The role of genotype specific anti-G antibodies in protection against severe human respiratory syncytial virus infection

Abstract

Human respiratory syncytial virus is a leading cause of lower respiratory tract infection in infants. Current prophylaxis with Palivizumab, a humanised anti-fusion glycoprotein monoclonal antibody, has only achieved moderate success. In animal models, immunization with G glycoprotein has led to largely genotype specific immunity. This suggests the hypothesis that anti-G antibodies to the infecting virus genotype, either acquired transplacentally or induced by immunisation, may contribute to protection of the infant in the early months of life. The aim of this study has been to test this hypothesis by measuring levels of maternally acquired antibody to the G glycoprotein of the infecting virus genotype in an index group of infants with severe HRSV infection and, to the same genotype, in a comparison group of infants with no history of HRSV infection. As only 2% of infants suffer severe disease on primary HRSV infection, the comparison group are assumed to be destined to be resistant to severe infection. A deficiency of anti-G antibodies in the index group will thus indicate a protective role for these antibodies. A phylogenetic study was carried out in Newcastle upon Tyne from autumn 2007 to spring 2010 revealing three circulating genotypes of HRSV, namely GA2, GA5 and BA4 although GA5 was not detected in the third epidemic. The G genes of GA2 and BA4 and the F gene of GA2 were cloned and expressed individually in modified vaccinia Ankara virus. The recombinant proteins were used in the development of a concanavalinA capture ELISA sufficiently sensitive to detect maternal antibodies in the acute sera of infants under 6 months of age. An attempt was made to refine the assay in order to separately detect antibodies to the conserved and variable epitopes of the G glycoprotein. However, mapping of conserved and variable epitopes revealed overlap epitopes precluding the development of a differentiation in a simple ELISA assay. An index group of infants with severe HRSV infection and a comparison group of infants of similar age with no history of HRSV infection were recruited with consent by their legal carer during the epidemic of 2009/2010. The infecting virus lineage of each infant in the index group, either GA2 or BA4, was identified by reverse transciptasepolymerase chain reaction of virus recovered from nasal secretions. Sera were collected from both groups, at the acute stage of infection for the index group, and screened for ii HRSV specific IgA by ELISA. Only those without detectable IgA were included in the study. IgG maternal antibodies to the recombinant G glycoprotein of the infecting virus genotype and the F glycoprotein of the GA2 genotype were measured in the sera of index and comparison group infants whilst maternal antibody levels to both F and G glycoproteins in the sera of both index and control sera decayed at similar rates with age, the index group possessed significantly more anti-G and anti-F antibody at all ages suggesting that infants hospitalized with severe HRSV infection were not deficient in antibodies to either glycoprotein contrary to the hypothesis under test. However, mean maternal antibody levels at birth have been shown to fall across the winter epidemic and the above analysis may be susceptible to bias introduced by uneven sampling of infants across the epidemic. To avoid this potential error, anti-F/anti-G antibody ratios, which give a measure of anti-G immunity independent of age and time of collection, were also compared in the sera of index and comparison group infants. Mean ratios were not significantly different between the two groups also rejecting the hypothesis that severely affected infants have a deficiency in maternal anti-G antibodies. These studies, therefore, fail to establish a role for anti-G glycoprotein antibodies in the protection of infants against severe lower respiratory tract disease.