Antigenic classification of Rickettsia felis by using monoclonal and polyclonal antibodies (original) (raw)

Rickettsia felis is a flea-transmitted rickettsia. There is a discrepancy between its reported phylogenic and phenotypic identifications. Following the first report of R. felis, it was considered by tests with serologic reagents to be closely related to another recognized flea-transmitted rickettia, R. typhi. Subsequently, it appeared to be more closely related to spotted fever group (SFG) rickettsiae by genetic analysis. In the present work, R. felis was studied by microimmunofluorescence (MIF) serologic typing and with monoclonal antibodies (MAbs). Mouse polyclonal antisera to R. felis cross-reacted only with SFG rickettsiae. A neighbor-joining analysis based on MIF indicated that R. felis is actually related to SFG rickettsiae antigenically, clustering with R. australis, R. akari, and R. montanensis. A panel of 21 MAbs was raised against a 120-kDa protein antigen or a 17-kDa polypeptide of R. felis. They cross-reacted with most members of the SFG rickettsiae but not with R. prowazekii, R. typhi, or R. canadensis of the typhus group (TG) rickettsiae. Sixty-four MAbs previously generated to seven other ricketttsial species were tested with R. felis. Three MAbs reacted with the 120-kDa antigen and were generated by R. africae, R. conorii, and R. akari, respectively. They exhibited cross-reactivities with R. felis. All our data show that R. felis harbors the antigenic profile of an SFG rickettsia. Rickettsia spp. are gram-negative and obligate intracellular bacteria (24). They are bacilli of 0.3 to 0.5 m in diameter and 0.8 to 0.2 m in length which retain basic fuchsin when they are stained by the method of Gimenez. Pathogenic rickettsiae are transmitted to humans by arthropods and cause clinical diseases that manifest typically as fever, rash, and vasculitis. At present, rickettsiae are divided into two groups, the spotted fever group (SFG) and the typhus group (TG), on the basis of their intracellular positions, optimal growth temperatures, percent GϩC DNA contents, clinical features, epidemiological aspects, and antigenic characteristics (24, 30). Recently, a novel rickettsia-like organism was observed in the midgut epithelial cells of cat fleas (Ctenocephalides felis) in California by electron microscopy (1). It was described as the ELB agent, for the EL Laboratory (Soquel, Calif.), where it was originally described (1, 23). In 1996, this flea-borne bacterium was proposed as a distinct species, "Rickettsia felis" (12), and was characterized as a typhus-like rickettsia. Monoclonal antibodies (MAbs) specific for R. typhi reacted with this organism (6). Its ultrastructure and tissue distribution in fleas resembled those of R. typhi (1, 6). However, molecular data, which were obtained by sequencing and phylogenetic analysis of a 17-kDa protein-encoding gene, a citrate synthase-encoding gene, a 155-kDa protein-encoding gene, a 120-kDa protein-encoding gene, and the metK, ftsY, polA, and dnaE genes, classified R. felis into the SFG rickettsiae (2, 6, 9, 26, 27, 29). Although advanced genetic techniques have been extensively used to classify rickettsial species, serotyping by indirect microimmunofluorescence (MIF) with mouse antisera is still considered valuable for its general applicability (20).