Therapeutic uses of post-partum tissue-derived mesenchymal stromal cell secretome

Introduction

Stem cell (SC) research has brought regenerative medicine to the forefront of cell-based clinical research and therapy due to promising outcomes. SC research has produced cells, tissues and whole organ-like structures in vitro using SCs to replace damaged or non-functional tissues or organs by transplantation1. Diseases such as autism2 and muscular dystrophies3 which are labelled as incurable have gained promising results using SC therapy.

Pluripotent embryonic SCs (ESCs) isolated from the blastocyst stage of embryos have ethical concerns than the multipotent adult SCs which are isolated from the bone marrow, brain and heart tissue; post-partum tissue (PPT) such as umbilical cord (UC), UC blood (UCB), amniotic membrane (AM) and placenta and surgical waste such as deciduous teeth pulp and adipose tissue (AD)4. As ESC sources are the stored embryos at assisted reproductive centres, varying ethical issues dependent on the country of use, in addition to their tumourigenic capacity have limited the use of ESCs in research5. Induced pluripotent SCs, adult cells which are reprogrammed to function as embryonic-like SCs, have also shown great potential in therapeutics6. Among the different types of adult SCs, bone marrow SCs (BM-SCs) have widely been used in research. With the ability of establishing SCs from biological and surgical waste, SC research has flourished mainly due to the minimum ethical considerations associated with the use of such waste starting material7. High availability8, absence of tumourigenicity and favourable immune-privileged effects9 are other reasons for the increased use of SCs in biological waste material to fulfil the demand of the ever-rising numbers of therapy trials. Mesenchymal stromal cells (MSCs) and haematopoietic SCs (HSCs) derived from PPT are such adult SC categories that are in the initial stages of clinical trials. As at November 2020, the US National Institutes of Health SC registry lists 5685 SC-related clinical trials, of which 86 are related to HSCs derived from cord blood and 58 trials related to MSCs from other PPTs10.

With the use of relatively easy isolation methods, low rejection rates, high availability and wide differentiation potential, PPT-SCs have gained attention in SC research. hMSCs can be isolated from all PPTs using either digestion or explant methods. UCB provides a source to isolate HSCs by selecting CD34+ cells and expanding them in a suitable medium supplemented with selected cytokines as non-adherent cultures11 or else UCB can be used to isolate MSCs by expanding mononuclear cells (MNCs) as adherent cultures12. UCB-hMSC showed significantly higher proliferation, clonality and/or significantly lower expression of p53, p21 and p16, well known markers of senescence, compared to BM and AD hMSCs11. However, successful isolation of hMSCs from UCB is believed to depend on the time between collection and isolation, the net volume of blood and the MNC count; hence, the isolation process itself becomes laborious and time-consuming, resulting in low yields of hMSCs13. Due to such difficulties confronted, as well as contemplation by parents on storing UCB in blood banks for future use of their child, the use of other types of PPT are considered. Although their self-renewal capacity when compared with other hMSCs was not significantly different, placenta-hMSC showed high proliferative capacity and better growth characteristics than bone marrow, adipose-derived and UCB-hMSCs11. Expression of stemness markers was not significantly different between these hMSCs, making UCB and placenta-hMSCs potential candidates for research akin to bone marrow- or adipose-derived hMSCs11. Although 5-50 per cent of SC marker-positive cells reside within the population of amniotic epithelial cells, a mere 0.01-0.1 per cent SCs are present within the other residing tissue types14, making the AM a rich source of SCs compared to somatic tissues. Human AM-MSCs, UC-MSCs and UCB-MSCs also demonstrate immunosuppressive properties151617. In an in vitro study, human placenta-MSCs have shown a significantly higher ability of immunosuppression compared to human UC-MSCs18.

It is believed that most of the therapeutic effects are due to different bioactive molecules such as growth factors, cytokines, chemokines and angiogenic factors that are secreted by SCs which are collectively known as the 'stem cell secretome’, and all these molecules have been thoroughly investigated19. It is reported that the UC-MSC secretome is significantly different from the bone marrow- and adipose-derived SC secretomes20. This article reviews the therapeutic effects of the UC-derived mesenchymal SC secretome and highlights the pros and cons of its applications, compared to other SC secretomes.

Composition of the PPT-MSC secretome

A typical hMSC secretome is known to contain growth factors, cytokines, extracellular vesicles, lipid mediators, extracellular membrane proteases and hormones21, causing differential effects on the treated cells.

Significance of the hPPT-MSC secretome

Of the MSC secretomes, the UC-MSC secretome has proven to be significantly different from BM-MSC and adipose-derived MSC secretomes31. UC-MSCs show significantly reduced synthesis of important proangiogenic factors but increased secretion of angiogenic growth factors and chemokines when compared to BM-MSCs and AD-MSCs323334. UC-MSCs have also demonstrated significantly higher increased secretion of neurotrophic factors35, important cytokines and haematopoietic growth factors than the BM-MSC and AD-MSC secretomes36, pointing towards the potential benefits of therapy specific to the UC-MSC secretome. A study comparing the effects of BM-MSC and WJ-MSC secretomes on neural differentiation demonstrated different temporal profiles regarding stimulation of neurite outgrowth and the gene expression of neuronal markers37. Although proteomic-based mass spectrometry has shown differences of protein profiles among the BM, AD and UC-MSC secretomes, it also confirmed that the UC-MSC secretome is a potential candidate for neuroregenerative research as much as the other two secretomes31. UC-MSCs cultured using post-partum waste and the resultant secretome obtained with ease, together with minimum ethical considerations, will augment its value in cell-free therapeutic procedures. The following subsections highlight research in which UC-MSC secretome was investigated in different therapeutic procedures against a wide range of diseases.

