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Gut microbiota is a constant source of antigens and stimuli to which the resident immune system has developed tolerance. However, the mechanisms by which mononuclear phagocytes, specifically monocytes/macrophages, cope with these usually pro-inflammatory signals are poorly understood. Here, we show that innate immune memory promotes anti-inflammatory homeostasis, using as model strains of the commensal bacterium Lactiplantibacillus plantarum. Priming of monocytes/ macrophages with bacteria, especially in its live form, enhances bacterial intracellular survival and decreases the release of pro-inflammatory signals to the environment, with lower production of TNF and higher levels of IL-10. Analysis of the transcriptomic landscape of these cells shows downregulation of pathways associated with the production of reactive oxygen species (ROS) and the release of cytokines, chemokines and antimicrobial peptides. Indeed, the induction of ROS prevents memory-induced bacterial survival. In addition, there is a dysregulation in gene expression of several metabolic pathways leading to decreased glycolytic and respiratory rates in memory cells. These data support commensal microbe-specific metabolic changes in innate immune memory cells that might contribute to homeostasis in the gut.

Genes involved in the transport and catabolism of carbohydrates are usually controlled through the binding of the catabolite control protein A (CcpA) to the catabolite-responsive elements (cre) of target genes in Gram-positive bacteria. In this work, we show how the elimination of the cre sites in Lactobacillus casei BL23 promoters induced by sorbitol (PgutF), maltose (PmalL), and myoinositol (PiolT) allowed the induction of gene expression in media supplemented with sorbitol, maltose, and myo-inositol, respectively, even in the presence of glucose. This was studied using plasmids encoding the anaerobic fluorescent protein evoglow-Pp1 as a reporter. In addition, gutF cre site was introduced into a bile inducible promoter (P16090) and into the constitutive promoter of the elongation factor P (PEf-P) of L. casei BL23. The existence of the cre site blocked gene expression in the P16090 inducible promoter in the presence of glucose, while it had no influence on the expression of the PEf-P constitutive one. These results demonstrated that the introduction or elimination of cre sites in inducible promoters allows the control and modification of their heterologous genetic expression, showing how the cre site, the transcriptional regulator, and CcpA interact to control gene expression in inducible genes. Key points• Cre sequences regulate gene expression in inducible promoters in L. casei BL23.• Cre sites do not affect gene expression in constitutive promoters in L. casei BL23.• Cre sequences could control heterologous genic expression in lactobacilli.Keywords catabolite response element . inducible promoter . constitutive promoter . genic expression . Lactobacillus

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