ABSTRACTNontyphoidal serovars ofSalmonella entericaare pathogenic bacteria that are common causes of food poisoning. WhereasSalmonellamechanisms of host cell invasion, inflammation, and pathogenesis are mostly well established, a new possible mechanism of immune evasion is being uncovered. Programmed death ligand 1 (PD-L1) is an immunosuppressive membrane protein that binds to activated T cells via their PD-1 receptor and thereby halts their activation. PD-L1 expression plays an essential role in the immunological tolerance of self-antigens but is also exploited for immune evasion by pathogen-infected cells and cancer cells. Here, we show for the first time thatSalmonellainfection of intestinal epithelial cells causes the induction of PD-L1. The increased expression of PD-L1 throughSalmonellainfection was seen in both human and rat intestinal epithelial cell lines. We determined that cellular invasion by the bacteria is necessary for PD-L1 induction, potentially indicating thatSalmonellastrains are delivering mediators from inside the host cell that trigger the increased PD-L1 expression. Using knockout mutants, we determined that this effect largely originates from theSalmonellapathogenicity island 2. We also show for the first time in any cell type thatSalmonellacombined with gamma interferon (IFN-γ) causes a synergistic induction of PD-L1. Finally, we show thatSalmonellaplus IFN-γ induction of PD-L1 decreased the cytokine production of activated T cells. UnderstandingSalmonellaimmune evasion strategies could generate new therapeutic targets and help to manipulate PD-L1 expression in other diseases.
A subset of epithelial cells mimics regulatory T cells and contributes to immune evasion during development of pancreatic adenocarcinoma
