Abstract
Botulinum toxin typically interacts with two types of cells to cause the disease botulism. The toxin initially interacts with epithelial cells in the gut or airway to undergo binding, transcytosis, and delivery to the general circulation. The toxin then interacts with peripheral cholinergic nerve endings to undergo binding, endocytosis, and delivery to the cytosol. The receptors for botulinum toxin on nerve cells have been identified, but receptors on epithelial cells remain unknown. The initial toxin binding site on nerve cells is a polysialoganglioside, so experiments were performed to determine whether polysialogangliosides are also receptors on epithelial cells. A series of single mutant and dimutant forms of the botulinum toxin type A binding domain (HC₅₀) were cloned and expressed. One of these (dimutant HC₅₀ A(W₁₂₆₆L,Y₁₂₆₇S)) was shown to have lost its ability to bind nerve cells (phrenic nerve-hemidiaphragm preparation), yet it retained its ability to bind and cross human epithelial monolayers (T-84 cells). In addition, the wild-type HC₅₀ and the dimutant HC₅₀ displayed the same ability to undergo binding and transcytosis (absorption) in a mouse model. The fact that the dimutant retained the ability to cross epithelial barriers but did not possess the ability to bind to nerve cells was exploited to create a mucosal vaccine that was non-neurotropic. The wild-type HC₅₀ and non-neurotropic HC₅₀ proved to be comparable in their abilities to: 1) evoke a circulating IgA and IgG response and 2) evoke protection against a substantial challenge dose of botulinum toxin.