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

Kinetics of Inflammatory Mediators in the Immune Response to Burn Injury: Systematic Review and Meta-Analysis of Animal Studies

Burns are often accompanied by a dysfunctional immune response, which can lead to systemic inflammation, shock, and excessive scarring. The objective of this study was to provide insight into inflammatory pathways associated with burn-related complications. Because detailed information on the various inflammatory mediators is scattered over individual studies, we systematically reviewed animal experimental data for all reported inflammatory mediators. Meta-analyses of 352 studies revealed a strong increase in cytokines, chemokines, and growth factors, particularly 19 mediators in blood and 12 in burn tissue. Temporal kinetics showed long-lasting surges of proinflammatory cytokines in blood and burn tissue. Significant time-dependent effects were seen for IL-1β, IL-6, TGF-β1, and CCL2. The response of anti-inflammatory mediators was limited. Burn technique had a profound impact on systemic response levels. Large burn size and scalds further increased systemic, but not local inflammation. Animal characteristics greatly affected inflammation, for example, IL-1β, IL-6, and TNF-α levels were highest in young, male rats. Time-dependent effects and dissimilarities in response demonstrate the importance of appropriate study design. Collectively, this review presents a general overview of the burn-induced immune response exposing inflammatory pathways that could be targeted through immunotherapy for burn patients and provides guidance for experimental set-ups to advance burn research.

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 Consequences of Aging On the Response to Injury and Critical Illness

Changing demographic trends have led to an increase in the overall geriatric trauma patient volume. Furthermore, the intersection of aging and injury can be problematic because geriatric patients have multiple comorbidities, geriatric-specific syndromes, and reduced physiological reserve. Despite mounting evidence that frail geriatric patients have inferior outcomes following trauma, very few studies have examined the effect of aging on the biological response to injury. In the present article, we review the current literature and explore the pathophysiological rationale underlying observed data, available evidence, and future directions on this topic.

Direct Peritoneal Resuscitation Improves Survival in a Murine Model of Combined Hemorrhage and Burn Injury

INTRODUCTION

Combined burn injury and hemorrhagic shock are a common cause of injury in wounded warfighters. Current protocols for resuscitation for isolated burn injury and isolated hemorrhagic shock are well defined, but the optimal strategy for combined injury is not fully established. Direct peritoneal resuscitation (DPR) has been shown to improve survival in rats after hemorrhagic shock, but its role in a combined burn/hemorrhage injury is unknown. We hypothesized that DPR would improve survival in mice subjected to combined burn injury and hemorrhage.

MATERIALS AND METHODS

Male C57/BL6J mice aged 8 weeks were subjected to a 7-second 30% total body surface area scald in a 90°C water bath. Following the scald, mice received DPR with 1.5 mL normal saline or 1.5 mL peritoneal dialysis solution (Delflex). Control mice received no peritoneal solution. Mice underwent a controlled hemorrhage shock via femoral artery cannulation to a systolic blood pressure of 25 mm Hg for 30 minutes. Mice were then resuscitated to a target blood pressure with either lactated Ringer's (LR) or a 1:1 ratio of packed red blood cells (pRBCs) and fresh frozen plasma (FFP). Mice were observed for 24 hours following injury.

RESULTS

Median survival time for mice with no DPR was 1.47 hours in combination with intravascular LR resuscitation and 2.08 hours with 1:1 pRBC:FFP. Median survival time significantly improved with the addition of intraperitoneal normal saline or Delflex. Mice that received DPR followed by 1:1 pRBC:FFP required less intravascular volume than mice that received DPR with LR, pRBC:FFP alone, and LR alone. Intraperitoneal Delflex was associated with higher levels of tumor necrosis factor alpha and macrophage inflammatory protein 1 alpha and lower levels of interleukin 10 and intestinal fatty acid binding protein. Intraperitoneal normal saline resulted in less lung injury 1 hour postresuscitation, but increased to similar severity of Delflex at 4 hours.

CONCLUSIONS

After a combined burn injury and hemorrhage, DPR leads to increased survival in mice. Survival was similar with the use of normal saline or Delflex. DPR with normal saline reduced the inflammatory response seen with Delflex and delayed the progression of acute lung injury. DPR may be a valuable strategy in the treatment of patients with combined burn injury and hemorrhage.

