Hindawi Publishing Corporation Stem Cells International Volume 2013, Article ID 656839, 11 pages http://dx.doi.org/10.1155/2013/656839 Research Article Dynamic Imaging of Marrow-Resident Granulocytes Interacting with Human Mesenchymal Stem Cells upon Systemic Lipopolysaccharide Challenge Jay T. Myers, 1 Deborah S. Barkauskas, 1 and Alex Y. Huang 1,2 1 Department of Pediatrics, Division of Pediatric Hematology/Oncology, Case Western Reserve University School of Medicine, Cleveland, OH 44106-7288, USA 2 Department of Pathology, Case Western Reserve University School of Medicine, Cleveland, OH 44106-7288, USA Correspondence should be addressed to Alex Y. Huang; alex.y.huang@case.edu Received 20 January 2013; Accepted 27 February 2013 Academic Editor: G. Phinney Donald Copyright © 2013 Jay T. Myers et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Human mesenchymal stem cells (hMSCs) have gained intense research interest due to their immune-modulatory, tissue diferentiating, and homing properties to sites of infammation. Despite evidence demonstrating the biodistribution of infused hMSCs in target organs using static fuorescence imaging or whole-body imaging techniques, surprisingly little is known about how hMSCs behave dynamically within host tissues on a single-cell level in vivo. Here, we infused fuorescently labeled clinical-grade hMSCs into immune-competent mice in which neutrophils and monocytes express a second fuorescent marker under the lysozyme M (LysM) promoter. Using intravital two-photon microscopy (TPM), we were able for the frst time to capture dynamic interactions between hMSCs and LysM + granulocytes in the calvarium bone marrow of recipient mice during systemic LPS challenge in real time. Interestingly, many of the infused hMSCs remained intact despite repeated cellular contacts with host neutrophils. However, we were able to observe the destruction and subsequent phagocytosis of some hMSCs by surrounding granulocytes. Tus, our imaging platform provides opportunities to gain insight into the biology and therapeutic mechanisms of hMSCs in vivo at a single- cell level within live hosts. 1. Introduction Human mesenchymal stem cells (hMSCs) are self-renewing precursor cells capable of diferentiating into bone, adipose tissue, cartilage, and stromal cells of the bone marrow depending on the stimuli [1]. Available data indicate that hMSCs are pericytes whose pleiotropic nature allows them to sense and respond to infammatory processes in the microenvironment [2]. Although hMSCs are found at low frequency in vivo in a variety of adult tissues including bone marrow, muscle, fat, and dermis, they can be expanded to large numbers under appropriate culture conditions. For this reason, hMSCs have been applied therapeutically in rapidly expanding clinical investigations including more than 200 currently active clinical trials worldwide [1, 3–6]. A wide range of diseases including diabetes, atherosclerosis, multiple sclerosis, systemic lupus, Crohn’s disease, myocardial infarc- tion, stroke, Parkinson’s disease, bone and cartilage repair, wound healing, and graf-versus-host disease [1, 3] have been treated using hMSCs as a cellular therapy. Tese clinical trials aimed to explore the therapeutic potential of hMSCs with regard to their immune-modulatory properties, tissue regenerative capacity, graf enhancement, tissue protection, and repair capabilities. Similarly, hMSCs have been applied in vivo for their efcacy in a variety of human disease models in immune-competent mice including skin and spinal cord repair [7], Huntington’s disease, [8], other demyelinating diseases [9], and graf-versus-host disease [10]. Despite intense research interest and active clinical appli- cations of hMSCs, there has been some controversy and little evidence regarding the biodistribution and actual cellular behavior of hMSCs upon infusion in vivo. Investigators have