Tracking / Imaging

Integral to understanding and assessing treatment efficacy using stem cells as vehicles for transgene products is a mechanism for surveillance. This is especially true in the case of systemic injection of stem cells due to the degree of migration possible. Being able to track stem cells can not only offer information about migratory behavior, but can also provide valuable insight into the development of iatrogenic tumors - an unwanted but possible side-effect of stem cell treatment.

One possible avenue for tracking stem cell migration is the usage of bioluminescent firefly luciferase (Fluc). Neural stem cells can be engineered to express Fluc and subsequently be implanted into mice. One group injected such cells into the cerebral hemisphere of mice that had had tumors grafted to this area previously. By monitoring these mice using a CCD camera and image processing software used to localize the emission spectrum from Fluc, the migration of neural stem cells could be monitored.(30) In a similar study, neural stem cells were again engineered to express Fluc, but gliomas were transduced with the gene for Renilla luciferase (Rluc). This allowed for not only the surveillance of stem cell migration, but also any metastatic or infiltrative action by the gliomas. The same group also recently incorporated green fluorescent protein (GPF) expression into the gliomas and DsRed2 expression into neural stem cells. Through the combination of both fluorescence and bioluminescence, one can analyze gross morphology changes as well and utilize laser scanning microscopy to resolve physiological changes at the cellular level.(31)

The inherent disadvantage of the above imaging techniques however, is their limitations in transition into human treatment analysis. As a result, more conventional imaging modalities such as magnetic resonance imaging (MRI) have been employed to monitor stem cell migration after injection into mice. It has been shown that embryonic stem cells which constitutively expressed GFP and which were labeled with MRI contrast agent would migrate toward a site of cerebral ischemia when injected intracranial and that this migration could be monitored with adequate resolution using MRI. These findings were confirmed via GFP registration. Other groups have attempted to improve upon this technique via the administration of a novel bifunctional contrast agent, gadolinium rhodamine dextran, which can be resolved by both MRI and fluorescence microscopy.(32)