Introduction

The field of cell therapy is currently positioned at a critical juncture of technological transformation, characterised by a transition from first-generation autologous approaches to next-generation allogeneic platforms. First-generation cell therapies, such as chimeric antigen receptor T (CAR-T) cells, have demonstrated transformative clinical efficacy, particularly in the treatment of haematological malignancies. However, their autologous nature—requiring patient-specific manufacturing—introduces significant limitations, including high production costs, extended lead times, and substantial interpatient variability. These challenges continue to constrain the scalability and accessibility of such therapies. Consequently, the development of universal, “off-the-shelf” cell therapies has emerged as a central objective within the field.

Natural killer (NK) cells, as key effectors of the innate immune system, are widely recognised as an optimal cellular chassis for the development of off-the-shelf therapies. This is attributable to their intrinsic biological characteristics, including the absence of prior sensitisation requirements, lack of major histocompatibility complex (MHC) restriction, and a low risk of graft-versus-host disease (GvHD). Unlike T cells, NK cells recognise target cells through a balance of activating and inhibitory receptors rather than through antigen-specific T-cell receptor (TCR) engagement. This fundamental distinction enables the safe application of NK cells in allogeneic settings. A pivotal study by Liu et al. (2020) marked a decisive shift in this paradigm, demonstrating that umbilical cord blood-derived CD19 CAR-NK cells achieved a 73% response rate in patients with relapsed or refractory lymphoma and leukaemia. Notably, no patients experienced cytokine release syndrome (CRS), neurotoxicity, or GvHD. These safety characteristics clearly differentiate NK cell-based therapies from CAR-T platforms and further highlight their compatibility with cryopreservation and off-the-shelf inventory models. Accordingly, NK cells represent an industrialisable and scalable immune cell therapy modality. However, when used as monotherapy, NK cells remain limited by two primary clinical challenges: limited in vivo persistence and susceptibility to functional exhaustion in complex pathological microenvironments.

Table 1: Autologous T Cells vs Allogeneic NK Cells

The MSC–NK Interplay: From Inhibition to Developmental Support

Mesenchymal stromal cells (MSCs) are currently among the most widely utilised allogeneic, off-the-shelf therapeutic cell types in cell therapy. MSCs are particularly well suited for allogeneic administration due to their low expression of HLA class II molecules, absence of co-stimulatory molecules such as CD80 and CD86, and extremely low inherent immunogenicity. In addition, their immunoregulatory capacity is enhanced following exposure to inflammatory stimuli, a process commonly referred to as “inflammatory licensing.” Owing to their pronounced tissue tropism, potent immunomodulatory properties, and low immunogenicity, MSCs occupy a distinctive position within regenerative medicine.

The International Society for Cell & Gene Therapy established minimal criteria for defining MSCs, thereby providing a foundation for standardised production and regulatory alignment (Dominici et al., 2006), which, in turn, facilitated their translation into large-scale good manufacturing practice (GMP) production and multicentre clinical evaluation. Early foundational studies demonstrated that MSCs inhibit T cell proliferation through soluble mediators (Aggarwal and Pittenger, 2005), while subsequent investigations extended these findings to NK cells. Specifically, MSCs were shown to suppress NK cell proliferation, interferon-γ (IFN-γ) secretion, and cytotoxic activity via mediators such as prostaglandin E2 (PGE2) and indoleamine 2,3-dioxygenase (IDO) (Spaggiari et al., 2008). Furthermore, MSCs have been demonstrated to regulate dendritic cell maturation and macrophage polarisation (English et al., 2009), functioning as dynamic immunomodulators rather than passive immunosuppressive agents.

In recent years, both scientific and industrial communities have increasingly recognised the programmable potential underlying MSC–NK interactions and have begun exploring combinatorial strategies. The central challenge has shifted from evaluating the individual efficacy of each cell type to rationally designing synergistic interactions between them. The plasticity of MSC–NK interplay extends beyond simple co-infusion approaches and instead involves the construction of advanced “cell-combining-cell” therapeutic systems through engineering methodologies.

