ALLORECOGNITION OF ARTIFICIAL NERVE GUIDES FILLED WITH HUMAN SCHWANN CELLS : An In Vitro Pilot Study (2000)

In the surgical repair of peripheral nerve injury, a gap between the proximal and the distal nerve stump is frequently encountered. The standard treatment involves bridging the gap with nerve autografts. These grafts, like the Wallerian degenerated distal nerve part, consist of empty tubes and Schwann cells (1). Regenerating axons are able to grow through these tubes and reconnect with their target organs. However, disadvantages of harvesting nerve autografts are donor site morbidity, limited supply, and prolonged surgical procedures (2). Therefore, research has been focused on the production of artificial nerve grafts to bridge these gaps. In optimizing these grafts, adding Schwann cells to the lumen has been proved successful in rats (2). Schwann cells secrete growth factors and produce extracellular matrix proteins, which are both stimulatory agents in nerve regeneration (3). Artificial tubes can easily be filled with in vitro cultured Schwann cells from inbred rat strains. Rejection of allografted rat Schwann cells, however, has previously been reported (4). Similarly, when artificial nerve guides filled with allogenic Schwann cells have to be implanted in humans, the problem of immunological recognition and rejection of donor material is raised. We performed a pilot experiment to test allorecognition of human Schwann cells of two different hosts using peripheral blood mononuclear cells (PBMC) of HLA-DR matched and mismatched individuals. Primary human Schwann cells were isolated and propagated as described before (5). Schwann cells were seeded into 96-well, roundbottom plates at a density of 500 cells per well. PBMC of 6 individuals were isolated using a Ficoll gradient and added at a density of 150.000 PBMC per well in triplicate. After 5 days, proliferation was measured by incorporation of 3H-Thymidine, which was added 16 hr before the assay was terminated. Significant differences were observed between proliferation of PBMC of matched and mismatched individuals (Fig. 1). Figure 1: Allorecognition of primary human Schwann cell cultures A and B by matched and mismatched PBMC. Results are depicted as mean counts/min±SEM. HLA-DR type of PBMC and Schwann cells are given in the graph. Allorecognition is equally present when PBMC are mismatched for one or two alleles and absent when matched.Allorecognition was absent in HLA-DR compatible combinations. However, when PBMC were mismatched in one or both alleles, an identical increase in response was observed. Our results strongly suggest that implanted artificial nerve guides filled with allogenic human Schwann cells are likely to be rejected. Even mismatching of one of the DR-alleles results in a huge immunological response, showing its forceful character. To avoid lifelong administration of immunosuppressive drugs, the problem of allorecognition and subsequent rejection of the graft in humans could, theoretically, be overcome in different ways. First, Schwann cells could be collected from HLA-DR typed donors (postmortem). Alternatively, a piece of a small cutaneous nerve with neglectible sensory deficit may be taken from the patient to generate an artificial graft filled with cultured autologous Schwann cells. This method, however, not only requires two surgical interventions, but also the certainty at the time of presentation of the case that immediate reconstruction is not likely to give better results. In elaborating this option, it would be interesting to analyze whether Schwann cell cultures can also be generated from the distal nerve stump, which could be tested in animal models. Eric Spierings1 Carmen L. A. M. Vleggeert-Lankamp Enrico Marani Ralph T. W. M. Thomeer Tom H. M. Ottenhoff