Disclosed herein are isolated nucleic acids, compositions of isolated nucleic acids, and compositions of polypeptides that are useful for the generation, enhancement, or improvement of an immune response to a target antigen. Some embodiments of the compositions include hepatitis B core antigen (HBcAg) protein and a heterologous protein antigen. In some embodiments, an isolated nucleic acid encoding hepatitis B core antigen (HBcAg) protein and a heterologous protein antigen is disclosed. Also disclosed herein are methods of administering the composition or isolated nucleic acid to generate an immune response, where HBcAg acts as adjuvant to improve the immune response to the heterologous protein. In certain embodiments, the HBcAg is as a stork or heron hepatitis antigen.