Transgenic Expression of Bcl-xL or Bcl-2 by Murine B Cells Enhances the In Vivo Antipolysaccharide, but Not Antiprotein, Response to IntactStreptococcus pneumoniae

Abstract

IgG antipolysaccharide (PS) and antiprotein responses to Streptococcus pneumoniae (Pn) are both CD4+ T cell dependent. However, the primary IgG anti-PS response terminates more quickly, uses a shorter period of T cell help, fails to generate memory, and is more dependent on membrane Ig (mIg) signaling. We thus determined whether this limited anti-PS response to Pn reflected a greater propensity of PS-specific B cells to undergo apoptosis. We used mice that constitutively expressed the antiapoptotic protein Bcl-xL or Bcl-2 as a B cell-specific transgene. Both transgenic (Tg) mice exhibited increased absolute numbers of splenic B-1 and peritoneal B-1b and B-2 cells, subsets implicated in anti-PS responses, but not in marginal zone B (MZB) cells. Both Tg mouse strains elicited, in an apparently Fas-independent manner, a more prolonged and higher peak primary IgM and IgG anti-PS, but not antiprotein, response to Pn, but without PS-specific memory. A similar effect was not observed using purified PS or pneumococcal conjugate vaccine. In vitro, both splenic MZB and follicular Tg B cells synthesized DNA at markedly higher levels than their wild-type counterparts, following mIg cross-linking. This was associated with increased clonal expansion and decreased apoptosis. Using Lsc−/− mice, the Pn-induced IgG response specific for the capsular PS was found to be almost entirely dependent on MZB cells. Collectively, these data suggest that apoptosis may limit mIg-dependent clonal expansion of PS-specific B cells during a primary immune response to an intact bacterium, as well as decrease the pool of PS-responding B cell subsets.

Parameters Underlying Distinct T Cell-Dependent Polysaccharide-Specific IgG Responses to an Intact Gram-Positive Bacterium versus a Soluble Conjugate Vaccine

“…The propensity of particulate Ags to be sequestered within the marginal zone (MZ) (28), and the observation that MZBs in relation to FB are programmed to favor plasma cell over memory cell differentiation (22), led us to hypothesize that the failure of intact Pn to generate PPS14-specific IgG memory was secondary to its selective usage of MZBs to generate this response. Indeed, we show that the PPS14-specific IgG response to intact Pn14, but not conjugate, is markedly reduced in Lsc Ϫ/Ϫ mice, in which MZBs are defective in migrating from the MZ and are thus unable to receive CD4 ϩ T cell help (7,10). These data strongly suggest that intact Pn and soluble conjugate differentially utilize MZBs and FBs, respectively, to generate the PPS14-specific IgG response.…”

“…8), radically changes the nature of the in vivo IgG anti-PPS14 response relative to both purified capsular PPS14 as well as intact Pn. Thus, the IgG anti-PPS14 response to soluble PPS14-PspA conjugate is largely similar to the IgG anti-PspA responses to both conjugate and intact Pn14, including prolonged primary kinetics of induction and the generation of a CD4 ϩ T cell-dependent, PPS14-specific IgG memory response (7,9). Collectively, these data indicate that PPS14 in association with intact Pn14 is immunologically distinct from a soluble covalent conjugate of PPS14 and an immunogenic pneumococcal protein, despite the CD4 ϩ T cell dependence of the IgG responses to both immunogens.…”

“…However, in contrast to the antiprotein response, the IgG anti-PS response to intact Pn exhibits attenuated primary kinetics, fails to generate immunological memory, and is dependent on a shorter period of T cell help and B7-dependent costimulation. Furthermore, the IgG anti-PS response to Pn is ICOS independent and extrafollicular (6) and more apoptosis prone (7), in contrast to the IgG antiprotein response. The mechanism underlying these differences, as well as the essential difference between PS expressed by an intact bacterium vs a conjugate vaccine, is unknown.…”

Distinct Mechanisms Underlie Boosted Polysaccharide-Specific IgG Responses Following Secondary Challenge with Intact Gram-Negative versus Gram-Positive Extracellular Bacteria

“…Previous studies using intact heat-killed S. pneumoniae and a soluble pneumococcal conjugate vaccine strongly suggested that PS-specific IgG responses were derived from marginal zone and follicular B cells, respectively (18,19), with elicitation of distinct idiotypes on the PS-specific (serotype 14) IgG (17). Indeed, mice primed with intact serotype 14 S. pneumoniae do not elicit an augmented PS-specific IgG response following booster immunization with a soluble type 14 pneumococcal conjugate vaccine, in which the carrier protein is pneumococcal surface protein A (33), but do elicit an augmented response following booster immunization with intact GBS-III expressing type 14 PS (21).…”

“…For example, systemic immunization with intact S. pneumoniae induced PS-specific IgG that derived from splenic marginal zone B cells and expressed a distinct idiotype, whereas a pneumococcal conjugate vaccine of the same PS serotype (type 14) induced PS-specific IgG largely lacking this idiotype and derived from splenic follicular B cells (17)(18)(19).…”