Phase 1Collection and separation
01MSC isolation from tissue (bone marrow, adipose tissue, umbilical cord) (harvest / primary isolation)
The starting step that releases the target cells from the source tissue and purifies them by adherence selection. Density gradient for bone marrow, enzymatic digestion for adipose tissue/umbilical cord.
- Standard method
- Mononuclear cell isolation by enzymatic digestion (collagenase) + density gradient centrifugation, followed by adherence selection
Why it is the standard. Isolation that satisfies item 1 of the ISCT minimal criteria, "plastic adherence", is the starting point. Density gradient + adherence is standard for bone marrow, and collagenase digestion for adipose tissue/umbilical cord. In GMP, the closed Sepax/Sefia automate the density gradient and secure sterility and traceability (ISCT Dominici 2006).
Check also. The best separation method differs greatly by tissue source (bone marrow, adipose tissue, umbilical cord), and lot-to-lot differences in enzyme and variation in activity in collagenase digestion affect recovery and cell quality. Residual enzyme must be removed by washing so that it is not carried into later steps, and for bone marrow the basic approach is density gradient + adherence selection without enzymes.
With Autologous / 2D planar flask scale-out
- A bank model does not hold for patient-specific lots
- Method: No 2-tier cell bank (MCB → WCB) is built; the starting point is a primary culture from each patient donor. No formal bank is established, and the required characterization is performed in-process and at release for each patient lot
- Check also: Donor-to-donor variability is large (doubling time differs by more than 2-fold, and final yield varies on the order of 10^7–10^9 cells), so lot specifications and yield are uneven from patient to patient
About this step and the alternatives
The source materials are bone marrow, adipose tissue and umbilical cord. Taking the target stem cells out of them is this step, the starting point of manufacturing.
Bone marrow is liquid, so mononuclear cells are scooped out by density gradient centrifugation, which separates layers by differences in specific gravity. Adipose tissue and umbilical cord are firm tissues, so an enzyme called collagenase loosens the fibers and releases the cells individually.
At this point the cells are a mixed bag. In the adherent culture that follows, only the "cells that stick to the vessel" are selected to raise purity; this is the preliminary sorting step.
The first step, taking the target stem cells out of the source tissue. For bone marrow, mononuclear cells are separated by density gradient; for adipose tissue or umbilical cord (Wharton's jelly), the tissue is broken up with enzymes to release the cells. A mixed cell population is separated here and purified in the subsequent adherent culture.
Alternatives in practice
- Explant method (tissue fragment outgrowth culture) · Corning CellBIND T-75 flasks / Sarstedt tissue culture flasks + surgical scalpel — Avoids enzymatic digestion and reduces cell stress. Effective for umbilical cord Wharton's jelly and synovium, but outgrowth takes time and yield varies widely
- Immunomagnetic positive selection (CD271/MSCA-1) · Miltenyi Biotec CliniMACS Prodigy + CliniMACS CD271 Reagent (LNGFR) / for research, CD271 MicroBead Kit, Anti-MSCA-1 (W8B2) MicroBeads + autoMACS Pro — Selects fresh MSC from bone marrow or lipoaspirate at high purity. Effective for purification before culture, but yield is limited
- Fully automated density gradient separation · Cytiva Sepax C-Pro (NeatCell/density protocol) / Fresenius Kabi Lovo — Closed system and GMP-compatible, eliminating operator-to-operator variability. An alternative to manual Ficoll
Cell bank establishment (MCB/WCB) (cell banking)
The basis for expanding the initial cells, freezing them in aliquots and supplying uniform starting material across lots over the long term.
- Standard method
- 2-tier cell bank (MCB → WCB) + characterization package
Why it is the standard. A tiered bank in accordance with ICH Q5D (origin, preparation and characterization of cell substrates) is the reference. Each bank is checked for identity, purity, sterility, mycoplasma and karyotype, forming the basis of lot-to-lot consistency. This is especially important for allogeneic products.
With Autologous / 2D planar flask scale-out
- Starts from primary isolation of patient tissue, not from thawing a bank
- Method: No bank vial thaw or seed train is performed; the patient tissue isolate from step [0] is connected directly to recovery and initial expansion. The thawing step itself, which is needed for DMSO removal, is omitted
- Check also: The starting material is patient-derived, its quality and quantity are not constant, and the cell number at the start of the seed train cannot be standardized
About this step and the alternatives
Rather than taking cells from a new donor for each manufacture, this step expands the cells isolated first and freezes them in aliquots, building a "stock of seeds", so to speak.
The key is the two-tier structure. First the original master (MCB) is made, and from it the working bank (WCB) for daily use is started. Think of locking the original in a safe and working from copies.
Because the starting material of every lot always has the same origin, quality variation is kept down and long-term stable supply is possible. Identity is also confirmed by characterization.
Rather than collecting from a new donor for every manufacture, the initially isolated cells are expanded and frozen in aliquots to make master and working cell banks. This makes the starting material of each lot uniform and allows cells of the same origin to be supplied over the long term.
