It was recently shown that bacterial thymus-independent (TI) antigens confer long-lasting immunity and generate memory B lymphocytes. However, reactivation of TI memory B cells is repressed in immunocompetent mice, thus raising the issue of the mechanism whereby TI vaccines confer immune protection. Here, we propose an explanation to this apparent paradox by showing that a Streptococcus pneumoniae capsular polysaccharide (PS) generates long-lived bone marrow (BM) plasma cells which frequency can be increased by CpG oligodeoxynucleotides (ODNs). The adjuvant effect of CpG ODNs on the PS3 Ab response is directly targeted to B cells and does not involve B-1a cells. We also demonstrated that BM plasma cells generated in response to the thymus-dependent (TD) form of the PS vaccine have a higher secretion capacity than those produced after immunization with the CpGadjuvanted PS vaccine. Finally, we show that the PS-specific BM plasma cell compartment is sufficient to confer full protection of vaccinated mice against S pneumoniae infection. Altogether, our results show that TI antigens like their TD counterparts can generate both the lymphoid and the plasma cell component of B-cell memory. They also provide a framework for the improvement and widespread usage of TI vaccines.
IntroductionDespite advances in antimicrobial therapy, infections with extracellular polysaccharide (PS)-encapsulated bacteria remain an important clinical problem and a primary source of death worldwide. Pathogenic bacteria including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae express capsular PSs that behave as prototypic thymus-independent (TI) antigens (Ags). These Ags have initially been defined on the basis of their ability to elicit an antibody (Ab) response in T cell-deficient or athymic mice by a mechanism that requires expression of a functional Bruton tyrosine kinase. 1 They are mostly nonpeptidic molecules, which in their great majority, are unable to follow the conventional Agprocessing pathway leading to presentation by MHC class I and II molecules. 2 To overcome systemic infections by encapsulated extracellular bacteria such as S pneumoniae, B cells rapidly generate a protective Ab response against capsular PS. B-1 and marginal zone B cells have been envisioned as key players in the humoral response against TI Ags. 3 More recently, 2 studies, using Borrelia hermsii or capsular PSs from S pneumoniae as immunogens, have shown that B-1b cells are critically involved in bacterial clearance in vaccinated mice. 4,5 It has long been thought that the generation of memory B cells is restricted to thymus-dependent (TD) responses. However, the recent observation that adoptive transfer of B-1b cells from donor mice immunized with whole bacteria can confer long-lasting TI immunity to immunodeficient mice shows that TI Ags generate B-cell memory and that this function can be assigned to the B-1b-cell subset. 5 Nonetheless, the concept of TI B-cell memory is at odds with the fact that, unlike TD Ags, TI Ags fail to...