a fluorescently tagged specific binding reagent for activated caspase-1. Once validated by in vitro studies, we measured caspase-1 activation by FACS in immune cell subsets prepared from the lymph nodes and spleens of sham- or burn-injured mice at different time points. Lastly, the functional significance of inflammasome activation following burn injury was tested in mice treated with the specific caspase-1 inhibitor, AC-YVAD-CMK. The results of in vitro studies indicated that adenosine triphosphate and lipopolysaccharide stimulation induced significant caspase-1 activation in dendritic cells, macrophages, and natural killer (NK) cells. This approach also revealed caspase-1 activation in CD4 and CD8 T cells as well as B cells. We then measured caspase-1 activation in cells prepared from the lymph nodes and spleens of sham- or burn-injured mice. Significant caspase-1 activation was detected in macrophages and dendritic cells by 4 h after injury and peaked by day 1 after injury. FLICA staining confirmed that caspase-1 activation occurred in these cells at 1 day after injury. We also found significant injury-induced caspase-1 activation in NK cells, CD4 T cells, and B cells, but CD8 T cells did not demonstrate caspase-1 activation. Surprisingly, we found that blocking caspase-1 activation with AC-YVAD-CMK in vivo caused significantly higher mortality in burn-injured mice (P < 0.01). Taken together, these findings document that injury induces inflammasome activation in many immune cell subsets, but primarily in macrophages, and that inflammasome activation plays a protective role in the host response to severe injury.

Perioperative nutritional support: immunonutrition, probiotics, and anabolic steroids

Nutritional support in surgical patients has evolved from simple provision of adequate calories to retard loss of lean body mass to the provision of specific nutrients in an attempt to manipulate metabolic and immune responses. Although still limited, the current understanding of this complex subject indicates that the type, route, amount, and composition of nutritional support provided to patients can affect their outcome. Further studies are, however, needed to better characterize the exact nutritional support that is most beneficial for a specific disease state and a specific patient.

Injury-induced GR-1+ macrophage expansion and activation occurs independently of CD4 T-cell influence

Burn injury initiates an enhanced inflammatory condition referred to as the systemic inflammatory response syndrome or the two-hit response phenotype. Prior reports indicated that macrophages respond to injury and demonstrate a heightened reactivity to Toll-like receptor stimulation. Since we and others observed a significant increase in splenic GR-1 F4/80 CD11b macrophages in burn-injured mice, we wished to test if these macrophages might be the primary macrophage subset that shows heightened LPS reactivity. We report here that burn injury promoted higher level TNF-α expression in GR-1, but not GR-1 macrophages, after LPS activation both in vivo and ex vivo. We next tested whether CD4 T cells, which are known to suppress injury-induced inflammatory responses, might control the activation and expansion of GR-1 macrophages. Interestingly, we found that GR-1 macrophage expansion and LPS-induced TNF-α expression were not significantly different between wild-type and CD4 T cell-deficient CD4(-/-) mice. However, further investigations showed that LPS-induced TNF-α production was significantly influenced by CD4 T cells. Taken together, these data indicate that GR-1 F4/80 CD11b macrophages represent the primary macrophage subset that expands in response to burn injury and that CD4 T cells do not influence the GR-1 macrophage expansion process, but do suppress LPS-induced TNF-α production. These data suggest that modulating GR-1 macrophage activation as well as CD4 T cell responses after severe injury may help control the development of systemic inflammatory response syndrome and the two-hit response phenotype.

Neutrophils, not monocyte/macrophages, are the major splenic source of postburn IL-10

