The antibiotic vitamin: deficiency in vitamin D may predispose people to infection.
"First, the ward below mine was quarantined, then the wards on my right, left, and across the hall," Cannell recalls. However, although the 32 men on his ward at Atascadero (Calif.) State Hospital had mingled with patients from infected wards before their quarantine, none developed the illness.
Cannell's ward was the only heavily exposed ward left unaffected. Was it by mere chance, Cannell wondered, that his patients dodged the sickness?
A few months later, Cannell ran across a possible answer in the scientific literature. In the July 2005 FASEB Journal, Adrian F. Gombart of the University of California, Los Angeles (UCLA) and his colleagues reported that vitamin D boosts production in white blood cells of one of the antimicrobial compounds that defends the body against germs.
Immediately, Cannell says, the proverbial lightbulb went on in his head: Maybe the high doses of vitamin D that he had been prescribing to virtually all the men on his ward had boosted their natural arsenal of the antimicrobial, called cathelicidin, and protected them from flu. Cannell had been administering the vitamin D because his patients, like many other people in the industrial world, had shown a deficiency:
The FASEB Journal article also triggered Cannell's recollection that children with rickets, a hallmark of vitamin D deficiency, tend to experience more infections than do kids without the bone disease. He shared his flu data with some well-known vitamin D researchers, and they urged him to investigate further.
On the basis of more than 100 articles that he collected, Cannell and seven other researchers now propose that vitamin D deficiency may underlie a vulnerability to infections by the microbes that cathelicidin targets. These include bacteria, viruses, and fungi, the group notes in a report available online for an upcoming Epidemiology and Infection.
This is only a hypothesis, "but a very credible one" that deserves testing, says immunologist Michael Zasloff of Georgetown University in Washington, D.C.
Behind the hypothesis are recent studies that link vitamin D intake to revved-up cathelicidin production. These investigations point to an infection-fighting role for vitamin D, which is produced in skin exposed to sunlight but is present in few foods.
A study published earlier this year that investigated the relation ship between vitamin D and susceptibility to tuberculosis also bolsters the idea proposed by Cannell's team. Scientists have already planned a handful of clinical trials to evaluate the antimicrobial benefits of vitamin D supplementation.
Zasloff argues that if studies support the hypothesis, "we can imagine one day treating infections not by giving somebody a drug, but by giving them safe and simple substances--like a vitamin."
INNATE PROTECTION Legions of germs come into contact with our bodies every day. Each microbe seeks a host in which it can multiply. Most would-be invaders, however, don't succeed; if not barred entry outright, they're destroyed by cellular recruits called up to participate in local immune militias.
Scientists hadn't been sure what serves as the call to arms for these immune cells and what triggers the production of their antibiotic arsenal, which includes several chemical weapons.
Over the past 5 years, a spate of studies began to shed light on the rollout of one of those munitions--cathelicidin. Dermatologist and immunologist Richard L. Gallo of the University of California, San Diego, a coauthor of many of these studies, explains that cathelicidin "targets the bad guys." It kills invaders by punching holes in the external membrane of a microbe, permitting its innards to leak out.
Molecular geneticist John H. White of McGill University in Montreal and his colleagues were the first to observe that cathelicidin production is ramped up by vitamin D--or, more specifically, by the hormone 1,25-D, the vitamin's active form (SN: 10/9/04, p. 232). Through a cascade of events, vitamin D transforms into a compound, called a prehormone, that circulates in blood and then is converted locally, as needed, into 1,25-D.
In the nucleus of cells, 1,25-D binds to short sequences of DNA. Known as response elements, these sequences switch on the activity of adjacent genes. "We wanted to find out what genes were next to the vitamin D response elements," White recalls.
Two of these response elements proved to be neighbors of genes that make antimicrobial peptides, cathelicidin and beta-defensin 2, the researchers reported in 2004. When the researchers administered 1,25-D to a variety of cells, production of beta-defensin 2 increased "modestly," White told Science News. In contrast, he says, the gene for making cathelicidin "went boom! Its induction was very, very strong."
Almost a year later, while hunting for triggers for cathelicidin production, Gombart confirmed the McGill finding. His group had been administering various natural signaling agents to white blood cells, which the immune system sends out to vanquish germs.
In these cells, "nothing turned on the cathelicidin gene to any degree except vitamin D. And it really turned that gene on--just cranked it up," Gombart says. "I was completely surprised."
Independently, dermatologist Mona Stahle of the Karolinska Institute in Stockholm reached a similar conclusion when she realized that both vitamin D and several antimicrobials, including cathelicidin, are produced in the skin. She says, "It just came to me--an intuitive thought--that maybe the sun, through vitamin D production, might help regulate the skin's antimicrobial response."
So, her team administered an ointment containing a drug mimic of 1,25-D to the skin of four healthy people. The salve hit "the jackpot, right away," Stahle says. In the May 2005 Journal of Investigatire Dermatology, her team reported that where the ointment had been applied, cathelicidin-gene activity skyrocketed as much as l00-fold. The team also found evidence of a localized increase in the concentration of cathelicidin.
