Cell therapy was first introduced by Swiss physician Dr. Paul Niehans in 1931. Dr. Niehans’s pioneering work began when he treated a critically ill patient suffering from refractory tetanus by injecting cells from calf parathyroid glands. Remarkably, this intervention not only saved the patient’s life but also resulted in long-term health improvements, demonstrating the potential of cellular therapies.

Following this initial success, Dr. Niehans expanded his methods to include the careful injection of various organ cells harvested from young animals, leading to further positive outcomes in severely ill patients. By 1937, he had begun using tissue cultures, solidifying his role as a pioneer in this field. His innovative approach laid the groundwork for what would become a significant therapeutic modality in modern medicine².

Cell therapy is a promising tool for improving organ transplantation outcomes and treating a range of diseases. Unlike conventional treatments that focus on pain relief or surgery, it harnesses the body’s natural healing abilities. Its significance lies in treating conditions such as blood cancer, neurological disorders, and various types of tissue damage. By using stem cells, immune cells, and other specialized cells, researchers are exploring new avenues for diseases once considered untreatable.

Types of transplantation in cell therapy

In cell therapy, transplantation involves transferring cells into a patient to restore function, repair damaged tissues, or modulate the immune system. The primary types of transplantation in cell therapy include:

Autologous transplantation

In autologous transplantation, cells are taken from the patient’s tissues, usually from blood and bone marrow. These cells are extracted, treated, and then reinfused after a conditioning regimen. This method eliminates the possibility of immunological rejection because such cells are not considered foreign bodies. Autologous stem cell transplants are widely used in the treatment of blood malignancies such as leukemia and lymphoma and some autoimmune diseases. Additionally, they are also being explored as treatments for conditions that require tissue regeneration, such as cartilage and muscle repair.

Allogeneic transplantation

In allogeneic transplantation, cells are taken from a donor who may be genetically related or unrelated to the patient. To ensure compatibility between the donor’s cells and the patient’s body, a process known as human leukocyte antigen (HLA) tissue typing is performed³. This test helps to ideincrease most compatible donor, as a close match between the donor’s and patient’s HLA markers increases the likelihood of transplant success and reduces the risk of complications.

Allogeneic stem cell transplants are most used to treat various types of blood cancers, including leukemia, lymphoma, and multiple myeloma, as well as bone marrow disorders like aplastic anemia and myelodysplastic syndromes.
In allogeneic HSCT, careful attention must be paid to the compatibility of HLA loci between the donor and recipient⁴. When compatibility is not ideal, complications such as graft rejection or graft-versus-host disease (GVHD) can occur.

GVHD is a condition where the transplanted immune cells from the donor attack the recipient’s tissues. In cases where no compatible family member is available, a matched unrelated donor (MUD) may be considered. However, MUD transplants tend to carry a higher risk of GVHD due to potential mismatches, making the procedure more challenging and often riskier for the recipient.

Both autologous and allogeneic transplants have their unique challenges and advantages, but both have made significant contributions to the treatment of diseases and the advancement of cell therapy.

Types of cell therapy

The therapeutic use of cells can be broadly categorized based on the origin of the cells (autologous or allogeneic) and the specific condition being treated⁵. The strategies used in cell therapy vary depending on the type of cells employed in cell therapy, the condition targeted, and the intended therapeutic effect. The primary types of cell therapy include:

Stem cell therapy

Stem cells are classified based on their differentiation potential, which refers to their ability to develop into various cell types. Understanding these varying potentials is vital for advancing regenerative medicine and developing targeted therapies for various diseases. The primary categories include:

Totipotent stem cells: These cells can differentiate into all cell types necessary for the development of an entire organism, including both embryonic and extra-embryonic tissues. An example is the zygote formed at fertilization.

Pluripotent stem cells: Derived from totipotent cells, pluripotent stem cells can give rise to nearly all cell types from the three germ layers, the endoderm, mesoderm, and ectoderm, but not extra-embryonic tissues like the placenta. Embryonic stem cells are a prime example.

Multipotent stem cells: These cells can differentiate into multiple, but limited, cell types within a specific lineage. For example, hematopoietic stem cells can develop into various blood cells, including lymphocytes and monocytes.

Oligopotent stem cells: With a more restricted differentiation potential than multipotent cells, oligopotent stem cells can give rise to a few closely related cell types. An example includes myeloid stem cells, which can differentiate into various types of blood cells.

Unipotent stem cells: These cells can differentiate into only one specific cell type but retain the property of self-renewal.

