Severity of burn injury and sepsis determines the cytokine responses of bone marrow progenitor-derived macrophages

Gut Microbial Changes and their Contribution to Post-Burn Pathology

ABSTRACT

Burn injuries are a common form of traumatic injury that leads to significant morbidity and mortality worldwide. Burn injuries are characterized by inflammatory processes and alterations in numerous organ systems and functions. Recently, it has become apparent that the gastrointestinal bacterial microbiome is a key component of regulating the immune response and recovery from burn and can also contribute to significant detrimental sequelae after injury, such as sepsis and multiple organ failure. Microbial dysbiosis has been linked to multiple disease states; however, its role in exacerbating acute traumatic injuries, such as burn, is poorly understood. In this article, we review studies that document changes in the intestinal microbiome after burn injury, assess the implications in post-burn pathogenesis, and the potential for further discovery and research.

Ventilator-Associated Pneumonia Prevention Bundle Significantly Reduces the Risk of Ventilator-Associated Pneumonia in Critically Ill Burn Patients

Ventilator-associated pneumonia (VAP) is a common cause of morbidity and mortality for critically ill burn patients. Prevention of VAP through bundled preventative measures may reduce the risk and incidence of VAP in burn patients. A retrospective chart review was performed of all mechanically ventilated adult (age ≥ 18 years) burn patients before and after VAP prevention bundle implementation. Data collected included age, TBSA, gender, diagnosis of inhalation injury, mechanism of injury, comorbid illnesses, length of mechanical ventilation, length of hospital stay, development of VAP, discharge disposition, and mortality. Burn patients with VAP had larger burn injuries (47.6 ± 22.2 vs 23.9 ± 23.01), more inhalation injuries (44.6% vs 27%), prolonged mechanical ventilation, and longer intensive care unit (ICU) and hospital stays. Mortality was also higher in burn patients who developed VAP (34% vs 19%). On multivariate regression analysis, TBSA and ventilator days were independent risk factors for VAP. In 2010, a VAP prevention bundle was implemented in the burn ICU and overseen by a nurse champion. Compliance with bundle implementation was more than 95%. By 2012, independent of age, TBSA, inhalation injury, ventilator days, ICU and hospital length of stay, VAP prevention bundles resulted in a significantly reduced risk of developing VAP (odds ratio of 0.15). Burn patients with an inhalation injury and a large burn injury are at increased risk of developing VAP. The incidence and risk of VAP can be significantly reduced in burn patients with VAP prevention bundles.

Association between early airway damage-associated molecular patterns and subsequent bacterial infection in patients with inhalational and burn injury

Bacterial infection is a major cause of morbidity affecting outcome following burn and inhalation injury. While experimental burn and inhalation injury animal models have suggested that mediators of cell damage and inflammation increase the risk of infection, few studies have been done on humans. This is a prospective, observational study of patients admitted to the North Carolina Jaycee Burn Center at the University of North Carolina who were intubated and on mechanical ventilation for treatment of burn and inhalational injury. Subjects were enrolled over a 2-yr period and followed till discharge or death. Serial bronchial washings from clinically indicated bronchoscopies were collected and analyzed for markers of tissue injury and inflammation. These include damage-associated molecular patterns (DAMPs) such as hyaluronic acid (HA), double-stranded DNA (dsDNA), heat-shock protein 70 (HSP-70), and high-mobility group protein B-1 (HMGB-1). The study population was comprised of 72 patients who had bacterial cultures obtained for clinical indications. Elevated HA, dsDNA, and IL-10 levels in bronchial washings obtained early (the first 72 h after injury) were significantly associated with positive bacterial respiratory cultures obtained during the first 14 days postinjury. Independent of initial inhalation injury severity and extent of surface burn, elevated levels of HA dsDNA and IL-10 in the central airways obtained early after injury are associated with subsequent positive bacterial respiratory cultures in patients intubated after acute burn/inhalation injury.

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.

