Gamma-delta (γδ) T cells make up on average 4–5% of T cells found in human peripheral blood (1). Two different types predominate; one that is largely found in organs and tissues, and another found in blood. Unlike regular T cells, γδ-T cells function independent of the major histocompatibility complex (MHC) and recognize a wide variety of ligand molecules using multiple T-cell (TCR), innate (e.g., NKG2D), toll-like, and Fc (CD16) receptors. As a result, they are involved in immune responses against both pathogens and tumor cells, while also playing a role in homeostasis, wound healing, and aging.
Both autologous and allogeneic γδ T-cell immunotherapies are in development to treat a range of cancers. Stimulation and activation of the T cells prior to administration is common. While approaches are being pursued that use unmodified γδ T-cells because of their independent ability to identify cancer cells, many candidates in preclinical and clinical development also seek to boost their killing capability via genetic engineering and/or using a two-pronged strategy for increasing the susceptibility of the target cancer cells. Solutions include the use of combinations with chemotherapeutics or antibodies and active tumor modification with gene therapy.
According to William Ho, director, president, CEO, and co-founder of IN8bio, γδ T cells have the potential to address many challenges towards targeting different tumors, and when combined with chemotherapy or other therapies, that potential is magnified. This type of strategy, could, he believes, “make cell therapies a more central feature of the cancer armamentarium and provide potentially more lasting treatments.”
Several advantages vs. other cell therapies
Chimeric antigen receptor (CAR) T-cell therapies have been a fantastic advance in cancer treatment options, says Jeff Boyle, CSO of American Gene Technologies and spinout Addimmune. They are limited, however, by the need to have a differentiated antigen on the cell surface and “take all the breaks off of regular T cells,” which often results in off-target toxicity issues, he adds.
Gamma-delta T cells are not antigen-specific in the same manner, Boyle continues. “They recognize molecules via more of a pattern-recognition type of response that is not peptide-based and tend to be focused on entities arising from infection with fungi and bacteria, or carbohydrate entities,” he observes. That is an advantage because, Boyle explains, many of the same pathways upregulated during fungal and bacterial infections get upregulated in cancer as well. In addition, the broad targeting ability means γδ T-cell immunotherapies could possibly be generic, their MHC independence means there is greater potential for allogeneic therapies with reduced concerns of graft-versus host disease (GvHD) or host-versus-graft (HvG) rejection, and their different mechanism of action suggests they could have safer profiles.
More specifically, according to Ho, γδ T cells are “primordial innate anticancer cells that bridge the innate and adaptive branches of the immune system and present a formidable multi-tiered weapons package to directly recognize and immediately respond to a broad range of cancer-associated stress antigens with direct killing and release of inflammatory cytokines. In addition, γδ T cells can kill or amplify indirect killing mechanisms such as antibody-dependent cellular cytotoxicity through both CD16 and CD3 pathways, which sets up the potential for therapeutic synergy with cancer-target antibodies or bispecific engagers,” he observes.
Normal alpha-beta (αβ) T cells emerge following the initial innate immune response and recognize threat-specific peptides unique to the cancer, but the number and heterogeneity of tumor associated-antigens (TAAs) increases rapidly as the tumor grows and ultimately exceeds their ability to respond, says Ho. With their redundant stress-signaling pathway, γδ-T cells can overcome the complexity of tumor heterogeneity, he notes. Like natural killer (NK) cells, Ho adds, γδ-T cells can kill tumor cells very effectively, but because they are T cells, they can create an adaptive or memory response and are more persistent than NK cells. Furthermore, he points out that because normal cells do not express stress antigens, many of the toxicities of αβ T-cell therapies like cytokine release syndrome (CRS), immune effector cell-associated neurotoxicity syndrome (ICANS) or neurotoxicity, and GvHD may be avoided.
Attractive for allogeneic treatments
Patient-specific, autologous cell therapies have manufacturing and logistics challenges, but do not present concerns of potentially lethal GvHD. Off-the-shelf, allogeneic versions are potentially easier to produce more cost-effectively, would be immediately available, and because they come from healthy donors should be more robust from an activity- or tumor-killing standpoint, Ho comments. With allogeneically delivered αβ-T cells, however, potential lethal immune toxicities such as GvHD must be overcome through significant genetic editing.
The γδ T-cell, MHC-independent mechanism of action may obviate those risks, says Ho. He adds that γδ T-cell therapies have traditionally been known not to initiate GvHD. “Our data has not yet demonstrated a concerning safety signal with either of the autologous or allogeneic delivered products IN8bio is developing, including not having observed any CRS or ICANS to date,” he contends.
