Stem cells can be found in an organism in embryos and adult cells; they are a type of unspecialized, self-renewable cells prepped to differentiate into any cell type and/or as many cell types (28). What dictates how many cell types stem cells can differentiate into is their developmental potency. Developmental potency represents a differentiation continuum starting with totipotency (i.e., highest differentiation potential; e.g., zygote) and dwindling to pluripotency (e.g., embryonic stem cells), multipotency (e.g., hematopoietic stem cells), oligopotency (e.g., myeloid stem cells), and unipotency (i.e., least differentiation potential; e.g., dermatocytes) (29, 30). During passage along this continuum of potency toward mature/specialized cells, stem cells lose their self-renewal and differentiation potential (30). However, this hierarchy can be artificially reversed by nuclear reprogramming methods, including the use of transcriptional factors, which can eventually induce pluripotency in any cell type (31, 32). Stem cell specialization is influenced by external signals (e.g., physical contact between cells, paracrine secretions of nearby tissue, and tissue type), internal signals (e.g., genes), and epigenetics (embryonic cell origin) (29, 33). Depending on the type of stem cells, stem cell specialization can be detected by in silico gene expression analysis [e.g., PluriTest bioinformatic assay (34)] and validated by several techniques, including microarrays, polymerase chain reaction (PCR), and immunocytochemistry (35–38). Specific surface markers, molecular markers (e.g., transcription factors, microRNAs, transcription regulators, histone modifiers, DNA methylation state, X chromosome functional state, key molecular signaling pathways) (39–45), functional assays (e.g., teratoma formation assay, in vitro differentiation assay, blastocyst chimerism) (46–48), and culture characteristics (e.g., morphology, tolerance to single cell dissociation by trypsinization) (33) also help guide the evaluation of developmental potency.

Stem cells used or targeted by cell therapy can be grouped into three categories: pluripotent stem cells (PSCs), adult stem cells (ASCs), and cancer stem cells (CSCs).

Among ACT products, TILs is a multicellular therapy that includes different lymphocyte lineages, including T cells and B cells (149). In cancer biology, lymphocytes recognize growing cancer cells and infiltrate the tumor. Once in the tumor, TILs try to initiate cancer killing. However, cancer cells can inactivate TILs to evade immunosurveillance by ligating their checkpoint receptors [e.g., programmed death 1 (PD-1), cytotoxic T lymphocyte-associated protein 4 (CTLA-4)], which are normally bound by specific ligands (e.g., programmed death-ligand 1 (PD-L1), B7) to control immune activity (152). Many tumors express PD-1 receptor ligands, such as PD-L1, permitting the mitigation of anti-tumor immunity (153, 154). Therefrom, immune checkpoint inhibitors were devised to lift off immune cell suppression and promote anti-cancer immunity (155). Indeed, it has been shown that TILs can promote tumor invasion and metastasis through several mechanisms, including cancer cell-leukocyte fusion and recruitment of regulatory T cells (Tregs) (156). The 1970's recorded the first attempts of lymphocyte isolation from tumor tissues (157, 158). In the next decade, IL-2-expanded isolated TILs showed significant antitumor activity in vivo (159), as opposed to TILs alone (160). TIL preparation involves tumor excision, digestion, culture with IL-2, and assessment for specific tumor recognition; tumor-specific TIL cultures are then expanded using anti-CD3 monoclonal antibody, high IL-2 concentrations, and irradiated allogeneic feeder cells (161). Characterization methods of TIL cellularity include gene expression analyses and analytical tools, such as CIBERSORT (162, 163). TIL products are heterogenous in terms of CD8⁺/CD4⁺ T cell ratios and T-cell differentiation stage and can be impacted by tumor biology (164). For an endowment of resistance to tumor suppression and/or enhanced tumor homing, TILs can be genetically modified either using different types of vectors or via gene editing technologies (165).

LAK cells are another ACT product composed of IL-2-activated PBMC's, mainly NK cells, NKT cells, and T cells, with non-specific cytotoxicity and non–MHC-restrictive cytotoxic effects (137). The use of LAK cells is limited to few cancer types due to their difficult amplification and associated adverse effects (166, 167). LAK cells have been reported to induce tumor cell killing by releasing cytolytic mediators, including perforin and granzymes (168).