Impact of oral resuscitation on circulating and splenic leukocytes after burns

BACKGROUND

Hemodynamic aberrations after severe burns are treated with aggressive intravenous (IV) fluid resuscitation however, oral resuscitation has been proposed in resource poor scenarios. Previously we have shown that animals receiving oral fluid following burns were able to recover kidney function. However, immune function such as circulating and splenic immune cell populations after oral or intravenous fluid administration was not examined. Herein, we perform a follow up analysis of splenic tissue and plasma from the previous animal study to examine the splenic response following these resuscitation strategies after burn injury.

METHODS

Eighteen anesthetized Yorkshire swine receiving 40%TBSA contact burns were randomized to receive either: (1) no fluids (Fluid Restricted; negative control), (2) 70 mL/kg/d Oral Rehydration Salt solution (Oral), or (3) 2 mL/kg/%TBSA/d of lactated Ringer's solution IV. Blood was drawn for blood cell analysis, and CT scans were performed before and 48 h post-burn, at which point spleens were harvested for histological, Western blot, and RT-PCR analyses.

RESULTS

Splenic artery diameter decreased by -0.97 ± 0.14 mm in fluid-restricted animals, while IV led to an increase of 0.68 ± 0.30 mm. No significant differences were detected in white and red pulp. IV fluids reduced the population of splenic monocytes (CD163; P = 0.001) and neutrophils (MPO protein; P = 0.13), as well as cytokines IL-8 (P = 0.003), IFN-γ (P = 0.11) and TNFα (P = 0.05). Additionally, withholding IV fluids consistently decreased the expression of FoxP3, CCR6, and IL17β in spleen, suggesting a shift in T-cell phenotype with IV resuscitation.

CONCLUSIONS

The route of fluid administration has a minor influence on the changes in circulating and splenic leukocytes post-burn in the acute phase. Further research is needed to help guide resuscitation approaches using immunologic markers of splenic function following burns.

Chronic Critical Illness and Persistent Inflammation: What can we Learn from the Elderly, Injured, Septic, and Malnourished?

Patients in the intensive care unit (ICU) who develop chronic critical illness significantly stress the clinical capacity and financial resources of healthcare systems. Although vast improvements have been made in critical care management, outcomes for this ICU subset remain poor. A hallmark for patients who progress to chronic critical illness is the development of persistent inflammation and immunosuppression. The risk factors associated with the development of chronic critical illness include increased age, medical comorbidities, severe injury, septic shock, and malnutrition. Interestingly, each of these clinical states bears strikingly similar immune defects, often resulting in the activation of a persistent inflammatory state. Strategies aimed at the prevention or early recognition of this state of immune compromise may help improve outcomes for these individuals and minimize the number who progress to chronic critical illness. This review explores the current knowledge regarding the immune defects associated with the development of persistent inflammation, the ways in which it can manifest clinically, attempted therapeutic interventions to date, and future insights into improving outcomes for this patient population.

Amitriptyline Usage Exacerbates the Immune Suppression Following Burn Injury

Currently, over 10% of the US population is taking antidepressants. Numerous antidepressants such as amitriptyline are known to inhibit acid sphingomyelinase (Asm), an enzyme that is known to mediate leukocyte function and homeostasis. Severe burn injury can lead to an immunosuppressive state that is characterized by decreased leukocyte function and numbers as well as increased susceptibility to infection. Based upon the intersection of these facts, we hypothesized that amitriptyline-treated, scald-injured mice would have an altered immune response to injury as compared with untreated scald mice. Prior to burn, mice were pretreated with amitriptyline. Drug- or saline-treated mice were subjected full thickness dorsal scald- or sham-injury. Immune cells from spleen, thymus, and bone marrow were subsequently harvested and characterized. We first observed that amitriptyline prior to burn injury increased body mass loss and spleen contraction. Both amitriptylinetreatment and burn injury resulted in a 40% decrease of leukocyte Asm activity. Following scald injury, we demonstrate increased reduction of lymphocyte precursors in the bone marrow and thymus, as well as mature leukocytes in the spleen in mice that were treated with amitriptyline. We also demonstrate that amitriptyline treatment prior to injury reduced neutrophil accumulation following peptidoglycan stimulus in scald-injured mice. These data show that Asm alterations can play a significant role in mediating alterations to the immune system after injury. The data further suggest that those taking antidepressants may be at a higher risk for complications following burn injury.

