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Applied Biotechnology, Clinical Trials, FDA, Public Health, Research, Stanford News

The best toxicology lab: a mouse with a human liver

The best toxicology lab: a mouse with a human liver

of mice and menA few years ago, Stanford pharmacogenomic expert Gary Peltz, MD, PhD, collaborating with researchers in Japan, developed a line of bioengineered mice whose livers were largely replaced with human liver cells that recapitulate the architecture and function of a human liver. Now, in a recent study published in PLoS Medicine, Peltz’s team has shown that routine use of this altered lab mouse in standard toxicology tests preceding clinical trials would save human lives.

Among the liver’s numerous other job responsibilities, one of the most important is chemically modifying drugs in various ways to make them easier for the body to get rid of. But some of those chemical products, or metabolites, can themselves be quite toxic if they reach high levels before they’ve been excreted.

The Food and Drug Administration requires that prior to human testing, a drug’s toxicological potential be assessed in at least two mammalian species. But we humans metabolize things differently from other mammals, because our livers are different. That can make for nasty surprises. All too often, drugs showing tremendous promise in preclinical animal assessments fail in human trials due to unforeseen liver toxicity, said Peltz, a former pharmaceutical executive who is intimately familiar with established preclinical testing procedures in the industry.

That’s what happened in 1993 when, after a short safety trial of a drug called FIAU concluded without incident, the comp0und was placed in a phase-2 clinical trial of a drug for hepatitis B. FIAU belongs to a class of drugs that can interfere with viral replication, so it was considered a great candidate for treating virally induced infections such as hepatitis B.

As I wrote in my release about the new study:

“FIAU was supposed to be a revolutionary drug,” Peltz said. “It looked very promising in preclinical tests. In phase 1, when the drug was administered to subjects for a short period of time, the human subjects seemed to do fairly well.” But the phase-2 trial was stopped after 13 weeks, when it became clear that FIAU was destroying patients’ livers.

In fact, nearly half the patients treated with FIAU in that trial died from complications of liver damage. Yet, before advancing to clinical trials FIAU had been tested for as long as six months in mice, rats, dogs and monkeys without any trace of toxicity. An investigation conducted by the National Academy of Sciences later determined that the drug had shown no signs of being dangerous during those rigorous preclinical toxicology tests.

In Peltz’s new study, though, FIAU caused unmistakable early signs of  severe liver toxicity in the bioengineered mice with human livers. This observation would have served as a bright red stop signal that would have prevented the drug from being administered to people.

Bonus item: Using bioengineered mice with human livers instead of mice with murine ones would no doubt result in the clinical and commercial success of some drugs that never got to the human-testing stage because they caused liver toxicity in mice.

Previously: Fortune teller: Mice with ‘humanized’ livers predict HCV drug candidate’s behavior in humans, Alchemy: From liposuction fluid to new liver cells and Immunology escapes from the mouse trap
Photo by erjkprunczyk

Aging, Genetics, Men's Health, Neuroscience, Research, Stanford News, Women's Health

Having a copy of ApoE4 gene variant doubles Alzheimer’s risk for women but not for men

Having a copy of ApoE4 gene variant doubles Alzheimer's risk for women but not for men

brain cactus - smallSince the early 1990s, when Duke University neurologist Allen Roses, MD, first broke the news, it’s been known that a person carrying the gene variant known as ApoE4 is at elevated risk of getting Alzheimer’s disease. To this day ApoE4 is the strongest known single genetic risk factor for Alzheimer’s, a progressive neurological syndrome that robs its victims of their memory and reasoning ability.

But only now is it looking certain that the increased Alzheimer’s risk ApoE4 confers is largely restricted to women. Men’s fates don’t seem to be altered nearly as much by the genetic bad penny that is ApoE4, according to a new Annals of Neurology study led by Mike Greicius, MD, medical director of the Stanford Center for Memory Disorders.

