What are Mesenchymal Stem Cells?
It is widely accepted that stem cells can be divided broadly into embryonic and non-embryonic stem cells. Embryonic stem cells (ESCs) are derived from the inner cell mass of blastocysts and are pluripotent, meaning they can differentiate into cells of all three germ layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). Conversely, non-embryonic stem cells are found in the extra-embryonic tissues (placenta, umbilical cord blood and amniotic fluid) and in all adult tissues, (i.e. bone marrow, fat, kidney, etc). Human mesenchymal stem cells (hMSCs) are an example of non-embryonic stem cells and were first isolated in the bone marrow and characterized by Friedenstein and his colleagues in 1974 (Amorin, 2014). hMSCs, also called mesenchymal stromal cells, are a subset of non-hematopoietic adult stem cells that originate from the mesoderm (Kim et al, 2013). They are considered to be multipotent; able to self-renew and generate progeny of several distinct cell types. Specifically, they produce cells found in the connective tissues such as cartilage cells (chondrocytes), bone cells (osteoblasts) and fat cells (adipocytes). They are defined as having these properties:
- remaining plastic-adherent under standard culture conditions with spindle cell morphology
- expressing CD105, CD73, and CD90 and failing to express CD45, CD34, CD14 or CD11b, CD79a or CD19, and major histocompatibility complex (MHC) class II molecules
- able to differentiate into osteoblasts, adipocytes, and chondrocytes in vitro [1-challenges/promise allogenic].
Mesenchymal stem cells are most commonly isolated from donor bone marrow (BM) but can also be isolated from adipose tissue, amniotic fluid, endometrium, dental tissues, and umbilical cord blood. hMSCs have some key characteristics that are of interest for developing therapeutics including:
hMSCs exhibit the ability to transdifferentiate – defined as the conversion of one cell type to another – giving rise to cells outside of the mesoderm lineage. It has been documented that MSCs in vitro can transdifferentiate into cells that are challenging to isolate and expand in culture to study, including neuron-like cells (ectoderm), and pancreatic islet-like cells (endoderm).
Migratory and Paracrine Effects
hMSCs have the ability to migrate to damaged tissue sites exhibiting inflammation. They secrete soluble factors (such as cytokines, chemokines and growth factors) necessary for survival and proliferation of adjacent cells in their microenvironment which is important to support tissue regeneration after injury has occurred.
hMSCs can modulate immune response through interaction with a wide range of lymphocytes associated with both the innate and adaptive immune systems (Squillaro, 2016). They are able to suppress the excessive response of T cells, B cells, dendritic cells, macrophages, and natural killer cells through secretion of soluble factors, which has important implications for the use of hMSCs in treatment of autoimmune diseases.
Most importantly, hMSCs express low levels of class I and no class II Human Leukocyte Antigens (HLAs), making them immunoprivileged and able to be used without HLA matching, ideal for allogeneic (where the donor is genetically similar but not identical to recipient) cell transplantation therapies where immune rejection is a risk (Herrmann, 2014).
Mesenchymal Cells in Regenerative Medicine
Their unique characteristics, as described above, have made hMSCs the most commonly used adult stem cells in regenerative medicine. According to the US National Institutes of Health database, 744 hMSC-based clinical trials have been reported worldwide as of June 20, 2017 evaluating their use for a diverse range of illnesses:
- bone and cartilage diseases (i.e. osteogenesis imperfect)
- cardiovascular diseases (i.e. myocardial infarction)
- liver diseases (i.e. cirrhosis)
- autoimmune diseases (i.e. Crohn’s disease, multiple sclerosis)
- cancer (the migratory ability of MSCs to sites of tumorigenesis makes them suitable as vehicles to deliver targeted Gene Therapy)
- graft versus host disease (GvHD)
Graft versus Host Disease (GvHD)
Currently, the most extensively studied therapeutic application of hMSC is for the treatment of GvHD. GvHD is a severe inflammatory condition that may occur following an allogeneic bone marrow transplant (BMT). The immune cells (pro-inflammatory T cells) from the transplant (the graft) regard the recipient (the host) as foreign or ‘non-self’ and mount an attack on the host’s cells resulting in tissue damage in the skin, gut and liver which is often fatal. In the clinical setting, the condition is divided into acute (aGvHD) and chronic (cGvHD) forms.
