MSC isolated from different tissues had immune regulation ability not only in vivo but in vitro and it might consist the "immune protection site" in human body[25, 26]. Considering their richness in source, availability for expansion, and most importantly, their robust immuno-modulatory activity, MSCs appear to be a primary candidate for cellular therapy in immune disorders[12, 16, 27]. In normal physiological conditions, MSCs are very scarce (one MSC per 10,000-100,000MNC), therefore, normal immune responses against foreign antigens are not affected. This is consistent with in vitro results showing that immuno-suppressive function was abolished when the ratio of MSC to T cells was less than 1:100. However, once a large number of MSCs were infused for immune therapy, influx of MSC in the circulation and bone marrow could bring the hypersensitive immune response to normal. Moreover, MSC infusion could not only modulate immune responses but enhance the hematopoietic microenvironment. Transplantation of MSCs offers bright prospects in developing new therapies for blood diseases caused by an abnormal immune system and impaired hematopoietic microenvironment. To date, MSCs have been used to treat GVHD, which is a disorder of hyper-immunoresponse, and shown to be effective clinically[28, 29].
Chronic myeloid leukemia is a clonal hematopoietic stem cell disorder characterized by the t(9;22) chromosome translocation and resultant production of the constitutively activated BCR/ABL tyrosine kinase. Interestingly, this BCR/ABL fusion gene, was also detected in the endothelial cells of patients with CML, suggesting that CML might originate from hemangioblastic progenitor cells that can give rise to both blood cells and endothelial cells. Although Interferon-α, Intimab(a BCR/ABL tyrosine kinase inhibitor) and stem cell transplantations are the standard therapeutic options, transplant-related morbidity from graft-versus-host disease and mortality rates of 10% to 20% have greatly reduced the allogeneic hematopoietic cell transplantation in clinics, while interferon-α is only effective in some patients to some degree and chemotherapeutic intervention does not result in prolonged overall survival[32, 33] and the reason is possibly due to some unknown biology of the CML immune regulation.
We conducted this study of CML patient-derived MSCs to evaluate the safety and effectiveness of autologous MSCs in treating CML. We tested the karyotype and genetic changes of in vitro-expanded MSCs for safety evaluation. The immuno-modulatory function of MSCs was also examined. The investigation of CML patient-derived MSCs could help to further elucidate etiology and pathology of CML. Specifically, the answers to questions of whether gene aberrations exist in MSCs and whether the functions of MSCs are impaired are crucial for understanding of CML development and finding effective treatments.
We utilised Flk1+CD31-CD34- MSCs from CML patients for 4-6 passages, and there were chromosomal abnormities, indicating that mutation of CML happened at the hematoangioblast level. We thereby hypothesized that malignant mutation existed in stem cells more primordial than HSCs. Data from functional tests proved that CML-derived MSCs had abnormal immuno-modulatory function, although their MSCs showed normal karyotype. An inhibitory effect on T cell proliferation is an important characteristic of MSC in immuno-modulatory action. A previous study, in accordance with another report, suggested that the inhibitory effect on T cell proliferation might be through cell cycle arrest. MSCs from healthy volunteers could obviously block T cells in G0/G1 phase. In this study, inhibitory effects of MDS-derived MSCs on T cell proliferation were obviously impaired. Moreover, no significant cell cycle arrest was observed in PHA-stimulated T cells cocultured with CML-derived MSCs. In addition, an inhibitory effect on T cell activation is another key point of immuno-modulatory function for MSCs, although there are still disputes[21, 22]. CD25, CD69 and CD44 are candidates for T cell activation in different phases. In our study, MSCs from healthy volunteers showed significant inhibitory effects on expression of T cell activation markers, but MSCs from CML patients showed very limited inhibitory effects. These results suggested that CML-derived MSCs have immunologic abnormalities and their application in immuno-modulation might be limited.
Normally, the invasion and metastasis by malignant tumor cells consists of three major steps: the receptor-mediated adhesion of tumor cells to the extracellular matrix, the degradation of the extracellular matrix by the proteinase secreted by the tumor cells, and the transfer and proliferation of tumor cells. So, the loose of ECM and secreted cytokines are important for the metastasis of the tumor cells from the primary tumor. Pathological conditions will change the tumor cell fate leading to invasion and metastasis, Local secretion of proteases have been implicated in this tumor-stroma crosstalk. Matrix Metalloproteinase-9 (MMP-9) is one of them which has the preferential ability to degrade denatured collagens (gelatin) and collagen type IV, the 2 main components of basement membranes and therefore plays a critical role in tumour progression and metastaisis. Moreover, its expression increases with the increased or greater proliferation of tumor cells.
We used a ds-RNA to interfere with the expression of MMP-9 gene in CML MSC and our findings support the conclusion that MMP-9 constitutes a trigger for the switch between adhesive and invasive states in CML MSC by changing the ICAM-1 from membrane-anchored state to solvable one leading to tumor cell immune evasion and metastasis.
In conclusion, the immune function of CML patient-derived MSCs showed that their immuno-modulatory ability, compared to MSCs from healthy volunteers, was impaired, whichmight be a cause for an abnormal hematopoietic environment. This indicates that autologous MSCs transplantation might be futile. Instead, allogenic MSCs transplantation might be a better choice to ameliorate CML.