Systemically Transplanted Human Bone Marrow Mesenchymal Stem Cells Primarily Traffic to Mesenteric Lymph Nodes

Abstract 2580

Intravenously administered mesenchymal stem cells (MSCs) are trapped in pulmonary vascular bed and only few MSCs home to bone or other tissues in physiological or pathological conditions. Following intracardiac injection MSCs pass the lung barrier but their homing to bone and tissue localization is uncertain. The aim of the study was to investigate trafficking and exact localization of human MSCs following intracardiac injection into unchallenged mice and a xenograft bone tumor model. MSCs were isolated from human fetal bones (ABR Inc, Alameda CA) and expanded in DMEM-LG medium supplemented with 10% FBS. Global gene expression profiling revealed that the cultured MSCs were devoid of hematopoietic cells and expressed typical mesenchymal markers such as CD166, CD146 and CD90. We have previously shown that these MSCs are capable of differentiation into osteoblasts and adipocytes and retain their differentiation potential after multiple passages (Haematologica 2006). The MSCs were transduced with a luciferase/GFP reporter in a lentiviral vector and were maximally passaged 8 times before used in vivo. Detection of MSCs in mice was determined by live-animal imaging and ex vivo bioluminescence activity using the IVIS system, by microscopic examination of GFP-expressing cells and by immunohistochemistry for GFP. MSCs (1×10⁶ cells/mouse) were intracardiacly injected into unconditioned SCID mice (n=8) using Dovetail Slide Micromanipulator that ensures accurate injection. Following 2 or 7 days after MSC injection to SCID mice, live-animal imaging revealed bioluminescence activity mainly in the mice abdomen but not bone, while ex vivo examination detected MSCs in various abdominal organs, primarily in reproductive organs, intestine and pancreas. Careful microscopic examination revealed localization of MSCs in draining lymph nodes attached to these organs by connective tissue. Immunohistochemistry showed GFP-expressing MSCs in the adjacent mesenteric lymph nodes but not within the organs. To confirm our findings, MSCs were intracardially injected into C57BL6 mice (n=6) that harbor functional lymph nodes. Evans blue dye which is known to accumulate in and identify lymph nodes, was injected into the rear footpad or lateral tail base of the mice, 3 hours after MSC injection and 30 minutes prior to bioluminescence and florescence analyses. The Evans blue dye and GFP positivity were co-localized, indicating specific trafficking of MSCs to lymph nodes. Culturing of the dissected lymph nodes resulted in release of GFP-expressing MSCs which regained their in vitro morphology. For testing MSCs trafficking in a xenograft model, we used our SCID-rab system constructed by implanting a 4-weeks old rabbit bone into which human myeloma cells were directly injected (Leukemia 2004; Blood 2007). In this model myeloma cells grow restrictively in the implanted bone. MSCs injected intracardiacly into SCID-rab mice were mostly found in mesenteric lymph nodes but were also detected in the myelomatous bone 72 hours after MSCs injection, validating the ability of tumor cells to attract MSCs and that these MSCs are capable of transmigration. We conclude that MSCs primarily traffic to draining lymph nodes, partially explaining their in vivo immunomodulatory activity, and that understanding the mechanism by which MSCs traffic to lymph nodes may help develop approaches to shift their homing to desired organs.