Cell therapies in the clinic

Abstract Cell therapies have emerged as a promising therapeutic modality with the potential to treat and even cure a diverse array of diseases. Cell therapies offer unique clinical and therapeutic advantages over conventional small molecules and the growing number of biologics. Particularly, living cells can simultaneously and dynamically perform complex biological functions in ways that conventional drugs cannot; cell therapies have expanded the spectrum of available therapeutic options to include key cellular functions and processes. As such, cell therapies are currently one of the most investigated therapeutic modalities in both preclinical and clinical settings, with many products having been approved and many more under active clinical investigation. Here, we highlight the diversity and key advantages of cell therapies and discuss their current clinical advances. In particular, we review 28 globally approved cell therapy products and their clinical use. We also analyze >1700 current active clinical trials of cell therapies, with an emphasis on discussing their therapeutic applications. Finally, we critically discuss the major biological, manufacturing, and regulatory challenges associated with the clinical translation of cell therapies.


| INTRODUCTION
Cell therapies represent a major frontier and paradigm shift in biotechnology. In contrast to conventional therapeutic modalities, cell therapies are living and can dynamically respond to biological cues to attack malignancies, regenerate tissues, restore impaired or lost biological functions, or otherwise augment the body's own capability to fight disease (e.g., vaccination, immunomodulation). 1 Cell therapies hold exceptional promise particularly because cells can function in ways that conventional small molecules and biologics cannot.
Uniquely, living cells can simultaneously respond to both systemic and local chemical, physical, and biological cues, readily breach biological barriers, 1 molecularly target and interact with specific cell types and tissues, 2 and serve as a platform for additional therapeutic functions (e.g., cellular hitchhiking, genetic engineering). [3][4][5] In this review, we have identified 28 cell therapy products approved for clinical use and Ninad Kumbhojkar and Neha Kapate contributed equally to this study. 1705 active clinical trials employing cells for therapeutic purposes.
We provide a snapshot of the clinical landscape of cell therapies by: (i) highlighting these approved products; (ii) summarizing and reviewing these current clinical trials based on their cell type, indication, source, and phase; and (iii) discussing the challenges associated with clinical translation. For mammalian cell-based therapies, we restricted our analysis to applications where cells are administered as a single cell-suspension (i.e., exclusion of tissue scaffolds and whole blood transplants). Our discussion is mainly focused on blood cells

(T cells, natural killer [NK] cells, red blood cells [RBCs], dendritic cells
[DCs], mononuclear cells, and platelets) and stem cells. We also provide an update on the clinical status of microbe therapeutics, which have recently emerged as a promising class of cell therapies for the treatment of infections and cancer.

