Currently, the majority of respiratory syncytial virus (RSV) vaccine candidates include the fusion (F) protein in different conformations. The antigenic site II found in these different F conformations is the target for palivizumab, the only approved monoclonal antibody (mAb) for the treatment of RSV. Serum palivizumab-like antibody (PLA) may therefore be a potential serologic correlate of immunity. The authors were interested in whether different conformations of F protein in a palivizumab competitive antibody (PCA) assay affect the PLA concentrations. They standardised four PCA assays using mAbs, each of which contained prefusion, postfusion or intermediate F forms. PLA concentrations were measured in acute and convalescent sera taken from 22 RSV/A and 18 RSV/B-infected adult haematopoietic stem cell transplant (HSCT) recipients. The authors found that the PCA assays measured significantly greater PLA concentrations in convalescent sera, with comparable increases in PLA concentration in both RSV/A and RSV/B-infected HSCT recipients and significantly reduced PLA concentrations in HSCT recipients who shed RSV ≥14 days. The authors also detected a significant positive correlation between PCA assays and RSV neutralising antibody titres. Therefore the F protein conformation does not appear to have a measurable impact on PCA assays for the measurement of PLA induced by RSV/A or RSV/B infection.
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