Burn induces myeloid-derived suppressor cells (MDSCs), a heterogeneous population of immature polymorphonuclear neutrophils (PMNs) and monocytes, which protect against infection. Previous work from our laboratory demonstrated that inflammatory monocytes (iMos) were the major MDSC source of TNF-α in the postburn spleen, and we hypothesized that they were also the major source of postburn IL-10. To test this hypothesis, we examined cytokine production by postburn CCR2 knockout (KO) mice, which have fewer iMos than burn wild-type (WT) splenocytes, but equal numbers of PMNs and F4/80 macrophages. Using cell sorting and/or intracellular cytokine techniques, we examined IL-10 production by postburn PMNs and iMos. Finally, we compared IL-10 production by postburn PMNs and iMos with culture-derived MDSCs. Splenocytes from postburn CCR2 KO mice produced less IL-6 and TNF-α than WT burn splenocytes in response to LPS, but KO and WT burn splenocytes produced equal amounts of IL-10 in response to peptidoglycan. Depletion of PMNs from postburn splenocytes led to reductions in IL-10 and increases in IL-6 and TNF-α in response to peptidoglycan, but not in response to LPS. Sorting or intracellular cytokine techniques gave consistent results: Burn PMNs made more IL-10 than sham PMNs and also more IL-10 than burn or sham iMos. Polymorphonuclear neutrophil and iMos subpopulations from culture-derived MDSCs produced the same cytokine profiles in response to LPS and peptidoglycan as did the PMNs and iMos from postburn spleens: PMNs made IL-10, whereas iMos made IL-6. Finally, LPS-induced mortality of burn mice was made worse by anti-Gr-1 depletion of all PMNs and 66% of iMos from burn mice. This suggests that PMNs play a primarily anti-inflammatory role in vitro and in vivo.

CLP-induced impairment of innate immune function is caused by exposure to the cecal lumenal contents and not the tissue trauma or tissue ischemia/necrosis components

When mice are subjected to a Pseudomonas aeruginosa challenge 5 days after cecal ligation and puncture (CLP), clearance of the Pseudomonas is diminished when compared to sham mice. The object of this study was to determine which component(s) of CLP contributed to the impairment of the innate immune response. Mice subjected to either trauma alone or cecal ischemia/necrosis alone did not have impaired ability to clear a subsequent Pseudomonas challenge (determined by colony-forming units (cfu's) after culture of spleen tissue). However, mice subjected to abdominal contamination with heat-killed cecal contents had reduced ability to clear the subsequent Pseudomonas challenge. In contrast to normobiotic mice, neither CLP performed in germ-free mice nor abdominal contamination of mice with cecal contents from germ-free mice adversely affected clearance of a subsequent Pseudomonas challenge. These data suggest that suppressed immune function after CLP is due to exposure to microbial ligands within the cecal lumen rather than tissue trauma, ischemia, or necrosis. However, suppression of immune function did not appear to be due to exposure to LPS as TLR4-deficient mice subject to abdominal contamination with cecal contents had diminished clearance of a Pseudomonas challenge similar to that seen in wild-type mice.

A ribonucleotide reductase inhibitor reverses burn-induced inflammatory defects

Immature myeloid cells have been implicated as a source of postburn inflammation, and the appearance of these cells correlates with enhanced upregulation of hematopoiesis. The role of proliferative cells in postburn immune changes has not been directly tested. Gemcitabine, a ribonucleotide reductase inhibitor, has been shown to deplete proliferative immature myeloid cells in tumor models while sparing mature cells, leading to restored lymphocyte function and tumor regression. We treated burn mice at postburn day 6 (PBD6) with 120 mg/kg gemcitabine. On PBD8, splenocytes were taken and stimulated with LPS, peptidoglycan, or concanavalin A. The blood and spleen cell populations were enumerated by flow cytometry or automated cell counter. In addition, mice treated with gemcitabine were given LPS or infected with Pseudomonas aeruginosa at PBD8, and mortality was monitored. Gemcitabine depleted burn-induced polymorphonuclear leukocytes and inflammatory monocytes without affecting mature F4/80 macrophages. This was accompanied by reduced TNFα, IL-6, and IL-10 production by burn splenocytes. Burn splenocytes stimulated with mitogens exhibited increased nitric oxide production relative to sham mice. In vivo treatment of burn mice with gemcitabine blocked these burn-induced changes without damaging lymphocyte function. Treatment of burn mice with gemcitabine ameliorated burn-induced susceptibility to LPS and infiltration of polymorphonuclear leukocytes into the liver and lung. Finally, gemcitabine treatment blocked the protective effect of burn injury upon P. aeruginosa infection. Our report shows that proliferative cells are major drivers of postburn immune changes and provides evidence that implicates immature myeloid cells in these processes.

