The Capsule Sensitizes Streptococcus pneumoniae to α-Defensins Human Neutrophil Proteins 1 to 3

ABSTRACT Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. Its polysaccharide capsule causes resistance to phagocytosis and interferes with the innate immune system's ability to clear infections at an early stage. Nevertheless, we found that encapsulated pneumococci are sensitive to killing by a human neutrophil granule extract. We fractionated the extract by high-performance liquid chromatography and identified α-defensins by mass spectrometry as the proteins responsible for killing pneumococci. Analysis of sensitivity to the commercial α-defensins human neutrophil proteins 1 to 3 (HNP1-3) confirmed these findings. We analyzed the sensitivities of different pneumococcal strains to HNP1-3 and found that encapsulated strains are efficiently killed at physiological concentrations (7.5 μg/ml). Surprisingly, nonencapsulated, nonvirulent pneumococci were significantly less sensitive to α-defensins. The proposed mechanisms of α-defensin resistance in nonencapsulated pneumococci is surface charge modification, e.g., by introduction of positive charge by d-alanylation of surface-exposed lipoteichoic acids. These mechanisms are surmounted by the presence of the capsule, which we hypothesize is masking these charge modifications. Hence, α-defensins in the phagolysosome of neutrophils possibly contribute to intracellular killing after antibody-mediated opsonophagocytosis of encapsulated pneumococci.