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

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.

Comprehensive single-cell analysis reveals novel anergic antigen-presenting cell subtypes in human sepsis

Background

Sepsis is a life-threatening condition with high mortality. A few studies have emerged utilizing single-cell RNA sequencing (scRNA-seq) to analyze gene expression at the single-cell resolution in sepsis, but a comprehensive high-resolution analysis of blood antigen-presenting cells has not been conducted.

Methods

All published human scRNA-seq data were downloaded from the single cell portal database. After manually curating the dataset, we extracted all antigen-presenting cells, including dendritic cells (DCs) and monocytes, for identification of cell subpopulations and their gene profiling and intercellular interactions between septic patients and healthy controls. Finally, we further validated the findings by performing deconvolution analysis on bulk RNA sequencing (RNA-seq) data and flow cytometry.

Results

Within the traditional DC populations, we discovered novel anergic DC subtypes characterized by low major histocompatibility complex class II expression. Notably, these anergic DC subtypes showed a significant increase in septic patients. Additionally, we found that a previously reported immunosuppressive monocyte subtype, Mono1, exhibited a similar gene expression profile to these anergic DCs. The consistency of our findings was confirmed through validation using bulk RNA-seq and flow cytometry, ensuring accurate identification of cell subtypes and gene expression patterns.

Conclusions

This study represents the first comprehensive single-cell analysis of antigen-presenting cells in human sepsis, revealing novel disease-associated anergic DC subtypes. These findings provide new insights into the cellular mechanisms of immune dysregulation in bacterial sepsis.

Advantages and Disadvantages of Using Small and Large Animals in Burn Research: Proceedings of the 2021 Research Special Interest Group

Multiple animal species and approaches have been used for modeling different aspects of burn care, with some strategies considered more appropriate or translatable than others. On April 15, 2021, the Research Special Interest Group of the American Burn Association held a virtual session as part of the agenda for the annual meeting. The session was set up as a pro/con debate on the use of small versus large animals for application to four important aspects of burn pathophysiology: burn healing/conversion; scarring; inhalation injury; and sepsis. For each of these topics, 2 experienced investigators (one each for small and large animal models) described the advantages and disadvantages of using these preclinical models. The use of swine as a large animal model was a common theme due to anatomic similarities with human skin. The exception to this was a well-defined ovine model of inhalation injury; both of these species have larger airways which allow for incorporation of clinical tools such as bronchoscopes. However, these models are expensive and demanding from labor and resource standpoints. Various strategies have been implemented to make the more inexpensive rodent models appropriate for answering specific questions of interest in burns. Moreover, modelling burn-sepsis in large animals has proven difficult. It was agreed that the use of both small and large animal models have merit for answering basic questions about the responses to burn injury. Expert opinion and the ensuing lively conversations are summarized herein, which we hope will help inform experimental design of future 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.

Immune Cells in Cutaneous Wound Healing: A Review of Functional Data from Animal Models

The healing of skin wounds involves the activation and recruitment of various immune cell types, many of which are believed to contribute significantly to different aspects of the repair process. Roles for immune cells have been described in practically all stages of wound healing, including hemostasis, inflammation, proliferation and scar formation/remodeling. Over the last decade, tools to deplete immune cell populations in animal models have become more advanced, leading to a surge in the number of studies examining the function of specific immune cell types in skin repair. In this review, we will summarize what is known about distinct immune cell types in cutaneous wound healing, with an emphasis on data from animal studies in which specific cell types have been targeted.

Control of innate immune response by biomaterial surface topography, energy, and stiffness

As the focus of implantable biomaterials has shifted from bioinert implants to bioactive designs, recent research has highlighted the complex interactions between cell physiologic systems and material properties, particularly physical cues. From the cells known to interact with implanted biomaterials, the response of the immune system has been a critical target of study recently. Here, we review studies characterizing the response of innate immune cells to various material cues, particularly of those at the surface of implanted materials.The innate immune system consists of cell types with various roles in inflammation. Neutrophils and macrophages serve both phagocytic and signaling roles, especially early in the inflammatory phase of biomaterial implantation. These cell types ultimately dictate the outcome of implants as chronic inflammation, fibrosis, or integration. Other cell types like dendritic cells, mast cells, natural killer cells, and innate lymphoid cells may also serve an immunomodulatory role in the biomaterial context. This review highlights recent advances in our understanding of the role of innate immunity in the response to implantable biomaterials as well as key mechanobiological findings in innate immune cells underpinning these advances. STATEMENT OF SIGNIFICANCE: This review highlights recent advances in the understanding of the role of innate immunity in the response to implantable biomaterials, especially in neutrophils and macrophages, as well as key mechanobiological findings in innate immune cells underpinning these advances. Here we discuss how physicochemical properties of biomaterials control innate immune cell behavior.

Post-injury immunosuppression and secondary infections are caused by an AIM2 inflammasome-driven signaling cascade

Loss of lymphocytes, particularly T cell apoptosis, is a central pathological event after severe tissue injury that is associated with increased susceptibility for life-threatening infections. The precise immunological mechanisms leading to T cell death after acute injury are largely unknown. Here, we identified a monocyte-T cell interaction driving bystander cell death of T cells in ischemic stroke and burn injury. Specifically, we found that stroke induced a FasL-expressing monocyte population, which led to extrinsic T cell apoptosis. This phenomenon was driven by AIM2 inflammasome-dependent interleukin-1β (IL-1β) secretion after sensing cell-free DNA. Pharmacological inhibition of this pathway improved T cell survival and reduced post-stroke bacterial infections. As such, this study describes inflammasome-dependent monocyte activation as a previously unstudied cause of T cell death after injury and challenges the current paradigms of post-injury lymphopenia.