Stem cell therapy or stem cell treatment is used to treat or manage a variety of medical problems. Stem cells are undifferentiated cells that can differentiate into several cell types in the body, including muscle, bone, and nerve cells, depending on the potency of the cell¹. This unique property makes stem cells ideal candidates for regenerative therapies, as they have the potential to restore damaged tissues and organs by producing cells that are needed for healing.

The most common therapies using stem cells include bone marrow, kidney, and liver transplants, all of which involve transplanting stem cells to replace damaged or diseased tissue. Blood-forming stem cells from bone marrow, known as hematopoietic stem cells (HSCs), were the first type of stem cells to be discovered and utilized in clinical trials⁶. These cells have proven to be effective in treating a variety of blood disorders, such as leukemia, lymphoma, and sickle cell anemia.

Types of stem cells used in therapy

Cell therapy uses a variety of stem cells, including embryonic stem cells (ESCs), induced pluripotent stem cells, neural stem cells, and adult stem cells (ASCs). Each type of stem cell possesses distinct characteristics that make it suitable for different applications^1,7.

ESCs are pluripotent cells that are derived from the inner cell mass of the embryo and can develop into any tissue in the body, making them highly versatile and suitable for a wide range of applications. However, the use of these cells in therapies has raised significant ethical discussion because an embryo is destroyed during the extraction process. As a result, these cells have not undergone clinical testing in the United States, and their usage in research is subject to stringent regulatory and ethical considerations. Nevertheless, they are being investigated as the basis for possible treatments for diabetes and Parkinson’s disease.

ASCs are undifferentiated cells found in various tissues throughout the adult body. Unlike ESCs, which can develop into any cell type, ASCs are typically multipotent, meaning they are restricted to differentiating into cell types related to the tissue from which they were derived.

ASCs are used in targeted regeneration therapies that usually target more specific tissue types. For example, hematopoietic stem cells (HSCs) from bone marrow are used to treat blood disorders. In contrast, mesenchymal stem cells (MSCs) from tissues such as adipose tissue and bone marrow can differentiate into bone, cartilage, and fat cells.

ASCs offer various advantages over embryonic stem cells. ASCs exhibit less differentiation potential. However, they are easier to obtain and pose fewer ethical concerns, making them more readily available for clinical applications in treating diseases within their tissue of origin.

For example, MSCs possess immunomodulatory properties, multipotency, and rapid proliferation, making them ideal for various treatments, such as immune modulation, bone and cartilage repair, myocardium regeneration, and Hurler syndrome, a disorder affecting the skeleton and nervous system. Additionally, researchers have explored MSCs for treating osteogenesis imperfecta (OI).

Adult mammalian neural stem cells possess unique characteristics, including their ability to differentiate into various neural cell types, self-renew, and enter a quiescent state⁸. These stem cells are primarily located in specific regions of the brain known as niches, such as the subventricular zone (SVZ) and the subgranular zone (SGZ) of the hippocampus. The SVZ is positioned along the ependymal cell layer, separating the ventricular area from the subventricular zone.

There are various sources of neural stem cells, including human embryonic stem cells, fetal brain-derived neural stem cells, human induced pluripotent stem cells (iPSCs), and astrocytes that have been directly reprogrammed into neural stem cells. Stem cell research has become an essential tool for both scientific exploration and therapeutic development. iPSCs hold significant potential for studying human neurons, enabling the generation of specific neuronal populations for the investigation of neurodevelopmental and neurodegenerative diseases, as well as ischemic injuries. Researchers use iPSCs to study conditions like Alzheimer’s disease, Parkinson’s disease, and genetic disorders, enabling the development of disease-specific cell models for testing potential treatments.

iPSCs represent another significant advancement in stem cell therapy. iPSCs are adult cells, usually skin or blood cells, that have been reprogrammed to resemble embryonic stem cells. This reprogramming is accomplished by inserting specific genes into adult cells, which effectively reset them to a pluripotent state. iPSCs possess the ability to differentiate into various cell types, similar to embryonic stem cells (ESCs), but without the ethical concerns associated with using embryonic cells.

Generating iPSCs from a patient's own cells holds significant promise for personalized medicine by potentially reducing the risk of immune rejection, making them suitable for autologous therapies. Moreover, iPSCs have numerous applications, including drug testing, tissue regeneration, and disease modeling. However, it is not possible to get iPSCs to differentiate into any type of cell yet, and studies have indicated that iPSCs may exhibit genetic and epigenetic variations, which could impact their utility in clinical applications,

Beyond stem cells, cell therapy is also advancing with differentiated cells that have minimal or no proliferative capacity but are specialized for specific functions. Examples include cardiomyocytes, which support heart function, and insulin-producing islet cells used in diabetes treatment.