Alternatives in practice
- Single bank (MCB only) · Same CryoMed CRF + vapor-phase LN2 tank — Omitting the WCB reduces development cost. For early clinical and small-scale production. Risk of vial depletion in commercial use
- Bankless (manufacture from fresh material each time) · Cytiva Sepax C-Pro processing line for each donor collection — Freezing and characterization can be skipped, but lot-to-lot variability and donor dependence are large and setting specifications is difficult
Phase 2Cell culture and production (upstream)
03Cell bank thawing and recovery + seed train expansion → a bank vial is thawed and the seed culture is built up in stages in flasks / shaking systems (thawing and seed train)
Direct large-scale culture from a frozen bank is not possible. Thaw recovery and a staged seed train are essential to secure the required inoculum cell quantity.
- Standard method
- Controlled thawing in a water bath / automated device, DMSO removal by centrifugation, recovery in static T-flask culture, and seed train expansion with sequential passaging into larger vessels
Why it is the standard. Bank thaw → staged seed train is a step that actual processes always go through as the stage before large-scale expansion. It secures post-thaw viability (generally ≥80%) and passage recovery, and meets the seeding density of the main culture.
About this step and the alternatives
The step that wakes the frozen vial and prepares the cells at the starting line of culture. A bank has limited quantity and viability, so it cannot go straight into a large vessel.
Thawing is the first key point. The vial is warmed evenly in a water bath or an automated device, and centrifugation washes out the cryoprotectant (DMSO) so that the cells recover. The feeling is of waking them carefully, without hurrying.
After that comes the seed train, starting in T-flasks and moving to successively larger vessels. The inoculum cell number and cleanliness needed for the next step are brought up stage by stage.
Frozen MCB/WCB vials have limited viability and cell number and cannot be inoculated directly into a large expansion vessel. This is the standard step that removes the cryoprotectant and allows recovery by controlled thawing, then expands in stages from T-flasks to shaking/multilayer systems, meeting the inoculum cell number and bioburden control needed for initial expansion and large-scale expansion.
Alternatives in practice
- Closed-system automated expansion, with thaw through recovery in one go · Miltenyi CliniMACS Prodigy (Adherent Cell Culture, ACC application) — Thawing, washing and initial expansion are completed in 1 closed-system instrument. Reduces contamination, but throughput is 1 lot at a time
- Planar seed train in multilayer flasks · Corning CellSTACK 1–5 layers / Thermo Nunc Cell Factory — Established adherent planar expansion. Scale is linear with the number of flasks, and it requires labor and floor space
Initial expansion (planar culture / passaging) (planar expansion culture)
Anchorage-dependent MSC are grown adherent on planar surfaces, keeping passage number and PDL low to secure bioreactor-scale quantities before senescence.
- Standard method
- Adherent, anchorage-dependent culture in multilayer flasks (xeno-free medium)
Why it is the standard. hPL is becoming the industry standard as a replacement for FBS (avoiding immunogenicity and bovine-derived infection risk). Multilayer flasks are technically mature and are the reference configuration for seed expansion. Passage number and PDL (population doublings) control suppress senescence and changes in properties.
Check also. MSC proliferate more slowly as passages accumulate, and they age (senescence) and lose quality, so expansion in planar culture has an upper limit on the "usable passage number". It is important to plan the initial seeding cell number and the expansion design in advance so that a clinical dose can be secured in few passages.
With Autologous / 2D planar flask scale-out
- Scale-out, not scale-up
- Method: Instead of microcarriers + single-use stirred-tank bioreactors, adherent culture is done in multilayer planar flasks (HYPERStack / CellSTACK, etc.) or closed automated planar culture systems (Terumo Quantum hollow fiber bioreactor, Lonza Cocoon), and the devices are run in parallel for each patient
- Equipment: Multilayer planar flasks / hollow fiber, closed automated planar culture systems (stirred-tank bioreactors not used)
- Check also: Uniform online control like that of a stirred tank (unified DO and pH control) is difficult, and lot size stays at 1 to a few doses
About this step and the alternatives
The step of steadily expanding a small number of cells first in planar vessels. MSC are "anchorage-dependent", that is, they can divide only after attaching to a surface.
A wide surface area is secured in multilayer flasks, and cells are passaged repeatedly by detaching them and transferring them. The idea is to gain numbers through surface area until there is enough to load into the next large-scale culture.
The key point here is not to be greedy with the passage number. If cells are expanded too far, they age and lose proliferative capacity and therapeutic effect. Holding the reins to keep quality takes skill.
The small number of isolated cells is first expanded in planar vessels. MSC are anchorage-dependent cells (they grow only after attaching), so they are attached to the surface and passaged repeatedly until the amount can be loaded into the downstream bioreactor. Keeping the passage number low and avoiding senescence is the key to maintaining quality.
