Specific regions of the ricin toxin A subunit are involved in eliciting protective antibodies: implications for vaccine design (53.5)

Abstract

Abstract Efforts to develop an effective vaccine against ricin are focused on engineering attenuated and stable recombinant forms of the toxin’s enzymatic A subunit (RTA). While several candidate antigens are in development, vaccine design and efficacy studies are being undertaken in the absence of a fundamental understanding of the regions of RTA that are critical in eliciting protective immunity. To address this issue, we recently identified six distinct immunodominant linear regions on RTA. Using monoclonal antibodies (MAbs) specific to each region, we determined that protective antibodies were directed against α-helices located in RTA folding domains 1 and 2, whereas non-neutralizing antibodies recognized random coils and loops that were primarily confined to folding domain 3. This suggests that neutralizing and non-neutralizing antibodies target distinct domains on RTA. In support of this hypothesis, we now report that the vast majority of conformation-dependent ricin neutralizing MAbs bind to RTA 1-33/44-198, a recombinant RTA deletion mutant lacking folding domain 3. Furthermore, preliminary studies in mice suggest that RTA 1-33/44-198 is more effective than an attenuated RTA point mutant at eliciting neutralizing antibodies. These data offer insights into the immunodominant and structural determinants on RTA that give rise to protective immunity, and for the first time provide an immunological rationale for ricin vaccine design.