The Role of Nestin⁺ Mesenchymal Stem Cells (MSCs) in Bone Marrow Chronic Graft-Versus-Host Disease

Chronic graft-versus-host disease (cGVHD) is a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). cGVHD is a syndrome of variable clinical features resembling autoimmune, involving not only the epithelial target tissues (skin, lung, gastrointestinal tract and liver) but also any other organ system (musculoskeletal, joint, ocular and genital tissues). Bone marrow (BM) suppression without infection is often observed in patients undergoing allo-HSCT as GVHD symptoms appear, suggesting that BM could be a target of GVHD (Yusuke S. et. al. Blood 2010). However, the BM cGVHD is rarely reported and the mechanism for BM cGVHD is still unclear. Nestin⁺ mesenchymal stem cells (MSCs) are capable of differentiating into cells of multiple cell lineages and involved in the formation of fibrotic lesions (Miaohua M. et. al. Cell Mol Life Sci 2016). To determine whether Nestin⁺ MSCs play an important role in the development of BM cGVHD. We used two murine models of cGVHD, including MHC-matched BALB/c (H-2^d) recipients given B10D2 (H-2^d) grafts, and MHC-mismatched BALB/c (H-2^d) recipients given C57BL/6 (H-2^b) grafts. We observed that BM displayed a significant increase in collagen and reticulin fibers deposition around blood vessels in cGVHD group compared to non-cGVHD group. The decreased absolute numbers of monocytes and neutrophils in the BM indicated the broad BM suppression in the cGVHD group. We further demonstrated that Nestin⁺MSCs located in perivascular BM niche underwent tremendous expansion in cGVHD group (p<0.001). Nestin⁺ MSCs in cGVHD group acquired expression of alpha smooth muscle actin (a-SMA), indicating myofibroblast differentiation. Moreover, transforming growth factor- beta1 (TGF-β1) concentrations also increased in cGVHD BM microenvironment (p<0.0001). High concentrations of TGF-β1 induced formation of Nestin⁺ MSC clusters and more transition from Nestin⁺ MSCs to a-SMA⁺ cells (p<0.05). Besides, expanded Nestin⁺ MSCs were surrounded by many macrophages and expressed high level of TGF-β1 receptor (p<0.001 and p<0.005). Taken together, these results indicate that Nestin⁺ MSCs participate BM cGVHD development through transition to a-SMA⁺ cells induced by high TGF-β1 in cGVHD microenvironment.