Regulation of C5 convertase activity by properdin, factors B and H

Structural studies offer a framework for understanding the role of properdin in the alternative pathway and beyond

Structures of alternative pathway proteins have offered a comprehensive structural basis for understanding the molecular mechanisms governing activation and regulation of the amplification pathway of the complement cascade. Although properdin (FP) is required in vivo to sustain a functional alternative pathway, structural studies have been lagging behind due to the extended structure and polydisperse nature of FP. We review recent progress with respect to structure determination of FP and its proconvertase/convertase complexes. These structures identify in detail regions in C3b, factor B and FP involved in their mutual interactions. Structures of FP oligomers obtained by integrative studies have shed light on how FP activity depends on its oligomerization state. The accumulated structural knowledge allows us to rationalize the effect of point mutations causing FP deficiency. The structural basis for FP inhibition by the tick CirpA proteins is reviewed and the potential of alphafold2 predictions for understanding the interaction of FP with other tick proteins and the NKp46 receptor on host immune cells is discussed. The accumulated structural knowledge forms a comprehensive basis for understanding molecular interactions involving FP, pathological conditions arising from low levels of FP, and the molecular strategies used by ticks to suppress the alternative pathway.

A Complement C3-Specific Nanobody for Modulation of the Alternative Cascade Identifies the C-Terminal Domain of C3b as Functional in C5 Convertase Activity

The complement system is an intricate cascade of the innate immune system and plays a key role in microbial defense, inflammation, organ development, and tissue regeneration. There is increasing interest in developing complement regulatory and inhibitory agents to treat complement dysfunction. In this study, we describe the nanobody hC3Nb3, which is specific for the C-terminal C345c domain of human and mouse complement component C3/C3b/C3c and potently inhibits C3 cleavage by the alternative pathway. A high-resolution structure of the hC3Nb3-C345c complex explains how the nanobody blocks proconvertase assembly. Surprisingly, although the nanobody does not affect classical pathway-mediated C3 cleavage, hC3Nb3 inhibits classical pathway-driven hemolysis, suggesting that the C-terminal domain of C3b has an important function in classical pathway C5 convertase activity. The hC3Nb3 nanobody binds C3 with low nanomolar affinity in an SDS-resistant complex, and the nanobody is demonstrated to be a powerful reagent for C3 detection in immunohistochemistry and flow cytometry. Overall, the hC3Nb3 nanobody represents a potent inhibitor of both the alternative pathway and the terminal pathway, with possible applications in complement research, diagnostics, and therapeutics.

Structural Basis for Properdin Oligomerization and Convertase Stimulation in the Human Complement System

Properdin (FP) is a positive regulator of the immune system stimulating the activity of the proteolytically active C3 convertase C3bBb in the alternative pathway of the complement system. Here we present two crystal structures of FP and two structures of convertase bound FP. A structural core formed by three thrombospondin repeats (TSRs) and a TB domain harbors the convertase binding site in FP that mainly interacts with C3b. Stabilization of the interaction between the C3b C-terminus and the MIDAS bound Mg2+ in the Bb protease by FP TSR5 is proposed to underlie FP convertase stabilization. Intermolecular contacts between FP and the convertase subunits suggested by the structure were confirmed by binding experiments. FP is shown to inhibit C3b degradation by FI due to a direct competition for a common binding site on C3b. FP oligomers are held together by two sets of intermolecular contacts, where the first is formed by the TB domain from one FP molecule and TSR4 from another. The second and largest interface is formed by TSR1 and TSR6 from the same two FP molecules. Flexibility at four hinges between thrombospondin repeats is suggested to enable the oligomeric, polydisperse, and extended architecture of FP. Our structures rationalize the effects of mutations associated with FP deficiencies and provide a structural basis for the analysis of FP function in convertases and its possible role in pattern recognition.

