Plant-based expression of a partially humanized neutralizing monoclonal IgG directed against an immunodominant epitope on the ricin toxin A subunit.

GD12 is a murine monoclonal IgG(1) (mAb) that recognizes an immunodominant linear neutralizing epitope (163-TLARSFIICIQM-174) on the A subunit (RTA) of ricin toxin. With the long-term goal of using GD12 as a potential countermeasure against ricin intoxication, we have produced a chimeric derivative of GD12 (cGD12) in which the murine heavy and light chain variable regions were fused to a human IgG(1) framework. The chimeric mAb, expressed and purified using a Nicotiana-based system demonstrated epitope specificity and ricin neutralizing activity similar to the parental murine mAb. Passive administration of cGD12 (10μg) to mice by intraperitoneal injection protected the animals against a systemic ricin challenge. In a post-exposure setting, the murine and chimeric mAbs administered as much as 6h after toxin challenge were each capable of rescuing mice from toxin-induced death, revealing the potential of GD12 to serve as both a prophylactic and therapeutic for ricin intoxication.