Cardiomyocytes are being investigated for their ability to repair damaged heart tissue, particularly in patients who have suffered heart attacks or other forms of cardiovascular damage⁹. Similarly, islet cells, which produce insulin in the pancreas, are being explored as potential treatments for diabetes, aiming to restore normal insulin regulation in diabetic patients¹⁰.

Cell-based immunotherapy

Cell-based immunotherapy involves using or modifying immune cells, such as T cells, natural killer (NK) cells, or dendritic cells, to target and destroy cancer cells more effectively. Chimeric antigen receptor-T (CAR-T) cell treatment uses genetically modified T cells to target cancer cells, whereas NK cell therapy employs natural killer cells to destroy malignancies. Dendritic cell treatment improves immune detection of cancers. These therapies have demonstrated encouraging results in terms of enhancing cancer treatment outcomes by increasing immune responses¹¹.

CAR-T cell therapy

CAR-T is an immunomodulatory therapy. It genetically changes T lymphocytes, allowing them to target and kill cancer cells¹². T cells are extracted from the patient’s blood and genetically modified in the laboratory to express a chimeric antigen receptor (CAR). The CAR helps the T cells attach to a specific cancer cell antigen. Such treatment has proven to be highly beneficial in a variety of blood cancers, including acute lymphoblastic leukemia and non-Hodgkin lymphoma. CAR-T cell therapy is also called cell-based gene therapy because it involves altering the genes inside T cells to help them attack cancer.

NK cell therapy

Natural killer (NK) cell therapy is a method that uses NK (lymphocyte) cells, a subset of the body’s immune system that fights cancer¹³. As a result, they may identify and kill tumor cells without the need for antigen presentation, making them a potentially helpful tool in the fight against cancer immunotherapy. Cytokine-induced killer cell¹⁴ (CIK) and lymphokine-activated killer cell¹⁵ (LAK) therapies consist of a mix of immune cells with features of natural killer T (NKT) cells or NK cells that can target and eliminate cancer cells.

Dendritic cell therapy

Dendritic cell therapy enhances antigen presentation to the immune system by ex vivo manipulation of dendritic cells¹⁶. Patient-derived dendritic cells are isolated from peripheral blood, cultured in vitro, and pulsed with tumor-specific antigens. These antigen-loaded dendritic cells are then reinfused into the patient, stimulating a robust immune response against cancer cells. Dendritic cell therapy has been tested in clinical trials for a variety of cancers, including melanoma, prostate cancer, and lung cancer.

While the approach has shown promise in some instances, the therapy is still being refined to increase its effectiveness and broad applicability. Dendritic cell therapy can be particularly valuable in cancers where immune evasion mechanisms are at play, helping the body recognize and target tumors that might otherwise remain undetected.

Clinical applications of cell therapy

Clinical applications of cell therapy have expanded significantly, showcasing its potential across various medical fields⁷. Here are some key areas where cell therapy is making a substantial impact.

Neurodegenerative disorders: Possible applications in Parkinson’s and Alzheimer’s disease

Cell therapy has the potential to treat neurological disorders, such as Parkinson’s and Alzheimer’s disease¹⁷. Stem cells may assist in rebuilding damaged neurons, delaying disease development, and restoring lost functions. Cell-based therapies seek to enhance motor abilities, cognition, and general quality of life by replacing or repairing damaged brain cells, providing new hope for individuals suffering from these debilitating disorders. Research is ongoing to explore the efficacy of various stem cell types in regenerating neural tissues and mitigating the symptoms associated with neurodegenerative diseases.

Cardiovascular diseases

Stem cell-based therapies are favorable treatments for cardiovascular disorders because they repair cardiac tissue damaged by infarctions, such as heart attacks¹⁸. These therapies can enhance heart function by promoting tissue regeneration and improving blood flow to affected areas. In conditions like heart failure, where the heart’s ability to pump blood is compromised, mesenchymal stem cells can help restore functionality by regenerating cardiac tissue and improving the structural integrity of the heart.

Clinical trials are investigating the use of different stem cell sources, including MSCs and cardiac progenitor cells, to determine their effectiveness in treating heart failure and other cardiovascular conditions¹⁹. Some studies have focused on cell therapy strategies that aim not only to replace damaged cells but also to modulate the heart’s microenvironment, promoting angiogenesis and reducing inflammation^20,21.

While these therapies hold great promise, their application is not without challenges, including issues with cell survival, integration into the heart tissue, and long-term safety. Nonetheless, the potential of stem cell therapies to treat cardiovascular diseases, especially those related to atherosclerosis and heart failure, offers new hope for patients with limited treatment options, advancing the future of regenerative medicine in cardiology²².

