By David N. Leff
Like spring styles, therapeutic molecules tend to come in trends or fashions. Remember interleukin-2? Tumor necrosis factor?
A few years ago, the word was out from a number of immunology labs that yet another cytokine, transforming growth factor-beta (TGF-ß), looked promising for treating multiple sclerosis (MS). Whereupon, the National Institutes of Health mounted a clinical trial, to see what TGF-ß could do for MS patients.
What it did was cause them kidney complications so severe that the study had to be stopped. And this setback sent immunologist Jeanette Thorbecke back to her TGF-ß drawing board. Thorbecke, a pioneer cytokine researcher, is a professor of pathology at New York University School of Medicine (NYU), and a past president of the American Association of Immunologists.
To sidestep the harmful side effects of the growth factor in humans, she turned to gene therapy in an animal model of MS — experimental allergic encephalomyelitis (EAE).
"MS is a demyelinating autoimmune disease," Thorbecke told BioWorld Today, "and EAE is an exact working model of that disease, except that we don't know the causative antigen in MS."
Thorbecke is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Oct. 13, 1998. Its title is: "Gene therapy in allergic encephalomyelitis using myelin basic protein-specific T cells engineered to express latent transforming growth-factor- ß1."
Her strategy took off from the fact that this key growth factor can't go into action until it gives the slip to a strict chaperone molecule, latency-associated protein (LAP).
Thorbecke explained: "TGF-ß is produced in latent form by many cell types, including T cells. Latency means it has something closely associated with it that prevents it from sitting on its receptor. So latent TGF-ß can do nothing until it gets activated — in other words, until LAP gets removed. Only then can it interact with its receptor.
"Giving the latent growth factor in cells that have specificity for an antigen that is in the demyelinating lesion's tissue," she went on, "that lesion would be targeting it to the tissue where you wanted it. And giving exogenous TGF-ß — engineered by T cells — as a supplement in latent form could have no bad side effects, because it really wouldn't do anything until it got activated. We were hoping that other cells in the lesion — not necessarily the T cells themselves — would help activate what the T cells made, and that might then modify the disease. And this seems to be what happened."
Here is how Thorbecke and her co-authors constructed their gene-delivery vector to reverse developing EAE disease in mice:
"We figured that we would transduce T cells with specificity for an autoantigen, myelin basic protein (MBP). The cDNA gene for TGF-ß would be under retroviral-promoter control, so it would be constitutively produced by these T cells.
Extra Dollop Of Growth Factor Halted Disease In Mice
"If we would now transfer those cells into animals that were developing EAE," she continued, "they might go into the lesions, especially activated by this antigen in vitro first — because activated T cells seem to migrate much better across blood vessels into lesions.
"So, we injected the cells intravenously into our mice, and hoped that they would find the lesions and do their beneficial effects there. We first immunized the animals to induce EAE with a myelin antigen, PLP (proteolipid protein), about two weeks before we gave them the cells, while their lesions were beginning to develop."
To monitor their murine patients, the team charted a five-stage score-card of EAE progression: limp tail = 1; partial hind-leg paralysis = 2; total hind-leg paralysis = 3; hind and front limb paralysis = 4; moribund = 5.
The result: Recipients of the activated MBP-specific T cells that had been transduced with latent TGF-ß experienced a significant decrease in the five-stage EAE score. Therapeutic effects persisted through 19 days following initial PLP triggering of the disease.
Thorbecke observed that T cells with specificity for myelin antigens can even be found in the peripheral blood of normal healthy human individuals. "It certainly seems that this would be a feasible approach in patients," she said. "One could take T cells from peripheral blood and expand them in clones, transfect them and give them back to the same individual."
Clinical Trials In View, But Not In Sight
"This would certainly be a very complex type of therapy," she allowed, "and very individually directed, but indeed for very long-lasting and debilitating diseases such as MS, I think clinical trials would be warranted. I've been talking to people about trying to think of what kind of vector one would want to use to do this in human trials. We have colleagues here at NYU who treat MS patients; they would certainly be quite interested.
"But this is an enormous enterprise," Thorbecke went on. "Before you have something like that going, you'd need, I think, a lot of commercial support to get such a trial on the way. At the moment we don't have that."
Asked if she would be open to suggestion from industry at this point, she replied: "Sure! I would really not know who would be the ideal commercial partner. I think you'd need a company that has enormous experience in human trials. Both biotechnology and major pharmaceutical companies have such experience by now.
"I was initially thinking about Hoffmann-La Roche," Thorbecke observed, "because they hold title to the gene for human TGF-ß. So they would be a logical candidate. But I haven't approached them yet." *