Comparative study of DNA-based immunization vectors: effect of secretion signals on the antibody responses in mice

The presence of a signal sequence preceding the gene encoding a target antigen in a DNA vaccine should facilitate secretion of the in vivo translated antigen. The immune responses elicited upon injection with such a vector could differ from those induced by the same vector lacking a signal sequence. In the present study, the humoral responses elicited in mice immunized with two plasmids, either containing or lacking the human tissue plasminogen activator signal sequence, were compared. Both plasmids encode the chimeric antigen ZZN4, containing a malaria antigen Pf332-derived sequence (N4) linked to a bacterial fusion partner (ZZ). In vitro transfection of COS cells with each plasmid and treatment of the transfectants with brefeldin A confirmed that secretion of ZZN4 via the endoplasmic reticulum and Golgi pathway only occurred in cells transfected with the signal peptide-encoding plasmid. Repeated intramuscular injections of mice with either of the plasmids elicited comparable antibody responses to ZZN4 with regard to kinetics, specific IgG levels and persistence. These results indicate that in vivo transfection of muscle cells by either of these two plasmids generated comparable levels of antigen available for B-cell recognition and for uptake by antigen-presenting cells, despite the differential intracellular targeting of the encoded antigen. The relevance of these findings for the design of DNA vaccine vectors is discussed.