Mesenchymal stem cells supply bone marrow stroma and have in vitro differentiation capacity into chondrocytes, osteoblasts, and adipocytes. As supporting cells in peripheral tissues, MSCs are closely related to their secretory functions in physiological homeostasis, tissue repair, and fibrosis. Additionally, MSCs exert immunoregulatory effects, improving multiple diseases, such as Graft Versus Host Disease (GVHD). MSCs have been immediately utilized after transplantation due to their low immunogenicity and the expression of MHC-class-I combination molecules, HLA-G, B7-H1, PD-L1, IDO, and prostaglandin E2, which can inhibit T cell and NK cell responses, particularly MHC-class-II negative peripheral blood MSCs, which can induce immune tolerance similar to fetal MSCs [
12]. However, these molecules and cytokine-mediated immunoregulatory functions are unstable and are easily influenced by the cell microenvironment, such as inflammation and hypoxia [
13]. Prior research found that affected tissues release more IL-1β, IL-6, and TNF-α when MSCs are transplanted. MSCs have a strong homing ability for inflammatory tissues and have abundant blood supply and rich nutrient sources [
13]. After 6 weeks of injection, a small number of MSCs are undifferentiable and can be detected in the lungs, kidneys, and liver. However, they can only be detected by detection techniques, such as luciferase, PCR, qPCR, lentivirus-GFP, and cyclophosphamide-GFP, and fewer studies have used animal models of heart disease specifically [
14]. In addition, MSCs possess a large, flat, spindle-shaped morphology, representing an adherence-independent characteristic of suspension culture, and are not sensitive to serum-containing medium or carrier contact. After a certain period of exponential growth, MSCs may enter the G0 phase, exhibiting cell senescence, cell aging, cell death, and G1/S blocking, thereby negatively impacting proliferation [
15]. Finally, MSCs are recognized as the nuclei and cells by karyotyping techniques, which change the chromosomal number and affect cell differentiation potential, the secretion of cytokines, biological characteristics, and tumorigenicity [
16].Bone marrow-derived mesenchymal stem cells and umbilical cord blood-derived mesenchymal stem cells are the only stem cell types that can be banked following plastic adherence and long-term expansion [
15]. Mesenchymal stem cells systemically suppress T and B cells as long as they express functional NIS but thereafter are partially replaced by functionally competent embryonic stem cell-derived mesenchymal stem cells [
16]. Co-transduction of NIS and thyroid peroxidase (TPO) for the non-immunogenic target-specific NIS-probe is another [
16]. Specifically, mesenchymal stem cell attachment onto NIS-expressing primary non-thyroid cells may be the easiest if the NIS expression level is checked prior to initial stem cell isolation [
16]. The potency by heterologous or allogeneic NIS-mesenchymal stem cells defined here should stimulate further in vivo NIS/gene delivery model development. In 2003, Herzog et al. proved that BM-derived MSCs could migrate to damaged brain tissues and transform into neurons, astrocytes, and oligodendrocytes [
17]. The observation showed MSCs participated in neurodegenerative disease treatments. The BM-derived MSCs could also cure the chronic spinal cord injury in mouse. Intravenous transplantation of MSCs reduced astrogliosis, induced oligodendrocyte synthesis, and promoted serotonergic axon regeneration [
17]. In 2003, Angelopoulou et al. investigated the property of BM-derived MSCs for expressing SP during the adipogenic differentiation [
18]. This finding identifies the direct action of MSCs to angiogenesis. Furthermore, BM-derived MSCs take part in the skeletal muscle repairing process. After muscle injury in mice, the injected BM-derived MSCs repair the muscle and downregulate migration inhibitory factor. MSCs could also participate in the recovery from lung injury [
19]. It created an antifibrogenic effect and restored cell proliferation of alveolar type II. Mesenchymal stem cells (MSCs) are self-renewing, pluripotent, and can differentiate into tissues like adipocytes, chondrocytes, osteoblasts, etc. Many kinds of MSCs could express normal tissue-specific proteins when seeded or transplanted in some kinds of tissues. Umbilical cord blood-derived MSCs are CD34-negative, CD45-negative, and HLA-DR-negative. Human umbilical cord blood-derived MSCs could differentiate into cells expressing liver-function proteins [
20]. Human placental- or umbilical cord blood-derived MSCs are suitable for obtaining CB, and the process of collecting CB sample is noninvasive. It avoids donor morbidity and infection risks so that the safety issue is reduced. Moreover, it has little HLA identity, so transplantation of embryonic stem cells (ESCs) or MSCs from CB may reduce the side effects of immune rejection [
21]. The characteristic of low immunogenicity and potent immunosuppression makes MSCs suppress the immune rejection, solve engraftment difficulties, and ameliorate the stem cell niche environment. Remarkably, the umbilical cord blood-derived MSCs sustain high paracrine activity with high inflammatory cytokine levels. Cell damage and cryopreservation strategies impact paracrine activity [
22].
