Introduction: Clinical Demand and Manufacturing Bottlenecks

Chimeric Antigen Receptor Natural Killer (CAR-NK) cell therapy stands as one of the most promising "off-the-shelf" cellular immunotherapies in modern oncology, rapidly reshaping the treatment landscape for hematological malignancies. However, unlike their CAR-T counterparts, NK cells have a remarkably short half-life in vivo. Allogeneic NK cells typically survive only days to weeks in patients, with a median survival of about seven days.

To maintain therapeutic efficacy against aggressive tumors, clinical protocols require high doses (often 10⁷ cells/kg body weight) and repeated infusions. This operational reality imposes stringent demands on in vitro manufacturing capacity. A viable commercial platform must possess the ability to produce billions—even tens of billions—of highly cytotoxic NK cells reliably and at an acceptable cost.

Addressing this challenge, a collaborative team led by Professor Jinyong Wang and Professor Mengyun Zhang from the Institute of Zoology (Chinese Academy of Sciences), alongside Professor Xiaofan Zhu from the Institute of Hematology & Blood Diseases Hospital (Chinese Academy of Medical Sciences), published a landmark study (Hu et al., 2025) in Nature Biomedical Engineering. The researchers developed an innovative, three-step method for the ultra-large-scale, low-cost production of induced NK (iNK) and CAR-iNK cells from cord blood-derived CD34⁺ hematopoietic stem/progenitor cells (HSPCs). This breakthrough eliminates a primary barrier to the widespread application of “off-the-shelf” NK cell therapies.

A Standardised Manufacturing Case Study from CD34⁺ HSPCs to CAR-NK Cells: A Breakthrough by the Institute of Zoology, Chinese Academy of Sciences

A study published on October 7, 2025, in Nature Biomedical Engineering by Professor Jinyong Wang's team at the Institute of Zoology, Chinese Academy of Sciences, in collaboration with Professor Xiaofan Zhu's team at the Institute of Hematology & Blood Diseases Hospital, Chinese Academy of Medical Sciences, presents a paradigm for upgrading traditional feeder-layer co-culture methods to achieve ultra-large-scale production. The essence of this method lies in an innovative three-step process that not only dramatically increases the yield of induced NK (iNK) cells but also opens new avenues for the low-cost, high-efficiency preparation of CAR-NK cells.

The Initial Goal of the Method: Overcoming Existing Technical Bottlenecks

Existing cytokine-based HSPC-NK differentiation systems produce only 1,879 to 4,450 NK cells per input HSPC, with extremely low terminal CD16 expression (approximately 3%). This severely limits their antibody-dependent cell-mediated cytotoxicity (ADCC) function. Furthermore, balancing CAR engineering with efficient differentiation has been a long-standing unresolved challenge. The research team precisely targeted these bottlenecks, designing an integrated closed-system process that combines high-density HSPC expansion, organoid-driven lymphoid lineage specification, and subsequent NK maturation.

The Core Three-Step Engineering Process 

Phase 1: Super-Expansion of HSPCs (Days 0–14)

The process begins with a single CD34⁺ HSPC isolated from umbilical cord blood. These cells are co-cultured with an irradiated mouse bone marrow stromal cell line, AFT024, in a medium supplemented with an optimized cytokine cocktail containing Stem Cell Factor (SCF), FLT3 Ligand (FLT3L), and Thrombopoietin (TPO). The AFT024 layer mimics the bone marrow hematopoietic niche, providing key self-renewal signals. Over 14 days, a single CD34⁺ HSPC achieves an astonishing 800- to 1000-fold expansion, establishing a massive pool of seed cells.

The Golden Window for Genetic Engineering: Retroviral transduction of the CAR gene is executed during this early expansion window. Because cells are engineered before massive biological amplification, the required viral vector volume is reduced by 140,000- to 600,000-fold compared to traditional methods that transduce mature NK cells. This drastic reduction in viral consumption removes one of the steepest cost barriers in cellular therapy commercialisation.

Phase 2: Lineage Differentiation in Organoids (Days 14–28)

The expanded HSPCs are next transferred to a co-culture system with the OP9 mouse bone marrow stromal cell line. Under these conditions, the cells spontaneously aggregate into three-dimensional, feeder-free organoid structures. This 3D architecture closely mimics the spatial microenvironment of embryonic hematopoiesis. Through complex cell-to-cell interactions—primarily the mechanical activation of the Jagged1/Notch1 signaling pathway—the system efficiently directs the multi-potent HSPCs to commit entirely to the lymphoid and NK cell lineages.

Phase 3: Maturation and Large-Scale Expansion (Days 28–49)

Once committed to the NK lineage, the progenitors are released from the organoids and transferred into closed, gas-permeable culture bags. Driven exclusively by a feeder-free cytokine regimen featuring Interleukin-15 (IL-15) and Interleukin-2 (IL-2), the cells undergo terminal maturation and monumental numerical expansion. The gas-permeable bag configuration ensures a closed system that complies directly with clinical Good Manufacturing Practice (GMP) standards.

