If malaria were a board game like Monopoly, instead of the world'sworst infectious killer, its rules of play might read:

First _ Let a hungry female Anopheles mosquito bite a malaria-infected person, suck up a meal of blood swarming with malarialparasites, and then buzz off.

Second _ Later, let that same insect bite a healthy person, and injecta shot of saliva, laced with those pathogens, in their sporozoite stage.

Third _ Point these sporozoites straight at their victim's liver, totake up residence in the hepatocytes.

Fourth _ Holed up in these liver cells, let them multiply mightily fora week or so, then sally forth as pre-blood-cell-stage parasites toinfect the blood, where they become merozoites.

Fifth _ By savaging and ravaging red blood cells, those merozoitestrigger the chills, fever and other symptoms of full-blown malaria.

Sixth _ Another Anopheles flies past "Go," lights on a malariavictim and sucks up those merozoites, to perpetuate the cycle.

Parasitologists, immunologists and genetic engineers have spentdecades aiming at antigenic targets of opportunity in that grim life-cycle, trying to contrive a vaccine against malaria. (See BioWorldToday, Oct. 13, 1994, p. 1.)

One of the latest efforts is reported in the current Proceedings of theNational Academy of Sciences (PNAS), dated April 25. Its title:"Maintenance of protective immunity against malaria by persistenthepatic parasites derived from irradiated sporozoites."

Post-doctoral immunologist Libia Scheller, then at the University ofMaryland School of Medicine in Baltimore, exposed Anophelesmosquitoes infected with Plasmodium berghei malarial parasites to10 kilorads of gamma radiation emanating from a Cesium-137isotopic source. Next, she extracted the irradiated sporozoites fromthe insects' salivary glands, and injected these putative antigens intothe bloodstream of rats and mice. As controls, she inoculated otherrodents with the residues of salivary glands emptied of irradiatedparasites.

So far, Scheller, the PNAS paper's first author, was essentiallyrepeating experiments with radiation-weakened sporozoites currentlygoing on in many laboratories around the world. Her results, though,broke new ground. They showed that rats thus immunized withcrippled but live liver-stage parasites survived challenge by full-strength sporozoites, but lost their immunity as soon as the irradiatedparasites were removed from the liver.

Protection against malaria is thought to be a cellular immuneresponse. T cells, not antibodies, presumably confer the immunity.Scheller and the head of her microbiology/immunology laboratory,Abdu Azad, suspected otherwise.

"We wanted to prove," Azad told BioWorld, "that it's not really theT cells. So if you wait long enough, and then remove the irradiatedform of the parasite, there is no memory cell around. And if youchallenge this animal, it should get infected. Which is what we provein this paper."

Azad explained: "When you challenge an animal immunized withirradiated sporozoites, which are in the hepatocyte, the challengingparasite gets into the liver, but is destroyed before it can form the pre-blood stage."

Sheller, now at the Naval Medical Research Program in Rockville,Md., told BioWorld Today: "We showed for the first time that liver-stage sporozoites can transform into [pre-blood-stage] trophozoitesthat stay in the liver for much longer times _ up to 60 days _ and ifremoved, abrogate protective immunity."

Anti-Malarial Protection Survives Sporozoite Removal

Primaquine is an antimalarial drug that effectively cleans out bothmultiplying and dormant sporozoites from liver cells. Sheller used itto eliminate the irradiated, antigenic sporozoites from vaccinated rats,and then challenge them with virulent parasites. The rodentscontinued to show 100 percent protection from infection seven daysafter primaquine treatment. Even after 90 days, 16 percent shruggedoff the challenge.

In a final experiment, Scheller vaccinated rats with irradiatedsporozoites, and homogenized those of their livers that contained pre-blood-stage forms. She then inoculated naive rodents with thisantigenic liver puree. They all escaped blood infection afterchallenge, but this one-shot protection was short-lived.

Sheller concluded: "That last experiment shows that in seekingantigens for purposes of immunization, and development of asynthetic vaccine or DNA vaccine of some sort, we should now lookat those antigens that are expressed by this irradiated parasite in itsdormant form."

William Trager, emeritus professor of parasitology at RockefellerUniversity, told BioWorld Today: "What Scheller's and Azad's workshows is that using irradiated sporozoites, these liver stages thatresulted and were unable to multiply, nevertheless survived for verylong times _ up to six months. That was what was responsible forproducing and maintaining the immunity." n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.