Electrostatic effects are ubiquitous in protein interactions and are found to be pervasive in the complement system as well. The interaction between complement fragment C3d and complement receptor 2 (CR2) has evolved to become a link between innate and adaptive immunity. Electrostatic interactions have been suggested to be the driving factor for the association of the C3d:CR2 complex. In this study, we investigate the effects of ionic strength and mutagenesis on the association of C3d:CR2 through Brownian dynamics simulations. We demonstrate that the formation of the C3d:CR2 complex is ionic strength-dependent, suggesting the presence of long-range electrostatic steering that accelerates the complex formation. Electrostatic steering occurs through the interaction of an acidic surface patch in C3d and the positively charged CR2 and is supported by the effects of mutations within the acidic patch of C3d that slow or diminish association. Our data are in agreement with previous experimental mutagenesis and binding studies and computational studies. Although the C3d acidic patch may be locally destabilizing because of unfavorable Coulombic interactions of like charges, it contributes to the acceleration of association. Therefore, acceleration of function through electrostatic steering takes precedence to stability. The site of interaction between C3d and CR2 has been the target for delivery of CR2-bound nanoparticle, antibody, and small molecule biomarkers, as well as potential therapeutics. A detailed knowledge of the physicochemical basis of C3d:CR2 association may be necessary to accelerate biomarker and drug discovery efforts.

Background

Engineered vaccine proteins incorporating both antigen and adjuvant components are constructed with the aim of combining functions to induce effective protective immunity. Bacterial flagellin is a strong candidate for an engineered vaccine scaffold as it is known to provide adjuvant activity through its TLR5 and inflammasome activation. Moreover, polymerized flagellin filaments can elicit a more robust immunoglobulin response than monomeric flagellin, and the multimeric antigen form can also promote T cell-independent antibody responses. Here, we aim to produce and test a covalently stabilized polymerized flagellar filament, providing additional immune efficacy through stabilization of its polymeric filament structure, as well as stabilization for long-term storage.

Results

Computational modeling of monomer packing in flagellin filaments helped identify amino acids with proximity to neighboring flagella protofilaments. Paired cysteine substitutions were made at amino acids predicted to form inter-monomer disulfide cross-links, and these substitutions were capable of forming flagella when transfected into a flagellin-negative strain of Salmonella enterica subspecies Typhimurium. Interestingly, each paired substitution stabilized different helical conformational polymorphisms; the stabilized filaments lost the ability to transition between conformations, reducing bacterial motility. More importantly, the paired substitutions enabled extensive disulfide cross links and intra-filament multimer formation, and in one of the three variants, permitted filament stability in high acidic and temperature conditions where wild-type filaments would normally rapidly depolymerize. In addition, with regard to potential adjuvant activity, all crosslinked flagella filaments were able to induce wild-type levels of epithelial NF-κB in a cell reporter system. Finally, bacterial virulence was unimpaired in epithelial adherence and invasion, and the cysteine substitutions also appeared to increase bacterial resistance to oxidizing and reducing conditions.

Conclusions

We identified amino acid pairs, with cysteine substitutions, were able to form intermolecular disulfide bonds that stabilized the resulting flagellar filaments in detergent, hydrochloric acid, and high temperatures while retaining its immunostimulatory function. Flagellar filaments with disulfide-stabilized protofilaments introduce new possibilities for the application of flagella as a vaccine adjuvant. Specifically, increased stability and heat tolerance permits long-term storage in a range of temperature environments, as well as delivery under a range of clinical conditions.

Purpose

To redesign a complement-inhibiting peptide with the potential to become a therapeutic for dry and wet age-related macular degeneration (AMD).

Methods

We present a new potent peptide (Peptide 2) of the compstatin family. The peptide is developed by rational design, based on a mechanistic binding hypothesis, and structural and physicochemical properties derived from molecular dynamics (MD) simulation. The inhibitory activity, efficacy, and solubility of Peptide 2 are evaluated using a hemolytic assay, a human RPE cell-based assay, and ultraviolet (UV) absorption properties, respectively, and compared to the respective properties of its parent peptide (Peptide 1).

Results

The sequence of Peptide 2 contains an arginine-serine N-terminal extension (a characteristic of parent Peptide 1) and a novel 8-polyethylene glycol (PEG) block C-terminal extension. Peptide 2 has significantly improved aqueous solubility compared to Peptide 1 and comparable complement inhibitory activity. In addition, Peptide 2 is more efficacious in inhibiting complement activation in a cell-based model that mimics the pathobiology of dry AMD.

Conclusions

We have designed a new peptide analog of compstatin that combines N-terminal polar amino acid extensions and C-terminal PEGylation extensions. This peptide demonstrates significantly improved aqueous solubility and complement inhibitory efficacy, compared to the parent peptide. The new peptide overcomes the aggregation limitation for clinical translation of previous compstatin analogs and is a candidate to become a therapeutic for the treatment of AMD.