It's long been thought that the body's own immune system, rather than reducing the symptoms, may make things worse. But now researchers at the Stanford University School of Medicine have shown that the immune system really does side with the victim, at least in four kinds of venom that were used in their experiments. Their findings will be published in the July 28 issue of Science.
Venom from three species of poisonous snakes and one species of honeybee were studied by a group led by Stephen Galli, MD, professor and chair of the Department of Pathology. Using mice, they analyzed how mast cells, a vital part of the immune system in mammals, reacted to the various venoms. The net effect of the mast cell response to the four venoms "is to enhance resistance to the toxicity and reduce mortality induced by the venom," said Galli, the paper's senior author.
This helpful mast cell response runs contrary to the conventional wisdom - that the immune system only added to the woes of snakebite victims. This assumption arose because of the way mast cells respond to certain other stimuli.
Mast cells synthesize a wide range of biological mediators - compounds that can promote inflammation and other tissue changes - that are selectively unleashed from the cells in response to various triggers, often intruders such as parasites or bacteria. In people who have been sensitized (i.e., made allergic) by prior exposure to substances such as peanuts or certain pollens, mast cells also respond to those stimuli. When mast cells overreact to allergens, they contribute to the effects associated with allergy attacks, such as a runny nose, sneezing, itching and red eyes. When they severely overreact, they can cause anaphylaxis, which can be fatal.
Given that tendency to overreact when stimulated by allergens, it seemed plausible that introducing venom into the body would trigger a similar response. But Galli and Martin Metz, MD, a postdoctoral scholar in pathology and first author of the study, have shown that when mast cells respond to selected venoms, they unleash proteins that break down some of the venoms' most toxic components.
The study was inspired by a 2004 paper in Nature, by Galli and a team of researchers including Metz, showing that mast cells reduced the mortality rate of mice suffering from bacterial peritonitis, a severe bacterial infection in the abdominal cavity that can also be fatal to humans. They found that mast cells released proteins that broke down a molecule called endothelin-1, one of the major toxins produced by the body during bacterial peritonitis or sepsis (bacterial infection in the blood).
In perusing the scientific literature, Metz noticed that endothelin-1 bore a striking similarity to sarafotoxin 6b, the most toxic component in the venom of the burrowing asp, or Israeli mole viper. Knowing also that mammalian mast cells had been shown to respond to many snake venoms by secreting some potent biologically active mediators, they hypothesized that mast cells might also act to degrade sarafotoxins and reduce the toxicity of the Israeli mole viper venom.
Galli and Metz first did experiments in vitro using isolated sarafotoxin 6b with mast cells from mice. "It worked as we thought it would," said Metz. The mast cells were activated, they released the expected proteins and the proteins degraded the sarafotoxin 6b. Mast cells also enhanced resistance of mice to sarafotoxin 6b when it was injected in vivo.
Next, Galli and Metz did experiments using the whole venom, not just the isolated toxin. Some of the mice they worked with were genetically deficient in mast cells, while others, called wild-type mice, had normal mast cells. "We saw the same results in the wild-type mice that we saw before with just the one component, sarafotoxin 6b," said Metz. The mast cells were activated via a particular receptor they had on the cell surface and released the appropriate proteins, which, Metz said, went on to "degrade and thus eliminate the venom, or at least make it less toxic."
The wild-type mice were able to withstand 10 times the dosage of this venom than the mast cell deficient mice could, further indicating that the mast cells were reducing the impact of the venom. To test whether mast cells could also reduce the toxicity of venom from snakes that didn't contain toxins comparable to sarafotoxin 6b, Galli and Metz tested the venom of the western diamondback rattlesnake and the southern copperhead. Again, the mast cells conferred a distinct protective edge.
Testing the mast cell response even further, they also experimented with the venom from honeybees, with the same positive result. "The mast cells significantly limit not only the toxicity, but also the mortality associated with the venom," said Galli.
But Galli called the battle between predators with venom and their prey "a kind of evolutionary arms race." He and Metz suspect that, given the broad range of venoms that have been developed by snakes and other creatures, mast cells probably won't perform as well against every type of venom.
"We expect that there will be some snake venoms that either are not affected by mast cells at all or perhaps even elicit more pathology due to their ability to activate mast cells. It all depends on the balance between the positive and negative effects of the mediators released by mast cells in response to a particular venom," said Galli.
Galli and Metz are embarking on a systematic survey of animal venoms. As more is learned about the natural defenses the mammalian immune system has against ve nom, it may someday even lead to better antivenins, though it will first have to be shown that human mast cells respond in the same way mouse mast cells do. The group has begun in vitro studies using human mast cells to evaluate this possibility.