Animal Tissues: The Long View
/Animals represent the largest source of young, healthy tissues for surgical replacement of injured human body parts. The problem of rejection by the host has prevented their widespread use. Here is an update on our solution to the rejection problem.
In 1996, I set out to humanize animal tissues in order to use them for cartilage, disc, heart valve, ligament, and tendon replacement. The primary problem was that animal tissues, due to a genetic mutation 25 million years ago, are loaded with a carbohydrate not present in human tissues. This carbohydrate, called the Gal epitope, was identified by immunologist Uri Galili, Ph.D.
A similar carbohydrate, present in human type A blood cells, was identified by scientists at the New York Blood Center. Their solution was to enzymatically wash away the epitope in order to produce type O blood: the universal donor, always in short supply for human transfusions.
Being a young and aggressive scientist, I simply called up the Blood Center and asked if I could use their enzyme to wash human tissues. “Sure,” they said. “But you will never get it into and all over the tissues.” “I’m an orthopaedic surgeon,” I replied. “I appreciate the challenge. I will figure it out.”
With Uri Galili as a partner, we then developed the enzymatic wash technique for cleaning the gal epitope from animal orthopaedic tissues. We tested it in the meniscus, articular cartilage, bone, tendon, and ligaments in a genetically engineered “knockout mouse” model (i.e. a model in which researchers have "knocked out" an existing lab mouse gene). We then tested the ligaments in a primate model that, similar to humans, does not have the gal epitope.
The technique worked. I then completed an IRB-approved human pilot study of an enzymatically-washed ACL reconstruction device called “Z-Lig.” The study was performed on 10 patients in 2003 and then underwent a wide clinical trial in Europe. These tests resulted in a CE mark approval to sell the Z-Lig device widely across the continent.
Unfortunately for some patients, and for the Z-Lig business, an error was made in the processing plant: The wrong water filter size was used, permitting the entry of contaminated water in one of the processing steps. As a result, a number of the devices failed, setting back the commercialization timeline. An impatient and unfriendly investor caused the Z-Lig company to fail. It has not yet been restarted.
Science and business do not always progress congenially in the right direction. Fortunately, we were able to follow the original patient group (now 20 years later) and publish the results in the Journal of Experimental Orthopaedics. These patients were selected not to optimize the results but to see just how well the devices could work in a highly athletic patient population, several presenting with multiple problems and previous surgeries.
This was a tactical error. Patients with less complicated knees and more predictable lifestyles would have made the approval process easier. Four patients suffered repeat injuries from sports, one was lost to follow-up after his 10-year exam, and one developed scar tissue that required a clean-out. Of the remaining four patients, one went on to win the Canadian Master’s Downhill Championships in skiing three times on his “pig lig” and continues to ski race today. The others continue to ski and play sports without complaint. The MRIs of the pig ligaments in these transplant patients, at 20 years of follow-up, look just like normal human reconstruction ligaments.
Over the years, genetic engineering has evolved to produce pigs that do not have the gal epitope. But they are extremely expensive, compared to just washing the tissue. The technology that we developed for humanizing animal tissues awaits commercialization. When that happens, I expect it will be used to help millions of people—people who would prefer to have young, healthy, strong, humanized pig tissues, rather than robbing one part of their body to rebuild another part.
Review SRF’s “Xenograft bone-patellar tendon-bone ACL reconstruction” paper in full in the Journal of Experimental Orthopaedics.
This prestigious journal is the official Open Access journal of the European Society of Sports Traumatology, Knee Surgery, and Arthroscopy (ESSKA) and “aims to bridge the gap between orthopaedic basic science and the clinics, and publishes papers across the entire orthopaedics field, including basic science and clinical research studies.”