By David N. Leff
Bites from sand flies infected with the Leishmania parasite afflict their human targets with severely disfiguring, disabling and sometimes-fatal disease.
But guess what: Bites from uninfected sand flies successfully vaccinated mice against leishman iasis. It was as if that graceful but deadly tropical insect, Phlebotomus papatasi, when it didn't happen to be carrying the Leishmania pathogen, immunized its victims against infection by sand flies that did deliver the parasite with its bite.
Sand flies and leishmania are allied to produce leishmaniasis, the way Anopheles mosquitoes and Plasmodium parasites team up to cause malaria, or deer ticks (Ixodes dammini) go into business with the Borrelia burgdorferi spirochete to sow Lyme disease in people.
With their long, graceful pair of oval wings and lengthy, jointed spidery legs, centimeter-size sand flies resemble Anopheles mosquitoes, poised for a meal of blood. They infest much of Latin America, Africa, south Asia and the Mediterranean basin - as does leishmaniasis. There's an epidemiological symbiosis between P. patatasi and Homo sapiens.
"But where do the flies get their infection in the first place?" asked immunologist David Sacks rhetorically. His answer: "They acquire it from people who themselves have been infected before. These flies need a source of blood for egg development. That's why it's only females that bite. They bite not only humans by also other mammals as a source of their blood meal."
Sacks heads the Laboratory of Intracellular Parasite Biology at the National Institute of Allergy and Infectious Diseases in Bethesda, Md.
"We all know that we develop inflammatory reactions in our skin when we get bitten by mosquitoes," he observed. "Sand flies elicit an even more powerful dermal reaction. Even when a leishmania-infected sand fly bites you, that infected insect is depositing in your skin not only one or more parasites, but also its own saliva, which elicits this inflammatory response. And in that setting, it turns out that it's very harmful to the parasite. So that animals, and presumably people," he pointed out, "that have been bitten by flies in the past will be more resistant to disease than people who have never been bitten by sand flies."
Uninfected Flies Confer Resistance To Parasites
Sacks is senior author of an article in the current issue of Science, dated Nov. 17, 2000. Its title: "Protection against cutaneous leishmaniasis resulting from bites of uninfected sand flies."
"These were studies in a mouse model of the disease," he told BioWorld Today, "in which we used infected sand flies to transmit infection, as opposed to the way it has always been done before, which is needle inoculation of parasites. Our main finding was that mice exposed to bites from uninfected flies, prior to receiving the bites of infected ones, developed resistance to leishmaniasis.
"We were trying to reproduce what we thought would be situations in endemic regions," he continued, "where many, if not most, people will be bitten by sand flies many times, and most of those insects won't carry parasites. So the hosts become sensitized to the saliva that they're exposed to when flies bite to obtain blood. And the tissue response to that saliva alone can be quite severe in people.
"Later, when these same individuals get bitten by an infected sand fly," Sacks went on, "there is an inflammatory reaction to the saliva previously deposited in the skin. In that inflammatory tissue environment, those parasites are killed.
"Their hosts are being immunized," he explained, "not to the parasites but to the fly's own saliva. It's a bystander effect. You have to remember," he pointed out, "that only a very few parasites are deposited in the skin to initiate infections in that tissue environment. It becomes very activated because the host is now making an immune response - an inflammatory reaction - to saliva. So within that site of the bite immune-defense cells take up the parasite and kill it."
As they reported in Science, he and his co-authors have demonstrated in mice the protective, parasite-free effects of sand fly saliva. "We exposed some of our mice," Sacks recounted, "to the bites of uninfected flies - trying to mimic the kind of exposures in these tropical field settings, where most flies don't harbor parasites, and people will be bitten many times by uninfected flies. And over time they develop inflammatory immune reactions to the saliva of those insects.
"So we exposed these mice to the bites of about 10 flies - collected from field specimens in Saudi Arabia - and two weeks later did it again. Two weeks after that we exposed the mice to the bites of infected sand flies, which are now capable of transmitting cutaneous leishmaniasis.
"Then we compared the severity of that disease in those mice that had never been bitten by sand flies before with those whose first exposure was to infected flies. We found the ones that had been exposed to the bites of the uninfected flies were strongly resistant to the development of cutaneous leishmaniasis."
If Phlebotomus papatasi can administer its own protective vaccine, Sacks suggests, this salivary strategy merits a shot at human vaccination.
Prepping For Vaccine Salivation
"Conventional vaccine development," he observed, "incorporates the antigens of the parasite. Those are the main efforts, and they should be. We're suggesting that this saliva aspect is a very different approach to vaccination, which may be incorporated in a conventional vaccine.
"There have been human trials of leishmaniasis vaccines employing a whole-cell killed antigen," Sacks went on, "in the Middle East and Venezuela. Those trials are still ongoing, but their initial results don't look all that promising. A more recent effort," he added, "with DNA-based vaccine, is being used in many other lab-based experiments as well, and they do look quite promising.
"What antigenic element the fly harbors in its saliva is not yet known," Sacks pointed out. "We aim to determine that when the various molecules of the saliva are cloned by genetic techniques. That's well under way, by co-workers in this department with whom we're working. So already we can use recombinant molecules to elicit these reactions.
"Vaccine testing is going on here in mice," Sacks concluded, "after which we'll go on to monkeys and eventually humans."