Stem cells are self-renewing cells that have the potential to become multiple different cell types in the body. Embryonic stem cells are isolated from developing embryos. Adult stem cells are isolated from blood or tissues. Induced pluripotent stem (iPS) cells, also abbreviated iPSCs, are a type of stem cell derived from adult cells that are reprogrammed in the lab.
Stem cell culture
Stem cell culture shares many of the same protocols and equipment as standard mammalian cell culture but there are special conditions needed to maintain them in an undifferentiated state such as supplying certain growth factors and growing cells in colonies on a feeder cell layer.While mouse-derived feeder cells are often used in research, human feeder cells (Xeno-free) or feeder-free culture systems are needed for clinical use to avoid patient exposure to animal pathogens.Stem cells are also used to generate cultured meat, a product that also aims to be grown feeder-free and serum-free to avoid relying on animal use. The Cultured Meat Foundation is aiming to create a bank of induceded pluripotent cells (iPS cells) from cow, pig, chicken, tuna, salmon and lobster that are grown serum free and feeder free.
Controlling stem cell differentiation
In development stem cells receive cues from their environment causing them to up regulate and down regulate genes which control stemness and cell differentiation into specialized cell types. In culture, stem cell scientists and engineers seek to program cells in a predictable way by supplying growth factors, physical cues or modifying the expression of genes that control stemness or key cell type specific genes.
- Gene delivery systems can be used to introduce genes that control differentiation
Tracking stem cells and their differentiation in culture
- CRISPR/Cas tools in live cell imaging include dCas9-GFP , CRISPR/MB (dCas9 based) , CLING (dCas9 based) , Cas13a fused with GFP for tracking RNA , CRISPR/Cas9 insertion of MS2 casette for RNA tracking.
Tracking stem cells in living tissues
- Direct Labeling. A labeling agent is introduced into the cells, which is incorporated or attached to the cells prior to transplantation. Radionuclides are used for direct labeling but allow only short-term monitoring because radiodecay and diffusion of signal though cell division and dispersion. Other direct labelling materials include nanoparticles or quantum dots. Depending on the radiotracer, imaging can be performed using single-photon emission computed tomography (SPECT) or positron emission tomography (PET). Magnetic resonance imaging (MRI) can track cells labeled with superparamagnetic iron oxide particles (SPIO).
- Indirect Labeling. Several reporter gene strategies have been used in pre-clinical stem cell transplant studies to study the long-term fate of transplanted cells. A reporter gene may be introduced into cells prior to transplantation that expresses fluorescent proteins such as green fluorescent protein or bioluminescence. The herpes simplex virus type 1 thymidine kinase (HSV1-tk) system phosphorylates an exogenously administered substrate. PET imaging can be used to detect a dopamine 2-like receptor reporter gene which codes for a cell membrane protein that binds an exogenously administered probe. Another reporter gene system uses a thyroid transmembrane protein, sodium-iodide symporter (NIS) which transports iodine into cells in exchange for sodium, which can be imaged with PET or SPECT with radioactive iodine or Technetium Tc-99m pertechnetate. MRI detected reporter genes are generally iron homeostasis proteins, reporter enzymes and reporter genes which generate chemical exchange saturation transfer. Reporter gene systems are not used in humans due to the potential for adverse effects of integration of foreign DNA into human cells. Safe methods for long-term monitoring of transplanted cells in human patients are needed.
Stem cell isolation
- Cell harvesting from blood or tissues or from cell culture
- Pre-plating makes use of the phenomenon that stem cells tend to adhere to culture plates and dishes more than the rest of the cells in a population. Pre-plating has been shown to enrich for mesenchymal stem cells (MSCs) from bone marrow or human adipose-derived stem cells from lipoaspirate from liposuction procedure.
- Density gradient centrifugation. Separation of cells by density gradient requires knowledge of the density of the target cell type. A density gradient is established in a test tube and centrifuged cells accumulate at a position where density of cells matches the density of medium. Ficoll-paque, Percoll and RosetteSep are used as media for stem cell separation by density gradient.
Generating induced pluripotent stem (iPS) cells
Induced Pluripotent Stem Cells, abbreviated as iPS cells or iPSCs, are generated in the laboratory by treating adult differentiated cells from sources such as skin, with factors that reprogram the cells. The pluripotent cells can then be treated with factors that signal the cells to differentiate into another cell type.
Stem cell standards and oversight
- International Society for Cellular Therapy
- International Stem Cell Forum
Stem cell characterization
Stem cell applications in medicine
Tissue engineering is the differentiation of cells into specific cell types using cues like growth factors, physical scaffolds and other 3D cell culture approaches to produce functional replacement tissue or organs. Synthetic biology approaches in tissue engineering are being used for programming cells to self assemble into tissues.
