Quantitative and phenotypic analysis of mesenchymal stromal cell graft survival and recognition by microglia and astrocytes in mouse brain.

Although cell transplantation is increasingly suggested to be beneficial for the treatment of various neurodegenerative diseases, the therapeutic application of such intervention is currently hindered by the limited knowledge regarding central nervous system (CNS) transplantation immunology. In this study, we aimed to investigate the early post transplantation innate immune events following grafting of autologous mesenchymal stromal cells (MSC) in the CNS of immune competent mice. First, the survival of grafted Luciferase/eGFP-expressing MSC (MSC-Luc/eGFP) was demonstrated to be stable from on day 3 post implantation using in vivo bioluminescence imaging (BLI), which was further confirmed by quantitative histological analysis of MSC-Luc/eGFP graft survival. Additional histological analyses at week 1 and week 2 post grafting revealed the appearance of (i) graft-surrounding/-invading Iba1+ microglia and (ii) graft-surrounding GFAP+ astrocytes, as compared to day 0 post grafting. While the density of graft-surrounding astrocytes and microglia did not change between week 1 and week 2 post grafting, the density of graft-invading microglia significantly decreased between week 1 and week 2 post implantation. However, despite the observed decrease in microglial density within the graft site, additional phenotypic analysis of graft-invading microglia, based on CD11b- and MHCII-expression, revealed >50% of graft-invading microglia at week 2 post implantation to display an activated status. Although microglial expression of CD11b and MHCII is already suggestive for a pro-inflammatory M1-oriented phenotype, the latter was further confirmed by: (i) the expression of NOS2 by microglia within the graft site, and (ii) the absence of arginase 1-expression, an enzyme known to suppress NO activity in M2-oriented microglia, on graft-surrounding and -invading microglia. In summary, we here provide a detailed phenotypic analysis of post transplantation innate immune events in the CNS of mice, and warrant that such intervention is associated with an M1-oriented microglia response and severe astrogliosis.