Streptococcus pneumoniaeCapsular Serotype 19F Is More Resistant to C3 Deposition and Less Sensitive to Opsonophagocytosis than Serotype 6B

Abstract

The polysaccharide capsule is a major virulence mechanism of Streptococcus pneumoniae, shielding the bacterium from phagocytes. Capsule types may differ in their abilities to resist immune defense. Antibodymediated complement activation and opsonophagocytosis are crucial in protection against pneumococcus. Conjugate vaccine trials suggest imperfect protection against 19F. We have previously shown that significantly more anti-19F than anti-6B antibody is needed for killing in the opsonophagocytic assay (OPA). In this study, we explored whether the amount of C3 deposited on serotype 6B and 19F pneumococcal strains reflects their sensitivity to opsonophagocytosis. We compared clinical 6B and 19F nasopharyngeal, middle ear, and blood isolates as well as reference OPA strains (n ‫؍‬ 16) for their sensitivity to opsonophagocytosis and C3 deposition. Sixfold anticapsular antibody concentrations were required for 50% opsonophagocytic killing of 19F compared to that of 6B strains. Serotype 19F was more resistant to C3 deposition than 6B. Complement deposition and opsonophagocytosis were dependent on the concentration of anticapsular antibodies. Differences between pneumococcal serotypes in antibody-mediated protection may partly be explained by the abilities of the capsules to resist complement deposition. These findings support previous studies suggesting that higher antibody concentrations to the capsular polysaccharide are needed for protection against disease caused by serotype 19F than that caused by 6B.

The Capsular Serotype ofStreptococcus pneumoniaeIs More Important than the Genetic Background for Resistance to Complement

“…We have previously found that pneumococcal isolates of certain serotypes, such as 1 and 5, associated with invasive disease were particularly resistant to complement deposition and opsonophagocytic killing, while serotypes such as 6B and 23F, associated with carriage, were more sensitive to deposition of C3 and opsonophagocytosis (30,31). We found significant variation in the magnitude of complement deposition between isolates expressing the same capsular serotype, suggesting a role of serotype-independent factors in the outcome of this particular interaction.…”

“…The functional activity of serum antibodies was measured by standard opsonophagocytic killing assay (40) using differentiated HL-60 cells (promyelotic leukemia cells, CCL240; American Type Culture Collection, Rockville, MD), as described previously (30). Shortly thereafter, polymorphonuclear cells were allowed to phagocytose bacteria in the presence of serum pools containing anticapsular antibodies and baby rabbit complement (PeelFreez Biologicals/Dynal) or, as a control, complement only.…”

“…The deposition of C3 on pneumococci was measured with a flow cytometric assay as described previously (30). In short, pneumococcal strains were incubated in 20% human serum with active complement components for 5 min, and bound C3 molecules (C3b and iC3b) were detected by incubating the bacteria with fluorescein isothiocyanate-conjugated rabbit polyclonal anti-human complement C3c (Dako Immunoglobulins, Denmark).…”

“…Frozen stocks of bacterial cultures were prepared for the opsonophagocytic (30,40), with minor modifications. Bacteria were cultured in THYE with 5% FBS until the early logarithmic growth phase, and culture stocks were frozen slowly in 15% glycerol at Ϫ70°C.…”

Impact of infant 13-valent pneumococcal conjugate vaccine on serotypes in adult pneumonia

“…National IPD surveillance data from the UK found that the majority of vaccine failures among the PCV7 serotypes were due to serotypes 6B and 19F in the post-PCV7 period [19]. Vaccine failures related to serotype 19F may be due to resistance to complement deposition and lower sensitivity to opsonophagocytic killing [25]. Inability to effectively clear nasopharyngeal carriage of serotype 19F in children may translate into a lack of "herd protection" against serotype 19F amongst adults.…”