Comparative sequence analysis of the P-, M- and L-coding region of the measles virus CAM-70 live attenuated vaccine strain

Genetic characterization of Chinese measles vaccines by analysis of complete genomic sequences

The complete genomic sequences of two Chinese measles vaccine viruses, Shanghai-191 (S-191) and Changchun-47 (C-47), were determined and compared to the sequences of other measles vaccine strains as well as the prototype measles strain, Edmonston wild-type (Edwt). Compared to Edwt, S-191 and C-47 had 49 and 43 nucleotide changes, respectively. These differences were found at 52 nucleotide positions that were not found in other vaccine strains. Phylogenetic analysis of the all of the available genomic sequences for measles vaccines showed that S-191 and C-47 were most closely related to the Leningrad-4 strain. S-191 and C-47 shared conserved vaccine virus-specific amino acid changes in the phosphoprotein (P), V, C, matrix (M), and hemagglutinin (H) that could represent important targets for future studies aimed at understanding the molecular basis of attenuation. In addition, S-191 and C-47 had several unique amino acid changes including 13 positions that differed from Edwt. This is the first comparison of the complete genomic sequences of Chinese measles vaccines to the sequences of other vaccine strains.

Adaptation to cell culture induces functional differences in measles virus proteins

Background

Live, attenuated measles virus (MeV) vaccine strains were generated by adaptation to cell culture. The genetic basis for the attenuation of the vaccine strains is unknown. We previously reported that adaptation of a pathogenic, wild-type MeV to Vero cells or primary chicken embryo fibroblasts (CEFs) resulted in a loss of pathogenicity in rhesus macaques. The CEF-adapted virus (D-CEF) contained single amino acid changes in the C and matrix (M) proteins and two substitutions in the shared amino terminal domain of the phosphoprotein (P) and V protein. The Vero-adapted virus (D-VI) had a mutation in the cytoplasmic tail of the hemagglutinin (H) protein.

Results

In vitro assays were used to test the functions of the wild-type and mutant proteins. The substitution in the C protein of D-CEF decreased its ability to inhibit mini-genome replication, while the wild-type and mutant M proteins inhibited replication to the same extent. The substitution in the cytoplasmic tail of the D-VI H protein resulted in reduced fusion in a quantitative fusion assay. Co-expression of M proteins with wild-type fusion and H proteins decreased fusion activity, but the mutation in the M protein of D-CEF did not affect this function. Both mutations in the P and V proteins of D-CEF reduced the ability of these proteins to inhibit type I and II interferon signaling.

Conclusion

Adaptation of a wild-type MeV to cell culture selected for genetic changes that caused measurable functional differences in viral proteins.

Study of the genetic stability of measles virus CAM-70 vaccine strain after serial passages in chicken embryo fibroblasts primary cultures

To evaluate the genetic stability of the CAM-70 measles vaccine strain we have performed 10 serial passages of the seed lot virus FMS-7 in chicken embryo fibroblasts primary cultures (CEF) under production conditions. The nucleotide sequences of the seed lot virus, the virus from a vaccine vial (third passage) and from the 10th passage were determined and compared with each other and with sequences from other sources. The full genome analysis of the CAM-70 vaccine still considers it as the most divergent among all vaccine strains. The nucleotide sequence analyses of viral genomes from the three CAM-70 passage levels have demonstrated that they are identical. This study shows that the measles CAM-70 vaccine virus is highly adapted to its cultivation conditions and that its genetic stability contributes, in part, to the safety profile of the vaccine.

Altered interaction of the matrix protein with the cytoplasmic tail of hemagglutinin modulates measles virus growth by affecting virus assembly and cell-cell fusion

Clinical isolates of measles virus (MV) use signaling lymphocyte activation molecule (SLAM) as a cellular receptor, whereas vaccine and laboratory strains may utilize the ubiquitously expressed CD46 as an additional receptor. MVs also infect, albeit inefficiently, SLAM(-) cells, via a SLAM- and CD46-independent pathway. Our previous study with recombinant chimeric viruses revealed that not only the receptor-binding hemagglutinin (H) but also the matrix (M) protein of the Edmonston vaccine strain can confer on an MV clinical isolate the ability to grow well in SLAM(-) Vero cells. Two substitutions (P64S and E89K) in the M protein which are present in many vaccine strains were found to be responsible for the efficient growth of recombinant virus in Vero cells. Here we show that the P64S and E89K substitutions allow a strong interaction of the M protein with the cytoplasmic tail of the H protein, thereby enhancing the assembly of infectious particles in Vero cells. These substitutions, however, are not necessarily advantageous for MVs, as they inhibit SLAM-dependent cell-cell fusion, thus reducing virus growth in SLAM(+) B-lymphoblastoid B95a cells. When the cytoplasmic tail of the H protein is deleted, a virus with an M protein possessing the P64S and E89K substitutions no longer grows well in Vero cells yet causes cell-cell fusion and replicates efficiently in B95a cells. These results reveal a novel mechanism of adaptation and attenuation of MV in which the altered interaction of the M protein with the cytoplasmic tail of the H protein modulates MV growth in different cell types.