Relapsing fever spirochaetes produce a serine protease that provides resistance to oxidative stress and killing by neutrophils.

The spirochaetes that cause tick-borne relapsing fever and Lyme disease are closely related human pathogens, yet they differ significantly in their ecology and pathogenicity. Genome sequencing of two species of relapsing fever spirochaetes, Borrelia hermsii and Borrelia turicatae, identified a chromosomal open reading frame, designated bhpA, not present in the Lyme disease spirochaete Borrelia burgdorferi. The predicted amino acid sequence of bhpA was homologous with the HtrA serine proteases, which are involved with stress responses and virulence in other bacteria. B. hermsii produced an active serine protease that was recognized by BhpA antibodies and the recombinant BhpA protein-degraded beta-casein. bhpA was transcribed in vitro at all growth temperatures and transcription levels were slightly elevated at higher temperatures. These results correlated with the synthesis of BhpA during B. hermsii infection in mice. With the exception of Borrelia recurrentis, the bhpA gene, protein and enzymatic activity were found in all relapsing fever spirochaetes, but not in Lyme disease or related spirochaetes. Heterologous expression of bhpA in B. burgdorferi increased the spirochaete's resistance to both oxidative stress and killing by human neutrophils. Therefore, we propose that bhpA encodes a unique and functional serine protease in relapsing fever spirochaetes. This periplasmic enzyme may prevent the accumulation of proteins damaged by the innate immune response and contribute to the ability of the relapsing fever spirochaetes to achieve high cell densities in blood.