Characterization of Bartonella clarridgeiae flagellin (FlaA) and detection of antiflagellin antibodies in patients with lymphadenopathy

Cat scratch disease (CSD) is a frequent clinical outcome of Bartonella henselae infection in humans. Recently, two case reports indicated Bartonella clarridgeiae as an additional causative agent of CSD. Both pathogens have been isolated from domestic cats, which are considered to be their natural reservoir, B. clarridgeiae and B. henselae can be distinguished phenotypically by the presence or absence of flagella, respectively. Separation of the protein content of purified flagella of B. clarridgeiae by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis indicated that the flagellar filament is mainly composed of a polypeptide with a mass of 41 kDa. N-terminal sequencing of 20 amino acids of this protein revealed a perfect match to the N-terminal sequence of flagellin (FlaA) as deduced from the sequence of the flaA gene cloned from B. clarridgeiae. The flagellin of B. clarridgeiae is closely related to flagellins of Bartonella bacilliformis and several Bartonella-related bacteria. Since flagellar proteins are often immunodominant antigens, we investigated whether antibodies specific for the FlaA protein of B. clarridgeiae are found in patients with CSD or lymphadenopathy. Immunoblotting with 724 sera of patients suffering from lymphadenopathy and 100 healthy controls indicated specific FlaA antibodies in 3.9% of the patients' sera but in none of the controls. B. clarridgeiae FlaA is thus antigenic and expressed in vivo, providing a valuable tool for serological testing. Our results further indicate that B. clarridgeiae might be a possible etiologic agent of CSD or lymphadenopathy. However, it remains to be clarified whether antibodies to the FlaA protein of B. clarridgeiae are a useful indicator of acute infection.