A novel probe for harpin receptor in nonhost plants: Monoclonal anti-idiotypic antibodies as internal images of HarpinXoo active sites

Harpins constitute one group of effector proteins which elicit a hypersensitive response in nonhost plants, but the subcellular localization and tissue distribution of harpin receptors are still controversial. Antigen mimicry by anti-idiotypic antibodies is employed as a reliable strategy to probe receptors that are present in very low concentrations in the organism. In this study, a monoclonal anti-idiotypic antibody (Ab2), named 6B2, was elicited by F(ab′)2 fragments digested from the purified polyclonal antibody specific for HarpinXoo (Ab1). 6B2 competed with HarpinXoo for binding to Ab1 and the total protein extracted from tobacco leaves indicated its anti-idiotypic character and internal image property. The relevance of antigen mimicry was further confirmed by eliciting a third generation antibody (Ab3), which was shown not only to bind to Ab2 competing with Ab1 but also to react with the original antigen, HarpinXoo. Taken together, these results demonstrate functional and biochemical mimicry of HarpinXoo by Ab2 and suggest that 6B2 can be a useful tool in probing its receptor in nonhost plants and other downstream studies.