title: Using a pseudotype-based virus neutralisation assay to examine the serological response to morbillivirus infection
Creators: name: Hu, Siyuan
Publisher: name: University of Glasgow
Year: 2022

description: Measles has been considered a candidate for eradication after smallpox and rinderpest, thanks to the worldwide immunisation campaigns that have been ongoing for more than half a century. However, measles cases have resurged recently, which raises the question of whether the current immunisation strategies, especially the childhood immunisation protocol, are still effective. Furthermore, increasing cases of cross-species transmission of animal morbilliviruses such as canine distemper virus (CDV) make it important to evaluate the level of immunity in the population that would cross-protect against potential zoonotic transmission of morbilliviruses. A pseudotype-based virus neutralisation assay was used to measure neutralising antibody (NAb) titres in serum samples collected from Thai infants at six timepoints before and after two-doses of MMR vaccination to verify the immunogenicity of the childhood immunisation strategy. It was observed that infants born with high levels of maternal immunity did not respond effectively by developing protective titres while infants with lower maternal immunity developed protective titres of antibody following vaccination. It was concluded that increasing the immunogenicity of the first measles immunisation by tailoring the vaccination schedule according to the maternal immunity level acquired by infants could lead to significant improvements in the induction of protective immunity against measles in vaccinated infants. NAb levels were also assessed in a group of sera that were collected from healthcare workers during a measles outbreak in Sheffield in 2016, to indicate the protection status in the “front-line” population. Antibodies against both the vaccine strain and the circulating strains were evaluated to determine the cross-genotypes neutralising activity in the population. The proportion of the samples tested with NAb titres within the “well-protected” range ranged from 82.5% to 93.6% for different genotypes, which fell below the 95% threshold required to eliminate measles. The finding suggested that although cross-neutralisation was observed between the different measles virus (MeV) genotypes tested, a vaccination rate greater than 95% will be required to achieve protective immunity against MeV infection in at least 95% of the population. Furthermore, ELISA were performed in the serum samples using two commercial kits (LIAISON® and EUROIMMUN®) to investigate a possible correlation between the levels of IgG and NAbs. Spearman’s rank correlation coefficients were calculated and ranged from 0.40 to 0.55 (P &lt; 0.05) by LIAISON® kit, or 0.71 to 0.79 (P &lt; 0.0001) by EUROIMMUN® kit. The finding demonstrated that it is feasible to use rapid IgG testing as a surrogate measure for neutralising activity to define clinical protection levels within population. Neutralising activities against four morbilliviruses (MeV, RPV, PPRV and CDV) were determined in serum samples collected from either individuals who were previously infected with measles or healthcare workers who had been immunised with measles vaccine. The results indicated that a group of NAbs that can cross-neutralise animal morbilliviruses exist in humans, and the strength of cross-species neutralising activity in humans correlated to the phylogenetic distance between the morbillivirus species. Therefore, maintaining a high MeV vaccination coverage rate across the world is desirable, even after measles eradication. Furthermore, NAbs titres against chimaeric pseudotypes bearing haemagglutinin (H) and fusion (F) proteins from MeV and CDV were tested in human sera, including naturally infected and vaccinated adults and Thai infants. A predominance of MeV-F as a target for NAbs present in both adults and infants was observed when these NAbs were induced by natural infection; however, the predominance of NAb recognising MeV-H was observed in sera collected from vaccinated adults or infants. The results indicated that the induction of NAbs by infection or vaccination could result in different proportion of NAb recognising the H or F glycoproteins and this could impact the cross-neutralisation of animal morbilliviruses. The results of this project suggest that increasing vaccine immunogenicity by targeting vaccination to those susceptible individuals in a population who are able to respond well to vaccination, may be more effective and cost-efficient than vaccinating the whole population indiscriminately. Providing IgG level could correlate with NAb level that is an indicator of clinical protection, it is a promising strategy of using IgG lateral flow test with required cut-off value, which is defined by protective NAb titres, to identify the susceptible individuals rapidly during measles outbreak. Furthermore, the cross-species neutralisation by NAbs recognising the F glycoprotein may be underestimated and warrants further investigation, which could be helpful for preventing the potential zoonotic transmission of animal morbilliviruses, such as CDV to humans.