New research suggests that applying "naked" DNA in aqueous solution to skin may be an effective, simple and low cost method of immunisation. However, efficacy appears to be dependent on the presence of normal hair follicles, according to Dr Paul Khavari and colleagues (Nature Biotechnology 1999;17:870). The authors found that topical, aqueous DNA was comparable to topical liposomal and adenoviral preparations, and to the intramuscular route, in generating specific IgG antibodies four weeks post-vaccination. Specific antibody and cellular responses to the hepatitis B surface antigen (HBsAg) were induced by topical application of the DNA vaccine to the same order of magnitude as those produced by intramuscular injection of a commercially available HBsAg vaccine.Normal hair follicles were found to be necessary to induce an immune response in mice, as topical administration to skin lacking normal follicles did not elicit antibodies to HBsAg. The authors believe that the vaccine is taken up via the hair follicle rather than through the epidermis. They speculate that normal hair follicles may be needed because they are among the most common routes of infection in humans and, consequently, may have efficient mechanisms for inducing immune responses to proteins in the follicle.
Dr Khavari and colleagues (Stanford University School of Medicine, California, US) say that previous successful topical gene transfer to skin involved shaving followed by physical abrasion, chemical treatment or other surface modification. They say that their results have proved this to be unnecessary. Vaccines made out of an aqueous solution of "naked" plasmids could be advantageous in terms of cost and compliance, they conclude.