A Recombinant Homotrimer, Composed of the α Helical Neck Region of Human Surfactant Protein D and C1q B Chain Globular Domain, Is an Inhibitor of the Classical Complement Pathway

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Abstract
The first step in the activation of the classical complement pathway by immune complexes involves the binding of the six globular heads of C1q to the Fc regions of IgG or IgM. The globular heads of C1q (gC1q domain) are located C-terminal to the six triple-helical stalks present in the molecule, each head being composed of the C-terminal halves of one A, one B, and one C chain. The gC1q modules are also found in a variety of noncomplement proteins, such as type VIII and X collagens, precerebellin, hibernation protein, multimerin, Acrp-30, and saccular collagen. In several of these proteins, the chains containing these gC1q modules appear to form a homotrimeric structure. Here, we report expression of an in-frame fusion of a trimerizing neck region of surfactant protein D with the globular head region of C1q B chain as a fusion to Escherichia coli maltose binding protein. Following cleavage by factor Xa and removal of the maltose binding protein, the neck and globular region, designated ghB3, formed a soluble, homotrimeric structure and could inhibit C1q-dependent hemolysis of IgG- and IgM-sensitized sheep erythrocytes. The functional properties of ghB3 indicate that the globular regions of C1q may adopt a modular organization in which each globular head of C1q may be composed of three structurally and functionally independent domains, thus retaining multivalency in the form of a heterotrimer. The finding that ghB3 is an inhibitor of C1q-mediated complement activation opens up the possibility of blocking activation at the first step of the classical complement pathway.