Figure 1: Schematic of the “three-step”protocol for large-scale production of iNK and CAR-iNK cells from cord blood CD34+ hematopoietic stem and progenitor 
Note. Adapted from “Scientists Unveil Breakthrough Method to Mass Produce Cancer-Fighting Natural Killer Cells", by Chinese Academy of Sciences (2026).

Industry-Defining Performance Metrics

The quantitative and qualitative yields of this three-step protocol represent a paradigm shift for cellular immunotherapies:

• Unprecedented Expansion Yields: By day 42, a single input CD34⁺ HSPC yields an average of 1.4 × 10⁷ ± 0.1 × 10⁷ iNK cells or 7.6 × 10⁶ ± 1.2 × 10⁶ CD19-CAR-iNK cells. By the final day 49 harvest, these numbers escalate to 8.3 × 10⁷ ± 0.7 × 10⁷ iNK cells or 3.2 × 10⁷ ± 0.2 × 10⁷ CAR-iNK cells. Given that a standard single cord blood unit yields roughly 1–2 × 10⁶ CD34⁺ cells, this platform holds the theoretical capacity to produce trillions of CAR-iNK cells—enough to generate thousands of therapeutic doses from just a fraction of a single cord blood donor unit.
• Exceptional Purity and Safety: The final cellular product exhibits a > 99% purity of CD45⁺ CD56⁺ CD16⁺ NK cells. Crucially, T-cell contamination is completely undetectable, mitigating the risk of Graft-versus-Host Disease (GvHD) and solidifying its safety profile as an allogeneic therapeutic.
• High Endogenous CD16 Expression: In contrast to traditional cytokine-induced methods yielding a nominal ~3% CD16 positivity, this protocol drives high endogenous CD16 expression. This guarantees robust ADCC functionality, allowing these cells to be paired synergistically with monoclonal antibodies for a dual-mechanism tumor assault.
• Sustained In Vivo Efficacy: In multiple mouse xenograft models of human B-cell acute lymphoblastic leukaemia (B-ALL), the generated CD19 CAR-iNK cells consistently halted tumour growth and significantly prolonged host survival. In rigid solid tumour models, cryopreserved-then-thawed CAR-iNK cells sustained active tumour-clearing kinetics in vivo for up to six months.

Translating Laboratory Breakthroughs into GMP-Compliant CAR-NK Manufacturing through Biomaterials and Tissue Engineering 

While Hu et al. provided a magnificent "quantity blueprint," translating this laboratory triumph into an industrialised, clinically accessible product requires resolving a significant regulatory bottleneck: the reliance on mouse feeder cells (AFT024 and OP9) in Phases 1 and 2. Murine components introduce viral safety risks, immunogenic concerns, and profound regulatory hurdles for commercial GMP approval.

By integrating concurrent advances in tissue engineering and biomaterials science, we can map an optimisation pathway to upgrade this protocol into a fully closed, automated, and xeno-free (animal-component-free) manufacturing platform.

1. Humanising the Niche: hMSCs on Scalable Microcarriers

The logical alternative to murine stroma is the use of Human Mesenchymal Stem Cells (hMSCs), the natural architectural support cells of the human bone marrow niche. Historical data confirms that co-culturing cord blood-derived NK progenitors with umbilical cord-derived MSCs (UC-MSCs) alongside an optimised cytokine mix yields a $64.7 \pm 8.4\text{-fold}$ expansion—a tenfold increase over cytokine-only controls ($6.4 \pm 1.5\text{-fold}$). This acceleration is heavily dependent on direct physical contact between the MSCs and the progenitor cells (Boissel et al., 2008).

To industrialise this interaction, microcarrier technology is essential. By seeding hMSCs onto macroscopic, porous microcarriers inside a stirred-tank bioreactor, developers can establish a dynamic, highly scalable suspension system. Because the hMSCs remain anchor-dependent and firmly attached to the microcarrier matrices while the expanding HSPCs grow fluidly in suspension, simple physical sedimentation or inline filtration can instantly separate the two populations at the end of Phase 1. This completely bypasses the need for complex flow cytometry sorting or destructive enzymatic harvesting.

2. Eliminating Organoids: Biomimetic 3D Microgels

Phase 2 of the current protocol depends on OP9-driven 3D organoid aggregation to trigger the Notch signalling cascade required for lymphoid differentiation. Recent milestones in biomaterials (Li et al., 2023; Liang et al., 2023) show that customised, engineered 3D microenvironments can replicate—and even exceed—these biological prompts without animal stroma.

