title: In macrophages, caspase-1 activation by SopE and the type III secretion system-1 of S. typhimurium can proceed in the absence of flagellin.
abstract: The innate immune system is of vital importance for protection against infectious pathogens. Inflammasome mediated caspase-1 activation and subsequent release of pro-inflammatory cytokines like IL-1beta and IL-18 is an important arm of the innate immune system. Salmonella enterica subspecies 1 serovar Typhimurium (S. Typhimurium, SL1344) is an enteropathogenic bacterium causing diarrheal diseases. Different reports have shown that in macrophages, S. Typhimurium may activate caspase-1 by at least three different types of stimuli: flagellin, the type III secretion system 1 (T1) and the T1 effector protein SopE. However, the relative importance and interdependence of the different factors in caspase-1 activation is still a matter of debate. Here, we have analyzed their relative contributions to caspase-1 activation in LPS-pretreated RAW264.7 macrophages. Using flagellar mutants (fliGHI, flgK) and centrifugation to mediate pathogen-host cell contact, we show that flagellins account for a small part of the caspase-1 activation in RAW264.7 cells. In addition, functional flagella are of key importance for motility and host cell attachment which is a prerequisite for mediating caspase-1 activation via these three stimuli. Using site directed mutants lacking several T1 effector proteins and flagellin expression, we found that SopE elicits caspase-1 activation even when flagellins are absent. In contrast, disruption of essential genes of the T1 protein injection system (invG, sipB) completely abolished caspase-1 activation. However, a robust level of caspase-1 activation is retained by the T1 system (or unidentified T1 effectors) in the absence of flagellin and SopE. T1-mediated inflammasome activation is in line with recent work by others and suggests that the T1 system itself may represent the basic caspase-1 activating stimulus in RAW264.7 macrophages which is further enhanced independently by SopE and/or flagellin.
authors.affiliation: Institute of Microbiology, D-BIOL, ETH Zürich, Zürich, Switzerland.
authors.forename: Claudia
authors.initials: C
authors.lastname: Hoffmann
authors.forename: Marlies
authors.initials: M
authors.lastname: Galle
authors.forename: Sabrina
authors.initials: S
authors.lastname: Dilling
authors.forename: Rina
authors.initials: R
authors.lastname: Käppeli
authors.forename: Andreas J
authors.initials: AJ
authors.lastname: Müller
authors.forename: Pascal
authors.initials: P
authors.lastname: Songhet
authors.forename: Rudi
authors.initials: R
authors.lastname: Beyaert
authors.forename: Wolf-Dietrich
authors.initials: WD
authors.lastname: Hardt
mesh_terms: D000818:Animals; D001422:Bacterial Adhesion; D001426:Bacterial Proteins / Q000378:metabolism / Q000494:pharmacology*; D055162:Biocatalysis; D020170:Caspase 1 / Q000378:metabolism*; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D005408:Flagellin; D008264:Macrophages / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D051379:Mice; D012486:Salmonella typhimurium / Q000502:physiology
chemical_list: D001426:Bacterial Proteins; C103987:SopE protein, Salmonella; D005408:Flagellin; D020170:Caspase 1
references.citation: Eur J Immunol. 2010 Mar;40(3):611-5
references.pmid: 20201013
references.citation: Immunol Rev. 2009 Jan;227(1):106-28
references.pmid: 19120480
references.citation: J Bacteriol. 1995 Jul;177(14):3965-71
references.pmid: 7608068
references.citation: Mol Microbiol. 2007 Aug;65(3):741-60
references.pmid: 17635190
references.citation: Annu Rev Immunol. 2009;27:229-65
references.pmid: 19302040