Autoimmune disorders

Stem cell therapy has the potential to treat autoimmune disorders such as multiple sclerosis and lupus23. In these conditions, the immune system mistakenly attacks healthy tissues, leading to inflammation and damage. Stem cells can help regulate the immune response, replace damaged tissues, and promote healing. Therapies aim to reduce inflammation, restore normal immune function, and improve patient outcomes.

In MS, for instance, stem cells can aid in repairing myelin damage, restoring neuronal function, and stimulating endogenous repair mechanisms. This approach can potentially reverse or halt the neurodegenerative processes that current immunosuppressive treatments struggle to address²⁴.

Similarly, in SLE, stem cell therapies, including hematopoietic stem cell transplantation (HSCT) and mesenchymal stem cell transplantation (MSCT), have shown promise in resetting the immune system and reducing systemic inflammation²⁵. These therapies may help to overcome the limitations of traditional immunosuppressive treatments, which often fail to prevent disease relapse and may lead to severe side effects.

Clinical studies are underway to assess the safety and efficacy of various stem cell approaches in managing autoimmune diseases, providing new avenues for treatment.

Orthopedics and tissue engineering

In orthopedics, stem cell therapy is being applied to treat musculoskeletal injuries, including cartilage damage and bone fractures²⁶. These therapies offer a less invasive alternative to traditional surgical interventions by promoting tissue regeneration and healing through the application of stem cells directly at the site of injury.

One of the most notable applications of stem cell therapy is in the treatment of osteoarthritis (OA), a degenerative joint disease that primarily affects cartilage²⁷. Current treatments for OA are primarily focused on symptom management, such as pain relief, and fail to address the underlying degeneration of joint tissues. Stem cell therapy, particularly using MSCs, has emerged as a potential solution to repair and regenerate damaged cartilage, offering a regenerative alternative to these symptomatic treatments.

Research is focused on optimizing the protocols for harvesting, processing, and administering stem cells to maximize their regenerative potential. By promoting tissue repair and reducing inflammation, stem cell therapy aims to slow or even reverse the progression of conditions like OA, as well as aid in the healing of sports-related injuries, fractures, and tendon damage²⁷.

Clinical trials are ongoing to evaluate the effectiveness and safety of these treatments, with the goal of improving long-term outcomes and offering patients more effective, non-invasive solutions for musculoskeletal injuries²⁸.

Corneal endothelial disease

Cell therapy offers a hopeful solution for treating corneal endothelial diseases, which are often managed by endothelial keratoplasty. Recent research has highlighted the successful use of stem cells and cadaveric corneal endothelial cells (CECs) to generate viable CEC populations for intracameral injection. Human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have also shown potential in restoring corneal function.

Additionally, studies on corneal endothelial progenitors, the viability of cryopreserved cells, and the use of non-cultured cells offer hope for treating corneal decompensation more effectively. The growing body of research on CEC physiology and the development of efficient cultivation and delivery methods have contributed to significant progress in this field²⁹. Continued advances in cell therapy for corneal diseases may provide a viable solution to corneal blindness worldwide.

Cell therapy systems and processing

Cell therapy systems use technologies to promote cell proliferation, isolation, differentiation, and transplantation. Examples of such devices include bioreactors, which provide the controlled environment necessary for cell multiplication, centrifuges for separating specific cell parts, and genetic modification equipment for modifications of cells, such as in the case of CAR-T cell therapy.

In the processing of cell therapies, methods to maintain cell viability, purity, and differentiation are essential to their success. Cell purification processes eliminate unwanted cell types that could affect the efficacy or safety of the treatment. Likewise, cell culture systems provide the right conditions for cells to grow and proliferate, ensuring the production of adequate cell numbers. Rigorous quality control is also paramount to assessing the potency and functionality of cells before they are used in therapy.

Manufacturing and scaling challenges in cell therapy

Manufacturing and scale-up challenges in cell therapy include maintaining cell potency during production, ensuring consistent quality, and overcoming difficulties in large-scale cell isolation, expansion, and modification³⁰.

Another challenging aspect of scaling up cell therapy manufacturing is ensuring that cells retain their therapeutic properties through each stage of production. Complex regulatory requirements and the need for cost-effective production methods also pose significant obstacles to widespread clinical adoption and commercialization.

The isolation and purification processes must be rigorous to ensure that only the best cells are used in therapy. These techniques help to eliminate unwanted cell types, such as those that may carry disease or that could hinder the immune response. Several methods, such as magnetic bead separation, flow cytometry, and density gradient centrifugation, are commonly employed to achieve high levels of cell purity. The precision of these techniques is essential for achieving the desired therapeutic outcomes.

