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Original Research ARTICLE

Front. Immunol. | doi: 10.3389/fimmu.2020.00080

Streptococcal extracellular membrane vesicles are rapidly internalized by immune cells and alter their cytokine release Provisionally accepted The final, formatted version of the article will be published soon. Notify me

 Mina Mehanny1, Marcus Koch2, Claus-Michael Lehr1 and  Gregor Fuhrmann1*
  • 1Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Germany
  • 2Leibniz Institut für Neue Materialien (LG), Germany

Extracellular vesicles are membranous structures shed by almost every living cell. Bacterial gram-negative outer membrane vesicles (OMVs) and gram-positive membrane vesicles (MVs) play important roles in adaptation to surrounding environment, cellular components exchange, transfer of antigens and virulence factors, and infection propagation. Streptococcus pneumoniae is considered one of the priority pathogens, with a global health impact due to increasing infection burden and growing antibiotic resistance.
We isolated MVs produced from Streptococcus pneumoniae reference strain (R6), and purified them via size exclusion chromatography (SEC) to remove soluble protein impurities. We characterized the isolated MVs by nanoparticle tracking analysis (NTA), and measured their particle size distribution and concentration. Isolated MVs showed a mean particle size range of 130-160 nm and a particle yield of around 1012 particle/mL. Cryogenic transmission electron microscopy (cryo-TEM) images revealed a very heterogeneous nature of isolated MVs with a broad size range and various morphologies, arrangements and contents.
We incubated streptococcal MVs with several mammalian somatic cells, namely human lung epithelial A549 and human keratinocytes HaCaT cell lines, and immune cells including differentiated macrophage-like dTHP-1 and murine dendritic DC2.4 cell lines. All cell lines displayed excellent viability profile and negligible cytotoxicity after 24 h incubation with MVs at concentrations reaching 106 MV/cell (somatic cells) and 105 MV/cell (immune cells).
We evaluated uptake of fluorescently labelled MVs into these four cell lines, using flow cytometry and confocal microscopy. Dendritic cells demonstrated prompt uptake after 30-minute incubation, whereas other cell lines showed increasing uptake after 2 h incubation and almost complete colocalization/internalization of MVs after only 4 h incubation. We assessed the influence of streptococcal MVs on antigen presenting cells e.g., dendritic cells using enzyme-linked immunosorbent assay (ELISA), and observed enhanced release of TNF-α, slight increase of Interleukin-10 (IL-10) secretion and no detectable effect on IL-12.
Our study provides better understanding of gram-positive streptococcal MVs, and show their potential to elicit a protective immune response. Therefore, they could offer an innovative avenue for safe and effective cell-free vaccination against pneumococcal infections.

Keywords: extracellular membrane vesicles (EMVs), Streptococcus pneumoniae (pneumococcus), Cytotoxicity, uptake, Cytokine - immunological terms, Vaccine

Received: 26 Sep 2019; Accepted: 13 Jan 2020.

Copyright: © 2020 Mehanny, Koch, Lehr and Fuhrmann. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Mx. Gregor Fuhrmann, Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrücken, 66123, Saarland, Germany, gregor.fuhrmann@helmholtz-hips.de