Toll-Like Receptor Signaling in Burn Wound Healing and Scarring

Abstract

Significance: Damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) emanate from burn-injured tissue and enter systemic circulation. Locally and systemically, they activate pattern-recognition receptors, including toll-like receptors (TLRs), to stimulate cytokine secretion, which in the severest burns typically results in extreme systemic cytokine levels, a dysfunctioning immune system, infection, impaired healing, and excessive scarring. This system-wide disruption of homeostasis can advance to life-threatening, multiorgan dysfunction syndrome. Knowledge of DAMP- and PAMP-TLR signaling may lead to treatments that ameliorate local and systemic inflammation and reduce scarring and other burn injury sequela. Recent Advances: Many PAMPs and DAMPs, the TLRs they activate, and their downstream signaling molecules have been shown to contribute to local and systemic inflammation and tissue damage following burn injury. Critical Issues: Whether TLR-pathway-targeting treatments applied at different times postburn injury might improve scarring remains an open question. The evaluation of this question requires the use of appropriate preclinical and clinical burn models carried out until after mature scar has formed. Future Directions: After TLR-pathway-targeting treatments are evaluated in porcine burn wound models and their safety is demonstrated, they can be tested in proof-of-concept clinical burn wound models.

Keywords: burns; experimental models; inflammation; scar; tissue death.

Kai P. Leung, PhD

Figure 1.

Depths of cutaneous burn injuries. Shown are superficial, partial-thickness, deep partial-thickness (DPT), and full-thickness (to hypodermis, shown by dotted line, or beyond) cutaneous burn injuries.

Figure 2.

Outlines of DAMP- and PAMP-TLR signaling resulting in the production of cytokines and interferons. For the details of these outlines, see O'Neill et al. (image modified from). DAMPs and PAMPs activate similar receptors and converge on similar signaling pathways. The transcription factor–activating kinases are prominent drug targets. Image of thermally injured patient was provided by Rodney K. Chan. DAMP, damage-associated molecular pattern; PAMP, pathogen-associated molecular pattern; TLR, toll-like receptor.

Figure 3.

Elevated gene expression in the margins of clinical partial-thickness burn wounds. These data were obtained from a clinical study by Greco and Nanney and colleagues that used the Affymetrix U133 plus 2.0 GeneChip™ to evaluate global gene expression in the wound edge of burns at postburn time periods: Early (0–3 days), Middle (4–7 days), and Late (7–17 days). The significantly elevated genes graphed were extracted from Gene Expression Omnibus accession record GSE8056 (using R). The genes shown were upregulated in cultured keratinocytes in response to high sodium, dependent on the activity of the sodium channel Na_x (scn7a).

Figure 4.

Correlation between burn size and systemic effects. The relationship is not necessarily linear.

Figure 5.

Disruption of systemic homeostasis following burn injury. Larger, deeper, and infected wounds produce uncontrolled inflammation with dysregulated systemic inflammation, susceptibility to infection, organ damage and dysfunction, impaired healing, and high risk for hypertrophic scar (red arrows).

Figure 6.

TLR gene expression in WBCs of severely burned patients (n = 244) over postburn time and healthy subjects (n = 35). Blood was sampled from patients with severe burns (>20% total body surface area; admitted within 96 h of injury; The Inflammation and the Host Response to Injury Large-Scale Collaborative Research Program). Gene expression was analyzed using the Affymetrix U133 plus 2.0 GeneChip. These data were extracted from Gene Expression Omnibus GSE37069. The healthy subject expression values are plotted along the y axis with their mean indicated by a dotted line. The burn-patient TLR expression values were fitted using locally estimated scatterplot smoothing regression, with the 95% confidence interval shown as gray shading. Interestingly, TLRs that heterodimerize (TLR1/TLR2 and TLR4/TLR5) were expressed similarly in WBCs over postburn time. In addition, TLR9 and TLR10 were expressed similarly after burn injury, and both are known to be expressed predominantly in human B cells and to be upregulated with similar kinetics after B cell activation, including activation by CpG DNA. WBC, white blood cell.