Plasmid DNA and protein vaccination of mice to the outer surface protein A of Borrelia burgdorferi leads to induction of T helper cells with specificity for a major epitope and augmentation of protective IgG antibodies in vivo.

Plasmid DNA-based vaccination is an efficient way to evoke various forms of protective immunity in laboratory animals. Our previous experiments have shown that mice immunized with either plasmid DNA encoding the outer surface lipoprotein A (pOspA) of Borrelia burgdorferi or the respective lipoprotein (Lip-OspA) produce protective antibodies against subsequent challenge with virulent spirochetes. In the present study, we compared the specificity and function of T cells generated in AKR/N mice previously immunized to either pOspA or Lip-OspA. T cell populations derived by either of the two protocols consistently responded by proliferation in vitro to one (residues 186-203; B4) out of a panel of 27 overlapping 20-mer peptides spanning the entire OspA molecule of strain ZS7. B4 was shown to express allele-specific ligand motifs for I-Ek. Most of the other peptides produced variable and much less pronounced or marginal proliferative T cell responses. T cells reactive to B4 as well as to some minor epitopes were CD4+CD8- T cells which produced IFN-gamma but no detectable IL-4 upon antigen stimulation in vitro. Priming of AKR/N mice with B4 but not with inactive peptides of OspA led to an enhanced production of IgG antibodies, mainly of the IgG1 isotype, including those to a prominent protective epitope (LA-2) upon subsequent challenge with Lip-OspA or intact spirochetes. The data demonstrate that both plasmid DNA and protein immunization with OspA results in T cell responses with specificity for a dominant OspA epitope and suggest that priming of mice with immunodominant peptides accelerates the appearance of protective antibodies in vivo. The identification of T helper cell epitopes relevant for the induction of protective antibodies will also facilitate the design of more potent vaccines against Lyme disease.