Receptor-independent, direct membrane binding leads to cell-surface lipid sorting and Syk kinase activation in dendritic cells.

Binding of particulate antigens by antigen-presenting cells is a critical step in immune activation. Previously, we demonstrated that uric acid crystals are potent adjuvants, initiating a robust adaptive immune response. However, the mechanisms of activation are unknown. By using atomic force microscopy as a tool for real-time single-cell activation analysis, we report that uric acid crystals could directly engage cellular membranes, particularly the cholesterol components, with a force substantially stronger than protein-based cellular contacts. Binding of particulate substances activated Syk kinase-dependent signaling in dendritic cells. These observations suggest a mechanism whereby immune cell activation can be triggered by solid structures via membrane lipid alteration without the requirement for specific cell-surface receptors, and a testable hypothesis for crystal-associated arthropathies, inflammation, and adjuvanticity.