CIK cells are a subset of T lymphocytes with an NKT cell phenotype, and can be expanded ex vivo from PBMC's or BM mononuclear cells. When activated, CIK cells stimulate the immune system to recognize and eradicate tumor cells in a non–MHC-restricted manner (137, 139).

Scaffold-based cellular products are engineered technologies that deliver different cell types (e.g., fibroblasts and keratinocytes) seeded within 3D biocompatible tissue analogs (9). Traditionally, scaffold-based cellular products employ biodegradable natural or synthetic polymers (e.g., bovine collagen, hydrogels, sponges) with sophisticated porous networks through which oxygen, nutrients, and metabolites can be exchanged (169, 170). Current scaffold-based cellular products with FDA approval are used for the treatment of diabetic foot ulcers (e.g., Apligraf^®, Dermagraft^®), burns (OrCel^®), and mucogingival conditions (GINTUIT) (9, 24, 171, 172). Scaffold-free cellular products are tissue analogs that are densely populated with cells carried and protected by their secreted, tissue-specific extracellular matrix (ECM) (9). This biotechnology can employ temperature-responsive polymers (e.g., pNIPAM, PVME) that transition between hydrophobic and hydrophilic states at certain temperatures, allowing the control of cell culture and growth and subsequently the deposition of ECM and the formation of cell sheets that adhere to biological surfaces (173–175). Several automated technologies (e.g., robots, bioreactors) can also be used to enhance the scalability, elevate the architectural biomimicry, or allow for the perfusion of these tissue analogs (176–178). An example of commercially available, FDA-approved scaffold-free cellular products is Epicel^® (cultured epidermal autografts), a petrolatum gauze composed of sheets of autologous keratinocytes and proliferation-arrested murine fibroblasts and indicated for deep/full burns (179). Generally, the specific mechanisms of action of scaffold-based or -free cellular products are unknown, but are surmised to involve the production of cytokines and growth factors similar to healthy human skin (180). Although these products represent important advances in regenerative medicine, they are still limited by their high costs and non-regenerative outcomes, including their inability to fully reconstitute the damaged skin architecture (181).

SVF is a heterogeneous mixture of stromal and vascular cells, including ASCs, granulocytes, monocytes, lymphocytes, pericytes, and endothelial progenitor cells (EPCs), obtained from the processing of adipose tissue (e.g., lipoaspirate, excised fat) (182–185). Besides its use as an investigational product in different clinical settings (186–188), SVF is used as a source to isolate ASCs (i.e., adipose-derived stem cells, or ADSCs), which can constitute up to 10% of its fraction depending on the processing technique, usually involving serial straining and centrifugation of SVF and cell culture in growth media (189). SVF composition can be identified immunophenotypically by flowcytometry, for example for the presence of ADSCs (CD45⁻CD235a⁻CD31⁻CD34⁺), which share several surface markers with BM-MSCs but are CD36⁺CD106⁻. Other techniques used for identifying the cellular composition of SVF include lineage-specific differentiation assays and biochemical or PCR evaluation (190). Due to its heterogeneous composition, SVF functions in various mechanisms, including paracrine signaling through cytokines, chemokines, and growth factors and cell-cell interactions, ultimately promoting neovascularization, cell repair, and immunomodulation (191).

Stem cell transplant is performed in settings that damage the body's stem cells, including hematologic malignancies (e.g., leukemia, lymphoma, multiple myeloma, neuroblastoma) or cancer therapy (e.g., high-dose chemotherapy, total body irradiation) (192). Stem cell transplant relies on 3 stem cell sources: bone marrow, peripheral blood, and umbilical cord blood (193).