Bronchoalveolar Lavage Microvesicles Protect Burn-Injured Mice from Pulmonary Infection

BACKGROUND

Pseudomonas aeruginosa is a major cause of morbidity and mortality among burn patients, despite antibiotic therapy. There is a need to identify innate immune defenses that prevent P aeruginosa infection in injured adults in an effort to find therapeutic alternatives to antibiotics. Here, we tested our hypothesis that microvesicles (MVs) in bronchoalveolar (BAL) fluid have a role in the immunity of the lung in response to pathogens.

STUDY DESIGN

Microvesicles were isolated from murine BAL fluid, quantified using Nanoparticle Tracking Analysis, and injected into burn-injured mice before P aeruginosa infection. Survival was assessed and BAL bacterial loads enumerated. Neutrophil number and interleukin 6 activity were determined. Lungs were harvested and sphingosine (SPH) content analyzed via immunohistochemistry. Antimicrobial effects of MVs and SPH-enriched MVs were assessed in an in vitro assay.

RESULTS

Burn-injured mice have reduced BAL MV number and SPH content compared with sham. When BAL MVs from healthy mice are administered to injured mice, survival and bacterial clearance are improved robustly. We also observed that intranasal administration of MVs restores SPH levels after burn injury, MVs kill bacteria directly, and this bacterial killing is increased when the MVs are supplemented with SPH.

CONCLUSIONS

Using a preclinical model, BAL MVs are reduced after scald injury and BAL MV restoration to injured mice improves survival and bacterial clearance. The antimicrobial mechanisms leading to improved survival include the quantity and SPH content of BAL MVs.

Frontline Science: Sphingosine rescues burn-injured mice from pulmonary Pseudomonas aeruginosa infection

Burn patients with concomitant pulmonary Pseudomonas aeruginosa (PA) infection have mortality rates as high as 50%, despite antibiotic therapy. Sphingosine is generated from ceramide via ceramidase and has been reported to have antimicrobial properties. We observed a reduction in sphingosine and a concurrent increase in ceramide in bronchial epithelial cells after burn injury. After PA inoculation, these mice had a significant decrease in survival compared to noninjured mice. However, when injured mice were pretreated with sphingosine or neutral ceramidase and subsequently infected, mortality and bacterial levels were robustly reduced. We further observed that sphingosine directly kills PA. Together, these results demonstrate that reduction in sphingosine is associated with an increased susceptibility to pulmonary infection after burn injury. Restoration of sphingosine levels through direct sphingosine administration or conversion of the increased ceramide to sphingosine by neutral ceramidase reduces mortality and mitigates pulmonary infection after burn injury.

Burn-Evoked Reactive Oxygen Species Immediately After Injury are Crucial to Restore the Neutrophil Function Against Postburn Infection in Mice

Although reactive oxygen species (ROS) basically play beneficial roles to maintain host homeostasis against external disturbance/stress including infection, excessive ROS generation by activated neutrophils can sometimes cause organ damage. We investigated the role of burn-induced ROS generation in the injured hosts, focusing on postburn infection. C57BL/6 mice received a 20% full-thickness burn injury. In these mice, the burn-induced ROS generation was inhibited during and immediately after injury by pretreatment with superoxide dismutase (at 1 h before and immediately before injury), or the subsequent ROS production was inhibited posttreatment with superoxide dismutase (at 1 and 2 h after injury), which could not scavenge the ROS produced immediately after injury. As expected, inhibition of ROS production during/immediately after injury reduced the burn-induced pulmonary damage at 6 h, whereas inhibition of the subsequent ROS production did not lead to any improvements. Burn injury rendered the mice susceptible to bacterial infection at 5 days after injury and impaired bactericidal activity of neutrophils. Nevertheless, inhibition of the ROS production during/immediately after injury did not improve the burn-induced susceptibility to infection or the neutrophil dysfunction. Interestingly, inhibition of the subsequent ROS production potently restored the neutrophil functions and hematopoietic function of the bone marrow myelocytes, thereby improving the postburn infection. Thus, although the inhibition of burn-evoked ROS generation is effective against burn-induced organ injury, it may be ineffective against postburn infection. Preservation of the immediate burn-evoked ROS production, but the inhibition of subsequent ROS production, may be crucial to protect against postburn infection.