| CLINICAL LANDSCAPE
The global market for cell therapy is predominantly shared by stem cells and tissue-specific cells (e.g., skin cells, chondrocytes), followed by blood cells. [6][7][8] Current approved stem-cell therapies include hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), and to a lesser extent limbal stem cells (LSCs). HSC products are predominantly approved for the treatment of blood disorders. MSC therapies are indicated for a broad variety of diseases, including cardiovascular diseases, graft versus host diseases (GvHD), degenerative disorders, and inflammatory bowel diseases. The single LSC product is approved for LSC deficiency. Distinct from stem cell products, terminally differentiated tissue-specific cells are mainly used for regenerative medicine and tissue engineering applications, such as autologous skin cells (i.e., keratinocytes, fibroblasts, and melanocytes) for the treatment of thermal burns, 9 bi-layers of living cellular skin substitute for venous leg ulcers and diabetic foot ulcers, 10 and autologous chondrocyte scaffolds for repair of cartilage defects. 11 These tissue-specific cell therapies are beyond the scope of this review because they are mostly applied as tissue scaffolds instead of as singlecell suspension and have been extensively reviewed elsewhere. [12][13][14] The third group of cell therapies consist of blood cells, including leukocytes, RBCs, and platelets; however, only T cells and DCs have been approved as therapeutic products in the market to date. Most approved T cell products are chimeric antigen receptor (CAR)-T therapies for hematologic malignancies, whereas DC products are used as vaccines for solid cancers. We should also note that RBCs and platelets, while not associated with a specific product, are widely used in clinical settings for blood transfusions. 15 In addition, the cell source of these approved products can be originated either from the patients themselves (autologous) or from the other donors (allogeneic).
Although stem cells and tissue-specific cells account for the vast majority of approved cell therapies in the current market, blood cells have emerged as the dominant cell type that is being developed and evaluated in clinical trials. Just 5 years ago, the number of trials for MSCs alone was greater than the number of trials for all lymphocytes and DCs combined. 14 Currently, T-cell trials individually outnumber all stem cell trials, and far exceed those for tissue-specific cells. This ongoing shift is driven primarily by the recent clinical success of CAR-T therapy, which is in turn a product of major breakthroughs in our understanding of how immune modulatory approaches can be used to treat disease. [16][17][18][19] In light of this trend, we collected and analyzed clinical trials that use blood cells, with additional focus on stem cells delivered as single-cell suspensions, and microbes (including non-single-cell suspension dosage forms), which have recently emerged as promising agents for the treatment of infections and cancer. Specifically, we identified the trials on clinicaltrials.gov by searching for each cell type (Figure 1) with the following key words (listed in parentheses) in the "Intervention/treatment" category: T cells ("T cell"; system also automatically searched for "T lymphocyte"), stem cells ("stem cell"; system also automatically searched for "progenitor cell"), natural killer cells ("natural killer," "NK"), dendritic cells ("dendritic cell," "DC"; system also automatically searched for "antigen presenting cell" and "cellular"), monocytes ("monocyte"; system also automatically searched for "monocytic"), macrophages ("macrophage"), bone marrow-derived mononuclear cells ("bone marrowderived mononuclear cell"), peripheral blood mononuclear cells ("peripheral blood mononuclear cell"; system also automatically searched for "peripheral blood," "blood," "whole blood"), red blood cells ("red blood cell"; system also automatically searched for "erythrocytes," "red cells," "whole blood," "RBC count," and "blood corpuscles"), platelets ("platelet"; system also automatically searched for "thrombocyte"), and microbes ("live biotherapeutic," "bacteria," "consortia"). In the "Status" category under "Recruitment," we selected trials with statuses of not yet recruiting, recruiting, enrolling by invitation, and active/not recruiting. The collected data capture the clinical landscape as of August 2020. We then manually filtered the trials to exclude entries that mentioned the cell types of interest but did not use them as therapeutic interventions. Finally, we excluded long-term follow-up studies that did not involve re-administration of the therapy. A list of abbreviations used through this manuscript is shown in Table S1.
F I G U R E 1 Various types of cell therapies in clinical trials. T cells dominate the current clinical studies of cell therapies, followed by stem cells, dendritic cells, natural killer cells, microbes, red blood cells, mononuclear cells, and platelets A total of 1705 unique, active cell therapy clinical trials have been identified and categorized according to cell type, general indication, trial phase, and cell source ( Figure 2). Among only leukocytes, T cells account for the largest portion of all current trials (767/1705, 45%), followed by DCs (136/1705, 8%), NK cells (116/1705, 7%), and the remaining mononuclear cells (27/1705, 2%). It is unsurprising that the main indication of T cells, DCs, and NK cells is cancer (85% in T cells; 93% in DCs; 95% in NK cells), as they play major roles in anti-cancer immunity. T cells are adaptive immune cells capable of directly eliminating mutated or infected host cells, activating other immune cells, and producing cytokines to regulate immune responses. 20 NK cells are innate immune cells that destroy tumor cells and virally infected cells via release of lytic molecules from granules and rapid production of pro-inflammatory cytokines. 21 DCs are professional antigen-presenting cells (APCs) that regulate adaptive immune cells by delivering antigens to draining lymph nodes and presenting them to cytotoxic and helper T cells. 