GRAFTS OF GENETICALLY MODIFIED SCHWANN CELLS TO THE SPINAL CORD: SURVIVAL, AXON GROWTH, AND MYELINATION MARK H. TUSZYNSKI,*²‡ 1 NORBERT WEIDNER,* MELISSA MCCORMACK,* IAN MILLER,* HENRY POWELL,‡ AND JAMES CONNER* Departments of *Neurosciences and ‡Pathology, University of California–San Diego, La Jolla, CA 92093-0608 and ²Veterans Affairs Medical Center, San Diego, CA 92161  Abstract — Schwann cells naturally support axonal re- generation after injury in the peripheral nervous system, and have also shown a significant, albeit limited, ability to support axonal growth and remyelination after grafting to the central nervous system (CNS). It is possible that Schwann cell-induced axonal growth in the CNS could be substantially increased by genetic manipulation to secrete augmented amounts of neurotrophic factors. To test this hypothesis, cultured primary adult rat Schwann cells were genetically modified using retroviral vectors to produce and secrete high levels of human nerve growth factor (NGF). These cells were then grafted to the midthoracic spinal cords of adult rats. Findings were compared to animals that received grafts of nontransduced Schwann cells. Spinal cord lesions were not placed prior to grafting because the primary aim of this study was to examine features of grafted Schwann cell survival, growth, and effects on host axons. In vitro prior to grafting, Schwann cells secreted 1.5  0.1 ng human NGF/ml/10 6 cells/day. Schwann cell transplants readily survived for 2 wk to 1 yr after in vivo placement. Some NGF-transduced grafts slowly increased in size over time compared to nontransduced grafts; the latter remained stable in size. NGF-transduced transplants were densely penetrated by primary sensory nociceptive axons originating from the dorsolateral fasciculus of the spinal cord, whereas control grafts showed significantly fewer penetrating sensory axons. Over time, Schwann cell grafts also became penetrated by TH- and DBH-labeled axons of putative coerulospinal origin, unlike control cell grafts. Ultrastructurally, axons in both graft types were extensively myelinated by Schwann cells. Grafted animals showed no changes in gross locomotor function. In vivo expression of the human NGF transgene was demonstrated for periods of at least 6 m. These findings demonstrate that primary adult Schwann cells 1) can be transduced to secrete augmented levels of neurotrophic factors, 2) survive grafting to the CNS for prolonged time periods, 3) elicit robust growth of host neurotrophin-responsive axons, 4) myelinate CNS axons, and 5) express the transgene for prolonged time periods in vivo. Some grafts slowly enlarge over time, a feature that may be attributable to the pro- pensity of Schwann cells to immortalize after multiple passages. Transduced Schwann cells merit further study as tools for promoting CNS regeneration. © 1998 Elsevier Science Inc.  Keywords — Spinal cord injury; Schwann cells; Nerve growth factor; Myelination; Regeneration; Transplanta- tion; Grafting. INTRODUCTION Schwann cells possess several intrinsic features that contribute to successful axonal regeneration in the in- jured peripheral nervous system. Schwann cells 1) se- crete growth factors, including NGF, brain-derived neu- rotrophic factor (BDNF), glial cell line– derived neurotrophic factor (GDNF), and ciliary neurotrophic factor (CNTF) (1,8,15,26,42); 2) produce and secrete molecules of the extracellular matrix, including laminin, to which injured axons can attach and extend (6,12–14); 3) guide regenerating axons in combination with other structural elements of the injured peripheral nerve (6,12– 14,18,20,33,34); and 4) remyelinate regenerating axons. These features could be useful in promoting axonal regeneration in the injured central nervous system. In- deed, several previous studies have demonstrated that Schwann cells grafted to the CNS as either cell suspen- sions (23) or attached to artificial surfaces (41) can remyelinate CNS axons (3,9,10,19) and support some degree of axonal growth (5,19,24,25,27,29 –31,40,41). Often, however, the degree of axonal growth detected within Schwann grafts in the CNS is modest, suggesting that alternative means are required to augment the suitability of the Schwann cell as a conducive medium for CNS axonal regeneration. Previously we and others have reported that primary fibroblasts transduced to produce and secrete neurotro- 1 Correspondence should be addressed to: Dr. M.H. Tuszyn- ski, University of California–San Diego, Department of Neu- rosciences, La Jolla, CA 92093-0626. Cell Transplantation, Vol. 7, No. 2, pp. 187–196, 1998 © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0963-6897/98 $19.00 + .00 PII S0963-6897(97)00166-8 187