Accessing two huge publicly available national databases, Greicius and his colleagues were able to amass medical records for some 8,000 people and show that initially healthy ApoE4-positive women were twice as likely to contract Alzheimer’s as their ApoE4-negative counterparts, while ApoE4-positive men’s risk for the syndrome was barely higher than that for ApoE-negative men.

What the heck is ApoE4 for, anyway? In my release on the new study, I wrote:

The ApoE gene is a recipe for a protein important for shuttling fatty substances throughout the body. This is particularly important in the central nervous system, as brain function depends on rapid rearrangement of such fatty substances along and among nerve cell membranes. The ApoE gene comes in three varieties — ApoE2, ApoE3 and ApoE4 — depending on inherited variations in the gene’s sequence. As a result, the protein that the gene specifies also comes in three versions, whose structures and fatty-substance-shuttling performance differ. Most people carry two copies of the ApoE3 gene variant (one from each parent). But about one in five people carries at least one copy of ApoE4, and a small percentage have two ApoE4 copies. Numerous studies … have confirmed that ApoE4 is a key risk factor for Alzheimer’s disease, with a single copy of ApoE4 increasing that risk twofold or fourfold. Carrying two copies confers 10 times the risk of Alzheimer’s.

Early hints in the medical literature that the ApoE4 variant exerted differential effects on women’s versus men’s brains were largely ignored until now, says Greicius. He says that’s because most of the seminal ApoE4/Alzheimer’s genetics research was conducted as case-control studies: The ApoE4 gene version’s frequency in people with Alzheimer’s was compared to its frequency in people without the disease. (About half of those with Alzheimer’s, but only about 15 percent without it, are positive for ApoE4.)

But that method has limitations, says Greicius: “About 10-15 percent of ‘normal’ 70-year-olds will develop Alzheimer’s if you wait five or ten years.” Their lurking in the “normal” group dilutes the results. Moreover, Greicius says,“these kinds of genetic studies are looking for needles in a haystack, so they require large numbers of subjects – thousands – to achieve statistical significance. If you want to further examine male/female differences, you have to double the sample size.” That’s costly.

And that’s how come the large government- and industry-supported repositories to which Greicius and his team resorted are such a great idea.

Previously: Estradiol – but not Premarin – prevents neurodegeneration in women at heightened dementia risk, Common genetic Alzheimer’s risk factor disrupts healthy older women’s brain function, but not men’s, Hormone therapy halts accelerated biological aging seen in women with Alzheimer’s genetic risk factor and A one-minute mind-reading machine? Brain-scan results distinguish mental states
Photo by Sean Michael Ragan

Aging, Genetics, Neuroscience, Research, Sleep, Stanford News

Restless legs syndrome, most common in old age, appears to be programmed in the womb

Restless legs syndrome, most common in old age, appears to be programmed in the womb

Restless legsWhile the sleep disorder called “restless legs syndrome” is more typical of older than younger people, it looks as though it’s programmed in the womb. And a group led by Stanford neurologist Juliane Winkelmann, MD, has pinpointed for the first time the anatomical region in the brain where the programming takes place.

Restless legs syndrome, or RLS, is just what it sounds like: a pattern of unpleasant sensations in the legs and the urge to move them. It has been described as a feeling similar to the urge to yawn, except that it’s situated in the legs or arms instead of the upper torso and head.

Estimates vary, but something on the order of one in ten Americans has RLS. Women are twice as likely as men, and older people more likely than young people, to have it. This urge to move around comes in the evening or nighttime, and can be relieved only by – wait for it – moving around. Needless to say, that can cause sleep disturbances. In addition, RLS can lead to depression, anxiety and increased cardiovascular risk.

Very little is known about what actually causes RLS, although it’s known to be highly heritable. Although a number of gene variants (tiny glitches in a person’s DNA sequence) associated with the condition have been discovered, each by itself appears to contribute only a smidgeon of the overall effect, and nobody knows how.

