Vitetta, E. | bioRxiv Science

bioRxiv2020

Endpoint and Epitope-specific Antibody Responses as Correlates of Vaccine-mediated Protection of Mice against Ricin Toxin

The successful licensure of vaccines for biodefense is contingent upon the availability of well-established correlates of protection (CoP) in at least two animal species that can then be applied to humans, without the need to assess efficacy in the clinic. In this report we describe a multivariate model that combines pre-challenge serum antibody endpoint titers (EPT) and values derived from an epitope profiling immune-competition capture (EPICC) assay as a predictor in mice of vaccine-mediated immunity against ricin toxin (RT), a Category B biothreat. EPICC is a modified competition ELISA in which serum samples from vaccinated mice were assessed for their ability to inhibit the capture of soluble, biotinylated (b)-RT by a panel of immobilized monoclonal antibodies (mAbs) directed against four immunodominant toxin-neutralizing regions on the enzymatic A chain (RTA) of RT. In a test cohort of mice (n=40) vaccinated with suboptimal doses of the RTA subunit vaccine, RiVax(R), we identified two mAbs, PB10 and SyH7, which had EPICC inhibition values in pre-challenge serum samples that correlated with survival following a challenge with 10 x LD50 of RT administered by intraperitoneal (IP) injection. Analysis of a larger cohort of mice (n=645) revealed that a multivariate model combining endpoint titers and an epitope-profiling immune-competition capture (EPICC) assay values for PB10 and SyH7 as predictive variables had significantly higher statistical power than any one of the independent variables alone. Establishing the correlates of vaccine-mediated protection in mice represents an important steppingstone in the development of RiVax(R) as a medical countermeasure under the United States Food and Drug Administrations "Two Animal Rule."

immunology↗

bioRxiv2018

Humanized Monoclonal Antibody Rescues Rhesus macaques from Aerosolized Ricin Toxin Exposure

Ricin toxin (RT) ranks at the top of the list of potential bioweapons of concern to civilian and military personnel alike due to its high potential for morbidity and mortality after inhalation. In non-human primates, aerosolized ricin triggers a severe acute respiratory distress characterized by perivascular and alveolar edema, neutrophilic infiltration, and severe necrotizing bronchiolitis and alveolitis. There are currently no approved countermeasures for ricin intoxication. In this report, we demonstrate the therapeutic potential of huPB10, a toxin-neutralizing humanized monoclonal antibody (MAb) against an immunodominant epitope on ricins enzymatic A chain (RTA). Five rhesus macaques that received intravenous huPB10 (10 mg/kg) four hours after lethal dose ricin aerosol exposure all survived the toxin challenge, as compared to control animals, which succumbed to ricin intoxication within 30 h. Antibody treatment at 12 h after ricin exposure resulted in the survival of only one of five monkeys, indicating that, in the majority of animals, ricin intoxication and local tissue damage had progressed beyond the point where huPB10 intervention was beneficial. Change in pro-inflammatory cytokine/chemokines levels in bronchial alveolar lavage fluids before and after toxin challenge successfully clustered monkeys based on survival, as well as treatment group. IL-6 was the most apparent marker of ricin intoxication. This study represents the first demonstration in nonhuman primates that the lethal effects of inhalational ricin exposure can be negated by a drug candidate and opens up a path forward for product development.

immunology↗