Serum Selection for Cytotoxicity Assays — ADCC, CDC, MTT, AlamarBlue, LDH & ELISpot
Serum is not a neutral background in cytotoxicity assays. It is the most biologically active variable in your assay system — containing complement proteins, immunoglobulins, growth factors, and lipid-binding proteins that interact directly with effector cells, target cells, fluorescent readout reagents, and test compounds. Using the wrong serum grade does not simply add variability. It changes the biological signal being measured.
This guide covers all major cytotoxicity assay formats and provides specific serum grade recommendations, interference mechanisms, and protocols for each.
1. Bovine IgG in standard FBS competes with therapeutic mAbs for FcγR on NK cells and macrophages — reduces measured ADCC/ADCP activity.
2. Active complement in non-HI serum causes spontaneous lysis of complement-sensitive cell lines — inflates background cytotoxicity in all viability assays.
3. FBS and BSA bind to fluorescent dyes (AlamarBlue, resazurin, calcein) causing spectral shifts — serum concentration must be identical in all wells including controls.
Quick Reference — Serum by Assay Type
| Assay Type | Recommended Serum | Standard FBS? | Critical Interference |
|---|---|---|---|
| ADCC (NK cell-mediated) | FBS Ultra Low IgG <5 µg/mL | ✗ No | Bovine IgG competes with mAb for FcγRIII on NK cells |
| CDC — active complement | Human Serum AB OTC (native) | ✗ No | Bovine complement ≠ human complement specificity |
| CDC — negative control | Human Serum AB Heat Inactivated | ✗ No | HI destroys complement — required as CDC negative control |
| MTT / MTS | FBS Low Endotoxin ≤5 EU/mL | ⚠ Only if LE | Endotoxin activates TLR4 → non-specific growth inhibition |
| AlamarBlue / Resazurin | FBS Low Endotoxin ≤5 EU/mL | ⚠ Only if LE | FBS/BSA quench fluorescence — identical serum % in ALL wells |
| LDH release | FBS Low Endotoxin, low haemoglobin | ⚠ Only if low Hgb | Haemolysis releases endogenous LDH → elevated background |
| ELISpot (IFN-γ, Granzyme B) | FBS Very Low Endotoxin ≤1 EU/mL or Human Serum AB HI | ✗ No | Endotoxin activates monocytes → non-specific spot background |
| Hepatotoxicity (HepG2, HepaRG) | FBS Low Endotoxin ≤5 EU/mL | ⚠ Only if LE | Endotoxin activates hepatoma innate immune responses |
| 3D Spheroid cytotoxicity | FBS Ultra Low IgG <5 µg/mL (2–5%) | ✗ No | Bovine IgG interferes with anti-tumour antibody readout |
| Neutralisation assay | FBS Very Low Endotoxin ≤1 EU/mL | ✗ No | Complement in non-HI FBS can neutralise enveloped viruses |
| BSA — blocking / detection buffer | BSA Low Endotoxin ≤5 EU/mg | N/A | High endotoxin BSA activates cells during blocking incubation |
ADCC — Antibody-Dependent Cell-Mediated Cytotoxicity
ADCC measures NK cell-mediated killing of antibody-opsonised target cells via FcγRIII (CD16) on NK cells. Standard FBS contains 200–800 µg/mL bovine IgG which binds directly to FcγRIII — competing with the therapeutic mAb and reducing measured cytotoxicity. FBS Ultra Low IgG <5 µg/mL eliminates this interference completely.
| Parameter | Recommendation | Note |
|---|---|---|
| Serum grade | FBS Ultra Low IgG <5 µg/mL | Use in effector cell expansion AND assay medium — consistent throughout |
| Concentration | 5–10% | Identical in all wells including controls |
| Effector cells | PBMC or purified NK cells | Expand in FBS Ultra Low IgG for ≥5 days before assay |
| Target cell wash | 2× wash before assay | Remove residual bovine IgG from target cell medium |
| Readout | LDH release, calcein AM, or flow (PI) | Spontaneous release control must use same serum concentration |
| Typical E:T ratio | 10:1 to 50:1 | Optimise per cell line and mAb concentration |
Why FBS Ultra Low IgG and not standard FBS?
Bovine IgG in standard FBS (200–800 µg/mL) occupies FcγRIII receptors on NK cells before the therapeutic antibody is added. This reduces the number of available Fc receptors for mAb binding — directly reducing measured ADCC activity. The interference cannot be corrected by controls. The only solution is to remove bovine IgG from the culture medium.
