Polymer-Encapsulated Genetically Modified Cells Continue to Secrete Human Nerve Growth Factor for over One Year in Rat Ventricles: Behavioral and Anatomical Consequences

Abstract

The long-term delivery of growth factors and other proteins into the CNS at putatively therapeutic yet safe levels continues to be technically constrained. In the present studies, the gene encoding human nerve growth factor (hNGF), introduced into a dihydrofolate reductase-based pNUT expression vector system, was engineered into a clonal baby hamster kidney (BHK) cell line. BHK-hNGF23 and mock-transfected cells were encapsulated in an immunoisolating polymeric device and transplanted into the lateral ventricles of healthy young adult rats for 13.5 months. As measured by ELISA, nanogram quantities of hNGF were released by encapsulated cells both prior to implantation (3.6±0.8 ng/device/24 h) and upon removal from rat lateral ventricles after 13.5 monthsin vivo(2.2±0.4 ng/device/24 h). In addition, the hNGF released into the tissue culture medium was biologically active. Long-term encapsulated cell survival was confirmed by histologic analysis. The presence of genomic DNAs (hNGF transgene), as determined by PCR analyses, revealed that the transgene copy number from the recovered BHK-hNGF23 cells after 13.5 monthsin vivowas equivalent to preimplant levels. No deleterious effects from hNGF were detectable on body weight, mortality rate, motor/ambulatory function, or cognitive function as assessed with the Morris water maze and delayed matching to position in healthy young adult rats. In addition, there was no evidence that hNGF from these encapsulated cells produced hyperalgesia. Only tests of somatosensory thresholds revealed statistically significant effects related to the hNGF delivered in the present study, and that effect was limited to a decrease in the number of trials to asymptote. Animals receiving BHK-hNGF23 implants exhibited a marked hypertrophy of cholinergic neurons within the striatum (22% increase) and nucleus basalis (7% increase) but not the medial septum ipsilateral to the capsule. Moreover a robust sprouting of cholinergic fibers was observed within the frontal cortex and lateral septum proximal to the implant. These results indicate that encapsulated xenogeneic cells provide a safe and effective method for the long-term delivery of hNGF and potentially other neurotrophic factors within the CNS.