Stimulating defensins to protect skin

March 1, 2007

National report - Cultured skin substitutes (CSS) are being engineered to perform better against infection through the stimulated production of beta defensins.

CSS are slower to vascularize than autografts, which makes them more susceptible to infection until vascularization occurs. Administering multiple antibiotics as part of the wound dressing fluids has helped to manage this problem, but it also has contributed to the emergence of resistant infection.

Dorothy M. Supp, Ph.D., and colleagues at the University of Cincinnati College of Medicine are working with the Shriners Burn Hospital "trying to make the cultured skin perform better" in protecting against infection.

Beta defensins are part of the innate immune system - small, cationic antimicrobial peptides that are thought to disrupt microbial membranes.

"The defensins have a different mechanism of action from antibiotics, so they can actually kill bacteria that are resistant to antibiotics," Dr. Supp says.

"Human beta-defensin four (HBD4) has very good activity against Pseudomonas aeruginosa," she tells Dermatology Times.

Dr. Supp chose to work with HBD4 not only because of its activity against Pseudomonas but also because "We were looking for a protein that wasn't found in the skin, to increase the overall amount of defensin protein in the skin."

Using gene therapy ex vivo, she has been able to generate expression of HBD4 by skin cells.

"We found that a fairly modest increase in protein resulted in a 25 percent increase in bacterial killing and reduction in bacterial numbers. There is definitely a dose-related response, so we are trying to increase that" with more efficient viral vectors to infect the cells and turn on the HBD4 gene, and with enrichment techniques that cull and expand those cells that do begin to manufacture that protein, she says.

Manipulating donor cells

One possible strategy that Dr. Supp is considering is a CSS mix of patient cells plus donor cells that have been manipulated to express HBD4.

The patient cells will engraft while the defensins produced by the modified cells help to protect the wound during vascularization. The donor cells eventually will be eliminated from the wound through normal turnover and rejection.

This strategy may finesse two separate potential issues: the body's rejection of foreign tissue as well as the Food and Drug Administration (FDA)'s heightened scrutiny of any procedure that might permanently alter a patient's DNA.

Much of the initial work is focusing on burn patients because of the Shriners' support for that research. Dr. Supp believes the approach will be equally useful in treating chronic wounds, such as leg and foot ulcers.

Dr. Supp says it will be at least five years before researchers will be ready to move into a phase 1 trial in humans.

"Gene therapy holds a lot of promise, but people are very nervous about it. To me, skin is the ideal tissue for gene therapy. It's right there, you see it, it's accessible, and if there are problems, it can be removed. But when you look to see what clinical trials are out there, there are hardly any for skin. I find that very unusual."