CDC — Complement-Dependent Cytotoxicity
CDC measures antibody-mediated activation of the complement cascade leading to membrane attack complex (MAC) formation and target cell lysis. Human complement must be used for human cell CDC assays — bovine complement has different specificity, activation kinetics, and regulatory protein interactions with human cells.
| Application | Serum | Why |
|---|---|---|
| CDC assay — active complement | Human Serum AB OTC (native) — 10–25% | Active human complement. Type AB — no anti-A/B antibodies, no blood group lysis of target cells. |
| CDC assay — complement-depleted control | Human Serum AB Heat Inactivated — same concentration | 56°C/30 min destroys complement completely. Paired native/HI OTC from same donor pool required for valid comparison. |
| CDC monitoring (patient samples) | Patient serum — test with Human Serum AB OTC as reference | Neuroblastoma anti-GD2, anti-CD20 rituximab, anti-CD52 alemtuzumab CDC assays |
MTT / MTS / AlamarBlue / CellTiter-Glo — General Cytotoxicity
Metabolic viability assays measure reduction of tetrazolium salts (MTT, MTS) or resazurin (AlamarBlue) by metabolically active cells. FBS and BSA bind to these dye reagents causing fluorescence quenching and spectral shifts — the identical serum concentration in all wells is non-negotiable.
| Application | Protocol | Note |
|---|---|---|
| General drug screening (HeLa, CHO, HEK293) | 5–10% FBS Low Endotoxin ≤5 EU/mL | Identical serum % in compound wells, vehicle control, maximum kill control, and serum-only blank |
| Hepatotoxicity (HepG2, HepaRG) | 5–10% FBS Low Endotoxin — or serum-free for long-term adapted HepG2 | Endotoxin activates innate immune responses in hepatoma lines — Low Endotoxin FBS reduces hepatocyte background stress |
| AlamarBlue background correction | Subtract serum-only blank (no cells, same serum %) before calculating % viability | FBS/BSA reduce resazurin signal — pre-read well before adding reagent for reference |
| BSA in assay diluent | BSA Low Endotoxin ≤5 EU/mg — 0.1–1% | Protein stabiliser for compound dilution series — Low Endotoxin grade prevents non-specific cell activation |
ELISpot — T Cell & NK Cell Effector Function
ELISpot measures cytokine secretion (IFN-γ, IL-2, Granzyme B) from individual effector cells. Endotoxin in standard FBS activates monocytes and macrophages in PBMC preparations non-specifically via TLR4 — generating background IFN-γ spots that mask antigen-specific T cell responses.
| Application | Serum | Note |
|---|---|---|
| IFN-γ ELISpot with PBMC (vaccine immunogenicity) | FBS Very Low Endotoxin ≤1 EU/mL or Human Serum AB HI | Human serum preferred for vaccine immunogenicity — species-matched matrix for human immune cells |
| Granzyme B ELISpot (CTL function) | FBS Very Low Endotoxin ≤1 EU/mL | Heat inactivated serum if complement-mediated PBMC lysis is observed during assay |
| NK cell degranulation (CD107a) | FBS Very Low Endotoxin ≤1 EU/mL | Endotoxin activates NK cells non-specifically via monocyte IL-12 — must be eliminated |
Three Serum Interference Mechanisms Explained
| Mechanism | Affected Assays | Serum Component | Solution |
|---|---|---|---|
| Fc Receptor Competition Bovine IgG occupies FcγRI/II/III on effector cells before mAb is added |
ADCC, ADCP, 3D spheroid antibody assays | IgG (200–800 µg/mL in standard FBS) | FBS Ultra Low IgG <5 µg/mL |
| Complement Activation Active complement lyses complement-sensitive cells spontaneously — independent of antibody |
CDC, ELISpot with PBMC, any assay with lymphoblast or RBC target cells | Complement proteins C1q–C9 (present in non-HI serum) | Heat inactivated serum (56°C/30 min) or Human Serum AB HI |
| Dye & Compound Binding Serum proteins bind fluorescent dyes and test compounds — reduces apparent signal and underestimates cytotoxicity |
AlamarBlue, calcein AM, MTT, any fluorescent viability assay | Albumin, IgG, lipid-binding proteins | Identical serum concentration in all wells. Serum-only blank subtraction. FBS Low Endotoxin |
Recommended Protocols
Protocol 1 — ADCC Assay with FBS Ultra Low IgG
1. Expand NK cells or PBMC for 5–7 days in 5–10% FBS Ultra Low IgG <5 µg/mL in RPMI-1640.
2. Prepare target cells in 5% FBS Ultra Low IgG — wash 2× before assay to remove residual bovine IgG.
3. Opsonise target cells with therapeutic mAb at 1–10 µg/mL for 30 min at 37°C.
4. Co-culture effector + target at E:T ratio 10:1–50:1 in 5% FBS Ultra Low IgG for 4–6 hours at 37°C.
5. Read LDH release, calcein AM, or PI flow — serum concentration identical in spontaneous and maximum lysis controls.
6. % Specific lysis = [(test − spontaneous) / (maximum − spontaneous)] × 100
Protocol 2 — CDC Assay with Human Serum AB (native + HI pair)
1. Prepare target cells (e.g. Daudi CD20+) in serum-free medium — wash 2×.
2. Add therapeutic mAb at 1–10 µg/mL — incubate 30 min on ice.
3. Add Human Serum AB OTC (native) at 10–25% as complement source. Pair with Human Serum AB HI at identical concentration as complement-depleted control.
