Immunomodulatory Effects of Pneumococcal Extracellular Vesicles on Cellular and Humoral Host Defenses

Abstract

Gram-positive bacteria, including the major respiratory pathogen Streptococcus pneumoniae, were recently shown to produce extracellular vesicles (EVs) that likely originate from the plasma membrane and are released into the extracellular environment. EVs may function as cargo for many bacterial proteins, however, their involvement in cellular processes and their interactions with the innate immune system are poorly understood. Here, EVs from pneumococci were characterized and their immunomodulatory effects investigated. Pneumococcal EVs were protruding from the bacterial surface and released into the medium as 25 to 250 nm lipid stained vesicles containing a large number of cytosolic, membrane, and surface-associated proteins. The cytosolic pore-forming toxin pneumolysin was significantly enriched in EVs compared to a total bacterial lysate but was not required for EV formation. Pneumococcal EVs were internalized into A549 lung epithelial cells and human monocyte-derived dendritic cells and induced proinflammatory cytokine responses irrespective of pneumolysin content. EVs from encapsulated pneumococci were recognized by serum proteins, resulting in C3b deposition and formation of C5b-9 membrane attack complexes as well as factor H recruitment, depending on the presence of the choline binding protein PspC. Addition of EVs to human serum decreased opsonophagocytic killing of encapsulated pneumococci. Our data suggest that EVs may act in an immunomodulatory manner by allowing delivery of vesicle-associated proteins and other macromolecules into host cells. In addition, EVs expose targets for complement factors in serum, promoting pneumococcal evasion of humoral host defense.

Grad‐seq in a Gram‐positive bacterium reveals exonucleolytic sRNA activation in competence control

“…Synthetic peptide production, immunization, and validation by ELISA were done by Innovagen (Lund, Sweden), following a 41‐day immunization protocol including two booster injections. It was further used in previous studies (Codemo et al , ; Pathak et al , ). Detection of the GAPDH‐specific serum was performed using an anti‐rabbit secondary antibody (Thermo Fisher Scientific, cat# 31460).…”

A capsular polysaccharide-expressing live vaccine suppresses streptococcal toxic shock-like syndrome and provides sequence type-independent protection during Streptococcus suis infection

“…As isotype-switched IgG plays an important role in the fight against encapsulated extracellular bacteria (35, 36), antibodies (Abs) to the entire encapsulated SC19 strain were assessed after 2015033 immunization. Immunized mice exhibited a significant increase in IgG, IgG1, and IgG2a, which remained at high levels until the end of the experiment, while mice injected with phosphate buffered saline (PBS) showed no detectable increases (Fig.…”

Autolysin-Independent DNA Release inStreptococcus pneumoniae in vitroBiofilms

“…Previous studies performed with different bacteria have suggested that, in addition to autolytic phenomena, eDNA might be associated with EV. Recent evidence indicates that S. pneumoniae , actively sheds extracellular nano-sized EV, as do many other microorganisms (39–41). Whether pneumococcal EV contains DNA, however, was never examined.…”

“…For example, the stability of EV under various conditions may vary and a vesicle that loses membrane integrity may ‘leak’ its constituents into the supernatant (vesicle destabilization), rather than break up, as reported in Bacillus anthracis , S. aureus and other microorganisms (61). Interestingly, a very recent study has reported that the LytA NAM-amidase is not needed for production of EV, although the presence of EV-associated DNA was not analyzed (41).…”