Viral pseudotyped particles (pp) are enveloped virus particles, typically derived from retroviruses or rhabdoviruses, that harbor heterologous envelope glycoproteins on their surface and a genome lacking essential genes. These synthetic viral particles are safer surrogates of native viruses and acquire the tropism and host entry pathway characteristics governed by the heterologous envelope glycoprotein used. They have proven to be very useful tools used in research with many applications, such as enabling the study of entry pathways of enveloped viruses and to generate effective gene-delivery vectors. The basis for their generation lies in the capacity of some viruses, such as murine leukemia virus (MLV), to incorporate envelope glycoproteins of other viruses into a pseudotyped virus particle. These can be engineered to contain reporter genes such as luciferase, enabling quantification of virus entry events upon pseudotyped particle infection with susceptible cells. Here, we detail a protocol enabling generation of MLV-based pseudotyped particles, using the Middle East respiratory syndrome coronavirus (MERS-CoV) spike (S) as an example of a heterologous envelope glycoprotein to be incorporated. We also describe how these particles are used to infect susceptible cells and to perform a quantitative infectivity readout by a luciferase assay.

Despite developed long time ago, plaque assay is still the gold standard for viral titer quantification in modern virology. The standard crystal violet-based plaque assay relies on virus’ ability to induce cytopathic effect (CPE) which limits the assay to lytic viruses. Alternative viral quantification assays such as 50% tissue culture infectious assay (TCID₅₀) and genetic material quantification by Q-PCR provide a different way of viral quantification with their own shortcoming. In here, we modified the fluorescent focus assay and developed an antibody-based immunoplaque assay which provides a reliable and reproducible viral quantification independent of CPE. Our assay not only allows accurate determination of viral titer, but also provides information on viral kinetics, genetic stability and purity of the virus population.