From the American Revolution to Vietnam, U.S. armed forces havefought nine major wars, and lost 578,245 troops killed in combat. Allof these battlefield casualties outnumber by only about 10 percent theestimated 548,000 Americans who died in just one year, 1918 to1919 _ victims of the influenza virus. In India, the pandemic killed12.5 million.
That viral aggressor deploys more treasons, stratagems and spoilsthan any strategist or tactician in military history.
Picture an underwater anti-ship mine, a metal ball studded withprojections fused to explode on contact. Orthomyxoviridae, theinfluenza virus, has the same spherical shape, but is only 100nanometers (i.e., 10 millionths of a meter) in diameter.
Its outer surface too is covered with spikes, consisting of twoglycoproteins designed to grapple the virion to ciliated epithelial cellsof the upper respiratory tract. One of these molecules, hemagglutinin,hooks the virion to receptors on the cell; the other, neuraminidase,then cleaves said receptor.
Those two proteinaceous protuberances represent the virus's ultra-evasion secret, but also the immediate targets for flu vaccines.
Hardly anyone needs to be reminded of influenza's symptoms: chills,fever, coughing, sneezing, runny nose, sore throat, muscle aches, awashed-out feeling and headache. Some of these miseries, notablycoughing and sneezing, help the virus directly to spread through apopulation, by propelling shed virions from the nostrils into the air,for new victims to inhale.
Indeed, its cunning seems almost human-like. Thus, the virion's pointof attack, the ciliated airway epithelium, cripples those cells' abilityto repel boarders. Cilia wave rhythmically, like the oars of a racingshell or ancient galley, to move virus-riddled mucous out of the body.Destroying these anti-pollutant paddles lets the virions hang in thereas they replicate.
But Orthomyxoviridae's ultimate evasive counter-measure against humans' sole defense, flu vaccines, is the proclivityof those antigenic surface spikes to change every two to four years,by undergoing point mutations.
Vaccine-Of-The-Year Race Runs In Place
Last winter's flu shot _ recommended for the very young, theelderly, and people at risk of chronic heart or pulmonary disease,such as cystic fibrosis _ may be useless this year. Which is why aworldwide anti-influenza espionage network, run by the WorldHealth Organization, ferrets out new antigenic strains to updateinactivated whole-virus or virion subunits for next winter'scommercial vaccine.
This annual tribute harks back to the first multivalent vaccine,formulated in 1934 by the American microbiologist andepidemiologist Thomas Francis, Jr. (1900-1969).
Now genetic engineering is pointing toward a new way ofimmunizing people against the influenza virus, by sidestepping itsshifty antigenic drift.
A paper in the current Nature Medicine, published June 1, reports thefirst non-replicating flu vaccine that reaches below the virion's now-you-see-it-now-you-don't surface antigens to target an internalnucleoprotein that drifts far more slowly. Its title: "Preclinicalefficacy of a prototype DNA vaccine: enhanced protection againstantigenic drift in influenza virus."
The article's first author, immunologist John Donnelly, is associateimmunology director in the cell biology department of MerckResearch Laboratories in West Point, Pa., He and seven othervirologists and immunologists report testing their naked-DNAinocula against commercial vaccines in monkeys, and also in ferrets(Mustela putorious furo), the animal model of choice for influenzavaccine research, because their antibody responses parallel those ofhumans.
When live flu viruses invade, the human immune system fights backwith both arms _ humoral and cellular: Neutralizing antibodiesgenerated by the humoral B cells attack the antigenic spikes, and Tcells move in to kill off virion-infected epithelial cells.
"Our results indicate," Donnelly told BioWorld Today, "that DNAvaccines appear promising, as a means of providing a greater breadthof protection of humans against influenza than do current vaccines,and for vaccination against other diseases."
Seeking To Scotch Flu Virus's `Gotcha!' Game
Merck's prototype flu vaccine directly delivers intramuscularinjections of naked DNA expression vectors. These encode thecurrent configuration of the hemagglutinin spike protein, as antibodytarget, and the deep down stable nucleoproteins as bull's eyes forcell-mediated scotching of viral replication.
Initially, the team vaccinated seven ferrets with DNA encodinghemagglutinin from that pioneer 1934 vaccine. Three of them shedno virions after challenge with live virus.
In another experiment, Donnelly continued, "Animals inoculated withthe surface antigen from a recent altered viral strain, then challengedwith that same live virus, showed overall shedding very significantlyreduced in comparison with non-immune ferrets. "As we expected,"he went on, "immunization with hemagglutinin DNA alone was mosteffective when the immunogen exactly matched the challenge strain."
Combining the recombinant surface antigen sequence with the 1496-base-pair gene encoding the internal nucleoprotein's 498 aminoacids, Donnelly observed, "substantially increased the efficacy ofimmunization.
The paper lists three potential advantages over current vaccines basedon whole inactivated virus:
* The combined surface and internal immunogens protect againstdrifted surface antigens.
* DNA inocula can be derived from human clinical specimens onlybriefly passaged in mammalian cell culture, as opposed tocommercial vaccines, cultured in chick eggs, which may selectslightly different antigens.
* Injected DNA encodes viral surface proteins "subjected to folding,assembly and post-translational modifications . . . of epitopes thatmay also be more similar to those found in native virus . . ."
The Merck group also spelled out the potential benefits of their nakedDNA over live attenuated virus vaccines.
Patents on the naked DNA flu vaccine are pending worldwide.
Donnelly and his co-authors caution that "The safety and efficacy ofDNA vaccines in humans are as yet unproven."
Vaccinologist Pierre Meulien of Pasteur-Merieux Serums et Vaccinsin Marcy L'Etoile, France, commented in Nature Medicine on theMerck work:
"This demonstrates for the first time," he wrote, "the role played byinternal proteins of the influenza virus in providing support forprotection . . . in a relevant flu model."
But Meulien cited "some major hurdles before it can become aclinical reality." These concern the immunological behavior ofmuscle cells receiving foreign DNA by the intramuscular injectionroute.
Yet, he concluded: ". . . we view DNA immunization as holdingsubstantial promise as an outstanding contribution of biotechnologyto modern medicine." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.