Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

Microscopic imaging and technolog

Understanding the first steps of the complement classical pathway: Structural studies on C1-antibody complexes

The complement system is a complex network of plasma and membrane-associated proteins, which can produce effective immune responses to infectious micro-organisms. Deficiencies of complement proteins cause diseases like systemic lupus erythematosus (SLE), rheumatoid arthritis and angioedema. Recently, C1q has been found to play an important role in synaptic pruning which is crucial for the brain functioning and related to central nervous system diseases like glaucoma or Alzheimer’s disease. Since complement system has such a broad range of functions, understanding the molecular details of complement activation is fundamental. Classical Pathway of Complement is initiated by a large macromolecular complex called C1 binding to antibody-antigen complexes on the surface. C1 is a 790-kDa complex, which consists of 6 recognition proteins C1q and a hetero-tetramer of serine proteases, C1r2C1s2. In this thesis, we addressed issues about assembly of C1 (i.e. C1r2s2 binding to C1q), auto-activation of C1r and C1 binding to antibodies. Using the latest advances in native mass spectrometry and cryo-electron microcopy, our data provide new insights into the assembly and activation of the C1 complex and structural details of C1 bound to activating antibodies

Understanding the first steps of the complement classical pathway: Structural studies on C1-antibody complexes

The complement system is a complex network of plasma and membrane-associated proteins, which can produce effective immune responses to infectious micro-organisms. Deficiencies of complement proteins cause diseases like systemic lupus erythematosus (SLE), rheumatoid arthritis and angioedema. Recently, C1q has been found to play an important role in synaptic pruning which is crucial for the brain functioning and related to central nervous system diseases like glaucoma or Alzheimer’s disease. Since complement system has such a broad range of functions, understanding the molecular details of complement activation is fundamental. Classical Pathway of Complement is initiated by a large macromolecular complex called C1 binding to antibody-antigen complexes on the surface. C1 is a 790-kDa complex, which consists of 6 recognition proteins C1q and a hetero-tetramer of serine proteases, C1r2C1s2. In this thesis, we addressed issues about assembly of C1 (i.e. C1r2s2 binding to C1q), auto-activation of C1r and C1 binding to antibodies. Using the latest advances in native mass spectrometry and cryo-electron microcopy, our data provide new insights into the assembly and activation of the C1 complex and structural details of C1 bound to activating antibodies

Structural diversity in the atomic resolution 3D fingerprint of the titin M-band segment.

In striated muscles, molecular filaments are largely composed of long protein chains with extensive arrays of identically folded domains, referred to as "beads-on-a-string". It remains a largely unresolved question how these domains have developed a unique molecular profile such that each carries out a distinct function without false-positive readout. This study focuses on the M-band segment of the sarcomeric protein titin, which comprises ten identically folded immunoglobulin domains. Comparative analysis of high-resolution structures of six of these domains ‒ M1, M3, M4, M5, M7, and M10 ‒ reveals considerable structural diversity within three distinct loops and a non-conserved pattern of exposed cysteines. Our data allow to structurally interpreting distinct pathological readouts that result from titinopathy-associated variants. Our findings support general principles that could be used to identify individual structural/functional profiles of hundreds of identically folded protein domains within the sarcomere and other densely crowded cellular environments

Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

Danger patterns on microbes or damaged host cells bind and activate C1, inducing innate immune responses and clearance through the complement cascade. How these patterns trigger complement initiation remains elusive. Here, we present cryo–electron microscopy analyses of C1 bound to monoclonal antibodies in which we observed heterogeneous structures of single and clustered C1–immunoglobulin G1 (IgG1) hexamer complexes. Distinct C1q binding sites are observed on the two Fc-CH2 domains of each IgG molecule. These are consistent with known interactions and also reveal additional interactions, which are supported by functional IgG1-mutant analysis. Upon antibody binding, the C1q arms condense, inducing rearrangements of the C1r2s2 proteases and tilting C1q’s cone-shaped stalk. The data suggest that C1r may activate C1s within single, strained C1 complexes or between neighboring C1 complexes on surfaces

Complement Is Activated by IgG Hexamers Assembled at the Cell Surface

Microscopic imaging and technolog

Complement Is Activated by IgG Hexamers Assembled at the Cell Surface

Complement activation by antibodies bound to pathogens, tumors, and self antigens is a critical feature of natural immune defense, a number of disease processes, and immunotherapies. How antibodies activate the complement cascade, however, is poorly understood. We found that specific noncovalent interactions between Fc segments of immunoglobulin G (IgG) antibodies resulted in the formation of ordered antibody hexamers after antigen binding on cells. These hexamers recruited and activated C1, the first component of complement, thereby triggering the complement cascade. The interactions between neighboring Fc segments could be manipulated to block, reconstitute, and enhance complement activation and killing of target cells, using all four human IgG subclasses. We offer a general model for understanding antibody-mediated complement activation and the design of antibody therapeutics with enhanced efficacy