The majority of marketed seasonal influenza vaccines are prepared using viruses that are chemically inactivated and treated with a surfactant. Treating with surfactants has important consequences: it produces 'split viruses' by solubilizing viral membranes, stabilizes free membrane proteins and ensures a low level of reactogenicity while retaining high vaccine potency. The formulation stability and potency of split influenza vaccines are largely determined by the specifics of this 'splitting' process; namely, the consequent conformational changes of proteins and interactions of solubilized particles, which may form aggregates. Robust methods to quantitatively determine the split ratio need to be developed before optimal splitting conditions can be investigated to streamline production of superior influenza vaccines.Here, we present a quantitative method, based on both steady-state and time-resolved fluorescence spectroscopy, to calculate the split ratio of the virus after surfactant treatment. We use the lipophilic dye Nile Red (NR) as a probe to elucidate molecular interactions and track changes in molecular environments. Inactivated whole influenza viruses obtained from a sucrose gradient were incubated with NR and subsequently treated with increasing concentrations of the surfactant Triton X-100 (TX-100) to induce virus splitting. NR's emission spectra showed that the addition of TX-100 caused~27 nm red-shifts in the emission peak, indicative of increasingly hydrophilic environments surrounding NR. The emission spectra of NR at different surfactant concentrations were analyzed with multi-peak fitting to ascertain the number of different micro-environments surrounding NR and track its population change in these different environments. Results from both the emission spectra and fluorescence lifetime spectroscopy revealed that NR showed presence in 3 distinct molecular environments. The split ratio of the virus was then calculated from the percentages of NR in these environments using both fluorescence emission and lifetime data. This study can pave the way for the development of robust methods to rapidly quantify splitting extent during vaccine manufacturing. KEYWORDS fluorescence emission; fluorescence lifetime; influenza split virus vaccine; influenza virus; membranesurfactant interactions; Nile Red; partitioning of dye in membranes; protein aggregation; split ratio quantification; surfactant; Triton X-100; vaccine formulation; vaccine manufacturing; virus splitting; virus-surfactant interactions
For modelling purposes it is of great importance to derive the speci®c growth rate as a function of time from biomass measurements. Traditional methods such as exponential or polynomial ®tting do not give satisfactory results nor do these methods take the noise characteristics of the biomass measurements into account. Standard recursive techniques, such as Kalman ®ltering, use only the data up to the time under consideration and are dependent of a good initial estimation. This paper describes a technique based on combining subsequent backward and forward extended Kalman ®l-tering to give a smoothing estimator for the speci®c growth rate. The estimator does not need an initial value and is shown to have a single tuning parameter. The applicability of the estimator is demonstrated on batch and fed-batch cultivations of two organisms: Bordetella pertussis and Neisseria meningitidis.
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