Activation of innate immunity by lysozyme fibrils is critically dependent on cross-β sheet structure.

Inflammation occurs in many amyloidoses, but its underlying mechanisms remain enigmatic. Here we show that amyloid fibrils of human lysozyme, which are associated with severe systemic amyloidoses, induce the secretion of pro-inflammatory cytokines through activation of the NLRP3 (NLR, pyrin domain containing 3) inflammasome and the Toll-like receptor 2, two innate immune receptors that may be involved in immune responses associated to amyloidoses. More importantly, our data clearly suggest that the induction of inflammatory responses by amyloid fibrils is linked to their intrinsic structure, because the monomeric form and a non-fibrillar type of lysozyme aggregates are both unable to trigger cytokine secretion. These lysozyme species lack the so-called cross-β structure, a characteristic structural motif common to all amyloid fibrils irrespective of their origin. Since fibrils of other bacterial and endogenous proteins have been shown to trigger immunological responses, our observations suggest that the cross-β structural signature might be recognized as a generic danger signal by the immune system.