Winkelmann has been exploring the genetic underpinnings of RLS at length and in depth. In a just-published paper in Genome Research, she and her colleagues have shown that one gene variant in particular depresses the expression of a protein involved in organ development and maintenance. The DNA abnormality Winkelmann’s team zeroed in on occurs not on the gene’s coding sequence – the part of the gene that contains the recipe for the protein for which the gene is a blueprint – but rather on a regulatory sequence: a part of the gene that regulates how much of that protein (in this case, the one involved in organ development and maintenance) gets made, and when.

The kicker (pardon my pun) is that the regulatory sequence in question seems to be active only during early brain development and only in a portion of brain that is destined to become the basal ganglia, a brain region well known to be involved in movement.

“Minor alterations in the developing forebrain during early embryonic development are probably leading to a predisposition in the [basal ganglion],” Winkelmann says. “Later in life, during aging, and together with environmental factors, these may lead to the manifestation of the disease.”

(Wondering if you’ve got RLS? Check this out.)

Previously: National poll reveals sleep disorders, use of sleeping aids among ethnic groups, Caucasian women most likely to have restless leg syndrome
Photo by Maxwell Hamilton

Global Health, Immunology, Infectious Disease, Microbiology, Research, Stanford News

Discovered: Why so many people with schistosomiasis (there’s a lot of them) are so vulnerable to bacterial co-infection

Discovered: Why so many people with schistosomiasis (there's a lot of them) are so vulnerable to bacterial co-infection

More than a billion people worldwide – almost all of them in developing countries – are infected by worm-like parasitic organisms called helminths. Organisms making up just a single genus of helminth, Schistosoma, account for one-quarter of those infections, which damage different body parts depending on what schistosomal species is doing the infecting. Some go for the lung. Others (card-carrying members of the species Schistosoma haematobium) head for the urinary tract, with one in ten infected patients suffering severe physical consequences.

People with schistosomiasis of the urinary tract are especially vulnerable to bacterial co-infections. Worse, these co-infections exacerbate an already heightened risk of bladder cancer in infected individuals, it’s believed. Unfortunately, considering the massive numbers of cases, surprisingly little is understood about the molecular aspects of the infection’s course.

A big reason for that relative ignorance has been the absence of an effective animal model enabling the detailed study of urinary-tract schistosomiasis. A couple of years ago, Stanford schistosomiasis expert Mike Hsieh, MD, PhD, developed the world’s first decent mouse model for the disease, allowing him to explore the molecular pathology that occurs early in the course of infection. Now, in a just-published study in Infection and Immunity, Hsieh has put that mouse model to work in coaxing out the cause of the curious collegiality of S. haematobium and co-infecting bacteria.

The secret, the scientists learned, is that S. haemotobium infection induces a spike in levels of a circulating immune-system signaling protein, or cytokine, called IL-4. That excess, in turn, results in a drop in the number and potency of a subset of immune cells that are important in fighting off bacterial infections. The discovery opens a pathway toward the development of new, non-antibiotic drug treatments for co-infected patients that won’t wreak havoc with their microbiomes, as antibiotics typically do.

Previously: Is the worm turning? Early stages of schistosomiasis bladder infection charted, Neglected story of schistosomiasis in Ghana, as told in a  sand animation and A good mouse model for a bad worm

Clinical Trials, Global Health, Infectious Disease, Pediatrics, Stanford News

Life-saving dollar-a-dose rotavirus vaccine attains clinical success in advanced India trial

Life-saving dollar-a-dose rotavirus vaccine attains clinical success in advanced India trial

dollar bill 2Nearly every child in the world has been infected with rotavirus at least once by the age of five. But kids in poor countries get the worst of it. Rotavirus mortality is low in the developed world, but in low-income countries it’s a killer, accounting for 85 percent of the estimated 180,000 to 400,000 annual deaths caused by the pathogen.

The disparity exists for at least two reasons.

First, widespread malnutrition results in a different epidemiology. For example, 70 percent of rotavirus hospitalizations in India happen the first year of life, compared with 40 percent in high- and middle-income countries.

