Intracarotid transplantation has shown potential for efficient stem cell delivery to the brain. However, reported complications, such as compromised cerebral blood flow (CBF), prompted us to perform further safety studies. Glial-restricted precursors (GRPs) and mesenchymal stem cells (MSCs) were transplanted into the internal carotid artery of rats (n ¼ 99), using a microcatheter. Magnetic resonance imaging was used to detect post-transplantation complications, including the development of stroke, for the following experimental variables: cell size, cell dose, cell infusion velocity, delay between artery occlusion and cell infusion, discordant versus concordant xenografting, and intracarotid transplantation with preserved versus compromised blood flow. Immunocompatibility and delayed infusion did not affect the number of complications. An infusion velocity over X1 mL/minute often resulted in stroke (27 out of 44 animals), even with an infusion of vehicle, whereas a lower velocity (0.2 mL/minute) was safe for the infusion of both vehicle and smaller cells (GRPs, diameter ¼ 15 mm). Infusion of larger cells (MSCs, diameter ¼ 25 mm) resulted in a profound decrease (75±17%) in CBF. Stroke lesions occurred frequently (12 out of 15 animals) when injecting 2 Â 10 6 MSCs, but not after lowering the dose to 1 Â 10 6 cells. The present results show that cell size and infusion velocity are critical factors in developing safe protocols for intracarotid stem cell transplantation. Journal of Cerebral Blood INTRODUCTIONThe intravascular route of stem cell delivery has met with increasing interest because of the minimally invasive nature of the procedure and the potential for broad cell distribution. Recent reports 1,2 have revealed positive effects of intravascular cell transplantation in animal models of neurologic disorders. The evaluation of neural cell distribution after intravenous cell transplantation has shown that most of the cells are initially entrapped within the lungs and do not travel to the brain.3 It has been hypothesized that an intraarterial approach would be a more efficient route of cell delivery to the brain, as this approach avoids the pulmonary circulation. This approach is particularly attractive when selecting and sorting cells for adhesion molecules, which can enhance cell homing and therapeutic outcome. 4 Moreover, it has been recently shown that transfection of adhesion molecules in progenitor cells results in a dramatic increase of their homing to inflamed brain endothelium.

The therapeutic effect of glial progenitor transplantation in diseases of dysmyelination is currently attributed to the formation of new myelin. Using magnetic resonance imaging (MRI), we show that the therapeutic outcome in dysmyelinated shiverer mice is dependent on the extent of cell migration but not presence of mature and compact myelin. Human or mouse glial restricted progenitors (GRPs) were transplanted into rag2−/− shiverer mouse neonates and followed for over one year. Mouse GRPs produced mature myelin as detected with multi-parametric MRI, but showed limited migration without extended animal lifespan. In sharp contrast, human GRPs migrated extensively and significantly increased animal survival, but production of mature myelin did not occur until 46 weeks post-grafting. We conclude that human GRPs can extend the survival of transplanted shiverer mice prior to production of mature myelin, while mouse GRPs fail to extend animal survival despite early presence of mature myelin. This paradox suggests that transplanted GRPs provide therapeutic benefits through biological processes other than mature myelin capable to facilitate rapid conduction, challenging the current dogma of the role of myelin in the function of the central nervous system.

Allografts continue to be used in clinical neurotransplantation studies, hence it is crucial to understand the mechanisms that govern allograft tolerance. We investigated the impact of transplantation site within the brain on graft survival. Mouse (FVB) glial precursors, transfected with luciferase have been injected (3×105) into the forceps minor (FM) or striatum (STR). Immunodeficient rag2−/− and immunocompetent BALB/c mice were used as recipients. Magnetic resonance imaging confirmed that cells were precisely deposited at the selected coordinates. The graft viability was assessed non-invasively with bioluminescent imaging for a period of 16 days. Regardless of implantation site all grafts (n=10) deposited in immunodeficient animals revealed excellent survival. In contrast, immunocompetent animals accepted all grafts only at STR site (n=10), while all FM grafts were rejected (n=10). To investigate the factors that led to rejection of FM grafts, with acceptance of STR grafts, another group of animals (n=19) was sacrificed during pre-rejection period, on day 5. Near-infrared fluorescence imaging with IRDye®800CW-PEG probe displayed similar blood-brain barrier disruption at both graft locations. The morphological distribution of FM grafts was cylindrical, parallel to the needle track, while cells transplanted into the STR accumulated along the border between the striatum and corpus callosum. There was a significantly less infiltration by both innate and adaptive immune cells in the STR grafts, especially along the calloso-striatal border. With allograft survival being dependent on the transplantation site, the anatomical coordinates of the graft target should always be taken into account as it may determine success or failure of therapy.