Age-related immune responses after burn and inhalation injury are associated with altered clinical outcomes

Burn-Induced Apoptosis in the Livers of Aged Mice Is Associated With Caspase Cleavage of Bcl-xL

INTRODUCTION

Older adult burn victims have poorer outcomes than younger burn victims. The liver is critical for the recovery of patients with burns. Postburn hepatic apoptosis in young individuals compromises liver integrity; however, this pathway has not yet been studied in older individuals. Because aged animals with burns suffer significant liver damage, we hypothesized that apoptosis is altered in these animals and may affect liver function. Understanding postburn hepatic apoptosis and its effects on liver function in aged animals may help improve outcomes in older patients.

METHODS

We compared the protein and gene expression levels in young and aged mice after a 15% total-body-surface-area burn. Liver and serum samples were collected at different time points after injury.

RESULTS

Caspase-9 expression in liver tissue was downregulated by 47% in young animals and upregulated by 62% in aged animals 9 h postburn (P < 0.05). The livers of aged mice showed a Bcl-extra-large (Bcl-xL) transcription increase only after 6 h; however, the livers of young mice exhibited 4.3-fold, 14.4-fold, and 7.8-fold Bcl-xL transcription increases at 3, 6, and 9 h postburn, respectively (P < 0.05). The livers of young mice showed no changes in Caspase-9, Caspase-3, or Bcl-xL protein levels during the early postburn period. In contrast, the livers of aged mice contained cleaved caspase-9, reduced full-length caspase-3, and an accumulation of ΔN-Bcl-x at 6 and 9 h postburn (P < 0.05). p21 expression decreased in aged mice; however, it was significantly increased in the liver tissue of young mice postburn (P < 0.05). Serum amyloid A1 and serum amyloid A2 serum protein levels were 5.2- and 3.1-fold higher in young mice than in aged mice, respectively, at 6 and 9 h postburn (P < 0.05).

CONCLUSIONS

Livers of aged mice exhibited different apoptotic processes compared to those of young mice early after burn injury. Collectively, burn-induced liver apoptosis in aged mice compromises hepatic serum protein production.

Aberrant inflammatory responses in intoxicated burn-injured patients parallel impaired cognitive function

Burn-injured patients with alcohol use disorder (AUD) have increased morbidity and mortality compared to alcohol-abstaining individuals with similar injuries. It is hypothesized that this is due, in part, to alcohol-induced dysregulation of the systemic inflammatory response, leading to worsened clinical outcomes, including increased susceptibility to infection, and heightened cognitive impairment. To examine the effects of alcohol on inflammatory markers after burn injury, we used multiplex assays to measure a panel of 48 cytokines, chemokines, and growth factors in the plasma of burn patents within 24 h of admission to the University of Colorado Burn Center. Thirty patients were enrolled between July 2018 to February 2020 and were stratified based on presence of AUD and total body surface area (TBSA) burn of ≥20% into four groups: [AUD-, TBSA <20%, N = 12], [AUD+, TBSA <20%, N = 3], [AUD-, TBSA ≥20%, N = 8], [AUD+, TBSA ≥20%, N = 7]. In addition, Confusion Assessment Method (CAM) scores were collected to evaluate patient delirium during the course of hospitalization. Multivariate statistical analysis demonstrated a number of cytokines and other factors that were significantly different between the groups. For example, the anti-inflammatory cytokine interleukin 1 receptor antagonist (IL-1ra) was dampened in the AUD+, TBSA ≥20% cohort with a 75.2% decrease compared to AUD-, TBSA ≥20%, and an 83.9% decrease compared to AUD-, TBSA <20% (p = 0.008). Additionally, plasma levels of the pro-inflammatory mediator CXCL12 (C-X-C motif chemokine ligand 12, also known as stromal cell-derived factor 1, SDF-1) was higher in the AUD + groups (p = 0.03) and similarly, IL-18 levels were greater in AUD+, TBSA ≥20% (p = 0.009). Eotaxin (also known as cytokine CC motif ligand 11, CCL11) was markedly elevated in the AUD+, TBSA ≥20% cohort with a 2.4-fold increase over the AUD-, TBSA ≥20%, and a 1.7-fold rise compared to the AUD-, TBSA <20% cohorts (p = 0.04). Interestingly, there was also a marked rise in CAM + delirium scores (85.7%) among the AUD + patients with TBSA ≥20% (p = 0.02). Not surprisingly, we found that hospital stays increased with AUD+ and larger burns (p = 0.0009). Our findings reveal that burn patients who misuse alcohol have aberrant inflammatory responses that may lead to greater immune dysregulation and worse clinical outcomes.

