Researchers showed previously (6
) that the majority of IgM and IgG OspC antibodies produced after infection with B. burgdorferi
sensu stricto were complement-dependent borreliacidal antibodies that killed the spirochetes without the necessity of scavenging by phagocytic cells. Moreover, the OspC borreliacidal antibodies were specific for an epitope(s) contained within the 50 C-terminal amino acids of the protein (16
). In this study, we demonstrate that the OspC borreliacidal antibodies are specific for a highly conserved epitope within the seven C-terminal amino acid residues.
This finding is remarkable. As an infected tick begins feeding on a human, OspC is expressed (27
) and becomes the major outer membrane protein expressed in early infection, regardless of the Borrelia
genospecies infecting the patient (11
). Correspondingly, OspC antibodies quickly become the dominant humoral immune response (22
); however, OspC ELISAs have lacked accuracy for confirming Lyme disease (12
). The shortcomings were explained most often by the heterogeneity of ospC
), even among isolates within the same genospecies (32
). The antibody responses induced by the highly variable regions would have widely variable specificities. Additionally, antibodies induced by other infections may also react to these regions. However, previous studies that characterized the ability of OspC antibodies to kill the spirochetes independent of scavenging by phagocytic cells have provided compelling evidence that the borreliacidal antibodies are specific for a conserved region of the protein. For example, a flow cytometric test that detects borreliacidal OspC antibodies by monitoring the ability of the antibodies to kill only B. burgdorferi
sensu stricto 50772 detected the response in >70% of early Lyme disease sera collected from throughout the United States (5
In this study, we extend these findings by demonstrating that the dominant OspC borreliacidal antibody epitope is located within the surface-exposed (22
) seven amino acid residues of the C-terminal end of the protein. More importantly, a BLAST search (1
) confirmed the region is highly conserved among infectious B. burgdorferi
sensu stricto, B. garinii
, and B. afzelii
isolates. The results therefore provide explanation for the increased accuracy (5
) of the flow cytometric borreliacidal antibody test. Moreover, the findings provide strong evidence that an ELISA comprised of the epitope would correlate directly with the highly specific borreliacidal antibody response. This should provide a sensitive and highly specific serodiagnostic confirmation of Lyme disease without the technical complexity and live organisms necessary to detect borreliacidal activity.
Mathiesen et al. (22
) also reported that the OspC antibodies in sera from patients with neuroborreliosis caused by infection with B. garinii
were specific predominantly for the identical sequence of 10 C-terminal amino acid residues of the protein. However, an ELISA with a synthetic peptide (21
), while more reactive than the rOspC ELISA, detected OspC antibodies in European sera significantly less often (36%) than in sera from U.S. patients (11
). The authors noted the distinct absence of IgG OspC antibodies in sera from European patients (34
) and suggested the failure of the response to reach maturity was because the epitope induced antibodies in a T-cell-independent fashion (22
). However, this is clearly not the case after infection with B. burgdorferi
sensu stricto. In this study, we detected high titers of borreliacidal IgG OspC antibodies, and numerous other investigators (5
) have also confirmed that IgG OspC antibodies are commonly present in all stages of the illness. It seems unlikely the identical epitope would induce both a T-cell-independent and a T-cell-dependent response.
Rather, we suspect the failure of the OspC antibodies to switch classes in the European Lyme disease patients is more likely due to the predilection of the spirochetes to colonize the central nervous system, which may inhibit the maturation of the antibody response. In support, European patients are often infected with B. garinii
or B. afzelii
, which commonly cause neuroborreliosis (23
). These genospecies are not found in the United States, where the patients are instead infected with B. burgdorferi
sensu stricto, which most often colonizes the skin or joints. Additional studies to evaluate this hypothesis remain necessary, but the collective results still highlight the immunodominance of the OspC antibody response specific for the C terminus of the protein and provide support for the development of serodiagnostic procedures to exploit this finding.
In addition, the results provide compelling evidence that the C terminus of OspC is a good human vaccine candidate. However, we failed to detect borreliacidal OspC antibodies in sera from the laboratory mice or hamsters, despite vigorous antibody responses against many other B. burgdorferi proteins. The sera instead contained only small concentrations of OspC antibodies detected by ELISA or Western blotting. More significantly, the OspC antibodies were not borreliacidal antibodies. The findings therefore confirmed a significant difference in the antigenic reactivities of B. burgdorferi sensu stricto spirochetes that infect human or rodent hosts. More importantly, the results cast considerable doubt on the ability of rodent animal models to provide experimental data that accurately predict the ability of vaccination with OspC to provide protection against human infection.
In conclusion, the immunodominant IgM and IgG OspC antibody response in human Lyme disease is specific for the highly conserved seven C-terminal amino acid residues. This finding provides valuable information for future efforts to develop effective serodiagnostic tests and vaccines, but an accurate assessment of their effectiveness cannot be obtained by using laboratory mice or hamsters.