Title: The human immune response to Streptococcus pneumoniae
Other Titles: De humane immuunrespons tegen Streptococcus pneumoniae
Authors: Borgers, Heleen
Issue Date: 11-Dec-2012
Abstract: Streptococcus pneumoniae are bacteria that colonize the human upper respiratory tract. Although colonisation with pneumococci is mostly asymptomatically, it can progress to a variety of diseases ranging from mild infections, such as otitis media and sinusitis, to diseases of greater severity such as pneumonia, septicemia and meningitis. Pneumococcal disease is a worldwide health problem. The mortality rate caused by invasive infection with S. pneumoniae is still very high, especially among children. Host protection against pneumococcal lung infection is characterized by an inflammatory reaction, initially involving alveolar macrophages and followed by lung infiltrating neutrophils. If pneumococci overcome these first-line defenses and gain entry to the blood stream, systemic protection is afforded by antibodies against capsular polysaccharides (caps-PS) and pneumococcal surface proteins. Anti-caps-PS or anti-protein antibodies either neutralize the activity of virulence factors or function as opsonins to enhance clearance of S. pneumoniae through phagocytosis (=opsonophagocytosis). Caps-PS, surrounding the bacteria, are the main virulence factor of pneumococci. More than 90 different capsule types of pneumococci (serotypes) have been identified. High levels of antibodies directed against caps-PS confer clinical protection against invasive infections and caps-PS-based vaccines are efficacious in reducing S. pneumoniae infection. In the first part of this thesis we focused on the determination and interpretation of the anti-caps-PS antibody response after immunization with the pneumococcal polysaccharide vaccine Pneumo23®. Immunization with Pneumo23® is usually used to assess the immunologic response to polysaccharide antigens in patients who are suspected of having an anti-polysaccharide antibody deficiency, e.g. SPAD. For the interpretation of the immune response to Pneumo23® several criteria are commonly used: the post-vaccination antibody concentration, the fold-increase, and the percentage of serotypes to which there is a good response. Currently there is no universal definition of an adequate response to pneumococcal polysaccharides. We tried to gain a better insight in this anti-polysaccharide antibody response for the determination of better standardized criteria.1. We evaluated a multiplexed bead-based assay (xMAP® Pneumococcal Immunity assay) for the simultaneous determination of antibodies against 14 capsular polysaccharides. Post-vaccination (Pneumo23®) antibody concentrations were measured in 35 healthy children, 40 healthy adults, 99 consecutive patients with increased susceptibility to respiratory infections, and 24 SPAD patients. The device scored well on technical aspects, such as linearity and reproducibility (especially for lower values). Concerning the interpretation of post-vaccination antibody titers, we determined cutoff values (5th percentile of healthy controls) for 14 pneumococcal serotypes. Healthy persons should beable to respond above a level of 0.23 to 1.66mg/l, depending on the serotype. After analysis of 24 previously diagnosed SPAD patients, we reported that such adequate response is necessary for at least 7 of the 14 serotypes tested. 2. We reported on serotype-specific antibody response to pneumococcal caps-PS in elderly. Pre- and post-vaccination (Pneumo23®) antibody concentrations to 14 serotypes were measured in 58 healthy elderly and 74 healthy young subjects. In general, post-vaccination polysaccharide-specific antibody levels were comparable to (or higher than) post-vaccination antibody levels in young subjects. In both age groups, low post-vaccination antibody concentrations were seen for serotype 1, 3, 4 and 12F. The highest (pre- and post-vaccination) antibody levels were found for serotype 14. 3. Although it is generally believed that children under 2 years of age do not mount adequate responses to pneumococcal polysaccharides, we found that 2 children of 6 month old were able to mount antibodies to serotype 3 after vaccination with Pneumo23®. 