Once isolated, cells must be expanded to generate sufficient quantities for therapeutic use. Cell expansion involves growing the cells in a controlled culture system where factors such as nutrient supply, oxygen levels, and temperature are optimized to promote cell division and growth. The expansion process must be carefully monitored to ensure that cells retain their functional properties and potency while proliferating. This requires precise control over the culture environment and a careful balance of growth factors. For stem cells or immune cells such as T cells, this is particularly important, as the cells must maintain their differentiation potential or functionality to be effective in therapy.

Techniques such as gene editing (eg, CRISPR/Cas9) and viral vector transduction require precise control and optimization. Scaling these processes necessitates advanced bioreactor systems and automation to maintain product quality and reduce contamination risks³¹.

Effective and safe cell therapy requires rigorous quality control and safety procedures due to its complexity. Ensuring that cells meet required potency, purity, and safety standards is important to prevent adverse effects, such as immune reactions, graft-versus-host disease (GVHD), or infections. Rigorous testing is performed at multiple stages of production to detect any potential contamination, confirm viability, and ensure the functional integrity of the cells. For example, cell cultures are routinely tested for microbial contamination and functional assays to assess whether the cells retain their therapeutic capabilities after expansion or modification. Additionally, sterility is essential in avoiding infections, as cell-based therapies involve introducing live cells directly into patients’ bodies.

Risks, challenges, and considerations

Like all medical treatments, cell therapy carries the risk of potential side effects and complications. These can range from mild, short-term issues to more severe reactions³².

• Common short-term side effects: Fatigue, headache, chills, nausea, and low-grade fever are among the most common short-term side effects of stem cell therapy.
• Immune reactions: One significant concern is the potential for immune reactions. The body’s immune system may recognize the introduced cells as foreign, triggering an inflammatory response or rejection.
• Cytokine release syndrome (CRS): CAR T-cell therapy can induce cytokine release syndrome (CRS), where the modified T-cells release large amounts of cytokines, leading to fever, low blood pressure, and organ dysfunction.
• Neurotoxicity: Some patients may experience neurotoxicity after receiving CAR-T cell therapy. This condition affects the central nervous system and can cause confusion, seizures, and difficulty speaking or walking.
• Other side effects and complications: Other potential side effects include allergic reactions, abnormal mineral levels, a weakened immune system, infections, low blood cell counts, increased risk of blood cancer, thromboembolism, fibrosis, and tumor formation.

Future perspectives on cell therapy

The future of cell therapy is favorable, with advances in gene editing, personalized medicine, and improved cell culture technologies. These innovations aim to enhance the precision, effectiveness, and scalability of treatments. As research progresses, cell therapies are expected to become more widely adopted, offering personalized, regenerative solutions for various diseases³².

Advances in gene editing technologies, such as CRISPR-Cas9, allow for precise alterations to such cells, making cell therapies more targeted and effective. Improvements in cell culture technologies and bioreactors may allow for more effective production of therapeutic cells.

Acellular therapies and extracellular vesicles

Acellular therapies, such as those utilizing extracellular vesicles (EVs), are gaining attention for their potential to mediate tissue repair and regeneration without the need for live cells³³. EVs, which are derived from stem cells, can transfer proteins, lipids, and nucleic acids to target tissues, thereby influencing cellular behavior and promoting regeneration. These therapies are particularly hopeful for conditions involving tissue damage or degenerative diseases. The future of cell therapy will likely see more integration of acellular therapies to complement or even replace traditional stem cell-based approaches in certain contexts.

FAQs

How is cell therapy different from gene therapy?

Cell therapy involves transplanting or modifying live cells to repair or replace damaged tissues, while gene therapy focuses on altering genetic material to treat diseases at their root cause. Cell therapy works by introducing healthy or engineered cells, whereas gene therapy modifies DNA to correct faulty genes. Some treatments, like CAR-T cell therapy, combine both approaches by genetically modifying a patient’s immune cells to fight diseases like cancer.

What cancers can be treated with CAR T cell therapy?

CAR T-cell therapy is an FDA-approved treatment for a variety of cancers, including34:

• B-cell acute lymphoblastic leukemia (ALL)
• Diffuse large B-cell lymphoma (DLBCL)
• Follicular lymphoma
• High-grade B-cell lymphoma (HGBL)
• Mantle cell lymphoma
• Multiple myeloma
• Primary mediastinal large B-cell lymphoma (PMBCL)

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