Assessing the immune status of critically ill trauma patients by flow cytometry

BACKGROUND

Unintentional injury or trauma remains the leading cause of death among young adults. About one fifth of these trauma patients require care in an intensive care unit (ICU) because of severity of injuries and comorbidities. Patients hospitalized in an ICU are at increased risk for nosocomial infections, such as urinary tract infections, pneumonia, bacteremia, and wound infections. Many of these patients will develop sepsis or septic shock, and some will progress to multiple organ failure and death. The balance between the proinflammatory and counterinflammatory immune response appears to be a driving factor in this progression. At present, there is no proposed method for the timely detection of the immune status in trauma patients, making rational decisions to use immune-altering therapies difficult.

OBJECTIVE

We demonstrate that flow cytometry, with its capabilities to characterize and/or enumerate (a) leukocyte subtypes, (b) leukocyte activation markers, (c) leukocyte-derived cytokines and microvesicles, and (d) leukocyte function is well suited to monitor the immune status of critically ill trauma patients.

METHODS

Information for the review was obtained from the extant literature.

DISCUSSION

We suggest that flow cytometry is a research method that might aid nurse scientists in investigating the immune status of critically ill patients, the recovery status of conditions such as hemorrhagic shock and tissue injury and the relationship between cancer disease progression and symptoms. Therefore, flow cytometry has the potential to broaden nursing research priority areas so that a comprehensive approach to understanding the cellular response is attained.

Thymic stromal lymphopoietin mediates the host response and increases mortality during sepsis

BACKGROUND

Sepsis and subsequent multiorgan system failure is associated with high rates of mortality and morbidity. Thymic stromal lymphopoietin (TSLP) is a cytokine that can be produced by keratinocytes and epithelial cells. Primarily, TSLP has been shown to promote counter-inflammatory processes. However, its potential expression or role in the pathogenesis of sepsis is largely unexplored. We hypothesized that TSLP is expressed during sepsis and TSLP blockade would alter the immune response and mortality.

MATERIALS AND METHODS

Mice underwent cecal ligation and puncture (CLP) to produce a physiologically relevant murine model for sepsis. Cohorts were either treated with neutralizing TSLP antibodies or isotype controls before the CLP to determine changes in survival, bacterial loads, cytokine levels, and neutrophil function.

RESULTS

It was observed that TSLP levels peaked at 6 h and remained detectable up to 48 h after CLP. Mice pretreated with neutralizing TSLP showed decreased mortality and bacterial load after CLP. Additionally, we determined that septic mice pretreated with the anti-TSLP antibody had increased tumor necrosis factor alpha and oxidative burst as well as increased interleukin 17 and neutrophil numbers compared with mice pretreated with isotype controls.

CONCLUSIONS

TSLP levels peak early but are sustained during the first 48 h of sepsis. We speculate that TSLP blunts the neutrophil response resulting in increased bacterial load and mortality.

Animal models in burn research

Burn injury is a severe form of trauma affecting more than 2 million people in North America each year. Burn trauma is not a single pathophysiological event but a devastating injury that causes structural and functional deficits in numerous organ systems. Due to its complexity and the involvement of multiple organs, in vitro experiments cannot capture this complexity nor address the pathophysiology. In the past two decades, a number of burn animal models have been developed to replicate the various aspects of burn injury, to elucidate the pathophysiology, and to explore potential treatment interventions. Understanding the advantages and limitations of these animal models is essential for the design and development of treatments that are clinically relevant to humans. This review aims to highlight the common animal models of burn injury in order to provide investigators with a better understanding of the benefits and limitations of these models for translational applications. While many animal models of burn exist, we limit our discussion to the skin healing of mouse, rat, and pig. Additionally, we briefly explain hypermetabolic characteristics of burn injury and the animal model utilized to study this phenomena. Finally, we discuss the economic costs associated with each of these models in order to guide decisions of choosing the appropriate animal model for burn research.

