The infection connection
Microbial triggers of chronic inflammation
What is the spark that lights the fires of chronic inflammation? Is it defective genes? Too many bacon-cheeseburgers? Toxic chemicals in our air, water and food?
Genetic and environmental factors certainly may contribute to inflammatory conditions, but there is another, albeit controversial explanation for them—persistent, and often silent, infections.
One of the most outspoken proponents of this theory, Paul Ewald, Ph.D., of the University of Louisville, argues that defective genes are an unlikely cause of chronic diseases because, over evolutionary time, they should have disappeared from the human population. Although there are exceptions in which a mutation may be beneficial (such as the mutation for sickle cell anemia which provides resistance to malaria), Ewald thinks that infectious agents are more likely culprits.
“Bad genes and bad environments have often been falsely accused, or, at least, they have taken more than their share of the blame. Viruses and bacteria are the primary offenders,” he writes in his 2002 book Plague Time: The new germ theory of disease.
Perhaps the most convincing evidence for his argument comes from the example of ulcer disease. For years, the medical community believed that stress and spicy food caused most ulcers. However, in the 1980s a team of researchers led by Barry Marshall, M.D., at the University of Western Australia made a radical proposal—that a curiously curvy bacterium called Helicobacter pylori was the primary cause of gastric ulcers.
Marshall proved that this bacterium was the cause of ulcers by guzzling an H. pylori cocktail. He subsequently developed gastritis, an inflammation of the stomach lining and precursor to ulcer disease, which was cured by a course of antibiotics.
Marshall’s revolutionary idea didn’t catch on immediately. “It probably took five or six years for the medical community to grasp the concept that ulcer disease was indeed an infectious disease,” says Richard Peek, M.D., chief of the Division of Gastroenterology, Hepatology and Nutrition at Vanderbilt University Medical Center.
According to the U.S. Centers for Disease Control and Prevention, H. pylori is now considered to be responsible for 80 percent to 90 percent of ulcers and is associated with a two- to six-fold increased risk of gastric cancers. Approximately two-thirds of the world’s population is infected with the bacterium.
Yet most people who are infected never develop ulcers or gastric cancers. Why not? Peek believes the body’s inflammatory response to the infection may be the answer.
“It appears that disease results from the dynamic interactions between a particular virulent strain (of H. pylori) and a susceptible host,” he says. “Most of these host genetic differences we are finding are within inflammatory genes.”
One bacterial virulence factor Peek and colleagues have identified is a group of linked genes called the cag island. Bacteria that express cag genes are able to trigger the production of inflammatory cytokines by gastric epithelial cells.
“Persons who have certain polymorphisms in cytokine genes can produce increased amounts of these molecules in response to the bacterial infection,” Peek says. This causes an enhanced inflammatory response, which is thought to be the direct cause of gastric ulcers.
This combination—of a highly virulent bacterium and a host that overreacts to the infection—might be the answer to this vexing problem.
While identifying the infectious agent responsible for ulcers and gastric cancer was relatively easy (because H. pylori is virtually the only bacterial species that can colonize the normal stomach), the picture gets much cloudier when dealing with other tissues. For example, Peek describes, “in the large bowel, there are approximately 1012 (one trillion) bacteria per gram of tissue, so trying to pinpoint one species that causes a chronic inflammatory disorder that affects this organ, such as Inflammatory Bowel Disease (IBD), is prohibitive.”
Finding the cause of inflammatory neurodegenerative diseases like Alzheimer’s disease, multiple sclerosis (MS) and the myasthenias (which cause muscle weakness) is hampered by the long time course—often 20 years—from the onset of disease to severe disability.
MS is a progressive demyelinating disease with periodic relapses and remissions in which the protective myelin around the nerves is destroyed or damaged, resulting in a range of neurological symptoms.
Although the cause is unknown, MS has been classified as an “autoimmune” disease in which the immune system mistakenly attacks body tissues that bear an auto-antigen it recognizes as foreign. For some diseases, like the myasthenias, an auto-antigen has been identified. But, so far, no convincing auto-antigen has been found for MS.