Cortisol exerts bi-phasic regulation of inflammation in humans

Natural and synthetic glucocorticoids (GCs) have been used for decades to suppress inflammation. In this paper, we re-examine the role of the endogenous GC, cortisol, as a primary homeostatic regulator of the human inflammatory response to injury. Our data show that cortisol regulation of innate immunity can be both pro-inflammatory and anti-inflammatory. Using a human model of in vivo cortisol depletion, we first show that baseline (diurnal) cortisol concentrations do not exert an anti-inflammatory effect. This is the first clue that cortisol regulation of inflammation is not represented by a linear dose-response relationship. We next show in surgical patients that cortisol does exert an acute anti-inflammatory effect over a carefully regulated range of physiologic cortisol concentrations. Finally, transient pre-treatment of healthy humans with cortisol induces a bi-phasic response during a later, delayed systemic inflammatory response: an intermediate cortisol concentration augments inflammation while a high cortisol concentration is neither pro- nor anti-inflammatory. Based on these findings and the work of others, we propose a new paradigm that identifies cortisol regulation of human inflammation as both dualistic-it is pro- and anti-inflammatory-and dynamic, it evolves over time.

Effect of major abdominal surgery on the host immune response to infection

PURPOSE OF REVIEW

The present review summarizes key studies on the effects of major abdominal surgery on the host response to infection published during the last 18 months.

RECENT FINDINGS

Surgical trauma causes stereotyped systemic proinflammatory and compensatory anti-inflammatory reactions. It is leukocyte reprogramming rather than general immune suppression. The list of recent findings is long. Preoperative infectious challenge was found to increase survival. Obesity is associated with increased production of interleukin-17A in peritonitis. Abdominal surgery alters expression of toll-like receptors (TLRs). The acute phase reaction down-regulates the transcription factor carbohydrate response element binding protein. Myosin light chain kinase activation is a final pathway of acute tight junction regulation of gut barrier and zonula occludens 1 protein is an essential effector. The brain is involved in regulating the immune and gut system. Elimination of lipopolysaccharide is challenging. Th1/Th2 ratio is lowered in patients with postoperative complications. Cholinergic anti-inflammatory pathways can inhibit tissue damage. The new substance PXL01 prevents adhesions. Postoperative infection causes incisional hernias. Hypothermia reduced human leukocyte antigen DR surface expression and delayed tumor necrosis factor clearance. Systems biology identified interferon regulatory factor 3 as the negative regulator of TLR signaling. Protective immunity could contribute defeating surgical infections.

SUMMARY

Systemic inflammation is the usual response to trauma. All organs seem to be involved and linked up in cybernetic systems aiming at reconstitution of homeostasis. Although knowledge is still fragmentary, it is already difficult to integrate known facts and new technologies are required for information processing. Defining criteria to develop therapeutic strategies requires much more insight into molecular mechanisms and cybernetics of organ systems.

Early infection during burn-induced inflammatory response results in increased mortality and p38-mediated neutrophil dysfunction

Following burn injury, the host is susceptible to bacterial infections normally cleared by healthy patients. We hypothesized that during the systemic immune response that follows scald injury, the host's altered immune status increases infection susceptibility. Using a murine model of scald injury under inhaled anesthesia followed by intraperitoneal infection, we observed increased neutrophil numbers and function at postburn day (PBD) 1 compared with sham-burned and PBD4 mice. Further, increased mortality, bacteremia, and serum IL-6 were observed in PBD1 mice after Pseudomonas aeruginosa (PA) infection compared with sham-burned and PBD4 mice infected with PA. To examine these disparate responses, we investigated neutrophils isolated at 5 and 24 h following PA infection from PBD1 and sham-burned mice. Five hours after infection, there was no significant difference in number of recruited neutrophils; however, neutrophils from injured mice had decreased activation, active-p38, and oxidative burst compared with sham-burned mice. In direct contrast, 24 h after infection, we observed increased numbers, active-p38, and oxidative burst of neutrophils from PBD1 mice. Finally, we demonstrated that in neutrophils isolated from PBD1 mice, the observed increase in oxidative burst was p38 dependent. Altogether, neutrophil activation and function from thermally injured mice are initially delayed and later exacerbated by a p38-dependent mechanism. This mechanism is likely key to the observed increase in bacterial load and mortality of PBD1 mice infected with PA.

