Selection of Affibody Affinity Proteins from Phagemid Libraries.

Herein, we describe a general protocol for the selection of target-binding affinity protein molecules from a phagemid-encoded library. The protocol is based on our experience with phage display selections of non-immunoglobulin affibody affinity proteins but can in principle be applied to perform biopanning experiments from any phage-displayed affinity protein library available in a similar phagemid vector. The procedure begins with an amplification of the library from frozen bacterial glycerol stocks via cultivation and helper phage superinfection, followed by a step-by-step instruction of target protein preparation, selection cycles, and post-selection analyses. The described procedures in this standard protocol are relatively conservative and rely on ordinary reagents and equipment available in most molecular biology laboratories.

Affibody-based hBCMA x CD16 dual engagers for NK cell-mediated killing of multiple myeloma cells.

We describe the development and characterization of the (to date) smallest Natural Killer (NK) cell re-directing human B Cell Maturation Antigen (hBCMA) x CD16 dual engagers for potential treatment of multiple myeloma, based on combinations of small 58 amino acid, non-immunoglobulin, affibody affinity proteins. Affibody molecules to human CD16a were selected from a combinatorial library by phage display resulting in the identification of three unique binders with affinities (KD) for CD16a in the range of 100 nM-3 µM. The affibody exhibiting the highest affinity demonstrated insensitivity towards the CD16a allotype (158F/V) and did not interfere with IgG (Fc) binding to CD16a. For the construction of hBCMA x CD16 dual engagers, different CD16a binding arms, including bi-paratopic affibody combinations, were genetically fused to a high-affinity hBCMA-specific affibody. Such 15-23 kDa dual engager constructs showed simultaneous hBCMA and CD16a binding ability and could efficiently activate resting primary NK cells and trigger specific lysis of a panel of hBCMA-positive multiple myeloma cell lines. Hence, we report a novel class of uniquely small NK cell engagers with specific binding properties and potent functional profiles.

Caprin-1 binding to the critical stress granule protein G3BP1 is influenced by pH.

G3BP is the central node within stress-induced protein-RNA interaction networks known as stress granules (SGs). The SG-associated proteins Caprin-1 and USP10 bind mutually exclusively to the NTF2 domain of G3BP1, promoting and inhibiting SG formation, respectively. Herein, we present the crystal structure of G3BP1-NTF2 in complex with a Caprin-1-derived short linear motif (SLiM). Caprin-1 interacts with His-31 and His-62 within a third NTF2-binding site outside those covered by USP10, as confirmed using biochemical and biophysical-binding assays. Nano-differential scanning fluorimetry revealed reduced thermal stability of G3BP1-NTF2 at acidic pH. This destabilization was counterbalanced significantly better by bound USP10 than Caprin-1. The G3BP1/USP10 complex immunoprecipated from human U2OS cells was more resistant to acidic buffer washes than G3BP1/Caprin-1. Acidification of cellular condensates by approximately 0.5 units relative to the cytosol was detected by ratiometric fluorescence analysis of pHluorin2 fused to G3BP1. Cells expressing a Caprin-1/FGDF chimera with higher G3BP1-binding affinity had reduced Caprin-1 levels and slightly reduced condensate sizes. This unexpected finding may suggest that binding of the USP10-derived SLiM to NTF2 reduces the propensity of G3BP1 to enter condensates.

Naturally occurring dipeptide from elite controllers with dual anti-HIV-1 mechanism.