22 In the case of cancer treatment, T and NK cells are employed as cytotoxic agents, while DCs primarily serve as cancer vaccines. From the perspective of cell source, autologous cells are mainly used in T cell (74%) and DC (87%) therapy, as allogeneic cells increase the risk of allograft rejection (recipient cells against donor cells) or, more considerably, GvHD (donor cells against recipient cells). 23 F I G U R E 2 Current landscape of cell therapies in clinical trials. In this review, all clinical trials that include blood cells and stem cells delivered as a suspension were cataloged. Trials using microbes (delivered via various routes and dosage forms) were also included, as they represent an emerging class of therapies for similar applications. The relevant cell types include T cells, NK cells, mononuclear cells, DCs, RBCs, platelets, stem cells, and microbes. Tissue-specific cells were excluded from the analysis. The total number of trials identified for each cell type is displayed in the figure, however the sum of these trials for all cell types (1760) exceeds the total number of analyzed trials (1705) because some trials use two or more cell therapies in combination. For phase classification, dual-phase trials (e.g., Phase 1/2) were counted as both Phase 1 and 2. Eleven broad indications were identified for the purpose of trial classification (i.e., cancer, infectious diseases, autoimmune diseases, nonautoimmune inflammatory diseases, cardiovascular diseases, transplant-related diseases, trauma, blood disorders, degenerative diseases, metabolic disorders, etc.), with relevant abbreviations listed in the box at the bottom of the figure. Because some trials are used to treat more than one of these conditions, the total number of indications used to generate each pie chart exceeds the total number of trials for each cell type While the aforementioned leukocytes are mainly indicated for the treatment of cancer, the remaining mononuclear cells are used mostly for cardiovascular diseases (39%) and cancer (29%). For the purpose of this review, we refer to mononuclear cells as belonging to one of the following cell populations: monocytes, macrophages, bone marrow-derived mononuclear cells (BMMCs), or peripheral blood mononuclear cells (PBMCs). Monocytes are circulatory cells of the innate immune system that extravasate into tissue in response to inflammation, infection, or injury. 24 Once in the tissue they terminally differentiate into macrophages, which are tissue-resident innate immune cells that (i) phagocytose dead cells, debris, and foreign materials/pathogens, (ii) modulate innate immune responses, and (iii) maintain homeostatic growth, repair, and metabolism. 25  The remaining blood cells, RBCs and platelets, are used as cell therapies for treatment of blood disorders and in trauma care via blood transfusions, and account for 2% (39/1705) and <0.4% (7/ 1705) of all current cell therapy trials, respectively ( Figure 2). Typically, they are used to replenish lost or dysfunctional cells to maintain homeostasis in the body. The RBC is a critical transporter of oxygen and nutrients to tissues as well as an inter-organ communicator, with additional roles in the regulation of pH, redox homeostasis, and molecular metabolism. 27 Hence, loss of RBC integrity and/or number can lead to severe pathologies and heighten the incidence of vascular disease. Similarly, the platelet serves as a key element in blood vessels by regulating hemostasis under normal conditions and thrombosis upon vascular damage. 28 Thrombocytopenia (i.e., platelet deficiency) that results from either trauma or blood disorders can lead to hemorrhage in tissues or uncontrolled bleeding of wounds. Both RBC and platelet therapy largely apply allogeneic cells (77% and 86%, respectively) in clinical settings. Still, the use of allogeneic RBCs requires blood type matching between donor and recipient. The major efforts in current RBC and platelet clinical trials are focused on optimizing transfusion protocols and verifying the durability of transfused cells.
Other than blood cells, stem cells account for 36% of current cell therapy trials (620/1705) as the second largest cell category of focus for this review. The trials of stem cell therapy, primarily those of HSCs and MSCs, encompass a wide range of indications covering 10 broad disease classifications ( Figure 2). HSCs are multipotent stem cells capable of self-renewing and differentiating into mature blood cells that form the myeloid and lymphoid cell lineages. As a result, hematopoietic stem cell transplantation (HSCT) can be used to reconstitute the hematopoietic and immunologic systems for the treatment of inherited and acquired blood disorders. HSCT is also used frequently to treat blood cancers after cancerous cells are eliminated by a myeloablative treatment. 29 While autologous HSCs or matched sibling   donor HSCs are the most ideal candidates for HSCT due to the   reduced risk of GvHD, graft rejection, and engraftment syndrome, 30 allogeneic HSCs have an advantage in cancer treatment because they can elicit graft-versus-tumor effects. 31 MSCs, also a type of multipotent stem cell, are capable of effectively differentiating into a wide variety of cell types in mesodermal (e.g., chondrocytes), ectodermal (e.g., neurocytes), and endodermal lineages (e.g., hepatocytes). 32 As a result, they have broad applications in clinical settings for the treatment of degenerative diseases, autoimmune diseases, inflammatory diseases, and trauma, among others. Notably, most stem cell therapy trials are in early stages with nearly equal representation in Phase 1 (44%) and Phase 2 (47%), showing their considerable potential to affect the future scope of cell therapies. Finally, microbes comprise 3% of the total trials (48/1705) with major indications including cancer (44%), infectious diseases (19%), and inflammatory diseases (13%). Although metabolic disorders account for only 8% of the indications for microbes, it is worth mentioning this unique niche, as very few cell therapies are investigated for this indication. Microbes exert therapeutic mechanisms of action by (i) displacing pathogenic microbiomes to restore symbiosis and (ii) producing therapeutic biomolecules, a function enabled by genetic modification. 33

