Acute respiratory distress syndrome (ARDS) is a neutrophil (polymorphonuclear leukocyte; PMN)-driven lung injury that is associated with fever and heat-stroke, and involves approximately 40% mortality. In murine models of acute lung injury (ALI), febrile-range hyperthermia (FRH) enhanced PMN accumulation, vascular permeability, and epithelial injury, in part by augmenting pulmonary cysteine-x-cysteine (CXC) chemokine expression. To determine whether FRH increases chemokine responsiveness within the lung, we used in vivo and in vitro models that bypass the endogenous generation of chemokines. We measured PMN transalveolar migration (TAM) in mice after intratracheal instillations of the human CXC chemokine IL-8 in vivo, and of IL-8-directed PMN transendothelial migration (TEM) through human lung microvascular endothelial cell (HMVEC-L) monolayers in vitro. Pre-exposure to FRH increased in vivo IL-8-directed PMN TAM by 23.5-fold and in vitro TEM by 7-fold. Adoptive PMN transfer demonstrated that enhanced PMN TAM required both PMN donors and recipients to be exposed to FRH, suggesting interdependent effects on PMNs and endothelium. FRH exposure caused the activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase in lung homogenates and circulating PMNs, with an associated increase in HSP27 phosphorylation and stress-fiber formation. The inhibition of these signaling pathways with U0126 and SB203580 blocked the effects of FRH on PMN extravasation in vivo and in vitro. Collectively, these results (1) demonstrate that FRH augments chemokine-directed PMN extravasation through direct effects on endothelium and PMNs, (2) identify ERK and p38 signaling pathways in the effect, and (3) underscore the complex effects of physiologic temperature change on innate immune function and its potential consequences for lung injury.Keywords: ARDS; neutrophil; endothelium; febrile-range hyperthermia; p38 MAP kinase Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) pose frequent complications in critically ill patients, and are responsible for significant morbidity, mortality, and related healthcare costs (1). To date, only two interventions, low-volume mechanical ventilation (2) and aggressive fluid management (3), have been shown to confer benefit in patients with ARDS. However, despite these advances in supportive care, mortality remains at approximately 40% in such patients (4). Fever is common in critically ill patients (5, 6), especially in those with ALI/ARDS (7), and ARDS is a common complication of heat stroke (8). Fever is associated with increased length of stay in the intensive care unit, prolonged mechanical ventilation, and increased mortality (5, 6, 9).Murine studies in which hyperthermia is induced by raising ambient temperature demonstrated that febrile-range hyperthermia (FRH; an z 2 8 C increase in core temperature) profoundly increases ALI/ARDS (10, 11). In murine models of ALI caused by exposure to hyperoxia (10) or an intratracheal instillation of LPS (11...