Need for activation
There are two important challenges to developing effective γδ T-cell therapies. They derive from the broad-natured activity of γδ-T cells, which is both a strength and a weakness, as they tend not to be as potent as desired at killing tumor cells, according to Boyle. Their killing capability is therefore often increased by increasing the activity of the γδ-T cells, and boosting the susceptibility of tumor cells to attack by these enhanced cells.
Ideally, Boyle observes, stimulation of the γδ-T cells occurs ex vivo prior to administration, which is then achieved in conjunction with another therapeutic agent of some kind (small molecule, antibody, gene therapy), that targets some of the tumor cells, the effect of which then spreads throughout the tumor.
Synthetic cell therapy
Many different types of therapies based on cell types that occur naturally in the body are under development today. Designing optimum immunotherapies based on complex living cells that represent only a fraction of the highly complex immune system has limitations, however (1,2). Researchers do not yet fully understand the operations of different cells, making precise engineering a challenge. Undesirable cellular activities can result in off-target effects, for instance. The differences in patient cells due to genetic and environmental factors can, meanwhile, greatly impact the effectiveness of living cell therapies.
An alternative approach involves bottom-up construction of synthetic cells, such as “cell-like lipid-based compartments in the form of giant unilamellar vesicles,” in a step-by-step manner that allows for incorporation of defined functional capabilities and attributes (2). Such synthetic cells would thus be much simpler and perform only specific functions, potentially with greater potency and without concern of unwanted side effects.
To date most synthetic cells have been used in a laboratory setting for research applications, but some have been designed as potential drug-delivery systems and others that produce drug substances are being investigated as in vivo treatments, including their implantation into tumors as anticancer therapies, their use to promote neuronal development, and a means to deliver insulin in vivo (2).
One recent paper reports the creation of immune-inspired synthetic cells (iiSCs) “that display an antigen-directed target specific cytotoxicity against cancerous cells” by incorporating aspects of NK, T, and B cells and adding a polyethylene glycol coating to prevent interactions with non-target cells (2). As a specific example, the researchers then immobilized immunoglobulins targeting leukemia cells onto the synthetic cells. While the system was built only to demonstrate the technology, the researchers believe that versions designed for in vivo application should be possible to make without too much difficulty given the charge-mediated assembly process employed (2).
Indeed, some believe that synthetic cells might prove to be optimal for the development and production of therapies for individuals or small patient populations, including on-demand, personalized cancer vaccines and drugs to treat rare infections (1). The ability to construct synthetic cells that are stable at room temperature would also facilitate eliminating the need for cold-chain management through their storage, transport, and delivery.
Regulatory clarity is still needed for treatments based on synthetic cells, however (1). While the guidelines for engineered cell therapies, such as CAR T-cell therapies, are applicable to some degree, synthetic cells are different and have unique aspects that will require specific guidance. To realize that goal will require close collaboration between developers of synthetic cell therapies and regulatory bodies such as the US Food and Drug Administration (1). Long-term security risks associated with development of synthetic cell therapies must also be considered and addressed (1).
References
1. Sampson, K.; Sorenson,C.; and Adamala, K. The FDA Needs to Get Ready to Evaluate Synthetic Cells, the Next Generation of Therapeutics. Stat News, July 26, 2022. https://www.statnews.com/2022/07/26/fda-develop-framework-evaluate-synthetic-cells/
2. Bücher et al. Bottom-up Assembly of Target-specific Cytotoxic Synthetic Cells. Biomaterials, 2022 Vol, 285, June, p. 121522. DOI: 10.1016/j.biomaterials.2022.121522
Expansion also necessary
The rarity of γδ-T cells in the immune milieu creates a third important challenge to developing γδ-T-cell therapies, notes Ho. Because they encompass approximately 5% of the total lymphocyte, or white-blood-cell population, obtaining sufficient cells to formulate multiple doses requires significant cell expansion.
Fortunately, though, the ability to grow γδ-T cells in large numbers is easier than growing standard effector T cells, according to Boyle. “Standard T cells once activated immediately begin upregulating all systems and processes, and it is necessary to shut those down in order to maintain effectiveness after longer periods of expansion. That is not an issue with γδ-T cells, which means it is much less difficult to maintain effectiveness during expansion compared to other types of T cells,” he explains.
Developing allogeneic and autologous treatments at IN8bio
IN8bio is a clinical-stage company with two active programs in glioblastoma (GBM) and hematologic malignancies. The GBM program has progressed to a Phase II study, and the company is completing its Phase I study in hematologic malignancies. “Our philosophy is to create a cancer therapy that achieves durability, tolerability, and addresses tumor heterogeneity to achieve the goal of our mission ... Cancer Zero, or the safe elimination of all cancer cells in every patient battling the disease,” Ho remarks.
