An Intranasally Administered Antibody Cocktail Prevents Ricin Toxin-Induced Lung Inflammation and Alveolar Macrophage Death

Abstract

Abstract Ricin toxin, a plant-derived, mannosylated glycoprotein, elicits an incapacitating and potentially lethal, pulmonary inflammatory response in the airways following inhalation. Uptake by alveolar macrophages (AM) and other pulmonary cell types occurs via ricin’s B subunit (RTB), a galactose-specific lectin, and the mannose receptor (MR; CD206). Ricin’s A subunit (RTA) is a ribosome-inactivating protein capable of inducing apoptosis in all mammalian cell types. It was recently reported that a single monoclonal antibody (MAb), PB10, directed against an immunodominant epitope on RTA, is able rescue Rhesus macaques against lethal dose of ricin administered by aerosol. In this study, we now demonstrate in mice that the effectiveness PB10 is significantly improved when combined with a second MAb, SylH3, against RTB. Mice treated with PB10 alone survived lethal-dose intranasal ricin challenge, but experienced significant weight loss, moderate pulmonary inflammation (e.g., elevated IL-1 and IL-6 levels, PMN influx), and apoptosis of alveolar and tissue macrophages. In contrast, mice treated with the PB10/SylH3 cocktail were essentially impervious to pulmonary ricin toxin exposure, as evidenced by no weight loss, no change in local IL-1 and IL-6 levels, retention of alveolar and tissue macrophage numbers, and a significant dampening of PMN recruitment to the bronchoalveolar lavage (BAL) fluids. The PB10/SylH3 cocktail only marginally reduced ricin binding to target cells in the BAL, suggesting that the antibody mixture neutralizes ricin by interfering with one or more steps in the RTB- and MR-dependent uptake pathways.