Stem cell transplant with bone marrow as the source of stem cells is known as bone marrow transplantation (BMT), which has been in practice since the 1960's (194). BMT entails BM aspiration (195) for harvesting HSCs (196) as well as progenitor cells, MSCs, lymphocytes, neutrophils, platelets, red blood cells, eosinophils, basophils, and monocytes (197, 198) (see Section BMA-derived therapies). Peripheral blood stem cell transplantation (PBSCT) is another type of hematopoietic stem cell transplantation (HSCT) that uses peripheral blood-derived HSCs (199). PBSCT came forth in the 1990's (196) as an alternative to bone marrow transplantation (BMT) (194), due to easier stem cell collection, higher stem cell yields, and faster patient recovery post-transplantation (150). In this procedure, autologous or HLA-matched allogeneic peripheral blood stem cells (PBSCs) are infused into the patient's bloodstream following a preparative conditioning regimen consisting of chemotherapy with/without total body irradiation that ensures immune tolerance of the engraftment. Once in the blood, PBSCs home toward the BM to repopulate lost blood cells or allow cancer remission (150, 200). PBSCs are collected by continuous-flow apheresis after mobilization using medications including granulocyte colony-stimulating factor (G-CSF) agents and chemokine receptor 4 (CXCR4) blockers (e.g., plerixafor); chemotherapy can also be used for mobilization (i.e., chemoembolization) (200). PBSCs are generally identified and quantified using flowcytometry via their immunophenotypic patterns (e.g., CD34⁺CD38⁻) (192). Besides CD34⁺ HSC subpopulations, PBSC grafts contain nucleated cells including DCs, T cells, B cells, NK cells, and monocytes (201). Compared to BM-derived stem cells, PBSCs express more lineage-specific differentiation antigens, are less metabolically active, and show higher clonogenicity (202). However, the clinical benefit/risk ratio of PBSCs vs. BM-derived stem cells is disputed, with the preference being dependent on the type of hematologic disease, the age of donors/recipients, and whether the HLA-matched donor is related or unrelated—factors which influence the incidence of graft vs. host disease (GVHD) or the patient's quality of life (196, 199). The differences in cellular composition (e.g., CD34⁺ and lymphocyte numbers) between both stem cell grafts is also associated with differences in their clinical outcomes (203).

Another source of stem cell transplantation is cord blood (CB), whose HSCs and hematopoietic progenitor cells are observed to differ from those of peripheral blood and BM in terms of surface markers, recovery speed of blood cells post-transplant, clinical outcomes, and GVHD incidence (204, 205). As of 2011 to date, eight allogeneic cord blood products have gained FDA approval for the treatment of hematopoietic system disorders. These products mainly contain HSCs and hematopoietic progenitor cells, which migrate to the BM where they divide, and their progeny cells mature and subsequently replace lost blood cell reservoirs. Notable, these products are also composed of monocytes, lymphocytes, and granulocytes, which render their mechanisms of action only partially known (10–17).

BMA-derived therapies are commonly termed concentrated bone marrow aspirate (cBMA), bone marrow concentrate (BMC), or bone marrow aspirate concentrate (BMAC) (151). BM aspiration is a procedure performed under local or general anesthesia, in which a liquid sample is collected from the BM of usually the anterior or posterior iliac crest among other bones (195). Since the early 1960's, the BM has been the chief source for harvesting HSCs for BMT procedures (196); however, its use in this context has diminished following the emergence of PBSCT (206, 207). BMA includes various cell types including HSCs, progenitor cells, MSCs, lymphocytes, neutrophils, platelets, red blood cells, eosinophils, basophils, and monocytes (197, 198). The processing of BMA to concentrate nucleated cell yields, such as MSCs which represent 0.001% of the non-hematopoietic, multipotent cellular portion of BMA, yields BMAC (198). The concentration of BMA can be performed by different techniques, including automated centrifugation systems (208) or cell filtration systems (209). In BMAC, concentrations of nucleated cells become 5-fold higher, and concentrations of MSCs 6-fold higher (210, 211). The composition of BMAC also includes HSCs, progenitor cells, white blood cells, platelets, and cytokines/growth factors (212, 213) and can be characterized by microscopy, flowcytometry, cytogenetic and molecular analyses, and cytochemical staining (214). Generally, the clinical application of BMAC spans orthopedic settings, in which it can be sterilely injected intra-articularly under the guidance of fluoroscopy or ultrasonography (215). Like SVF, BMAC functions in various mechanisms involving paracrine signaling by MSCs and nucleated cells that drive tissue repair and immunomodulation (66, 209, 210) and by growth factors and cytokines that induce tissue growth and promote reparative processes (198, 216).