| CLINICALLY APPROVED PRODUCTS
In the following sub-sections, we provide additional details on approved cell therapy products that presently include T cells, stem cells, and DCs (Table 1). We also discuss closely related modalities, namely the applications of donor blood products and microbe-based therapies in the clinic. Of note, many of these approved cell therapies are being developed and evaluated in current trials for additional indications, as summarized in Table S2.

| T cells
A total of four T-cell products have been approved globally as of 2020, three by the FDA (USA) and one by the Korea Food & Drug Administration (KFDA) ( Table 1). All FDA-approved T-cell products are for CAR-T therapy, which is a form of immunotherapy that uses T cells genetically modified with a CAR to recognize and destroy cancer cells. 34 The two essential components of a CAR include (i) an extracellular target binding domain used to identify surface antigens on cancer cells and (ii) an intracellular signaling portion comprised of costimulatory and activation domains that initiate processes including activation, clonal expansion, and cell killing. 35 New functional domains are now being explored in both preclinical and clinical settings with the aim of providing safer and more effective CAR-T therapies. Of note, all approved CAR-T products are autologous and contain CARs targeting CD19, a biomarker that is selectively expressed on the surface of B cells. Accordingly, T A B L E 1 Clinically approved cell therapies, grouped by cell type (IL-2) and anti-CD3 antibody, 40 to collect activated T lymphocytes.
ImmunCell-LC ® showed an increased rate of recurrence-free and overall survival in patients who underwent tumor resection. 41 Additional clinical trials of ImmunCell-LC ® are underway for hepatocellular carcinoma.

| Stem cells
Our search revealed a total of 21 stem cell products that have been approved globally, with 12 approved by the FDA (USA) or European Medicines Agency (EMA, Europe). The remaining nine products are approved in other countries, particularly in Asia (Table 1). Notably, all but one product are composed of HSCs or MSCs.