BACKGROUND: Enhanced levels of a dipeptide, WG-am, have been reported among elite controllers - patients who spontaneously control their HIV-1 infection. This study aimed to evaluate anti-HIV-1 activity and mechanism of action of WG-am. METHODS: Drug sensitivity assays in TZM.bl cells, PBMCs and ACH-2 cells using WT and mutated HIV-1 strainswere performed to evaluate the antiviral mechanism of WG-am. Mass spectrometry-based proteomics and Real-time PCR analysis of reverse transcription steps were performed to unravel the second anti-HIV-1 mechanism of WG-am. RESULTS: The data suggest that WG-am binds to the CD4 binding pocket of HIV-1 gp120 and blocks its binding to the host cell receptors. Additionally, the time course assay showed that WG-am also inhibited HIV-1 at 4-6 hours post-infection, suggesting a second antiviral mechanism. Drug sensitivity assays under acidic wash conditions confirmed the ability of WG-am to internalise into the host cell in an HIV independent manner. Proteomic studies showed a clustering of all samples treated with WG-am independent of the number of doses or presence or absence of HIV-1. Differentially expressed proteins due to the WG-am treatment indicated an effect on HIV-1 reverse transcription, which was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). CONCLUSION: Naturally occurring in HIV-1 elite controllers, WG-am stands out as a new kind of antiviral compound with two independent inhibitory mechanisms of action on HIV-1 replication. WG-am halts HIV-1 entry to the host cell by binding to HIV-1 gp120, thereby blocking the binding of HIV-1 to the host cell. WG-am also exerts a post-entry but pre-integration antiviral effect related to RT-activity.

The Wittig bioconjugation of maleimide derived, water soluble phosphonium ylides to aldehyde-tagged proteins.

Herein we disclose the transformation of maleimides into water-soluble tris(2-carboxyethyl)phosphonium ylides and their subsequent application in the bioconjugation of protein- and peptide-linked aldehydes. The new entry into Wittig bioconjugate chemistry proceeds under mild conditions and relies on highly water soluble reagents, which are likely already part of most biochemists' inventory.

Progress and Future Directions with Peptide-Drug Conjugates for Targeted Cancer Therapy.

We review drug conjugates combining a tumor-selective moiety with a cytotoxic agent as cancer treatments. Currently, antibody-drug conjugates (ADCs) are the most common drug conjugates used clinically as cancer treatments. While providing both efficacy and favorable tolerability, ADCs have limitations due to their size and complexity. Peptides as tumor-targeting carriers in peptide-drug conjugates (PDCs) offer a number of benefits. Melphalan flufenamide (melflufen) is a highly lipophilic PDC that takes a novel approach by utilizing increased aminopeptidase activity to selectively increase the release and concentration of cytotoxic alkylating agents inside tumor cells. The only other PDC approved currently for clinical use is 177Lu-dotatate, a targeted form of radiotherapy combining a somatostatin analog with a radionuclide. It is approved as a treatment for gastroenteropancreatic neuroendocrine tumors. Results with other PDCs combining synthetic analogs of natural peptide ligands with cytotoxic agents have been mixed. The field of drug conjugates as drug delivery systems for the treatment of cancer continues to advance with the application of new technologies. Melflufen provides a paradigm for rational PDC design, with a targeted mechanism of action and the potential for deepening responses to treatment, maintaining remissions, and eradicating therapy-resistant stem cells.

Discovery, optimization and biodistribution of an Affibody molecule for imaging of CD69.

Due to the wide scale of inflammatory processes in different types of disease, more sensitive and specific biomarkers are required to improve prevention and treatment. Cluster of differentiation 69 (CD69) is one of the earliest cell surface proteins expressed by activated leukocytes. Here we characterize and optimize potential new imaging probes, Affibody molecules targeting CD69 for imaging of activated immune cells. Analysis of candidates isolated in a previously performed selection from a Z variant E. coli library to the recombinant extracellular domain of human CD69, identified one cross-reactive Z variant with affinity to murine and human CD69. Affinity maturation was performed by randomization of the primary Z variant, followed by selections from the library. The resulting Z variants were evaluated for affinity towards human and murine CD69 and thermal stability. The in vivo biodistribution was assessed by SPECT/CT in rats following conjugation of the Z variants by a DOTA chelator and radiolabeling with Indium-111. A primary Z variant with a Kd of approximately 50 nM affinity to human and murine CD69 was identified. Affinity maturation generated 5 additional Z variants with improved or similar affinity. All clones exhibited suitable stability. Radiolabeling and in vivo biodistribution in rat demonstrated rapid renal clearance for all variants, while the background uptake and washout varied. The variant ZCD69:4 had the highest affinity for human and murine CD69 (34 nM) as well as the lowest in vivo background binding. In summary, we describe the discovery, optimization and evaluation of novel Affibody molecules with affinity for CD69. Affibody molecule ZCD69:4 is suitable for further development for imaging of activated immune cells.

