Human macrophage response to the emerging enteric pathogen <i>Aeromonas veronii</i> : Inflammation, apoptosis, and downregulation of histones

This study investigated the pathogenic mechanisms of <i>Aeromonas veronii</i> in macrophages. THP-1 derived macrophages were used as a human macrophage model and were treated with <i>A. veronii</i> strain AS1 isolated from intestinal biopsies of an IBD patient, or <i>Escherichia coli</i> strain K-12. RNA was extracted and subjected to RNA sequencing and comparative transcriptomic analyses. Protein levels of IL-8, IL-1β, IL-18, and TNFα were measured using ELISA, and apoptosis was assessed using caspase 3/7 assays. Both <i>A. veronii</i> AS1 and <i>E. coli</i> K-12 significantly upregulated the expression of many genes involving inflammation. At the protein level, <i>A. veronii</i> AS1 induced significantly higher levels of IL-8, TNFα, mature IL-18 and IL-1β than <i>E. coli</i> K-12, and led to greater elevation of caspase 3/7 activities. Both <i>A. veronii</i> AS1 and <i>E. coli</i> K-12 upregulated the expression of <i>CASP5</i>, but not other caspase genes. <i>A. veronii</i> AS1 significantly downregulated the expression of 20 genes encoding histone proteins that <i>E. coli</i> K-12 did not. The more profound pathogenic effects of <i>A. veronii</i> in inducing inflammation and apoptosis in macrophages than <i>E. coli</i> K-12 are consistent with its role as a human enteric pathogen. The upregulated expression of <i>CASP5</i> and increased release of IL-1β and IL-18 support the role of <i>CASP5</i> in activation of non-canonical inflammasome. The downregulation of histone genes by <i>A. veronii</i> suggests a unique impact on host cell gene expression, which may represent a novel virulence strategy. These findings advance the understanding of pathogenic mechanisms of the emerging human enteric pathogen <i>A. veronii.</i>