Liang et al. developed an injectable, gelatin-based 3D microgel system that beautifully mirrors the structural architecture of bone marrow. When applied to expanding human haematopoietic stem cells, these biomimetic micro-niches yielded powerful translational insights:

• Autonomous Signal Regulation: The physical constraints, stiffness, and internal topography of these 3D micro-scaffolds naturally stimulate the Notch signalling pathway—the precise mechanism previously requiring OP9 cell interaction.
• Unassisted Proliferation Kinetics: Under optimised parameters (e.g., dynamic 40 rpm agitation over 3 days), the addition of engineered 3D microcarriers alone (without any feeder cell layers) expanded fundamental HSPC subsets up to 179-fold.
• Superior Stemness Preservation: The concentration of highly prized Long-Term Haematopoietic Stem Cells (LT-HSCs) remained 3.5 times higher in 3D biomimetic matrices than in traditional 2D flatbed liquid cultures.
• In Vivo Proof of Concept: In lethal radiation mouse models (9.0 GY), hosts transplanted with cells cultivated within these engineered 3D micro-niches achieved an 80% survival rate, whereas all control animals succumbed to bone marrow failure.

3. Engineering Technological Synergy

Melding these independent discoveries yields a powerful compounding effect for CAR-NK manufacturing:

Future Outlook: A Fully Enclosed, Automated, Continuous-Flow Bioreactor Production Line

When the aforementioned optimisation strategies are seamlessly integrated, a comprehensive blueprint for the next-generation iNK/CAR-iNK manufacturing platform comes sharply into focus:

• Phase I (Bioreactor Expansion): Within a stirred-tank bioreactor, CD34+ haematopoietic stem and progenitor cells (HSPCs) are co-cultured with human mesenchymal stem cells (hMSCs) grown on dissolvable, porous microcarriers. By adopting the physical parameters optimised by Liang et al. (stiffness ~11.2 kPa, pore size ~80 μm), an ideal expansion microenvironment is constructed. The enormous surface-area-to-volume ratio afforded by the 3D microcarriers renders ultra-large-scale expansion of HSPCs feasible within a remarkably compact footprint.
• Phase II (In Situ Directed Differentiation): The microcarriers are retained within the reactor using an appropriate large-pore filtration membrane, whilst the expanded suspension of CD34+ HSPCs or CAR-HSPCs is transferred to a subsequent bioreactor. There, fresh 3D microcarriers—this time devoid of hMSCs—are introduced, and the culture medium composition is switched by withdrawing expansion factors and adding differentiation factors. This directly initiates directed differentiation towards the NK cell lineage within the bioreactor itself. At this stage, the biomimetic physical architecture and surface properties of the microcarriers take over the function of OP9 stromal cells, guiding the efficient specification of HSPCs by activating the Notch signalling pathway—a key mechanism confirmed by Liang and colleagues.
• Phase III (Microcarrier Dissolution and Terminal Culture): Once lineage commitment is complete, a specific enzyme is introduced into the bioreactor to completely dissolve the microcarriers. The liberated NK precursor cells are then either transferred or directly perfused into gas-permeable culture bags or a new downstream bioreactor, where they undergo final maturation and expansion driven by IL-15 and IL-2.

Conclusion

The work of Hu et al. provides a profound quantitative answer to the cell therapy dilemma, demonstrating that a single stem cell can be amplified into an arsenal capable of treating hundreds of cancer patients. By matching this cellular blueprint with advanced humanised biomaterials—such as GMP-grade 3D TableTrix™ gelatin or RecomTrix™ recombinant collagen microcarriers—the industry can build a highly compliant, xeno-free production infrastructure.

Transitioning from manual, artisanal cell culture protocols dependent on murine feeder cells to automated, continuous-flow bioreactors signals the "industrial brewing" era of cell therapy. This evolution drastically drives down manufacturing overheads, minimises batch-to-batch variation, and ensures that highly potent, off-the-shelf CAR-NK immunotherapies move smoothly from laboratory benchmarks to affordable, life-saving realities for cancer patients worldwide.

References

Hu, F., Li, J., Wang, Y., Lin, Y., Zhang, J., Xu, J., Zheng, X., Weng, Q., Liu, X., Geng, Y., Wu, H., Liu, L., Peng, H., Wu, B., Huang, D., Xia, C., Wang, T., Du, X., Zeng, H., ... Wang, J. (2025). Large-scale generation of iNK and CAR-iNK cells from CD34+ haematopoietic stem and progenitor cells for adoptive immunotherapy. Nature Biomedical Engineering, 1-20.

Li, Y., He, M., Zhang, W., Liu, W., Xu, H., Yang, M., & Gao, Y. (2023). Expansion of human megakaryocyte-biased hematopoietic stem cells by biomimetic Microniche. Nature Communications, *14*(1), Article 2207.

Liang, H., Ao, Y., Li, W., Liang, K., Tang, B., Li, J., Wang, L., & Du, Y. (2023). Injectable bone marrow microniches by co-culture of HSPCs with MSCs in 3D microscaffolds promote hematopoietic reconstitution from acute lethal radiation. Bioactive Materials, *22*, 453-465.