A Murine Model of Full-Thickness Scald Burn Injury with Subsequent Wound and Systemic Bacterial Infection

Infection is the leading cause of death and prolonged hospitalization in severely burned patients that survive the acute phase of injury. Here we describe a murine model of severe burn injury followed by subsequent postburn infection, both local and systemic, that leads to sepsis. A detailed description of the full-thickness scald burn procedure is provided, followed by description of infection with two common burn-associated nosocomial pathogens, Pseudomonas aeruginosa and Staphylococcus aureus.

Novel Therapeutics for the Treatment of Burn Infection

Background: Burn injury continues to be a significant cause of morbidity and death, with infectious complications being the primary cause of death. Patients are susceptible to overwhelming infection secondary to both the physical breakdown of the skin and mucosal barrier and the immune dysfunction that accompanies the inflammatory response to a major burn. With resistance to traditional antibiosis looming as a serious threat to patient outcome, advancement in the treatment of burn infections is imperative. Methods: Between February 15 and March 15, 2020, a search of Pubmed and clinicaltrials.gov was performed using search terms such as "burn immunotherapy," "therapeutic microorganisms in burn," "burn infection clinical trials," and applicable variations. Results: Topical antimicrobial drugs continue to be standard of care for burn wound injuries, but personalized and molecular treatments that rely on immune manipulation of the host show great promise. We discuss novel therapeutics for the treatment of burn infection: Probiotics and therapeutic microorganisms, immune modulators, tailored monoclonal antibodies, and extracellular vesicles and proteins. Conclusions: The treatment strategies discussed employ manipulation of structure and function in host immune cells and pathogen virulence for improved outcomes in burn infection.

Myeloid cells in sepsis-acquired immunodeficiency

On May 2017, the World Health Organization recognized sepsis as a global health priority. Sepsis profoundly perturbs immune homeostasis by initiating a complex response that varies over time, with the concomitant occurrence of pro- and anti-inflammatory mechanisms. Sepsis deeply impacts myeloid cell response. Different mechanisms are at play, such as apoptosis, endotoxin tolerance, metabolic failure, epigenetic reprogramming, and central regulation. This induces systemic effects on circulating immune cells and impacts progenitors locally in lymphoid organs. In the bone marrow, a progressive shift toward the release of immature myeloid cells (including myeloid-derived suppressor cells), at the expense of mature neutrophils, takes place. Circulating dendritic cell number remains dramatically low and monocytes/macrophages display an anti-inflammatory phenotype and reduced antigen presentation capacity. Intensity and persistence of these alterations are associated with increased risk of deleterious outcomes in patients. Thus, myeloid cells dysfunctions play a prominent role in the occurrence of sepsis-acquired immunodeficiency. For the most immunosuppressed patients, this paves the way for clinical trials evaluating immunoadjuvant molecules (granulocyte-macrophage colony-stimulating factor and interferon gamma) aimed at restoring homeostatic myeloid cell response. Our review offers a summary of sepsis-induced myeloid cell dysfunctions and current therapeutic strategies proposed to target these defects in patients.

Crosstalk between Dendritic Cells and Immune Modulatory Agents against Sepsis

Dendritic cells (DCs) play a critical role in the immune system which sense pathogens and present their antigens to prime the adaptive immune responses. As the progression of sepsis occurs, DCs are capable of orchestrating the aberrant innate immune response by sustaining the Th1/Th2 responses that are essential for host survival. Hence, an in-depth understanding of the characteristics of DCs would have a beneficial effect in overcoming the obstacle occurring in sepsis. This paper focuses on the role of DCs in the progression of sepsis and we also discuss the reverse sepsis-induced immunosuppression through manipulating the DC function. In addition, we highlight some potent immunotherapies that could be used as a novel strategy in the early treatment of sepsis.

H2S, a Bacterial Defense Mechanism against the Host Immune Response

The biological mediator hydrogen sulfide (H₂S) is produced by bacteria and has been shown to be cytoprotective against oxidative stress and to increase the sensitivity of various bacteria to a range of antibiotic drugs. Here we evaluated whether bacterial H₂S provides resistance against the immune response, using two bacterial species that are common sources of nosocomial infections, Escherichia coli and Staphylococcus aureus Elevations in H₂S levels increased the resistance of both species to immune-mediated killing. Clearances of infections with wild-type and genetically H₂S-deficient E. coli and S. aureus were compared in vitro and in mouse models of abdominal sepsis and burn wound infection. Also, inhibitors of H₂S-producing enzymes were used to assess bacterial killing by leukocytes. We found that inhibition of bacterial H₂S production can increase the susceptibility of both bacterial species to rapid killing by immune cells and can improve bacterial clearance after severe burn, an injury that increases susceptibility to opportunistic infections. These findings support the role of H₂S as a bacterial defense mechanism against the host response and implicate bacterial H₂S inhibition as a potential therapeutic intervention in the prevention or treatment of infections.

Alteration of Flt3-Ligand-dependent de novo generation of conventional dendritic cells during influenza infection contributes to respiratory bacterial superinfection

Secondary bacterial infections contribute to the excess morbidity and mortality of influenza A virus (IAV) infection. Disruption of lung integrity and impaired antibacterial immunity during IAV infection participate in colonization and dissemination of the bacteria out of the lungs. One key feature of IAV infection is the profound alteration of lung myeloid cells, characterized by the recruitment of deleterious inflammatory monocytes. We herein report that IAV infection causes a transient decrease of lung conventional dendritic cells (cDCs) (both cDC1 and cDC2) peaking at day 7 post-infection. While triggering emergency monopoiesis, IAV transiently altered the differentiation of cDCs in the bone marrow, the cDC1-biaised pre-DCs being particularly affected. The impaired cDC differentiation during IAV infection was independent of type I interferons (IFNs), IFN-γ, TNFα and IL-6 and was not due to an intrinsic dysfunction of cDC precursors. The alteration of cDC differentiation was associated with a drop of local and systemic production of Fms-like tyrosine kinase 3 ligand (Flt3-L), a critical cDC differentiation factor. Overexpression of Flt3-L during IAV infection boosted the cDC progenitors' production in the BM, replenished cDCs in the lungs, decreased inflammatory monocytes' infiltration and lowered lung damages. This was associated with partial protection against secondary pneumococcal infection, as reflected by reduced bacterial dissemination and prolonged survival. These findings highlight the impact of distal viral infection on cDC genesis in the BM and suggest that Flt3-L may have potential applications in the control of secondary infections.

