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

Cat scratch disease (CSD) is a frequent clinical outcome of Bartonella hens elae infection in humans. Recently, two case reports indicated Bartonella c larridgeiae as an additional causative agent of CSD. Both pathogens have be en isolated from domestic cats, which are considered to be their natural re servoir, B. clarridgeiae and B. henselae can be distinguished phenotypicall y by the presence or absence of flagella, respectively. Separation of the p rotein content of purified flagella of B. clarridgeiae by sodium dodecyl su lfate-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 reveale d 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 flagel lin of B. clarridgeiae is closely related to flagellins of Bartonella bacil liformis and several Bartonella-related bacteria. Since flagellar proteins are often immunodominant antigens, we investigated whether antibodies speci fic 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 antibodie s in 3.9% of the patients' sera but in none of the controls. B. clarridgeia e FlaA is thus antigenic and expressed in vivo, providing a valuable tool f or serological testing. Our results further indicate that B. clarridgeiae m ight be a possible etiologic agent of CSD or lymphadenopathy. However, it r emains to be clarified whether antibodies to the FlaA protein of B. clarrid geiae are a useful indicator of acute infection.