4. We evaluated the fold increase in antibody concentrations over pre-immunization titers (Pneumo23®) in 132 healthy controls (children-adults-elderly). The majority of the individuals with a low pre-vaccination antibody concentration mounted a 2-fold increase rather than a 4-fold increase. In addition, the fold increase was age and serotype dependent. There is a subgroup of controls with a 2-fold or 4-fold increase, but an inadequate post-vaccination antibody concentration. Therefore the fold-increase always has to be interpreted in combination with the post-vaccination antibody concentration. To conclude, we believe that an adequate post-vaccination antibody response for a defined amount of serotypes is the best criterion for the interpretation of an anti-caps-PS antibody response after immunization with Pneumo23®. Measurement of fold increase can be used, but has to be interpreted together with the post-vaccination titer. When evaluating post-vaccination antibody concentrations, one should take serotype specificity and age into account. However, more research is necessary to validate the influence of the latter. In a second part, we tried to gain more insight in the immune mechanisms that are involved in the innate or/and adaptive (human) immune response to S. pneumoniae. This might be helpful for the development of better strategies to control pneumococcal infection. We tested candidate pneumococcal proteins for vaccination by using a newly described screening method. Current pneumococcal vaccines protect against a limited amount of pneumococcal serotypes and the incidence of pneumococcal disease caused by non-vaccine serotypes is increasing. In addition, although antibiotics are usually effective against S. pneumoniae, resistant strains are rapidly developing. In this thesis we studied different components of the immune system and their possible role in the (human) immune response to S. pneumoniae.1. We found a significant correlation between the post-vaccination IgG (but not IgM) antibody response and absolute numbers of (i) naïve B cells, (ii) IgM memory B cells, and (iii) switched memory B cells.2. We investigated the role of CD56 expressing cells in the human innate and adaptive immune response to S. pneumoniae. CD56 is a surface marker of Natural Killer (NK) and Natural Killer T (NKT) cells. In an in vitro system, soluble caps-PS (Pneumo23®) were unable to stimulate IFNgamma production by human CD56+ cells, cocultured with monocytes. On the other hand, we found that heat-inactivated (HK) S. pneumoniae did induce IFNgamma production. The antibody response to pneumococcal antigens (caps-PS and protein) after immunization with HK S. pneumoniae or Pneumo23® in SCID/SCID mice that had received PBMC (=humanization) was comparable to the antibody response in SCID/SCID mice that were humanized with PBMC without CD56+ cells. There was no correlation between the number of NK cells and the post-vaccination (Pneumo23®) levels in patients with increased susceptibility to respiratory infection. SCID/SCID mice humanized with PBMC showed longer survival after intranasal challenge with S. pneumoniae (=administration of viable pneumococci) compared to SCID mice humanized with PBMC without CD56+ cells. 3. We studied whether the acute phase protein haptoglobin (HP) influences the immune response to S. pneumoniae. The antibody concentrations to pneumococcal antigens (caps-PS and protein), measured after immunization with Pneumo23® and HK S. pneumoniae, did not differ between HP Knockout (-/-) mice (= no production of HP) and Wild Type (WT) mice (= control). In humans, Hp-2-2 phenotype has been linked to poor disease outcomes. Anti-caps-PS antibody concentrations did not differ significantly between patients with HP1-1, HP1-2 or HP2-2 phenotype. After intranasal challenge of HP-/- mice and WT mice, HP-/- mice showed prolonged survival compared with WT mice.4.The measurement of IL17A production is described as an evaluation method for candidate pneumococcal vaccines that might influence colonization and immunity. We tested two pneumococcal proteins (SP_1683 and SP_1386) in an in vitro system (human PBMC). SP_1386 induced very low concentrations of IL17A; IL17A concentrations after stimulation with were higher. Although we gained new insights in the immune response to S. pneumoniae, several questions still remain unsolved. The results from the correlation study between anti-caps-PS antibody concentrations and amount of different B cell subsets, showed an intricate relationship between both groups. Further research is necessary to investigate which B cell subset produces the anti-caps-PS antibodies. Our results showed no apparent role for CD56+ cells and haptoglobin in the (human) antibody response to S. pneumoniae, but a potential role in the innate immune response to pneumococcal lung infection. Future experiments will have to be done to elucidate the potential role of CD56+ cells and haptoglobin in the initial (human) host response to pneumococcal lung infection. It would be interesting to investigate whether vaccination with SP_1683 influences colonization and immunity and the possible role of IL17A here in.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Experimental Laboratory Immunology
Department of Health and Technology - UC Leuven

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