In Vivo Cellular Imaging Pinpoints the Role of Reactive Oxygen Species in the Early Steps of Adult Hematopoietic Reconstitution

Progress in regenerative medicine will be greatly helped by the characterization of the in vivo processes that lead to tissue reconstitution by adult somatic stem cells. Many major advances have been achieved in the purification and the ex vivo amplification of the adult somatic stem cells, but very few data are available on the pre-requisites that will enhance their in vivo biological activities. The best studied model of tissue reconstitution is the long-term reconstitution of hematopoiesis after transplantation of adult hematopoietic stem cells (HSCs) following lethal irradiation and imaging strategies are amongst the most promising techniques to reveal the in vivo cellular dynamics of this process. Here, we present a new imaging technique to track labeled cells in living animals and we show that this technique can visualize the hematopoietic reconstitution within the femoral cavity after lethal irradiation. We use a fibered in vivo confocal fluorescence imaging system that can navigate inside the femoral cavity from the knee to the femoral head. Imaging is done in living mice, does not alter their viability, does not interfere with hematopoietic reconstitution and can easily be used to follow the temporal dynamics of tissue reconstitution in the mouse at a cellular level. Using this imaging system, we track the temporal and topological cellular dynamics of in vivo engraftment and proliferation of Lin-Sca+c−Kit+CD34− cells (LSKCD34−), enriched in HSCs, transplanted into lethally irradiated mice. Homing and engraftment of LSKCD34− hematopoietic cells first occur in the two femoral epiphyses where these cells are found associated with vascularized structures as early as one hour after transplantation. Reactive oxygen species (ROS) regulate

• the initial homing of transplanted HSCs through the modulation of VCAM-1 expression by the endothelial cells and

• the proliferation of transplanted HSCs.