One of biotechnology's early successes is the recombinant vaccineagainst hepatitis B virus. One of its continuing failures is the inabilityto provide a user-friendly version of the vaccine to the people whoneed it most.

In the U.S., hepatitis B virus infects at least 24,000 persons a year. Inthe rest of the world, the Third World in particular, the numberinfected is at least 300 million, half of them in mainland China. Manyof these sustain permanent, debilitating liver damage and hepaticcancer.

Recombivax HB is the name of the recombinant vaccine, for whichMerck & Co. Inc., of Whitehouse Station, N.J., obtained FDAapproval in July 1986. Its payload consists mainly of the geneencoding a viral envelope subunit, cloned into yeast host cells.Protection against infection requires three shots a few months apart,at a cost per course varying from perhaps $50 to $100 in the U.S. to ausually subsidized price of $10 to $20 in many overseas countries.

The vaccine requires refrigeration for shipping and storage.

These three drawbacks _ injectable route, cold-chain and cost _ putthis crown jewel of genetic engineering beyond the reach of themillions, especially children, most at risk of hepatitis B infection andits sequelae.

Out of Texas comes a likely solution to all three roadblocks. It'sheralded by a paper in the current Proceedings of the NationalAcademy of Sciences (PNAS) dated April 11, titled:"Immunogenicity of transgenic plant-derived hepatitis B surfaceantigen."

The article's third author is plant molecular biologist Hugh Mason,on the faculty of Texas A&M University's Institute of Biosciencesand Technology. He told BioWorld Today how the institute's plantbiotechnology program leader, Charles Arntzen, spearheaded aproject to insert the hepatitis B epitope's gene into botanical species,beginning with tobacco.

"That subunit epitope," Mason said, "is a virus-likeparticle, which we think will enhance its ability to produce immuneresponses. After transforming the plants," he went on, "we extractedmaterial from the tobacco leaves, and sent it to our co-authors atRoswell Park Cancer Institute in Buffalo. They injected it into mice,and compared the antibody increase with responses obtained byinjecting the Recombivax vaccine. In general those B-cell responseslooked fairly similar."

T cell epitopes "were faithfully preserved in the plant-derivedmaterial," Mason continued. "That's important in cell-mediatedimmunity, where recognition by specific T cells can kill virus-infected cells."

Moreover, "Both the yeast vaccine and tobacco protovaccine can berecognized by the same antibodies, because they are sharing the sameepitopes."

Whatever its current popular reputation, tobacco, Nicotiana tabacum,is a favorite model for preliminary plant research. But for futureanimal trials of their vaccine, the Texans turned to Solanumtuberosum, the edible potato.

"We have potatoes now," Mason said, "that are expressing HBsurface antigens _ and a number of other antigens. I can't really talkabout the others too much," he added, "because publications are justcoming out in the near future." He observed that the potato "lookslike it's going to be a very good model system to use with mice andsome other test animals."

Because chimpanzees and humans are the only two speciessusceptible to hepatitis B infection, Mason said, "we have arrangedfor a primate colony, and identified a potential academiccollaborator, but have not yet made final arrangements for suchchimp studies."

He observed that "It's still very early stage. We have not shownanything about actual vaccine protective efficacy. We haven't doneany study like challenging an immunized animal with the pathogen."

Even in Third World countries, people don't usually eat theirpotatoes raw, although presumably chimps will. The heat of cookingwould destroy the subunit's immunogenicity, so the A&M team isplanning to create transgenic bananas, Musa acuminata, to delivertheir vaccines.

"One banana has approximately one gram of protein," Mason pointedout. If we can engineer that fruit to contain only 1 percent of thatgram in the form of recombinant protein _ candidate vaccines _ inmost cases that should be enough to have a dose."

Up to now, he continued, "nothing has been published on introducinga foreign gene into bananas. So we first had to develop that system.Now that work has been done, and is workable."

Their paper on that subject is coming out next month inBio/technology magazine, titled "Generation of transgenic banana(Musa acuminata) plants via agrobacterium-mediatedtransformation."

At this point, Mason and his colleagues are "in other stages oflooking for genetic regulatory elements that we can use in bananas todrive the expression of foreign genes."

All well and good, but would such transgenic bananas have to beshipped to recipient countries from centralized, sophisticated geneticengineering laboratories? "Actually," Mason responded, "much of itcan be done in those banana republics, I think. The places wherebananas are grown already have facilities for vegetativelypropagating the plants." He doubts that they would require cold-chainlevel refrigeration.

Oral Administration Still Needs Work

Of course, the best known oral vaccine to date is that against polio. Abanana-borne, ingestible subunit immunogen would be "similar, butnot exactly the same," Mason observed, "because the oral poliovaccine is a live attenuated virus; ours a subunit."

He and his co-authors have tested the ability of certain other (non-hepatitis B) antigens cloned in potato to stimulate antibodies in serumand mucosal secretions, when fed to mice. "It looks very promising,"he said.

Texas A&M launched its plant vaccine program in response to theWorld Health Organization's "Children's Vaccine Initiative." Masonpointed out that an edible, user-friendly hepatitis B vaccine would bea boon to countries where the infection is virtually endemic. Even atsubsidized prices, he observed, "the Chinese Peoples' Republic can'tafford to vaccinate their entire population."

Last spring, Merck announced it was licensing its expertise, andtransferring equipment, for making recombinant hepatitis B vaccineto China royalty-free. (See BioWorld Today, June 10, 1994, p. 3.)

Charles Arntzen, the PNAS paper's lead author, on Thursdayaddressed an international symposium on "New Approaches toVaccine Development" at the University of Vienna.

He reported on "Transgenic plants as an inexpensive means forproduction and delivery of vaccines for diarrheal disease," anddeclared: "Oral immunogens have the potential to be some of themost versatile, cost effective, and potent vaccines for stimulatingmucosal immune responses." n

-- David N. Leff Science Editor

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