Insights Into the Structure-Function Relationships of Dimeric C3d Fragments

Identification and structural analysis of dimeric chicken complement component 3d and its binding with chicken complement receptor 2

Complement component 3d (C3d), the final cleavage product of complement component C3, interacts with CR2 and thus plays a crucial role in linking the innate and adaptive immune systems. Additionally, human C3d executes various functions in its dimeric form, which is more effective than its monomeric form. In this study, we aimed to explored whether chicken C3d (chC3d) exhibits similar characteristics, namely dimerization and binding of dimeric chC3d to chicken CR2 (chCR2). We investigated the interaction and co-localization of chC3d with itself using coimmunoprecipitation and confocal laser scanning microscopy, respectively. Then, dimeric chC3d was detected using native polyacrylamide gel electrophoresis and western blotting, and its equilibrium dissociation constant KD (827 nM) was determined using surface plasmon resonance. Finally, the interaction modes of dimeric chC3d were identified using molecular docking simulations, which revealed that dimeric chC3d could crosslink with chCR2 receptor. Overall, our findings will facilitate future explorations of the chicken complement system.

Eluding the immune system's frontline defense: Secreted complement evasion factors of pathogenic Gram-positive cocci

The human complement system is an important part of the innate immune response in the fight against invasive bacteria. Complement responses can be activated independently by the classical pathway, the lectin pathway, or the alternative pathway, each resulting in the formation of a C3 convertase that produces the anaphylatoxin C3a and the opsonin C3b by specifically cutting C3. Other important features of complement are the production of the chemotactic C5a peptide and the generation of the membrane attack complex to lyse intruding pathogens. Invasive pathogens like Staphylococcus aureus and several species of the genus Streptococcus have developed a variety of complement evasion strategies to resist complement activity thereby increasing their virulence and potential to cause disease. In this review, we focus on secreted complement evasion factors that assist the bacteria to avoid opsonization and terminal pathway lysis. We also briefly discuss the potential role of complement evasion factors for the development of vaccines and therapeutic interventions.

Staphylococcal Complement Evasion Protein Sbi Stabilises C3d Dimers by Inducing an N-Terminal Helix Swap

Staphylococcus aureus is an opportunistic pathogen that is able to thwart an effective host immune response by producing a range of immune evasion molecules, including S. aureus binder of IgG (Sbi) which interacts directly with the central complement component C3, its fragments and associated regulators. Recently we reported the first structure of a disulfide-linked human C3d^17C dimer and highlighted its potential role in modulating B-cell activation. Here we present an X-ray crystal structure of a disulfide-linked human C3d^17C dimer, which undergoes a structurally stabilising N-terminal 3D domain swap when in complex with Sbi. These structural studies, in combination with circular dichroism and fluorescence spectroscopic analyses, reveal the mechanism underpinning this unique helix swap event and could explain the origins of a previously discovered N-terminally truncated C3dg dimer isolated from rat serum. Overall, our study unveils a novel staphylococcal complement evasion mechanism which enables the pathogen to harness the ability of dimeric C3d to modulate B-cell activation.

Complement component C3: A structural perspective and potential therapeutic implications

As the most abundant component of the complement system, C3 and its proteolytic derivatives serve essential roles in the function of all three complement pathways. Central to this is a network of protein-protein interactions made possible by the sequential proteolysis and far-reaching structural changes that accompany C3 activation. Beginning with the crystal structures of C3, C3b, and C3c nearly twenty years ago, the physical transformations underlying C3 function that had long been suspected were finally revealed. In the years that followed, a compendium of crystallographic information on C3 derivatives bound to various enzymes, regulators, receptors, and inhibitors generated new levels of insight into the structure and function of the C3 molecule. This Review provides a concise classification, summary, and interpretation of the more than 50 unique crystal structure determinations for human C3. It also highlights other salient features of C3 structure that were made possible through solution-based methods, including Hydrogen/Deuterium Exchange and Small Angle X-ray Scattering. At this pivotal time when the first C3-targeted therapeutics begin to see use in the clinic, some perspectives are also offered on how this continually growing body of structural information might be leveraged for future development of next-generation C3 inhibitors.