Mycoplasma Signal and an Extra STR Peak in a Three-Year-Old Master Cell Bank
A 200-vial CHO-derived master cell bank (MCB) was manufactured three years ago and is stored in a liquid-nitrogen tank that also holds a partner company's MCB on the same rack. Eight months ago the tank's level alarm sounded and the automatic fill failed for about 60 hours before manual filling; no inventory was lost and the event was closed. The company has now received the MCB release panel. The contract laboratory reports 'mycoplasma positive' by qPCR (Ct 34, internal control passed) but negative by culture after 28 days. STR profiling gives a 92% match to the donor tissue and shows one extra peak at D7S820. An engineering run for a new product is scheduled in nine days using this MCB. Two GMP batches made from the same bank were released in the past 14 months, and one of them is still in the clinic.
1. How should the mycoplasma and STR results be interpreted and triaged before any conclusion is drawn?
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Do not treat either result as a verdict; treat them as unresolved signals.
Mycoplasma: qPCR positive at Ct 34 is a late amplification, close to the limit of reliable detection, while culture is negative at 28 days. Plausible explanations include: non-viable organism or free DNA in the sample; reagent or laboratory contamination (ask the contract lab for its negative controls and contamination trend); a fastidious or non-cultivable species that will not grow in the standard medium; or a genuine low-level contamination below the culture method's sensitivity. The actions are: quarantine the bank, do not destroy vials, and retest from at least two independently retrieved vials, ideally from different canes and positions, at a second qualified laboratory, by both qPCR and the compendial culture method. Include a viability-discriminating step (for example a DNA-binding dye treatment before qPCR) and inhibition controls. Review raw-material history (serum, media, trypsin) and whether the same lab has had other positives.
STR: a 92% match with one extra peak is not a clean identity result, but it is also not proof of cross-contamination. CHO cells are aneuploid with an unstable karyotype, so reference profiles are inherently approximate. Repeat STR on a fresh vial, compare with the parental line and with the partner's bank, and consider SNP profiling or karyotyping if the extra peak persists. Document the reasoning rather than declaring a mismatch on one peak.
2. What must happen to the engineering run, the stored inventory and the released batches?
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Engineering run: it does not start. The bank is not released, and nine days is irrelevant to that decision. Either the run is deferred until the investigation is closed, or a substitute released bank is used after it passes the full panel.
Inventory: quarantine the entire bank physically and in the inventory system, with a unique status flag so no vial can be issued. Segregate the rack from the partner's material; inspect vials for cracks and frost damage, and record LN2 level history for the past year. Do not discard until the investigation is complete, because the vials are the evidence.
Released batches: perform a documented look-back. If contamination is confirmed, every lot derived from the MCB is potentially affected, including the clinical lot. Quarantine remaining stocks, hold further shipment, and perform a health-hazard assessment; mycoplasma contamination of an injectable is a patient-safety issue, not a specification deviation. Review the release testing and environmental monitoring data for those lots, remembering that the sterility or mycoplasma test samples only a small number of units and provides limited assurance. Then apply the local requirements for notification, and consider recall. If the investigation shows the signal was a false positive, document that conclusion with the retest data before releasing anything. The two released batches are not exonerated simply because they passed at the time of release.
3. The partner company asks whether their bank could be affected. What is your answer and its basis?
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Answer in two parts: a factual statement and a commitment.
The factual basis: liquid nitrogen is not a plausible route for microorganisms to travel into a sealed, heat-sealed or welded cryovial. Storage cross-contamination risk comes from vial-to-vial contact through liquid nitrogen only when a vial is cracked or the seal is compromised, or from handling errors such as reusing a contaminated aliquot. Sharing a rack, or sharing the vapour phase of the same tank, does not by itself create a transfer route. There is currently no evidence linking the partner's bank to the finding, and no shared vial has been opened.
What to offer: notify the partner formally and promptly rather than waiting for a conclusion; share the investigation plan; allow them to inspect the inventory and vial integrity and to retrieve and test their own vials; keep the two banks physically segregated while the investigation runs; and confirm that any future shared storage uses separate canes with documented inventory and level monitoring.
What not to say: do not state categorically that their bank is 'fine' before the investigation is closed, and do not disclose your own test data in a way that breaches confidentiality. The answer should be proportionate, documented and transparent, because the credibility cost of a late notification is higher than the cost of an early one.