Complement activation, regulation, and molecular basis for complement-related diseases

The complement system is an essential element of the innate immune response that becomes activated upon recognition of molecular patterns associated with microorganisms, abnormal host cells, and modified molecules in the extracellular environment. The resulting proteolytic cascade tags the complement activator for elimination and elicits a pro-inflammatory response leading to recruitment and activation of immune cells from both the innate and adaptive branches of the immune system. Through these activities, complement functions in the first line of defense against pathogens but also contributes significantly to the maintenance of homeostasis and prevention of autoimmunity. Activation of complement and the subsequent biological responses occur primarily in the extracellular environment. However, recent studies have demonstrated autocrine signaling by complement activation in intracellular vesicles, while the presence of a cytoplasmic receptor serves to detect complement-opsonized intracellular pathogens. Furthermore, breakthroughs in both functional and structural studies now make it possible to describe many of the intricate molecular mechanisms underlying complement activation and the subsequent downstream events, as well as its cross talk with, for example, signaling pathways, the coagulation system, and adaptive immunity. We present an integrated and updated view of complement based on structural and functional data and describe the new roles attributed to complement. Finally, we discuss how the structural and mechanistic understanding of the complement system rationalizes the genetic defects conferring uncontrolled activation or other undesirable effects of complement.

Structural Basis for the Function of Complement Component C4 within the Classical and Lectin Pathways of Complement

Complement component C4 is a central protein in the classical and lectin pathways within the complement system. During activation of complement, its major fragment C4b becomes covalently attached to the surface of pathogens and altered self-tissue, where it acts as an opsonin marking the surface for removal. Moreover, C4b provides a platform for assembly of the proteolytically active convertases that mediate downstream complement activation by cleavage of C3 and C5. In this article, we present the crystal and solution structures of the 195-kDa C4b. Our results provide the molecular details of the rearrangement accompanying C4 cleavage and suggest intramolecular flexibility of C4b. The conformations of C4b and its paralogue C3b are shown to be remarkably conserved, suggesting that the convertases from the classical and alternative pathways are likely to share their overall architecture and mode of substrate recognition. We propose an overall molecular model for the classical pathway C5 convertase in complex with C5, suggesting that C3b increases the affinity for the substrate by inducing conformational changes in C4b rather than a direct interaction with C5. C4b-specific features revealed by our structural studies are probably involved in the assembly of the classical pathway C3/C5 convertases and C4b binding to regulators.

Hemolytic Uremic Syndrome: A Factor H Mutation (E1172Stop) Causes Defective Complement Control at the Surface of Endothelial Cells

Defective complement regulation results in hemolytic uremic syndrome (HUS), a disease that is characterized by microangiopathy, thrombocytopenia, and acute renal failure and that causes endothelial cell damage. For characterization of how defective complement regulation relates to the pathophysiology, the role of the complement regulator factor H and also of a mutant factor H protein was studied on the surface of human umbilical vein endothelial cells. The mutant 145-kD factor H protein was purified to homogeneity, from plasma of a patient with HUS, who is heterozygous for a factor H gene mutation G3587T, which introduces a stop codon at position 1172. Functional analyses show that the lack of the most C-terminal domain short consensus repeats 20 severely affected recognition functions (i.e., binding to heparin, C3b, C3d, and the surface of endothelial cells). Wild-type factor H as well as the mutant protein formed dimers in solution as shown by cross-linking studies and mass spectroscopy. When assayed in fluid phase, the complement regulatory activity of the mutant protein was normal and comparable to wild-type factor H. However, on the surface of endothelial cells, the mutant factor H protein showed severely reduced regulatory activities and lacked protective functions. Similarly, with the use of sheep erythrocytes, the mutant protein lacked the protective activity and caused increased hemolysis when it was added to factor H-depleted plasma. This study shows how a mutation that affects the C-terminal region of the factor H protein leads to defective complement control on cell surfaces and damage to endothelial cells in patients with HUS. These effects explain how mutant factor H causes defective complement control and in HUS-particularly under condition of inflammation and complement activation-causes endothelial cell damage.