[Activation of the alternative complement pathway in pleural fluid of patients with pneumonia].

It has been reported that the opsonic activity of pleural fluid is decreased in patients with pneumonia complicated by infected effusions (empyema) compared to those with sterile (metapneumonic) effusions. In this paper some parameters of complement activity in 12 patients with empyema and in 13 patients with metapneumonic effusions were compared. The hemolytic activity of the alternative pathway and of Factor B in empyemas (15 +/- 10% and 25+15%, respectively) was less than that measured in metapneumonic effusions (55 +/- 38% and 71 +/- 21%; p less than 0.01). Both parameters showed a significant correlation (r=0.76; p less than 0.01). The Ba fragment, a catabolic product of Factor B, was increased in empyema (125 +/- 59%) compared to that in metapneumonic effusions (49 +/- 24%; p less than 0.01). The levels of C3, protein of both, alternative and classical pathways, were decreased in empyema (17 +/- 29%) when compared with the levels in metapneumonic pleural effusion (42 +/- 20%; p less than 0.01). The levels of this protein were correlated with the activity of alternative complement pathway (r=0.68; p less than 0.01), as well as with those of Factor B (r=0.75; p less than 0.01). Finally, the hemolytic activity of C4, as a representative element of the classical pathway, has also shown to be decreased in empyema (6 +/- 6%) compared to metapneumonic fluids (27 +/- 28%; p less than 0.01). Significant correlations between the hemolytic activity of C4 and levels of C3 (r=0.75; p less than 0.01), the hemolytic activity of the alternative pathway (r=0.63; p less than 0.01) and the levels of Factor B (r=0.73; p less than 0.01) were demonstrated. This last suggest a simultaneous activation of both, classical and alternative pathways of complement. The complement decrease and particularly, the activation of the alternative pathway in patients with empyema could be a determinant factor of the deficient bacterial opsonization. This can explain the survival of bacteria in an empyema despite the large number of neutrophils present.