Flt3 Ligand Treatment Attenuates T Cell Dysfunction and Improves Survival in a Murine Model of Burn Wound Sepsis

INTRODUCTION

Sepsis is a leading cause of death among severely burned patients. Burn injury disrupts the protective skin barrier and causes immunological dysfunction. In our previous studies, we found that burn injury and wound infection causes a significant decline in lymphocyte populations, implying adaptive immune system dysfunction. In the present study, we examined the effect of treatment with Fms-like tyrosine kinase-3 Ligand (Flt3L) on T cell phenotype and function in a model of burn wound sepsis. FLt3L is an essential cytokine required for hematopoietic progenitor cell development and expansion of both myeloid and lymphoid lineages. Flt3L has been shown to potentiate innate immune functions of dendritic cells and neutrophils during burn wound sepsis. However, the ability of Flt3L to improve T cell function during burn wound sepsis has not been previously evaluated.

METHODS

Mice underwent 35% total body surface area scald burn and were treated with Flt3L (10 μg) or vehicle daily via the intraperitoneal route starting 1 day after burn injury. On day 4 after burn injury, Pseudomonas aeruginosa was used to induce wound infection. Leukocytes in spleen and wound draining lymph nodes were characterized using flow cytometry. Bacterial clearance, organ injury, and survival were also assessed.

RESULTS

Flt3L treatment prevented the decline in splenic CD4 and CD8 T cells caused by burn injury and infection. Flt3L treatment also attenuated the decline in CD28 expression on CD4 and CD8 T cells and IFNγ production by CD8 T cells in the spleen and wound draining lymph nodes. Furthermore, Flt3L decreased the levels of programmed death ligand 1 expression on splenic dendritic cells and macrophages. Flt3 treatment improved systemic bacterial clearance, decreased liver and kidney injury, and significantly improved survival in mice with burn wound sepsis.

CONCLUSION

Burn injury and associated sepsis causes significant loss of T cells and evidence of T cell dysfunction. Flt3L attenuates T cell dysfunction and improves host resistance to burn wound sepsis in mice.

Septic predictor index: A novel platform to identify thermally injured patients susceptible to sepsis

BACKGROUND

During the past decades' sepsis has become the major cause of death in severely burned patients. Despite the importance of burn sepsis, its diagnosis, let alone its prediction, is difficult if not impossible. Recently, we have demonstrated burn patients have increased NLRP3 inflammasome activation in white adipose tissue. We aimed to delineate a unique immune profile that can be used to identify septic outcomes in severely burned patients.

METHODS

Adult burn patients (n = 37) admitted to our burn center between June 2013-2015 were enrolled in this study. White adipose tissue from the site of injury and plasma were collected from severely burned patients (>20% total body surface area) within 96 hours after thermal injury, indiscriminate of sex or age.

RESULTS

We found that patients exhibiting aberrantly high levels of proinflammatory interleukin-1β and decreased macrophages at the site of injury are highly susceptible to development of sepsis. Septic patients also had increased anti-inflammatory (interleukin-10, interleukin-1RA) cytokines in plasma. The Septic Predictor Index was generated as a quotient for the site of injury macrophage proportion and interleukin-1β production. All patients who eventually develop sepsis had septic predictor index values >0.5. Septic patients with Septic Predictor Index values >1 all had sepsis onset within 12 days post-injury, whereas patients with Septic Predictor Index values between 0.5-1 all had later onset (>12 days).

CONCLUSION

The Septic Predictor Index can determine sepsis onset accurately in thermally injured patients a priori and further enables surgeons to develop clinical studies and focused therapies specifically designed for septic cohorts.

Fms-like tyrosine kinase-3 ligand increases resistance to burn wound infection through effects on plasmacytoid dendritic cells

Background

Patients experiencing large thermal injuries are susceptible to opportunistic infections that can delay recovery and lead to sepsis. Dendritic cells (DC) are important for the detection of pathogens and activation of the innate and acquired immune responses. DCs are significantly decreased in burn patients early after injury, and the development of sepsis is associated with persistent DC depletion. In a murine model of burn wound infection, the enhancement of DCs after injury by treatment with the DC growth factor Fms-like tyrosine kinase-3 ligand (FL) enhances neutrophil migration to infection, improves bacterial clearance, and increases survival in a DC-dependent manner. FL expands the production of both conventional DCs (cDC) and plasmacytoid DCs (pDC). It has been established that cDCs are required for some of the protective effects of FL after burn injury. This study was designed to determine the contribution of the pDC subset.

Methods

Mice were subjected to full-thickness scald burns under deep anesthesia and were provided analgesia. pDCs were depleted by injection of anti-plasmacytoid dendritic cell antigen-1 antibodies. Survival, bacterial clearance, and neutrophil responses in vivo and in vitro were measured.

