Enhancement of antibody responses by DNA immunization using expression vectors mediating efficient antigen secretion

The immune responses elicited in mice, after intradermal (i.d.) immunization with plasmids encoding secreted or intracellular forms of HIV-1 nef, HIV-1 tat or C. pneumoniae omp2 proteins, respectively, were compared. To mediate secretion of these proteins the genes were fused to a heterologous signal sequence from murine heavy chain IgG. The nef- and omp2-specific antibody responses were dramatically increased when mice were inoculated with the plasmid encoding the secreted form of these proteins. In contrast, HIV-1 tat comprising an internal strong nuclear targeting sequence could not be induced to secretion and subsequently no enhanced antibody response was observed. Slight improvement of the HIV-1 nef antibody response was achieved after co-inoculation with a granulocyte-macrophage colony-stimulating factor (GM-CSF) expression vector. Further, nef-specific T-cell responses were induced after nef DNA injections, and were of Th1-like phenotype regardless of whether the nef protein was secreted or not. The system described in this study, using a plasmid vector with a strong heterologous signal sequence that mediate efficient antigen secretion in vivo, may have wide applicability for the induction of high antibody levels to normally non-secreted antigens.