Murine caspase-11, and its human homologs caspase-4 and caspase-5, are mammalian intracellular receptor proteases activated by TLR4 and TLR3 signaling during the innate immune response. Caspase-11, also termed the non-canonical inflammasome, is activated by TLR3/TLR4-TRIF signaling and directly binds cytosolic lipopolysaccharide (LPS), a major structural element of Gram-negative bacterial cell walls. Activation of caspase-11 by LPS is known to cause the activation of other caspase proteins, leading to septic shock, pyroptosis, and often organismal death.

LPS is a known activator of innate immune responses. Extracellular LPS binds specifically to the cell surface receptor TLR4. LPS binding to TLR4 subsequently causes initiation of the MyD88 and TRIF signaling pathways, leading to expression of pro-inflammatory molecules and cytokines. These inflammatory mediators cause host toxic shock and sepsis as a result of an overactive immune response to LPS. Until recently, TLR4 was considered the sole receptor for LPS.

However, in 2013 it was shown that TLR4 knockout mice treated with the TLR3 ligand poly I:C still die of toxic shock induced by LPS treatment. Conversely, it was also found that poly I:C treated TLR4 and caspase-11 double knockout mice do not develop toxic shock in response to LPS. These results suggest that TLR4 is not the sole LPS receptor but that caspase-11 also responds to the presence of LPS. Caspase-11 was subsequently shown to be a cytosolic protein that responds solely to intracellular, cytosolic LPS.

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