2.2. Immunomodulatory Properties
Human mesenchymal stem cells (hMSCs) are self-renewing progenitors best known for differentiation to mesenchymal lineage cells such as osteocytes, adipocytes, and chondrocytes. hMSCs can also differentiate to endodermic-like cells, for example hepatocytes, and neurogenic-like cells, for example neurons and glia; these cells can participate in tissue repair and regeneration mainly through paracrine effects. Surrounding the MSC and microenvironment in which they are located significantly affect their potential role in the immune response [
27]. Endogenous MSCs are found in bone marrow, pericytes from various tissues, fibroblasts, and neural cells. Due to the low number of MSCs in human tissues, explantation techniques are used to expand these cells in in vitro cultures. In clinical trials, MSCs have been shown to be safe and hold particular promise for their immunophysiological properties in the treatment of various diseases [
28].
MSCs-associated features include their apparent exclusion from the different types of lymphoid and myeloid cells; they are immunosuppressive. These facts, together with the property of suppressing proliferation of peripheral blood lymphocytes that do not produce lymphokines, can absorb or modulate immune responses [
29]. Although these effects have been extensively studied in a large number of in vitro allogeneic and xenogeneic studies, MSC actions in vivo are less defined. In particular, labeling, ex vivo expansion, and reimplantation of MSCs showed that, a few days later, they were rejected in a reaction mediated by T-cells or NK cells. Lower immunosuppressive impacts in vivo were observed, although one working mechanism of MSC action includes their participation in the suppression of lymphopoiesis in the bone marrow [
30]. Control of lymphopoiesis is achieved through withdrawal of IL-7 and CXCL12 support from the stem/progenitor cells. MSCs lack HLA class II expression. Even in conditions in which major histocompatibility complex class I expression was enhanced, few NK-activating molecules of MSCs were observed [
31]. Most of the evidence is consistent with an immune escape mechanism. MSC immunosuppression was effective either in the presence or absence of a descending number of specific patient lymphoid cells. Furthermore, the suppressive effects of patient hMSCs were maintained in peripheral blood lymphocyte samples harvested after transplant, suggesting a resistance to tolerance factors [
32,
33]. The mechanism(s) that MSCs use to suppress immune responses are complex, but mainly involve localized effects via release of soluble factors. The immunosuppressive pattern of MSCs was also involved in the selective accumulation of these cells in some tumors [
34]. Okolicsanyi and Griffiths assessed the isolation, expansion, and differentiation of WJ-MSCs and U-CMSCs isolated from different gestational age Wharton’s jelly (WJ) into osteoblasts, adipocytes, and neural progenitor cells and the hMSC cell surface markers, revealing that WJ-MSCs may be a potential stem cell therapy source for nervous system injury, and that the immunophenotypes of the expanded isolated U-CMSCs did not change with in vitro culturing [
35]. These cells maintained an extraordinary growth rate, while the WJ-MSCs maintained their structural and functional features. Balakrishnan assessed the immunomodulatory properties of different sources of hMSCs priced the comparison based on the intensity of expression of immunomodulatory and cytokine receptors, and concluded that WJMSCs demonstrate intense anti-inflammatory and anti-apoptotic mechanisms, such as release of immunoregulatory and pro-angiogenic factors [
36]. In summary, the results presented by Balakrishnan and Okolicsanyi & Griffiths provide evidence for the potential role of umbilical cords in using U-CMSCs for immune system regulation in different disorders.
