Interactions between Streptococcus pneumoniae and the human host

Abstract

Streptococcus pneumoniae, the pneumococcus, is an important human pathogen causing considerable morbidity and mortality worldwide. This thesis addresses several interactions between pneumococcus and man. The first part of the thesis deals with the host immune response against pneumococci. We studied the anticapsular antibody response during pneumonia and found that only 45% of adult pneumococcal pneumonia patients mount a significant serotype-specific antibody response. We showed that corticosteroid use during pneumonia does not affect anticapsular pneumococcal antibody formation. In the second part of the thesis diagnostic options for pneumococcal pneumonia are discussed. We investigated the measurement of antibodies as an alternative method to detect pneumococcal pneumonia as well as two additional newly developed techniques: a quantitative PCR followed by capsular sequence typing, and a serotype-specific urinary antigen assay. Using all three techniques we were able to detect the pneumococcus in samples of 56% more patients compared to conventional methods. Furthermore, we investigated whether a urinary antigen test (UAT) could serve as a method to diagnose invasive pneumococcal pneumonia, classically defined as pneumonia in which S. pneumoniae is cultured from a normally sterile body site, by comparing the outcome of bacteraemic and non-bacteraemic/UAT-positive patients. A non-significant association with mortality was found for bacteraemia. In the third part of the thesis various techniques for distinguishing and typing cultured pneumococci are described. Two molecular serotyping techniques, a real-time multiplex PCR (rmPCR) assay detecting 21 different serotypes/-groups and a sequetyping assay, based on the sequence of the cpsB gene within the pneumococcal capsulation locus, were compared. The rmPCR assay was shown to perform well for the 21 serotypes/-groups included in the assay and for serotype mixtures. The sequetyping assay performs well, with specific exceptions, and may be more useful for regions where vaccine serotypes are less common. To gain more insight in the genetic background of pneumococci, we developed a mixed genome microarray containing 470 DNA fragments and evaluated genetic differences between 445 invasive and non-invasive pneumococcal isolates. The microarray was able to genetically distinguish pneumococci at a deeper level than the typing techniques generally used. The fourth and last part of the thesis focuses on pneumococcal vaccination. We have studied the impact of the seven-valent pneumococcal conjugate vaccine (PCV7) introduction in the Netherlands on the incidence and presentation of invasive pneumococcal disease (IPD). Four years after introduction of the vaccine, significant declines in overall IPD incidence were observed in children <2 (60%) and in persons ≥65 (13%) years of age. In adults, the proportion of immunocompromised persons increased among IPD patients. Overall, deaths from IPD decreased from 16% to 12%. An increase was observed in the incidence of serotype-1 disease in young female adults in the Netherlands, which was not observed in men of the same age. It remains uncertain whether or not there is an association between the observed increase in serotype-1 disease in young female adults and the implementation of PCV7 in the Netherlands.