Deep Exploration of Synergistic Mechanisms: A Complex Dialogue Beyond "1+1"

Any rigorous evaluation of the MSC–NK combination strategies must begin with a comprehensive mechanistic framework. Moloudizargari et al. (2021) provided one of the most detailed analyses of MSC–NK intercellular communication, describing this relationship as a dynamic and bidirectional regulatory system rather than a purely inhibitory interaction.

Early studies reported MSC-mediated suppression of NK cell activity, leading to initial interpretations of functional incompatibility. Spaggiari et al. (2008) demonstrated that MSCs downregulate the expression of NK cell activating receptors, reduce IFN-γ secretion, inhibit cytotoxic function, and alter intracellular signalling pathways through soluble mediators—including PGE2, IDO, transforming growth factor-β (TGF-β), interleukin-10 (IL-10), and HLA-G—as well as through direct cell–cell interactions. Importantly, this suppressive effect is highly context-dependent and varies according to cytokine milieu, NK cell activation status, MSC-to-NK cell ratio, and inflammatory conditions, with enhanced suppression observed following inflammatory priming (e.g., exposure to IFN-γ).

Conversely, NK cells can exert cytotoxic effects on MSCs. Under inflammatory conditions, activated NK cells can induce MSC apoptosis, particularly when MSCs express stress-associated ligands (Sotiropoulou et al., 2006). These findings demonstrate that MSCs are not inherently immune-privileged and that the MSC–NK interaction is fundamentally reciprocal.

Figure 3: MSC-NK interaction outcome depends on cytokine milieu and activation state.

Note. Adapted from “IFN-Γ stimulated human Umbilical-Tissue-Derived cells potently suppress NK activation and resist NK-Mediated cytotoxicity in vitro”, by Noone, et. al. (2013).

However, mechanistic studies have revealed a more nuanced interaction. The relationship between MSCs and NK cells is not only functional but also developmental. A landmark study by Stecher et al. (2025) published in Nature Communications demonstrated that bone marrow MSC subtypes differentially regulate IL-15-dependent immune lineages. Using Il15 reporter and conditional knockout mouse models, they found that while both are IL-15-high MSCs, Osx⁺ MSCs support NK cell precursors and memory CD8⁺ T cells, whereas Lepr⁺ MSCs sustain the survival of mature NK cells in the bone marrow. Further investigation revealed that IL-15-expressing sinusoidal endothelial cells maintain blood NK cells but do not support bone marrow NK cells. Therefore, MSC heterogeneity, mediated by IL-15 niche biology, controls NK cell development, survival, and immune homeostasis (Stecher et al., 2025). This discovery redefines MSCs as developmental architects of innate immunity, regulators of NK lineage homeostasis, and providers of cellular niches.

Consequently, the MSC-NK interaction is not unidirectional immunosuppression but a tunable immune thermostat (Moloudizargari et al., 2021). Abbasi et al. (2022) further systematically summarised the mechanisms of MSC-NK interaction and their potential clinical applications, emphasising that this bidirectional regulation offers a rich design space for developing novel cell therapies (Abbasi et al., 2022). The MSC-NK axis can be engineered, primed, or genetically modified to favour regeneration, anti-tumour activity, or immune reconstitution depending on the disease context (Moloudizargari et al., 2021). For example, selecting or engineering IL-15-expressing MSCs could modulate NK cell development and function in vivo; the choice of MSC subtype may dictate the quality of NK cell expansion; and off-the-shelf MSC-NK combinations could mimic physiological immune niches to enhance therapeutic efficacy.

The Ideal Partner for Off-the-Shelf Therapy: Why Are MSCs the "Perfect Match" for NK Cells?

Understanding this complex bidirectional dialogue highlights that to build a successful off-the-shelf cell combination, the key lies in solving the problems of "who navigates" and "who fights."

NK cells are potent "killing machines," but they are "lost" in vivo, lacking precise homing ability. Following intravenous infusion, the vast majority of NK cells are trapped in the capillary beds of organs such as the lungs, liver, and spleen, with only a minuscule fraction truly reaching the lesion site. Furthermore, NK cells are short-lived effector cells; without sustained cytokine support, their half-life in vivo is typically only a few days, making it difficult to achieve durable therapeutic effects.