Alternatives in practice
- Manual culture in single-layer T-flasks · Corning/Sarstedt T-75 and T-175 flasks — Flexible with minimal equipment, but labor-intensive and limited in scale. For research to very early development
- Automated adherent culture system · Sartorius (formerly TAP) CompacT SelecT / ambr automated cell — Robotic automated passaging of multiple flasks. Improves reproducibility, but instrument cost is high
- Hollow fiber bioreactor · Terumo BCT Quantum Flex — Closed system, high surface area to footprint ratio. High density through perfusion. Limited sampling and observability
Large-scale expansion (microcarrier suspension bioreactor) (microcarrier suspension culture)
Obtains clinical doses, for which planar surface area is insufficient, at a multi-billion scale with high reproducibility by controlled culture in a stirred tank + microcarriers.
- Standard method
- Culture with microcarriers + single-use stirred-tank bioreactor
Why it is the standard. The standard scalable strategy for expanding anchorage-dependent MSC to the multi-billion scale. Stirred tank + microcarriers has a high surface-to-volume ratio, controls dissolved oxygen, pH and agitation through instrumentation, and gives higher lot-to-lot consistency than planar culture (Cytotherapy/PMC5895685).
Check also. Microcarrier suspension culture is the practical standard for scale-up, but not every cell line behaves the same way when moved over from planar culture, and the transfer conditions need to be worked out. Cells are easily damaged by agitation shear and by handling at detachment, so validating operating conditions that preserve viability and quality is a prerequisite.
About this step and the alternatives
The dose used clinically is several hundred million to several billion cells. Planar culture lacks the surface area, so the stage moves here to a stirred-tank bioreactor.
The trick is to "let the cells attach to tiny beads (microcarriers) and float in the liquid". The idea is to have anchorage-dependent MSC swim along with their scaffold.
Dissolved oxygen, pH and agitation are controlled to achieve uniform, highly reproducible growth. This is the heart of mass production, gaining large cell numbers on a small floor area.
Planar culture does not provide enough surface area to obtain a clinical dose (several hundred million to several billion cells). In a stirred-tank bioreactor, cells are attached to microcarriers (tiny beads) and cultured in large quantities in suspension. Dissolved oxygen, pH and agitation are controlled to achieve uniform, highly reproducible growth.
Alternatives in practice
- Fixed-bed / packed-bed bioreactor · Cytiva iCELLis nano / iCELLis 500+ (packed with polyester microfibers) — High surface area in a small footprint. Detachment and carrier separation are unnecessary or simplified, but uniform cell recovery is an issue
- Hollow fiber bioreactor · Terumo BCT Quantum Flex — High density by perfusion, closed system. Scale is handled by running units in parallel
- Spinner flask · Corning single-use spinner / Eppendorf (DASGIP) spinner — For PD and small-scale studies. Control of agitation shear and oxygen supply is inferior to a stirred tank
In-process monitoring (cell number, viability, metabolism) (in-process monitoring)
Tracks cell number, viability and metabolites to judge the timing of medium exchange and harvest and to detect growth abnormalities early.
- Standard method
- Automated cell counting (fluorescent staining) + offline metabolite analysis
- Reagents and materials
- Via1-Cassette for ChemoMetec NC-200 and Via2-Cassette for NC-202 (both with immobilized acridine orange + DAPI). Solution 13 (AO•DAPI) is used separately for free-form viability staining. Trypan blue reagent packs for Beckman Vi-CELL. Glucose/lactate/glutamine/ammonia/LDH enzyme reagent cartridges for Roche Cedex Bio, Nova BioProfile reagent packs
Why it is the standard. NucleoCounter NC-200 distinguishes live and total cells by AO/DAPI fluorescence, is more objective and reproducible than visual trypan blue counting, and is the de facto reference instrument for MSC counting. Metabolite control optimizes the medium feeding strategy.
With Autologous / 2D planar flask scale-out
- No microcarriers are used, so no separation step is needed
- Method: Detachment from the planar surface with a recombinant enzyme only. Because there are no microcarriers, the carrier separation step using mesh/in-line filters disappears
- Equipment: Mesh/in-line filters for carrier separation are not used (detachment may be completed inside the closed system)
About this step and the alternatives
A step that measures, on the fly, whether the culture is going well. It continuously follows how many cells have grown and how many are alive.
Live and dead cells are distinguished by fluorescent staining and counted automatically, and glucose consumption (nutrient) and accumulation of lactate and ammonia (waste) are analyzed at the same time.
These become the yardstick for deciding when to exchange medium and harvest. They are also an early warning that detects growth abnormalities quickly and saves the lot.
Tracks how much the cells are growing, how many are alive, and whether they are consuming nutrients and releasing waste during culture. Glucose consumption and lactate/ammonia accumulation are used to decide the timing of medium exchange and harvest, and growth abnormalities are detected early.