“The inference that an inflammatory disease is autoimmune is largely by default since we are unable to find an infectious agent,” says Subramanian Sriram, M.D., professor of Neurology, and Microbiology & Immunology at Vanderbilt.
Just because scientists haven’t found a causative organism(s) doesn’t mean it doesn’t exist, however. “We may have overlooked them,” Sriram says. “To use the cliché, ‘absence of evidence is not evidence of absence.’”
Both a virus (Human Herpesvirus 6) and a bacterium (Chlamydia pneumoniae) have been proposed as inciting silent infections that may underlie MS. However, no definitive link has yet been shown for either of these candidates.
Although at least five labs, including Sriram’s, have found C. pneumoniae in the spinal fluid of MS patients, several others have not. While he thinks that the bacterium definitely plays a role in the disease, the nature of MS makes the link difficult to establish.
“We do not think it’s the cause of MS,” Sriram says. “We think it’s a cofactor—a secondary mediator, or possibly one of the polymicrobial infectious agents in the disease process.”
Sriram and his colleagues recently completed a pilot study, which suggested that a six-month course of antibiotics might stabilize the brain lesions and brain atrophy characteristic of MS. The researchers are planning a larger, longer-term study, which will be necessary to prove a pathogenic basis for MS.
“In my studies with MS, the problem in finding a link is that we have very little tissue of patients with MS in early stages. MS is not a fatal disease, and so we obtain postmortem brain tissue from people who’ve had the disease for 40 to 50 years,” Sriram said.
“It is impossible to identify what factors caused these lesions to develop 30 to 40 years ago. What we see is the result of something having happened—‘the aftermath of a battle’.”
To address this problem, the Mid South Chapter of the National MS Society is setting up a donor program to collect brain tissue from MS patients who die of causes unrelated to the disease. This tissue will allow scientists to examine the initial events that cause MS.
As in the H. pylori story, the disease process likely depends on an interaction between the germs and the host’s response to them. In general, Sriram agrees with Ewald’s theory of infectious agents causing chronic diseases but adds, “we can’t blame it all on the pathogen. The host bears some responsibility in the ultimate outcome.”
Premature labor, the leading cause of perinatal morbidity and mortality worldwide, also appears to have an inflammatory origin. According to Roberto Romero, M.D., chief of the Perinatology Research Branch at the National Institute of Child Health and Human Development, “one of every four premature babies are born to women with subclinical infections of the amniotic cavity.”
Romero’s studies have shown that chronic, often silent infections within the amniotic cavity incite inflammation of the membranes and the fetus. The most frequent offenders are the normal bacterial residents of the vagina—Ureaplasma urealyticum, Mycoplasma hominis, and Fusobacterium species.
“For unclear reasons, bacteria from the lower genital tract cross the cervix, get into the uterus, and cross intact fetal membranes to gain access to the amniotic cavity. Microorganisms within the amniotic cavity may infiltrate the fetus when it breathes in infected amniotic fluid, through the ear or even through the skin,” Romero says. This prompts the fetus to mount an exaggerated inflammatory response, known as Fetal Inflammatory Response Syndrome.
Inflammatory cytokines produced by intrauterine tissues and the fetus are believed to mediate the key aspects of normal labor and delivery: uterine contractions, cervical dilation, and rupture of the membranes. However, if the fetus has a systemic inflammatory response due to intra-amniotic infection, this response may trigger premature birth.
Romero and colleagues have found increased levels of several cytokines in the amniotic fluid of women with infection. These cytokines stimulate the synthesis of prostaglandins, which can induce contractions, cervical dilation and rupture of membranes.
“A unique circumstance of the fetus is that it must live in an infected environment. That problem is often solved by initiating labor,” he says. Premature delivery has its own attendant risks, however, including respiratory distress syndrome, heart problems and blindness.
Antibiotic therapy has not been effective in preventing preterm labor, possibly because the infection is not typically found until preterm labor has already begun. Instead, studies in animals suggest that anti-inflammatory therapies—including the use of anti-oxidants and COX-2 inhibitors—may be the best bet for stopping premature labor and improving neonatal outcome.
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