Complement activation by ligand-driven juxtaposition of discrete pattern recognition complexes

Significance A salient feature of the immune system is its ability to discriminate self from nonself. We define the molecular mechanism governing activation of an ancient and central component: the lectin pathway of complement. The basis is the association of two proteases in distinct complexes with at least five pattern recognition molecules. Clustering of these complexes on ligand surfaces allows cross-activation of the proteases, which subsequently activate downstream factors to initiate a proteolytic cascade. This is conceptually similar to signaling by cellular receptors and could be viewed as cellular signaling turned inside out. Different pattern recognition complexes “talk to each other” to coordinate immune activation, which may impart differential activation based on recognition of simple vs. complex ligand patterns.