By David N. Leff
Why should the NIH's National Institute of Dental Research (NIDR) be treating arthritis?
"Even though rheumatoid arthritis [RA] may seem a bit far afield," explained NIDR immunologist Sharon Wahl, "actually, it's not.
"The pathways of inflammation and tissue destruction," she pointed out, "whether they are in the joint or the mouth, really are very parallel, because in periodontal disease we get destruction of bone and of the surrounding tissues. The same thing occurs in the synovial joints of arthritis, where we have inflammation and destruction of cartilage and bone.
"In both cases," Wahl added, "the preclinical model of RA that we're working with is initiated by bacterial components, and so is periodontal disease in the mouth." Wahl is chief of NIDR's Oral Infection and Immunity branch.
She is senior author of a paper published June 15, 1998, in the twice-monthly Journal of Clinical Investigation. Its title: "Plasmid DNA encoding transforming growth factor-ß1 suppresses chronic disease in a streptococcal cell wall-induced arthritis model."
When researchers inject fragments of S. pyogenes' cell wall into rats, they induce a fair imitation of the swollen, tender joints that mark RA. But not in all rats.
"In the rat model," Wahl recounted, "there are strains susceptible to bacterial cell-wall induction of arthritis, and strains that are not. The particular rats we use in our work are genetically susceptible."
Whether humans also divide along RA-prone and non-prone lines is an open question. "RA is probably a multifactorial disease," Wahl pointed out. "Its cause is not known. Some people say it's genetic; others, bacterial; still others, viral. I think perhaps all of them are right."
As a therapeutic to alleviate RA, Wahl places her bets on a molecule in the immune system called human transforming growth factor-beta (TGF-ß). This near-ubiquitous cytokine switches on inflammatory cells when and where they're called for in the body. Then the same TGF-ß calls off that inflammation after it's done its job.
"Actually," Wahl pointed out, "one of the growth factor's major sources is platelets. So if you have an injury or infection of some sort, and you get platelet aggregation, TGF-ß is released. And interestingly, it can then activate those inflammatory cells to make more TGF-ß.
"Once those inflammatory cells get activated at the immune or inflammation site, TGF-ß will turn them on, and if you have activated cells, it will turn them off. It's quite remarkable."
Wahl found in previous experiments that "if we put TGF-ß directly into the arthritic joints of rodents, we actually made their arthritis worse. If we give it systemically into the bloodstream, it makes their symptoms better." But exposing the whole body to repeated doses of the growth factor risked serious side effects. So she and her co-authors turned to gene therapy as a strategy to get the rats making their own TGF-ß.
Naked DNA Sidesteps Viral Gene Carriers
They planted the growth factor's naked DNA sequence in a circular plasmid from the genome of the Escherichia coli bacterium, as a vehicle for entering cells. To this construct they added a cytomegalovirus promoter to trigger the TGF-ß expression in vivo.
"So we had the combination of all these different pieces," Wahl recalled. "The vector, the CMV promoter and the growth-factor gene. Then, when this construct gets into the muscle cells, it does its thing.
"What we did," Wahl recounted, "was to inject the plasmid directly into the rats' muscle, where it gets produced, and the expressed growth factor moves into the circulation. So it's acting more as if we had injected TGF-ß into the bloodstream, rather than directly into the joint."
A single 300-microgram dose of plasmid naked DNA into muscle tissue produced a striking reduction in the number of arthritis-affected joints and the amount of disabling swelling and deformity. Inflammation subsided, and no cartilage or bone loss occurred. And a single injection kept symptoms at bay for up to three months after treatment.
Control animals got the plasmid minus its naked DNA gene, and had no therapeutic reaction.
"I think that one of the important issues we were able to identify from this experiment," Wahl observed, "is the first evidence that one can administer the growth factor as a naked DNA gene, at a site different from the inflammation, and see a biologically active effect.
"Obviously, one of the reasons we're in this business," Wahl observed, "is ultimately to have an impact on human health." But that "ultimately" will be at the end of other preclinical animal trials.
"We are now starting a collaboration," she revealed, "to try some additional inflammatory-disease models, to be sure our gene-therapy approach is not unique to this particular rat and disease." The researchers will try it next in inflammatory bowel disease — colitis — partnered with gastroenterologist Balfour Sartor at the University of North Carolina, Chapel Hill.
"We plan a similar type of treatment," Wahl said, "with growth factor administered to rats with induced colitis. If we can see that it has a more widespread application, then I think that will give us the incentive we need to move further into other kinds of experimental models, and ultimately into translational — extrapolated — studies.
"Studies," Wahl concluded, "in which we take our basic research and preclinical research and try them out in certain types of human diseases." *