Second, price. Vaccination is second only to gaining access to potable water as a low-cost, high-payoff  strategy for ensuring children’s health. But many vaccines are far too pricey for families living on incomes in the neighborhood of $1,500 per year. As a result, most childhood deaths from vaccine-preventable diseases happen in low-income countries. India has the most rotavirus deaths in the world, estimated at about 75,000-122,000 per year (close to a quarter of the worldwide total.)

So it’s great news that a new rotavirus vaccine developed by Indians for Indians has leaped the safety and efficacy thresholds of a late-stage clinical trial, in which more than 6,500 Indian infants were inoculated, and will likely become available in that country for less than a dollar a dose. (The full immunization procedure requires three separate doses.)

The results appear in a study just published in The Lancet and co-authored by a team including veteran rotavirus-vaccine developer Harry Greenberg, MD. An accompanying perspective piece co-written by Greenberg, who also directs the Stanford Center for Clinical and Translational Research and Education, states:

[P]roof of the efficacy of the… vaccine against a disease that affects almost every child in India, leads to millions of clinic visits and hundreds of thousands of hospital admissions, and kills roughly one child in every 175-200 born in India before their fifth birthday is cause for celebration.

The new vaccine was the first to be fully tested for efficacy in a randomized, double-blind, placebo-controlled clinical trial in India. Interestingly, its development began with the discovery, by an Indian pediatrician, that newborns were getting rotavirus infections in the hospital but not getting sick. The strain they were infected with turned out to be an attenuated mutant virus that turns on the body’s immune response without causing symptoms: in short, the ideal vaccine candidate.

Ultimately spearheaded by a young Indian biotechnology company, Bharat Biotech, the effort to capitalize on this promising episode of serendipity drew financial support from the Bill & Melinda Gates Foundation and technical assistance from the Government of India’s Department of Biotechnology, the United States’ Centers for Disease Control and Prevention, and Stanford, among others.  This international team of collaborators then spent more than 15 years turning the promise into a reality.

Previously: Trials, and tribulations, of a rotavirus vaccine
Photo by David Guo

Clinical Trials, Imaging, Neuroscience, Research, Stanford News, Women's Health

Estradiol – but not Premarin – prevents neurodegeneration in women at heightened dementia risk

Estradiol - but not Premarin - prevents neurodegeneration in women at heightened dementia risk

bottle of pillsWomen near the age of menopause and at elevated risk for dementia – owing, say, to a family history of Alzheimer’s disease, a personal history of major depression, or a genotype positive for the infamous Alzheimer’s-predisposing gene variant, ApoE4 – may want to consider talking to their doctor about estrogen-based hormone therapy.

In a brain-imaging study just published in PLOS ONE, hormone therapy protected key “early warning” brain regions from metabolic decline in women who fit that description – but only if they started therapy shortly after reaching menopause, and only if the pill they took contained just estradiol, the dominant female sex-steroid hormone. Premarin, a more widely used hormone-therapy formulation derived from the urine of pregnant mares, was far less protective.

Premarin contains more than 30 substances, with estradiol accounting for only about 17 percent. Other components exert various endocrinological effects on different tissues. In my release on the new study, I wrote:

More than 20 million women in the United States are between 45 and 55 years old – an age range at which many once were considered prime candidates for Premarin. Hormone therapy… was… widely heralded as protecting postmenopausal women from heart disease, osteoporosis and even cognitive decline.

Indeed, from 1992 through 2001 Premarin was the most widely prescribed drug in the United States. Then came the deluge. Here’s the backstory:

In July 2002, a large multicenter study of hormone therapy’s effects was abruptly halted when – contrary to expectations – woman assigned to PremPro (Premarin plus progestin, a synthetic version of progesterone, another important female steroid hormone) developed more cardiovascular disease than those getting a placebo. Within 18 months, about half of American women who’d been on hormone therapy abandoned it. Its use has since plunged considerably further.

Then in 2003, an ancillary study called WHIMS (“Women’s Health Initiative Memory Study”) reported that dementia incidence among 65- to 79-year-old women randomly assigned to PremPro was double  that of women on placebo. This disappointing finding was widely covered in the media.