TACKLING TB AND MORE Those studies, though suggestive, didn't reveal whether vitamin D directly reduced infection risk in people. Together with Gallo, microbial immunologist Robert Modlin of UCLA and his colleagues moved closer to that goal: They evaluated the vitamin's role in fending off the tuberculosis (TB) germ Mycobacterium tuberculosis.
This group, working independently of Gombart's team, had been focusing on macrophages, a type of white blood cell deployed by the immune system to gobble up and destroy germs. These defense cells have features, called toll-like receptors, that identify biochemical patterns characteristic of invading microbes. If the receptors sense an invader, they can trigger cathelicidin production.
Modlin's team showed that before making that antibiotic, those cells briefly boosted their production of vitamin D receptors and of an enzyme that converts the vitamin D prehormone into 1,25-D. However, the data suggested that significant concentrations of 1,25-D would develop only in the presence of the TB bacteria. This indicated that the microbe, and perhaps other germs, must be present for the enzyme to maximize its production of 1,25-D, Modlin says.
His group then tested whether people's blood concentrations of the prehormone are high enough to drive the production of germ-killing concentrations of cathelicidin. Black people, because of the sun-filtering effect of dark pigments in their skin, are far more likely than whites to be vitamin D deficient (SN: 10/16/04, p. 248). Furthermore, blacks tend to be more susceptible to TB than whites and to develop a more severe illness when infected.
The team collected blood serum from white people and from blacks. When the researchers added TB bacteria, macrophages in the serum from black participants produced 63 percent less cathelicidin--and were less likely to kill the germs--than were macrophages incubated in serum from whites.
The scientists then added vitamin D to the serum from blacks until concentrations of the prehormone matched those in the serum from whites. This boosted the macrophages' cathelicidin production and rates of TB-microbe killing to those seen when such cells were incubated in serum from whites. Modlin's group reported its findings in the March 24 Science.
The new data may explain the difference between blacks and whites in TB susceptibility. Modlin says, "We showed that serum from African American individuals did not support the production of the antibiotic by immune cells, until the serum received supplemental vitamin D."
"We're now planning to do a clinical trial and treat African Americans who are deficient with vitamin D to correct their serum levels [of the prehormone] and see if this will change their antimicrobial response," Modlin says.
Gallo is also planning a new trial. His group will compare the effectiveness of supplemental vitamin D in elevating cathelicidin concentrations when administered as oral supplements or as a skin treatment.
The team expects to see the biggest benefit in skin wounds. However, Gallo predicts that even healthy skin will exhibit somewhat elevated antimicrobial concentrations, signaling an improved resistance to infection.
Sun exposure--in moderation--might also prove therapeutic, Stahle's team suggested in the November 2005 Journal of Investigative Dermatology. The scientists showed that in eight fair-skinned people, a single dose of ultraviolet-B radiation--just enough to evoke some skin reddening the next day--activated the vitamin D receptor and the cathelicidin gene in the exposed skin.
Stahle is now beginning a trial of people with skin infections. A drug analog of 1,25-D will be applied to see whether it speeds wound healing.
FLU TOO? Many other findings also suggested to Cannell's team that flu vulnerability might be tempered by adequate vitamin D intake. The researchers have marshaled data, gleaned from 120 or so reports over the past 70 years, suggesting a link between vitamin D and resistance to infections.
For instance, the researchers point to studies showing that in winter, colds, flu, and other respiratory diseases are more common and more likely to be deadly than they are in summer. During winter, ultraviolet-light exposure tends to be low because people spend more time indoors and the atmosphere filters out more of the sun's rays, especially at mid and high latitudes.
Cannell's group cites a 1997 study showing that the rate of pneumonia in Ethiopian children with rickets, and therefore a likely vitamin D deficiency, was 13 times as high as in children without that disease. The researchers also point to five studies since the 1930s that have linked reduced risks of infectious disease to dietary supplementation with cod liver oil, a rich source of vitamin D.
Although the arguments in the paper by Cannell's group "are provocative," White says, "I find them believable."
So does Gallo. "There are many microbes out there that rarely-to-never cause disease in immunocompetent individuals. It's not because the microbes don't choose to infect us," he notes. "It's because the body's immune defense against the microbes is sufficient to control their proliferation."
It's possible, he says, that a shortfall in vitamin D might seriously compromise that defense.
Gombart's group is developing rodents in which vitamin D modulates cathelicidin.
Until such lab animals are available, vitamin D's impact--even on flu risk--"should be explored in clinical trials," Zasloff says, because the treatment poses little risk to people.
Moreover, he argues, the payoff from any positive finding "would be amazing. Imagine being able to block the spread of epidemic flu with appropriate doses of this vitamin."
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|Date:||Nov 11, 2006|
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