Results

Depletion of preexisting pDCs, but not FL-expanded pDCs, abrogated the beneficial effects of FL on survival, bacterial clearance, and neutrophil migration in response to burn wound infection. This requisite role of pDCs for FL-mediated enhancement of neutrophil migratory capacity is not due to direct effects of pDCs on neutrophils. cDCs, but not pDCs, directly increased neutrophil migratory capacity after co-culture.

Conclusions

The protective effects of FL treatment after burn injury are mediated by both pDCs and cDCs. Pharmacological enhancement of both DC subtypes by FL is a potential therapeutic intervention to enhance immune responses to infection and improve outcome after burn injury.

Polymicrobial Sepsis Diminishes Dendritic Cell Numbers and Function Directly Contributing to Impaired Primary CD8 T Cell Responses In Vivo

Patients surviving acute stages of sepsis often display impaired adaptive-immune responses. Using the cecal ligation and puncture model, we demonstrated that sepsis leads to substantial and long-lasting changes in the naive CD8 T cell repertoire, affecting the capacity of the host to respond to new infections. However, the identity of CD8 T cell-extrinsic factor(s) and mechanism(s) that contribute to impaired CD8 T cell responses after sepsis is unknown. Priming of naive CD8 T cells is critically dependent on the ability of dendritic cells (DCs) to provide Ag, costimulation, and inflammatory signal 3 cytokines; therefore, the sepsis-induced changes in the DC compartment might represent a contributing factor leading to diminished CD8 T cell immunity in septic hosts. In a direct test of this hypothesis, we show that, in addition to numerical decline, sepsis leads to functional impairments in DCs, diminishing their capacity to produce cytokines upon TLR stimulation in vitro or postinfection in vivo. Importantly, we demonstrated a direct link between DC dysfunction and impairments in CD8 T cell immunity after sepsis by directly targeting Ag to DCs. Finally, postsepsis Flt3 ligand treatment increased the number of DCs and improved DC function, including the ability to sense inflammation and produce IL-12, leading to improved primary CD8 T cell responses to newly encountered Ags. Thus, sepsis-induced numerical and functional loss of DCs contributes to the observed defects in CD8 T cell immunity, and therapeutic approaches designed to improve the status of the DC compartment after sepsis might facilitate the recovery of CD8 T cell immunity.

Macrophage polarization and MRSA infection in burned mice

Mortality associated with Staphylococcus aureus infection remains high during the sub-acute phase of burn injury. In this study, we aimed to improve antibacterial resistance of sub-acutely burned mice through macrophage polarization. Sepsis did not develop in mice at the sub-acute phase of 5% total body surface area (TBSA) burn after being infected with methicillin-resistant S. aureus (MRSA), and M1 macrophages (interleukin (IL)-10^-IL-12⁺ inducible nitric oxide synthase⁺ Mφ) were isolated from these mice. In contrast, predominantly M2b macrophages (C-C motif chemokine ligand 1 (CCL1)⁺IL-10⁺IL-12^- Mφ) were isolated from mice with >15% TBSA burn, and all of these mice died after the same MRSA infection. Comparing NOD scid gamma mice inoculated with Mφ with 25% TBSA burns, all mice treated with CCL1 antisense oligodeoxynucleotide (ODN) survived after MRSA infection, whereas all untreated mice given the same infection died within 4 days. CCL1 antisense ODN has been characterized as a specific polarizer of M2bMφ. M1Mφ were isolated from MRSA-infected mice with 25% TBSA burn after treatment with CCL1 antisense ODN, and these mice were shown to be resistant against a lethal dose of MRSA infection. M1Mφ were also isolated from 25% TBSA-burned mice infected with MRSA when the ODN was administered therapeutically, and subsequent sepsis was effectively controlled in these mice. These results indicate that the M2bMφ polarizer is beneficial for controlling MRSA infection in mice at the sub-acute phase of severe burn injury.

Immunotherapy: A promising approach to reverse sepsis-induced immunosuppression

Sepsis is defined as life-threatening organ dysfunction caused by dysregulated host responses to infection (Third International Consensus definition for Sepsis and septic shock). Despite decades of research, sepsis remains the leading cause of death in intensive care units. More than 40 clinical trials, most of which have targeted the sepsis-associated pro-inflammatory response, have failed. Thus, antibiotics and fluid resuscitation remain the mainstays of supportive care and there is intense need to discover and develop novel, targeted therapies to treat sepsis. Both pre-clinical and clinical studies over the past decade demonstrate unequivocally that sepsis not only causes hyper-inflammation, but also leads to simultaneous adaptive immune system dysfunction and impaired antimicrobial immunity. Evidences for immunosuppression include immune cell depletion (T cells most affected), compromised T cell effector functions, T cell exhaustion, impaired antigen presentation, increased susceptibility to opportunistic nosocomial infections, dysregulated cytokine secretion, and reactivation of latent viruses. Therefore, targeting immunosuppression provides a logical approach to treat protracted sepsis. Numerous pre-clinical studies using immunomodulatory agents such as interleukin-7, anti-programmed cell death 1 antibody (anti-PD-1), anti-programmed cell death 1 ligand antibody (anti-PD-L1), and others have demonstrated reversal of T cell dysfunction and improved survival. Therefore, identifying immunosuppressed patients with the help of specific biomarkers and administering specific immunomodulators holds significant potential for sepsis therapy in the future. This review focusses on T cell dysfunction during sepsis and discusses the potential immunotherapeutic agents to boost T cell function during sepsis and improve host resistance to infection.

