Endotoxin tolerance in mast cells, its consequences for IgE-mediated signalling, and the effects of BCL3 deficiency

Stimulation with lipopolysaccharide (LPS; endotoxin) not only causes rapid production of proinflammatory cytokines, but also induces a state of LPS hypo-responsiveness to a second LPS stimulation (endotoxin tolerance (ET)). Murine bone marrow-derived MCs (BMMCs) and peritoneal MCs (PMCs) developed ET as shown by an abrogated production of Il6/Tnf RNAs and IL-6/TNF-α proteins. In naive BMMCs, LPS stimulation induced a transient decline in the trimethylation of lysine 9 of the core histone H3 (H3K9me3), a suppressive chromatin mark, at the Il6/Tnf promoters, which correlated with p50(NFκB) and p65(NFκB) binding. Both demethylation and NFκB binding were abrogated in tolerant cells. In addition, cytosolic NFκB activation was suppressed in tolerant BMMCs. Intriguingly, antigen stimulation of naive and tolerant MCs induced comparable production of Il6/Tnf and IL-6/TNF-α, although ET also affected antigen-triggered activation of NFκB; pharmacological analysis indicated the importance of Ca²⁺-dependent transcription in this respect. In macrophages, the IκB member BCL3 is induced by LPS and known to be involved in ET, which was not corroborated comparing wild-type and Bcl3-deficient BMMCs. Interestingly, Bcl3-deficient PMCs produce markedly increased amounts of IL-6/TNF-α after LPS stimulation. Collectively, ET in MCs is BCL3-independent, however, in PMCs, BCL3 negatively regulates immediate LPS-induced cytokine production and quantitatively affects ET.