- The acute form of the disease is observed ≤ 100 days post-transplantion and is a leading cause of transplant-related mortality
- The chronic form of graft-versus-host-disease occurs > 100 days post-transplant
Corticosteroids, such as prednisone, are the standard treatment course, but have a limited success rate of only 30-50%. Nearly 50% of all allogeneic BMT patients develop aGvHD with mortality rates of up to 85%.
The use of hMSCs as a treatment for steroid-resistant aGvHD was first published by Le Blanc et al in 2004. The patient, a 9 year old boy unresponsive to other therapies, showed a dramatic improvement of the condition after infusion with repeated doses of haploidentical (donor has an identical set of HLAs as the recipient) bone marrow-derived MSCs. Subsequently, hMSCs have been used in a number of Phase I and II trials in acute and chronic GvHD trials with success (Hermann, 2014). However, it is a challenge to make comparisons between the studies because of the variable preparation, dosing, and frequency of MSC infusions. Additionally, there are no standardized potency assays to assess the efficacy of these cells in vitro. But in general, hMSC infusions were well tolerated and none of the published reports have shown an adverse safety effect.
First Generation MSC-based Therapeutics
Osiris Therapeutics Inc. (NASDAQ: OSIR) located in Columbia, MD, USA, was the first to develop an ‘off-the-shelf ‘intravenous hMSC product known as Prochymal® for the treatment of aGvHD. It is an FDA-approved allogeneic stem therapy using BM-derived hMSCs from healthy adult donors between 18 and 30 years of age. In May 2012, it became the first ever stem Cell Therapy approved for use in Canada for the management of steroid-refractory pediatric aGvHD. Osiris was acquired by Australian company Mesoblast Limited (NASDAQ: MESO, ASX:MSB) in 2013 and Prochymal® was renamed TEMCELL®. Recently, in February 2016, Mesoblast’s Japanese licensee JCR Pharmaceuticals Co. Ltd. announced that TEMCELL® was approved as the first allogeneic Cell Therapy in Japan for treatment of aGvHD. However, the main drawback in these 1st generation programs is the inherent batch variability from different BM donors, which can affect the quality and efficacy/potency of the subsequent hMSC-based therapeutics. Still, the commercialization and approval of Prochymal™/TEMCELL® as a Cell Therapy represented an important milestone for regenerative medicine. The lack of standardization has been acknowledged by regulatory bodies such as the FDA and others with a vested interest in cell therapies but until recently, it has not been adequately addressed.
Second Generation MSC-based Therapeutics
Australian stem cell and regenerative medicine company, Cynata Therapeutics Limited (ASX: CYP) has made major advances in translational technology to address the issue of standardization by switching from primary BM cells to a replicable, induced pluripotent stem cell (iPSC) source to produce their therapeutic hMSCs. On September 19, 2016, Cynata received approval from the UK Medicines and Healthcare Products Regulatory Agency (MHRA) to proceed with its Phase 1 clinical trial of CYP-001 – Cynata’s Cymerus™ hMSC product – in patients with steroid-refractory GvHD. The trial, entitled “An Open-Label Phase 1 Study to Investigate the Safety and Efficacy of CYP-001 for the Treatment of Adults With Steroid-Resistant Acute Graft Versus Host Disease”, is the world’s first clinical trial using a therapeutic product derived from allogeneic iPSCs and establishes Cynata as a leader in the production of 2nd generation stem cell products. The trial is to be conducted at a number of leading clinical centers in the UK and Australia (NCT02923375).
“We are delighted that the MHRA has approved our clinical trial. Not only does this enable us to start providing our highly promising therapy to patients with a particularly devastating disease, it also provides clear validation of our manufacturing process and preclinical development program, from one of the most highly regarded regulatory authorities worldwide,” said Cynata Vice President of Product Development, Dr Kilian Kelly.
“The treatment of the first patient [May 16, 2017] in this trial marks the beginning of an exciting new chapter in stem cell therapeutics and the future of regenerative medicine,” said Cynata Managing Director and Chief Executive Officer Dr. Ross Macdonald.
A total of 16 patients, divided into Cohort A and B, are expected to participate in the phase 1 trial will receive two infusions of CYP-001, with a week between doses. The main objective of this trial is to assess safety and tolerability, while the secondary objective is to evaluate the efficacy of two infusions of CYP-001 in adults with steroid-resistant GvHD. The primary evaluation period will conclude 100 days after the first dose in each patient.
