Agglutinating antibody titers to members of the family Legionellaceae in cystic fibrosis patients as a result of cross-reacting antibodies to Pseudomonas aeruginosa.

The objective of this study was to evaluate the prevalence and significance of antibody titers to organisms in the family Legionellaceae in 128 serum samples collected from cystic fibrosis patients at routine examinations. Antibody titers were determined for 10 antigenic types of Legionellaceae; Legionella pneumophila serogroups 1 to 6, Fluoribacter (Legionella) bozemanae, Fluoribacter (Legionella) dumoffii, Fluoribacter (Legionella) gormanii, and Tatlockia (Legionella) micdadei. The method of antibody titer determination was the microagglutination test. Elevated titers (greater than or equal to 1:64) to one or more antigens were found in 41.3% of cystic fibrosis patients but in only 9.7% of 103 normal control subjects (P less than 0.01). Titers to 8 of the 10 antigens were directly correlated with the number of Pseudomonas aeruginosa precipitating antibodies in patient sera, as determined by crossed immunoelectrophoresis (correlation coefficients, greater than or equal to 0.74). Cross-reactions between P. aeruginosa and L. pneumophila were substantiated by crossed immunoelectrophoresis of hyperimmune rabbit serum as well as patient sera against P. aeruginosa and Legionellaceae antigens. Monospecific antibody to the "common antigen" of P. aeruginosa was used to demonstrate the presence of this antigen in L. pneumophila. The presence of cross-reacting antibodies in cystic fibrosis patients chronically infected with P. aeruginosa emphasizes the need for cautious interpretation of antibody titers to members of the family Legionellaceae.