| Hematopoietic stem cells
There are 10 approved HSC products globally, with eight approved by the FDA and the remaining two by the EMA (

| Dendritic cells
There are currently three DC products in the global market with approvals by the FDA, KFDA, and Indian FDA ( 3.4 | Other cell-based therapies (transfusions, transplants, and supplements) While donor blood products have a long history in the treatment of some blood disorders and deficiencies, 15 there are no specific approved products for RBCs and platelets. RBCs are administered to patients who are anemic due to a blood disorder (i.e., thalassemia, sickle cell disease, iron or other vitamin deficiency, aplastic anemia), or as a result of trauma or injury. Prior to intravenous administration, blood must be ABO blood type and Rhesus D (RhD) matched. Packed RBC infusions are given most commonly, although whole blood can also be administered. In many cases, autologous blood is isolated prior to a surgical procedure in anticipation of potential blood loss. Currently, drugs cannot be mixed with donor blood prior to infusion. beyond the scope of this review because they are not being developed as individual drug products. [53][54][55] The second type of therapy, probiotics, includes living microorganisms that are widely available over the counter and can also be prescribed by clinicians. [56][57][58] However, they are also beyond the scope of this review because they are typically categorized as foods, functional foods, or supplements, and as such do not undergo the same regulatory process as pharmaceuticals. 59

| CURRENT CLINICAL TRIALS
In the following sub-sections, we categorize and discuss current clinical trials employing blood cells and stem cells administered as single-cell suspensions, and on microbes administered in various dosage forms.
We define current clinical trials as those that appear on clinicaltrials.gov with a status of not yet recruiting, recruiting, enrolling by invitation, or active/not recruiting. These sub-sections account for data that capture the current clinical landscape as of August 2020. The overall summary of our analysis and additional details are shown in Figure 2.

| T cells
The clinical landscape of T-cell therapies has rapidly diversified over  (Table S3). The next most abundant targets include BCMA, CD22, and CD20, all of which are also exclusively found on B cells (Table S3). Together with CD19, they comprise 58% of the targets that are currently investigated in CAR-T clinical trials, reflecting the continued prevalence of B-cell cancer indications in the field. One new approach for the treatment of liquid cancers is dual CAR-T therapy, in which two different CARs are presented on the same cell or two distinct CAR-T products are co-infused. 60 This strategy has the potential to reduce relapse rates by targeting and eliminating cancer cells that are resistant to CD19-targeted therapy (NCT04049383).
Historically, CAR-T therapy been ineffective in the treatment of solid tumors due to a lack of defined extracellular antigen targets,  (Table S3). Based on our analysis, intravenous administration remains the most common route of administration for CAR-T therapy against solid tumors (75%), though other administration routes are also being investigated, such as intraperitoneal, intraventricular/intracavitary, intratumoral, intra-hepatic artery, intrapancreatic (via splenic vein or artery), and intrapleural administration.
The emergence of strategies to prevent T-cell exhaustion, improve target specificity, and promote tissue infiltration may expand the current application of CAR-T therapies against solid tumors (Section 5). 62 The next major class of GM T cell is the TCR-T cell, which comprises about 12% of trials involving GM T cells (Figure 3(a)). While   64 The most common targets for this application are cytomegalovirus (CMV) and EBV (Table S5)

| HSCs
HSCs comprise 44% of the current stem cell trials (Figure 4(a)). These trials apply HSCT, which has been widely utilized in the clinic since it was first reported in 1957. 66 The basic process for HSCT includes collection of mobilized stem cells from peripheral blood or cord blood, ex vivo purification and engineering of the cells, and finally infusion. 67 Prior to transplantation, patients undergo a regimen of chemotherapeutics, broadly classified into myeloablative and reduced intensity conditioning (RIC), to deplete native lymphocytes in the bone marrow.
This conditioning step is required for the successful engraftment of the infused cells. 68 The choice of regimen depends on parameters such as disease severity, risk, age group, and other factors. RIC leads to reduced neutropenic periods, speedier engraftment, and improved recovery of the immune system compared to myeloablative conditioning. As a result, RIC has widened the potential applications of HSCT to a greater patient population, especially older patients. 69