IL-15 Superagonist Expands mCD8+ T, NK and NKT Cells after Burn Injury but Fails to Improve Outcome during Burn Wound Infection

Background

Severely burned patients are highly susceptible to opportunistic infections and sepsis, owing to the loss of the protective skin barrier and immunological dysfunction. Interleukin-15 (IL-15) belongs to the IL-2 family of common gamma chain cytokines and stimulates the proliferation and activation of T (specifically memory CD8), NK and NKT cells. It has been shown to preserve T cell function and improve survival during cecal ligation and puncture (CLP)-induced sepsis in mice. However, the therapeutic efficacy of IL-15 or IL-15 superagonist (SA) during infection after burn injury has not been evaluated. Moreover, very few, if any, studies have examined, in detail, the effect of burn injury and infection on the adaptive immune system. Thus, we examined the effect of burn and sepsis on adaptive immune cell populations and the effect of IL-15 SA treatment on the host response to infection.

Methods

Mice were subjected to a 35% total body surface area burn, followed by wound infection with Pseudomonas aeruginosa. In some experiments, IL-15 SA was administered after burn injury, but before infection. Leukocytes in spleen, liver and peritoneal cavity were characterized using flow cytometry. Bacterial clearance, organ injury and survival were also assessed.

Results

Burn wound infection led to a significant decline in total white blood cell and lymphocyte counts and induced organ injury and sepsis. Burn injury caused decline in CD4⁺ and CD8⁺ T cells in the spleen, which was worsened by infection. IL-15 treatment inhibited this decline and significantly increased cell numbers and activation, as determined by CD69 expression, of CD4⁺, CD8⁺, B, NK and NKT cells in the spleen and liver after burn injury. However, IL-15 SA treatment failed to prevent burn wound sepsis-induced loss of CD4⁺, CD8⁺, B, NK and NKT cells and failed to improve bacterial clearance and survival.

Conclusion

Cutaneous burn injury and infection cause significant adaptive immune dysfunction. IL-15 SA does not augment host resistance to burn wound sepsis in mice despite inducing proliferation and activation of lymphocyte subsets.

Role of G-CSF in monophosphoryl lipid A-mediated augmentation of neutrophil functions after burn injury

Infection is the leading cause of death in severely burned patients that survive the acute phase of injury. Neutrophils are the first line of defense against infections, but hospitalized burn patients frequently cannot mount an appropriate innate response to infection. Thus, immune therapeutic approaches aimed at improving neutrophil functions after burn injury may be beneficial. Prophylactic treatment with the TLR4 agonist monophosphoryl lipid A is known to augment resistance to infection by enhancing neutrophil recruitment and facilitating bacterial clearance. This study aimed to define mechanisms by which monophosphoryl lipid A treatment improves bacterial clearance and survival in a model of burn-wound sepsis. Burn-injured mice were treated with monophosphoryl lipid A or vehicle, and neutrophil mobilization was evaluated in the presence or absence of Pseudomonas aeruginosa infection. Monophosphoryl lipid A treatment induced significant mobilization of neutrophils from the bone marrow into the blood and sites of infection. Neutrophil mobilization was associated with decreased bone marrow neutrophil CXCR4 expression and increased plasma G-CSF concentrations. Neutralization of G-CSF before monophosphoryl lipid A administration blocked monophosphoryl lipid A-induced expansion of bone marrow myeloid progenitors and mobilization of neutrophils into the blood and their recruitment to the site of infection. G-CSF neutralization ablated the enhanced bacterial clearance and survival benefit endowed by monophosphoryl lipid A in burn-wound-infected mice. Our findings provide convincing evidence that monophosphoryl lipid A-induced G-CSF facilitates early expansion, mobilization, and recruitment of neutrophils to the site of infection after burn injury, allowing for a robust immune response to infection.

Adoptive transfer of bone marrow-derived dendritic cells decreases inhibitory and regulatory T-cell differentiation and improves survival in murine polymicrobial sepsis

A decrease in the number of dendritic cells (DCs) is a major cause of post-sepsis immunosuppression and opportunistic infection and is closely associated with poor prognosis. Increasing the number of DCs to replenish their numbers post sepsis can improve the condition. This therapeutic approach could improve recovery after sepsis. Eighty C57BL/6 mice were subjected to sham or caecal ligation and puncture (CLP) surgery. Mice were divided into four groups: (i) Sham + vehicle, (ii) Sham + DC, (iii) CLP + vehicle, and (iv) CLP + DC. Bone-marrow-derived DCs (BMDCs) were administered at 6, 12 and 24 hr after surgery. After 3 days, we assessed serum indices of organ function (alanine aminotransferase, aspartate aminotransferase, creatinine, amylase and lipase), organ tissue histopathology (haematoxylin and eosin staining), cytokine [interferon-γ (IFN-γ), tumour necrosis factor-α, interleukin-12p70 (IL-12p70), IL-6 and IL-10] levels in the serum, programmed death-1 (PD-1) expression on T cells, regulatory T-cell differentiation in the spleen, and the survival rate (monitored for 7 days). BMDC transfer resulted in the following changes: a significant reduction in damage to the liver, kidney and pancreas in the CLP-septic mice as well as in the pathological changes seen in the liver, lung, small intestine and pancreas; significantly elevated levels of the T helper type 1 (Th1) cytokines IFN-γ and IL-12p70 in the serum; decreased levels of the Th2 cytokines IL-6 and IL-10 in the serum; reduced expression of PD-1 molecules on CD4(+) T cells; reduced the proliferation and differentiation of splenic suppressor T cells and CD4(+)  CD25(+)  Foxp3(+) regulatory T cells, and a significant increase in the survival rate of the septic animals. These results show that administration of BMDCs may have modulated the differentiation and immune function of T cells and contributed to alleviate immunosuppression, hence reducing organ damage and mortality post sepsis. Hence, the immunoregulatory effect of BMDC treatment has potential for the treatment of sepsis.

