Effects of Streptococcus pneumoniae Strain Background on Complement Resistance

Abstract

BackgroundImmunity to infections caused by Streptococcus pneumoniae is dependent on complement. There are wide variations in sensitivity to complement between S. pneumoniae strains that could affect their ability to cause invasive infections. Although capsular serotype is one important factor causing differences in complement resistance between strains, there is also considerable other genetic variation between S. pneumoniae strains that may affect complement-mediated immunity. We have therefore investigated whether genetically distinct S. pneumoniae strains with the same capsular serotype vary in their sensitivity to complement mediated immunity.Methodology and Principal FindingsC3b/iC3b deposition and neutrophil association were measured using flow cytometry assays for S. pneumoniae strains with different genetic backgrounds for each of eight capsular serotypes. For some capsular serotypes there was marked variation in C3b/iC3b deposition between different strains that was independent of capsule thickness and correlated closely to susceptibility to neutrophil association. C3b/iC3b deposition results also correlated weakly with the degree of IgG binding to each strain. However, the binding of C1q (the first component of the classical pathway) correlated more closely with C3b/iC3b deposition, and large differences remained in complement sensitivity between strains with the same capsular serotype in sera in which IgG had been cleaved with IdeS.ConclusionsThese data demonstrate that bacterial factors independent of the capsule and recognition by IgG have strong effects on the susceptibility of S. pneumoniae to complement, and could therefore potentially account for some of the differences in virulence between strains.

Streptococcus pneumoniae Capsular Serotype Invasiveness Correlates with the Degree of Factor H Binding and Opsonization with C3b/iC3b

“…The isogenic capsular transparent-phase variants of the S. pneumoniae strain TIGR4 used in this study have been described previously (2,4,23,24). The 33 clinical strains used represent common multilocus sequence type (MLST) clones for 8 serotypes, and all isolates were obtained from nasopharyngeal cultures of asymptomatic children or from invasive S. pneumoniae infections in adults and children and were described by Hyams et al (19) (kind gifts from Brian Spratt, Imperial College, and Birgitta Henriques-Normak, Karolinska). PspC mutants of capsule-switched variants of TIGR4 were created by transformation using conventional methods with a pspC deletion construct made by overlap extension PCR as described previously (22).…”

“…Capsule-specific antibody titers for clinically relevant serotypes were measured using standardized enzyme-linked immunosorbent assays (ELISAs) with preabsorption with C-polysaccharide and serotype 22F capsular polysaccharide as described (http://www .vaccine.uab.edu/ELISA%20Protocol.pdf). Total IgG binding to S. pneumoniae (representing both anticapsular and antiprotein antigen activity), C3b/iC3b deposition, iC3b, FH, and Bf binding to S. pneumoniae after incubation in 20% serum for 20 min at 37°C were measured using flow cytometry assays and R-phycoerythrin goat anti-human IgG (Jackson ImmunoResearch) or fluorescein isothiocyanate (FITC)-conjugated antihuman C3 or iC3b, anti-FH, or anti-Bf (ICN) as described previously (2,12,15,16,19). As complement factor binding to S. pneumoniae is often biphasic with strongly positive and weakly positive populations of bacteria, results are presented as a fluorescence index (FI; percentage of positive bacteria multiplied by the geometric mean fluorescence index (MFI) in arbitrary units) (2,12,15); this ensures both intensity (geometric mean MFI) of complement factor binding and the proportion of positive bacteria are included in the data analysis.…”

“…To further support the hypothesis that differences in FH binding might influence strain invasiveness, FH binding was analyzed using a set of 33 clinical isolates from 8 common serotypes with different genetic backgrounds (19). FH binding varied significantly between strains from different serotypes (e.g., serotype 1 strain versus serotype 6A strain) and between strains expressing the same capsular serotype (e.g., serotype 4 and serotype 14) (Fig.…”

“…Complement is vital for protection against septicemia (14)(15)(16), suggesting that the effect of the capsule on complement resistance is one major reason why the capsule is vital for the development of invasive disease. Complement resistance varies markedly between S. pneumoniae strains, and this is related to both the capsular serotype (2,3,17,18) and other genetic variation between strains (18,19). Infection with serotypes that were resistant to complement resulted in greater bacterial CFU in the blood in mouse models of sepsis (2,4), supporting a hypothesis that resistance to complement-mediated immunity could partially explain differences in invasiveness between serotypes.…”

Wzy-Dependent Bacterial Capsules as Potential Drug Targets

“…While numerous virulence factors undoubtedly contribute to the pathogen's ability to resist complement deposition and phagocytosis [23][24][25], two recent studies have illustrated the central dominance of the capsule in this strategy [24,26]. Both of these studies showed the fundamental importance of capsule by leaving the genetic background the same and only changing the capsule type.…”