In a previous study, all convalescent-phase sera from patients with culture-confirmed
legionellosis reacted on immunoblots with a Legionella genus-wide 58-kilodalton (kDa)
protein antigen (J.S. Sampson, B.B. Plikaytis, and H.W. Wilkinson, J. Clin. Microbiol. 23:92-99, 1986). The present study was done to immunologically characterize and determine the diagnostic relevance of this purified
antigen. The
antigen was precipitated from enriched
cell extracts with
ammonium sulfate and purified by high-pressure liquid chromatography. High-titered rabbit antiserum produced to the purified
protein was used to show its presence on immunoblots in the 60-kDa range in 38 Legionella serogroups, representing 23 species, and in 39 non-Legionella bacteria. The antiserum was made specific for Legionella strains by sequential absorptions with Bordetella pertussis, Pseudomonas aeruginosa, and Pseudomonas fluorescens whole cells. Serum from
legionellosis patients reacted with both specific and nonspecific
epitopes. Results of indirect immunofluorescence experiments showed that neither specific nor nonspecific
epitopes of the 60-kDa
protein were surface exposed on Legionella cells and that cross-reactive
epitopes were variably exposed on non-Legionella bacteria. The 60-kDa
protein antigen should be useful in diagnostic tests for
legionellosis if care is taken to expose cryptic
epitopes and if the tests use or measure only the Legionella-specific
epitopes.