Regenerative medicine deploys a body's own cells and growth factors to repair tissues by restoring their lost functions (111). Several cell therapies in regenerative medicine have become either established practices or commercially available with FDA approval, such as keratinocyte- and/or fibroblast-derived skin substitutes for treatment of diabetic foot ulcers (172, 233) or burns (179); keratinocyte- and fibroblast-containing scaffold products for treatment of surgically created vascular wound beds in the oral cavity (24); fibroblast intradermal injections for improvement of appearance of nasolabial fold wrinkles (25); chondrocyte-containing scaffold implants for treatment of knee cartilage defects (26); and cord blood-derived HSC/hematopoietic progenitor cell products for treatment of hematopoietic system disorders that are inherited, acquired, or result from myeloablative treatment (10–17). Although commercial cell therapies are beneficial in repairing tissues, they are unable yet to regenerate them (234). Clinical development is also an arduous process that hinders the introduction of new products into the market (235, 236). This can be seen in the proportion of approved biologics over a 9 year period, which reached 23% of all approved drugs. Additionally, biologics in the US are granted 12 years of exclusivity protection vs. ~7 years for new chemical entities (237).

In clinical investigation settings, multiple cell therapies, as well as acellular therapies with cellular components, have been assessed for their safety and efficacy in a regenerative context, including PRP, ESCs, iPSCs, SVF, ADSCs, MSCs, and BMAC (190, 234, 238, 239).

PRP is widely evaluated in orthopedics due to its enriched composition of cytokines, growth factors, and platelets, which establish an anti-inflammatory environment at the site of injection and promote skeletal and connective tissue regeneration and reconstruction (225, 226). For instance, PRP preparations have demonstrated efficacy and safety in tendon injuries (240, 241), rotator cuff tears (242), osteoarthritis (OA) of the knee or hip (243, 244), and muscle injuries (245), with benefits being mostly symptomatic relief. The cellularity of PRP can also dictate clinical outcomes, thus classifying PRP preparations into leukocyte-rich PRP (LR-PRP)—with leukocyte concentrations exceeding baseline levels—and leukocyte-poor PRP (LP-PRP) —with leukocyte concentrations below baseline levels (241). Accordingly, it is recommended that PRP be analyzed for its leukocyte content and used in accordance with the catabolic vs. anabolic requirements of the treated condition (246).

The clinical application of ESCs is restricted by ethical concerns, regulatory bodies, and the lack of preclinical evidence supporting their use (53, 54). However, few successful outcomes in regenerative medicine merit acknowledgment. For example, human embryonic stem cells (hESCs) have improved the vision of patients with macular degeneration and macular dystrophy by differentiating into photoreceptors and retinal pigment epithelial cells (55). In a case report, cardiomyocytes derived from hESCs have also improved the ejection fraction of a 68 year old patient with severe heart failure and without inducing subsequent complications (56).

Despite presenting several advantages over ESCs (e.g., non-invasive collection, less immune rejection, ethically unrestricted nature), the use of iPSCs in clinical settings is still farfetched due to lack of therapeutic evidence as well as other preparation and standardization obstacles (57). Indeed, a data compilation in 2018 showed that the fraction of clinical trials investigating the aptness of iPSCs as a treatment modality constitutes only 11% of the total clinical trials of iPSCs, including those terminated (58). The first and potentially only reported clinical benefits of iPSC-based therapy are minimal and date to 2017 in a patient with neovascular age-related macular degeneration (59). On the other hand, iPSCs—like ESCs—have been extensively employed as research tools for drug toxicity testing (e.g., drug-induced QT prolongation) and—unlike ESCs—have been useful for disease modeling and drug discovery studies (247).

SVF was first clinically investigated in reconstructive surgeries with the rationale of promoting adipose tissue survival and thus provide structural tissue support (248). Multiple other studies followed, in which SVF was investigated for its healing and regenerative abilities. For example, SVF has promoted neovascularization and improved tissue hydration in patients with radiotherapy-induced lesions, owing partially to its ADSC composition (249). Few other examples of regenerative settings in which SVF has provided patients with clinical benefits include knee OA (250, 251), chronic wound healing (252), urogenital conditions (253), and systemic sclerosis (SS)-associated facial handicap (254).

In regenerative medicine, ASCs are considered the most promising among cell therapies, and ADSCs constitute an ideal option due to their ease of harvest requiring minimal invasiveness; multi-lineage differentiation potential; and anti-inflammatory and proangiogenic secretome (255). To date, there are 11 active or recruiting registered studies involving ADSCs as an intervention in conditions such as knee OA and chronic kidney disease (256). Data disclosed hitherto, mostly by pilot studies, show that—despite not living up fully to their regenerative rationale—ADSCs have shown a promising potential in multiple settings, including ischemic heart disease (73, 74), acute myocardial infarction (257), knee OA (258, 259), peripheral vascular disease (260), SS-associated ulcers (261, 262), ischemic diabetic feet (252), urogenital conditions (263, 264), and breast cancer-associated lymphedema (265). For example, an early-phase placebo-controlled trial has shown that the intracoronary infusion of ADSCs is well tolerated, improves left ventricular ejection fraction (LVEF), and reduces infarct size after 6 months of follow-up in patients with acute myocardial infarction (257). Similarly, the intramyocardial delivery of ADSCs has been shown to significantly improve LVEF and exercise tolerance in patients with chronic ischemic heart disease (73, 74).

