Mononuclear cells are critical to the eradication of invading organisms. The mechanism in which these innate immune cells respond to these invaders is through the activation of a series of pattern recognition receptors or Toll-like receptors (TLRs). Activation of these receptors, on specialized plasma membrane microdomains is complex and poorly elucidated. Based on previous work by us, we hypothesize that formation of these complexes requires breakdown of plasma membrane sphingolipids into ceramide leading to the formation of lipid raft macrodomains and the formation of TLR complexes. As a result, specific infectious factors are presented to these pattern recognition receptors leading to cellular activation. Although these responses may be life saving, severe trauma is know to result in reprogramming and alterations in innate immunity. These altered phenotypes, rather than leading to host protection, are responsible for increased susceptibility to invading organisms leading to the development of sepsis and organ failure. This state has been recreated in vitro by subjecting mononuclear cells to factors induced by trauma, including platelet activating factor, oxidant stress and complement 5a. Although the mechanism(s) responsible for this reprogramming remain unknown, previous work has demonstrated that this process is associated with alterations in the lipid and protein content within the plasma membrane. These alterations are hypothesized to occur on lipid rafts. Following injury, we hypothesize that factors induced by trauma result in the production of ceramide, but to a lesser degree than that seen during activation. Ceramide once produced fuses within rafts leading to the formation of macrodomains similar to that which occurs with activation. Additionally, ceramide leads to the mobilization of calcium leading to the activation of CaMK II. Activation of these cellular messengers is associated with the formation of focal adhesion-like complexes that contain some but not all of the TLR components. We hypothesize that assembly of these complexes and changes in lipid raft ceramide content are responsible for subsequent reprogramming that induces enhanced activation in response to subsequent infection. Thus, this proposal sets out to determine more fully the molecular mechanisms responsible for reprogramming and activation following trauma by exploring the effects of ceramide, calcium and CaMK II in vitro, and in severely injured trauma patients.
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