TCID50 Endpoint-Dilution Titration of Influenza A Virus on MDCK Cells with CPE Scoring and Spearman-Karber Calculation

Eight serial 10-fold dilutions (1e-1 to 1e-8) of the virus stock are each plated into 8 replicate wells of a 96-well plate (rows A-H, one dilution per column block). Each plate includes 8 cell-only (mock) wells and 8 positive-control wells inoculated with a previously titered reference stock. Two independent plates (technical replicates) are run per assay; the entire assay is repeated on 3 separate days (biological replicates) with freshly thawed cell passages. Plates are read at 72 h. Wells are scored binary (CPE positive/negative) by an operator blinded to dilution layout via a coded plate map.

BSL-2; influenza A is a Risk Group 2 respiratory pathogen. Perform all virus manipulations in a certified Class II biosafety cabinet wearing a lab coat, gloves, and eye protection. Decontaminate the BSC and all surfaces with freshly prepared 10% bleach (10 min contact) or 70% ethanol. Treat all liquid waste with bleach to 1% final before disposal; autoclave solid waste. TPCK trypsin and crystal violet/formalin are irritants/fixatives - handle in the BSC, avoid skin contact. Institutional Biosafety Committee approval and seasonal influenza vaccination of personnel are recommended.

Positive control: a reference influenza stock of known titer run in parallel must produce CPE within +/- 0.5 log10 of its historical value, validating cell permissiveness and trypsin activity. Negative/mock control: 8 cell-only wells receiving infection medium without virus must remain CPE-negative (confirms no contamination, no trypsin overdigestion, no nonspecific monolayer loss). Trypsin control: a no-trypsin well set confirms the trypsin dependence of influenza CPE. Highest-dilution wells (1e-8) are expected to be CPE-negative, anchoring the lower end of the endpoint curve.

A clean assay shows full CPE (8/8 wells positive) at low dilutions (1e-1 to 1e-4), a transition zone (e.g., 6/8, 3/8, 1/8) across two to three dilutions, and 0/8 at the highest dilutions. The 50% endpoint typically falls between 1e-5 and 1e-7. Expected titer for a healthy PR8 stock is 1e6-1e8 TCID50/mL. A well-behaved curve has a monotonic, sigmoidal decrease in positive fraction; non-monotonic 'skips' indicate pipetting error.

• MDCK cells (ATCC CCL-34), passage 4-20, maintained in DMEM + 10% FBS
• Influenza A/PR/8/34 (H1N1) test stock, stored at -80 C in single-use aliquots
• Reference influenza stock of known titer (positive control)
• Infection medium: DMEM (high glucose), 0% FBS, 1 ug/mL TPCK-treated trypsin, 0.2% BSA, 1x penicillin/streptomycin
• TPCK-treated trypsin, 1 mg/mL stock in PBS
• DPBS without Ca2+/Mg2+
• 0.25% trypsin-EDTA for cell passaging
• Flat-bottom 96-well tissue culture plates
• Crystal violet 0.1% in 10% neutral buffered formalin (optional fixation/visualization)

To accurately quantify the infectious titer of an influenza A virus stock by serial endpoint dilution on Madin-Darby canine kidney (MDCK) cells, reading cytopathic effect (CPE) at 72 h post-infection and computing the 50% tissue culture infectious dose (TCID50/mL) via the Spearman-Karber method. This assay supports stock characterization, MOI calculations for downstream infections, and lot-to-lot consistency tracking for neutralization and antiviral studies.

1. Seed MDCK cells at 2e4 cells/well in 100 uL DMEM+10% FBS into 96-well plates; incubate 24 h at 37 C, 5% CO2 to ~90-95% confluent monolayers.
2. Thaw a virus aliquot on ice. Prepare 10-fold serial dilutions in infection medium: add 100 uL stock to 900 uL infection medium (1e-1), mix, carry 100 uL forward through tubes to 1e-8, changing tips each step.
3. Aspirate growth medium from the plate; wash each well once with 150 uL warm DPBS.
4. Add 100 uL of each dilution to its 8 replicate wells. Add 100 uL infection medium only to mock wells; add reference stock to positive-control wells.
5. Incubate 1 h at 37 C, 5% CO2 for adsorption, rocking the plate gently every 15 min.
6. Without removing inoculum, add 100 uL infection medium per well (final TPCK trypsin 1 ug/mL). Incubate 72 h at 37 C, 5% CO2.
7. At 72 h, examine each well by inverted phase-contrast microscopy (10x) for CPE: cell rounding, detachment, monolayer destruction. Score each well +/-.
8. Optionally fix and stain: remove medium, add 50 uL 0.1% crystal violet/formalin for 15 min, rinse with water, air-dry, and read intact (stained) vs destroyed (clear) monolayers.

Independent variable: log10 virus dilution (1e-1 through 1e-8). Dependent variable: proportion of CPE-positive wells per dilution, and the derived TCID50/mL. Controlled variables: MDCK passage number and seeding density (2e4/well), monolayer confluency at infection (90-95%), TPCK trypsin concentration (1 ug/mL), adsorption time (1 h), incubation duration (72 h), temperature/CO2 (37 C/5%), and operator-blinded scoring.

If the virus stock contains replication-competent influenza A, then serial 10-fold dilutions will produce a sigmoidal transition from wells showing complete CPE to wells with no CPE, and the dilution at which 50% of replicate wells are infected will yield a reproducible titer (expected ~1e6 to 1e8 TCID50/mL) consistent to within +/- 0.5 log10 across independent assays performed by different operators.

Record the number of CPE-positive wells per dilution. Compute the 50% endpoint by the Spearman-Karber method: log10(TCID50) = x0 - d/2 + d*(sum of fraction positive across dilutions), where x0 is the log10 of the highest dilution giving 100% positive, d is the log10 dilution interval (1.0), and the sum runs over all dilutions from x0. Multiply by the inoculum volume correction (per mL) to express TCID50/mL. Reed-Muench can be reported as a cross-check. Plate maps and scores are recorded in a validated spreadsheet template; conversion to PFU-equivalent uses ~0.7 PFU per TCID50 only when noted.

1. No CPE at any dilution: trypsin inactive or omitted - verify TPCK trypsin lot activity and that it was added; confirm virus aliquot was not freeze-thawed repeatedly. 2. CPE in mock wells: contamination or trypsin overdigestion - reduce trypsin to 0.5 ug/mL, re-confirm sterility, use fresh cells. 3. Non-monotonic positive fractions (skips): pipetting/carryover error - change tips every dilution, mix thoroughly, vortex tubes briefly. 4. Titer drifting >0.5 log10 between assays: standardize MDCK passage/confluency and use single-use aliquots. 5. Ambiguous low-grade CPE: confirm by crystal violet staining or a hemagglutination spot test on supernatant.

Report titer as mean log10 TCID50/mL +/- SD across the 3 biological replicates (n=3 days, each with 2 technical plates). Inter-operator and inter-day variability are assessed by one-way ANOVA on log10 titers (alpha=0.05); acceptance requires SD <= 0.5 log10. For comparing two stocks, use an unpaired t-test on log10 titers with Welch's correction. Power analysis: to detect a 0.5 log10 difference with SD=0.3 at 80% power and alpha=0.05, n=3 independent assays per group is sufficient.