Discussion

Despite the data obtained by many researchers in their studies, it is apparent that MSCs from different sources show heterogeneity, particularly when expanded in vitro and express a great variety of chemokines and their receptors, which can lead to preferred transendothelial migration and homing of various MSCs into damaged tissues. Also, available knowledge from leukocytes or hematopoietic progenitor cells cannot be extrapolated to fully explain homing and migration of MSCs. Therefore, it is important to take into consideration the conditions of MSC isolation, cultivation, possibly priming and the number of passages undergone (number of times MSCs were removed from the culture and undergone a subculture process) before analysis.

Honczarenko et al., (2006) conducted a study on in vitro-cultured, second passage MSCs, which were positive for CCR1, CCR7, CCR9, CXCR4, CXCR5, and CXCR6 cell surface receptors and secreted following chemokine ligands: CCL2, CCL4, CCL5, CCL20, CXCL12, CXCL8, CX3CL1. Some of these ligands can activate the MAP kinase and focal adhesion kinase (FAK) signaling pathways, by phosphorylation. These pathways are important for transcription of integrin signalling molecules and therefore cell migration. For example, CCL5 activates STAT-1 and CXCL12 selectively triggers STAT-5 activation, inducing F-actin polymerization in the cytoskeleton. MSCs in culture can gain or lose some surface receptors, which influences their transendothelial migratory ability. Studies that used culture-expanded MSCs showed loss of cell surface chemokine receptors at passage 12-16, leading to reduced chemotactic response, decreased presence of ICAM-1, ICAM-2, and VCAM-1 adhesion molecules. This phenotypic change is related to slowing of cell growth during long-term cultivation and spontaneous apoptosis, dependent on cell confluency and the environmental conditions (e.g. hypoxic or normoxic) during incubation.

In the Ruster et al., (2006) study, MSCs were found expressing high rolling velocities, which could be due to the lack of in vitro activation with TNF-α, which is required for the expression of cell adhesion receptors that mediate rolling in inflamed or injured epithelia. Freshly isolated MSCs show improved homing capability, than culture-expanded MSCs. (Rombouts and Ploemacher, 2003). This data suggests that several chemokine axes exist in MSC biology and it is important to validate MSCs intended for regenerative therapy prior to their use.

FAK regulates extracellular migration, which is integral for angiogenesis, therefore, increased expression of FAK can be correlated with increased metastasis, possible tumour formation and reduced safety of MSC treatment. Similarly to ichaemically damaged regions, different tumours also produce inflammatory mediators (cytokines, chemokines, chemoattractant molecules), which could attract MSCs to migrate towards the tumour environment, most likely directed via SDF-1/CXCR4 ineraction. Zhu et al., (2006) show that both fetal bone marrow MSCs and adult MSCs could contribute to tumour growth in vivo, when transplanted subcutaneously into mice. Tumour tissues showed rich vascularisation and general invasion and necrosis of the surrounding normal tissues and formation of tumour-associated stroma. This progression of carcinogenesis could be due to spontaneous transformation of MSCs, that have reached primary tumour sites. These findings illustrate the necessity of assessing long-term safety of MSC therapy. However, it is possible to genetically transform MSCs to produce anti-cancer mediators that would impede tumour growth. Chen et al., (2006) have used adenoviral engineering to persuede MSCs to secrete interleukin-12, creating AdIL-12-MSCs. Interleukin-12 in combinaion with interferon γ, natural killer cells and CD4 , CD8 T cells encourages anti-carcinogenesis activity. The AdIL-12-MSCs were infused into mice, before tumour injection. The results show that AdIL-12-MSCs prevented 12 (out of 12, 100% response) mice from developing tumours in the B16 melanoma group, 10 (out of 12, 83%) mice in the Lewis lung cancer group and 11 (out of 12, 92%) mice in the hepatocellular carcinoma group. The conrol groups showed 100% carcinogenesis, suggesting that AdIL-12-MSCs demonstrated preventive properties on tumorigenesis.

Lee et al., (2010) conducted a long-term, follow-up study in patients who received intravenous autologous MSC therapy after ischaemic stroke, to assess the long-term safety and efficacy of the treatment. Based on 5 years of follow-up, the results of this study showed that IV application of in vitro cultured MSCs is safe. No differences in comorbidities between the control and the MSC groups were found. Limited acute toxicity was reported and the mortality in the MSC group was lower; 4 out of 16 patients (25%) died in the MSC group and 21 out of 36 (58.3%) died in the control group during the follow-up period. Serious adverse effects associated with MSC therapy included: arrhythmia, supraventricular tachyarrhythmia, seizures, caused by abnormal excitation from freshly formed neural circuits and vascular occlusion, caused by MSCs size during IV infusion. The occurrence of these adverse effects did not differ between the groups. Researchers reported, that the level of SDF-1 in blood plasma during the infusion of MSCs, directly correlated with clinical improvements. In patients with ischaemic stroke circulating SDF-1 may be isolated within the vascular endothelium or infarcted area, increasing the local SDF-1 level. This emphasizes the importance of SDF-1 in post-stroke functional recovery following MSC treatment. It has also been suggested, that using serum SDF-1 levels could be useful for selecting patients who are likely to have favourable outcomes after MSC therapy.

Explaining MSC migration and engraftment is challenging, because to date, no clear, universally accepted criteria for classification of MSC phenotype and functional properties are available. More detailed research examining specific molecular signalling pathways mediating MSC migration would aid novel therapeutic agent production and increase their efficacy in clinical practice and regenerative medicine. (Kholodenko et al.,2013) Also, the method of MSC culturing has proven to affect MSC performance. The opportunity of modulating cultured MSCs with cytokines or using retroviral transduction opens broad possibilities in creating successful treatment, which will be capable of directed tropism of greater number of transfused MSCs towards the site of the injury (Chen et al., 2006).