With an in vitro system that used a luminescent strain of Klebsiella pneumoniae to assess bacterial metabolic activity in near-real-time, we investigated the dynamics of complement-mediated attack in healthy individuals and in patients presenting to the emergency department with community-acquired severe sepsis. A novel mathematical/statistical model was developed to simplify light output trajectories over time into two fitted parameters, the rate of complement activation and the delay from activation to the onset of killing. Using Factor B-depleted serum, the alternative pathway was found to be the primary bactericidal effector: In the absence of B, C3 opsonization as measured by flow cytometry did not progress and bacteria proliferated near exponentially. Defects in bacterial killing were easily demonstrable in patients with severe sepsis compared with healthy volunteers. In most patients with sepsis, the rate of activation was higher than in normal subjects but was associated with a prolonged delay between activation and bacterial killing (P , 0.05 for both). Theoretical modeling suggested that this combination of accentuated but delayed function should allow successful bacterial killing but with significantly greater complement activation. The use of luminescent bacteria allowed for the development of a novel and powerful tool for assessing complement immunology for the purposes of mechanistic study and patient evaluation.Keywords: alternative complement pathway; theoretical models; nonlinear dynamics; sepsisThe complement cascade fills a crucial niche in early host defense against bacterial infection. Activating by fully innate means (i.e., through the alternative and mannose-binding lectin pathways) or in conjunction with acquired humoral immunity (via the classical pathway), the system contributes to defense by enzymatically labeling microorganisms for phagocytosis through C3 opsonization, by generating a series of anaphylatoxins that alert cellular defenses to the presence of a pathogen, and, in some instances, by directly killing the invading cell by permeabilizing its cell wall with membrane attack complex (C5b6789 n or MAC).During experimental or human sepsis, globally assessing the complement system is typically done in one of two ways. Most often, traces of previous activation are sought by measuring concentrations of complement proteins (such as C3, which falls with activation) or species elaborated during activation such as C3a, C5a, or soluble MAC (1). Less commonly, function is directly assessed. This is typically done by measuring complement hemolytic activity, with the canonical example being the CH 50 method. CH 50 is known to fall in severe human sepsis and multiorgan failure (2). How best to interpret this capacity (i.e., the complement-mediated cytolysis of an enucleate and otherspecies blood cell fully opsonized by a third species' IgG) in the setting of an overwhelming infection is not self-evident.A more relevant functional assay of the complement system's performance in the presence of...
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