Expression of Mincle is known to be upregulated by macrophages in response to cell stress, but until now its ligand, signalling pathway and function have been unknown. Mincle was shown to associate selectively with the immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor FcRγ (Fc receptor γ-chain) through a conserved arginine (Arg42) in the transmembrane domain of Mincle, and the signalling capacity of Mincle was Arg42 dependent. Antibody-mediated crosslinking of Mincle on peritoneal macrophages induced the production of several pro-inflammatory mediators, including CXC-chemokine ligand 2 (CXCL2; also known as MIP2), through a phosphorylation cascade that was triggered by spleen tyrosine kinase (SYK). This pathway was markedly abrogated in FcRγ-deficient cells. In line with the known role of CARD9 (caspase-recruitment domain protein 9) in linking SYK activation to inflammatory responses, the production of CXCL2 in response to Mincle activation was also abrogated in CARD9-deficient macrophages. So, the authors conclude that Mincle activates macrophages through a FcRγ–SYK–CARD9 signalling pathway.
But what is the physiological ligand that triggers signalling through this pathway? The authors established a T-cell hybridoma that expresses both Mincle and FcRγ, as well as a reporter construct that produces green fluorescent protein (GFP) in response to ITAM-mediated signalling. When these cells were cultured alone without changing the medium, an increase in cell death was associated with an increase in the number of GFP+ cells. This effect was almost completely inhibited by Mincle-specific blocking antibodies, which indicates that dead cells can activate ITAM signalling through Mincle. A soluble form of Mincle was produced and used to screen lysates of dead cells; spliceosome-associated protein 130 (SAP130; also known as SF3B3), which is a component of a small nuclear ribonucleoprotein, was shown to selectively bind Mincle, and recombinant SAP130 activated Mincle-expressing cells. SAP130 from living cells and dead cells bound to Mincle with equal efficiency, which indicates that it is the release of SAP130 from its normal nuclear localization, rather than protein modification, during cell death that activates Mincle.
This is a preview of subscription content, access via your institution