Pneumolysin suppresses the initial macrophage pro-inflammatory response to Streptococcus pneumoniae

Area of Science:

• Immunology
• Microbiology
• Pathogenesis

Background:

• The Streptococcus pneumoniae virulence factor Pneumolysin (Ply) exhibits conflicting roles in host inflammatory responses.
• Previous studies indicate Ply can activate the inflammasome but also suppress dendritic cell responses by binding MRC-1.

Purpose of the Study:

• To determine if pro- or anti-inflammatory effects of Ply dominate the early macrophage response to S. pneumoniae.
• To investigate the consequences of Ply's immunomodulatory effects during early lung infection.

Main Methods:

• In vitro infection model using human monocyte-derived macrophages (MDMs).
• Mouse model of pneumonia.
• Analysis of cytokine production (TNF, IL-6) and gene expression.
• Assessment of bacterial clearance and host response.

Main Results:

• S. pneumoniae expressing Ply suppressed TNF and IL-6 production in MDMs compared to ply-deficient strains.
• Ply-expressing S. pneumoniae induced a quantitatively lower MDM transcriptional response, including reduced TNF and type I IFN-inducible genes.
• Inhibition of Suppressor of Cytokine Signaling 1 (SOCS1) prevented the suppression of MDM inflammatory response.
• In vivo, the TNF response was enhanced and bacterial clearance increased in mice infected with ply-deficient S. pneumoniae.

Conclusions:

• Ply inhibits the initial macrophage inflammatory response to S. pneumoniae, likely mediated through SOCS1.
• This Ply-mediated suppression of inflammation is associated with improved bacterial immune evasion during early lung infection.