Selective effect of burn injury on splenic CD11c(+) dendritic cells and CD8alpha(+)CD4(-)CD11c(+) dendritic cell subsets

Burn injury causes an immunosuppression associated with suppressed adaptive immune function. Dendritic cells (DCs) are APCs for which signaling via their Toll-like receptors (TLRs) induces their maturation and activation, which is essential for the adaptive immune response. In this study, we examined if burn injury alters the TLR activity of splenic DCs. After injury, we noticed that DC functions were impaired, characterized by a suppressed capacity to prime naive T cells when triggering the TLR4 signaling cascade using specific ligands (LPS or rHSP60). The observed perturbations on LPS-primed DCs isolated from burned mice exhibited significantly diminished IL-12p40 production and enhanced IL-10 secretion-associated impairment in mitogen-activated protein kinase activation. Interestingly, we observed a decrease of TLR4/MD-2 expression on the CD8alpha(+) DC subset that persisted following LPS stimulation. The altered TLR4 expression on LPS-stimulated CD8alpha(+) DCs was associated with reduced capacity to produce IL-12 after stimulation. Our results suggested that TLR4 reactivity on DCs, especially CD8alpha(+) DCs, is disturbed after burn injury.

Pretreatment with stress cortisol enhances the human systemic inflammatory response to bacterial endotoxin

OBJECTIVE

There is continuing controversy regarding the effect of glucocorticoids on a systemic inflammatory process. Based ona model of glucocorticoid action that includes both pro- and anti-inflammatory effects, we used the human experimental endotoxemia model to test the hypothesis that a transient elevation of plasma cortisol to stress-associated levels would enhance a subsequent (delayed) systemic inflammatory response to bacterial endotoxin.

DESIGN

Prospective, randomized, double-blind, placebo-controlled clinical investigation.

SETTING

Academic medical center.

SUBJECTS

Thirty-six healthy human volunteers.

INTERVENTIONS

Participants were randomized to receive a 6-hr intravenous infusion of saline (control), an intermediate dose of cortisol (Cort80; 6.3 mg/hr/70 kg), or a high dose of cortisol (Cort160; 12.6 mg/hr/70 kg) on day 1. On day 2, participants received an intravenous injection of 2 ng/kg Escherichia coli endotoxin followed by serial measurements of plasma cytokine concentrations.

MEASUREMENTS AND MAIN RESULTS

Baseline participant characteristics and cortisol and cytokine concentrations were similar in all three groups. The plasma cortisol response to endotoxemia on day 2 was similar in all three groups. The interleukin-6 response to endotoxemia was significantly increased in the Cort80 Group compared with the control Group (p = .004), whereas the interleukin-10 response was significantly suppressed (p = .034). Corresponding results for the Cort160 Group were not significantly different from control Group values.

CONCLUSIONS

In this study, transient elevation of in vivo cortisol concentrations to levels that are observed during major systemic stress enhanced a subsequent, delayed in vivo inflammatory response to endotoxin. This appeared to be a dose-dependent effect that was more prominent at intermediate concentrations of cortisol than at higher concentrations of cortisol.

Kinetics of lethal factor and poly-D-glutamic acid antigenemia during inhalation anthrax in rhesus macaques

