A cytokine called interleukin-12 has helped outwit a smartparasite that breaks, enters and occupies macrophages PP thevery immune cells programmed to trash such pathogenicinvaders.
That intracellular parasite is Leishmania major, a sand fly-borne protozoan that causes a worldwide tropical scourge,leishmaniasis. The cytokine, IL-12, is currently a hot topicamong immunologists trying to develop a vaccine against thelikes of L. major, and who are looking for an effective adjuvant.
This week's Science carries a paper by immuno-parasitologistPhilip Scott et al. titled "The Adjuvant Effect of Interleukin-12in a Vaccine Against Leishmania major." Scott, whoselaboratory at the University of Pennsylvania Veterinary Schooltested his vaccine in mice, told BioWorld: "We are reportingthat if you use IL-12 as a vaccine adjuvant, it can induce notonly the helper T cells that generate an effective cellular-immunity response, but complete protection against thedisease."
More specifically, Scott stated, the recently identified cytokineenhances T helper-cell development in vivo and can replacebacterial adjuvants in a vaccine against leishmaniasis.
"What's probably as important here is the issue of not justleishmaniasis, but any disease where you want to induce agood cellular (as opposed to humoral) immune response," Scottadded. Besides the tropical disease proliferated by L. major,other intracellular bacterial invaders, notably tuberculosis andleprosy, depend on a strong cellular counterattack (seeBioWorld, Jan 7).
In vaccine development, top billing usually goes to the antigenthat triggers the immune defenses. A key supporting player,the adjuvant, gets only a footnote.
Thus, in an impending Phase I clinical trial of an experimentalvaccine against leishmaniasis sponsored by the World HealthOrganization (WHO), the antigen consists of the whole parasite,finely minced. The adjuvant is Bacillus Calmette-Guerin.
BCG was developed in the early years of this century by twoFrench scientists, who used a non-virulent, cow-infectingorganism, Mycobacterium bovis, as a decoy vaccine to preventMycobacterium tuberculosis from causing TB. (A similarstrategy immunized people against cowpox to foil smallpox.)
BCG didn't work very well at TB prophylaxis but turned out tobe fairly effective as an adjuvant in other vaccines. So did adifferent microorganism, Corynebacterium parvum, whichcauses acne. These two bacterial adjuvants have beenmainstays in vaccine development.
"How adjuvants work is poorly understood," Scott explained."We think their essential role is to direct T cell subsetdifferentiation. They supposedly function in several ways,including slowing the release of antigen, targeting antigen tomacrophages and activating macrophages."
Scott believes that many of the functions of bacterial adjuvantsmay lie in their ability to induce IL-12. He added that hisexperiments reported in Science "suggest that IL-12 cansubstitute for bacterial adjuvants, and that the induction of thiscytokine may be the key component in vaccine efficacy whencell-mediated immunity is required."
To prove this concept, Scott and his co-workers injected thefoot pads of one group of mice with soluble leishmania antigenand a second cohort with the same pathogenic substance plusmurine IL-12 (donated by Stanley Wolf, who oversees IL-12research at Genetics Institute).
After three days, the latter group of animals was producinghigh levels of interferon-gamma PP the cellular-system cytokinethat actually kills the parasite PP but almost no IL-4, whichdrives the humoral limb of the immune response, which is hereundesirable.
Ten days later, immune system cells from both mouse groupsgot a second dose of SLA. This in vitro challenge turned on thehigh IL-4 response, meaning no protection from the parasite.But SLA plus IL-12 reversed this pattern; it elevated INF-gwith little if any IL-4. However, IL-12 without the antigen hadno effect on INF-g output.
In mice, leishmanial infection is initially less a matter of lifeand death than of healing and non-healing skin lesions. Thenon-healing sores eventually metastasize from skin to visceralorgans and kill the animals. The healing infection overcomesthe fatal fate of the non-healers because of sufficient INF-gpre-infection.
Later, in an in vivo challenge, mice injected in the left foot padwith half-a-million L. major promastigotes (the parasite forminside the sand fly) were completely protected against disease.Control mice, which had received SLA without IL-12, hadleishmanial skin lesions averaging four millimeters in diameter;their vaccinated PP hence resistant PP peers had none and werechurning out INF-g in quantity.
"The immune system is as complicated as people want to makeit," Scott observed. One complication he and others areunraveling is how IL-12 singles out the cell-mediated subset ofthe body's two-pronged defenses from the cellular subset,which is ineffective against such intracellular invaders asleishmania.
Essentially, IL-12 energizes natural killer cells to activate TH1,the subset of helper T cells that produce primarily INF-g in thecellular response. TH2 helpers, on the other hand, are turnedon by antigen plus IL-4 instead of IL-12. They trigger theantibody-producing humoral system, which leishmania shrugsoff.
If IL-12 works as a super-adjuvant in mice, will it alsoimmunize monkeys and eventually humans? Scott said hehopes the vaccine will be tried in simians this summer or fall atthe Primate Research Center in Kenya under WHO sponsorship.
Meanwhile, Genetics Institute (GI), which has had a cross-licensing understanding with Hoffmann-La Roche on IL-12research, will start Phase I clinical trials of its recombinanthuman cytokine by the middle of this year, said Gail Brazier,GI's corporate communications manager. GI will try IL-12 asdirect therapy for HIV infection and metastatic cancer. Othertests as a vaccine adjuvant may follow, she told BioWorld.
"Our current plans are to continue studying the compound incancer patients with metastatic disease," said Rochespokeswoman Gail Levinson. "However, we have not ruled outother areas."
Another key corporate player in the basic immunology of IL-12 is DNAX Research Institute of Palo Alto, Calif., an arm ofSchering Plough. "This is sort of nitty-gritty immunology," saidDNAX senior scientist Anne O'Garra, speaking of Scott'sadjuvant concept for IL-12. She finds it "one of the mostthoughtful pieces of work regarding the treatment or inhibitionof infectious disease with IL-12." Its virtue, O'Garra toldBioWorld, lies in using the cytokine as a vaccine adjuvant,implying one or two doses, rather than as a drug administeredlong-term, for direct treatment of particular diseases, "like anumber of other cytokines, which I won't mention, becausethey involve pathology."
At DNAX, O'Garra and her team are "working out further IL-12mechanisms for not only obtaining cell-mediated immunity butfor controlling it." She confirmed that DNAX's parent company,Schering-Plough, "ultimately intends to put the cytokine intoclinical trials."
How soon is "ultimately?" O'Garra replied, "Phase-I-imminentyears, as opposed to long-distance years."
Scott's original mentor was Alan Sher, in whose Laboratory ofParasitic Diseases at the National Institute of Allergy andInfectious Diseases he studied leishmania five years ago." Now,Sher calls Scott "a golden boy," and told BioWorld, "His paper inScience is a very important finding. It's the first I know ofshowing cytokines to manipulate vaccines PP a whole new field."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.