The differences in the size of syncytia following F and HN

Co-expression of batMuV F and HN resulted in the formation of syncytia, which were noticeably smaller in comparison to those obtained after the co-expression of hMuV F and HN. When compared to a MuV consensus sequence, that is generated by sequence analysis of 26 different MuV strains and isolates, the corresponding glycoproteins of both human and bat-derived MuV show high levels of sequence homology on aa level: The hMuV F and HN proteins showed 98.9 % or 99.2 % homology to the consensus sequence; batMuV F and HN showed 94.6 % or 91 % homology, respectively (Kruger et al., submitted). Even in the case that a different aa residue is present at a certain position, it is mostly an aa residue with similar characteristics. For the batMuV F protein it was observed, that the total aa homology of the F1 subunit (96.5 %) is higher than that of the F2 subunit (86.5%). In the latter, especially the first 20 aa of the N-terminus of batMuV F, which have the characteristics of a signal peptide (SP; calculated by the SignalIP software), shined out in regard of aa conservation (69 %) in comparison to the corresponding aa residues of the consensus sequence. In contrast to this, the hMuV F2 subunit and the SP showed a high homology to the MuV consensus sequence (97.4 % and 96 %, respectively). To analyze whether the differences in this part of the polypeptides have an influence on the fusogenicity, chimeric F proteins with an exchange of the complete F2 subunit or only the predicted SP were generated und investigated in a fusion assay. In this way it was shown, that F proteins harbouring either the complete hMuV F2 subunit linked to the batMuV F1, or only the hMuV F SP-related aa residues linked to the remaining portion of the batMuV F, mediated the formation of huge syncytia following co-expression with both hMuV or batMuV HN (Kruger et al., submitted).

Those syncytia were larger in size when compared to those obtained after co-expression of batMuV F an HN, and comparable to those which were formed after hMuV F and HN co-expression. In contrast, F protein chimeras comprising the batMuV F2 subunit and the hMuV F1, or the batMuV F SP-related aa residues followed by the aa residues of the hMuV F (without the SP-related portion) induced the formation of smaller syncytia, resembling the phenotype achieved by co-expression of batMuV F and HN (Kruger et al., submitted).

From these results it is concluded, that the SP of the hMuV F enables a more efficient processing and transport of the F protein along the secretory pathway than the batMuV F SP.

7.2.4. Summary

It has been shown that there is a high relatedness between the glycoproteins of batMuV and human MuV strains, not only based on the highly conserved aa sequences, but also on the serological similarity and the functional activity. So far, no infectious MuV has been isolated from bats, but nevertheless the herein before mentioned studies provide some indications for a zoonotic potential of batMuV: (i) The fusion activity is not restricted to the reservoir host. (ii) Based on past studies, the ability to mediate cell-to-cell fusion is associated with the virulence of MuV strains and therefore, the syncytium formation induced after batMuV F and HN co-expression should be considered an important factor for the estimation of the virulence of the batMuV. (iii) Furthermore, the interaction with sialic acids (and the potential usage of them as a cellular receptor for virus entry) provides a widely distributed receptor determinant which is usually present in many animal species.

The in silico analysis of the phylogenetic relatedness of the small hydrophobic protein (SH) of batMuV and MuV genotypes indicates that the batMuV is not a variant of an already known human strain (that might have been introduced into the chiropteran species by accident), but rather a novel MuV strain of an origin different from humans.

In this light, it is noteworthy to mention that a MuV vaccination is not included into national immunization programs in African countries and therefore, the people are not protected against MuV infections. Given that monoclonal antibodies directed against the F protein of different MuV strains were able to detect both, hMuV and batMuV F, it is likely that hMuV and batMuV are conspecific and belong to one serogroup and that further a vaccination with the common MuV vaccine could elicit a protective immunity against the batMuV strain investigated in this study.

Based on the data obtained during this study it is feasible, that a batMuV has already been able to infect humans in local areas, although there are no data that support this assumption.

But this is a general problem for infectious diseases with non-specific symptoms on the African continent, as febrile illnesses are often attributed to malaria, yellow fever, dengue fever, etc., treated with therapeutics against the disease with the most-fitting symptoms.

Further testing for very rare, uncommon, or new infectious diseases is either not performed or impossible due to the high costs, missing equipment, lack of medical/ or immunological training, or a lack of tests (especially for so far unknown pathogens).

To get more information about the prevalence of (bat)MuV in Africa, it would be necessary to screen serum samples from humans as well as different mammalian species for the presence of MuV antibodies or RNA. This would give an overview about the distribution and diversity of (bat)MuV strains in Africa and would provide valuable informations for possible epidemiological follow-up studies.