Alternatives in practice
- Trypan blue + hemocytometer (visual) · Hirschmann/Marienfeld improved Neubauer hemocytometer + inverted microscope — Reference method with minimal equipment. Subjective and low throughput, so unsuited to routine in-process use
- Inline permittivity/capacitance viable cell density measurement · Hamilton Incyte Arc / Aber Futura biomass probe — Real-time, non-destructive VCD monitoring. Absolute value calibration is required for microcarrier culture
- Imaging cytometer · Nexcelom (Revvity) Cellaca MX / Cellometer — High-throughput counting of many samples. Live/dead judgment with two fluorescent colors. Mid-range equipment cost
Phase 3Harvest and clarification
07Cell detachment and microcarrier separation (detachment / carrier separation)
Short enzymatic detachment, then mesh filtration separates beads from cells by size difference to obtain a pure cell suspension.
- Standard method
- Detachment with a recombinant enzyme + carrier separation with mesh/in-line filters
Why it is the standard. TrypLE Select is animal-component-free with high lot consistency and is the standard detachment reagent for xeno-free processes. Separating the size difference between microcarriers (100–300 µm) and cells (~15–20 µm) by mesh filtration is the reference approach.
With Autologous / 2D planar flask scale-out
- Small lots, so no large-capacity harvest equipment is needed
- Method: Does not depend on large-volume counterflow centrifugation (kSep, etc.); processed by smaller-scale washing and concentration (e.g., centrifugal washing with Terumo LOVO) or by integrated washing inside the closed system
- Equipment: Small- to mid-scale automated washing and concentration (LOVO, etc.) / integrated washing in the device (large counterflow harvesters are often unnecessary)
About this step and the alternatives
The step that detaches the expanded cells from the microcarriers they were attached to and recovers them. It is the separation work at the exit of the culture.
A recombinant enzyme dissolves the glue of attachment so that the cells float free, and then mesh or in-line filters strain out only the beads.
The key point is to cut the enzyme treatment short. Long exposure damages the cell surface and lowers viability and quality, so working quickly protects quality.
The expanded cells are pulled off the microcarriers, separating beads and cells. After an enzyme releases the attachment, the microcarriers are removed with filters or sieves to give a pure cell suspension. It is important to keep the enzyme treatment at detachment short so as not to damage the cells.
Alternatives in practice
- Dissolvable microcarriers to skip the separation · Corning Dissolvable Microcarrier / Cytiva — Completely dissolved by an enzyme, so physical carrier separation is unnecessary. High cost, with restrictions on applicable media
- Inertial microfluidic filtration · (Research stage) spiral/inertial microfluidic devices — Continuous, sieve-free separation without clogging. Limited track record at GMP scale
- Carrier separation by gravity sedimentation · Closed-system sedimentation bag / conical sedimentation chamber — Simple and low shear. Takes time, and cell loss needs to be managed
Washing, concentration and medium exchange (washing and concentration)
Closed-system counterflow centrifugation removes enzyme and medium, exchanges into formulation buffer with low shear and concentrates to the target concentration.
- Standard method
- Automated washing and concentration by counterflow centrifugation
- Reagents and materials
- Human serum albumin for cell cryopreservation
Why it is the standard. kSep counterflow centrifugation washes and concentrates with high recovery and high viability while limiting shear, and is closed, single-use and GMP-compatible. It is the downstream standard for large-scale allogeneic MSC. For small to mid scale, Sefia/Sepax are widely used (Sartorius/ISCT 2021).
About this step and the alternatives
The step that washes away the "extras" such as the enzyme used for detachment and the old medium, swaps the cells into the formulation liquid and concentrates them to the target density.
By a method called counterflow centrifugation, washing and concentration are automated inside closed tubing. Because there is no contact with outside air, it is easy to maintain sterility, which is the advantage.
Cells are sensitive to strong liquid flow (shear stress), so they are handled gently to protect viability. This step acts as the bridge that passes the baton to the next step, formulation.
Removes the enzyme used for detachment, medium components and excess liquid before DMSO, replaces the cells into the formulation buffer and concentrates them to the target concentration. Doing this in a closed, automated way maintains sterility and protects viability by limiting shear stress.
Alternatives in practice
- Tangential flow filtration (hollow fiber TFF) · Repligen KrosFlo KR2i / KML, Cytiva hollow fiber cartridges — Continuous concentration and diafiltration for medium exchange. Shear and membrane fouling need optimization
- Batch centrifugation (manual) · Thermo Sorvall / Eppendorf 5910Ri + conical tubes — Minimal equipment. Open handling carries contamination and human-error risk. For early development
- Closed-system automated washing · Cytiva Sepax C-Pro (Wash protocol) — GMP closed washing at small to mid scale. Throughput is smaller than kSep
Phase 4Formulation and fill-finish
09Formulation (formulation and final buffer preparation) (formulation)
Adjusts the cells to a composition suitable for administration or freezing, designing the cryoprotectant and concentration according to the administration route.
- Standard method
- Formulation into a final drug product containing a cryoprotectant (ready-made cryopreservation medium)
Why it is the standard. CryoStor is a GMP-grade ready-made cryoprotectant solution with a defined composition, excels in lot consistency and regulatory compliance, and is the de facto standard for allogeneic MSC products. A DMSO concentration of 5–10% is common and is designed in balance with toxicity at administration.
