Recombinant nucleic acid molecules are described. The molecules have two nucleic acid sequences, wherein the first nucleic acid sequence is a truncated A subunit coding region obtained or derived from a bacterial ADP-ribosylating exotoxin, and the second nucleic acid sequence is a truncated B subunit coding region. Vectors and compositions containing these molecules are also described. Methods for enhancing an immune response against an antigen of interest using these recombinant nucleic acid molecules and compositions are also described.