The struggle between malarial parasites and malariologists is like the perennial contest for the baseball World Series. The scientists win a lot of games, but the mosquito-borne protozoans forever lead the series.

For three decades, parasitologist Ruth Nussenzweig, of New York University, has kept her eye on the ball of a vaccine against infection by Plasmodium falciparum, the deadliest human malarial parasite. Her latest at-bat is a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated March 31, 1998. Its title: "Recombinant viruses expressing a human malaria antigen can elicit potentially protective immune CD8+ responses in mice."

Rats and mice don't come down with human malaria, inflicted by P. falciparum. Instead, they succumb to a rodent-specific parasite, Plasmodium yoelii, but serve as suitable animal models for human immune responses to parasitic attack.

In man, these antimalarial defenses involve both arms of the human immune system, antibodies, made by B cells, and cytotoxic T lymphocytes (CTLs). The latter, killer T cells, attack and slay alien enemy cells, such as in organ transplants and malignant tumors. Malarial vaccinologists, notably Nussenzweig, have been trying for years to train rookie CTLs and B cells to become heavy hitters against the antigenic targets on P. falciparum .

In their most recent experiments, she and her co-authors introduced two plasmids into mice, one for the influenza virus, the other for vaccinia. Both carried genes expressing immunogenic proteins of the parasite's sporozoite stage in the victim's liver. (See BioWorld Today, May 28, 1997, p. 1.)

"The influenza virus," Nussenzweig told BioWorld Today, "would not permit large inserts, but could express just the esential part of the circumsporozoite [CS] surface protein, namely a B cell epitope responsible for antibody induction, plus a CTL epitope.

"But in the vaccinia vector," she continued, "which is a large virus permitting large inserts, we expressed the entire CS surface protein. We knew from earlier work that this protein is important for protection against malaria, not only in mice but in humans."

First, Nussenzweig and her co-authors sensitized, or primed, the mice with the recombinant flu virus plasmid, carrying the human P. falciparum CTL epitope of that CS and its protein. "The flu virus by itself is not protective," she explained.

Three weeks later, they injected the animals with the recombinant vaccinia vector. "Then," she recounted, "we waited two weeks and tested the mice for CTL production. Because we used the human parasite protein, we could not test them directly for protection against infection."

Method Produces 'Very Striking Protection'

Instead, in a base-stealing play, she "used an indirect testing method by injecting a highly virulent vaccinia virus intracerebrally, which would normally kill the mice. As a result of this challenge, the mice did not have any detectable vaccinia virus in their brains at the point in time when all the control animals did. So what it achieved was to delay very significantly a rapid lethal viral infection from the other virus. It was very striking protection."

Nussenzweig made the point that "the only thing this vaccinia virus had in common with the prior immunization was the CTL epitope."

She also emphasized another unexpected finding: "We found that the order of iimmunzation was very important. If we inverted it -- gave the vaccinia virus first and then the influenza virus -- there would be absolutely no protection in our rodent malaria model."

Nussenzweig concluded: "It might be possible in the future to immunize humans with a very, very attenuated vaccinia virus, which is minimally, if at all, pathogenic -- much less so than smallpox vaccination in the past. And also a highly attenuated influenza virus. We are working with these now in mice, and if we get the same protective effect, hopefully this will go to man." *

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