Flagellin treatment prevents increased susceptibility to systemic bacterial infection after injury by inhibiting anti-inflammatory IL-10+ IL-12- neutrophil polarization

Severe trauma renders patients susceptible to infection. In sepsis, defective bacterial clearance has been linked to specific deviations in the innate immune response. We hypothesized that innate immune modulations observed during sepsis also contribute to increased bacterial susceptibility after severe trauma. A well-established murine model of burn injury, used to replicate infection following trauma, showed that wound inoculation with P. aeruginosa quickly spreads systemically. The systemic IL-10/IL-12 axis was skewed after burn injury with infection as indicated by a significant elevation in serum IL-10 and polarization of neutrophils into an anti-inflammatory ("N2"; IL-10(+) IL-12(-)) phenotype. Infection with an attenuated P. aeruginosa strain (ΔCyaB) was cleared better than the wildtype strain and was associated with an increased pro-inflammatory neutrophil ("N1"; IL-10(-)IL-12(+)) response in burn mice. This suggests that neutrophil polarization influences bacterial clearance after burn injury. Administration of a TLR5 agonist, flagellin, after burn injury restored the neutrophil response towards a N1 phenotype resulting in an increased clearance of wildtype P. aeruginosa after wound inoculation. This study details specific alterations in innate cell populations after burn injury that contribute to increased susceptibility to bacterial infection. In addition, for the first time, it identifies neutrophil polarization as a therapeutic target for the reversal of bacterial susceptibility after injury.

Th17 cells: critical mediators of host responses to burn injury and sepsis

Th cells have long been recognized as vital components of the adaptive immune system. Until recently, CD3(+)CD4(+) Th cells were divided into cell-mediated Th1 or humoral Th2 responses. However, the Th1-Th2 hypothesis failed to accommodate the more recently described Th17 cells. Today, the major Th cell subsets include Th1, Th2, Th9, Th17, Th22, and Tregs, each of which produce specific effector cytokines under unique transcriptional regulation. Specifically, Th17 cells produce effector cytokines IL-17, IL-21, and IL-22 under the regulation of ROR-γt. Th17 lymphocytes were first described as orchestrators of neutrophil recruitment and activation and as key players in chronic inflammation and autoimmunity. More recent evidence suggest that Th17 lymphocytes and their effector cytokines play a crucial role in maintaining mucosal immunity and barrier integrity, including the skin, lung, and gut. Burn injury induces global changes to the systemic immune response, including suppressed immune function and increased susceptibility to infection. Moreover, burn trauma is associated with remote organ injury. This relationship between burn and remote organ injury supports the hypothesis that immune suppression may facilitate the development of sepsis, systemic inflammatory response syndrome, and multiple organ dysfunction syndrome in critically ill burn patients. Herein, we discuss this emerging adaptive cell subset in critical care settings, including burn injury and clinical sepsis, and highlight the potential therapeutic role of IL-22.

Burn injury dampens erythroid cell production through reprioritizing bone marrow hematopoietic response

BACKGROUND

Anemia in burn patients is due to surgical blood loss and anemia of critical illness. Because the commitment paradigm of common bone marrow progenitors dictates the production of erythroid, myeloid, and lymphoid cells, we hypothesized that skewed bone marrow lineage commitment decreases red cell production and causes anemia after a burn injury.

METHODS

After anesthesia, B(6)D(2)F(1) mice received a 15% total body surface area dorsal scald burn. The sham group did not receive scald burn. Femoral bone marrow was harvested on 2, 5, 7, 14, and 21 postburn days (PBD). Total bone marrow cells were labeled with specific antibodies to erythroid (CD71/Ter119), myeloid (CD11b), and lymphoid (CD19) lineages and analyzed by flow cytometry. To test whether erythropoietin (EPO) could increase red blood cell production, EPO was administered to sham and burn animals and their reticulocyte response was measured on PBD 2 and PBD 7.

RESULTS

Burn injury reduced the erythroid cells of the bone marrow from 35% in sham to 17% by PBD 5 and remained at similar level until PBD 21. Myeloid cells, however, increased from 42% in sham to 60% on PBD 5 and 77% on PBD 21. Burn injury reduced reticulocyte counts on PBD 2 and PBD 7 indicating that the erythroid compartment is severely depleted. This depleted compartment, however, responded to EPO but was not sufficient to change red cell production.

CONCLUSION

Burn injury skews the bone marrow hematopoietic commitment away from erythroid and toward myeloid cells. Shrinkage of the erythroid compartment contributes to resistance to EPO and the anemia of critical illness.

