Antiferritin VL homodimer binds human spleen ferritin with high specificity

Structural, Biophysical, and Computational Studies of a Murine Light Chain Dimer

Antibodies are widely used in medicinal and scientific research due to their ability to bind to a specific antigen. Most often, antibodies are composed of heavy and light chain domains. Under physiological conditions, light chains are produced in excess, as compared to the heavy chain. It is now known that light chains are not silent partners of the heavy chain and can modulate the immune response independently. In this work, the first crystal structure of a light chain dimer originating from mice is described. It represents the light chain dimer of 6A8, a monoclonal antibody specific to the allergen Der f 1. Building on the unexpected occurrence of this kind of dimer, we have demonstrated that this light chain is stable in solution alone. Moreover, enzyme-linked immunosorbent assays (ELISA) have revealed that, when the light chain is not partnered to its corresponding heavy chain, it interacts non-specifically with a wide range of proteins. Computational studies were used to provide insight on the role of the 6A8 heavy chain domain in the specific binding to Der f 1. Overall, this work demonstrates and supports the ongoing notion that light chains can function by themselves and are not silent partners of heavy chains.

Structure, dynamics and transferability of the metal-dependent polyhistidine tetramerization motif TetrHis for single-chain Fv antibodies

The polyhistidine (6XHis) motif is one of the most ubiquitous protein purification tags. The 6XHis motif enables the binding of tagged proteins to various metals, which can be advantageously used for purification with immobilized metal affinity chromatography. Despite its popularity, protein structures encompassing metal-bound 6XHis are rare. Here, we obtained a 2.5 Å resolution crystal structure of a single chain Fv antibody (scFv) bearing a C-terminal sortase motif, 6XHis and TwinStrep tags (LPETGHHHHHHWSHPQFEK[G₃S]₃WSHPQFEK). The structure, obtained in the presence of cobalt, reveals a unique tetramerization motif (TetrHis) stabilized by 8 Co²⁺ ions. The TetrHis motif contains four 6 residues-long β-strands, and each metal center coordinates 3 to 5 residues, including all 6XHis histidines. By combining dynamic light scattering, small angle x-ray scattering and molecular dynamics simulations, We investigated the influence of Co²⁺ on the conformational dynamics of scFv 2A2, observing an open/close equilibrium of the monomer and the formation of cobalt-stabilized tetramers. By using a similar scFv design, we demonstrate the transferability of the tetramerization property. This novel metal-dependent tetramerization motif might be used as a fiducial marker for cryoelectron microscopy of scFv complexes, or even provide a starting point for designing metal-loaded biomaterials.

Therapeutic treatment with scFv-PLGA nanoparticles decreases pulmonary fungal load in a murine model of paracoccidioidomycosis

Paracoccidioidomycosis (PCM) is a systemic mycosis with lymphatic dissemination that is caused by Paracoccidioides species. Treatment of PCM consists of chemotherapeutics such as itraconazole, trimethoprim, sulfamethoxazole or amphotericin B. However, several studies are aiming to develop therapeutic alternatives for the treatment of fungal infection using new molecules as adjuvants. The single-chain variable fragments (scFv) from an antibody that mimics the main fungal component incorporated within poly(lactide-co-glycolic) acid (PLGA) nanoparticles helped treat the fungal disease. After expressing the scFv in Picchia pastoris (P. pastoris), the recombinant molecules were coupled with PLGA, and the BALB/c mice were immunized before or after infection with yeast Paracoccidioides brasiliensis (P. brasiliensis). Our results showed decreased disease progression and decreased fungal burden. Taken together, our results showed an increased of IFN-γ and IL-12 cytokine production and an increased number of macrophages and dendritic cells in the pulmonary tissue of BALB/c mice treated with a high concentration of our molecule. Our data further confirm that the scFv plays an important role in the treatment of experimental PCM.

scFv-based fluorogen activating proteins and variable domain inhibitors as fluorescent biosensor platforms

Single chain antibodies (scFvs) are engineered proteins composed of IgG variable heavy (V(H)) and variable light (V(L)) domains tethered together by a flexible peptide linker. We have characterized the individual V(H) or V(L) domain activities of several scFvs isolated from a yeast surface-display library for their ability to bind environmentally sensitive fluorogenic dyes causing them to fluoresce. For many of the scFvs, both V(H) and V(L) domains are required for dye binding and fluorescence. The analysis of other scFvs, however, revealed that either the V(H) or the V(L) domain alone is sufficient to cause the fluorogenic dye activation. Furthermore, the inactive complementary domains in the original scFvs either contribute nothing to, or actually inhibit the activity of these active single domains. We have explored the interactions between active variable domains and inactive complementary domains by extensive variable domain swapping through in vitro gene manipulations to create hybrid scFvs. In this study, we demonstrate that significant alteration of the fluorogenic dye activation by the active V(H) or V(L) domains can occur by partnering with different V(H) or V(L) complementary domains in the scFv format. Hybrid scFvs can be generated that have fluorogen-activating domains that are completely inhibited by interactions with other domains. Such hybrid scFvs are excellent platforms for the development of several types of genetically encoded, fluorescence-generating biosensors.

Antigen Binding and Solubility Effects upon the Veneering of a Camel VHH in Framework-2 to Mimic a VH

Heavy chain only antibodies of camelids bind their antigens with a single domain, the VHH, which acquired adaptations relative to classical VHs to function in the absence of a VL partner. Additional CDR loop conformations, outside the canonical loop structures of VHs, broaden the repertoire of the antigen-binding site. The combined effects of part of the CDR3 that folds over the "former" VL binding site and framework-2 mutations to more hydrophilic amino acids, enhance the solubility of VHH domains and prevent VL pairing. cAbAn33, a VHH domain specific for the carbohydrate moiety of the variant surface glycoprotein of trypanosomes, has a short CDR3 loop that does not cover the former VL binding site as well as a VH-specific Trp47 instead of the VHH-specific Gly47. Resurfacing its framework-2 region (mutations Tyr37Val, Glu44Gly and Arg45Leu) to mimic that of a human VH restores the VL binding capacity. In solution, the humanised VHH behaves as a soluble, monomeric entity, albeit with reduced thermodynamic stability and affinity for its antigen. Comparison of the crystal structures of cAbAn33 and its humanised derivative reveals steric hindrance exerted by VHH-specific residues Tyr37 and Arg45 that prevent the VL domain pairing, whereas Glu44 and Arg45 are key elements to avoid insolubility of the domain.