TRP75-mediated STAT3 activation promotes anti-apoptotic signaling and <i>Ehrlichia chaffeensis</i> infection

<i>Ehrlichia chaffeensis</i> is an obligately intracellular bacterium that manipulates mononuclear phagocytes by hijacking host cell signaling pathways to promote infection. Previous studies from our laboratory have shown that multiple signal transducer and activator of transcription (STAT) family members interact with <i>E. chaffeensis</i> effector proteins. However, the functional role of STATs during infection remains poorly understood. Notably, STAT3, a highly immunomodulatory and pro-survival factor, interacts with the <i>E. chaffeensis</i> effector protein TRP75. In this study, we examined activation of STAT family members and transcription of STAT target genes during <i>E. chaffeensis</i> infection. We observed significant activation of multiple STATs (STAT1, STAT3, STAT5, and STAT6), with STAT3 showing the highest level of activation. Therefore, we further investigated STAT3 activation dynamics and effects of its inhibition on infection. STAT3 phosphorylation and nuclear translocation were detected beginning 48 h post-infection, coinciding with upregulation of STAT3 target genes, including the anti-apoptotic gene <i>MCL-1</i>. Pharmacological inhibition of STAT3 significantly reduced <i>MCL-1</i> expression and increased caspase cleavage, implicating STAT3 as a regulator of anti-apoptotic signaling during infection. Furthermore, both pharmacological inhibition and genetic knockout of STAT3 significantly reduced bacterial load, highlighting its critical role in supporting infection. Ectopic expression of TRP75 in human embryonic kidney 293 cells induced STAT3 phosphorylation, demonstrating a specific role for TRP75 in STAT3 activation. Collectively, these findings support a model in which <i>E. chaffeensis</i> exploits STAT3 via the TRP75 effector to activate an anti-apoptotic program and other cellular pathways that promote infection.