Evidence for in vivo but not in vitro expression of a Borrelia burgdorferi outer surface protein F (OspF) homologue

Abstract

Protein export signals from the low-passage 297 strain of Borrelia burgdorferi were cloned as fusions with an Escherichia coli alkaline phosphatase (PhoA) reporter lacking a signal sequence. One PhoA+ clone (BbK2.10-PhoA) was derived from a borrelial lipoprotein. Although the polypeptide encoded by the full-length bbk2.10 gene had 76% similarity and 56% identity to outer surface protein F (OspF) from B. burgdorferi strain N40, antibodies directed against recombinant forms of the two proteins revealed that they were not cross-reactive. The nucleotide sequences of bbk2.10 and ospF from the N40 and 297 strains, respectively, were determined to confirm that the N40 and 297 strains each contained both genes. Southern blot analysis revealed that bbk2.10 is a single-copy gene and that the B. burgdorferi strain 297 and N40 genomes appeared to contain one other gene more closely related to ospF than bbk2.10. It was particularly noteworthy that ospF, but not bbk2.10, was expressed in vitro while B. burgdorferi-infected mice generated antibodies reactive with both lipoproteins. To help confirm that the BbK2.10-reactive antibodies produced by the B. burgdorferi-infected mice were specific for that protein, a second gene, bbk2.11, which hybridized with the ospF probe was cloned; the corresponding polypeptide reacted strongly with OspF antisera but failed to react with BbK2.10-specific antisera. Taken together, these data demonstrate that BbK2.10, BbK2.11, and OspF comprise a B. burgdorferi lipoprotein family and that at least one member (BbK2.10) appears to be expressed only during infection.