Check also. For injectable cell suspensions, it is standard to pass the suspension through a closed-system in-line cell strainer (e.g., 70 µm) just before filling to remove aggregates, ensuring passage through the needle and particle control.
About this step and the alternatives
The step that tailors the washed and concentrated cells into a "formulation" suited to administration or cryopreservation. In cooking terms, it is the stage of seasoning and plating.
Cell number and viability are matched to the target specification, and cryoprotectants and stabilizing components are added and mixed uniformly. Concentration and volume are also designed according to the route of administration, whether infusion or local injection.
The composition decided here determines the fate of the cells through transport, freezing and administration. It is the design step that decides the "form" of the final drug product.
Washed and concentrated cells are adjusted to a composition suitable for administration or cryopreservation. Cell number and viability are matched to the target specification, and protectants and stabilizing components are added and mixed uniformly. Concentration and volume are designed according to the administration route (infusion, local).
Alternatives in practice
- Fresh (non-frozen) formulation · Closed-system filling bag + PlasmaLyte A dilution line — Avoids freeze damage and needs no thawing at administration, but the shelf life is short at several hours to several days and logistics are demanding
- DMSO-free cryoprotectant · BioLife HypoThermosol / Sigma sugar- and polymer-based (trehalose/PVP) formulations — Avoids DMSO toxicity and adverse reactions at administration. Post-thaw performance must be verified
- In-house prepared DMSO/HSA mixture · In-hospital/in-facility bag mixing — Low cost, but validating composition and sterility each time is a burden. Ready-made solutions are mainstream
Filling and controlled-rate freezing (filling and controlled-rate freezing)
Aseptically filled into administration units and frozen at a controlled rate of about 1 °C/min to limit ice crystal damage and assure post-thaw survival and function.
- Standard method
- Closed-system aseptic filling + controlled-rate freezing (CRF) → vapor-phase liquid nitrogen storage
Why it is the standard. CryoMed CRF freezes with a defined profile that includes latent heat compensation, minimizing ice crystal damage and assuring reproducibility of post-thaw viability and function; it is the standard freezing instrument for cell therapy. Vapor-phase LN2 storage is recommended over liquid immersion to suppress cross-contamination.
About this step and the alternatives
The step that aseptically fills the finished product into containers that serve as administration units, such as bags and vials, then freezes and stores it.
If freezing is rough, ice crystals pierce the cells. To avoid this, controlled-rate freezing (CRF) is used, which lowers the temperature precisely at about 1 °C per minute and also corrects for the heat released at the moment of freezing.
After freezing, the product is stored long-term in the vapor phase of liquid nitrogen. It is the final storage step that keeps the cells asleep, time and all, until the day of thawing to maintain quality.
The final drug product is aseptically filled into administration units (bags, vials, cryobags), and the temperature is lowered precisely at about 1 °C per minute to limit ice crystal damage during freezing. Freezing follows a defined profile that includes compensation for the latent heat of fusion, and long-term storage is in the vapor phase of liquid nitrogen.
Alternatives in practice
- Passive freezing container (-1 °C/min) · Thermo Scientific Mr. Frosty / CoolCell — Simple, inexpensive and effective for small scale, but the rate profile cannot be recorded, making it unsuitable for GMP final products
- Direct freezing in a mechanical ultra-low-temperature freezer · Thermo Scientific TSX / Forma -80 °C freezers — Simple because the CRF is omitted, but the freezing rate is not controlled. Risk of lower MSC viability; for research/intermediate storage
Phase 5Quality attributes and analytics
11Surface marker phenotype (ISCT minimal criteria) (surface marker analysis)
ISCT criterion 2 for judging MSC identity: CD73/90/105 positive ≥95%, hematopoietic markers/HLA-DR negative ≤2%.
- Standard method
- Surface marker quantification by multicolor flow cytometry
- Makers
- BD, Beckman Coulter, Cytek, BioLegend
Why it is the standard. The standard identity test that satisfies item 2 of the ISCT minimal criteria (Dominici 2006). Judges CD73/CD90/CD105 ≥95% positive and hematopoietic markers/HLA-DR ≤2% negative. The Stemflow kit corresponds directly to this criterion, and WHO is also considering standardization of the MSC phenotype by flow cytometry (WHO/BS/2019.2376).
Check also. The international criterion (ISCT minimal criteria) of CD73, CD90 and CD105 positive and CD45 etc. negative is a necessary condition showing that the cells are MSC, but it does not guarantee the efficacy (potency) of the product. Pass/fail is not decided by surface markers alone; quality is judged together with the downstream potency assays.
About this step and the alternatives
A test that tells by the "name tags (markers)" on the cell surface whether the cells that were made really are MSC.
Multicolor flow cytometry judges all of this at once. The pass criteria are CD73, CD90 and CD105 positive (95% or more) and the hematopoietic CD45 and CD34 and HLA-DR negative (2% or less).
This combination of positives and negatives is itself the international ISCT definition of MSC. It serves as an ID card that objectively shows the cells' identity.
