SUMMARY The efficacy of DNA-based immunization in conferring protective immunity against certain microbial pathogens including human immunodeficiency virus type 1 (HIV-1) has been described. The potential advantage of DNA-based immunization over the traditional vaccines largely results from its capacity to efficiently induce Th1-biased immune responses against an encoded antigen. We describe how Th1-biased immune responses are induced by DNA-based immunization, using a DNA vaccine construct encoding HIV-1 gp160 cDNA and an eukaryotic expression plasmid carrying murine IFN-γ cDNA. Transfection of an eukaryotic expression plasmid carrying immunostimulatory sequences (ISS) as well as a gene of interest (DNA vaccine) into professional antigen presenting cells (APC) induced transactivation of IL-12 mRNA, which resulted in antigen-specific Th1-biased immune responses against the encoded antigen. Th1-biased immune responses induced by DNA-based immunization were substantially upregulated by a codelivery of an ectopic IFN-γ expression system, and this augmentation was mediated via action on professional antigen presenting cells to upregulate IL-12 production. Taken together, it appears likely that Th1-biased immune responses induced by DNA-based immunization are mediated via action on professional antigen-presenting cells to produce IL-12. Interestingly, the model provided strikingly resembles that previously described in infection with Listeria monocytogenes, an intracellular Gram-positive bacterium that induces strong Th1-biased immune responses. The result suggests that DNA-based immunization mimics certain aspects of natural infection with microbial organisms like attenuated vaccines, which in turn provides a rationale to the question of why DNA-based immunization so efficiently induces protective immunity against these microbial pathogens.