Systemic administration of monovalent follistatin-like 3-Fc-fusion protein increases muscle mass in mice.

Targeting the signaling pathway of growth differentiation factor 8 (GDF8), also known as myostatin, has been regarded as a promising strategy to increase muscle mass in the elderly and in patients. Accumulating evidence in animal models and clinical trials has indicated that a rational approach is to inhibit a limited number of transforming growth factor ß (TGF-ß) family ligands, including GDF8 and activin A, without affecting other members. Here, we focused on one of the endogenous antagonists against TGF-ß family ligands, follistatin-like 3 (FSTL3), which mainly binds and neutralizes activins, GDF8, and GDF11. Although bivalent human FSTL3 Fc-fusion protein was rapidly cleared from mouse circulation similar to follistatin (FST)-Fc, monovalent FSTL3-Fc (mono-FSTL3-Fc) generated with the knobs-into-holes technology exhibited longer serum half-life. Systemic administration of mono-FSTL3-Fc in mice induced muscle fiber hypertrophy and increased muscle mass in vivo. Our results indicate that the monovalent FSTL3-based therapy overcomes the difficulties of current anti-GDF8 therapies.

Efficient Labeling of Native Human IgG by Proximity-Based Sortase-Mediated Isopeptide Ligation.

Antibody-drug conjugates (ADCs) have demonstrated great therapeutic potential due to their ability to target the delivery of potent cytotoxins. However, the heterogeneous nature of conventional drug conjugation strategies can affect the safety, efficacy, and stability of ADCs. Site-specific conjugations can resolve these issues, but often require genetic modification of Immunoglobulin G (IgG), which can impact yield or cost of production, or require undesirable chemical linkages. Here, we describe a near-traceless conjugation method that enables the efficient modification of native IgG, without the need for genetic engineering or glycan modification. This method utilizes engineered variants of sortase A to catalyze noncanonical isopeptide ligation. Sortase A was fused to an antibody-binding domain to improve ligation efficiency. Antibody labeling is limited to five lysine residues on the heavy chain and one on the light chain of human IgG1. The ADCs exhibit conserved antigen and Fc-receptor interactions, as well as potent cytolytic activity.

Tuning antiviral CD8 T-cell response via proline-altered peptide ligand vaccination.

Viral escape from CD8+ cytotoxic T lymphocyte responses correlates with disease progression and represents a significant challenge for vaccination. Here, we demonstrate that CD8+ T cell recognition of the naturally occurring MHC-I-restricted LCMV-associated immune escape variant Y4F is restored following vaccination with a proline-altered peptide ligand (APL). The APL increases MHC/peptide (pMHC) complex stability, rigidifies the peptide and facilitates T cell receptor (TCR) recognition through reduced entropy costs. Structural analyses of pMHC complexes before and after TCR binding, combined with biophysical analyses, revealed that although the TCR binds similarly to all complexes, the p3P modification alters the conformations of a very limited amount of specific MHC and peptide residues, facilitating efficient TCR recognition. This approach can be easily introduced in peptides restricted to other MHC alleles, and can be combined with currently available and future vaccination protocols in order to prevent viral immune escape.

Assigned NMR backbone resonances of the ligand-binding region domain of the pneumococcal serine-rich repeat protein (PsrP-BR) reveal a rigid monomer in solution.

The pneumococcal serine rich repeat protein (PsrP) is displayed on the surface of Streptococcus pneumoniae with a suggested role in colonization in the human upper respiratory tract. Full-length PsrP is a 4000 residue-long multi-domain protein comprising a positively charged functional binding region (BR) domain for interaction with keratin and extracellular DNA during pneumococcal adhesion and biofilm formation, respectively. The previously determined crystal structure of the BR domain revealed a flat compressed barrel comprising two sides with an extended ß-sheet on one side, and another ß-sheet that is distorted by loops and ß-turns on the other side. Crystallographic B-factors indicated a relatively high mobility of loop regions that were hypothesized to be important for binding. Furthermore, the crystal structure revealed an inter-molecular ß-sheet formed between edge strands of two symmetry-related molecules, which could promote bacterial aggregation during biofilm formation. Here we report the near complete 15N/13C/1H backbone resonance assignment of the BR domain of PsrP, revealing a secondary structure profile that is almost identical to the X-ray structure. Dynamic 15N-T1, T2 and NOE data suggest a monomeric and rigid structure of BR with disordered residues only at the N- and C-termini. The presented peak assignment will allow us to identify BR residues that are crucial for ligand binding.