But Rasgon and her colleagues’ findings are consistent with other analyses indicating that women initiating hormone therapy within five years of their last menstrual cycle experienced beneficial brain effects. In fact, major differences in trial design may explain the discrepancy between WHIMS’s decidedly negative results and the new study’s more nuanced ones.

The WHIMS women were older, on average, than those in Rasgon’s study and were beginning hormone therapy after a long hiatus during which their bodies were no longer producing substantial quantities of estrodiol. Moreover, the PremPro given to women in the active arms of WHIMS contained progestin – which, the new study shows, speeds metabolic deterioration in at least dementia-prone women’s brains.

Natalie Rasgon, MD, PhD, director of the Stanford Center for Neuroscience in Women’s Health and the study’s lead author, puts it plainly. “Hormone therapy’s neurological effect on women at risk for dementia depends critically on when they begin therapy and on whether they use estradiol or Premarin.”

Previously: Hormone therapy halts accelerated biological aging seen in women with Alzheimer’s genetic risk factor, Hormone therapy soon after menopause onset may reduce Alzheimer’s risk and Study shows common genetic risk factor for Alzheimer’s disrupts brain function in healthy older women, but not men
Photo by Canned Muffins

Genetics, Neuroscience, Research, Stanford News

X marks the spot, and so does Y: Brain differences, missing or extra sex chromosomes, and gene dosage

X marks the spot, and so does Y: Brain differences, missing or extra sex chromosomes, and gene dosage

X and Ys - smallHow is a gene like a drug? The more there is of it, the bigger the effect. You have to be careful how you spoon it out. Of course, gene “doses” don’t come in teaspoons, they come in chromosomal copy numbers.

You typically have two copies of each gene – one on the chromosome dad gave you, and one on the chromosome you got from mom – although, it must be said, the “flavors” of these copies many not be identical (e.g., specifying blue versus brown eye color).

And sometimes – in fact, often – one copy of a gene is “turned off” altogether, its activation more or less blocked by biochemical stop-signs. That’s about the same as having only one copy, until and unless the light turns green at some point. A particularly pronounced case of single-dose-itis (my word) occurs on the sex chromosomes, designated either X (for female) or Y (for male) because if you view them under a microscope, that’s sort of what they look like. Unlike the other 22 pairs of paternally and maternally derived chromosomes contained in each human cell, X and Y chromosomes actually look noticeably different from one another even at the gross-inspection stage. “Viewed” closer up with the tools of molecular biology, the two versions of the sex chromosome turn out to have large numbers of lengthy stretches that really are different and indeed may be entirely absent on the Y chromosome. Those differences make every cell in a woman’s body different from every cell in a man’s, as UC-Berkeley biologist Art Arnold, PhD, once pointed out at a particularly lively Stanford symposium on gender differences last year.

Still, X and Y chromosomes share plenty of common regions. So a deviation from the usual double chromosome count, even when the extra or missing chromosome is an X or a Y, can make a big difference in the dosages for plenty of genes. One genetic defect called Klinefelter syndrome, characterized by the presence of a Y and two X chromosomes in each cell, leads to an excessive dose of many genes (three copies instead of two, to be specific). Another genetic defect, Turner syndrome, results in each cell containing only a solitary X chromosome – and only a single copy of numerous genes. Both Turner and Klinefelter syndromes are marked by characteristic cognitive deficits.

Allan Reiss, MD, PhD, director of Stanford’s Center for Interdisciplinary Brain Sciences Research, and his colleagues compared the brains of people with Klinefelter and Turner syndromes with those of individuals with normal sex-chromosome counts. They showed in this imaging study in the Journal of Neuroscience, that anatomical aberrations in particular brain regions among people with extra or absent copies of the sex chromosome closely track the neurological deviations associated with these syndromes – and, importantly, that these aberrations may be caused by the gene-dosage differences resulting from variant sex-chromosome counts.

