Persistent Systemic Inflammation in Patients With Severe Burn Injury Is Accompanied by Influx of Immature Neutrophils and Shifts in T Cell Subsets and Cytokine Profiles

Abstract

Severe burn injury causes local and systemic immune responses that can persist up to months, and can lead to systemic inflammatory response syndrome, organ damage and long-term sequalae such as hypertrophic scarring. To prevent these pathological conditions, a better understanding of the underlying mechanisms is essential. In this longitudinal study, we analyzed the temporal peripheral blood immune profile of 20 burn wound patients admitted to the intensive care by flow cytometry and secretome profiling, and compared this to data from 20 healthy subjects. The patient cohort showed signs of systemic inflammation and persistently high levels of pro-inflammatory soluble mediators, such as IL-6, IL-8, MCP-1, MIP-1β, and MIP-3α, were measured. Using both unsupervised and supervised flow cytometry techniques, we observed a continuous release of neutrophils and monocytes into the blood for at least 39 days. Increased numbers of immature neutrophils were present in peripheral blood in the first three weeks after injury (0.1–2.8 × 106/ml after burn vs. 5 × 103/ml in healthy controls). Total lymphocyte numbers did not increase, but numbers of effector T cells as well as regulatory T cells were increased from the second week onward. Within the CD4+ T cell population, elevated numbers of CCR4+CCR6- and CCR4+CCR6+ cells were found. Altogether, these data reveal that severe burn injury induced a persistent innate inflammatory response, including a release of immature neutrophils, and shifts in the T cell composition toward an overall more pro-inflammatory phenotype, thereby continuing systemic inflammation and increasing the risk of secondary complications.

Severe Traumatic Injury Induces Phenotypic and Functional Changes of Neutrophils and Monocytes

“…Emergency granulopoiesis, which has been extensively described following traumatic injury and microbial challenge [27,28], is a part of the first line of cellular defense that crucially modulate subsequent repair processes after tissue damage [29]. However, on the other hand, an exaggerated release of neutrophils leads to bone marrow exhaustion and in turn, can impair the innate immune response to secondary hit such as surgery or infection [21].…”

“…Further, we observed a massive presence of immature (CD16 dim CD62L bright ) neutrophils in severely injured patients, whereas healthy individuals lack this population. It is assumed that immature neutrophils have impaired functional ability [21]. Moreover, Spijkerman et al have shown that the ratio of immature neutrophils positively correlates with the severity of the injury and the development of infectious complication [28].…”

“…This indicates that the above-described positive correlation between the elevated ratio of immature neutrophils and the development of inflammatory complications does not depend on defective phagocytosis. Moreover, as severe traumatic injury leads to neutrophilia with subsequent bone marrow exhaustion and the CD62L dim subset appears within the first hour after the injury [21,27,28,30], this may together with the incapability of CD62L dim neutrophils to adequately kill the bacteria [37] contribute to the development of infectious complications in later time course, because the immune system might not adequate respond to the bacterial escape from the CD62L dim neutrophils.…”

“…Although the production of reactive oxygen species (ROS) is essential for the pathogen clearance, an exaggerated release of proteases and oxygen radicals may lead to collateral tissue damage and contribute to the development of secondary complications [20,21]. In the context of acute lung injury or its most severe form ARDS, excessive generation of ROS leads to the loss of junctional integrity of vascular microvessels, promoting the migration of polymorphonuclear leukocytes and the transition of fluids in the alveolar lumen, causing pulmonary edema [7].…”

TPPU treatment of burned mice dampens inflammation and generation of bioactive DHET which impairs neutrophil function

“…Burn injury results in a robust systemic activation of both innate and adaptive immunity 14 . Murine burn injury acutely increases relative and absolute numbers of neutrophils 15 , induces acute activation and increase in numbers of Regulatory T cells 15 , 16 and impairs CD4 and CD8 T cell function 17 . Neutrophils are a key cellular component for the resolution of the injury as they are first recruited to the site of injury in quantitatively high numbers 18 .…”