Confirms by the expression pattern of surface markers that the obtained cells really are MSC. Positivity for CD73, CD90 and CD105 (≥95%) and negativity for hematopoietic markers (CD45, CD34, etc.) and HLA-DR (≤2%) is the international criterion that defines MSC.
Alternatives in practice
- Single-stain, individual antibody flow cytometry · Same flow cytometer as above + individual BioLegend/BD CD73/CD90/CD105 antibodies — Antibodies and fluorophores can be freely optimized. Preparing multiple tubes increases labor and variability
- Immunocytochemistry / immunofluorescence · Fluorescence microscope (Zeiss/Nikon) or high-content Revvity Operetta — Localization can also be assessed. Quantification and standardization are inferior to flow cytometry
- Spectral flow cytometry · Cytek Aurora (5L) — Strong for multicolor panels and autofluorescence separation. For detailed MSC phenotyping; the analysis is more demanding
Trilineage differentiation potential (multipotency) (trilineage differentiation test)
ISCT criterion 3: confirms trilineage differentiation into bone, fat and cartilage by staining, supporting multipotency as stem cells.
- Standard method
- Trilineage differentiation induction + lineage-specific staining (+ RT-PCR as needed)
Why it is the standard. The standard functional/identity test that satisfies item 3 of the ISCT minimal criteria (differentiation into osteoblasts, adipocytes and chondrocytes). Established stains for each lineage (Alizarin Red S / Oil Red O / Alcian Blue) are widely adopted and can be supplemented by RT-PCR (RUNX2/PPARγ/SOX9, etc.).
About this step and the alternatives
A step to confirm the multipotency, one of the definitions of MSC, of being able to change form in three directions: bone, fat and cartilage.
Cells are cultured in the differentiation induction medium for each of the three lineages, and the changes are confirmed by staining. The markers are calcium deposition for bone, lipid droplets for fat and glycosaminoglycans for cartilage.
Gene expression (RT-PCR) is also used as needed. It is proof of character that supports the cells still having the stem-cell-like "power to transform".
The third part of the MSC definition is "multipotency, the ability to differentiate into bone, fat and cartilage". Cells are cultured in differentiation induction media, and calcium deposition for bone, lipid droplets for fat and glycosaminoglycans for cartilage are confirmed by staining, supporting the stem-cell character.
Alternatives in practice
- Quantitative differentiation assay (dye extraction and absorbance) · Plate reader (SpectraMax) + cetylpyridinium salt/isopropanol extraction — Alizarin Red / Oil Red O is extracted and semi-quantified by absorbance. More objective than visual staining
- Differentiation marker qRT-PCR · Thermo Applied Biosystems QuantStudio / Bio-Rad CFX Opus — Quantitative evaluation by expression of genes such as RUNX2/PPARG/SOX9. More sensitive and specific than staining
Potency assay: immunomodulatory capacity (potency assay)
A potency assay that measures IDO/PD-L1 induction on IFN-γ stimulation and suppression of T cell proliferation, assuring the therapeutic potency of immunomodulation.
- Standard method
- IDO-1/PD-L1 expression after IFN-γ stimulation (flow cytometry) + T cell proliferation suppression assay
Why it is the standard. IDO is induced only on IFN-γ licensing and is both a major mediator and a sentinel marker of MSC immunosuppression. Flow cytometric quantification of IDO-1/PD-L1 is rapid and easy to build into the manufacturing process, and a multi-indicator matrix potency assay that correlates with a functional assay (suppression of T cell proliferation) is the reference (Cytotherapy 2018; PMC11630899).
Check also. A potency assay has to be designed to fit the mechanism of action (therapeutic mechanism) of each product, and a single indicator such as IDO or PD-L1 may not adequately represent clinical efficacy. Assay variability is also large, so it is safer to cross-check with multiple indicators and T cell suppression capacity.
About this step and the alternatives
Does this product really work? This is the test that goes deepest into the substance, measuring the "efficacy" of immunomodulation, the therapeutic mechanism.
Cells are stimulated (licensing) with IFN-γ, a signal of inflammation, and how much IDO-1 and PD-L1 are induced is measured by flow cytometry. How much T cell proliferation can be suppressed is also evaluated.
This potency value is the basis for assuring the efficacy of the lot. It is a checkpoint to avoid shipping "cells that cannot work" even though they look the same.
For many MSC products, immunomodulation is the therapeutic mechanism. The potency assay measures that "efficacy". It measures how much IDO and PD-L1 are induced on stimulation with IFN-γ (licensing), or how much T cell proliferation is suppressed, assuring the efficacy of the lot.
