Epitopes shared by unrelated antigens of Borrelia burgdorferi

Abstract

An immunoglobulin M kappa-chain murine monoclonal antibody (CAB) reacted in a Western blot (immunoblot) with approximately 30 polypeptides from a whole-cell lysate of several American and European Borrelia burgdorferi strains. The reactive antigen with the highest M(r) was measured at 93 kDa (p93) and had an NH2-terminal sequence identical to the one previously reported for this antigen. The lowest reactive antigen had an M(r) of 16,000. All antigens recognized by CAB had isoelectric points within a narrow acidic range, between 5.4 and 6.2. Thus, the objective of this study was to determine whether the broad reactivity of CAB could be due to degradation of the antigen with the highest M(r), since such spontaneous degradation of p93 has already been reported, and to determine whether CAB could recognize shared epitopes in different antigens. Treatment of B. burgdorferi with protease inhibitors did not result in changes in CAB reactivity, indicating that if such degradation existed, it was most likely not due to the action of endogenous proteases. Likewise, protease treatment of intact organisms and recovery of the antigens in the insoluble fraction of a Triton X-114 partition indicated that they were internal and thus less likely to be degraded by experimental procedures. Amino-terminal sequences of other reactive polypeptides showed one approximately 72-kDa polypeptide to be identical to the DnaK homolog of B. burgdorferi. Two other antigens at approximately 49 and 47 kDa were blocked to Edman degradation. Finally, one sequenced polypeptide with a molecular mass of approximately 38.5 kDa had a strong identity with glyceraldehyde-3-phosphate dehydrogenase of other bacteria and vertebrates. Thus, while it cannot be ruled out that some of the CAB reactivity may be due to fragmentation of p93, there is strong evidence to indicate the presence of a shared epitope in at least three, possibly five, unrelated antigens of B. burgdorferi. A linear epitope within amino acid residues 357 to 371 of p93 was identified. Evidence is presented for a discontinuous epitope in the carboxy-terminal region of the DnaK homolog, which bears strong amino acid identity with the p93 epitope. The conserved amino acid sequences necessary for these shared epitopes indicate possible genetic and/or functional relatedness among these various antigens.