Initial graft size and not the innate immune response limit survival of engrafted neural stem cells

Transplantation of neural stem cells (NSCs) appears to be a promising regenerative therapy for a variety of neurological disorders. Nevertheless, NSC engraftment is limited by the number of surviving cells. To maximize stem cell‐mediated effects, timing of implantation and cell number have to be precisely evaluated. Here, a transgenic murine NSC line was optimized for high expression levels of the imaging reporters Luc2 and copGFP. NSCs of 150 000, 75 000, 15 000 or 1500 cells or Hanks buffered salt solution were implanted into the striatum of nude mice. The survival of NSCs was monitored with in vivo bioluminescence imaging (BLI) over 2 weeks and brain sections were histologically analysed for glial cells of the innate immune system. The longitudinal in vivo BLI data revealed a significantly reduced viability with the highest rate for 150 000 engrafted NSCs. The cell loss was not correlated with the number of Iba‐1⁺ immune cells nor GFAP⁺ astrocytes. Histological quantification of copGFP⁺ cells at 14 days postimplantation confirmed the in vivo data with the highest density of copGFP⁺ cells in the 150 000‐cell graft and the highest survival rate for 1500 cells/graft. In conclusion, regenerative therapies should strictly evaluate the maximal number of stem cells to be transplanted in one location, as the results suggest that there is a critical limit of cells able to survive in the adult brain. Survival is limited by availability of oxygen and nutrients but not the inflammatory response induced by the implantation.