Cutaneous burn injury induces neuroinflammation and reactive astrocyte activation in the hippocampus of aged mice

BACKGROUND

By 2050, one in six people globally will be 65 or older. Confusion and delirium are significant complications after burn injury, especially in the elderly population. The etiology is still unknown, however complications may be driven by pro-inflammatory activation of astrocytes within the hippocampus (HPC) after burn injury. Reduced levels of phosphorylated cyclic-AMP response binding element (pCREB), caused by elevated systemic pro-inflammatory cytokines, could lead to cognitive decline and memory impairment.

METHODS

To examine the effects of remote injury on neuroinflammation in advanced age, young and aged mice were subjected to a 15 % total body surface area scald burn or sham injury, and euthanized after 24 h. Expression of ccl2 and tnfa were measured by qPCR in the whole brain and HPC. Astrocyte activation was measured by immunofluorescence within the HPC. pCREB was measured by immunohistochemistry in the dentate gyrus.

RESULTS

We saw an 80-fold increase in ccl2 and a 30-fold elevation in tnfa after injury in the whole brain of aged mice compared to young groups and aged sham mice (p < 0.05 and p < 0.05, respectively). Additionally, there was a 30-fold increase in ccl2 within isolated HPC of aged injured mice when compared to sham injured animals (p < 0.05). When investigating specific HPC regions, immunofluorescence staining showed a >20 % rise in glial fibrillary acidic protein (GFAP) positive astrocytes within the cornu ammonis 3 (CA3) of aged injured mice when compared to all other groups (p < 0.05). Lastly, we observed a >20 % decrease in pCREB staining by immunohistochemistry in the dentate gyrus of aged mice compared to young regardless of injury (p < 0.05).

CONCLUSIONS

These novel data suggest that remote injury in aged, but not young, mice is associated with neuroinflammation and astrocyte activation within the HPC. These factors, paired with an age related reduction in pCREB, could help explain the increased cognitive decline seen in burn patients of advanced age. To our knowledge, we are the first group to examine the impact of advanced age combined with burn injury on inflammation and astrocyte activation within the brain.

Advanced age exacerbates intestinal epithelial permeability after burn injury in mice

BACKGROUND

Advanced age is an independent risk factor for morbidity and mortality after burn injury. Following burn, the intestines can become permeable leading to the leakage of bacteria and their products from the lumen of the ileum to the portal and systemic circulation. Here, we sought to determine the effects of advanced age on intestinal permeability post burn injury and assess intestinal inflammatory biomarkers.

METHODS

Young (4-5 months) and aged (18-22 months) female BALB/cBy mice were subjected to a 12-15% total body surface area (TBSA) sham or burn injury. 24 h after injury, mice were euthanized, and organs collected. Colony-forming units (CFU) were counted from plated mesenteric lymph nodes (MLN). Gene expression of ileal tight junctional proteins, occludin and zonula occludens 1 (ZO-1), in addition to ileal damage associated molecular pattern (DAMP) proteins, S100A8 and S100A9, as well as ileal inflammatory markers IL-6 and TNF-α were measured by qPCR. Intestinal cell death was measured by ELISA. Intestinal permeability was determined by FITC fluorescence in serum; 4kD FITC-dextran was given by oral gavage 3 h before euthanasia.

