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Toll-Like Receptor 2 Mediates Cellular Activation by the B Subunits of Type II Heat-Labile Enterotoxins

Hajishengallis, George, Tapping, Richard I., Martin, Michael H., Nawar, Hesham, Lyle, Elizabeth A., Russell, Michael W., Connell, Terry D.
Infection and immunity 2005 v.73 no.3 pp. 1343-1349
Escherichia coli, adjuvants, agonists, amino acid sequences, cholera toxin, enterotoxins, gangliosides, genes, humans, interleukin-1, interleukin-6, interleukin-8, kidneys, luciferase, macrophages, mice, mutants, point mutation, tumor necrosis factor-alpha
The type II heat-labile enterotoxins (LT-IIa and LT-IIb) of Escherichia coli have an AB₅ subunit structure similar to that of cholera toxin (CT) and other type I enterotoxins, despite significant differences in the amino acid sequences of their B subunits and different ganglioside receptor specificities. LT-II holotoxins and their nontoxic B subunits display unique properties as immunological adjuvants distinct from those of CT and its B subunits. In contrast to type II holotoxins, the corresponding pentameric B subunits, LT-IIaB and LT-IIbB, stimulated cytokine release in both human and mouse cells dependent upon Toll-like receptor 2 (TLR2). Induction of interleukin-1{szligbeta} (IL-1{szligbeta}), IL-6, IL-8, or tumor necrosis factor alpha in human THP-1 cells by LT-IIaB or LT-IIbB was inhibited by anti-TLR2 but not by anti-TLR4 antibody. Furthermore, transient expression of TLR1 and TLR2 in human embryonic kidney 293 cells resulted in activation of a nuclear factor-[kappa]B-dependent luciferase gene in response to LT-IIaB or LT-IIbB. Moreover, peritoneal macrophages from TLR2-deficient mice failed to respond to LT-IIaB or LT-IIbB, in contrast to wild-type or TLR4-deficient cells. These results demonstrate that besides their established binding to gangliosides, the B subunits of type II enterotoxins also interact with TLR2. Although a ganglioside-nonbinding mutant (T34I) of LT-IIaB effectively induced cytokine release, a phenotypically similar point mutation (T13I) in LT-IIbB abrogated cytokine induction, suggesting a variable requirement for gangliosides as coreceptors in TLR2 agonist activity. TLR2-dependent activation of mononuclear cells by type II enterotoxin B subunits appears to be a novel mechanism whereby these molecules may exert their immunomodulatory and adjuvant activities.