references.citation: Semin Immunopathol. 2007 Sep;29(3):261-74
references.pmid: 17717669
references.citation: J Bacteriol. 2001 Apr;183(7):2348-58
references.pmid: 11244077
references.citation: PLoS Pathog. 2007 Aug 10;3(8):e111
references.pmid: 17696608
references.citation: J Exp Med. 2007 Dec 24;204(13):3235-45
references.pmid: 18070936
references.citation: Cell. 2008 Mar 7;132(5):818-31
references.pmid: 18329368
references.citation: Cell Microbiol. 2007 Dec;9(12):2893-902
references.pmid: 17991047
references.citation: Infect Immun. 2000 May;68(5):2735-43
references.pmid: 10768967
references.citation: J Bacteriol. 2004 Aug;186(16):5486-95
references.pmid: 15292151
references.citation: Nat Immunol. 2006 Jun;7(6):576-82
references.pmid: 16648852
references.citation: Infect Immun. 2004 Feb;72(2):795-809
references.pmid: 14742523
references.citation: J Cell Mol Med. 2008 Sep-Oct;12(5A):1767-76
references.pmid: 18081695
references.citation: Eur J Immunol. 2007 Nov;37(11):3030-9
references.pmid: 17935074
references.citation: J Exp Med. 2006 Jun 12;203(6):1407-12
references.pmid: 16717117
references.citation: J Exp Med. 2006 Apr 17;203(4):1093-104
references.pmid: 16606669
references.citation: Proc Natl Acad Sci U S A. 1999 Aug 17;96(17):9845-50
references.pmid: 10449782
references.citation: Cell. 2010 Mar 19;140(6):821-32
references.pmid: 20303873
references.citation: Mol Gen Genet. 1972;119(1):75-88
references.pmid: 4564719
references.citation: Cell. 1998 May 29;93(5):815-26
references.pmid: 9630225
references.citation: Infect Immun. 1999 Mar;67(3):1093-9
references.pmid: 10024548
references.citation: J Bacteriol. 1995 Dec;177(24):7078-85
references.pmid: 8522512
references.citation: J Biol Chem. 2001 Sep 7;276(36):34035-40
references.pmid: 11440999
references.citation: J Biol Chem. 2007 Nov 23;282(47):33897-901
references.pmid: 17911114
references.citation: Proc Natl Acad Sci U S A. 1997 Sep 2;94(18):9887-92
references.pmid: 9275221
references.citation: Microbes Infect. 2001 Nov-Dec;3(14-15):1201-12
references.pmid: 11755408
references.citation: Mol Cell Biol. 1994 Feb;14(2):1333-46
references.pmid: 8289811
references.citation: J Immunol. 2003 Dec 15;171(12):6742-9
references.pmid: 14662878
references.citation: Proc Natl Acad Sci U S A. 2000 Jun 6;97(12):6640-5
references.pmid: 10829079
references.citation: Proc Natl Acad Sci U S A. 2010 Feb 16;107(7):3076-80
references.pmid: 20133635
references.citation: Mol Microbiol. 2000 Jun;36(6):1206-21
references.pmid: 10931274
references.citation: Nature. 1981 May 21;291(5812):238-9
references.pmid: 7015147
references.citation: J Bacteriol. 2003 Dec;185(23):6950-67
references.pmid: 14617659
references.citation: Infect Immun. 2004 Jul;72(7):4138-50
references.pmid: 15213159
references.citation: Cell Host Microbe. 2009 Aug 20;6(2):125-36
references.pmid: 19683679
references.citation: J Gen Microbiol. 1981 Dec;127(2):351-60
references.pmid: 6123543
references.citation: Mol Microbiol. 2001 Jan;39(2):248-59
references.pmid: 11136447
references.citation: Cell Microbiol. 2006 Nov;8(11):1812-25
references.pmid: 16824040
references.citation: Nat Immunol. 2006 Jun;7(6):569-75
references.pmid: 16648853
references.citation: Mol Cell. 2002 Aug;10(2):417-26
references.pmid: 12191486
references.citation: Science. 1999 Mar 26;283(5410):2092-5
references.pmid: 10092234
references.citation: J Bacteriol. 2004 Feb;186(4):1215-9
references.pmid: 14762020
references.citation: Mol Microbiol. 1994 Aug;13(4):555-68
references.pmid: 7997169
references.citation: Proc Natl Acad Sci U S A. 1999 Mar 2;96(5):2396-401
references.pmid: 10051653
references.citation: J Immunol Methods. 1990 Dec 31;135(1-2):25-32
references.pmid: 2273259