Modulation function of sphingomyelin molecular species in TLR4 signaling and cell death

The innate immune system provides the first line of defense against pathogens. However, the mechanisms underlying its endogenous regulation remain unclear. We identified sphingomyelin (SM) as a novel immunomodulatory ligand. SM in serum consists of various fatty acid species. Our findings reveal that SM species regulate inflammatory cell death and cytokine release in an acyl-chain-dependent manner via Toll-like receptor 4 (TLR4)-myeloid differentiation factor-2 (MD-2) signaling. Specifically, N-lauroyl-D-erythro-sphingosylphosphorylcholine (SM C12) and N-myristoyl-D-erythro-sphingosylphosphorylcholine (SM C14) induce inflammatory cell death and pyroptosis in mouse macrophages. The activation of human caspase-4, mouse caspase-11, and gasdermin D underlies this pyroptotic response, and we have identified SM C12 as a ligand for caspase-4. Our results suggest a dual role for SM in mediating inflammation or suppressing LPS-stimulated inflammation through both cell surface TLR4/MD-2 interaction and intracellular caspase signaling pathways. This newfound understanding of SM's immunomodulatory properties opens avenues for exploring its therapeutic potential in modulating innate immune responses.