Acts as a negative regulator of the Toll-like and IL-1R receptor signaling pathways. Attenuates the recruitment of receptor-proximal signaling components to the TLR4 receptor, probably through an TIR-TIR domain interaction with TLR4. Through its extracellular domain interferes with the heterodimerization of Il1R1 and IL1RAP (By similarity). Interacts with IL1R1, IRAK1, TLR4, TLR5, TLR9 and TRAF6. Upon IL-1 stimulation found in a complex at least composed of IL1R1, SIGIRR, MYD88, IRAK1 and TRAF6. Upon stimulation with LPC found in a complex at least composed of TLR4, SIG1IR, MYD88, IRAK1 and TRAF6. Interacts with PALM3 (By similarity). Expressed at high levels in kidney, and at moderate levels in colon, small intestine, lung, spleen and liver. Not expressed in brain and muscle. Expressed at high levels in epithelial cells, at moderate levels in splenocytes, and at low or undetectable levels in fibroblasts or endothelial cells. Expressed in mucosal and dendritic cells. Down-regulated by LPS. Down-regulated during inflammation by inhibition of an SP1-mediated pathway. Belongs to the interleukin-1 receptor family.