Naturally existing CD11c(low)CD45RB(high) dendritic cells protect mice from acute severe inflammatory response induced by thermal injury

Despite a good understanding of the process that initiates and promotes host inflammation induced by acute injury, little is known about the host immune cells responsible for the inhibition of inflammatory response to thermal injury. The aim of this study was to investigate the potential effect of naturally existing CD11c(low)CD45RB(high) dendritic cells (CD11c(low)CD45RB(high) DCs) on acute severe inflammatory response and mortality rate in burned mice. Changes in the percentage of distinct subsets of splenic DCs and production of cytokines (IL-6, TNF-α, CCL-2) as well as CC chemokine (CCL)-2 were measured by FACS at various time points. The influence of CD11c(low)CD45RB(high) DCs on IL-6, TNF-α, CCL-2 as well as IL-10 levels and mortality rate was observed following a single intraperitoneal injection of DCs to scald mice. Levels of IL-6, CCL-2 and TNF-α were peaked at postburn hours (PBHs) 12, and percentages of CD11c(low)CD45RB(high) DCs were elevated at PBH 12-24. A single intraperitoneal injection of CD11c(low)CD45RB(high) DCs to 15% total body surface area in scald mice led to significant decrease in IL-6, TNF-α and CCL-2, and lethality, but up-regulation of IL-10 compared with untreated mice from PBH 0 to 48. CD11c(low)CD45RB(high) DCs can effectively down-regulate the production of inflammatory cytokines and reduce the mortality rate in 15% TBSA scald injury mice.

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.

Propranolol restores the tumor necrosis factor-alpha response of circulating inflammatory monocytes and granulocytes after burn injury and sepsis

Beta-adrenergic blockade ameliorates the hypermetabolism and catabolism in severe burn injury. Despite the salutary effects of beta-adrenergic blockade, the immunologic responses that accompany beta-blockade are not known. We have shown that burn sepsis is associated with increased sympathetic activation leading to altered monocytopoiesis and cytokine release in macrophages (MØ). Recent evidence suggests that murine MØ expressing F4/80+Gr1+ are the inflammatory phenotype. Here, we report that propranolol given after burn sepsis modulates the number and function of myeloid cells in circulation. B6D2F1 male mice were divided into sham (S), burn (B), and burn sepsis (BS) groups. Dorsal hair was shaved from S, B, and BS; B and BS received 15% scald burn; BS was inoculated with Pseudomonas Aeruginosa (PA 14, 4000-5000 colony-forming units) at the burn site. Mice from each group were then subjected to two different treatment regimens. One set received subcutaneous injections of propranolol (5 mg/kg body weight) at 24 and 48 hours after the injury while the control groups received saline. Blood was collected by cardiac puncture at 72 hours. The distribution of total F4/80+ monocyte population was determined by flow cytometry. Inflammatory monocyte subset was gated on Gr1+ expression in the F4/80+ fraction. Lipopolysaccharide-stimulated intracellular tumor necrosis factor (TNF)-alpha (ic-TNF) was also measured as an indicator of inflammatory response. The total F4/80+ monocyte fraction was significantly increased in BS (45 +/- 0.8%) vs S and B (10 +/- 0.8%; 9.5 +/- 0.6%). Propranolol treatment for 2 days reduced the number of circulating monocytes by 60% in BS. The mean fluorescent intensity (MFI) of ic-TNF produced per cell (F4/80+Gr1+ MØ) was significantly decreased in B and BS (S: 3043 +/- 213, B: 1638 +/- 343, BS: 1463 +/- 67). Of importance, propranolol treatment partially restored the MFI of ic-TNF (2177 +/- 114) and increased the percentage of inflammatory monocyte subset (F4/80+Gr1+) in BS by 70% compared with saline treatment. In contrast, beta-blockade after BS increased the percentage of granulocytes in circulation (28.4 +/- 3.6% in BS propranolol vs 15.4 +/- 0.3% in BS saline; P < .05) and augmented their TNF production (MFI = 903 +/- 102 in BS propranolol vs 644 +/- 5 in BS saline; P < .05). Propranolol reverses burn sepsis-induced monocytosis and simultaneously increases the number of granulocytes and enhances the inflammatory potential of the granulocytes and inflammatory monocyte subsets in circulation suggesting that monitoring MØ subsets and granulocytes in blood is a reliable biomarker to predict the efficacy of beta-blockade.

