Evaluation of plasma SC5b-9 (SMAC) in patients with ARDS

Abstract

The adult respiratory distress syndrome (ARDS) occurs in patients with various underlying illnesses. It has been suggested that complement activation may play a crucial role in the pathophysiology of ARDS by direct actions of complement fragments and/or by release of mediators including endotoxin, superoxide anion, and arachidonate metabolites. Therefore, complement evaluation was performed in patients with ARDS, with emphasis on SC5b-9 (Membrane Attack Complex). Complement split products, C4d, Bb, and SC5b-9, respectively from the classical, alternative, and terminal complement pathway were measured by enzyme immunoassay (EIA) using respective monoclonal antibodies. Plasma S-protein (vitronectin) concentration was also evaluated by EIA. We could not detect SMAC in normal controls or in a C5-deficient patient with abdominal infection. In contrast, most patients with ARDS and patients at risk showed elevated SMAC, and an increase in the values in association with exacerbation of the disease. However, two patients with ARDS exhibited lower values of SMAC in spite of deterioration and subsequent death. It was suggested that in these two ARDS patients, liver dysfunction may have contributed to the reduced SMAC formation. The finding that S-protein concentration (a MAC inhibitor) in the plasma of these two patients was reduced supports this speculation. It is suggested that simultaneous evaluation of SMAC and S-protein in plasma is helpful in determining the prognosis of critically ill patients.