Mitochondrial ROS production by neutrophils is required for host antimicrobial function against Streptococcus pneumoniae and is controlled by A2B adenosine receptor signaling

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Polymorphonuclear cells (PMNs) control Streptococcus pneumoniae (pneumococcus) infection through various antimicrobial activities. We previously found that reactive oxygen species (ROS) were required for optimal antibacterial function, however, the NADPH oxidase is known to be dispensable for the ability of PMNs to kill pneumococci. In this study, we explored the role of ROS produced by the mitochondria in PMN antimicrobial defense against pneumococci. We found that the mitochondria are an important source of overall intracellular ROS produced by murine PMNs in response to infection. We investigated the host and bacterial factors involved and found that mitochondrial ROS (MitROS) are produced independent of bacterial capsule or pneumolysin but presence of live bacteria that are in direct contact with PMNs enhanced the response. We further found that MyD88 -/- PMNs failed to produce MitROS in response to pneumococcal infection suggesting that released bacterial products acting as TLR ligands are sufficient for inducing MitROS production in PMNs. To test the role of MitROS in PMN function, we used an opsonophagocytic killing assay and found that MitROS were required for the ability of PMNs to kill pneumococci. We then investigated the role of MitROS in host resistance and found that MitROS are produced by PMNs in response to pneumococcal infection. Importantly, treatment of mice with a MitROS scavenger prior to systemic challenge resulted in reduced survival of infected hosts. In exploring host pathways that control MitROS, we focused on extracellular adenosine, which is known to control PMN anti-pneumococcal activity, and found that signaling through the A2B adenosine receptor inhibits MitROS production by PMNs. A2BR -/- mice produced more MitROS and were significantly more resistant to infection. Finally, we verified the clinical relevance of our findings using human PMNs. In summary, we identified a novel pathway that controls MitROS production by PMNs, shaping host resistance against S. pneumoniae .

Author summary

Despite the presence of antibiotics and vaccines, Streptococcus pneumoniae infections remain a serious cause of mortality and morbidity globally. Understanding protective host responses is key for designing improved therapies against infection. Neutrophils are innate immune cells that are crucial for control of S. pneumoniae infection. In this study we explored the mechanisms by which neutrophils kill S. pneumoniae . We found that the mitochondria, whose primary role is energy production, also produce reactive oxygen species (ROS) that are critical for the ability of neutrophils to kill S. pneumoniae. We explored the bacterial and host factors involved in mitochondrial ROS (MitROS) production by neutrophils. We found that recognition of bacterial products by neutrophils triggers this response and that the host A2B adenosine receptor regulates it. Importantly, MitROS were required for host resistance against S. pneumoniae . This study describes a novel pathway that controls anti-microbial responses and can be a future therapeutic target.

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