A Novel Surface Antigen of Relapsing Fever Spirochetes Can Discriminate between Relapsing Fever and Lyme Borreliosis

In a previous immunoproteome analysis of Borrelia hermsii, candidate antigens that bound IgM antibodies from mice and patients infected with relapsing fever spirochetes were identified. One candidate that was identified is a hypothetical protein with a molecular mass of 57 kDa that we have designated Borrelia immunogenic protein A (BipA). This protein was further investigated as a potential diagnostic antigen for B. hermsii given that it is absent from the Borrelia burgdorferi genome. The bipA locus was amplified and sequenced from 39 isolates of B. hermsii that had been acquired from western North America. bipA was also expressed as a recombinant fusion protein. Serum samples from mice and patients infected with B. hermsii or B. burgdorferi were used to confirm the immunogenicity of the recombinant protein in patients infected with relapsing fever spirochetes. Lastly, in silico and experimental analysis indicated that BipA is a surface-exposed lipoprotein in B. hermsii. These findings enhance the capabilities of diagnosing infection with relapsing fever spirochetes. The first outbreak of tick-borne relapsing fever in the United States was observed in Colorado in 1915 (39). Since then this disease continues to be under-reported and misdiagnosed, while regions where the disease is endemic continue to be identified in the Western and Southern United States (20, 51, 53). The three causative agents of relapsing fever in the United States are Borrelia hermsii, Borrelia turicatae, and Borrelia parkeri; the Borrelia spirochetes are maintained in enzoonotic cycles with their respective tick vectors Ornithodoros hermsi, Ornithodoros turicata, and Ornithodoros parkeri (17, 22). Most reported cases of this disease and all human isolates collected in the United States are due to B. hermsii. Although B. parkeri and B. turicatae have not been isolated from a human, B. turicatae has been implicated in several outbreaks in Texas because of the association of its tick vector with an area where an outbreak occurred (46). B. turicatae has also been isolated from sick dogs (52, 58), further supporting the likelihood of infection to humans. The spirochetemia during mammalian infection is cyclic with spirochetes escaping the immune response by genetic variation of surface proteins known as variable major proteins (Vmps) (5, 8, 16, 25, 47, 56). During episodes of spirochetemia, the bacteria can reach upwards of 10 7 spirochetes per ml of blood