Among ASCs, MSCs are gaining considerable attention as a cell therapy intervention in human studies, especially with the production of good manufacturing practice (GMP)-compliant human MSCs (66, 266). To date, there are at least 180 active or recruiting registered studies involving MSCs as an intervention in various conditions (69). In regenerative settings, MSCs have exhibited a promising potential in osteogenesis imperfecta (70), Crohn's disease (71), deep burns (72), periodontal defects (267), chondral/bone defects (268, 269), and diabetic foot (270). The benefits of MSCs in clinical investigations are mostly linked, not to their multi-lineage differentiation potential, but rather to their secretome, which establishes a nutritive microenvironment, promoting autocrine and paracrine signaling that inhibits apoptosis and dictates angiogenesis, local tissue mitosis, and cross-communication with resident stem cells (66, 271–273).

BMAC has emerged as a potential alternative for regenerative therapy, owing mainly to its enriched composition of growth factors and MSCs among other cell types (216). In regenerative settings—mainly orthopedics—BMAC has demonstrated a promising potential, as it provided patients with clinical benefits and/or improved diagnostic imaging outcomes of patients (209, 274, 275). To date, there are at least 14 active or recruiting registered studies involving BMAC as an intervention mainly in orthopedic conditions (276). The mechanism of action of BMAC remains unclear, and no serious attempts have been made to delineate the interactions between the different components of BMAC, which might collectively be at the origins of BMAC outcomes (277, 278). Among the components theorized to contribute to the therapeutic potential of BMAC are MSCs, which are endowed with tissue function-enhancing regenerative and immunologic properties (66, 209, 210); growth factors, which promote tissue growth; and nucleated cells (e.g., lymphocytes), which secrete various reparative cytokines and growth factors that act via paracrine pathways (198, 216). However, the conclusions drawn by these reports about the role of BMAC components—specifically MSCs—in driving clinical outcomes are based on rather extrapolations than benchwork. Hence, further molecular investigations are necessary to fully understand the degree of contribution of each of these components in the observed orthopedic benefits. Noteworthy, BMAC could still become an established therapy even with a partially understood mechanism of action and without having to erroneously suggest that its beneficial outcomes are driven by MSCs. This possibility could be observed with HSCT, which has become an established therapy for treating immune diseases despite its elusive mechanism of action (75, 89).

