Article from the April 11 1995 New York Times

Article

Microbe DNA Seen as Alien By Immune Cells By NATALIE ANGIER DEMONSTRATING vividly that the immune system is cleverer than the peo- ple who study it, researchers have discovered an entirely new facet of the body's genius, for detecting microbial intrud- ers. The discovery, once it is confirmed and better under- Bill Radl stood, could have broad impli- Dr. Arthur Krieg cations for the development of vaccines and stimulants for the immune system, as well as for treatments against autoimmune disorders and oth- er medical therapies. Reporting in the current issue of the journal Nature, Dr. Arthur M. Krieg of the University of Iowa College of Medicine in Iowa City and his colleagues have learned that the immune system recognizes bacteria and viruses not only by telltale features of their protein coats, as scientists have long known, but by simple yet distinct patterns in the microbes' DNA. The patterns are very different from the genetic codes found in the cells of humans and other mammals, and thus offer an easy opportunity for the immune system to know that there are alien life forms lurking about and that it had better get busy. "This is a really exciting discovery," said Dr. Jeffrey T. Holt of Vanderbilt University Medical School in Nashville. "It's one of those papers that changes the way you look at nature. If Krieg is right, and this a major part of the body's immune response, then he has found a completely unexplored area" in the broad and crowded field of immunology. Dr. Holt has written a commentary about the new report that will appear next month in the journal Nature Medicine. During a telephone interview he returned repeatedly to the issue of how significant the new work was, how surprising and how refreshing. "It's easy in science to focus so much Continued on Page C6 Continued Front Page Cl on individual problems that you get stuck in a rut," he said. "Reading a paper like this, which describes something completely new, reminds you of why you went into science in the first place." Other immunologists were more reserved in their judgment of the significance of the work. They said the new experiments were done in test tubes and laboratory mice, and the scientists wondered whether the findings reflect what happens during a genuine infection of a patient, whose immune system is significantly different from that of a rodent. "This is very interesting and very novel, but I question whether it happens normally in a human immune response," said Dr. Subbarao Bondada, an immunologist at the University of Kentucky School of Medicine in Lexington. He and others also point out that the mechanism through which the immune system might recognize microbial DNA is completely unknown. "But this is just a first paper," said Dr. Bondada. "I'm sure they'll be doing more work to try to answer these many problems." In the past, immunologists have focused on understanding how the immune system recognizes external traits of a microbe, like the rather stiff cell wall that surrounds a bacterium. They have also explored how the immune cells called macrophages pulverize a microbe and then show bits of the foreigners' protein carcass to other immune warriors, the louder to sound the alarm of an invasion and get additional troops sent in. The new work suggests that the immune system not only uses protein pieces, or antigens, from the microorganisms to galvanize itself into action, but also seizes on informative genetic material as evidence of trouble. In the latest experiments, Dr. Krieg and his co-workers determined first that bacterial DNA, when applied to a dish of important immune cells called B cells, dramatcally steps up their growth. By contrast, feeding the cells a sample of mammalian DNA has no such stimulatory impact. Going further, to see what it is about the bacterial DNA that sets off the growth spurt, the researchers have identified a particular sequence of genetic building blocks responsible for the arousal. The DNA of all living organisms is built of four subunits, the nucleic acids cytosine, adenine, guanine and thymine - or C, A, G and T repeated in varying patterns millions of times depending on the species. Scientists have known for some time that the sequence CG, a cytosine followed by a guanine, is much rarer in mammalian DNA than would be expected by random distribution alone, although the reason for that scarcity was not known. Moreover, those CG pairs that do crop up in mammalian DNA are chemically modified, given little crowns of molecules in a process called methylation. By contrast, the CG twosome commonly crops up in bacterial and viral DNA, and when it is found, it is not methylated. As it turns out, this difference is what B cells seize upon to know that they are confronting microbial DNA, rather than genes from their fellow body cells. The uncrowned CG motif, in other words, appears to be yet another way that the immune system discriminates self from nonself, and thereby attacks interlopers while leaving its own tissue alone. The behavior of the B cells toward the motif is extraordinary, said Dr. Krieg. Within half an hour of detecting the CG pattern, about 95 percent of the cells in a dish start to divide. As the B cells proliferate, they release a glut of all-purpose antibody proteins able to start neutralizing microbes. They also start secreting chemical signals designed to alert other immune soldiers like T cells. The phenomenon is no mere lab dish curiosity. When the scientists injected mice with synthetic gene sequences composed of linked C's and G's, they responded within 24 hours with a threefold to sixfold increase in B-cell production. Given the early and vigorous response of B cells to the sight of the CG pattern, Dr. Krieg suspects that the identification of microbial DNA is among the first steps in the body's elaborate defense against pathogens. "We believe on the basis of our data that the immune system has evolved a way to recognize foreign DNA, and we suspect it's one of the defense mechanisms acting early in an infection," said Dr. Krieg. As infection progresses, he said, the body becomes more refined in its methods, targeting the microbe with antibodies able to recognize pieces of that particular infiltrator. However, it remains entirely unknown how the microbial DNA is exposed to the immune system in the first place, or how the B cells sense the nucleic acids at all. Immunologists know that immune cells recognize foreign proteins by using fingerlike receptors on their surface, but no such receptors for DNA have been found. With the clarity of 20/20 hindsight, biologists admit that it makes profound sense for the immune system to use hallmarks of bacterial DNA to recognize an enemy; but they also confess they never thought of such an obvious idea before. "It's an intelligent thing for the immune system to have capitalized on," said Dr. Philippa Marrack, a Howard Hughes investigator at the National Jewish Center for Immunology and Respiratory Medicine in, Denver. "I've been thinking about these problems for years and years, and I must admit the idea never occurred to me. "I suppose we're not as smart as think we are, or at least I wasn't," she added. "I'd guess these guys weren't either, since all the best things are discovered by accident." Her assumption is correct. "It was a total accident," said Dr. Krieg. The project began because he had been working for years, in the field of antisense DNA, attempting to turn off unwanted genes like those involved in cancer by giving the body artificial copies of the opposite versions of the genes. He found that certain of the synthetic antisense sequences had an inflammatory effect in lab animals, indicating an immune reaction against these parts of the genes. Systematically analyzing which gene sequences might be causing the trouble, he and his co-workers ended up implicating the CG pattern. That led them to the discovery of the essential differences between mam. malian and microbial DNA, and to the realization that B cells respond to such discrepancies. The work has vast clinical potential, said Dr. Holt. There is some evidence, for example, that patients with the degenerative autoimmune disease systemic lupus erythematosus, in which the immune system attacks the body's own tissues, may have a defect in their ability to methylate their DNA, to put the little A discovery could have implications for the development of vaccines. protective chemical crowns on the appropriate spots of the chrortiosomes. As a result, the CG's within their DNA may look like bacterial DNA to the immune system, prompting a huge, misguided attack. If some way to increase methvlation could be found, the therapy could offer hope for an otherwise incurable illness. In other cases, synthesized pairs of CG's might be used as immune stimulants to fire up flagging B-cell production, or the pairs might be given along with vaccines or other drugs to augment their effectiveness. Because nucleic acids are natural parts of the body, in theory they would be much safer than drugs now used to enhance B cell output. At the moment, though, the researchers are struggling to understand the hows and wberefores of what they have stumbled upon. Like explorers,who have landed on the shore of an uncharted island, the first thing they must do is start sketching out a map. Science Times E-Mail Science Times has an elecironic mail address for comments from readers about published articles and suggested areas of coverage: scitimes@nytimes.com. .