Virus like particles are important tools in the medical field for vaccination or drug delivery systems. They mimic the outside structure of a virus but cannot reproduce themselves anymore. The purification thereof is still a challenge, but some techniques are published already.
When we hear the word virus we often start to panic even if the virus type we discuss about cannot hurt us. And, as the viruses themselves are not scary enough, the scientists came out with an even more creepy word combination – virus like particles (VLPs). No, these are not viruses that like particles. In fact there is a significant difference between viruses and VLPs. While viruses carry the genetic material that enables them to replicate in the host cells (because of this we could easily get infected), VLPs mimic the outside structure of a real virus but are empty inside and cannot reproduce themselves. Just like viruses, also VLPs are produced in the specially designed host cell organisms, where virus structural proteins are generated and assembled into the VLP without any genetic material. So, on the outside they look “dangerous” like viruses, but they are only empty shells that cannot harm living organisms. This is why they became attractive in the medical field, especially for vaccination or as drug delivery systems.
The advantage of VLPs over the viruses regarding safety issue causes, however, a serious disadvantage in the production process, or being more precise, in their analysis. A functional VLP must consist of the right proteins, which are correctly assembled and sometimes covered with a lipid bilayer of the host cell membrane. Therefore, to undoubtedly determine VLPs in the sample, several structural parameters, like specific protein content and particle size, need to be analyzed. Detection and quantification of VLPs becomes especially challenging in the cell culture broth of the host organism, where VLPs are surrounded by various structurally similar biomolecules or bioparticles, such as host cell proteins or non-assembled VLP structural proteins and exosomes. So just looking like a virus without being an infectious virus makes virus quantification techniques useless. As the VLPs do not contain any genetic material DNA or RNA detection methods cannot be used, either. Specific VLPs have specific proteins on their surface and therefore some immunoassays might be a possibility. However, how can we be sure that we are measuring the proteins assembled in the VLP structure and not the ones freely swimming around in the solution? Sometimes samples are dried and VLPs are visualized by transmission electron microscope (TEM), however as such they cannot be quantified and also the structure in the dried state might be very different to the structure in the solution. This puzzle sometimes gives researchers a headache, since there is no single straightforward method for VLP detection and quantification available.
After taking some aspirins acib experts joined forces with scientists from BOKU and examined this problem more closely. They cross-checked several biochemical and biophysical methods for VLP characterization and quantification and in combination with the analytical results came out with an elegant solution. They were playing around with particle characterization methods (that can measure particle morphology, size and size distribution) in combination with size-exclusion chromatography (SEC). Comparing three selected methods: nanoparticle tracking analysis (NTA), SEC with ultraviolet spectroscopy (UV) detection and detection with multi-angle light scattering (MALS), they developed a useful and reliable tool for rapid and automated quantification of VLPs in various sample matrixes. They discovered that using MALS detection after SEC, they could increase the working concentration range compared to NTA measurements 100-fold and were able to substantially reduce the effort for sample preparation before the analysis. Additionally, the combination with SEC provides further information on sample purity. And last but not least, by using a MALS detection system they could avoid time consuming preparation of the calibration curves since MALS allows direct quantification of particles in the sample.
So, now when the analytics of VLPs is resolved we can start with setting up the VLP production process. But leave this for the next time, shall we?
This work is based on:
P. Steppert, D. Burgstaller, M. Klausberger, A. Tover, E. Berger, A. Jungbauer: Quantification and characterization of virus-like particles by size-exclusion chromatography and nanoparticle tracking analysis. J Chromatography A
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