For the foreseeable future it is anticipated that the global supply of influenza vaccine will be manufactured predominantly in eggs. Vaccine production relies on a global network of public health, academic and industrial laboratories that work in concert to ensure the rapid update of vaccine composition when antigenic variants become dominant in the world [5]. The present study was designed to evaluate the performance characteristics of several cell lines which are already certified for or are currently being evaluated by national regulatory authorities to determine their suitability for human influenza vaccine manufacturing.
In general, MDCK cells appear to be the most permissive cell line for isolation and propagation of human and animal influenza viruses [45] and [46]. In the present study, the three MDCK cell lines used for primary isolation of influenza A and B viruses from clinical specimens proved to be highly sensitive. After one blind passage, all 20 isolates were detected in one of the two anchorage-dependent MDCK lines (MDCK-3) and in the suspension MDCK line. The anchorage-dependent MDCK-1 cells appeared to be slightly less sensitive, as two influenza A(H3N2) viruses and two influenza B viruses of the Yamagata lineage remained undetected. Recent influenza A(H3N2) may not grow or require one or more blind passages before the virus can be detected in culture. In this study eggs achieved a 45% isolation rate overall and 40% and 20% for A (H3N2) and B-Yamagata viruses, respectively, however during the last decade, the proportion of H3N2 viruses that has been recoverable in eggs has declined to 4 fold) with low frequency.
As soon as vaccine manufacturers adopt the use of cell culture–isolated influenza viruses in vaccine production, one or more of the approved cell lines could be made available to WHO Collaborating Centers for the isolation of viruses from virus-positive samples received from National Influenza Centers. These qualified cell lines could provide an alternative to eggs in the event that isolation of a suitable virus for vaccine production has not been possible. Preliminary results from a follow-up studies show that H3N2 viruses with high infectivity harvested from MDCK cultures can be propagated in eggs. Results of egg based studies will be the subject of a separate report.
To estimate the potential performance of viruses isolated in various cell lines in cell-based vaccine manufacturing, one influenza A virus of each subtype and one influenza B virus of each lineage isolated in each of the three MDCK cell lines was grown in a small-scale production experiment using the three MDCK and the VERO cell lines at the corresponding vaccine manufacturing sites. Infectivity titers in cell culture supernatants were determined using different methods at each manufacturing plant, which makes quantitative comparisons unfeasible. However, antigen amounts as well as infectivity titers did not vary significantly in the different combinations of isolation and production cell lines. It is thus likely that viruses isolated in certified cell lines by WHO Collaborating Centers can be successfully propagated in any of the cell lines currently used by different vaccine producers.
Virus protein yields were determined after concentration and purification of virus from small-scale production. In these experiments the MDCK-2 cell line, in accordance to routine production procedures at this manufacturing plant, was used at one order of magnitude lower cell density than the other cell lines. As a consequence, protein yields from this cell line were approximately 2 to 10 times lower than those observed from the other cell lines. Protein yields from the other two MDCK cell lines did not differ significantly from each other. For the influenza A(H1N1) virus, the highest protein yields were obtained with the VERO cell line. However, with influenza A(H3N2) and influenza B viruses of both lineages, protein yields from the VERO cell line were 1.5 to 10-fold lower than those obtained with the MDCK-1 and MDCK-3 cell lines.
These experiments were designed as a proof of concept that influenza viruses isolated in cell cultures could be successfully used for production of influenza vaccines in certified mammalian cell lines selected by vaccine manufacturers. The MDCK cell lines proved to be sensitive for primary isolation of influenza A and B viruses. The viruses studied retained their genetic and antigenic properties well during propagation in the cell lines. Antigen and protein yields were comparable in all different combinations of cell lines for primary isolation and for production. The scarcity of positive clinical specimens with a sufficiently high virus titer and/or volume to allow for performance of all the experiments limited the total number of isolates tested. However, influenza viruses isolated in certified cell lines fulfilled all of the requirements needed for acceptable vaccine seed viruses. Although the A(H1N1) seasonal viruses used in the present study have been replaced by the A(H1N1)pdm09 viruses since the 2009 pandemic, these results may be applicable to the new lineage as well. The feasibility of influenza viruses isolated in certified cell lines for use in egg-based production platform is currently under evaluation and those results will be presented elsewhere.
Isolation of recent influenza A (H3N2) viruses is becoming increasingly difficult in eggs, which severely limits the number of available virus candidates that could be evaluated for vaccine production. Alternative strategies must therefore be designed, tested, and evaluated including the use of viruses isolated in approved cell lines for further propagation in both cell-based and egg-based influenza vaccine manufacturing. The promising results obtained in the present study may assist decision making by public health laboratories, regulatory agencies and industry regarding the generation of virus isolates for cell-based manufacturing of influenza vaccines