Lysis of Staphylococcal Cells by Modular Lysin Domains Linked via a Non-covalent Barnase-Barstar Interaction Bridge.

Bacteriophage endolysins and bacterial exolysins are capable of enzymatic degradation of the cell wall peptidoglycan layer and thus show promise as a new class of antimicrobials. Both exolysins and endolysins often consist of different modules, which are responsible for enzymatic functions and cell wall binding, respectively. Individual modules from different endo- or exolysins with different binding and enzymatic activities, can via gene fusion technology be re-combined into novel variants for investigations of arrangements of potential clinical interest. The aim of this study was to investigate if separately produced cell wall binding and enzyme modules could be assembled into a functional lysin via a non-covalent affinity interaction bridge composed of the barnase ribonuclease from Bacillus amyloliquefaciens and its cognate inhibitor barstar, known to form a stable heterodimeric complex. In a proof-of-principle study, using surface plasmon resonance, flow cytometry and turbidity reduction assays, we show that separately produced modules of a lysin cysteine/histidine-dependent amidohydrolase/peptidase (CHAP) from Staphylococcus aureus bacteriophage K endolysin (LysK) fused to barnase and a cell wall binding Src homology 3 domain (SH3b) from the S. simulans exolysin lysostaphin fused to barstar can be non-covalently assembled into a functional lysin showing both cell wall binding and staphylolytic activity. We hypothesize that the described principle for assembly of functional lysins from separate modules through appended hetero-dimerization domains has a potential for investigations of also other combinations of enzymatically active and cell wall binding domains for desired applications.

Successive crystal structure snapshots suggest the basis for MHC class I peptide loading and editing by tapasin.

MHC-I epitope presentation to CD8+ T cells is directly dependent on peptide loading and selection during antigen processing. However, the exact molecular bases underlying peptide selection and binding by MHC-I remain largely unknown. Within the peptide-loading complex, the peptide editor tapasin is key to the selection of MHC-I-bound peptides. Here, we have determined an ensemble of crystal structures of MHC-I in complex with the peptide exchange-associated dipeptide GL, as well as the tapasin-associated scoop loop, alone or in combination with candidate epitopes. These results combined with mutation analyses allow us to propose a molecular model underlying MHC-I peptide selection by tapasin. The N termini of bound peptides most probably bind first in the N-terminal and middle region of the MHC-I peptide binding cleft, upon which the peptide C termini are tested for their capacity to dislodge the tapasin scoop loop from the F pocket of the MHC-I cleft. Our results also indicate important differences in peptide selection between different MHC-I alleles.

Chromophore pre-maturation for improved speed and sensitivity of split-GFP monitoring of protein secretion.

Complementation-dependent fluorescence is a powerful way to study co-localization or interactions between biomolecules. A split-GFP variant, involving the self-associating GFP 1-10 and GFP 11, has previously provided a convenient approach to measure recombinant protein titers in cell supernatants. A limitation of this approach is the slow chromophore formation after complementation. Here, we alleviate this lag in signal generation by allowing the GFP 1-10 chromophore to mature on a solid support containing GFP 11 before applying GFP 1-10 in analyses. The pre-maturated GFP 1-10 provided up to 150-fold faster signal generation compared to the non-maturated version. Moreover, pre-maturated GFP 1-10 significantly improved the ability of discriminating between Chinese hamster ovary (CHO) cell lines secreting GFP 11-tagged erythropoietin protein at varying rates. Its improved kinetics make the pre-maturated GFP 1-10 a suitable reporter molecule for cell biology research in general, especially for ranking individual cell lines based on secretion rates of recombinant proteins.

