Dynamic Imaging of Marrow-Resident Granulocytes Interacting with Human Mesenchymal Stem Cells upon Systemic Lipopolysaccharide Challenge

Human mesenchymal stem cells (hMSCs) have gained intense research interest due to their immune-modulatory, tissue differentiating, and homing properties to sites of inflammation. Despite evidence demonstrating the biodistribution of infused hMSCs in target organs using static fluorescence 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 fluorescently labeled clinical-grade hMSCs into immune-competent mice in which neutrophils and monocytes express a second fluorescent marker under the lysozyme M (LysM) promoter. Using intravital two-photon microscopy (TPM), we were able for the first 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. Thus, 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.