Alternatives in practice
- IDO enzyme activity by kynurenine production · Plate reader (SpectraMax) + Ehrlich reagent, or LC-MS/MS (Agilent/Sciex) — Directly quantifies IDO function by supernatant kynurenine concentration. Simpler and more objective than flow cytometry
- Suppression of mixed lymphocyte reaction (MLR) · Flow cytometer (CFSE) / 3H-thymidine incorporation — Functionally evaluates immunosuppressive capacity. Donor PBMC variability is large and standardization is an issue
- Secreted factor profile (PGE2/TSG-6, etc.) · R&D Systems/Thermo ELISA / Luminex (PGE2, TSG-6, IL-6) — Multi-item quantification of mechanism-related factors. Correlation with potency must be established
Cell number and viability (final product strength) (final product cell number and viability)
Viable cell number and viability including after thawing (≥70% as a guide) define dose and efficacy; the core of release testing.
- Standard method
- Automated viable cell counting by two-color fluorescent staining (including after thawing)
- Makers
- Bio-Rad, ChemoMetec, Beckman Coulter, BD
- Reagents and materials
- Via1-Cassette for ChemoMetec NC-200 and Via2-Cassette for NC-202 (both with immobilized acridine orange + DAPI). Solution 13 (AO•DAPI) separately for free-form staining. Trypan blue reagent for Vi-CELL as a reference
Why it is the standard. The AO/DAPI two-color method clearly distinguishes live cells from dead cells and debris, is more reproducible than visual trypan blue counting and is suitable for post-thaw samples. Viable cell number and viability are the dose specification and the core of release testing. In many cases a viability of ≥70% is generally set as the specification.
About this step and the alternatives
How many cells are administered, and what fraction of them are alive. This is the most basic and indispensable specification test, which determines dose and effect.
Only live cells are counted automatically by two-color fluorescent staining. The key is to measure the value after thawing as well. The number that survives freezing and thawing is the power that actually works (potency).
Only lots that meet the specified viable cell number and viability (generally 70% or more) go on to shipment. This step draws the minimum guarantee line of the product.
The number of cells administered and the proportion of them that are alive are the most basic specification that determines dose and efficacy. Measured including after thawing, only lots that meet the specified viable cell number and viability (generally ≥70%) are shipped.
Alternatives in practice
- Trypan blue + automated counter · Thermo Countess 3 / Bio-Rad TC20 — Simple and low cost. Stains only dead cells, so the resolution of live/dead discrimination is lower than with two-color fluorescence
- Viability by flow cytometry · BD FACSLyric / Beckman CytoFLEX + 7-AAD/Annexin V — Can distinguish apoptosis from necrosis. Useful for evaluating delayed cell death right after thawing; low throughput
Sterility, mycoplasma and endotoxin (sterility / mycoplasma / endotoxin)
To ensure the safety of living-cell products that cannot be terminally sterilized and have a short shelf life, sterility, mycoplasma and endotoxin are tested by rapid methods.
- Standard method
- Pharmacopoeial sterility test + nucleic acid amplification (NAT) for mycoplasma + horseshoe crab reagent for endotoxin
Why it is the standard. USP <71>/EP 2.6.1 for sterility, USP <63>/EP 2.6.7 for mycoplasma and USP <85>/EP 2.6.14 for endotoxin are the release criteria. Because of the short shelf life, rapid methods such as automated culture (BacT/ALERT) and qPCR for mycoplasma are widely adopted (USP; rapidmicromethods).
About this step and the alternatives
Living-cell products cannot be heat-sterilized at the end and have a short shelf life, which is exactly why confirming that they are "not contaminated" is the lifeline of safety.
Testing is done from three directions. The pharmacopoeial sterility test looks for bacteria and fungi, nucleic acid amplification for mycoplasma, and horseshoe crab-derived reagent for endotoxin (a pyrogen).
The shelf life is short, so the results cannot be waited for. Rapid methods are therefore used alongside, assuring sterility at a speed that meets the release decision.
Living-cell products cannot be terminally sterilized and have a short shelf life, so confirming the absence of contamination is essential for safety. Bacteria and fungi (sterility test), mycoplasma and endotoxin are tested, with rapid methods used alongside to cope with the short shelf life.
Alternatives in practice
- Classical mycoplasma culture + indicator cell method · Agar/broth culture + Vero/Hoechst staining fluorescence microscopy — Pharmacopoeial reference method requiring 28 days. Unsuitable for the short shelf life of the final product
- Recombinant Factor C (rFC) endotoxin assay · Lonza PyroGene rFC (fluorescence plate reader) / bioMérieux ENDONEXT — Not dependent on horseshoe crab, addressing animal welfare. Avoids β-glucan cross-reactivity
- Rapid sterility (non-growth-based) · Sartorius Milliflex Rapid / Charles River Celsis (ATP bioluminescence) — Shortens the time to results to days. Speeds up final product release; validation is required
Testing to rule out adventitious viruses → in vitro indicator cell assay and specific virus PCR (NGS as needed) on unprocessed bulk / cell banks (adventitious virus exclusion testing)
Virus safety confirmation is essential for human-derived cell products. Apart from sterility, mycoplasma and endotoxin, exclusion of adventitious agents per ICH Q5A is required.