Previously: Tomayto, tomahto: Separate genes exert control over differential male and female behaviors, Humor as a mate-selection strategy for women? and Brain imaging, and the image-management cells that make it possible
Photo by Naberacka

Applied Biotechnology, Genetics, Research, Science, Stanford News

RNA Rosetta stone? Molecules’ second, structural language predicted from their first, linear one

RNA Rosetta stone? Molecules' second, structural language predicted from their first, linear one

Rosetta stoneThe RNA whisperer is at it again.

In a study just published in Nature, Stanford’s Howard Chang, MD, PhD – an expert in all things RNA – and his colleagues detail how they were able to translate from one language spoken by this versatile biomolecule to another, more obscure but important one.

RNA is best known as the intermediate material in classic protein production. A so-called “messenger RNA” molecule serves as a mobile, short-lived copy of its more durable lookalike, DNA, the stuff genes are made of. Gene-reading machines in a cell’s nucleus produce RNA copies of protein-coding genes. Unlike a gene, which is a sequence of chemical letters situated somewhere on a big, bulky chromosome, a messenger RNA molecule can float out of the nucleus to the cell’s watery cytoplasm where proteins get made, and transmit a gene’s instructions to the protein-making machinery.

But RNA does more than simply specify which proteins are going to get made. A messenger RNA molecule’s 3-dimensional shape, for example, conveys bountiful information telling the cell’s protein-producing proletariat where to bring it, what to do with it when it gets there, and when and and how much protein to make from it.

DNA is famously double-stranded. That’s because, of its four component chemical “letters,” two in particular share a strong mutual attraction, biophysically speaking. Happily, the other two letters have a chemical crush on one another as well. So, when the letters composing one DNA strand are complementary to those on a closely opposed strand (and they virtually always are), the two strands lock in a lasting embrace to form a stable double helix.

RNA molecules are strings of four different chemical letters almost identical to those constituting DNA. But unlike DNA, an RNA molecule typically travels solo, as a single-stranded chain of those four chemical letters. It is thus a rather playful, floppy molecule. Nonetheless, the same alphabetical affinities that produce DNA’s double helix are at work in an RNA molecule, albeit in a more fleeting form: Small sequences of chemical letters along an RNA molecule find themselves attracted to complementary sequences elsewhere on the same molecule, causing it to fold into so-called secondary structures featuring pinched double-stranded sections alternating with bulges and loops, hairpins and hinges.

Chang’s gang has figured out how to predict, based on an RNA molecule’s linear chemical sequence, the way it will fold up into its secondary structure. They were able to do this for  thousands of differently shaped RNA molecules found in one type of human cell – about a thousandfold increase over the number of such structures that had been laboriously determined to date, Chang told me. That has consequences for understanding disease mechanisms and, potentially, for drug discovery as well.

Looks like RNA research is shaping up.

Previously: Night of the living dead gene: Pseudogene wakes up, puts chill on inflammation, New job description for RNA, oldest professional biomolecule and iPhone app shows 2D structures of thousands of RNA molecules
Photo by OliBac

Chronic Disease, Immunology, Research, Stanford News

Another piece of the pulmonary-hypertension puzzle gets plugged into place

Another piece of the pulmonary-hypertension puzzle gets plugged into place

puzzle piecePulmonary hypertension, a dangerous increase in the pressure of blood vessels in the lung, is one nasty disease, as I wrote in a Stanford Medicine article a few years ago. As many as three times as many women – many of them quite young – as men are diagnosed with the spontaneous form of PH (which can also arise from scleroderma or bad pharmaceuticals). While an increasing number of pharmaceutical treatments and advocacy groups have made the diagnosis more palatable, there is still no outright cure. Largely, this is because the molecular mechanisms of pulmonary hypertension have remained mysterious.

But recent work has strongly implicated inflammation in pushing predisposed tissues over the edge into the diseased state. And this week, a Journal of Experimental Medicine study led by PH specialist Marlene Rabinovitch, MD, and her colleagues at Stanford’s PH-focused Vera Moulton Wall Center plunks a potentially pivotal piece of the puzzle into place. Rabinovitch and her associates showed that levels of a pro-inflammatory growth factor usually designated by the acronym GM-CSF (if you really must know, it’s “granulocyte-macrophage colony stimulating factor”) rise substantially when a cell-surface receptor with a heavyweight acronym of its own, BMPR2 (for “bone morphogenic protein receptor”) isn’t functioning properly. That can be due to mutations in the gene that codes for the receptor (as occurs in familial versions of PH), to various environmental causes, or the interaction of the two.

