Discrimination between clinical and environmental strains of Legionella pneumophila by a monoclonal antibody.

Legionellae are widely spread in natural and man-made habitats. In many instances contaminated tap water has been linked to sporadic or endemic cases of human pulmonary infections, but it is not known why, in spite of frequent occurrence, legionellae only rarely cause disease. Monoclonal antibodies against Legionella pneumophila serogroup 1 (Philadelphia 1) were prepared in order to distinguish between subtypes of this serogroup. Balb/c mice were immunized i.v. three times with heat inactivated bacteria. Antibody formation was detected by an enzyme-linked immunosorbent assay (ELISA) technique using peroxidase-conjugated antimouse IgG. Spleen cells were then fused with NS-1 myeloma cells and cloned by limiting dilution. Four monoclonal antibodies were studied in detail. The study included 47 strains of L. pneumophila: 19 strains were of human origin and 28 were isolated from different environmental sources. Most were from tap water, but none from natural habitats. All strains belonged to serogroup 1 as defined by direct immunofluorescence (DFA) using monospecific FITC-labelled polyclonal antisera from rabbits. The strains were further characterized by beta-lactamase production, activity of catalase, oxidase and proteases, analysis of ubiquinones, and demonstration of membrane protein patterns by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A strong homogenicity between all the strains could be revealed by these methods independent of their origin. One of the monoclonal antibodies (B-1) was able to distinguish between human and environmental isolates. Eighteen of the 19 human strains reacted very strongly in DFA using antimouse immunoglobulin. No reaction, however, was seen with all of the environmental strains. Immunoblots were performed for characterization of the distinguishing feature using membrane complexes of all strains on nitrocellulose strips. The blots were incubated with antibody B-1, and immune complexes were detected by 125I-protein A. Broad intense blackening was seen between 22 and 70 kilodalton. This result suggests that no single protein, but rather a smaller component such as an oligosaccharide attached to constituents of different molecular weights, might be responsible for the discriminating reaction.