Antilipopolysaccharide antibodies and differential diagnosis of chronic Pseudomonas aeruginosa lung infection in cystic fibrosis

Chronic lung infection in cystic fibrosis is characteristically associated with polyagglutinable, serum-sensitive, mucoid strains of Pseudomonas aeruginosa. Enzyme-linked immunosorbent assay (ELISA) methods for standard-free quantitation of immunoglobulin G (IgG) and IgM antibodies to P. aeruginosa lipopolysaccharides (LPSs) have been developed. We now report the development of assays for quantitation of monomer and dimer total IgA and IgA anti-LPS antibodies. Use of these methods in diagnosis of early chronic P. aeruginosa lung infection was assessed. IgG and IgA anti-LPS levels increased significantly at the onset of chronic infection and continued to increase to very high levels in the later stages of infection. IgM anti-LPS levels also rose at the onset of chronic infection but did not increase further. The function of true- and false-positive rates was illustrated by using various concentrations of IgG, IgA, and IgM anti-LPS for discrimination of patients. Values that gave optimum separations were used for statistical evaluation of the diagnostic sensitivities and specificities of anti-LPS antibody concentrations. The results obtained in these assays were compared with a diagnosis, based on the number of precipitins in crossed immunoelectrophoresis, of serum samples from cystic fibrosis patients. In 64 paired serum samples taken before and immediately after the onset of chronic infection, as defined by crossed immunoelectrophoresis precipitins, the predictive values of a positive ELISA were 86% for IgG and 89% for IgA. The predictive values for a negative ELISA were 98% for IgG and 97% for IgA. Results of the IgM anti-LPS ELISA had a lower predictive value. Immunoblotting and absorption studies showed that IgG anti-LPS antibodies were directed specifically against LPS of P. aeuruginosa. ELISAs were developed to determine the specific IgG sublclasses involved. The increase in IgG anti-LPS involved all four subclasses. Highest anti-LPS titers were seen with IgG1 and IgG4, but the largest relative increases were seen with IgG2 and IgG3.