AUTHOR=Tartaglia Natayme R. , Breyne Koen , Meyer Evelyne , Cauty Chantal , Jardin Julien , Chrétien Denis , Dupont Aurélien , Demeyere Kristel , Berkova Nadia , Azevedo Vasco , Guédon Eric , Le Loir Yves 

TITLE=Staphylococcus aureus Extracellular Vesicles Elicit an Immunostimulatory Response in vivo on the Murine Mammary Gland

JOURNAL=Frontiers in Cellular and Infection Microbiology

VOLUME=8

YEAR=2018

URL=https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2018.00277

DOI=10.3389/fcimb.2018.00277

ISSN=2235-2988

ABSTRACT=<p><italic>Staphylococcus aureus</italic> is a major pathogen responsible for bovine mastitis, the most common and costly disease affecting dairy cattle. <italic>S. aureus</italic> naturally releases extracellular vesicles (EVs) during its growth. EVs play an important role in the bacteria-bacteria and bacteria-host interactions and are notably considered as nanocarriers that deliver virulence factors to the host tissues. Whether EVs play a role in a mastitis context is still unknown. In this work, we showed that <italic>S. aureus</italic> Newbould 305 (N305), a bovine mastitis isolate, has the ability to generate EVs <italic>in vitro</italic> with a designated protein content. Purified <italic>S. aureus</italic> N305-secreted EVs were not cytotoxic when tested <italic>in vitro</italic> on MAC-T and PS, two bovine mammary epithelial cell lines. However, they induced the gene expression of inflammatory cytokines at levels similar to those induced by live <italic>S. aureus</italic> N305. The <italic>in vivo</italic> immune response to purified <italic>S. aureus</italic> N305-secreted EVs was tested in a mouse model for bovine mastitis and their immunogenic effect was compared to that of live <italic>S. aureus</italic> N305, heat-killed <italic>S. aureus</italic> N305 and to <italic>S. aureus</italic> lipoteichoic acid (LTA). Clinical and histopathological signs were evaluated and pro-inflammatory and chemotactic cytokine levels were measured in the mammary gland 24 h post-inoculation. Live <italic>S. aureus</italic> induced a significantly stronger inflammatory response than that of any other condition tested. Nevertheless, <italic>S. aureus</italic> N305-secreted EVs induced a dose-dependent neutrophil recruitment and the production of a selected set of pro-inflammatory mediators as well as chemokines. This immune response elicited by intramammary <italic>S. aureus</italic> N305-secreted EVs was comparable to that of heat-killed <italic>S. aureus</italic> N305 and, partly, by LTA. These results demonstrated that <italic>S. aureus</italic> N305-secreted EVs induce a mild inflammatory response distinct from the live pathogen after intramammary injection. Overall, our combined <italic>in vitro</italic> and <italic>in vivo</italic> data suggest that EVs are worth to be investigated to better understand the <italic>S. aureus</italic> pathogenesis and are relevant tools to develop strategies against bovine <italic>S. aureus</italic> mastitis.</p>