Minor differences in the untranslated regions of measles vector additional transcription units are reflected by differential immunogenicity of encoded MERS-CoV Spike antigen

The recombinant measles virus (MeV) is among those vaccine platform technologies, which are tested for the fast generation of effective vaccines against other pathogens, such as emerging virus diseases. Different live-attenuated vaccine strains have become available as backbones, with few differences. Here, we analyze how minor differences of recombinant Moraten (MV _vac2 ) or Schwarz (MV _Schw ) constructs have an impact on the performance of respective derived Middle East respiratory syndrome (MERS) vaccine candidates. MV _vac2 or MV _Schw viruses, both encoding the Spike glycoprotein (S) of the MERS-coronavirus (MERS-CoV) in an additional transcription unit (ATU) between the hemagglutinin gene and the polymerase gene L cassette, were tested. Both robustly expressed the additional gene. However, approximately twofold to threefold higher amounts of S proteins were observed in cells infected by the MV _Schw -MERS-S(H) virus, correlating with the same relative ratio of S mRNA found in these cells. Exchange of the short genomic 3′ UTR ATU sequence in MV _vac2 against the sequence in MV _Schw enhanced gene expression by about the same factor and demonstrates causality. MeV-susceptible IFNAR ^−/− -CD46Ge mice vaccinated with either candidate reveal that higher relative S antigen expression by MV _Schw -MERS-S(H) tended toward induction of higher specific antibody titers, but lower numbers of S antigen-specific T cells. In subsequent challenge experiments, both candidates were effective. Thus, minor differences in the genetic composition of recombinant MeV’s ATU may modulate the gene expression pattern of additionally encoded proteins that are reflected by changes in the respective immunogenicities, but which were not decisive for protection in the chosen disease model.

IMPORTANCE

In case of emerging or re-emerging infections, vaccine platform technologies are needed to rapidly develop effective vaccines to aid public healthcare in pandemics. Besides mRNA vaccines, also viral platform technologies, that is, the adenovirus-derived vaccines Vaxzevria and JCOVDEN, have proven to be of immense value during the COVID-19 pandemic. For future pandemics, it is crucial to understand the factors in vector design that modulate immunogenicity. This knowledge allows the tailoring of vaccine vectors to fit specific target product profiles, for example, to build vectors which trigger an accentuated T cell or, alternatively, antibody response against an antigen of interest. Our study using the live-attenuated measles vaccine backbone as a promising example is therefore crucial in demonstrating that very minor differences in the vaccine backbone can alter the antigen expression profile of the vector-antigen system and impact the relative induction of T-cell or antibody responses against the added target antigen