Streptococcus pneumoniae (S. pneumoniae, pneumococcus) is a major cause of morbidity and mortality worldwide. Achieving long-term immunity against S. pneumoniae through immunization is an important public health priority. Long-term protection after immunization is thought to rely both on protective serum antibody levels and immunological memory in the form of antigen-specific memory B cells (MBCs). Although the ability to achieve protective antibody levels shortly after pneumococcal vaccination has been well documented for the various infant immunization schedules currently in use worldwide, the examination of immunological memory in the form of antigen-specific MBCs has been much more limited. Such responses are critical for long-term protection against pneumococcal colonization and disease. This review summarizes the published literature on the MBC response to primary or booster immunization with either pneumococcal polysaccharide vaccine (PPV23) or pneumococcal conjugate vaccines (PCVs), aiming to elucidate the immunological mechanisms that determine the magnitude and longevity of vaccine protection against pneumococcus. There is evidence that PCVs induce the production of antigen-specific MBCs, whereas immunization with PPV23 does not result in the formation of MBCs. Increased understanding of the immunological factors that facilitate the induction, maintenance and recall of MBCs in response to pneumococcal vaccination could enable the use of MBC enumeration as novel correlates of protection against S. pneumoniae. Ongoing studies that examine MBC response to pneumococcal vaccination in high burden settings will be extremely important in our understanding of long-term protection induced by pneumococcal conjugate vaccines.

Antigen-specific B cell identification by flow cytometry is crucial for investigating their immunophenotype, subset distribution, and kinetics post-infection or immunization. Methods using biotinylated polysaccharide antigens have been described, but there is still room for improvement regarding sensitivity and applicability. The aim of this study was the development and validation of a multimer bead-based method for detecting pneumococcal polysaccharide serotypes (PS)-specific B cells following pneumococcal immunization. PS was chemically biotinylated and mounted on anti-biotin beads, and labeled with phycoerythrin (PE)-conjugated anti-biotin antibody to form a PS-multimer used for cell staining. Labeled beads were washed to remove excess fluorochrome and diminish nonspecific labeling and background noise. Optimal ratios of PS-bead conjugate to PE and PS-multimer to cells were determined with titration assays. Comparison between the PSmultimer and a PS-PE monomer revealed enhanced detection of PS-specific cells and considerable signal amplification, attributed to the multimeric form of the detection probe and increased availability of antigen epitopes. To validate the specificity of the method, a competition assay using unbound PS was performed. Following pre-incubation with increasing PS concentrations, detection of PS-specific B cells with the PS-multimer was inhibited in a stepwise manner. Pre-incubation with excess PS completely blocked the fluorescent signal. This novel bead-based flow cytometry approach is a sensitive method demonstrating high specificity. It generated enhanced signals, provided clear-cut results, and was easily applicable, not requiring B cell pre-enrichment. It could be modified to adapt other antigens of interest, especially polysaccharides and proteins that could be used to probe antigen-specific B cell responses. The study of such responses may elucidate the underlying mechanisms involved in the establishment of long-term protection, provide evidence-based rationale for improving currently available vaccines and vaccination strategies, and pave the way for future vaccine development.