Salmonella Effector SteE Reprogrammes the Macrophage Regulatory Network to Drive Specific Hyperactivation of STAT3 Target Genes

The ability of Salmonella Typhimurium to exploit macrophages as a niche for survival, replication and dissemination is central to its pathogenesis. The effector SteE, which polarises macrophages into an anti-inflammatory state, is critical during invasive disease. SteE operates via an unprecedented mechanism, reprogramming the host serine/threonine kinase GSK3 to perform tyrosyl-directed phosphorylation of neosubstrates, including the immune transcription factors STAT1 and STAT3. Here, we demonstrate that SteE-driven transcriptional reprogramming relies critically and specifically on STAT3 phosphorylation and DNA binding. By activating STAT3 via a non-canonical pathway, bypassing endogenous negative feedback mechanisms, SteE drives hyperactivation of STAT3 target genes, surpassing the effects of canonical IL10 signalling. Hyperactivation correlates with elevated phosphorylated STAT3 in the macrophage nucleus, facilitating opening of chromatin regions not accessible during endogenous cytokine signalling. Overall, our study illustrates how hijacking of a signalling pathway by SteE dramatically reshapes the macrophage gene regulatory network to enhance Salmonella immune evasion.