Most immune system disorders develop due to excessive immune responses or autoimmune attacks (279). Primary treatment thus aims to alleviate inflammation, minimize symptoms, and prevent relapse (280). The rationale of exploiting cell therapy in immune system disorders extends beyond immune suppression and symptomatic relief to immune system resetting as a permanent cure (281). As of the late 1990's, BMT/HSCT has become the most established cell-based therapy for treating immune system disorders (75). HSCT has been shown to elicit durable outcomes in severe SS with acceptable rates of transplant-related mortality (76). In controlled phase 2/3 trials, HSCT has demonstrated efficacy in patients with autoimmune disorders, resulting for instance in 79% improvement in the disability status and marked improvement in disease relapse rates, MRI lesions, and quality of life in patients with multiple sclerosis (83). HSCT, including PBSCT, has also alleviated disease activity and stabilized/reversed organ dysfunction in patients with systemic lupus erythematosus (SLE) (84–86). While the benefits of HSCT in SLE are mostly reported by retrospective studies, prospective trials are limited and have not found significant benefits (83). On the other hand, HSCT has only shown transient responses or partial benefits in other immune diseases, such as rheumatoid arthritis (RA), vasculitis, and Crohn's disease (87, 88). Despite its benefits seen in most immune system disorders, HSCT's underlying mechanism of action remains elusive, with non-specific benefits being also omnipresent and pertaining to the accompanying regimen of lymphotoxic chemotherapy that reduces autoreactive antibodies (89). MSCs have also had their share of clinical successes in immune system disorders, specifically in GVHD, amyotrophic lateral sclerosis, and Crohn's anal fistula (77–81, 90). These benefits are linked to the immunomodulatory actions of MSCs originating mostly from their immune inhibitory secretome (82). Although several MSC-based therapies are approved worldwide for the treatment of immune diseases (including in Canada and Japan), they have not yet received FDA approval (66). Albeit to a less documented extent than BM-MSCs, ADSCs have also shown a promising potential as a cell therapy for the treatment of immune system disorders, such as GVHD, Crohn's disease, psoriasis, and SS (282–285). Other cell-based therapies with less reported benefits in immune diseases include PRP, which has been shown to reduce pain and inflammation with ultrasound imaging evidence in patients with RA (286), and Tregs, which have been shown to reduce the incidence of acute GVHD (287). DCs are another type of immunotherapies exploited in the treatment of patients with immune system disorders. For instance, tolerogenic DCs—a type of immature DCs that induce T-cell anergy and Treg differentiation causing peripheral tolerance (288)—have been reported to stabilize disease (289) or reduce inflammation and disease scores in RA (290). Other somatic cell-based therapies like pancreatic islet cell transplantation have resulted in substantial benefits in type 1 diabetes. For example, a single-arm phase 3 trial has shown that pancreatic islet transplantation leads to glycemic control and protection against severe hypoglycemic events in patients with type 1 diabetes (120). Therefore, it has become recommended globally that pancreatic islet transplantation be considered for patients whose problematic hypoglycemia persists despite insulin infusion or glycemic monitoring (291). Notable, clinical outcomes with pancreatic islet cell transplantation have been found to be associated with islet availability and engraftment success rates, which are elevated for instance in allogeneic transplants, where islets can be isolated from multiple donors (121). Other immunotherapies like CAR-T cells have not yet been reported to provide clinical benefits in immune conditions (292), despite promising preclinical outcomes (293).

The rationale of cancer therapy has evolved from the systemic targeting of tumors with chemotherapy/radiotherapy to a more targeted approach using novel biologic treatments, including monoclonal antibodies, oncolytic viruses, and cell therapy, such as antigen presenting cell (APC)-based anticancer vaccines and CAR-T cells (294). Another therapeutic approach in patients with cancer is the aforementioned regeneration of immune effectors specifically in hematological malignancies, in which case HSCT has long been the standard treatment (91). The year 2010 witnessed the FDA approval of PROVENGE^® (sipuleucel-T), the first and only APC-based anticancer vaccine indicated for the treatment of metastatic castration-resistant prostate cancer (23). Later in 2017, KYMRIAH™ (tisagenlecleucel) became the first CAR-T cell therapy to receive FDA approval for the treatment of relapsed or refractory B-cell precursor acute lymphoblastic leukemia and large B-cell lymphoma (19). Following tisagenlecleucel's steps, other autologous, CD19-directed CAR-T cell therapies then entered the market with the indication of treating relapsed or refractory large B-cell lymphoma or mantle cell lymphoma (Table 1) (18, 20, 21). In 2021, ABECMA^® (idecabtagene vicleucel) became the first B-cell maturation antigen (BCMA)-directed CAR-T cell therapy to receive FDA approval for the treatment of relapsed or refractory large B-cell lymphoma (22), with distinctive selectivity conferred by the highly selective expression of BCMA by malignant plasma cells (295).

Besides commercial cell therapy products, a multitude of cell therapies have been investigated for treating cancer in clinical settings. Among APC-based anticancer vaccines, DC-based anticancer vaccines—either created with primary CD1c⁺ myeloid DCs or engineered by fusion with patient-derived tumor cells, pulsation with tumor peptides/lysate, or electroporation with tumor associated antigen-encoding mRNA—have elucidated promising immunologic and/or clinical responses in B-cell lymphoma (296), multiple myeloma (297), acute myeloid leukemia (298), glioblastoma (299), and metastatic melanoma (300–302). CD34⁺ HSC-derived modified/manipulated DCs have also been clinically investigated in cancer settings with promising outcomes, such as generation of tumor-specific immunity and/or induction of tumor regression in patients with metastatic melanoma (92, 93). Using chemicals like polyethylene glycol, autologous primary DCs can be fused with irradiated, resected tumor cells to create tumor-DC hybrids whose subsequent bioengineering and administration to patients with glioblastoma receiving standard chemotherapy has been shown to improve clinical responses (303). Similarly, the vaccination of patients with acute myeloid leukemia who achieved remission following chemotherapy using autologous primary DCs fused with autologous cancer cells has led to the expansion of tumor-reactive T cell subsets and prolonged remission (304).

