Science Editor

Weakened strains of the smallpox vaccinia virus may provide as much protection as the traditional vaccine, but with less chance of damaging side effects. This counterintuitive finding informs a paper in today's issue of the Proceedings of the National Academy of Sciences (PNAS), released online July 14-18, 2003.

The article is titled "Shared modes of protection against poxvirus infection by attenuated and conventional smallpox vaccine viruses." Its senior author is immunologist Jay Berzofsky, chief of the molecular immunogenetics and vaccine research section at the National Cancer Institute.

He made the point that, "although smallpox was officially pronounced eradicated by the World Health Organization more than 20 years ago, the deadly and contagious smallpox variola virus could reemerge as a bioweapon". Berzofsky pointed out that "immunization with the molecularly related but nonlethal vaccinia virus effectively prevents smallpox infection. This live vaccinia virus reproduces inside the human body and stimulates its immune defenses. However," he added, "vaccinia virus can cause severe side effects in patients with weakened immune systems. (See BioWorld Today, Nov. 19, 2002.)

"The licensed smallpox vaccine currently used in the U.S.," Berzofsky continued, "is burdened by a risk of adverse effects and even some mortality. Hence we consider it very risky to immunize the population that is immunocompromised by AIDS, cancer chemotherapy or immunosuppression following organ transplant."

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"The mechanisms of protection from smallpox by these poxvirus vaccines is not clear," he commented. "And now there are new attenuated vaccines like the MVA [modified vaccinia Ankara] and NYVAC [New York vaccinia]. We have been studying them as potential vaccines that would be safer than the existing smallpox vaccines. Yet they can't really be tested for protection against smallpox in human clinical trials because you can't ethically give smallpox to a volunteer. Therefore, we reasoned that one comparison might be helpful in deciding whether these attenuated and safer viruses would be suitable.

"We could do this most easily in a small animal model such as the mouse, which can't catch smallpox. Neither of those two new entries replicates very well if at all in mammalian cells, human or mouse. Therefore, they are much safer than the conventional vaccine strain of vaccinia, which can replicate and potentially cause a disseminated infection in immunodeficient people for example. So we compared the mechanism of protection and found that both depended primarily on inducing antibodies in humans and small animals.

"Mice deficient in cells that can't make antibodies got disease but they still had a T-cell response that could prevent death and allow for recovery," Berzofsky explained, "In the absence of antibody, the vaccine could also induce a T lymphocyte for cellular immune response, which could mediate recovery and prevent death. So we concluded that because the mechanisms of protection are very similar, that information might be useful in deciding whether these were attenuated viruses suitable to be used for vaccines in humans. And that study couldn't be done in humans because you can't challenge people with smallpox variola virus.

"We studied the ability of MVA or NYVAC to protect laboratory mice against WR - a mouse pathogenic strain of vaccinia virus. We immunized groups of five mice intranasally or intramuscularly with MVA or NYVAC at a range of doses. As a positive control, we immunized with the conventional Wyeth DryVax vaccine by tail scratch to the mouse tail vein [corresponding to skin scratch used for human vaccination].

"One month after inoculation," Berzofsky said, "we challenged the mice with WR intranasally. A moderate dose induced death of unimmunized animals seven to nine days after challenge. We measured clinical protection by prevention of weight loss. Doses of conventional Wyeth DryVax induced complete protection against challenge with WR. Similar protection was seen with attenuated strains of both MVA and NYVAC."

Surviving Antibodies Sufficient To Prevent Death

"If we looked at mice that have a genetic deficiency in making B lymphocytes, which make antibodies," Berzofsky continued, "those mice are not protected against disease. But in contrast, completely unimmunized animals, although they lost weight, eventually recovered after a week or so - regaining weight and surviving.

"But they had a second mechanism that was not dependent on antibodies," he went on. "By depleting those mice of T lymphocytes we showed that in a second mechanism, basically the antibodies were necessary for protection against disease. But the T lymphocytes were sufficient to prevent death and mediate recovery in the absence of antibody." The same mechanism applied to both the attenuated MVA and the licensed vaccine strains.

As a crucial element in the role of antibodies, the PNAS paper introduced the Jh locus. "This," Berzofsky explained, "is a gene complex that encodes the J segment of the heavy chain of the immune globulin. Mice that lack that gene cannot make immunoglobulin because they need that J segment. So if those mice have those genes and their locus knocked out, those mice are B-cell deficient.

"In ongoing further research," Berzofsky volunteered, "we have an interest in inducing mucosal immunity. This because the natural route of smallpox transmission is into the lungs through the respiratory tract. So this route may be very important in protecting against disease. We're studying whether a vaccine delivered through such a mucosal route might be more effective. We would like to look at the immunity that's actually induced in the lung itself. So that's one of the next things we plan to do," he concluded.