Therapeutic effects of hPPT-MSC secretome

Use of hPPT-MSC secretome in therapeutic procedures against various other diseases

In vitro differentiated cells have the advantage of aiding fast recovery of the patient, rather than the time-consuming method of transplanting undifferentiated SCs which would differentiate and then replace the non-functional or injured tissue. Procedures to differentiate SCs into a variety of mature cell types in vitro and in vivo under different stimulated conditions such as by adding synthetic and natural compounds have been investigated; and, the hMSC secretome rich in various growth factors and cytokines has also been explored. In 2014, a phase 1 clinical trial was set up with 20 patients, to investigate the effect of microvesicles derived from UCB-MSCs, to decrease the inflammatory state and enhance the β-cell mass as well as the glycaemic control62. Two recent studies have also been registered on NIH clinical trials, US data base, on uses of secretome of adipose derived MSC for the treatment of Osteoarthritis and for Articular Regeneration and using hypoxia-MSC secretome to treat COVID-19 patients6364. Table III summarizes such research where the secretome was used in cell differentiation and cell protection protocols for therapeutic applications in cartilage disorders, Parkinson's disease, ischaemia, cardiotoxicity, acute myocardial infarction, pulmonary artery hypertension, chronic renal disease and skeletal muscle atrophy.

In addition, HSCs were also reported to increase their proliferation rates due to paracrine factors secreted by WJ-hMSCs such as IL1a, IL-6, IL-7, IL-8 cytokines, hyaluronic acid, cell adhesion molecules, cadherins and growth factors [stem cell factor and hepatocyte growth factor (HGF)] which are secreted in high amounts than BM-hMSC3677. Figure presents the gist of the review in a nutshell.

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Figure

Human post-partum tissue mesenchymal stromal cells secrete growth factors, chemokines, cytokines and extracellular vesicles which form the components of its secretome. These secretome components manifest different therapeutic effects. List of advantages and disadvantages of the therapeutic use of human post-partum tissue mesenchymal stromal cell secretome over current cell therapy methods is provided.

Export to PPT

Limitations and the way forward with the hPPT-MSC secretome

Although many potential beneficial therapeutic advantages of the PPT-MSC secretome are apparent, yet the most important issue involved is controlling the MSCs to continuously secrete the required factors in adequate amounts, because the secretion of secretome factors varies due to the state of the cells and the passage number of the cell line7879. In therapeutic procedures where hPPT-MSCs are transplanted, the short half-life of the secreted factors, such as HGF which only remains viable for 3-5 min, raises another concern; administering continuous doses of such secreted stimulants with extremely short half-lives to patients is required for positive therapeutic effects80. Solution to this problem was provided with the use of genome-editing technologies, where the genome of UCB-hMSCs was edited to render the cells continuously secrete HGF, but in an induced manner76. Furthermore, treatment of hMSCs with different cytokine cocktails modified the hMSC secretome by directing hMSCs to secrete specific required factors to render considerable therapeutic effects against, for example, liver inflammation by improving the immunomodulatory capacity81.

Existing literature supporting the presence of the therapeutic effects of EVs is another future aspect of the hMSC secretome to be examined. Purification of these EVs from the secretome is important as a study demonstrated that the purity of EVS secreted by UC-hMSCs is a limiting factor for their immunosuppressive effects82. Provision of solutions to issues related to the therapeutic uses of MSC secretome by means of gene editing, cytokine therapy and extrapurification procedures may however, lead to increased charges of such therapeutics, rendering these unavailable for the developing world. This single reason could mar the beneficial use of the hMSC secretome or its secreted components in therapy; hence, when producing at the commercial scale, it is crucial to adapt to procedures where the current limitations will be overcome in a cost-effective manner. Furthermore, the mRNAs transported by exosomes had been reported to be mediators of cancer communication and also associated with a number of neurodegeneration disorders; hence, further analysis of these derivatives should be done before clinical applications83. Use of standardized herbal extracts as an alternative may be an inexpensive option as a range of herbal extracts have shown proliferation and differentiation abilities when used on SCs7, suggestive of induced changes to the secretome. Countries rich in biodiversity and traditional medicine knowledge can actively contribute to achieve this goal, collaborating with countries which possess cutting edge technological advances, so that ‘induced secretome therapy’ may be affordable globally.

Conclusions

The properties of the hPPT-MSC secretome, provided through a strong and growing body of evidence, bear ample testimony to the potential therapeutic usage of it. However, extensive clinical trials are warranted to reinforce facts and figures obtained by in vitro and in vivo animal studies. Limiting factors of the hPPT-MSC secretome in therapeutic usage can be surmounted by strategies with the help of cutting-edge technologies. However, there is a risk in decreasing the cost-effectiveness of the proposed secretome therapy by the use of such novel technological advances using expensive reagents and equipment; hence, as an alternative, standardized herbal extracts may be used which are naturally available, cheaper, non-toxic and scientifically proven and are effective on hMSCs that will render these cells and their secretome therapeutically feasible, by inducing hMSCs to secrete its components selectively, in a continuous manner. If the current limitations posed may be successfully overcome, the secretome of PPT MSCs inclusive of its EVs may become an effective therapeutic agent which could be used against a wide range of diseases/disorders.