Cynata mass produces these 2nd generation cell products by establishing rigorous manufacturing and quality control programs to produce a consistent, high quality, economically viable hMSC product, CYP-001. The product is manufactured in a strictly controlled, xeno-free (contains no animal-derived components) process produced at the clinical grade good manufacturing (GMP) facility of Waisman Biomanufacturing in Madison, Wisconsin. Clinical-grade iPSCs sourced from Cellular Dynamics International (CDI, Fujifilm subsidiary company) derived from a single blood donation (one donor) using CDI’s non-viral, non-integrating episomal reprogramming method act as the starting material for this process. “A number of previous studies have demonstrated that MSCs can be an effective treatment for GvHD, but producing consistent MSCs in sufficient numbers for clinical use has been a major challenge until now. The Cymerus™ process offers a solution to this problem, by enabling the large-scale manufacture of a uniform MSC product derived from a single, one-time donor”, said Dr Adrian Bloor, Consultant Haematologist at The Christie Hospital, Manchester, and the UK Chief Investigator for this trial.
The CymerusTM technology is based on discoveries made at the University of Wisconsin-Madison in Dr. James Thomson’s lab (Vodyanik et al, 2010). The iPSCs are directed to the mesodermal lineage through the generation of key precursor cells called mesenchymoangioblast (MCAs) that are then used to manufacture the hMSC therapeutic product CYP-001. Cynata’s approach has made huge inroads to achieving the highest quality product through establishment of uniform manufacturing protocols to meet the strictest criteria for regulatory acceptance; this is a crucial step to furthering the field of regenerative medicine.
Cynata’s CYP-001 clinical trial is now one of two programs worldwide using iPSC technology for human clinical studies. The second study is for the treatment of macular degeneration (AMD) with iPSC-derived retinal pigment epithelial (RPE) cells spearheaded by Japanese researcher Dr. Masayo Takahashi. This is actually a restart of the pioneering clinical trial from 2014 which was the first time autologous iPSCs-derived cells were used in transplantation therapy. Due to some unexpected genetic anomologies identified during the initial trial, Dr. Takahashi and her team redesigned the second study and switched from an autologous to allogeneic approach using cells from an anonymous donor to generate the RPE cells. The first patient was treated with the new protocol on March 28, 2017 (Cyranowski, 2017). In both studies, researchers have been strategic in their choice to pursue allogeneic transplantation. For Cynata, hMSCs have been found to be immunoprivileged with numerous clinical studies showing little to no adverse effect with allogeneic transplantation (BM-derived hMSCs). In the case of the Japanese AMD trial, the eye is well-established site with immune privileged properties conducive for allogeneic transplantation.
The switch to allogeneic iPSC sources to generate therapeutics has the potential to broaden the accessibility of stem cell transplantation to a wider population. In addition, the ability to use ‘off-the-shelf’ cell products makes them available for use immediately and decreases the cost of treatment compared to using the patient’s own cells to generate autologous therapies, which can be time consuming and costly. Still, there is debate in the scientific community over using iPSCs derived from an allogeneic donor because of the potential immune rejection. Progress has been made to address this concern through identification of HLA superdonors, individuals whose HLA profiles make their cells widely compatible, for donation to unrelated patients. Companies worldwide, such as Lonza (Basel, Switzerland) and Cellular Dynamics are generating and banking HLA superdonor iPSC lines using good manufacturing practices (GMP) for therapeutic pursuits. Additionally, Shinya Yamanaka, the pioneer of iPSC technology and a Nobel-prizewinning stem-cell scientist at Kyoto University, is establishing his own cell bank, the iPS Cell Stock for Regenerative Medicine, which he hopes by March 2018, to have HLA-characterized cell lines that match 30–50% of Japan’s population (Cyranowski, 2017).
The results of these two closely watched allogeneic iPSC clinical trials could provide much needed ’proof of concept’ information for these next-generation therapies. Researchers are cautiously optimistic for what they symbolize for the future of Cell Therapy. In the meantime, Cynata is forging ahead to test Cymerus™ hMSCs for other conditions such as asthma, cardiovascular diseases and cancer that would benefit from novel cellular therapies.