- Standard method
- In vitro adventitious virus assay using indicator cells + specific qPCR panel for human pathogenic viruses, with high-sensitivity NGS / TEM evaluation of RVLP as needed
Why it is the standard. ICH Q5A(R2) calls for adventitious virus testing of cell banks and unprocessed bulk, and this is a standard part of release decisions and characterization for human-derived cell products. In vitro indicator cell assays + specific PCR are the backbone of actual practice.
Check also. The risk of virus contamination via human- and animal-derived materials cannot be detected by sterility tests, so independent testing of unprocessed bulk and cell banks is typically required under the ICH Q5A(R2) framework. Indicator cell assays and specific PCR have limits in detection range, and are supplemented as needed by high-sensitivity NGS and the like.
With Autologous / 2D planar flask scale-out
- The basis of identity changes from the "banked line" to the "patient themselves"
- Method: STR analysis is used not to confirm the identity of a bank line but to verify that the product derives from the patient in question (chain of identity / mix-up prevention). Karyotyping is performed not on a bank line but on each patient's primary expansion
- Check also: Identity verification is required for every patient lot, which increases the testing burden, while the viewpoint of continuous monitoring of genetic stability over long-term passaging is more limited than for a bank line
About this step and the alternatives
As long as the raw materials are of human or animal origin, the risk of viruses slipping in from outside cannot be avoided. This is a specialized test that rules them out as "not present".
An in vitro assay using indicator cells casts a wide net, and qPCR specific to human pathogenic viruses searches in a targeted way. High-sensitivity NGS is also brought in if necessary.
The targets are unprocessed bulk and cell banks. These are adversaries that sterility tests cannot catch, so the test is independently required under ICH Q5A(R2), the last bastion of safety.
Allogeneic (and autologous) cell therapy products carry the risk of adventitious and endogenous viruses via human- and animal-derived materials, and testing of unprocessed bulk harvest and cell banks is typically required under the "sourcing, testing and clearance" framework of ICH Q5A(R2). Because sterility tests and the like cannot detect them, this is essential as an independent step.
Alternatives in practice
- Broad adventitious agent screening by untargeted NGS · Illumina NextSeq 2000 + metagenomic analysis — Comprehensively searches for known and unknown viruses. Regulators are highly interested in it as an alternative to in vitro/in vivo methods; bioinformatics analysis is the key
- Retrovirus-like particle quantification by TEM · Transmission electron microscope (JEOL JEM / Thermo Talos) — Directly observes and counts particle morphology. Low sensitivity and throughput; specialist skills required
- In vivo inoculation (traditional method) · Laboratory animals (suckling mice, guinea pigs, embryonated hen eggs) — Classical pharmacopoeial method. Requires animal use and a long period, and is being replaced by in vitro/NGS methods
Genetic stability and identity of origin (karyotype and identity)
Karyotype checks chromosomal abnormalities (tumorigenicity) and STR checks identity of origin, assuring safety and traceability.
- Standard method
- G-banding karyotype analysis + STR (short tandem repeat) identity confirmation
Why it is the standard. G-banding is the most widely used chromosomal stability test in the ICH Q5D context and is effective for monitoring tumorigenicity and transformation. STR is the standard for identity of origin and prevention of mix-ups. Periodic monitoring such as every 10 passages is recommended (PMC4109748; WiCell).
Check also. G-banding karyotype analysis examines abnormalities in chromosome number and large structural changes, and cannot capture small deletions or copy number changes. To confirm tumorigenicity risk, it is desirable to cross-check with analyses of a different principle, such as arrays or NGS, according to product risk.
About this step and the alternatives
A step that watches from two angles whether the cells have "changed in nature" during long culture. It assures safety and traceability at the same time.
One is the chromosomes. G-banding karyotype analysis checks for abnormalities in number or structure and rules out the risk of tumorigenesis. It is a test that peeks into the pitfalls of long-term culture.
The other is origin. STR (short tandem repeat) analysis checks whether the cells match the donor's genetic fingerprint, proving that no cells were mixed up.
Confirms that no chromosomal abnormalities (tumorigenicity risk) have arisen during long-term culture and that the origin of the cells has not been mixed up. Karyotype analysis checks the number and structure of chromosomes and STR analysis checks the match to the donor origin, assuring safety and traceability.
Alternatives in practice
- FISH / SNP microarray · Thermo KaryoStat+ / CytoScan, MetaSystems FISH probes — Detects microdeletions and mosaicism at high resolution. More objective and higher resolution than G-banding, but balanced translocations can be missed
- iFISH-based rapid karyotyping · Interphase nuclear FISH (MetaSystems/ASI probes) — Rapid because metaphase is not required. Limited to specific target chromosomes, so there is no whole-genome coverage
- Tumorigenicity / soft agar colony / in vivo · Soft agar colony formation assay / inoculation into immunodeficient mice (NOG/NSG) — Evaluates the functional consequences (transformation) of genetic abnormalities. Requires a long period and animals
The reference (“gold standard”) method is the one broadly recognized in published practice and guidelines; your process, product and regulator may justify another. Makers and products are examples, not endorsements. This map is translated from the Japanese edition, where it is updated first (Japanese version).