Assessment of polysaccharide response to the 23-valent pneumococcal polysaccharide vaccine (PPV23) can be biased by previous exposure to the conjugate vaccine (PCV). We applied a modification to the existing pneumococcal IgG ELISA by pre-incubating serum with PCV to adsorb out PCV-specific serotypes and allow for detection of serotypes unique to PPV23.
Anti-pneumococcal capsular polysaccharide (PCP) IgG antibodies were measured using the VaccZymeTM anti–PCP IgG ELISA (The Binding Site Group Ltd, Birmingham, UK) in PCV-adsorbed and non-adsorbed serum samples. Paired (pre and post-vaccination) sera from 76 patients were analysed. For each sample, baseline anti-PCP IgG was subtracted from the post-vaccination value to measure vaccine responsiveness. Absolute change in titres and fold changes were compared between both methods.
PCV serotype-specific adsorption was confirmed using a multiplexed bead-based immunoassay performed on 10 paired samples. In PCV-vaccinated controls (n=28), pre-adsorption with PCV significantly reduced vaccine response compared to non-adsorbed sera [median titres: 6.41mg/L and 18.20mg/L, respectively; p=0.03 and median fold change: 1.23 and 1.89, respectively; p=0.01)]. In PPV23-vaccinated immunocompetent subjects (n=30), there was a significant difference in anti-PCP-IgG responses between the conventional and the modified ELISA [median values: 57.7mg/L and 38.01mg/L, respectively; p=0.04]. Antibody-deficient patients (n=18) displayed poor PPV23 responses. Although PPV23 responsiveness was not statistically different between both methods, we observed a trend for lower anti-PCP-IgG titres in PCV-adsorbed sera compared to non-adsorbed.
Pre-analytical modification to anti-PCP IgG ELISA by removing the PCV-specific serotypes potentially differentiates true polysaccharide response from memory response induced by previous PCV vaccination.