Murine leukocyte dysfunction in response to a non-lethal flame burn

In a non-lethal, 10% total body surface area, full-thickness flame mouse model, infections with <i>Pseudomonas aeruginosa</i> (PA) increased mortality post-burn, suggesting an impaired host immune response. The presence of a seroma beneath the burn wound sequesters CD45⁺ cells. Furthermore, in the case of burn and infection, PA was found to be in proximity to these cells but was not phagocytosed, suggesting leukocyte dysfunction. In this study, leukocytes isolated from the circulation and seroma of burned mice had a decreased ability to kill PA compared to the circulating leukocytes of Sham mice. Both Sham and burned mouse leukocytes lost the ability to kill when incubated <i>in vitro</i> with seroma fluid. Leukocytes from the seroma had a decreased ability to produce reactive oxygen species (ROS) following stimulation when compared to leukocytes isolated from the circulation of the same burned mice. Sham leukocytes incubated with sera from burned mice and burned and infected mice, but not with sera from Sham mice, significantly produce ROS at rest, which may be correlated with the pro-inflammatory danger-associated molecular pattern (DAMP) HMGB1 in the sera of burned mice. These data suggest that a non-lethal burn can prematurely activate leukocytes while in circulation, reducing their functionality at the infected burn site, and that leukocytes at the burn site (seroma) also have impaired function. We conclude that an otherwise non-lethal burn prematurely activates circulating leukocytes and that the seroma environment further inhibits the leukocytes that arrive at the burn site. This results in an impaired immune response and the development of lethal sepsis.