Mesenchymal Stem Cells Negatively Regulate Dendritic Lineage Commitment of Umbilical-Cord-Blood-Derived Hematopoietic Stem Cells: An Unappreciated Mechanism as Immunomodulators Hsiu-Yu Lai, M.Sc., 1,2 Ming-Jie Yang, M.D., 3 Kuo-Chang Wen, M.D., 3 Kuan-Chong Chao, M.D., 3 Chu-Chih Shih, Ph.D., 4 and Oscar K. Lee, M.D., Ph.D. 2,5 Due to their immunomodulatory functions, mesenchymal stem cells (MSCs) have great potential for clinical applications to prevent rejection in organ transplantation and to prevent graft-versus-host disease in hemato- poietic stem cell (HSC) transplantation. Since dendritic cells (DCs) play an important role in modulating diverse T cell responses, including rejection and graft-versus-host disease, the goal of this study was to investigate whether MSCs modulate DC differentiation from HSCs and if this effect could be one of the mechanisms for MSCs’ immune-modulating functions. Our results demonstrate that differentiation of HSCs into mature DCs is inhibited in the presence of MSCs. Similar frequency of dendritic precursors in the cultures, either with or without MSCs, suggests that the inhibition of MSCs on the differentiation of mature DCs from HSCs could be due to the arresting of maturation at the dendritic precursor step. Reduced levels of cyclic AMP, adenosine 3 0 ,5 0 -cyclic monophosphate (cAMP) and b-catenin in DC-like cells from the cocultures are detected, suggesting that induction of apoptosis and inhibition of differentiation could be the basis for the inhibition of mature DCs from HSCs by MSCs. Further, our results demonstrate that DCs derived from HSCs in the presence of MSCs are functionally impaired, especially for those after direct contact with MSCs. To investigate the basis of functional impairment, our data show down- regulated tumor necrosis factor-a and transforming growth factor-b1 secretion and upregulated interleukin-6 (IL6) and IL1b secretion in the cultures with MSCs. Together, MSCs can inhibit differentiation of mature DCs from HSCs by arresting them at the precursor stage and induce their apoptosis. Further, HSC-derived DCs in the presence of MSCs are functionally impaired, which could be partly due to the upregulation of IL6 secretion. Introduction M esenchymal stem cells (MSCs) are an attractive candidate for tissue engineering and regenerative medicine due to their multipotential ability to differentiate into various cell lineages. 1–3 In addition, since the discovery of their immunomodulatory capacity, 4 the potential of using MSCs in autoimmune diseases, organ transplantation, or graft-versus-host disease (GvHD) treatment has been widely appreciated, particularly for steroid-refractory GvHD. 5–9 At present, clinical trials using ex vivo cultured MSCs to treat patients with steroid-refractory acute GvHD or to prevent organ rejection are being carried out. 9,10 However, due to conflicting results in preclinical studies, 11 further investiga- tion is needed to discover the optimal conditions for clinical applications to treat or prevent acute GvHD. Further, the application of MSCs in the treatment of chronic GvHD or protection of transplanted tissue-engineered construct is yet to be developed. MSCs can suppress various immune cells, 12 possibly through secretion of soluble immunosuppressive factors such as hepatocyte growth factor, prostaglandin E2, trans- forming growth factor-b1 (TGF-b1), indoleamine 2,3- dioxygenase, nitric oxide, and interleukin-10 (IL10), 4,13–15 by MSCs themselves or immune cells in response to MSCs. Cell– cell contact also plays an important role in the immuno- suppressive activities of MSCs. 16 Antigen presenting cells are initiators of immune re- sponses because they direct cellular and humoral immune responses against self and nonself antigens. 17 Immature 1 Institute of Clinical Medicine and 2 Stem Cell Research Center, National Yang-Ming University, Taipei, Taiwan. 3 Department of Obstetrics and Gynecology, Taipei Veterans General Hospital, Taipei, Taiwan. 4 Division of Hematology/Hematopoietic Cell Transplantation, City of Hope National Medical Center, Duarte, California. 5 Department of Orthopaedics and Traumatology, Taipei Veterans General Hospital, Taipei, Taiwan. TISSUE ENGINEERING: Part A Volume 16, Number 9, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089/ten.tea.2009.0731 2987