Cellular mechanisms of the systemic inflammatory response following resuscitated hemorrhagic shock: The role of reactive oxygen species and Toll-like receptor 4

Acute Respiratory Distress Syndrome (ARDS) following hemorrhagic shock/resuscitation (S/R) is an important contributor to late morbidity and mortality in trauma patients. S/R promotes ARDS by inducing oxidative stress that primes cells of the innate immune system for excessive responsiveness to small inflammatory stimuli, termed the "two-hit" hypothesis. Activated alveolar macrophages (AM) play a central role and when recovered from S/R animals exhibit an exaggerated responsiveness to lipopolysaccharide (LPS) with increased activation of the proinflammatory transcription factor NF-kappaB, and augmented expression of cytokines. LPS triggers AM signalling through Toll like receptor 4 (TLR4), which resides in plasma membrane lipid rafts. The objective of this work is to define cellular mechanisms of macrophage priming by oxidative stress following shock resuscitation. The main hypothesis investigated is that altered cellular distribution of TLR4 can lead to macrophage priming and antioxidant resuscitation strategies can diminish these effects.;Collectively, these studies suggest a novel mechanism whereby oxidative stress might prime the responsiveness of cells of the innate immune system. Targeting the TLR4 signalling pathway early during shock resuscitation may represent an anti-inflammatory strategy able to ameliorate late morbidity and mortality following S/R.;AM of rodents, exposed in vivo to oxidant stress following S/R, increase their surface levels of TLR4, which in turn results in augmented NF-kappaB translocation in response to small doses of LPS. Furthermore, in vitro H2O2 treatment of RAW 264.7 macrophages results in similar TLR4 surface translocation. Depletion of intracellular calcium, disruption of the cytoskeleton or inhibition of the Src kinases prevents the H2O2-induced TLR4 translocation, suggesting the involvement of receptor exocytosis. Further, fluorescent resonance energy transfer between TLR4 and lipid rafts as well as biochemical raft analysis demonstrated that oxidative stress redistributes TLR4 to surface lipid rafts. Preventing the oxidant-induced movement of TLR4 to lipid rafts using methyl-beta-cyclodextrin precluded the increased responsiveness of cells to LPS after H2O 2 treatment. Further, AM priming by oxidative stress can be diminished by early exposure to resuscitation regimens with direct or indirect systemic antioxidant effects, such as 25% albumin, N-acetylcysteine and hypertonic saline. Hyperosmolarity was found to modulate AM TLR4 gene and protein expression.