The Borrelia hermsii factor H binding protein FhbA is not required for infectivity in mice or for resistance to human complement in vitro.
[CanLyme note: Currently, testing and symptomotology knowledge are not nearly accurate enough to distinguish between a human B. hermsii infection and a B.burgdorferi infection yet many people are told they do not have a treatable borreliosis simply because they do not have a positive Borrelia burgdorferi strain B31 when we have many other borrelia, and some we have not identified yet.]
Infect Immun. 2014 Aug;82(8):3324-32. doi: 10.1128/IAI.01892-14. Epub 2014 May 27
The primary causative agent of tick-borne relapsing fever in North America is Borrelia hermsii. It has been hypothesized that B. hermsii evades complement-mediated destruction by binding factor H (FH), a host-derived negative regulator of complement. In vitro, B. hermsii produces a single FH binding protein designated FhbA (FH binding protein A). The properties and ligand binding activity of FhbA suggest that it plays multiple roles in pathogenesis. It binds plasminogen and has been identified as a significant target of a B1b B cell-mediated IgM response in mice. FhbA has also been explored as a potential diagnostic antigen for B. hermsii infection in humans. The ability to test the hypothesis that FhbA is a critical virulence factor in vivo has been hampered by the lack of well-developed systems for the genetic manipulation of the relapsing fever spirochetes. In this report, we have successfully generated a B. hermsii fhbA deletion mutant (the B. hermsii YORΔfhbA strain) through allelic exchange mutagenesis. Deletion of fhbA abolished FH binding by the YORΔfhbA strain and eliminated cleavage of C3b on the cell surface. However, the YORΔfhbA strain remained infectious in mice and retained resistance to killing in vitro by human complement. Collectively, these results indicate that B. hermsii employs an FhbA/FH-independent mechanism of complement evasion that allows for resistance to killing by human complement and persistence in mice.