The A subunit of type IIb enterotoxin (LT-IIb) suppresses the proinflammatory potential of the B subunit and its ability to recruit and interact with TLR2.
The type IIb heat-labile enterotoxin of Escherichia coli (LT-IIb) and its nontoxic pentameric B subunit (LT-IIb-B(5)) display different immunomodulatory activities, the mechanisms of which are poorly understood. We investigated mechanisms whereby the absence of the catalytically active A subunit from LT-IIb-B(5) renders this molecule immunostimulatory through TLR2. LT-IIb-B(5), but not LT-IIb, induced TLR2-mediated NF-kappaB activation and TNF-alpha production. These LT-IIb-B(5) activities were antagonized by LT-IIb; however, inhibitors of adenylate cyclase or protein kinase A reversed this antagonism. The LT-IIb antagonistic effect is thus likely dependent upon the catalytic activity of its A subunit, which causes elevation of intracellular cAMP and activates cAMP-dependent protein kinase A. Consistent with this, a membrane-permeable cAMP analog and a cAMP-elevating agonist, but not catalytically defective point mutants of LT-IIb, mimicked the antagonistic action of wild-type LT-IIb. The mutants moreover displayed increased proinflammatory activity compared with wild-type LT-IIb. Additional mechanisms for the divergent effects on TLR2 activation by LT-IIb and LT-IIb-B(5) were suggested by findings that the latter was significantly stronger in inducing lipid raft recruitment of TLR2 and interacting with this receptor. The selective use of TLR2 by LT-IIb-B(5) was confirmed in an assay for IL-10, which is inducible by both LT-IIb and LT-IIb-B(5) at comparable levels; TLR2-deficient macrophages failed to induce IL-10 in response to LT-IIb-B(5) but not in response to LT-IIb. These differential immunomodulatory effects by LT-IIb and LT-IIb-B(5) have important implications for adjuvant development and, furthermore, suggest that enterotoxic E. coli may suppress TLR-mediated innate immunity through the action of the enterotoxin A subunit.
Liang
Shuang
S
Center for Oral Health and Systemic Disease, Department of Periodontics, University of Louisville Health Sciences Center, Louisville, KY 40292, USA.
Wang
Min
M
Triantafilou
Kathy
K
Triantafilou
Martha
M
Nawar
Hesham F
HF
Russell
Michael W
MW
Connell
Terry D
TD
Hajishengallis
George
G
D000818:Animals; D001427:Bacterial Toxins / Q000737:chemistry / Q000494:pharmacology*; D016466:CHO Cells; D002478:Cells, Cultured; D006224:Cricetinae; D003412:Cricetulus; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D016207:Cytokines / Q000096:biosynthesis; D004768:Enterotoxins / Q000737:chemistry / Q000494:pharmacology*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D007113:Immunity, Innate; D007249:Inflammation / Q000517:prevention & control*; D016328:NF-kappa B / Q000378:metabolism; D021122:Protein Subunits; D015398:Signal Transduction; D051195:Toll-Like Receptor 2 / Q000037:antagonists & inhibitors*; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis
D001427:Bacterial Toxins; D016207:Cytokines; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D016328:NF-kappa B; D021122:Protein Subunits; C495342:TLR2 protein, human; D051195:Toll-Like Receptor 2; D014409:Tumor Necrosis Factor-alpha; C027538:heat-labile enterotoxin, E coli; D017868:Cyclic AMP-Dependent Protein Kinases