Inhibition of multidrug-resistant Acinetobacter baumannii by nonviral expression of hCAP-18 in a bioengineered human skin tissue

When skin is compromised, a cascade of signals initiates the rapid repair of the epidermis to prevent fluid loss and provide defense against invading microbes. During this response, keratinocytes produce host defense peptides (HDPs) that have antimicrobial activity against a diverse set of pathogens. Using nonviral vectors we have genetically modified the novel, nontumorigenic, pathogen-free human keratinocyte progenitor cell line (NIKS) to express the human cathelicidin HDP in a tissue-specific manner. NIKS skin tissue that expresses elevated levels of cathelicidin possesses key histological features of normal epidermis and displays enhanced antimicrobial activity against bacteria in vitro. Moreover, in an in vivo infected burn wound model, this tissue results in a two log reduction in a clinical isolate of multidrug-resistant Acinetobacter baumannii. Taken together, these results suggest that this genetically engineered human tissue could be applied to burns and ulcers to counteract bacterial contamination and prevent infection.

Prophylactic treatment with fms-like tyrosine kinase-3 ligand after burn injury enhances global immune responses to infection

Severely burned patients are susceptible to infections with opportunistic organisms due to altered immune responses and frequent wound contamination. Immunomodulation to enhance systemic and local responses to wound infections may be protective after burn injury. We previously demonstrated that pretreatments with fms-like tyrosine kinase-3 (Flt3) ligand (Flt3L), a dendritic cell growth factor, increase the resistance of mice to a subsequent burn injury and wound infection by a dendritic cell-dependent mechanism. This study was designed to test the hypothesis that Flt3L administration after burn injury decreases susceptibility to wound infections by enhancing global immune cell activation. Mice were treated with Flt3L after burn injury and examined for survival, wound and systemic bacterial clearance, and immune cell activation after wound inoculation with Pseudomonas aeruginosa. To gain insight into the local effects of Flt3L at the burn wound, localization of Langerhans cells was examined. Mice treated with Flt3L had significantly greater numbers of CD25-expressing T cells and CD69-expressing T and B cells, neutrophils, and macrophages after, but not before, infection. Overall leukocyte apoptosis in response to infection was decreased with Flt3L treatment. Survival and local and systemic bacterial clearance were enhanced by Flt3L. Langerhans cells appeared in the dermis of skin bordering the burn wound, and further increased in response to wound infection. Flt3L augmented the appearance of Langerhans cells in response to both injury and infection. These data suggest that dendritic cell enhancement by Flt3L treatments after burn injury protects against opportunistic infections through promotion of local and systemic immune responses to infection.

Endotoxemia down-regulates bone marrow lymphopoiesis but stimulates myelopoiesis: the effect of G6PD deficiency

Bone marrow (BM) dysfunction is an important component of immunomodulation. This study investigated alterations in cell content, apoptotic responses, and cell proliferation in BM, blood, and spleen in endotoxemic mice (LPS from Escherichia coli). As the decreased antioxidant status associated with glucose-6-phosphate dehydrogenase (G6PD) deficiency has been shown to modulate the innate immune response, we also tested whether a G6PD mutation (80% decrease in cellular enzyme activity) alters BM responses during endotoxemia. LPS decreased BM myeloid (CD45(+)CD11b(+)) and B lymphoid (CD45(+)CD19(+)CD11b(-)) cell content compared with controls. In contrast, LPS increased CD11b(+) myeloid but decreased T and B cell counts in the circulation. Endotoxemia inhibited spontaneous, heat shock, and H(2)O(2)-induced apoptosis as well as proliferative activity in BM lymphoid cells. In contrast, BM myeloid cell apoptosis was not altered, and their proliferative activity was increased during endotoxemia. Following LPS, splenic myeloid cell content was increased, and T and B cell content was unchanged; furthermore, splenocytes showed increased apoptosis compared with controls. BM cell content, including lymphoid and myeloid cells, was greater in G6PD mutant than wild-type (WT) mice, and LPS decreased BM cell counts to a greater degree in mutant than WT mice. Endotoxemia caused widespread inhibition of BM cytokine and chemokine production; however, IL-6 production was increased compared with controls. LPS-induced IL-6 production was decreased in G6PD mutant animals compared with WT. This study indicates that endotoxin inversely affects BM myeloid and lymphoid cell production. LPS-induced down-regulation of B cell production contributes to the generalized lymphopenia and lymphocyte dysfunction observed following nonspecific immune challenges.