Elevated GM-CSF levels in pulmonary tissue work like a siren to call various hot-tempered inflammatory cells to the vasculature of the lung, resulting in thickened vessel walls and commensurately narrowed blood vessels. Conversely, by finding ways to compensate for BMPR2 under-performance, perhaps researchers will be able to develop therapeutics that keep GM-CSF levels within safe limits, modifying the course of incipient PH or even arresting it.

Several million actual and potential sufferers await that Big If with bated breath.

Previously: Researchers reverse pulmonary hypertension in rats by blocking inflammation-producing pathway, New arterial insights portend potential treatments for life-threatening diseases, and Man receives life-saving transplant thanks to health-care reform and a truck
Photo by ePublicist

Evolution, Immunology, Infectious Disease, Men's Health, Research, Stanford News

In men, a high testosterone count can mean a low immune response

In men, a high testosterone count can mean a low immune response

alpha maleMen have deeper voices and tons more facial and body hair than women. They are (usually) bigger, stronger, and much more likely to risk their lives on a whim. I, for example, have been known to bite a full-sized salami in half with a single snap of my jaws when hungry, angry or threatened. Or just for the hell of it.

But when it comes to immune response, men are wimps. It’s well documented that, for reasons that aren’t clear, men are more susceptible to bacterial, viral, fungal and parasitic infection than women are and that men’s immune systems don’t respond as strongly as women’s to vaccinations against influenza, yellow fever, measles, hepatitis and many other infectious diseases.

A new study just published in the Proceedings of the National Academy of Sciences by immunologist Mark Davis, PhD, who directs Stanford’s Institute for Immunity, Transplantation and Infection, and his colleagues may explain why. The same steroid hormone that makes a man’s beard, bones and muscles grow operates – albeit it in a slightly indirect way – to shrink immune responsiveness. Yep, we’re talking about (sigh…) that much-maligned male molecule, testosterone. In a nutshell, high circulating testosterone levels boost the activity of a clutch of genes that, among other things, dial down the aggressiveness with which our immune systems fight back against invading pathogens.

Now why, we ask ourselves, would evolution be so perverse as to have designed a hormone that on the one hand enhances classic male secondary sexual characteristics such as muscle strength, beard growth (or antler size, as the case may be) and risk-taking propensity – the very hallmarks of the alpha male – but on the other hand wussifies men’s immune systems?

Here’s what I got from talking at length (and, I admit, in an uncharacteristically high-pitched voice) to Davis in preparation for the news release I wrote about the study:

The evolutionary selection pressure for male characteristics ranging from peacocks’ plumage to deer’s antlers to fighter pilots’ heroism is pretty obvious: Females, especially at mating-cycle peaks, prefer males with prodigious testosterone-driven traits. Davis speculates that high testosterone may provide another, less obvious evolutionary advantage… Men are prone to suffer wounds from their competitive encounters, not to mention from their traditional roles in hunting, defending kin and hauling things around, increasing their infection risk. While it’s good to have a decent immune response to pathogens, an overreaction to them — as occurs in highly virulent influenza strains, SARS, dengue and many other diseases — can be more damaging than the pathogen itself. Women, with their robust immune responses, are twice as susceptible as men to death from the systemic inflammatory overdrive called sepsis. So perhaps, Davis suggests, having a somewhat weakened (but not too weak) immune system can prove more lifesaving than life-threatening for a dominant male in the prime of life.

Previously: Best thing since sliced bread? A (potential) new diagnostic for celiac disease, Deja Vu: Adults’ immune systems “remember” microscopic monsters they’ve seen before, Immunology escapes from the mouse trap and Immunology meets infotech
Photo by Craig Sunter *Click-64*

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