Capsule and PspA Cooperatively Confer Resistance of Streptococcus pneumoniae to the Human Defensin HNP-1

Streptococcus pneumoniae resists host defenses through multiple surface factors, yet their specific contribution to protection against antimicrobial peptides remains incompletely understood. We examined the role of pneumococcal surface protein A (PspA) and the polysaccharide capsule in protection against the human defensin HNP-1. PspA conferred increased resistance to HNP-1-induced killing, shown by a decreased killing in the presence of purified recombinant PspA and an increased sensitivity when PspA was deficient from the surface of strains of two different genetic backgrounds or when anti-PspA antibody was present. The capsule also conferred protection against HNP-1, which was serotype-dependent, with type 2 protecting better than type 4, and free polysaccharides acted as decoys by sequestering HNP-1. Removal of surface PspA from capsule-deficient mutants revealed additive contributions of both factors to survival. Molecular docking analysis suggests a potential electrostatic interaction between PspA and HNP-1. These findings highlight the independent and complementary roles of PspA and the capsule in pneumococcal resistance to HNP-1 and provide novel insights that may inform future vaccine design and antimicrobial strategies.