A trail blazed through DNA vaccine, noninvasive vaccine, and innate-adaptive immunity duo

I entered the field of vaccinology when I joined Dr. Stephen Johnston’s laboratory as a postdoc in 1990. He and Dr. John Sanford co-designed and fabricated the first hand-held gene gun and my job was to transfect live animals by delivering DNA-coated gold particles into a restricted subset of tissues under the gun point. After bombarding a recombinant plasmid encoding a human protein into the outer layer of murine skin, we demonstrated that serum antibodies to the human protein were elicited in transfected mice within a few weeks. This demonstration marked the first chapter of “DNA vaccine”; with the technique originally dubbed “genetic immunization” by Dr. Johnston. A year later, others reported that mice could be immunized following intramuscular injection of DNA with a conventional syringe needle. Comparative studies subsequently showed that gene gun inoculation outperformed intramuscular injection in mobilizing the immune repertoire toward beneficial protection of animals. To circumvent the low immunogenicity associated with needle injection, development of in vivo electroporation has provided another technique capable of immunizing animals with DNA vaccines to approximately the same caliber as that achievable by a gene gun. To date, human subjects have been effectively immunized following in situ DNA delivery by either electroporation or gene gun inoculation. In Dr. Johnston’s laboratory, we made an array of serendipitous observations that could hardly be explained by textbook knowledge. It was demonstrated that mice were effectively immunized when DNA was inoculated into the outermost layer of skin without going deep, as shown by β-galactosidase reporter expression within epidermis one day post-immunization. In conventional wisdom, transfected cutaneous cells (visible transfected cells as shown by reporter expression one day post-immunization were all keratinocytes) should broadcast a signal to recruit antigen-presenting cells (APC) for capturing the exogenous proteins made in transfected cells in conjunction with subsequent antigen presentation to T lymphocytes for eliciting an immune response; thus cross priming may not occur until the exogenous protein is expressed from the man-made DNA in vivo which takes approximately one day to reach its peak level. To our surprise, mice (the so-called van Gogh mice) could be effectively immunized when their transfected ear pinnae were surgically removed 5 min after inoculating DNA into the pinnae using a gene gun—an anti-dogmatic observation that potentially may lead research to a terra incognita. We subsequently identified dynamic relocation of discrete reporter spots from the transfected pinna to scalp, neck skin, and skin on the dorsal side; but never found such mobile reporter spots in skin on the ventral side of the animal one day following transfection of ear pinnae with the luciferase reporter gene using a gene gun (unpublished results). Although keratinocytes in an ear pinna could be transfected in large numbers under gun point, evidence implied that keratinocytes in the outermost layer of epidermis may not qualitatively be a A trail blazed through DNA vaccine, noninvasive vaccine, and innate-adaptive immunity duo