Effects of Antigen and Genetic Adjuvants on Immune Responses to Human Immunodeficiency Virus DNA Vaccines in Mice

The effects of genetic adjuvants on humoral and cell-mediated immunity to two human immunodeficiency virus antigens, Env and Nef, have been examined in mice. Despite similar levels of gene expression and the same gene delivery vector, the immune responses to these two gene products differed following DNA immunization. Intramuscular immunization with a Nef expression vector plasmid generated a humoral response and antigen-specific gamma interferon (IFN-gamma) production but little cytotoxic-T-lymphocyte (CTL) immunity. In contrast, immunization with an Env vector stimulated CTL activity but did not induce a high-titer antibody response. The ability to modify these antigen-specific immune responses was investigated by coinjection of DNA plasmids encoding cytokine and/or hematopoietic growth factors, interleukin-2 (IL-2), IL-12, IL-15, Flt3 ligand (FL), and granulocyte-macrophage colony-stimulating factor (GM-CSF). Coadministration of these genes largely altered the immune responses quantitatively but not qualitatively. IL-12 induced the greatest increase in IFN-gamma and immunoglobulin G responses to Nef, and GM-CSF induced the strongest IFN-gamma and CTL responses to Env. A dual approach of expanding innate immunity by administering the FL gene, together with a cytokine that enhances adaptive immune responses, IL-2, IL-12, or IL-15, generated the most potent immune response at the lowest doses of Nef antigen. These findings suggest that intrinsic properties of the antigen determine the character of immune reactivity for this method of immunization and that specific combination of innate and adaptive immune cytokine genes can increase the magnitude of the response to DNA vaccines.