Systemic anthrax manifests as toxemia, rapidly disseminating septicemia, immune collapse, and death. Virulence factors include the anti-phagocytic gamma-linked poly-d-glutamic acid (PGA) capsule and two binary toxins, complexes of protective antigen (PA) with lethal factor (LF) and edema factor. We report the characterization of LF, PA, and PGA levels during the course of inhalation anthrax in five rhesus macaques. We describe bacteremia, blood differentials, and detection of the PA gene (pagA) by PCR analysis of the blood as confirmation of infection. For four of five animals tested, LF exhibited a triphasic kinetic profile. LF levels (mean +/- standard error [SE] between animals) were low at 24 h postchallenge (0.03 +/- 1.82 ng/ml), increased at 48 h to 39.53 +/- 0.12 ng/ml (phase 1), declined at 72 h to 13.31 +/- 0.24 ng/ml (phase 2), and increased at 96 h (82.78 +/- 2.01 ng/ml) and 120 h (185.12 +/- 5.68 ng/ml; phase 3). The fifth animal had an extended phase 2. PGA levels were triphasic; they were nondetectable at 24 h, increased at 48 h (2,037 +/- 2 ng/ml), declined at 72 h (14 +/- 0.2 ng/ml), and then increased at 96 h (3,401 +/- 8 ng/ml) and 120 h (6,004 +/- 187 ng/ml). Bacteremia was also triphasic: positive at 48 h, negative at 72 h, and positive at euthanasia. Blood neutrophils increased from preexposure (34.4% +/- 0.13%) to 48 h (75.6% +/- 0.08%) and declined at 72 h (62.4% +/- 0.05%). The 72-h declines may establish a "go/no go" turning point in infection, after which systemic bacteremia ensues and the host's condition deteriorates. This study emphasizes the value of LF detection as a tool for early diagnosis of inhalation anthrax before the onset of fulminant systemic infection.

Differential immunological phenotypes are exhibited after scald and flame burns

A dysfunctional immune system is known to be part of the pathophysiology after burn trauma. However, reports that support this have used a variety of methods, with numerous variables, to induce thermal injury. We hypothesized that, all other parameters being equal, an injury infliction by a scald would yield different immunological responses than one inflicted by a flame. Here, we demonstrated that both burn methods produced a full-thickness burn, yet there was more of an increase in subdermal temperature, hematocrit, mortality, and serum IL-6 concentrations associated with the scald burn. On postinjury day 1, the scald-burned mice showed diminished lymphocyte numbers, interferon gamma production, and lymphocyte T-bet expression as compared with sham- and flame-burned mice. On postburn day 8, spleens from both sets of thermally injured animals showed an increase in proinflammatory myeloid cells as compared with sham-burned mice. Furthermore, the T-cell numbers, T-bet expression, and phenotype were changed such that interferon gamma production was higher in scald-burned mice than in sham- and flame-burned mice. Altogether, the data show that differential immunological phenotypes were observed depending on the thermal injury method used.

Mechanism of the salutary effects of estrogen on kupffer cell phagocytic capacity following trauma-hemorrhage: pivotal role of Akt activation

Kupffer cells are macrophages in the liver whose major role is to clear circulating pathogens. Decreased phagocytic capacity of Kupffer cells may result in severe systemic infection. We tested the hypothesis that the depressed Kupffer cell phagocytic capacity following trauma-hemorrhage is enhanced by estrogen administration and this occurs due to maintenance of Fc receptor expression and cellular ATP content via the activation of Akt. Male C3H/HeN mice were subjected to sham operation or trauma-hemorrhage and sacrificed 2 h thereafter. Estrogen, with or without an estrogen receptor antagonist (ICI 182,780), a PI3K inhibitor (Wortmannin), or vehicle, was injected during resuscitation. Kupffer cell phagocytic capacity was tested in vivo. The expression of Fc receptors, of Akt phosphorylation, of p38 MAPK phosphorylation, of DNA binding activity of NF-kappaB and ATP content of Kupffer cells were also determined. Trauma-hemorrhage suppressed Kupffer cell phagocytosis by decreasing Fc receptor expression and Akt activation; however, it induced p38 MAPK activation and increased NF-kappaB activity. Cellular ATP levels were also decreased following trauma-hemorrhage. Administration of estrogen following trauma-hemorrhage increased phospho-Akt levels and normalized all the parameters described as well as plasma levels of TNF-alpha, IL-6, and IL-10. Coadministration of ICI 182,780 or Wortmannin abolished the beneficial effects of estrogen in improving the phagocytic capacity of Kupffer cells following trauma-hemorrhage. Thus, activation of Akt plays a crucial role in mediating the salutary effect of estrogen in restoring trauma-hemorrhage-induced suppression of Kupffer cell phagocytosis.