Chromosomal characterization of MRC-5 cell banks utilizing G-banding technique.

We have performed chromosomal monitoring on 18 MRC-5 cell banks using the G-banding technique. A higher frequency of structural abnormalities and hyperdiploidy was detected with respect to the numerical values that were established with conventional staining techniques and reported in the 1979 Ad Hoc Committee guideline on karyology controls of human cell substrates. These numerical criteria have been adopted by international regulatory agencies to release human diploid cell banks (WI-38 and MRC-5) for vaccine manufacture. In the process of characterizing these 18 cell banks, a 7;12 translocation clone was detected in 7 cell banks at a frequency ranging from 0.2 to 5.6%. The presence of the t(7;12) appears to be biphasic. At low population doubling levels (PDLs) (< 30), t(7;12) is rarely observed. However, the incidence of t(7;12) increases and plateaus between PDL 40-50. A decrease in frequency is observed at higher PDLs. Before senescence of the cell bank, t(7;12) is not observed. Investigation of the growth characteristics of MRC-5 cells revealed that cell banks containing the translocation senesced at similar PDLs compared to MRC-5 cells with no detectable 7;12 translocation. In addition, MRC-5 cell banks containing the t(7;12) have successfully completed tumorigenicity testing in a nude mouse model. We conclude that MRC-5 cells obtained from either National Institute for Biological Standards and Control (NIBSC) or American Type Culture Collection (ATCC) contain a 7;12 translocation at a low frequency. This abnormality does not provide MRC-5 cell bank mass cultures with a growth advantage nor is it tumorigenic in nude mice. Furthermore, the presence of this clone and employment of the G-banding technique may be responsible for the increased observation of structural abnormalities in our laboratories. In view of this information, the cytogenetic criteria that were established in 1979 with conventional staining techniques are not appropriate for human diploid cell banks that are examined with more sensitive methodology. Since it has been recognized that MRC-5 and WI-38 cells are safe biological substrates, we recommend that MRC-5 and WI-38 cell banks should only be identified by using an appropriate identity test and should not require any chromosomal analysis before being used as a cell substrate for the manufacture of live virus vaccines.