That mission is being implemented, according to Ho, by coordinating γδ T-cell therapy with other therapies available to patients to ensure that cancer cells are attacked when they are at their lowest in numbers and are the most vulnerable. “With their unique status as cells that sit at the nexus of the adaptive and innate immune systems, γδ T cells address the critical concerns limiting the utility of cell therapies in solid tumors (i.e., durability, addressing tumor heterogeneity, and safety) and have demonstrated less potential for immune toxicities like CRS or ICANS than traditional αβ T-cell therapies,” he comments.
Specifically, IN8bio has developed proprietary technologies for the expansion, activation, and genetic engineering of γδ-T cells. The company has created γδ-T cells that are chemotherapy-resistant for treatment of solid tumors using autologous cells and plans on doing the same with allogeneic cells, according to Ho. For leukemia patients, it has optimized methods for expanding and activating γδ T cells ex vivo from allogeneic donors without additional modification. IN8bio is also working to create non-signaling CAR-T (nsCAR) therapies out of γδ T cells that allow them to distinguish between tumor cells and healthy tissue, thus avoiding many of the on-target, but off-tumor toxicities reported with αβ CAR T-cell therapies.
Another area of development for IN8bio is the creation of induced pluripotent stem cell (iPSC)-derived γδ T cells to allow the potential production of cell banks with multiple billions of cells, Ho says. Finally, the company is also investigating opportunities for combining γδ T cells with other novel therapies to maximize their potential as immunotherapies.
“IN8bio has unique expertise based on the seminal work of our CSO, Dr. Lawrence Lamb, who has studied γδ T cells for nearly three decades. His research has led to our unique capability in isolating, expanding, and activating these cells to maximize their efficacy, as well as modifying them into the variety of modalities possible with γδ T cells,” Ho says. He adds that through optimization efforts, the company has reduced the manufacturing time and cost and limited manufacturing failures for its autologous products.
One specific clinical program mentioned by Ho is IN8bio’s Vd2+ γδ T-cell program. He notes that this particular type of γδ T cell is prone to exhaustion or death during the expansion and engineering process. “Their production at scale requires some finesse, and thus we are the only group to date that has brought genetically modified Vd2+ γδ-T cells to patients in the clinic through the FDA,” he comments. The company also intends to demonstrate the utility and risk: benefit ratio of both its autologous and allogeneic γδ T-cell therapies in its Phase II INB-400 trial.
Leveraging gene therapy at American Gene Technologies
The γδ T-cell therapy program at American Gene Technologies has been reduced in scope for the past 18 months as the company has focused on developing solutions for human immunodeficiency virus (HIV) infections. The company recently announced the spinout of its HIV-related assets to a new company (Addimmune). Once the spinout is completed, American Gene Technologies will be continuing with a different executive team and renewing its focus on γδ T-cell therapy.
American Gene Technology’s approach involves activation of a large number of highly purified γδ T cells ex vivo combined with administration of a gene therapy designed to make the target tumor cells hyper-sensitive to the γδ-T cells. “In vivo tumor targeting using a gene therapy makes it possible to add molecular mechanisms within the tumor that make is susceptible to the specific type of γδ-T cells that we are administering,” Boyle explains.
Gene therapy as a combination treatment was selected, Boyle says, because it creates the opportunity to make one type of γδ T-cell therapy broadly applicable to many different tumor targets. “We can lock down the expansion and activation of the γδ T cells and then use those cells for different cancer treatments by changing the tumor-targeting ability of the gene therapy,” he notes.
In addition, gene therapy provides molecular control of expression within the tumor through selection of different knockdowns, according to Boyle. Use of a small-molecule as an adjunct treatment is problematic, he believes, because these treatments are delivered systemically, which increases the chances for off-target effects. Antibodies as adjuncts, however, are more promising from Boyd’s perspective, particularly as antibody-based technologies continue to advance.
Reference
1. Ling Ma; Yanmin Feng; and Zishan Zhou. A Close Look at Current γδ T-cell Immunotherapy. Front Immunol. 2023; 14: 1140623. DOI: 10.3389/fimmu.2023.1140623.
About the author
Cynthia A. Challener, PhD, is a contributing editor to Pharmaceutical Technology®.
Article details
Pharmaceutical Technology®
Vol. 48, No. 3
March 2024
Pages: 16-18
Citation
When referring to this article, please cite it as Challener, C. A. Development of Gamma-Delta T-Cell Therapies. Pharmaceutical Technology 2024 48 (3).
