Background: In spite of the large number of studies that have evaluated DNA-based immunization, few have directly compared the immune responses generated by different routes of immunization, particularly in non-human primates. Here we examine the ability of a hepatitis B surface antigen (HBsAg)-encoding plasmid to induce immune responses in mice and non-human primates (rhesus monkeys: Macaca mulatta) after delivery by a number of routes.
Materials and methods: Eight different injected [intraperitoneal (IP), intradermal (ID), intravenous (IV), intramuscular (IM), intraperineal (IPER), subcutaneous (SC), sublingual (SL), vaginal wall (VW)] and six noninjected [intranasal inhalation (INH), intranasal instillation (INS), intrarectal (IR), intravaginal (IVAG), ocular (Oc), oral feeding (oral)] routes and the gene gun (GG) were used to deliver HBsAg-expressing plasmid DNA to BALB/c mice. Sera were assessed for HBsAg-specific antibodies (anti-HBs, IgG, IgG1, IgG2a) and cytotoxic T lymphocyte (CTL) activity measured. Three of the most commonly used routes (IM, ID, GG) were compared in rhesus monkeys, also using HBsAg-expressing vectors. Monkeys were immunized with short (0-, 4- and 8-week) or long (0-, 12- and 24-week) intervals between boosts, and in the case of GG, also with different doses, and their sera were assessed for anti-HBs.
Results: In one study, anti-HBs were detected in plasma of mice treated by five of eight of the injected and none of the six noninjected routes. The highest levels of anti-HBs were induced by IM and IV injections, although significant titers were also obtained with SL and ID. Each of these routes also induced CTL, as did IPER and VW and one noninjected route (INH) that failed to induce antibodies. In a second study, GG (1.6 microg) was compared to ID and IM (100 microg) delivery. Significant titers were obtained by all routes after only one boost, with the highest levels detected by IM. Delivery to the skin by GG induced exclusively IgG1 antibodies (Th2-like) at 4 weeks and only very low IgG2a levels at later times; ID-immunized mice had predominantly IgG1 at 4 weeks and this changed to mixed IgG1/IgG2a over time. Responses with IM injection (in the leg or tongue) were predominantly IgG2a (Th1-like) at all times. IV injection gave mixed IgG1/IgG2a responses. In monkeys, in the first experiment, 1 mg DNA IM or ID at 0, 4, and 8 weeks gave equivalent anti-HB titers and 0.4 microg at the same times by GG induced lower titers. In the second experiment, 1 mg DNA IM or ID, or 3.2 microg by GG, at 0, 12, and 24 weeks, gave anti-HB values in the hierarchy of GG > IM > ID. Furthermore, high titers were retained after a single immunization in mice but fell off over time in the monkeys, even after boost.
Conclusions: Route of administration of plasmid DNA vaccines influences the strength and nature of immune responses in mice and non-human primates. However, the results in mice were not always predictive of those in monkeys and this is likely true for humans as well. Optimal dose and immunization schedule will most likely vary between species. It is not clear whether results in non-human primates will be predictive of results in humans, thus additional studies are required. http://link.springer-ny.com/link/service/journals/00020/bibs /5n5p287. html