Timing of cell transplantation after stroke significantly influences the cell-host interaction

Background and aims: Growing evidence suggests that cell transplantation holds great potential as a therapy for stroke. Several groups have shown that transplanted cells can survive in the stroke brain, migrate towards the injured tissue and improve neurological function. However, many questions must be answered before moving this therapy to the clinic. One fundamental variable that needs to be defined is the optimal time after stroke for transplantation. This study looks at two time points of cell delivery and describes the difference that the time of transplantation makes to the host response and graft biology. Methods: Primary medial ganglionic eminence (MGE) neural precursors were isolated from E15 rat embryos carrying the transgene for enhanced green fluorescent protein. Stroke was induced in rats by distal middle cerebral artery occlusion (dMCAo). A suspension of MGE cells was transplanted into the rat cortex at day 2 (n=2) or day 14 (n=3) after stroke. Animals were sacrificed at day 42 post-dMCAo and brain sections stained for neural, monocytic and blood vessel markers. Results: Cell survival differed drastically between the two transplant groups. Cells transplanted at day 2 post-stroke showed very little survival at day 42; the cells in the core appeared to be dead but a thin layer of cells survived around the periphery of the graft. In contrast, when examined at day 42, the cells transplanted at d14 exhibited much more robust survival throughout the graft and formed a much more elongated graft. There was a significant difference in transplanted cell survival between the two transplant groups ('day-2' group; 0.7 ± 0.2 ×10∧5 cells/mm3, 'day-14' group; 2.0 ± 0.3 ×10∧5 cells/mm3, P=0.02). No difference in blood vessel density in and around the grafts was found between the groups. Therefore, differential vasculature did not account for the difference in cell survival. There was, however, a significant difference in the inflammatory response between the two transplant groups. The day-2 group showed a robust inflammatory response at 6 weeks after transplantation, with Iba1-positive (monocytic) cells both within and surrounding the graft. In the day-14 group at six weeks post transplantation, there were very few Iba1-positve cells in the graft and a dramatic lack of Iba1 cells immediately surrounding the graft, much fewer than in the rest of the hemisphere. However, in this Iba1-sparse area there were many host astrocytes which appeared to form a barrier around the graft precluding the monocytes. Conclusions: Fetal neural precursors derived from the MGE survive poorly when transplanted two days post-dMCAo, possibly due to a prolonged host immune response. However, cells transplanted at 14 days post-dMCAo survived to a greater degree; they appeared to be protected from the monocytes by a wall of host astrocytes. Thus, the timing of transplantation after stroke has a long-term effect on the host response to the graft. This response can dramatically affect the graft's microenvironment and ultimately determine the success of cell-based therapy.