Enhancement of wound closure in diabetic mice by ex vivo expanded cord blood CD34+ cells

Diabetes can impair wound closure, which can give rise to major clinical problems. Most treatments for wound repair in diabetes remain ineffective. This study aimed to investigate the influence on wound closure of treatments using expanded human cord blood CD34⁺ cells (CB-CD34⁺ cells), freshly isolated CB-CD34⁺ cells and a cytokine cocktail. The test subjects were mice with streptozotocin-induced diabetes. Wounds treated with fresh CB-CD34⁺ cells showed more rapid repair than mice given the PBS control. Injection of expanded CB-CD34⁺ cells improved wound closure significantly, whereas the injection of the cytokine cocktail alone did not improve wound repair. The results also demonstrated a significant decrease in epithelial gaps and advanced re-epithelialization over the wound bed area after treatment with either expanded CB-CD34⁺ cells or freshly isolated cells compared with the control. In addition, treatments with both CB-CD34⁺ cells and the cytokine cocktail were shown to promote recruitment of CD31⁺-endothelial cells in the wounds. Both the CB-CD34⁺ cell population and the cytokine treatments also enhanced the recruitment of CD68-positive cells in the early stages (day 3) of treatment compared with PBS control, although the degree of this enhancement was found to decline in the later stages (day 9). These results demonstrated that expanded CB-CD34⁺ cells or freshly isolated CB-CD34⁺ cells could accelerate wound repair by increasing the recruitment of macrophages and capillaries and the reepithelialization over the wound bed area. Our data suggest an effective role in wound closure for both ex vivo expanded CB-CD34⁺ cells and freshly isolated cells, and these may serve as therapeutic options for wound treatment for diabetic patients. Wound closure acceleration by expanded CB-CD34⁺ cells also breaks the insufficient quantity obstacle of stem cells per unit of cord blood and other stem cell sources, which indicates a broader potential for autologous transplantation.

Treatment with Fms-like tyrosine kinase 3 ligand reverses lung dendritic cell immunoparalysis and ameliorates zymosan-induced secondary lung injury in mice

Depletion and dysfunction of dendritic cells in the lung can induce local immunoparalysis, which often leads to multiple organ dysfunction syndrome (MODS)-associated mortality. A therapeutic strategy that reverses this immunoparalysis is required. In the present study, we examined the effects of in vivo Fms-like tyrosine kinase 3 ligand (Flt3L) treatment on zymosan (zym)-induced secondary lung injury and dendritic cell (DC) immunoparalysis. BALBc mice were divided randomly into four groups (20/group): (1) sham [intraperitoneal (i.p.) saline] + vehicle [subcutaneous (s.c.) 0·01% mouse serum albumin]; (2) sham + Flt3L (s.c.); (3) zym (i.p.) + vehicle; and (4) zym + Flt3L. Injections were for 9 consecutive days; 12 days later we examined: survival rate (monitored for 12 days); lung tissue histopathology (haematoxylin and eosin staining); plasma indices of lung function (pH, PaO(2) , PaCO(2) , HCO(3) (-) ); DC subsets in lung tissue; and lung DCs production of interleukin (IL)-12p70 and IL-10. Zym administration resulted in increased mortality associated with significant lung histopathological changes and abnormal blood gas indices; however, these pathological changes were ameliorated by Flt3L treatment. Zym injections also resulted in significant reductions in DC subsets recovered from lungs [CD11c(+) major histocompatibility complex (MHC)-II/I-A(d+) , CD11c(+) CD11b(+) and CD11c(+) B220(+) ]. Importantly, in-vivo Flt3L treatment reversed these trends for DC immunoparalysis by increasing the percentages of recovered DC subsets concomitant with increased DC production of IL-12 p70 and decreased IL-10 production. These results suggest that Flt3L may have therapeutic potential for reversing DC immunoparalysis and ameliorating lung injury secondary to MODS.

FMS-like tyrosine kinase 3 ligand treatment of mice aggravates acute lung injury in response to Streptococcus pneumoniae: role of pneumolysin

FMS-like tyrosine kinase-3 ligand (Flt3L) is a dendritic cell (DC) growth and differentiation factor with potential in antitumor therapies and antibacterial immunization strategies. However, the effect of systemic Flt3L treatment on lung-protective immunity against bacterial infection is incompletely defined. Here, we examined the impact of deficient (in Flt3L knockout [KO] mice), normal (in wild-type [WT] mice), or increased Flt3L availability (in WT mice pretreated with Flt3L for 3, 5, or 7 days) on lung DC subset profiles and lung-protective immunity against the major lung-tropic pathogen, Streptococcus pneumoniae. Although in Flt3L-deficient mice the numbers of DCs positive for CD11b (CD11b(pos) DCs) and for CD103 (CD103(pos) DCs) were diminished, lung permeability, a marker of injury, was unaltered in response to S. pneumoniae. In contrast, WT mice pretreated with Flt3L particularly responded with increased numbers of CD11b(pos) DCs and with less pronounced numbers of CD103(pos) DCs and impaired bacterial clearance and with increased lung permeability following S. pneumoniae challenge. Notably, infection of Flt3L-pretreated mice with S. pneumoniae lacking the pore-forming toxin, pneumolysin (PLY), resulted in substantially less lung CD11b(pos) DCs activation and reduced lung permeability. Collectively, this study establishes that Flt3L treatment enhances the accumulation of proinflammatory activated lung CD11b(pos) DCs which contribute to acute lung injury in response to PLY released by S. pneumoniae.

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.