Delayed topical p38 MAPK inhibition attenuates full-thickness burn wound inflammatory signaling

Inflammatory signaling pathways, such as p38 mitogen-activated protein kinase (MAPK) play a central role in host responses to injury. In previous studies by the authors, topical p38 MAPK inhibitors effectively attenuated inflammatory signaling in a partial-thickness scald burn model, when applied to the burn wound immediately after injury. However, clinically relevant full-thickness scald burn wounds may act as a barrier to topical immune modulators, and delayed application of topical p38 MAPK inhibitors may not be effective. In this study, the authors evaluate the efficacy of topical p38 MAPK inhibition on full-thickness scald burns with immediate and delayed treatment. C57/BL6 mice received "Sham" or 30% TBSA full-thickness scald burn injury. After injury, the burn wounds were treated with a topical p38 MAPK inhibitor or vehicle. The treatment group received topical p38 MAPK inhibitor either immediately after burn or 4 hours (delayed) after injury. All animals were killed at 12 or 24 hours. Burn wounds underwent histological analyses. Skin and plasma were analyzed by enzyme-linked immunosorbent assay or real-time quantitative polymerase chain reaction for cytokine expression. Full-thickness scald burns resulted from immersion in 62°C water for 25 seconds. Topical p38 MAPK inhibitor attenuated dermal interleukin (IL)-6, MIP-2, and IL-1β expression and plasma IL-6 and MIP-2 cytokine expression. In addition, delayed application of topical p38 MAPK inhibitors significantly reduced dermal and plasma cytokine expression compared with vehicle control. Topical p38 MAPK inhibitors remain potent in reducing full-thickness burn wound inflammatory signaling, even when treatment is delayed by several hours postinjury. Topical application of p38 MAPK inhibitor may be a clinically viable treatment after burn injury.

Roles of hepatocyte and myeloid CXC chemokine receptor-2 in liver recovery and regeneration after ischemia/reperfusion in mice

Previous studies have demonstrated the significance of signaling through the CXC chemokine receptor-2 (CXCR2) receptor in the process of recovery and regeneration of functional liver mass after hepatic ischemia/reperfusion (I/R). CXCR2 is constitutively expressed on both neutrophils and hepatocytes; however, the cell-specific roles of this receptor are unknown. In the present study, chimeric mice were created through bone marrow transplantation (BMT) using wild-type and CXCR2-knockout mice, yielding selective expression of CXCR2 on hepatocytes (Hep) and/or myeloid cells (My) in the following combinations: Hep+/My+; Hep-/My+; Hep+/My-; and Hep-/My-. These tools allowed us to assess the contributions of myeloid and hepatocyte CXCR2 in the recovery of the liver after I/R injury. Flow cytometry confirmed the adoption of the donor phenotype in neutrophils. Interestingly, Kupffer cells from all chimeras lacked CXCR2 expression. Recovery/regeneration of hepatic parenchyma was assessed by histologic assessment and measurement of hepatocyte proliferation. CXCR2(Hep+/My+) mice showed the least amount of liver recovery and hepatocyte proliferation, whereas CXCR2(Hep-/My-) mice had the greatest liver recovery and hepatocyte proliferation. CXCR2(Hep+/My-) mice had enhanced liver recovery, with hepatocyte proliferation similar to CXCR2(Hep-/My-) mice. Myeloid expression of CXCR2 directly regulated CXC chemokine expression levels after hepatic I/R, such that mice lacking myeloid CXCR2 had markedly increased chemokine expression, compared with mice expressing CXCR2 on myeloid cells.

CONCLUSION

The data suggest that CXCR2 on myeloid cells is the predominant regulator of liver recovery and regeneration after I/R injury, whereas hepatocyte CXCR2 plays a minor, secondary role. These findings suggest that myeloid cell-directed therapy may significantly affect liver regeneration after liver resection or transplantation.

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.