Role of Pseudomonas aeruginosa exoenzymes in lung infections of patients with cystic fibrosis

We investigated the role of Pseudomonas aeruginosa exoenzymes in cystic fibrosis lung infection in the presence and absence of specific serum antibodies. In sputa of 21 cystic fibrosis patients, concentrations of P. aeruginosa proteases and exotoxin A were determined by sensitive radioimmunoassays. In all sputa, detection of exoenzymes was negative (less than or equal to 10 ng). Positive serum antibody titers to bacterial exoenzymes were found in the majority of patients. Purified immunoglobulin G (IgG) preparations from the sera of two patients revealing specific antibody titers to the bacterial proteases neutralized these enzymes at ratios of 1,000:1 to 5,600:1 (wt/wt). Above the neutralizing capacity of IgG, proteases caused cleavage of IgG; below that level, no enzymatic activity was observed. In vitro incubation of P. aeruginosa elastase, alkaline protease, or exotoxin A with elastase derived from polymorphonuclear leukocytes showed that polymorphonuclear leukocyte elastase: (i) was cleaved by bacterial elastase, (ii) was not inactivated by alkaline protease, and (iii) inactivated exotoxin A. The results suggest that soon after the onset of P. aeruginosa lung infection in cystic fibrosis patients, bacterial proteases, but not exotoxin A, become important virulence factors. The results also suggest that exoenzymes do not directly contribute to lung damage after immune response to bacterial antigens has begun.