Modulation of FLT3 signaling targets conventional dendritic cells to attenuate acute lung injury

Conventional dendritic cells (cDCs) have been reported to participate in the pathophysiology of acute lung injury (ALI). Fms-like tyrosine kinase 3 (FLT3) signaling represents a highly specific pathway for the manipulation of cDCs in vivo. The purpose of this study was to clarify the effect of FLT3 signaling on the accumulation and maturation of pulmonary cDCs, and whether inhibition of FLT3 signaling may attenuate acute lung inflammation and lung injury. C57BL/6 mice were pretreated with FLT3-ligand (FLT3L) and lestaurtinib separately for five consecutive days. A murine model of ALI was subsequently generated by intra-tracheal instillation of lipopolysaccharide (LPS) and lung specimens were harvested 24 h later. Flow cytometry was conducted to measure the accumulation and maturation of pulmonary cDCs. IL-6, IFN-γ, IL-4, MPO activity and transcription factor T-bet/GATA-3 mRNA ratio were quantified to evaluate lung inflammation. Lung injury was estimated by lung wet weight/body weight ratio (LWW/BW) and histopathological analysis. LPS challenge resulted in rapid accumulation and maturation of pulmonary cDCs. FLT3L pretreatment further stimulated the accumulation and maturation of pulmonary cDCs, leading to a markedly increased LWW/BW and aggravated lung histopathology. Meanwhile, lung MPO activity, T-bet/GATA-3 mRNA ratio and concentrations of IL-6 and IFN-γ were elevated by FLT3L administration. In contrast, lestaurtinib pretreatment inhibited the accumulation and maturation of pulmonary cDCs, leading to a significantly decreased LWW/BW and improved lung histopathology. Lestaurtinib administration also suppressed lung MPO activity, T-bet/GATA-3 mRNA ratio and production of IL-6 and IFN-γ. Our findings show that FLT3 signaling ameliorates ALI by regulating the accumulation and maturation of pulmonary cDCs, suggesting an innovative pharmacotherapy for ALI.

High MafB expression following burn augments monocyte commitment and inhibits DC differentiation in hemopoietic progenitors

We have previously shown that perturbed bone marrow progenitor development promotes hyporesponsive monocytes following experimental burn sepsis. Clinical and experimental sepsis is associated with monocyte deactivation and depletion of mDCs. Decrease in circulating DCs is reported in burn patients who develop sepsis. In our 15% TBSA scald burn model, we demonstrate a significant reduction in the circulating MHC-II(+) population and mDCs (Gr1(neg)CD11b(+)CD11c(+)) with a corresponding decrease in bone marrow MHC-II(+) cells and mDCs for up to 14 days following burn. We explored the underlying mechanism(s) that regulate bone marrow development of monocytes and DCs following burn injury. We found a robust bone marrow response with a significant increase in multipotential HSCs (LSK) and bipotential GMPs following burn injury. GMPs from burn mice exhibit a significant reduction in GATA-1, which is essential for DC development, but express high levels of MafB and M-CSFRs, both associated with monocyte production. GMPs obtained from burn mice differentiated 1.7 times more into Mϕ and 1.6-fold less into DCs compared with sham. Monocytes and DCs expressed 50% less MHC-II in burn versus sham. Increased monocyte commitment in burn GMPs was a result of high MafB and M-CSFR expressions. Transient silencing of MafB (siRNA) in GMP-derived monocytes from burn mice partially restored DC differentiation deficits and increased GATA-1 expression. We provide evidence that high MafB following burn plays an inhibitory role in monocyte-derived DC differentiation by regulating M-CSFR and GATA-1 expressions.

Dendritic cell modification of neutrophil responses to infection after burn injury

Burn patients are highly susceptible to infections due to increased exposure through wounds and impairments in a number of immune functions. Dendritic cells (DCs) are important in activation of numerous immune responses that are essential for the clearance of infections. We have found that prophylactic treatment of burn-injured mice with the DC growth factor FLT3 ligand (FL) significantly increases resistance to burn wound infections in a DC-dependent manner that is correlated closely with enhanced bacterial clearance. However, as DCs are not typically microbicidal, the mechanisms by which DC modulation enhances bacterial clearance are not known. Due to the rapid response of neutrophils to cutaneous wounds, and the reported interactions between DCs and neutrophils, we investigated the role of neutrophils in FL-mediated resistance to burn wound infection. This was examined both in vivo and in vitro through neutrophil depletion, supplementation of neutrophils, and assessment of neutrophil chemotaxis following FL treatment. To test the involvement of DCs, CD11c-diphtheria toxin receptor transgenic mice were used to deplete DCs during FL treatment. Studies revealed that neutrophils do play a critical role in FL-mediated resistance to a burn wound infection. Additionally, treatment with FL after a burn injury enhances neutrophil-mediated control of bacterial spread, neutrophil migratory capacity, and myeloperoxidase production in a DC-dependent manner. The results of this study provide new insight into immunological mechanisms that can offer protection against infection after burn injury.

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.

Insulin increases resistance to burn wound infection-associated sepsis

OBJECTIVE

This study was designed to determine the ability of insulin to improve outcome following a Pseudomonas aeruginosa wound infection in a rodent model of severe burn injury.

BACKGROUND

Severe burn injury predisposes patients to burn wound infections that can disseminate, lead to uncontrolled inflammation, and induce septic shock. Whereas insulin administration has been extensively discussed to improve morbidity and mortality in critically ill patients, the ability of insulin to improve outcomes of severely burned patients with infected burn wounds is not known.

DESIGN

Sprague-Dawley rats.

SETTING

University setting.

INTERVENTION

Burn-injured Sprague Dawley rats were randomized into treatment groups that received either saline or insulin. Burn wounds were topically inoculated with a lethal dose of Pseudomonas aeruginosa 6 days after injury.

