Light a fire to blight a fire.
That's the well-known strategy of setting a blaze in the path of an advancing forest fire to halt it by consuming the biomass it feeds on. Now this backfire ploy shows signs of working to starve HIV before it can replicate.
Immunologist and pathologist David Kaplan, of Case Western Reserve University, in Cleveland, describes his basic concept this way:
"If you're thinking in terms of the virus, the virion needs the cells it infects to remain alive. It can't replicate without living, functional, happy cells. So viruses come into cells, and shut down their ability to kill themselves.
"From the immune perspective," he continued, "the cell says: 'I've got a virus. The best thing for my organism is if I kill myself, so I'm not being used as a viral factory.'
"And that is the insight," Kaplan told BioWorld Today, "that I think we are bringing to this picture."
He paints that picture full-length in the current Proceedings of the National Academy of Sciences (PNAS), dated May 27, 1997. The article, of which he is senior author, bears the title: "Fas ligand deficiency in HIV disease."
Fas (a.k.a. CD95) has been called Doctor Death of the human immune system. (See BioWorld Today, July 9, 1996, p. 1.) On the surface of T cells, a Fas molecule binds to its receptor, the Fas ligand, and transmits its apoptotic message into the cell: Drop dead.
Normally, Fas ligand performs its cell-executioner's task to impose suicide on T cells rendered redundant after the immune system has finished dealing with an intruding antigen. Otherwise, the now-idle army of lymphocytes might turn against its host organism and trigger auto-immune disease.
T lymphocytes aren't the only immune-system cells targeted by HIV virions. They also infect monocyte macrophages, which, among other chores, eat up and spit out cellular debris. These cells, which circulate in the bloodstream, also carry Fas ligands on their surface, poised to commit the cell to suicide on demand.
Paradox: HIV Saves Immune Cells From Death
One might suppose -- as many researchers in the field do -- that if HIV's primary mission is to destroy its host's immune system, then it should encourage immune-system cells to embrace programmed cell death, courtesy of their Fas ligands.
But Kaplan and his co-authors report the contrary in their PNAS paper. "On mononuclear cells from the blood of HIV patients," he said, "we found the surface expression of Fas ligand to be inhibited." The team went on to demonstrate "that Fas ligand activity is very important in limiting viral replication."
The unknown mechanism that accounts for that inhibition of Fas ligand surface expression on monocytes "is obviously very key," Kaplan observed. "And that's what we're after right now."
He recalled that back in 1991, "there was a published hypothesis that in HIV disease there is an 'inappropriate induction of apoptosis of T cells.' This was before Fas ligand was discovered."
Then in 1995, he said, "some authors said that the mechanism whereby T cells kill themselves, normally, physiologically, is through the Fas-Fas ligand interaction."
Whereupon, Kaplan and his co-authors developed a cytotoxicity assay to look at Fas ligand activity in sickness and in health. "We found that there's activity in normal blood cells," he said. "Then we looked at HIV-infected patients, expecting that, very possibly, activity would be elevated. Instead, we found an incredibly marked decrease in those Fas ligand levels.
"About half of the patients had no activity. Zero. That's a 100 percent drop. The other half had low levels, in the range of 50 percent to 90 percent inhibition."
Finding this deficiency, which rebutted received wisdom, led his team to try restoring the lost apoptosis potential. "We reconstituted the deficiency in vitro," he reported, "which had the effect of markedly decreasing viral levels."
When they went on to add a functional anti-Fas antibody to the blood cells of HIV-positive individuals, viral production plummeted by more than 90 percent.
Therapy Possible But Problematic
It's a long way from such cellular reconstitution to human therapy, Kaplan observed, the more so because primate models of HIV disease are so inadequate and expensive.
"A whole human body," he observed, "is a whole lot different than what we got in a culture dish. There are a thousand reasons why you could not successfully reconstitute this kind of activity in a person. But having said that, I think there's a reasonable possibility, though I know there are major caveats out there."
Among the most major is that Fas ligand is known to be toxic, especially to the liver. "Can we get around that?" Kaplan asked rhetorically. "My answer would be: Our objective here is not to wipe out the liver with an overdose of Fas ligand. It's to reconstitute a normal level, and that doesn't mean giving a lot, necessarily."
At this point, his team has made a soluble recombinant human Fas ligand as a putative therapeutic, and is working toward toxicity testing. "In fact," he said, "we have given a molecule with Fas ligand activity to two mice, and did not kill them. Rather, we got a therapeutic effect. But it's not an HIV model.
"Other people," he observed, "have demonstrated that you can kill a mouse with Fas ligand. We're going to try to demonstrate how one might be able to keep a mouse alive."
One way to sidestep liver toxicity would be to administer the Fas ligand molecule by inhalation. "We have given fair amounts of the molecule intranasally in a different model, not HIV, without any adverse effects on the animals, in fact with therapeutic effects. There is a way to give it," he added. "You don't have to kill the beast."
Kaplan pointed out that "this would be a therapy based on diminishing viral titers, not on inhibiting a specific HIV protein. In that sense, it's not likely to engender viral resistance, like a protease inhibitor would. That's where a lot of the interest is for me."
He concluded: "This is a possibility, and I'd like to pursue that possibility. That's what we're working toward." *