2.3. Tumor-Tropic Migration
MSCs have the unique ability to migrate towards tumors. Studies have determined the migration ability of both mouse MSCs and human MSCs towards a variety of tumor types, both in vitro and in vivo [
33]. As a consequence, MSCs were found to localize and engraft in various tumor models following transplantation into animals. Interestingly, MSCs possess the ability to specifically migrate towards sites of sterile inflammation and ischemia, and to migrate towards sites of allogeneic hematopoietic stem cell transplantation [
37,
38]. Although the natural physiological function of MSCs to migrate towards sites of injury and inflammation is beneficial in tissue repair, it has been suggested that MSCs could potentially enhance tumor metastasis through their migration tropism [
39]. Some groups hypothesized that MSCs have anti-tumor abilities based on evidence showing that some cytokine-activated MSCs can inhibit the growth of various tumor type cells in vitro [
40]. Filing of MSCs with some cytokines, such as interferon-c (IFN-c) can enhance the in vitro anti-tumor ability of MSCs. In addition, results from a number of in vivo studies clearly show that MSCs can inhibit tumor growth if they are coadministered with appropriate cytokines such as GM-CSF or IFN-c [
41]. There is much controversial data on the interaction between MSCs and tumor cells: MSCs can enhance or inhibit tumor cell invasiveness in the presence or absence of tumor necrosis factor-a (TNF-a). The existing knowledge about the interaction between MSCs and different types of tumor cells imply that the design of MSC-based gene delivery as an “effector” therapy against systemic malignant tumors should take into account not only the tumor-tropism of MSCs, but also the effect of the tumor on the MSCs themselves, in particular, how MSCs are activated, reprogrammed or get sick when they meet tumor cells, and how these phenomena would synergistically impact on the ultimate gene therapy efficacy [
42]. Due to their innate ability to locate tumors, mesenchymal stem cells (MSC) are used as vehicles for tumor treatment. Growth factors, chemokines, and inflammatory cytokines are all produced at higher levels in tumors, which encourages MSCs to actively recruit into the tumor microenvironment and aid in the formation of the tumor stroma [
43]. Because MSCs are easily extracted, amplified, and transplanted across the allogenic barrier, they are highly suited for therapeutic uses. MSCs that have undergone genetic engineering hold promise as delivery systems for therapeutic genes like NIS. In early-stage human clinical studies, the use of modified MSCs to treat solid tumors is currently being investigated [
44]. Preclinical research utilizing xenograft tumor mouse models has shown the potential of CMV (cytomegalovirus) promoter driven MSC-mediated NIS gene delivery, with successful selective NIS expression in tumors and metastases as well as a strong therapeutic response following [131I]NaI application [
45]. MSC engineering has been exploited for cell-mediated gene therapy and for carrying therapeutic transgenes that protect the engrafting cells from the conditioning cytotoxic insult or that reinforce the functions of the stem/progenitor cells in the host recipients, promoting their engraftment, the differentiation processes, or conditioning a permissive local environment [
46]. However, the beneficial effects have been obtained only in a subset of the genetically manipulated cells, and the strategies available are still very expensive and potentially associated with risk factors. Some laboratories have reported results with a more efficient MSC transduction, alongside with the use of different transduction systems such as commercial viral vectors or the production of viral vectors derived from in-house developed stable packaging cells [
47,
48]. Radioisotope therapy can be successfully carried out with cells expressing the sodium iodide symporter (pNIS). Mesenchymal stem cells (MSCs) have been put forward as desirable vehicles for NIS gene transfer since they can be transplanted in a variety of different ways [
49]. They can contribute to viral clearing in the in vivo experiments and do not replicate in the target tissue, therefore providing tumor-specific pNIS expression [
50]. This latter property can significantly reduce the potential toxicity when the NIS-targeted malignant cells are efficiently radiouptaken with gamma emitter radionucleotides.