Full title Capsule and PspA Cooperatively Confer Resistance of Streptococcus pneumoniae to the Human defensin HNP-1

Abstract

Streptococcus pneumoniae resists host defenses through multiple virulence factors, yet their combined influence on the action of antimicrobial peptides remains unclear. We examined the role of Pneumococcal surface protein A (PspA) and the polysaccharide capsule in modulating susceptibility to the human defensin HNP-1. PspA-deficient strains of two different genetic backgrounds displayed increased sensitivity, while recombinant PspA neutralized peptide activity and anti-PspA antibodies enhanced bacterial killing. The capsule conferred serotype-dependent protection, with type 2 being more effective 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. These findings highlight the complementary roles of capsule and PspA in pneumococcal resistance to HNP-1 and suggest that targeting these mechanisms could potentiate innate immune clearance and provide novel insights that may inform future vaccine design and antimicrobial strategies. AUTHOR SUMMARYStreptococcus pneumoniae causes serious infections such as pneumonia and meningitis, in part by evading the human immune system. One key component of our immune defense is antimicrobial peptides like HNP-1, which directly kill bacteria. In this study, we investigated how two major pneumococcal virulence factors - the surface protein PspA and the sugar capsule - help the bacterium resist killing by HNP-1. We found that removing PspA made the bacteria more susceptible, while adding purified PspA or blocking it with antibodies increased the peptide activity. The protective effect of the capsule depended on the sugar composition, and purified capsule sugars could bind and neutralize HNP-1, limiting its activity against pneumococci. When we removed PspA from bacteria lacking a capsule, the bacteria became even more sensitive, showing that both factors contribute to resisting immune attack. These results reveal how S. pneumoniae uses multiple strategies to survive innate immune defenses and highlight potential targets for improved vaccines or new treatments.