MEASUREMENTS AND MAIN RESULTS

Survival, systemic dissemination of bacteria, systemic inflammation, and immune activation were examined. Insulin decreased the early inflammatory response to a severe burn injury. Treatment with low doses of insulin following burn injury improved the outcome of rats in response to a lethal burn wound infection. Specifically, survival was improved and systemic dissemination of bacteria from the wound was decreased. Systemic inflammation, indicated by serum interleukin-6 levels, was significantly decreased by insulin treatments after injury. Additionally, insulin treatments were associated with alterations in B and T lymphocyte responses to wound infection.

CONCLUSIONS

Although the mechanisms by which insulin improves outcome following a lethal burn wound infection are not known, the data suggest that immunologic responses to infection may be altered by postburn insulin treatments.

Fms-like tyrosine kinase-3 ligand alters antigen-specific responses to infections after severe burn injury

Burn patients are susceptible to opportunistic infections partly because of decreased immune functions, especially TH1-driven antigen-specific responses, which are regulated by dendritic cells. The dendritic cell growth factor, fms-like tyrosine kinase-3 ligand (FL), has been shown to increase resistance to Pseudomonas aeruginosa, in a dendritic cell-dependent manner, in a mouse model of burn wound infection. The specific mechanisms of protection are not known. This study tested the hypothesis that FL can enhance production of P. aeruginosa-specific antibodies after burn wound infection. Mice that had been previously exposed to P. aeruginosa were infected after burn injury by wound inoculation or challenged by intraperitoneal injection of heat-killed P. aeruginosa. In response to wound infection, FL treatments enhanced bacterial clearance and induced a shift from immunoglobulin (Ig) M toward IgG and IgA. However, serum levels of neither P. aeruginosa-specific antibodies nor interferon gamma (IFN-gamma) were significantly increased by FL, possibly because of decreased systemic exposure to bacteria. After challenge with heat-killed bacteria, which ensured equal exposures, FL-treated mice produced significantly greater levels of P. aeruginosa-specific IgG2a, which correlated with an increase in serum levels of interferon gamma and enhanced opsonization capacity. IL-12, IL-10, and transforming growth factor beta were significantly increased in FL-treated mice, regardless of the type of challenge. These findings indicate that FL treatments after burn injury enhance cytokine responses to recall antigens and increase bacterial clearance. In addition, through its ability to promote TH1-associated antigen-specific responses, FL may have potential as an immunotherapy to enhance adaptive immunity after severe burn injury.

Endogenous Fms-like tyrosine kinase-3 ligand levels are not altered in mice after a severe burn and infection

Background

Fms-like tyrosine kinase-3 ligand (Flt3L) is a hemopoietic cytokine and dendritic cell (DC) growth factor that promotes the proliferation and differentiation of progenitor cells into DCs. We have previously found that treatment of severely burned mice with recombinant Flt3L significantly enhances DC production and bacterial clearance from infected burn wounds, and increases global immune cell activation and survival in response to a burn wound infection. These significant benefits of Flt3L treatment after burn injury have prompted the question of whether or not severe burn injury induces deficits in endogenous Flt3L levels that could affect DCs and subsequent responses to infection.

Results

To address this, male BALB/c mice received a 30% total body surface area scald burn. Blood, spleens, and wound-draining lymph nodes were harvested at various time-points after injury. Some mice received a wound inoculation with P. aeruginosa. Murine Flt3L and G-CSF levels were measured by ELISA. Burn injury had no significant effect on Flt3L levels at any post-burn time-point examined compared to normal Flt3L levels in the sera, spleen, or lymph nodes. Additionally, Flt3L levels in the sera, spleen, and lymph nodes were not significantly altered when wounds were inoculated on the day of burn injury or at post-burn time points examined. Alternatively, levels of G-CSF were increased in response to burn injury and burn wound infection. Additionally, DC numbers and functions were not altered following burn injury alone. There was no significant difference between the number of DCs in the spleens of sham-injured mice and mice at 5 days after burn injury. When naïve T cells from sham-injured mice were co-cultured with DCs from either sham- or burn-injured mice, IFN-γ production was similar, however, IFN-γ levels produced by T cells harvested from burn-injured mice were significantly lower than those produced by T cells from sham mice, regardless of which DC group, sham or burn, was used in the coculture.

Conclusion

These data suggest that the beneficial effects of Flt3L treatments after burn injury are not due to correction of a burn-associated Flt3L deficiency but rather, are likely due to supplementary stimulation of DC production and immune responses to infection.

Murine dendritic cell antigen-presenting cell function is not altered by burn injury

Severe injury disrupts normal immune regulation causing a transient hyperinflammatory reaction and suppressed adaptive immune function. This report addresses the potential contribution of dendritic cells (DC) to changes in adaptive immune function after injury by specifically measuring injury-induced changes in splenic DC numbers and subsets, cell-surface markers, TLR responses, and APC function. Using a mouse burn injury model, we found that injury did not markedly alter the relative percentage of lymphoid, myeloid, or plasmacytoid DC in the spleens of burn-injured mice. Moreover, we did not observe a significant reduction in cell-surface expression of several major costimulatory molecules, CD40, CD80, CD86, programmed death 1 ligand, ICOS ligand, and B7-H3, on DC. Instead, we observed increased cell-surface expression of CD86 at 1 day after injury with no significant changes in costimulatory molecule expression at 7 days after injury, suggesting that burn injury causes an early activation of DC. In addition, injury did not suppress DC reactivity to TLR2, TLR4, or TLR9 agonists. Most important, DC prepared from injured mice were able to present peptide antigen to naive OTII TCR transgenic CD4+ T cells as efficiently and effectively as DC from sham-injured mice. We also found that CD4 T cells stimulated with antigen presented by DC from sham or burn mice showed similar levels of IL-2, IFN-gamma, IL-10, and IL-13 production. Taken together, these findings support the conclusion that DC do not acquire a suppressive phenotype following severe injury in mice.