cells, (c) neural stem cells, (d) bone marrow-derived stem cells and (e) others (cardiac stem cells, limbal stem cells, and endothelial progenitor cells)
HSC-based therapies are also clinically investigated for nonmalignant blood disorders associated with WBCs (BD w/WBCs) and immunodeficiency disorders, such as Wiskott-Aldrich syndrome (WAS), severe combined immunodeficiency (SCID), and leukocyte adhesion disorder (LAD). 69% of these trials utilize GM HSCs, as the intention is to correct mutations responsible for disease (Figure 4(a) (iii)). While most of the trials (95%) are still in Phase 1 or 2 (Figure 4(a) (ii)), there is one trial in Phase 3, where elivaldogene autotemcel, a therapy that uses GM HSCs endowed with functional human adrenoleukodystrophy protein, is used to treat cerebral adrenoleukodystrophy (NCT03852498). HSCs are also being explored for the treatment of autoimmune disorders in a small subset of trials (4%) (Figure 4(a)).
The specific indications are diverse and include multiple sclerosis

| MSCs
MSC therapies make up 46% of total current stem cell clinical trials ( Figure 4(b)). MSCs are derived from multiple tissues, including bone marrow, adipose tissue, the umbilical cord, Wharton's jelly, and the placenta. Interestingly, the majority (65%) of the investigated MSC therapies are allogeneic, reflecting the trend of next-generation "offthe-shelf" manufacturing ( Figure 4(b)). 73  Trauma is another important indication for MSC tissue repair applications, with 23 active trials (8%) (Figure 4(b)) underway for traumatic brain injury, spinal cord injury, acute kidney injury, ischemia reperfusion injury, and others. While the therapeutic mechanism of MSCs in these degenerative diseases seems to be related to their multipotent differentiation capabilities, their immunomodulatory potential may also play an important role. 75 Immunomodulatory capabilities of MSCs can also be leveraged as the primary mechanism for disease management. 45

| NK cells NK cells have drawn increased attention over the past few years due
to their natural cytotoxic functions, 85 non-MHC restricted activity, and "off-the-shelf" use capability. 86,87 These features allow them to offer a potent alternative to T-cell therapy. Currently, NK cell trials occupy 7% of total cell therapy trials (Figure 2), with a representative selection of these trials listed in Table 5. Usage of DCreg following the recent approval of CAR-T products ( Figure 5(b) (i)).

CAR-NK cells are generally regarded as safer than CAR-T cells due to
their more limited persistence in circulation. 89

| Red blood cells and platelets
Clinical trials are also being conducted on the intravenous infusion of donor blood products, such as packed RBCs, whole blood, and platelets. Here, we focus our scope on the trials that use a single cell type, either RBCs (2% of the total trials) or platelets (0.4% of the total trials)  Figure 2). A representative selection of these trials is given in Table 7.
Of all the clinical trials involving RBCs, the most common indications are blood disorders such as anemia (26%) and sickle cell disease (18%). In the majority of these trials, as well as those focused on treating hemorrhage, organ injury, and trauma, the unmodified RBCs

| Microbes
Forty-eight microbe-based clinical trials were identified (3% of the total trials) ( Figure 2) and separated into four main categories based upon consortia-based/single strain-based therapy status and indication (Table 8)

| Biological challenges
There are many common challenges related to the biological activity of cell therapies. These include safety, functional heterogeneity, the T A B L E 6 (Continued)

| Functional heterogeneity
Maintaining and tuning cell functionality in vivo is also a major biological challenge, especially for cells in the immunotherapy space. One general limitation is functional heterogeneity, which introduces batchto-batch variation in the product and therefore patient-to-patient variation in the functional response. These factors add to the complexity of treating heterogeneous disease microenvironments that exist within each patient. As a result, outcomes observed in clinical trials can be highly variable. These inconsistencies make it difficult to assess translational potential and properly identify which patients might benefit the most from therapy. MSCs in particular are impacted by these discrepancies, with differences in donor sources, tissue sources, subpopulation isolation procedures, cell storage conditions, and other manufacturing processes leading to substantial variation both within and among trials. 120,121 Future approaches to address the functional heterogeneity challenge should consider both the manufacturing and clinical trial design perspectives. Better characterization and quality control of cell products during the manufacturing process will be required to maintain functional homogeneity. In addition, new manufacturing strategies, such as iPSC-derived cell manufacturing, should be further explored. 122 Finally, clinical trial design will need to include more comprehensive, standardized documentation of the properties of injected cells. Using these data, cross-analyses of the relationships between cell identities/properties and safety/efficacy may be better performed to identify key attributes and inform future directions.