Hematopoietic stem cell transplants are routinely used to treat patients with cancers and other blood and immune system disorders. Hematopoietic cells can come from bone marrow, blood or umbilical cords. Many cell therapies use MSCs (mesenchymal stromal cells/mesenchymal stem cells) or other stem cells which may be effective due to their ability to differentiate into various tissues or due to secretion of paracrine factors such as growth factors, anti-inflammatory or pro-angiogenic factors.Cells may also be used as carriers of therapeutic agents.
Immunotherapy with engineered cells
Cell therapies being used or in development for treating cancer or autoimmune diseases use T cells and regulatory T cells (Tregs) respectively engineered with chimeric antigen receptors (CARs). CAR-T and TCR-T are engineered T cells and CAR-Treg and TCR-Treg therapies are engineered Tregs. FDA-approved CAR-T products are generated from harvesting autologous T cells from patients, followed by gene editing and infusion back into the patients. Allogeneic or universal cell therapy products, also called off-the-shelf cell therapy products would allow a broader implementation of these cell therapies. One method being researched is to derive natural killer (NK) cells from iPSCs and engineer iPSC-derived NK cells to target and kill cancer cell similarly to CAR-T cells.
Platelets carrying immunotherapy agents such as antibodies against immune checkpoint protein PD-1 have been tethered to hematopoietic stem cells to bring this immunotherapy into the bone marrow where it is needed to treat acute myeloid leukemia (AML). The technology has been developed in the lab of Zhen Gu, Founder of Zencapsule and Professor of bioengineering at UCLA Samueli School of Engineering.
Companies developing cell therapies
Bone and cartilage repair
Bone marrow transplants
- Alofisel, MSC product
Graft verus host disease (GvHD)
Cell therapy companies that target multiple diseases
- Athersys developed MultiStem®, a patented, adult-derived “off-the-shelf” stem cell product platform, for multiple diseases including neurological, cardiovascular, and inflammatory and immune disease areas, as well as other indications where there is unmet medical need
- Pluristem Therapeutics produces placenta-derived off-the-shelf products to treat inflammation, ischemia and radiation damage
- Vericel uses autologous adult stem cells from bone marrow to treat cartilage defects and burns
- Orig3n is working in regenerative medicine and also offers DNA testing where people can opt to share genetic information and donate blood for their human cell bank which they use to generate iPS cells
Cells engineered to carry biotherapeutics
While one approach to gene therapy is to deliver genes or gene editing components in vivo, the other approach is to transduce the therapeutic gene or gene editing components into patient-derived cells ex vivo. Then the patient’s gene edited or genetically engineered cells are transplanted back into the patient. Hematopoetic stem cells and iPS cells are often used for this approach.
Disease modeling, drug development and drug testing
Stem cells are used to grow organoids and organ-on-a-chip devices for disease modeling, drug development, drug testing and personized medicine.
Extracellular vesicles derived from stem cells
The secretome is the set of molecules such as proteins, nucleic acids, lipids and extracellular vesicles secreted to the extracellular space. Microvesicles and exosomes are two classes of extracellular vesicles. Exosomes are smaller and are released by endosome fusion with the plasma membrane and microvesicles are shed from the plasma membrane. Exosomes were once thought to be a method for cells to discard unwanted proteins but are now known to have a role in intercellular communication with both neighboring and distant cells in the body.
Extracellular vesicles can be isolated from cell culture media and from body fluids.Since the regenerative effects of MSCs are partly mediated by secreted exosomes which carry proteins, ncRNA, RNA and lipids, MSC exosomes are being explored as a cell-free therapy in regenerative medicine.
Conditioned medium or cell culture medium previously used to grow MSCs and other stem cells could be used to extract therapeutic components. Conditioned medium derived from MSCs has been shown to be as effective as MSCs in many animal models for human disease.
Stem cell therapies aimed at treating injured myocardium in patients that had experienced heart failure were intended to differentiate into cardiomyocytes, engraft and integrate with the host tissue. The stem cells rarely differentiated into heart muscle and integrated into the host tissue, cardiac function was restored, due secretion of factors from the exosomes of the transplanted induce pluripotent cell (iPSC) derived cardiomyocytes.
Aegle Therapeutics - isolates extracellular vesicles from bone marrow derived MSCs to treat dermatological conditions
Exopharm - developing therapies based on exosomes from platelets and stem cells
Regeneus - clinical stage regenerative medicine company using MSCs and their secretions to treat knee osteoarthritis
Cell manufacturing for research and cell therapy
In contrast to drugs, which are chemically defined products that can be replicated, cell therapy and tissue engineering products are defined by their process. In order to standardize cell products the process needs to be well defined. Production may be scaled up so that the cell product is manufactured in one or two facilities and then shipped to locations where they are needed. Alternatively, production may be scaled out to multiple facilities such as dedicated hospital clean rooms, where it is produced in smaller batches on site.
- Good Manufacturing Practice (GMP)
- Cellular Dynamics International manufactures human cells derived from iPS cells
Extracellular vesicle manufacturing
Companies providing supplies for stem cell culture
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