Pneumolysin binds to the mannose receptor C type 1 (MRC-1) leading to anti-inflammatory responses and enhanced pneumococcal survival.

Streptococcus pneumoniae (the pneumococcus) is a major cause of mortality and morbidity globally, and the leading cause of death in children under 5 years old. The pneumococcal cytolysin pneumolysin (PLY) is a major virulence determinant known to induce pore-dependent pro-inflammatory responses. These inflammatory responses are driven by PLY-host cell membrane cholesterol interactions, but binding to a host cell receptor has not been previously demonstrated. Here, we discovered a receptor for PLY, whereby pro-inflammatory cytokine responses and Toll-like receptor signalling are inhibited following PLY binding to the mannose receptor C type 1 (MRC-1) in human dendritic cells and mouse alveolar macrophages. The cytokine suppressor SOCS1 is also upregulated. Moreover, PLY-MRC-1 interactions mediate pneumococcal internalization into non-lysosomal compartments and polarize naive T cells into an interferon-γlow, interleukin-4high and FoxP3+ immunoregulatory phenotype. In mice, PLY-expressing pneumococci colocalize with MRC-1 in alveolar macrophages, induce lower pro-inflammatory cytokine responses and reduce neutrophil infiltration compared with a PLY mutant. In vivo, reduced bacterial loads occur in the airways of MRC-1-deficient mice and in mice in which MRC-1 is inhibited using blocking antibodies. In conclusion, we show that pneumococci use PLY-MRC-1 interactions to downregulate inflammation and enhance bacterial survival in the airways. These findings have important implications for future vaccine design.

Site-Specific Photoconjugation of Beta-Lactamase Fragments to Monoclonal Antibodies Enables Sensitive Analyte Detection via Split-Enzyme Complementation.

Protein fragment complementation assays (PCA) rely on a proximity-driven reconstitution of a split reporter protein activity, typically via interaction between bait and prey units separately fused to the reporter protein halves. The PCA principle can also be formatted for use in immunossays for analyte detection, e.g., via the use of small immunoglobulin binding proteins (IgBp) as fusion partners to split-reporter protein fragments for conversion of pairs of antibodies into split-protein half-probes. However, the non-covalent binding between IgBp and antibodies is not ideal for development of robust assays. Here, the authors describe how split-enzyme reporter halves can be both site-specifically and covalently photoconjugated at antibody Fc-parts for use in homogeneous dual-antibody in vitro immunoassays based on analyte-dependent split-enzyme fragment complementation. The half-probes consist of parts of a beta-lactamase split-protein reporter fused to an immunoglobulin Fc binding domain equipped with a unique cysteine residue at which a photoactivable maleimide benzophenone group (MBP) is attached. Using such antibody conjugates the authors obtain an analyte-driven complementation of the reporter enzyme fragments monitored via conversion of a chromogenic substrate. Results from detection of human interferon-gamma and the extracellular domain of HER2 is shown. The described principles for site-specific conjugation of proteins to antibodies should be broadly applicable.

Affibody Molecules in Biotechnological and Medical Applications.

Affibody molecules are small (6.5-kDa) affinity proteins based on a three-helix bundle domain framework. Since their introduction 20 years ago as an alternative to antibodies for biotechnological applications, the first therapeutic affibody molecules have now entered clinical development and more than 400 studies have been published in which affibody molecules have been developed and used in a variety of contexts. In this review, we focus primarily on efforts over the past 5 years to explore the potential of affibody molecules for medical applications in oncology, neurodegenerative, and inflammation disorders, including molecular imaging, receptor signal blocking, and delivery of toxic payloads. In addition, we describe recent examples of biotechnological applications, in which affibody molecules have been exploited as modular affinity fusion partners.

The BR domain of PsrP interacts with extracellular DNA to promote bacterial aggregation; structural insights into pneumococcal biofilm formation.