| Maintenance of biological activities
The efficacy of cell therapies is highly dependent on the ability of cells

| Delivery challenges
Targeted delivery is another biological challenge for many cell therapies, particularly those designed to treat diseases in solid tissues. Both cell migration to and localization at the target site are important considerations, and these factors may depend heavily on the initial delivery route. For example, intravenously administered T cells must sense biological cues, such as chemokine gradients and endothelial markers, to extravasate into a solid tumor. One major approach that is being explored to improve delivery is altering the administration route.
CAR-T cells indicated for solid cancers are currently administered via a variety of routes (see Section 4) to accomplish direct delivery into target tissue, a nearby artery, or cerebrospinal fluid (for neurologic tumors). New NK cell administration routes, including intracranial and intratumoral, are also being explored for the treatment of glioblastoma (NCT03383978, NCT04489420). While T and NK cells must reach the site of a tumor or infection, DCs must reach the lymph nodes to execute their antigen-presenting functions. Injection routes including intradermal, intranodal, and subcutaneous are currently being explored to improve lymph node trafficking. 131 For HSCs, intra-bone infusions are currently being explored as an alternative to IV administration to speed HSC delivery and graft reconstitution in the bone marrow. 132 While the outcomes of this approach are not yet conclusive, this administration route may reduce acute GvHD, providing a solution to this longstanding biological challenge.

| Manufacturing challenges
The development of closed, automated manufacturing processes is a necessary yet challenging endeavor for the production of safe, high- ineffective. 135 After samples are taken for quality assurance (QA) measures, the cells are concentrated and packaged by dose before being shipped to the site where the therapy will be administered. 133 The process for allogeneic manufacturing is streamlined by the exclusion of the first extraction and shipping steps. Multiple challenges exist at each step in this workflow, though future technologies have shown promise for potential integration into a closed, automated process. Current areas of focus include cryopreservation, cell selection and activation workflows, automated batch monitoring for QA purposes, and the adoption of electronic records systems. 126,134,136 These interventions have the potential to decrease the heterogeneity of cell therapies and reduce production time. In the future, decentralization of the manufacturing process will expand the global scale of cell therapies to reach patients who are currently inaccessible.
Another closely related factor is the cost associated with developing, manufacturing, and distributing cell therapies. The steep price tags of some therapies, such as CAR-T therapy Kymriah's list price of $475,000, 137 have drawn criticism due to concerns about patient accessibility. Advances in manufacturing procedures, particularly automation, along with widespread adoption of allogeneic therapies may help drive down direct costs in the future.

| Regulatory challenges
In addition to the aforementioned considerations, future advancements in the clinical translation of cell therapies will rely on the development of industry-wide and regulatory approval standards. As of 2020, there is no universal system for the classification of cell therapies. Not only does this insufficiency impede the regulatory approval process, but it also allows for a substantial degree of ambiguity in the reporting of quality attributes and clinical outcomes. 138 In light of these concerns, the FDA released a warning regarding the dangers of unapproved stem-cell therapies in 2020. 139 Notably, regulatory standards related to product purity are rapidly changing in the more recently developed field of microbe-based therapy. In early 2020, alerts were issued regarding the risks of transmission of pathogenic microorganisms and SARS-CoV-2 via FMT. 140 Given the rapid emergence of many new cell therapies with novel features, it is imperative that products be categorized in a central location for the benefit of the research, medical, and patient communities. In addition, this endeavor will enable more comprehensive future studies to identify associations between cell properties and clinical efficacy. These studies may then inform decision-making at the regulatory, clinical, and preclinical levels to accelerate the development of novel therapies.
Eventually, these efforts may enable the standardization of critical quality attributes, which has the potential to streamline the development and approval processes.