Lyme disease is an inflammatory process resulting from infection by the Borrelia burgdorferi spirochete, via Ixodes (tick) vector. During the Borrelia life cycle, it variably expresses many of its surface proteins, such as Outer Surface Proteins A through C, and VlsE (Vmp-like sequence, Expressed), an antigenically variable surface protein.
The subject of our analysis, OspA , is highly expressed on the spirochetal surface during its maturation in the tick midgut and in lab strains of Borrelia. However, while it is downregulated in the infected host, OspA does generate an antibody-mediated immune response which can be detected by Western Blot and ELISA. Currently, recombinant forms of the protein are being used as preventative therapies for Lyme disease in humans. Although the protein has yet to be completely characterized vis-a-vis structure and function, here we focus on its role as an antigen.
OspA is a 31 kDa protein that is anchored to the outer membrane of the spirochete via a lipidated cystein in the N-terminus. Its crystal structure is shown here in complex with mouse mAB 184.1 (Fab) (1). This monoclonal antibody recognizes an N-terminal epitope with a surface area of 800 angstroms-squared. However, it is not readily accessible on intact spirochetes due to the method used to generate the Fab (innoculation with sonicated spirochetes). As such, this epitope would be non-protective in an infection. This epitope is at the opposite end from the C-terminal domain , which is a target for most neutralizing antibodies against OspA. This protective epitope also includes higly conserved residue Trp 216 .
Other studies have attempted to further characterize the antigenicity of OspA. HPLC-coupled mass spectrometric analysis was used to map the epitope of another murine monoclonal antibody, 4C10C2 (2). Two regions were identified as belonging to the antibody:antigen binding interface, specifically Val145-Glu152 and Thr160-Gln166 .
The calculated surface area of this epitope is approximately 1400 angstroms-squared, and interestingly, the monoclonal appears to be specific to the American strain of Borrelia, B31.