Abstract:Previous attempts to produce a vaccine for ricin toxin have been hampered by safety concerns arising from residual toxicity and the undesirable aggregation or precipitation caused by exposure of hydrophobic surfaces on the ricin A-chain (RTA) in the absence of its natural B-chain partner. We undertook a structure-based solution to this problem by reversing evolutionary selection on the 'ribosome inactivating protein' fold of RTA to arrive at a non-functional, compacted single-domain scaffold (sequence RTA1-198… Show more
“…FGA12 is the only non-neutralizing MAb that we have identified to date that recognizes an epitope within folding domain 1. Interestingly, the FGA12 epitope lies within the exact exposed hydrophobic loop that was deleted by McHugh and colleagues in an effort to stabilize the recombinant derivative of RTA [21, 22]. While our data suggest an intriguing correlation between epitope structure and neutralizing activity, additional MAbs against each of these regions are needed to fully validate this conclusion.…”
Section: Discussionmentioning
confidence: 58%
“…The first is a recombinant derivative of RTA that contains a point mutation in the toxin’s active site, as well as a mutation in a region of RTA attributed to eliciting vascular leak syndrome [16]. The second is a recombinant form of RTA known as 1-33/44-198 that carries a deletion of an exposed 10 amino acid hydrophobic loop (T34-P43) within folding domain 1, as well as a truncation of all of folding domain 3 (A199 - F267) [20–22]. In both cases, however, RTA has been engineered in the absence of a fundamental understanding of the regions of the toxin that are critical in eliciting protective immunity.…”
Section: Discussionmentioning
confidence: 99%
“…RiVax is currently in Phase I clinical trials (E. Vitetta and J. Smallshaw, UT Southwestern, personal communication). The second RTA derivative being pursued as a vaccine was developed by the U.S. Army and is called RTA 1-33/44-198 [20–22]. This variant carries a deletion of an N-terminal proximal exposed loop region (T34-P43) as well as a truncation of the C-terminus (A199- F267).…”
Efforts to develop an effective vaccine against ricin are focused on the engineering of 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 those regions of RTA that are critical in eliciting protective immunity. In this present study, we produced and characterized a collection of monoclonal antibodies (MAbs) directed against five distinct immunodominant regions on RTA, and used these MAbs to identify several key neutralizing epitopes on the toxin. Protective MAbs 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. 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.
“…FGA12 is the only non-neutralizing MAb that we have identified to date that recognizes an epitope within folding domain 1. Interestingly, the FGA12 epitope lies within the exact exposed hydrophobic loop that was deleted by McHugh and colleagues in an effort to stabilize the recombinant derivative of RTA [21, 22]. While our data suggest an intriguing correlation between epitope structure and neutralizing activity, additional MAbs against each of these regions are needed to fully validate this conclusion.…”
Section: Discussionmentioning
confidence: 58%
“…The first is a recombinant derivative of RTA that contains a point mutation in the toxin’s active site, as well as a mutation in a region of RTA attributed to eliciting vascular leak syndrome [16]. The second is a recombinant form of RTA known as 1-33/44-198 that carries a deletion of an exposed 10 amino acid hydrophobic loop (T34-P43) within folding domain 1, as well as a truncation of all of folding domain 3 (A199 - F267) [20–22]. In both cases, however, RTA has been engineered in the absence of a fundamental understanding of the regions of the toxin that are critical in eliciting protective immunity.…”
Section: Discussionmentioning
confidence: 99%
“…RiVax is currently in Phase I clinical trials (E. Vitetta and J. Smallshaw, UT Southwestern, personal communication). The second RTA derivative being pursued as a vaccine was developed by the U.S. Army and is called RTA 1-33/44-198 [20–22]. This variant carries a deletion of an N-terminal proximal exposed loop region (T34-P43) as well as a truncation of the C-terminus (A199- F267).…”
Efforts to develop an effective vaccine against ricin are focused on the engineering of 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 those regions of RTA that are critical in eliciting protective immunity. In this present study, we produced and characterized a collection of monoclonal antibodies (MAbs) directed against five distinct immunodominant regions on RTA, and used these MAbs to identify several key neutralizing epitopes on the toxin. Protective MAbs 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. 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.
“…The use of recombinant RTA containing a 25 amino acid inactivating insertion plus alum was immunogenic and protective in mice, but residual catalytic activity makes it an unlikely candidate for a human vaccine [10]. Another vaccine developed by United States Army Medical Research Institute for infectious diseases (USAMRIID) utilizes a truncated RTA, which is structurally very stable, immunogenic and is a highly promising vaccine [11]. However, none of these vaccines eliminates the vascular leak-inducing site found in RTA and therefore, immunization could cause local tissue damage.…”
Ricin toxin is a CDC level B biothreat. We have developed a ricin vaccine, RiVax, which is a recombinant mutant of ricin A chain. RiVax is safe, immunogenic and protective in mice when administered intramuscularly (IM). We have now attempted to increase the utility and immunogenicity of RiVax by administering it intradermally (ID) with or without alum. Without alum, Rivax administered by the ID and IM routes was equally immunogenic and protective. With alum, ID vaccinations were more immunogenic and protective against both systemic and mucosal challenge with ricin and superior in protecting animals from ricin-induced lung damage.
“…USAMRIID has developed a recombinant RTA vaccine 1–33/44–198 (rRTA 1–33/44–198) (RV Ec ) produced in Escherichia coli [30–32]. Based on preclinical studies, including a pivotal repeated-dose toxicology study in New Zealand white rabbits conducted under GLP [33], this product was determined to have a reasonable safety profile for use in human studies.…”
Section: Progress Toward a Prophylactic Ricin Vaccinementioning
Ricin toxin, an extremely potent and heat-stable toxin produced from the bean of the ubiquitous Ricinus communis (castor bean plant), has been categorized by the US Centers for Disease Control and Prevention (CDC) as a category B biothreat agent that is moderately easy to disseminate. Ricin has the potential to be used as an agent of biological warfare and bioterrorism. Therefore, there is a critical need for continued development of ricin countermeasures. A safe and effective prophylactic vaccine against ricin that was FDA approved for “at risk” individuals would be an important first step in assuring the availability of medical countermeasures against ricin.
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