MSCs, conversely, possess innate attributes ideally suited to compensate for these deficiencies:

Built-in GPS Navigation: MSCs have inherent injury-induced chemotaxis. Chemokine gradients (e.g., SDF-1/CXCL12) released by pathological tissues—whether inflammation, ischaemia, trauma, or tumours—act as beacons, actively attracting MSCs to migrate and engraft.

Low Immunogenicity "Universal Donor" Profile: MSCs do not express MHC class II molecules or costimulatory molecules (e.g., CD40, CD80, CD86). This means that allogeneic MSCs rarely provoke host immune rejection or rapid clearance by the recipient's own NK cells or T cells. This characteristic makes them ideal carriers for the development of off-the-shelf combination therapies.

Innate "Bioreactor" Potential: MSCs are amenable to genetic engineering and can be designed as "living factories" that continuously secrete specific therapeutic proteins (e.g., cytokines, enzymes, or antibody fragments). Upon reaching the lesion, they release these agents in situ, enabling localised, long-term treatment.

Therefore, combining MSCs and NK cells synergistically essentially constructs a composite functional entity: an "intelligent navigation system + precise killing ammunition + logistical supply base".

Empowering Through Engineering: From Physical Anchoring to In Situ Activation – Enabling NK Cells to "Hitch a Ride" into the Lesion

Having clarified the bidirectional regulatory mechanisms of MSCs and NK cells, the unique advantages of MSCs as carriers, and the profound impact of the IL-15 niche on NK development, we can now explore how to translate these foundational insights into actionable therapeutic strategies through engineering. The most compelling real-world application of MSC-NK synergy emerges from breakthrough research in this area.

Physical Synergy: Cell Surface Engineering to Construct "Super-complexes" and the "Hitchhiking" Strategy

Traditional cell combination therapies often involve sequential or mixed infusion, which can lead to asynchronous biodistribution—MSCs may reach the lesion while NK cells fail to follow. To address this, a team led by Jiong Wang at Anhui Medical University developed a genetically engineered cell-cell complex using bioorthogonal chemistry-based cell anchoring technology, creating a stable MSC-NK complex (Zhang et al., 2025). Using adipose-derived MSCs (ADSCs) as carriers, they overexpressed IL-15 via mRNA electroporation and efficiently and stably anchored NK cells onto the MSC surface using bioorthogonal click chemistry (e.g., covalent conjugation of azide and DBCO). This formed a functionally integrated MSC-NK complex in vitro. This design directly targets two core challenges of NK therapy: poor tumour infiltration due to limited NK cell homing capacity, and rapid inactivation upon entering the tumour microenvironment.

The researchers vividly termed this the "hitchhiking" strategy—NK cells "hitch a ride" on MSCs, leveraging MSCs' inherent tumour tropism to bypass the dense stromal barrier of solid tumours and precisely reach deep within the lesion. This physical conjugation directly endows NK cells with the MSCs' "navigation" capability. When the complex reaches the lesion site via the bloodstream, MSCs first sense chemotactic signals, extravasate across the endothelium, and subsequently drag NK cells deep into the diseased tissue. Experimental data showed that this anchoring strategy increased NK cell infiltration at the lesion site by more than 3.5-fold, addressing a critical industry challenge: insufficient effector cell homing (Zhang et al., 2025).

Functional Synergy: MSCs as In Situ "Cytokine Factories"

Even if NK cells successfully "hitchhike" to the lesion, they can rapidly lose activity if facing an ischaemic, hypoxic, or immunosuppressive microenvironment. Sustained activation signals are required to maintain NK cell combat effectiveness. This is where the ingenious use of IL-15 engineering in the aforementioned study comes into play. After homing to the lesion as part of the complex, these mRNA-transfected MSCs (IL-15-MSC) continuously secrete IL-15 within the local microenvironment, creating a high-concentration cytokine "nest." This in situ delivery strategy offers dual advantages:

Precise Dosing: It avoids the severe toxicities associated with systemic IL-15 administration, such as capillary leak syndrome and hypotension.

