Hyporesponsiveness Following Booster Immunization With Bacterial Polysaccharides Is Caused by Apoptosis of Memory B Cells

Background. Repeated immunizations with polysaccharide ( PS) vaccines cause hyporesponsiveness through undefined mechanisms. We assessed the effects of a PS booster on immune responses, frequency, and survival of PS-specific B-cell subpopulations in spleen and bone marrow. Methods. Neonatal mice were primed with meningococcus serotype C (MenC) conjugate MenC-CRM197+CpG1826, boosted with MenC-CRM197, MenC-PS, or saline; subsequently, bromodeoxyuridine (BrdU) was injected daily intraperitoneally. MenC-PS-specific cells were labeled with fluorescent MenC-PS and phenotyped by flow cytometry. Results. After MenC-PS booster, proliferating (BrdU(+)) MenC-PS-specific naive B cells (CD138(-)/B220(+); P = .0003) and plasma cells (CD138(+)/B220(-); P = .0002) in spleen were fewer than after saline booster. BrdU(+) MenC-PS-specific plasma cells were also reduced in bone marrow (P = .0308). Compared to saline, MenC-PS booster reduced BrdU(+) IgG(+) MenC-PS-specific B cells in spleen (P = .0002). Twelve hours after the MenC-PS booster, an increased frequency of apoptotic (AnnexinV(+)) MenC-PS-specific B cells in spleen was observed compared with MenC-CRM197 (P = .0286) or saline (P = .001) boosters. Conclusions. We demonstrated that the MenC-PS booster significantly reduced the frequency of newly activated MenC-PS-specific B cells-mostly switched IgG(+) memory cells-by driving them into apoptosis. It shows directly that apoptosis of PS-specific memory cells is the cause of PS-induced hyporesponsiveness. These results should be taken into account prior to consideration of the use of PS vaccines.