NEW YORK (Reuters Health) - Specialized nerve cells transplanted from pigs' noses into rats greatly enhance the regrowth of severed spinal cords without generating a rejection response, researchers report.

Such OECs--for olfactory ensheathing cells--had previously been shown to enhance nerve cell regrowth, as had Schwann cells, which are specialized support cells in the nervous system. But the problem with transplanting cells between species is rejection of the foreign tissue by the recipient, according to Toshio Imaizumi from Yale University School of Medicine in New Haven, Connecticut, and associates.

These researchers used genetic engineering techniques to create pigs whose OECs and Schwann cells carried a blocker of human complement, which is a key instigator of the body's rejection reaction.

Weeks after these cells were transplanted into rats whose spinal cords had been severed, signals were able to cross the previously interrupted nerve pathway, the authors report. In untransplanted rats, no signal crossed.

In fact, the speed of the signal across the gap was faster in the transplanted rats than in rats that had never been injured, according to the report in the September issue of Nature Biotechnology.

Microscopic studies of the injury site confirm that the transplanted rats had regenerated normal-appearing nerve cells that grew several millimeters in both directions, as if they were smelling out the correct path for nerve growth.

Moreover, transplanted rats had more than twice as many new nerve cells in the injured area as the untreated rats did.

The donor cells survived in the rats for at least 4 weeks after transplantation without signs of rejection, the report indicates.

Besides offering hope for a variety of human conditions, genetically engineered pig cells ``...have several advantages over human fetal tissue, such as availability, quality control of the tissues obtained, and timing of cell collection and reproducibility,'' Imaizumi and colleagues conclude.

``Though unthinkable only a decade or two ago, it now appears that reparative treatment for spinal cord injury may be within reach,'' agrees Lars Olson from Karolinska Institute in Stockholm, Sweden, in a related commentary.

SOURCE: Nature Biotechnology 2000;18:949-953, 925-927.