Sustained Empowerment: After engrafting at the lesion site, IL-15-MSCs function as "bioreactors" for days to weeks, continuously providing activation signals to co-localised NK cells, significantly prolonging their survival and functional status in vivo.

Validation of Synergistic Effects

In functional validation, this engineered complex demonstrated multiple synergistic advantages (Zhang et al., 2025):

• Enhanced Tumour Localisation: In an orthotopic lung cancer model, the MSC-NK complex showed significantly greater enrichment in tumour tissue than NK cells alone.
• Sustained IL-15 Signalling: NK cells in the complex exhibited sustained STAT5 phosphorylation, providing direct evidence of IL-15 pathway activation.
• Improved Cytotoxic Function: NK cells within the complex maintained high levels of granzyme B and perforin expression, preserving their killing activity.
• Synergistic Response to Immune Checkpoint Blockade: When combined with an anti-Galectin-9 blocking antibody, the complex therapy achieved over 85% tumour inhibition in a patient-derived ovarian cancer xenograft (PDX) model.

Figure 2: Engineered MSC–NK Complex for Solid Tumours

Note. Adapted from “Engineered mesenchymal Stem Cell–NK cell complexes for spatially targeted and functionally revitalized cancer immunotherapy”, by Zhang et al. (2025).

This series of studies confirms that by deeply integrating the "navigation" and "supply" capabilities of MSCs with the "killing" function of NK cells through engineering, we can construct functionally integrated, microenvironment-adaptive "intelligent cell combinations." Schwarz & Leonard (2016) systematically outlined, in their review, how to engineer cell therapies for robust interaction with host physiology, providing a crucial theoretical framework for the design of such intelligent cell combinations.

From Scientific Concept to Industrial Practice: Engineering Breakthroughs Demand Industrialisation Support

The groundbreaking work led by Jiong Wang at Anhui Medical University has advanced the synergy between MSCs and NK cells from a "natural interaction" to an "engineered design" phase. Central to this advancement is the “hitchhiking” strategy, which integrates the intrinsic navigational capacity of mesenchymal stromal cells (MSCs) with the cytotoxic function of natural killer (NK) cells via cell-surface engineering. This approach reframes the therapeutic concept beyond that of a conventional drug, instead establishing an intelligent, flexible, and programmable off-the-shelf cellular platform.

​However, despite the conceptual and experimental promise of these strategies—whether based on engineered cell–cell complexes or genetically modified MSCs functioning as localised cytokine sources—their translation into clinically accessible and economically viable therapies remains contingent upon the development of scalable, standardised, and automated manufacturing systems. The successful progression from laboratory innovation to clinical application depends fundamentally on the ability to ensure reproducibility, efficiency, and cost-effectiveness at an industrial scale.

Addressing this challenge constitutes a central objective for industry stakeholders. The transition of MSC–NK combination therapies from bespoke, laboratory-based constructs to standardised, off-the-shelf products requires the establishment of robust, economically sustainable production platforms for both cell types. Within this context, CytoNiche Biotech leverages its core expertise in scalable, automated cell-processing technologies to deliver integrated solutions encompassing MSC expansion, harvesting, and formulation. Standardisation and reproducibility in MSC manufacturing are essential for providing a consistent, high-quality cellular foundation upon which complex combination therapies can be reliably constructed. In parallel, the development of cost-effective NK cell culture systems with defined compositions and optimised processes aims to reduce technical barriers and manufacturing costs associated with NK cell expansion. Such advancements enable research and clinical manufacturing platforms to access highly active, high-purity effector cell populations, thereby facilitating a greater focus on upstream therapeutic innovation.

The continued evolution of next-generation cell therapies will require not only advances in fundamental biological understanding but also the parallel maturation of enabling industrial technologies. By integrating scalable culture systems and standardised manufacturing frameworks, it becomes possible to bridge the gap between academic discovery and clinical implementation. This convergence is essential for advancing complex cellular therapies, such as MSC–NK combinations, towards broader clinical adoption and therapeutic impact.

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