The invention provides a method for stimulating an immune response by administering to a lymphoid tissue a nucleic acid molecule comprising an expression element operationally linked to a nucleic acid sequence encoding one or more heterologous epitopes. The heterologous epitope can be inserted into a complementarity-determining region of an immunoglobulin molecule. The invention also provides a nucleic acid molecule comprising a hematopoietic expression element operationally linked to a nucleic acid sequence encoding a heterologous polypeptide. The invention additionally provides a method of treating a condition by administering a nucleic acid molecule comprising a hematopoietic cell expression element operationally linked to a nucleic acid sequence encoding a heterologous polypeptide, wherein the nucleic acid molecule is targeted to a hematopoietic cell.