Investigational CAR-T cells have also shown high antitumor activity in relapsed/refractory multiple myeloma by targeting BCMA (305). Unlike CD-19- or BMAC-directed CAR-T cell therapy for hematologic malignancies, CAR-T cells directed against solid tumor antigens, such as PD-L1 and prostate-specific membrane antigen (PSMA), have had less clinical success due to obstacles pertaining to the suppressive nature of the tumor microenvironment and therapy persistence within the tumor (306). In small-scale studies, the use of bispecific CAR-T cells directed against CD19/BCMA in multiple myeloma has been met with promising patient responses (307). Among ACT, TCR-modified T cells directed against tumor-specific antigens have also had promising outcomes in cancer therapy, as they induced cancer regression in patients with melanoma (308, 309) and reduced metastases in patients with synovial cell sarcoma (310, 311). Similarly, TILs and LAK cells have been reported to induce tumor regression in patients with metastatic cancers (312–314). Additionally, LAK cells have improved the survival of patients with melanoma and patients with glioblastoma (315, 316), and TILs have augmented the rates of objective clinical responses of patients with metastatic melanoma (317). CIK cells were also reported to reduce disease recurrence or improve overall survival in patients with hepatocellular carcinoma and to augment the progression-free survival and overall survival in patients with renal cell carcinoma (318–320). The mechanism of action of CIK cells is observed to involve perforin-mediated tumor killing (321). On the other hand, although few phase 1 clinical trials have demonstrated the benefits of γδ T cells as a cancer immunotherapy, other studies have reported contrasting outcomes, revealing the suppressive façade of this T-cell subset and linking its presence within the tumor microenvironment to negative outcomes (137). What's more in ACT, allogeneic NK cells have only provided modest benefits to patients with acute myeloid leukemia (322) and patients with recurrent ovarian and breast cancer (323), generally due to their inhibition by host Tregs and/or the tumor as well as the high toxicity of IL-2 (137). In combined cell therapy approaches, CIK cells and tumor lysate-pulsed DCs infused intravenously at different time intervals have been shown to significantly prolong the median survival time at a rate comparable to chemotherapy in patients with colorectal cancer (114) and improve the overall survival and the quality of life in patients with advanced colorectal cancer (324). Similarly, DC-CIK immunotherapy has been reported to significantly prolong the overall survival and improve the quality of life in patients with advanced non-small cell lung cancer (325). Other combinatorial approaches include tumor lysate-loaded DC and TIL immunotherapy, which has revealed a promising potential based on evaluating objective clinical responses in a phase 1 study in patients with advanced melanoma (326). Patients with metastatic melanoma have also experienced immunologic responses and tumor regression upon treatment with the combined therapy comprising TCR-modified T cells directed against melanoma-associated antigen recognized by T cells (MART-1) and DCs (327).

Albeit to a less documented extent compared to ACT products or other cellular therapies, genetically engineered MSCs have been investigated as a cancer treatment due to their ease of obtainment and demonstrated tumor tropism and anti-tumorigenic properties; however, no benefits have been reported to date with patient-administered MSCs (66, 94), possibly due to their insufficient cell homing to tumors as reported in a phase 1 study (95) or their paradoxical pro-tumorigenic potential seen in several preclinical studies (66, 328). Noteworthy, based on previous promising preliminary data (329, 330), the first-in-human, first-in-child clinical trial for Celyvir—an autologous MSC-based therapy carrying oncolytic adenoviruses—has reported disease stabilization in two pediatric patients with neuroblastoma (96). Following the steps of this trial, other groups are exploring the potential of bioengineered MSCs carrying oncolytic viruses—viruses that evade immune surveillance and can conditionally replicate in tumor cells, unlike traditional attenuated viruses (97)—in patients with glioblastoma (NCT03896568) and patients with ovarian carcinomas (NCT02068794). Finally, several CSC-targeting agents for cancer treatment have been approved (e.g., vismodegib, ivosidenib, venetoclax) or are still under investigation, with mechanisms of action entailing the involvement with CSC pathways (109, 110).