Immortal cells are different from cultured primary cells in that they have a genetic change that renders them immortal. Often, tumor cells show aberrant chromosome numbers, lack contact inhibition, and display pleomorphism. However, not all tumor cell lines lose chromosomes and behave erratically. An intriguing tumor cell line has been isolated from a human testicular teratocarcinoma isolated from a metastasis in a patient (Andrews et al., 1982). This immortal cell line is pleuripotent and can be induced to stop dividing and to differentiate into a neuronal phenotype using retinoic acid (Andrews, 1984). This cell line is being investigated in the treatment of ischemic stroke, and animal data suggest that the postinjury transplantation of this cell line into an infarcted area can improve recovery (Saporta et al., 1999). The mechanisms surrounding this effect are unclear. Nevertheless, this cell line is currently being transplanted in the United States into patients with lacunar stroke in a small safety trial (12 patients) at the University of Pittsburg (P. Sanberg, personal communication). The transplantation of a cell line derived from a human cancer has obvious associated risks. However, the approval of this trial demonstrates that cell transplantation for severe neurological disorders is seen as a reasonable strategy by the regulatory agency, as long as safety and efficacy can be demonstrated in animal models.
Other groups are investigating the transplantation of immortal cell lines but are using genetic engineering to immortalize cells. Advances in genetic engineering have made it possible to extend the number of doublings a primary cell line can go through by inserting various oncogenes and cell cycle regulators. This allows for the selection, clonal expansion, and banking of a large number of cells. Besides the genetic modification, these cells retain otherwise normal genotypic characteristics.
One group in England has developed an undifferentiated neuroepithelial stem cell line immortalized with the simian virus (SV40) large T oncogene under temperature-sensitive regulation (Sinden et al., 1997). Using these cells, they have demonstrated positive effects on the sensory neglect and motor asymmetries in a rodent ischemic stroke model. Based on reports in the literature, various human neural cell lines have been created using the same immortalization strategy. Similar to the Pittsburgh study, the initial clinical target is stroke and clinical trials are anticipated to start in late 2000.
Other investigators have immortalized mouse and human neural stem cells using retroviral transduction of the v-myc oncogene (Snyder, 1995; Flax et al., 1998). These cells have been transplanted to rodent models and are able to survive, migrate, integrate, and differentiate into neurons, astrocytes, and oligodendrocytes (Snyder et al., 1997; Taylor and Snyder, 1997; Billinghurst et al., 1998).
Engineered immortal cell lines could potentially be made and expanded to treat millions of patients. However, genetically engineered cells may also show a greater propensity to form tumors compared with unmodified cells. To ensure safety, cells may need to be engineered with genes that render the cells mitotically inactive or that commit “suicide” if exposed to particular drugs. Extensive safety studies will be required before these cells can be tested in humans.