RESULTS

Aged mice subjected to burn injury had increased intestinal permeability as evidenced by a 5.8-fold higher level of FITC-dextran in their serum when compared to all other groups (p < 0.05). In addition, aged burn-injured mice exhibited heightened bacterial accumulation in the MLN with a 15.5-fold increase over all other groups (p < 0.05). Histology of ileum failed to show differences in villus length among all groups. Analysis of ileal tight junctional proteins and inflammatory marker gene expression revealed no difference in Ocln, Tjp1, Il6, or Tnf expression among all groups, but 2.3 and 2.9-fold upregulation of S100a8 and S100a9, respectively, in aged burn-injured mice relative to both young groups and aged sham-injured mice (p < 0.05). Lastly, cell death in the ileum was elevated more than two-fold in aged burn-injured mice relative to young animals regardless of injury (p < 0.05).

CONCLUSIONS

These data demonstrate that advanced age exacerbates intestinal epithelial permeability after burn injury. Heightened apoptosis may be responsible for the elevated intestinal leakiness and accumulation of bacteria in mesenteric lymph nodes. In addition, S100a8/9 may serve as a biomarker of elevated inflammation within the intestine.

Age and Injury Size Influence the Magnitude of Fecal Dysbiosis in Adult Burn Patients

Clinical studies have demonstrated that age 50 years or older is an independent risk factor associated with poor prognosis after burn injury, the second leading cause of traumatic injuries in the aged population. While mechanisms driving age-dependent postburn mortality are perplexing, changes in the intestinal microbiome, may contribute to the heightened, dysregulated systemic response seen in aging burn patients. The fecal microbiome from 22 patients admitted to a verified burn center from July 2018 to February 2019 was stratified based on the age of 50 years and total burn surface area (TBSA) size of ≥10%. Significant differences (P = .014) in overall microbiota community composition (ie, beta diversity) were measured across the four patient groups: young <10% TBSA, young ≥10% TBSA, older <10% TBSA, and older ≥10% TBSA. Differences in beta diversity were driven by %TBSA (P = .013) and trended with age (P = .087). Alpha diversity components, richness, evenness, and Shannon diversity were measured. We observed significant differences in bacterial species evenness (P = .0023) and Shannon diversity (P = .0033) between the groups. There were significant correlations between individual bacterial species and levels of short-chain fatty acids. Specifically, levels of fecal butyrate correlated with the presence of Enterobacteriaceae, an opportunistic gut pathogen, when elevated in burn patients lead to worsen outcomes. Overall, our findings reveal that age-specific changes in the fecal microbiome following burn injuries may contribute to immune system dysregulation in patients with varying TBSA burns and potentially lead to worsened clinical outcomes with heightened morbidity and mortality.

Inhalation Injury is Associated with Endotheliopathy and Abnormal Fibrinolytic Phenotypes in Burn Patients: A Cohort Study

Burn injury is associated with endothelial dysfunction and coagulopathy and concomitant inhalation injury increases morbidity and mortality. The aim of this work is to identify associations between inhalation injury (IHI), coagulation homeostasis, vascular endothelium, and clinical outcomes in burn patients. One-hundred and twelve patients presenting to a regional burn center were included in this retrospective cohort study. Whole blood was collected at set intervals from admission through 24 hours and underwent viscoelastic assay with rapid TEG (rTEG). Syndecan-1 (SDC-1) on admission was quantified by ELISA. Patients were grouped by the presence (n=28) or absence (n=84) of concomitant IHI and rTEG parameters, fibrinolytic phenotypes, SDC-1, and clinical outcomes were compared. Of the 112 thermally injured patients, 28 (25%) had IHI. Most patients were male (68.8%) with a median age of 40 (IQR, 29-57) years. Patients with IHI had higher overall mortality (42.68% vs. 8.3%; p<0.0001). rTEG LY30 was lower in patients with IHI at hours 4 and 12 (p<0.05). There was a pattern of increased abnormal fibrinolytic phenotypes among IHI patients. There was a greater proportion of IHI patients with endotheliopathy (SDC-1 > 34 ng/mL) (64.7% vs. 26.4%; p=0.008). There was a pattern of increased mortality among patients with inhalation injury and endotheliopathy (0% vs. 72.7%; p=0.004). Significant differences between patients with and without IHI were found in measures assessing fibrinolytic potential and endotheliopathy. Mortality was associated with abnormal fibrinolysis, endotheliopathy, and inhalation injury. However, the extent to which IHI associated dysfunction is independent of TBSA burn size remains to be elucidated.

