Serologic Cross-Reactivity between the Mumps Virus Vaccine Genotype A Strain and the Circulating Genotype G Strain

Abstract

Recent mumps outbreaks have been observed in vaccinated young adults due to the mumps virus (MuV) of genotype G, whereas the current vaccine is a mixture of two genotype A strains. These outbreaks could be attributed to waning vaccine immunity or the antigenic differences between the HN and F glycoproteins in the vaccine and circulating MuV. These glycoproteins are essential targets for the immune system, and antigenic variations may reduce the recognition of mumps antibodies, rendering the population susceptible to the MuV. We established stable cell lines expressing the MuV glycoproteins to study cross-reactivity between genotype A and genotype G. Cross-reactivity between the genotypes was evaluated via immunofluorescence using patient sera from vaccinated individuals, infected individuals, and vaccinated individuals infected with genotype G. Titer ratios showed that the vaccinated individuals exhibited a titer 3.68 times higher for the HN protein and 2.3 times higher for the F protein when comparing genotype A with genotype G. In contrast, the infected individuals showed a lower titer for genotype A compared with genotype G, at 0.43 and 0.33 for the HN and F proteins, respectively. No difference in titer ratio was observed for individuals vaccinated and subsequently infected with mumps. These findings suggest that antigenic variations between the two genotypes may potentially result in immune escape of the circulating strain, resulting in individuals susceptible to the MuV.

Keywords: antigenic variation; cross-reactivity; genotype A; genotype G; immunofluorescence; mumps virus; serology.

Figure 1

(A) Stable cell lines expressing the HN protein incubated with the positive control serum 186-89. (B) Stable cell lines expressing the F protein incubated with the positive control serum 186-89. The serum 186-89 was used as positive control as it had a high ND₅₀ for PRNT and the titers for this serum were read as positive at a 512 and at a 1024 fold dilution for HN protein genotype A and genotype G, respectively. For the F proteins, the titers were read as positive at a 512 fold dilution for both genotype A and genotype G. (C) Stable cells negative control serum F for HN protein. (D) Stable cells negative control serum F for F protein. Serum F was used as a serum-negative control as it was negative for the MuV antibody by ELISA and BioPlex 2200 and showed no signal (antibody binding) for the HN and F proteins. FLAG antibody (FLAG) was used without anti-mumps serum to check for protein expression. ‘All’ contains DAPI, the green channel (FLAG), and the red channel (HN Protein) all merged together.

Figure 2

(A) GMT for the HN protein antibodies. (B) GMT for the F protein antibodies. (C) Titer ratio for the HN protein. (D) Titer ratio for the F protein. Titer ratios were calculated by dividing genotype A over genotype G (A/G). The vaccinated group includes individuals vaccinated for mumps (n = 19). The infected group includes individuals infected with mumps and no history of vaccination (n = 9). The vaccinated –infected group includes individuals vaccinated for mumps and subsequently infected with MuV genotype G strains (n = 10). The error bars represent the 95% confidence intervals. Statistical significance is presented as p-value < 0.05 represented by *, p-value < 0.01 is represented by **, and p-value < 0.0002 is represented by ***, and p-value <0.0001 ****.