Pneumococcus drives STAT3 activation of lower airway epithelium in a strain and burden-dependent manner

Crosstalk between respiratory bacteria and human airway epithelial cells initiates cellular immunity, yet the molecular determinants of asymptomatic carriage versus inflammation remain incompletely understood. A key regulator of airway epithelial cellular immunity is the Signal Transducer and Activator of Transcription (STAT3). Once activated by phosphorylation this host transcription factor signals through two cascades, canonical by phosphorylation at tyrosine 705 and non-canonical by phosphorylation at serine 727, which contributes to bacterial clearance and immune defense in pneumonia. Despite this, its role in epithelial responses to Streptococcus pneumoniae has not been defined. Revealing these processes are particularly important for understanding the variability in pathogenic potential for the pathobiont Streptococcus pneumoniae which triggers either host commensal-like carriage or susceptibility at this interface. Here we investigated the role of STAT3 signaling during pneumococcal challenge using lower airway epithelial cells and two pneumococcal isolates favoring either commensal-like carriage, 6B ST90, or with pathogenic potential, TIGR4. We show the invasive TIGR4 strain drives canonical STAT3 phosphorylation and suppresses non-canonical STAT3 in a burden-dependent manner. This contrasts with the 6B ST90 isolate which requires a 20-fold increase in burden to elicit minimal STAT3 responses. TIGR4 activation correlated with SOCS3 expression, while knockdown of endogenous STAT3 did not alter pneumococcal adherence or epithelial membrane integrity. Mechanistically, STAT3 activation required live bacteria and pneumolysin production but was independent of bacterial hydrogen peroxide. Altogether we reveal previously unrecognized divergence in STAT3 signaling during pneumococcal infection.