Bacteria use small secreted chemicals or peptides as auto-inducers to coordinately regulate gene expression within a population in a process called quorum sensing. Quorum sensing controls several important functions in different bacterial species, including the production of virulence factors and biofilm formation in Pseudomonas aeruginosa and bioluminescence in Vibrio fischeri. Many gram-negative bacterial species use acyl homoserine lactones as auto-inducers that function as ligands for transcriptional regulatory proteins. Several recent reports indicate that bacterial acyl homoserine lactones can also affect gene expression in host cells. Direct signaling also appears to function in the opposite direction as some eukaryotic cell types produce mimics that interact with quorum sensing systems in bacteria. Here, we will describe the evidence to support the existence of bi-directional inter-kingdom signaling via acyl homoserine lactones and eukaryotic mimics and discuss the potential molecular mechanisms that mediate these responses. The functional consequences of inter-kingdom signaling will be discussed in relation to both pathogenic and non-pathogenic bacterial-host interactions.

SummaryThe opportunistic pathogen Pseudomonas aeruginosa utilizes a cell density-dependent signalling phenomenon known as quorum sensing (QS) to regulate several virulence factors needed for infection. Acylated homoserine lactones, or autoinducers, are the primary signal molecules that mediate QS in P. aeruginosa . The autoinducer N-3O-dodecanoylhomoserine lactone (3O-C12) exerts effects on mammalian cells, including upregulation of proinflammatory mediators and induction of apoptosis. However, the mechanism(s) by which 3O-C12 affects mammalian cell responses is unknown. Here we report that 3O-C12 induces apoptosis and modulates the expression of immune mediators in murine fibroblasts and human vascular endothelial cells (HUVEC). The effects of 3O-C12 were accompanied by increases in cytosolic calcium levels that were mobilized from intracellular stores in the endoplasmic reticulum (ER). Calcium release was blocked by an inhibitor of phospholipase C, suggesting that release occurred through inositol triphosphate (IP 3 ) receptors in the ER. Apoptosis, but not immunodulatory gene activation, was blocked when 3O-C12-exposed cells were co-incubated with inhibitors of calcium signalling. This study indicates that 3O-C12 can activate at least two independent signal transduction pathways in mammalian cells, one that involves increases in intracellular calcium levels and leads to apoptosis, and a second pathway that results in modulation of the inflammatory response.

BackgroundImmune checkpoint inhibition has dramatically transformed the treatment of malignant melanoma. With increasing use, their unique spectrum of immune-mediated toxicity has become apparent.Case presentationWe describe a case of sequential immune-related adverse events (irAEs) in a patient with metastatic melanoma treated with single-agent anti-programmed cell death-1 (PD-1) therapy, pembrolizumab. Although numerous cases of irAEs have been reported, sequential multi-organ involvement, including progressive atopic dermatitis, vitiligo, autoimmune nephritis, autoimmune hepatitis, and autoimmune encephalitis after cessation of therapy, has not been previously documented.ConclusionsImmunosuppression resulted in clinical remission of each irAE, highlighting the importance of vigilance for autoimmune complications in patients treated with checkpoint inhibition, even after immunotherapy cessation.

Quorum sensing (QS) is a cell density-dependent signaling system used by bacteria to coordinate gene expression within a population. QS systems in Gram negative bacteria consist of transcription factors of the LuxR family and their acyl homoserine lactone (AHL) ligands. We describe here a method for examining QS signaling systems in mammalian cells that uses engineered LuxR-type proteins from the opportunistic pathogen, Pseudomonas aeruginosa, which can function as AHL-dependent transcription factors. The engineered proteins respond to their cognate ligands and display sequence specific DNA binding properties. This system has several potential biotechnological and biological applications. It may be used to characterize any LuxR-type protein, screen animal and plant cell extracts or exudates for compounds that mimic or interfere with AHL signaling or to screen different cell types for AHL inactivating activities