4. Incubate 37°C, 60 min.
5. Read PI exclusion by flow cytometry or LDH release.
6. Complement-specific lysis = CDC (native) − CDC (HI).
Protocol 3 — AlamarBlue with Consistent Serum Control
1. Seed 2,000–10,000 cells/well in 96-well plate in 5–10% FBS Low Endotoxin ≤5 EU/mL.
2. Add test compound in serial dilution in medium with identical serum concentration.
3. Include: vehicle control, maximum kill control (0.1% Triton X-100), serum-only blank (no cells) — all with identical serum %.
4. Add AlamarBlue reagent at 1/10 volume — incubate 2–4 hours at 37°C.
5. Read fluorescence (Ex 560 / Em 590 nm). Subtract serum-only blank from all wells before calculating % viability.
Products for Cytotoxicity Assays
| Product | Key Specification | Primary Assay Use | Order |
|---|---|---|---|
| FBS Ultra Low IgG | <5 µg/mL IgG — chromatographic depletion | ADCC, ADCP, 3D spheroid antibody assays, hybridoma | Order → |
| FBS Low Endotoxin ≤5 EU/mL | ≤5 EU/mL (LAL tested per lot) | MTT, AlamarBlue, LDH, hepatotoxicity, ELISpot background reduction | Order → |
| FBS Very Low Endotoxin ≤1 EU/mL | ≤1 EU/mL (LAL tested per lot) | PBMC assays, ELISpot, macrophage and dendritic cell culture | Order → |
| Human Serum AB OTC (native) | Type AB, male donors, complement active | CDC — active complement source. Complement killing assays. Plasmodium falciparum culture. | Request → |
| Human Serum AB Heat Inactivated | Type AB, 56°C/30 min, complement inactive | CDC negative control. ELISpot with PBMC. Immune cell culture without complement background. | Request → |
| BSA Low Endotoxin ≤5 EU/mg | ≥98% purity, ≤5 EU/mg | ELISA blocking, antibody conjugation buffer, compound dilution series, protein stabiliser | Request → |
Frequently Asked Questions
Can I use standard FBS for ADCC if I add the right controls?
No. Controls cannot correct for FcγR occupancy by bovine IgG. The interference occurs before the assay reads out — NK cells pre-loaded with bovine IgG at CD16 have reduced mAb binding capacity. This reduces the measured cytotoxicity signal, not the background. FBS Ultra Low IgG <5 µg/mL is the only solution.
Why does CDC require human serum instead of FBS?
Complement is highly species-specific. Human cells express regulatory proteins (CD46, CD55, CD59) that interact specifically with human complement components. Bovine complement activates through different pathways with different kinetics. CDC data generated with bovine serum does not predict human complement-mediated killing in clinical or translational settings.
What is the correct serum concentration for AlamarBlue?
The concentration must be identical in every well — including cell-free blanks, vehicle controls, maximum kill controls, and all compound dilutions. FBS and BSA reduce resazurin signal concentration-dependently. If serum concentration differs between wells, apparent cytotoxicity or protection is generated as a measurement artefact. Subtract the serum-only blank from all wells before calculating % viability.
Which serum for ELISpot with PBMCs from vaccinated donors?
For vaccine immunogenicity IFN-γ ELISpot, use FBS Very Low Endotoxin ≤1 EU/mL to eliminate monocyte TLR4 activation. Human Serum AB Heat Inactivated is preferred when a species-matched human matrix is needed or when complement-mediated PBMC lysis is observed. Heat inactivation is mandatory — active complement lyses lymphoblasts and activated immune cells during the assay period.
Can NCS or Calf Serum be used for cytotoxicity screening?
For high-throughput compound screening in standard immortalised cell lines (CHO, HeLa, HEK293, Vero) where IgG levels and complement sensitivity are not critical — yes. Newborn Calf Serum and Calf Serum perform equivalently to FBS at lower cost and are suitable for general MTT/AlamarBlue assays. For ADCC, CDC, PBMC or ELISpot assays, use the specific grades listed in this guide.
Further Information
Product-specific technical information, batch documentation and application guides:
- FBS Ultra Low IgG — product page
- FBS Low & Very Low Endotoxin — product page
- Human Serum portfolio overview
- BSA portfolio overview
Technical enquiries, batch reservation, custom documentation: info@seamlessbio.de | +49 851 37932226