IL-7-dependent B lymphocytes are essential for the anti-polysaccharide response and protective immunity to Streptococcus pneumoniae.

Young children are impaired in their response to T cell-independent (TI) Ags, such as pneumococcal polysaccharide (PPS). B lymphopoeisis early in life is IL-7 independent, whereas in adults it is IL-7 dependent. Therefore, we hypothesized that IL-7-driven B lymphopoiesis plays a critical role in promoting Ab responses to TI Ags. Young but not adult mice are impaired in responses to PPS vaccination and to 4-hydroxy-3-nitrophenyl-acetyl-Ficoll, a widely studied model TI Ag, and B1b cells generate Ab responses to these Ags. In this paper, we show that, despite having B1b, B1a, and MZ B cells-all of which are involved in TI responses-young wild-type or adult mice deficient either in IL-7 or in IL-7Ralpha are severely impaired in anti-PPS responses and do not survive Streptococcus pneumoniae challenge, indicating IL-7-dependent B cells are required for TI immunity. Consistent with this, PPS immunization induced a robust TI response in young IL-7 transgenic mice that was comparable to adult wild-type responses. Moreover, immunized young or adult IL-7 transgenic mice were completely resistant to S. pneumoniae challenge. Our data indicate that activating the IL-7 signaling pathway could restore impaired TI responses in the young.

B cell mediated priming following pneumococcal colonization.

The primary reservoir for Streptococcus pneumoniae is the human nasopharynx, and colonization is often the initial step in pathogenesis. Recently we have demonstrated that pneumococcal colonization primes the immune response to subsequent vaccination with the pneumococcal conjugate vaccine (CPV). In this study we wished to determine if colonization stimulates the production of B cell memory that is activated following vaccination with CPV. To test this hypothesis, we colonized mice with S. pneumoniae serotype 14, adoptively transferred their B cells and CD4+ T cells into naïve recipients, and vaccinated the recipients with CPV. Our results indicate that pneumococcal colonization stimulates the production of memory B cells which are responsible for enhancing the immune response to CPV vaccination.

Analysis of the young and elderly variable gene repertoire in response to pneumococcal polysaccharides using a reconstituted SCID mouse model.

Significant changes in anti-pneumococcal polysaccharide (PPS) variable gene usage occur with aging and may be influenced by changes in cytokine environment. Severe combined immunodeficient (SCID) mice were engrafted with B cells obtained from young and elderly donors, supplemented with human cytokines and immunized with the pneumococcal polysaccharide vaccine. B cells specific for PPS serotypes 4 and 14 were isolated from mice and immunized donors, and variable region sequences analyzed. Significant differences in variable heavy and light chain gene usage were observed between young and elderly adults despite a more constant cytokine environment. Due to the limitations of the hu-PBL-SCID model, the use of alternative systems would be beneficial in the elucidation of mechanisms underlying the reduced vaccine efficacy in the elderly.

Comparison of the human immune response to conjugate and polysaccharide pneumococcal vaccination using a reconstituted SCID mouse model.

The pneumococcal conjugate vaccine (CV), although highly immunogenic in infants and young children, does not consistently demonstrate an advantage over the pneumococcal polysaccharide vaccine (PPV) in older adults. To further elucidate the adult immune response to CV, we compared its response to PPV on a molecular level using a severe combined immunodeficient (SCID) mouse model. This model allowed us to analyze a single individual's response to two different forms of antigen and define differences in gene usage elicited by these vaccines. We reconstituted SCID mice with human lymphocytes derived from an unimmunized donor; the mice were divided into two groups and immunized with either the PPV or CV. Our results demonstrate significant differences in variable gene usage in SCID mice immunized with PPV versus CV and suggest that the nature of the immunizing agent has a significant impact on gene usage and therefore influences antibody function and vaccine efficacy.

Immune response to pneumococcal polysaccharides 4 and 14 in elderly and young adults: analysis of the variable light chain repertoire.

Streptococcus pneumoniae is a human bacterial pathogen responsible for serious infections including pneumonia. The currently licensed polysaccharide vaccine provides 60 to 80% protection in young adults, but in the elderly the vaccine efficacy is drastically reduced despite normal antibody levels. We hypothesized that the reduced vaccine efficacy in the elderly results from altered variable gene family usage. We have analyzed the light chain gene usage in 20 young (20 to 30 years of age) and 20 elderly (65 to 86 years of age) adults in response to pneumococcal polysaccharide 4 (PPS4) and PPS14. We generated a variable light chain library using B cells specific for PPS4 and PPS14 from each vaccinated individual. We determined complete sequences and somatic mutation frequencies in all isolated variable light chain fragments. Six gene families, kappa1, kappa2, kappa3, kappa4, lambda1, and lambda3, were identified in response to PPS4 and PPS14 in both age groups. Comparison of young and elderly adults demonstrated significant differences in kappa4, lambda1, and lambda3 gene usage in response to PPS4 and PPS14. With aging, there was a significant increase in kappa4 gene usage and a significant decrease in lambda1 and lambda3 gene usage in response to both PPS4 and PPS14. Although both Vkappa1 and Vlambda3 gene products demonstrated extensive mutations, there was no age-related difference in mutational frequency per gene family. These findings suggest an age-related change in light chain gene usage in response to PPS4 and PPS14.

Immune response to pneumococcal polysaccharides 4 and 14 in elderly and young adults. I. Antibody concentrations, avidity and functional activity.

Streptococcus pneumoniae is a serious worldwide pathogen and the focus of numerous vaccine development projects. Currently the most widely accepted surrogate marker for evaluating the efficacy of a given vaccine is to utilize ELISA. Measurement of antibody concentration by ELISA without reduction in cross-reactive antibodies causes an overestimation of antibody concentration and therefore protection, this is most notable in the aged, an at risk group for this infection. We compared the immune response to the pneumococcal polysaccharides (PPS) 4 and 14 of 20 young to 20 elderly adults. Pre-and post-vaccination IgG antibody concentrations and antibody avidity against PPS4 and PPS14 were measured using two different enzyme-linked immunosorbant assay (ELISA) absorption protocols. All sera were pre-absorbed with either cell-wall polysaccharide (CPS), or CPS and serotype 22F polysaccharide. Pre- and post-vaccination IgG antibody concentrations for serotype 4, but not 14, were significantly lowered with the additional absorption with serotype 22F polysaccharide in both age groups. Young and elderly demonstrated a significant increase from pre- to post-immunization antibody concentration, using either absorption method; and opsonophagocytic antibody titers in response to both PPS4 and PPS14. The correlation coefficients between ELISA and opsonophagocytic assays were improved by additional absorption with serotype 22F in response to serotype 4, but not serotype 14 in all age groups. Opsonophagocytic antibody titers in a sub-group of elderly (>77 years of age) were significantly lower than the opsonophagocytic antibody concentrations in young adults. These results suggest the importance of eliminating cross-reactive antibodies from ELISA measurements by absorption of serum and an age-related impairment in the antibody response to pneumococcal polysaccharides.