Inhalation Injury: Unmet Clinical Needs and Future Research

Pulmonary and systemic insults from inhalation injury can complicate the care of burn patients and contribute to significant morbidity and mortality. However, recent progress in diagnosis and treatment of inhalation injury has not kept pace with the care of cutaneous thermal injury. There are many challenges unique to inhalation injury that have slowed advancement, including deficiencies in our understanding of its pathophysiology, the relative difficulty and subjectivity of bronchoscopic diagnosis, the lack of diagnostic biomarkers, the necessarily urgent manner in which decisions are made about intubation, and the lack of universal recommendations for the application of mucolytics, anticoagulants, bronchodilators, modified ventilator strategies, and other measures. This review represents a summary of critical shortcomings in our understanding and management of inhalation injury identified by the American Burn Association's working group on Cutaneous Thermal Injury and Inhalation Injury in 2018. It addresses our current understanding of the diagnosis, pathophysiology, and treatment of inhalation injury and highlights topics in need of additional research, including 1) airway repair mechanisms; 2) the airway microbiome in health and after injury; and 3) candidate biomarkers of inhalation injury.

How Inflammation Blunts Innate Immunity in Aging

The collective loss of immune protection during aging leads to poor vaccine responses and an increased severity of infection for the elderly. Here, we review our current understanding of effects of aging on the cellular and molecular dysregulation of innate immune cells as well as the relevant tissue milieu which influences their functions. The innate immune system is composed of multiple cell types which provide distinct and essential roles in tissue surveillance and antigen presentation as well as early responses to infection or injury. Functional defects that arise during aging lead to a reduced dynamic range of responsiveness, altered cytokine dynamics, and impaired tissue repair. Heightened inflammation influences both the dysregulation of innate immune responses as well as surrounding tissue microenvironments which have a critical role in development of a functional immune response. In particular, age-related physical and inflammatory changes in the skin, lung, lymph nodes, and adipose tissue reflect disrupted architecture and spatial organization contributing to diminished immune responsiveness. Underlying mechanisms include altered transcriptional programming and dysregulation of critical innate immune signaling cascades. Further, we identify signaling functions of bioactive lipid mediators which address chronic inflammation and may contribute to the resolution of inflammation to improve innate immunity during aging.

Aging leads to dysfunctional innate immune responses to TLR2 and TLR4 agonists

BACKGROUND

Sepsis is more common in the elderly. TNF⍺ is recognized as an important mediator in sepsis and Toll-like receptors (TLRs) play an important role in initiating signaling cascades to produce TNF⍺. Little is known about how innate immunity is altered in healthy human aging that predisposes to sepsis.

AIMS AND METHODS

We tested the hypothesis that aging dysregulates the innate immune response to TLR 2 and 4 ligands. We performed whole blood assays on 554 healthy subjects aged 40-80 years. TNFα production was measured at baseline and after stimulation with the TLR2 agonists: peptidoglycan, lipoteichoic acid, Pam3CysK, Zymosan A and the TLR4 agonist lipopolysaccharide (LPS). In a subset of subjects (n = 250), we measured Toll-like receptor (TLR) 2, 4 and MyD88 expression using real-time PCR.

RESULTS AND DISCUSSION

We measured a 2.5% increase per year in basal secretion of TNFα with aging (n = 554 p = 0.02). Likewise, TNFα secretion was increased with aging after stimulation with peptidoglycan (1.3% increase/year; p = 0.0005) and zymosan A (1.1% increase/year p = 0.03). We also examined the difference between baseline and stimulated TNFα for each individual. We found that the increase was driven by the elevated baseline levels. In fact, there was a diminished stimulated response to LPS (1.9% decrease/year; p = 0.05), lipoteichoic acid (2.1% decrease/year p = 0.03), and Pam3CysK (2.6% decrease/year p = 0.0007). There were no differences in TLR or MyD88 mRNA expression with aging, however, there was an inverse relationship between TLR expression and stimulated TNFα production.

CONCLUSIONS

With aging, circulating leukocytes produce high levels of TNFα at baseline and have inadequate responses to TLR2 and TLR4 agonists. These defects likely contribute to the increased susceptibility to sepsis in older adults.