The major human pathogen Streptococcus pneumoniae is a leading cause of disease and death worldwide. Pneumococcal biofilm formation within the nasopharynx leads to long-term colonization and persistence within the host. We have previously demonstrated that the capsular surface-associated pneumococcal serine rich repeat protein (PsrP), key factor for biofilm formation, binds to keratin-10 (KRT10) through its microbial surface component recognizing adhesive matrix molecule (MSCRAMM)-related globular binding region domain (BR187-385). Here, we show that BR187-385 also binds to DNA, as demonstrated by electrophoretic mobility shift assays and size exclusion chromatography. Further, heterologous expression of BR187-378 or the longer BR120-378 construct on the surface of a Gram-positive model host bacterium resulted in the formation of cellular aggregates that was significantly enhanced in the presence of DNA. Crystal structure analyses revealed the formation of BR187-385 homo-dimers via an intermolecular ß-sheet, resulting in a positively charged concave surface, shaped to accommodate the acidic helical DNA structure. Furthermore, small angle X-ray scattering and circular dichroism studies indicate that the aggregate-enhancing N-terminal region of BR120-166 adopts an extended, non-globular structure. Altogether, our results suggest that PsrP adheres to extracellular DNA in the biofilm matrix and thus promotes pneumococcal biofilm formation.

An affibody-adalimumab hybrid blocks combined IL-6 and TNF-triggered serum amyloid A secretion in vivo.

In inflammatory disease conditions, the regulation of the cytokine system is impaired, leading to tissue damages. Here, we used protein engineering to develop biologicals suitable for blocking a combination of inflammation driving cytokines by a single construct. From a set of interleukin (IL)-6-binding affibody molecules selected by phage display, five variants with a capability of blocking the interaction between complexes of soluble IL-6 receptor α (sIL-6Rα) and IL-6 and the co-receptor gp130 were identified. In cell assays designed to analyze any blocking capacity of the classical or the alternative (trans) signaling IL-6 pathways, one variant, ZIL-6_13 with an affinity (KD) for IL-6 of ∼500 pM, showed the best performance. To construct fusion proteins ("AffiMabs") with dual cytokine specificities, ZIL-6_13 was fused to either the N- or C-terminus of both the heavy and light chains of the anti-tumor necrosis factor (TNF) monoclonal antibody adalimumab (Humira®). One AffiMab construct with ZIL-6_13 positioned at the N-terminus of the heavy chain, denoted ZIL-6_13-HCAda, was determined to be the most optimal, and it was subsequently evaluated in an acute Serum Amyloid A (SAA) model in mice. Administration of the AffiMab or adalimumab prior to challenge with a mix of IL-6 and TNF reduced the levels of serum SAA in a dose-dependent manner. Interestingly, the highest dose (70 mg/kg body weight) of adalimumab only resulted in a 50% reduction of SAA-levels, whereas the corresponding dose of the ZIL-6_13-HCAda AffiMab with combined IL-6/TNF specificity, resulted in SAA levels below the detection limit.

Recombinant spider silk genetically functionalized with affinity domains.

Functionalization of biocompatible materials for presentation of active protein domains is an area of growing interest. Herein, we describe a strategy for functionalization of recombinant spider silk via gene fusion to affinity domains of broad biotechnological use. Four affinity domains of different origin and structure; the IgG-binding domains Z and C2, the albumin-binding domain ABD, and the biotin-binding domain M4, were all successfully produced as soluble silk fusion proteins under nondenaturing purification conditions. Silk films and fibers produced from the fusion proteins were demonstrated to be chemically and thermally stable. Still, the bioactive domains are concluded to be folded and accessible, since their respective targets could be selectively captured from complex samples, including rabbit serum and human plasma. Interestingly, materials produced from mixtures of two different silk fusion proteins displayed combined binding properties, suggesting that tailor-made materials with desired stoichiometry and surface distributions of several binding domains can be produced. Further, use of the IgG binding ability as a general mean for presentation of desired biomolecules could be demonstrated for a human vascular endothelial growth factor (hVEGF) model system, via a first capture of anti-VEGF IgG to silk containing the Z-domain, followed by incubation with hVEGF. Taken together, this study demonstrates the potential of recombinant silk, genetically functionalized with affinity domains, for construction of biomaterials capable of presentation of almost any desired biomolecule.