Consequently, to produce an abrin-subunit specific antigen possessing proper abrin-subunit conformational structure for rabbit immunization, we generated chimeric abrin-ricin toxins composed of Aabrinor Babrinlinked via disulfide bond to their non-homologous counterparts, subunit Bricinor Aricin, respectively. AricinBabrinconferred exceptionally high protection levels to mice following intranasal exposure to a a lethal dose of abrin, suggesting that the high level of protection conferred by Rabbit monoclonal to IgG (H+L)(Biotin) anti-abrin antibodies is not related to the neutralization of a particular subunit. Keywords:abrin, ricin, chimeric toxins, A-subunit, B-subunit, polyclonal antibodies == 1. Introduction == Abrin and ricin, plant toxins derived from rosary peas (Abrus precatorius) and castor bean plant (Ricinus communis), respectively, belong to the type 2 ribosome inactivating (RIP-2) group of proteins. The heterodimeric glycoprotein of both toxins comprises subunit A, carrying the enzymatic activity of the toxin, and subunit B, which is a lectin that binds to eukaryotic cell surfaces via galactose residues and mediates toxin internalization [1]. Upon entry to the cytosol, the A-subunits of both ricin and abrin inactivate ribosomes irreversibly by site-specific depurination of a specific adenine nucleotide within the 28S rRNA, thereby precipitating cessation of cellular protein synthesis [2]. Due to their high availability, ease of preparation and high toxicity, alpha-Boswellic acid ricin and abrin are considered potential bioterror agents and are classified as Category B agents by the US Center for Disease Control and Prevention (CDC). To date, there are no approved post-exposure therapeutic countermeasures for either toxin. The toxicity of ricin and abrin depends on the route of exposure, inhalatory exposure being considered the most fatal [2]. The clinical alpha-Boswellic acid manifestation following intranasal exposure of mice to these toxins is the onset of a localized yet severe pulmonary edematous inflammation, which is accompanied by massive recruitment of neutrophils to the lungs and onset of a turbulent pro-inflammatory cytokine storm within this organ. Pulmonary (intranasal) ricin and abrin intoxications in mice, are similar with regard to pathological features and kinetics [3,4]. However, despite their resemblance in morbidity and mortality, the ability to protect mice against ricin and abrin intoxications by post-exposure antibody-mediated treatment differs radically. In the case of lethal ricin intoxication, rabbit-derived alpha-Boswellic acid polyclonal anti-ricin antibody-based treatment up to three hours post exposure conferred protection to 90% of the mice, nonetheless following treatment at 24 hours survival rates declined dramatically, to 34% [3]. When antibody treatment was administered at 48 hours post exposure to ricin, protection was no more than marginal [5]. In sharp contrast, the administration of polyclonal anti-abrin antibodies to mice intranasally exposed to a lethal dose of abrin, led to very high survival rates (~70%80%), even when the antibodies were applied as late as 72 hours after intoxication [4]. The goal of the present work was to elucidate whether the efficient protection by polyclonal anti-abrin antibodies can be attributed to the neutralization of a single subunit, in other words, to define the critical neutralizing event that confers high-level of protection: prevention of toxin binding via the B subunit or abolishment of the A subunit-related catalytic activity. A specific conclusion could assist us, for example, to produce a subunit-directed abrin vaccine, which would be as efficient as holotoxin-based vaccine, but much safer. In addition, determining the subunit whose neutralization is critical for protection could assist in designing a more effective anti-ricin immunization protocol. Production of effective polyclonal antibodies against a specific subunit of the abrin (or ricin) toxin is complicated by the tendency of isolated subunits to adopt an unstable conformation (in particular, Babrinis insoluble in aqueous solutions when separated from the Aabrin, due to its hydrophobicity [6]). To overcome this difficulty, we incorporated each of the abrin subunits into a dimeric AB molecular fold by generating two chimeric heterologous toxins, AabrinBricinand AricinBabrin, utilizing the monomeric.