Monoclonal Antibodies Neutralize Bacillus cereus Nhe Enterotoxin by Inhibiting Ordered Binding of Its Three Exoprotein Components

The Nhe enterotoxin from Bacillus cereus is known to induce cytotoxicity on Vero and CaCo-2 cells by ordered binding of its single components NheA, NheB, and NheC. This study aimed to elucidate functional sites on NheB by identifying the epitopes of the neutralizing monoclonal antibodies 1E11 and 2B11. The binding regions of both antibodies were determined by using recombinant NheB fragments and synthetic peptides. The antigenic site of antibody 1E11 was located within the amino acids 321 to 341 of NheB, whereas reactivity of antibody 2B11 was dependent on the presence of amino acids 122 to 150 and on conformation. Both antibodies were able to bind simultaneously to NheB and did not interfere with target cell binding as shown by immunofluorescence microscopy. A set of neutralization assays revealed that antibody 2B11 most likely interfered with the interaction between NheB and NheC both on the epithelium cell surface and in solution. In contrast, antibody 1E11 inhibited association between NheA and cell-bound NheB in a competitive manner, and effectively neutralized Nhe cytotoxicity on a variety of human cell lines. This distinct mechanism further supports that NheA is the key component during the Nhe mode of action and the C-terminal epitope recognized by antibody 1E11 points to an important functional region of NheB.