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sion of the virus to man. Hemagglutinin-inhibiting and JEV-neutralizing antibodies were induced on vaccination. The nonvaccinated challenged animals succumbed to JEV infection, whereas the vaccinated group had levels of JEV challenge viremia insufficient to be transmitted by mosquitoes (28) Both a NYVAC- and a canarypox-based Japanese encephalitis recombinant are currently being evaluated in human clinical trials.

A NYVAC vector has been engineered to express the rabies glycoprotein gene. In mice, cats, and dogs, the recombinant was shown to be safe and to provide protection against a lethal rabies virus challenge. The recombinant is now being evaluated in phase I human clinical trials for safety and immunogenicity.

Pox virus vectors have been used to determine the immunogenic potential of antigens from Plasmodium spp. in an effort to understand the design of an effective vaccine against malarial infections. In this regard a NYVAC vector reconstituted with the K1L host range gene was constructed to express intact or mutated forms of the circumsporozoite protein of Plasmodium berghei. Vaccination of the target host, the mouse, induced both binding antibody and CTL. Vaccinated and control mice were challenged either by the intravenous injection of sporozoites or by allowing infected mosquitoes to feed on the subjects. Protection was scored as the absence of blood stage parasetemia as determined by microscopic analysis of blood films from individual mice from 5–15 days after challenge. In a number of challenge experiments, ≈80% protection was obtained. This is to be compared with the consistent 100% level of protection obtained by vaccination with irradiated sporozoites. Protection in the recombinant virus-immunized mice apparently did not correlate with antibodies but a good correlation was established between CTL and protection. In vivo antibody depletion of CD8+ T cells before challenge abrogated protection (29).

With this data as an inducement, a complex NYVAC-based recombinant was constructed to express multiple antigens from P. falciparum. To address the multiple stages of the parasite life cycle, multiple antigens from the various stages were used. Thus, a recombinant expressing seven parasite antigens was provided. This recombinant was evaluated in rodents and in monkeys where safety and immunogenicity were established (30). This recombinant is now being evaluated in clinical trials where the vaccinated subjects are exposed to the bites of infected mosquitoes. Appearance of parasites in the blood of the infected volunteers will terminate the challenge followed by administration of antimalarial drugs to thwart further replication of the parasite. Since ethical and medical considerations require treatment on appearance of blood-stage parasites, only the antisporozoite and liver-stage immunity engendered by the vaccine can be evaluated. Full evaluation of blood-stage and transmission-blocking immunity cannot be evaluated in this limited clinical setting.

To date, all the above-mentioned abstracted data provided from human clinical trials using NYVAC-based vectors have described a good safety profile and the induction of some level of immunity to the expressed heterologous antigens.

Other Applications of Pox Virus-Based Vectors

The use of pox virus-based vectors as recombinant vaccines for heterologous bacterial, viral, or parasitic pathogens was the first practical application of this technology deriving from the fact that vaccinia virus was an established vaccine. However, the pox virus vectors can be looked at as general delivery systems for genes for other applications. For example, these vectors can be used in vitro to stimulate and expand CTL reactivities from the peripheral blood of chronically infected or tumor-bearing individuals (31). The antigen-specific stimulation and expansion of such cultures might provide some therapeutic benefit when reintroduced to the donor patient.

For cancer immunotherapy, numerous pox virus-based recombinants expressing tumor-associated antigens or biological response modifiers have been described (32). Of particular note, recombinants expressing the carcinoembryonic antigen were shown to elicit both antibody and cellular immune responses in mice and monkeys and to protect mice from tumor cell challenge (33, 34). Whether vaccinia or canarypox-based recombinants expressing the carcinoembryonic antigen will have any therapeutic benefit is currently being investigated in the clinic in patients with colorectal carcinomas.

A recent publication (4) reported the protection of mice vaccinated with a p53 expressing recombinant against challenge with an isogenic and highly tumorigenic mouse fibroblast tumor cell line expressing high levels of a mutant human p53 but lacking endogenous murine p53. Expression of the mutant form of p53 in the recombinant virus was not essential since the wild-type p53 afforded similar efficacy. This may be an important observation since p53 is an attractive target for cancer immunotherapy. Mutations of p53 represent the most common genetic changes demonstrated in human tumors.


The excitement of the 1982 proposal to use pox virus-based vectors as heterologous vaccines and the ensuing years of extensive pursuit of this idea have provided numerous working examples in laboratory animal model systems as well as in target species. In the veterinary field, products have now been licensed for commercialization and a significant number of clinical studies have been and continue to be pursued for both infectious diseases, ex vivo therapies, and cancer immunotherapy. The immediate future looks to be as exciting as the recent past.

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