Cross-reactive humoral epitopes of EF and LF are identified but do not constitute major anthrax toxin neutralization responses following EF and LF vaccination (132.20)

BACKGROUND: Anthrax Lethal and Edema Toxins (LTx and ETx) are formed by binding of Lethal Factor (LF) or Edema Factor (EF) to the pore-forming moiety, Protective Antigen (PA). Immunity to LF and EF neutralize anthrax toxins. AIM: LF and EF immune sera were used to identify cross-reactive B cell epitopes in their conserved PA-binding domains and to determine the relative contribution of such antibodies to LTx and ETx neutralization. METHODS: A/J mice were immunized with recombinant (r) LF or EF. EF-binding antibodies were purified from LF immune sera, and LF-binding antibodies from EF immune sera by column absorption. Whole immune sera and purified cross-reactive antibodies were tested for IgG to rLF or rEF by ELISA, in vitro neutralization of LTx and ETx, and binding to solid phase, overlapping LF and EF decapeptides. RESULTS: Cross-reactive epitopes were identified as EF aa 98-257 and 257-268 and LF aa 265-274. Whole LF and EF immune sera neutralized LTx and ETx, respectively. However, LF sera did not neutralize ETx nor did EF sera neutralize LTx. Purified cross-reactive IgG also failed to cross-neutralize. CONCLUSIONS: Cross-reactive B cell epitopes in the PA-binding domains of whole rLF and rEF occur and have been identified; however, the major anthrax toxin-neutralizing humoral responses to these antigens are constituted by non-cross-reactive epitopes that likely lie outside of their PA-binding sites.