Besides Kaposi's sarcoma and Pneumocystis carinii pneumonia,another grim reaper that afflicts AIDS victims is cerebraltoxoplasmosis. Half of the human race, give or take a billion orso people, are thought to carry antibodies against thisintracellular protozoan, Toxoplasma gondii.
Most people who harbor it don't even realize they have theinfection. But when it first invades an adult's nervous system,as in AIDS, the latent parasite can become lethal.
Despite its near ubiquity, T. gondii remains something of amystery bug. Now, parasitologists and immunologists at theNational Institute of Allergy and Infectious Diseases (NIAID)are unraveling a skein of the intricately tangled immunemeshwork that the body deploys against the invasivepathogen.
The title of their report in the current issue of Proceedings ofthe National Academy of Science (PNAS) is: "Interleukin 12 isrequired for the T-lymphocyte-independent induction ofinterferon-y by an intracellular parasite, and inducesresistance in T-cell deficient hosts."
The paper's authors suggest that IL-12, a novel and (untillately) little-researched cytokine, "has major potential for theinduction of microbial resistance in immunocompromisedhosts," notably HIV-1-infected individuals.
If diagnosis is half the cure, a short report in last week's Lancetoffers the prospect of diagnosing toxoplasmic infection in AIDSpatients in time to quell the parasite with often-effectiveantibiotics before its cysts can hunker down fatally in thebrain.
French scientists at the Institute of Biology and the teachinghospital in Montpellier, France, reported in this week's Lanceton 124 HIV-1-positive adults. Of 21 with cerebraltoxoplasmosis, 20 spontaneously secreted T. gondii antibodiesin cultures of their peripheral blood lymphocytes. So didanother 19 of 103 patients without any signs or symptoms ofthe opportunistic infection. Significantly, five of these, whostopped prophylaxis for toxoplasmic encephalitis, showedantibody production three to 15 months before onset of thebrain disease.
The French researchers concluded: "In vitro production oftoxoplasma-specific antibodies could improve the diagnosis oftoxoplasmic encephalitis in HIV-1-infected patients."
The immune defense duel with toxoplasma begins when aninterloping microbial pathogen encounters the macrophage, afront-line defender. At the same time, the invader confrontsnatural killer cells (NK), which move into action well before Tcells react to the invasion. In four to eight hours, themacrophage releases IL-12 and tumor necrosis factor-alpha(TNF-alpha). These two cytokines then synergistically stimulatethe NK cells to produce IFN-y, which activates macrophages toa microbicidal state, checking the spread of the organism.
At NIAID, immunologist Ricardo Gazzinelli and his co-workersinjected SCID (severe combined immune deficiency) mice with10 T. gondii cysts each.
In the average mouse or person with a functioning immunesystem, the rear-echelon B and T cells mobilize a full-courtpress of antibodies and killer T cells. These immune-systemheavies can usually neutralize the T. gondii invasionharmlessly. In the SCID mice, which have NKs but nofunctioning T or B cells, those that received a dose ofrecombinant IL-12 survived 30 days or more; those that didn'tsuccumbed in nine to 16 days.
This result, pointed out Alan Sher, who heads NIAID'simmunology and cell biology section, demonstrated IL-12'sability to stimulate the T cell-independent NK pathway, tosynthesize IFN-y. Gamma interferon is a key player in theresponse of NK cells to T gondii infection.
Gazzinelli acquired the IL-12, a novel cytokine first identifiedat the Wistar Institute only four or five years ago, frommolecular biologist Stanley Wolf of Genetics Institute Inc. ofCambridge, Mass. Wolf, a co-author of the PNAS paper, toldBioWorld that he has "manufactured a significant amount of IL-12" at GI and distributes it to researchers worldwide, as well asantibodies to the cytokine, cDNA sequences andoligonucleotides for polymerase chain reaction (PCR) use.
In his collaboration with NIAID, Wolf added, "What was veryimportant and interesting to Gazzinelli and his people was thatIL-12 is a monocyte product." He helped them establish thatthe monocytes from their SCID mice will actually produce thenovel cytokine in response to toxoplasmosis.
Wolf emphasized that "Hoffmann-La Roche cloned the IL-12cDNA and supplied it to GI in a co-development arrangement."He added, "We generated the cell lines, which is a non-trivialfeat, and purified the injectable material." Other biotechnologycompanies also contributed to the NIAID's IL-12 R&D.Genentech Inc. supplied human IL-2; Cetus Corp. (now mergedwith Chiron Corp.) the interferon-y; Genzyme Corp therecombinant TNF-a.
Where does interleukin 12 go from here? "Everybody isthinking ahead to human trials," Gazzinelli told BioWorld."Stanley Wolf is doing some of the first experiments inprimates."
Confirming this, Wolf said that he and his co-workers "havesome primate studies ongoing, and we are considering others."But he cautioned, "We're still early on in development of IL-12."
Gazzinelli observed that "before going to humans, we have toexclude the possibility that the IL-12 molecule may be toxic."Once this is done, he sees its use in combination with otherdrugs, initially chemotherapeutics, to treat AIDS patients withopportunistic infections.
Further down the pike, IL-12 could be teamed up with theantibody to another cytokine, interleukin-10, which down-regulates the activation of macrophages and production ofgamma interferon.Humans get toxoplasmosis from the feces of a small mammal,Felis domesticus, better known as the common cat. There are atleast 54 million household cats in the U.S.; most of themprobably harbor the T. gondii parasite.
Three years ago, Paravax, Inc., now of Fort Collins, Colo.,announced plans to develop a cat vaccine against T. gondiijointly with Bayer AG's Mobay Corp. Animal Health Division inKansas City, Kan. (see BioWorld, Oct. 17, 1990). Its time linethen called for filing an application with the U.S. Department ofAgriculture in 1993.
Such a vaccine would protect not only pets but also people --particularly pregnant women -- who risk passing the infectiouscysts to their unborn offspring with danger of miscarriage orfetal birth defects.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.