Multi‐constraint computational design suggests that native sequences of germline antibody H3 loops are nearly optimal for conformational flexibility

The limited size of the germline antibody repertoire has to recognize a far larger number of potential antigens. The ability of a single antibody to bind multiple ligands due to conformational flexibility in the antigen‐binding site can significantly enlarge the repertoire. Among the six complementarity determining regions (CDRs) that generally comprise the binding site, the CDR H3 loop is particularly variable. Computational protein design studies showed that predicted low energy sequences compatible with a given backbone structure often have considerable similarity to the corresponding native sequences of naturally occurring proteins, indicating that native protein sequences are close to optimal for their structures. Here, we take a step forward to determine whether conformational flexibility, believed to play a key functional role in germline antibodies, is also central in shaping their native sequence. In particular, we use a multi‐constraint computational design strategy, along with the Rosetta scoring function, to propose that the native sequences of CDR H3 loops from germline antibodies are nearly optimal for conformational flexibility. Moreover, we find that antibody maturation may lead to sequences with a higher degree of optimization for a single conformation, while disfavoring sequences that are intrinsically flexible. In addition, this computational strategy allows us to predict mutations in the CDR H3 loop to stabilize the antigen‐bound conformation, a computational mimic of affinity maturation, that may increase antigen binding affinity by preorganizing the antigen binding loop. In vivo affinity maturation data are consistent with our predictions. The method described here can be useful to design antibodies with higher selectivity and affinity by reducing conformational diversity. Proteins 2009. © 2008 Wiley‐Liss, Inc.     

Production of anti-idiotypic antibodies by immunization with a pair of complementary peptides

Previous investigations have suggested that pairs of peptides specified by complementary RNA sequences (termed complementary peptides) can interact with one another. In the light of this finding, we hypothesized that an antibody directed against a peptide might interact with an antibody against its complementary peptide at the antigen combining site. To address this possibility, polyclonal antibodies against a peptide, Leu-Glu-Arg-Ile-Leu-Leu (LERILL), and its complementary peptide, Glu-Leu-Cys-Asp-Asp-Asp (ELCDDD), were made monospecific by affinity chromatography. Using radioimmunoassays, anti-ELCDDD antibodies were shown to interact with 125I-anti-LERILL antibodies but not with 125I-control antibodies. More importantly, the interaction of the two antibodies could be blocked using either peptide antigen, but not with control peptides. Furthermore, 125I-anti-LERILL binding to LERILL could be blocked with anti-ELCDDD antibody and vice versa. We concluded therefore that antibody/antibody binding occurred at or near the antigen combining site, demonstrating that this interaction was an idiotypic/anti-idiotypic one.     

Engineered pH-dependent recycling antibodies enhance elimination of Staphylococcal enterotoxin B superantigen in mice

A new modality in antibody engineering has emerged in which the antigen affinity is designed to be pH dependent (PHD). In particular, combining high affinity binding at neutral pH with low affinity binding at acidic pH leads to a novel antibody that can more effectively neutralize the target antigen while avoiding antibody-mediated antigen accumulation. Here, we studied how the in vivo pharmacokinetics of the superantigen, Staphylococcal enterotoxin B (SEB), is affected by an engineered antibody with pH-dependent binding. PHD anti-SEB antibodies were engineered by introducing mutations into a high affinity anti-SEB antibody, 3E2, by rational design and directed evolution. Three antibody mutants engineered in the study have an affinity at pH 6.0 that is up to 68-fold weaker than the control antibody. The pH dependency of each mutant, measured as the pH-dependent affinity ratio (PAR – ratio of affinity at pH 7.4 and pH 6.0), ranged from 6.7–11.5 compared to 1.5 for the control antibody. The antibodies were characterized in mice by measuring their effects on the pharmacodynamics and pharmacokinetics (PK) of SEB after co-administration. All antibodies were effective in neutralizing the toxin and reducing the toxin-induced cytokine production. However, engineered PHD antibodies led to significantly faster elimination of the toxin from the circulation than wild type 3E2. The area under the curve computed from the SEB PK profile correlated well with the PAR value of antibody, indicating the importance of fine tuning the pH dependency of binding. These results suggest that a PHD recycling antibody may be useful to treat intoxication from a bacterial toxin by accelerating its clearance.     

A combinatorial approach to engineering a dual-specific metal switch antibody

There is considerable interest in understanding how multiple binding events can be mediated through a single protein interface. Here, a synthetic library approach was developed to generate a novel dual-specific antibody. Using a combinatorial histidine-scanning phage display library, potential metal binding sites were introduced throughout an anti-RNase A antibody interface. Stepwise selection of RNase A and metal binding produced a dual-specific antibody that retained near wild-type affinity for its target antigen while acquiring a competitive metal binding site that is capable of controlling the antibody-antigen interaction. Structure analysis of the original antibody-RNase A complex suggested peripheral interface residues and loop flexibility are key contributors for obtaining the dual specificity.     

Anti-idiotype antibodies prevent antibody binding to mouse sperm and antibody-mediated inhibition of fertilization

Antibodies to components of sperm can interfere with sperm function and prevent fertilization by blocking specific steps of gamete interaction. It can be proposed that anti-idiotype antibodies (anti-Ids) that recognize determinants located close to or within the antigen-binding site of an anti-sperm antibody could block antibody binding to sperm antigen and antibody-mediated inhibition of fertilization. To test this hypothesis, rabbit polyclonal antibodies to idiotypic determinants of the monoclonal anti-sperm antibody M42.15 were developed and characterized. Previous studies demonstrated that M42.15 monoclonal antibody (mAb) inhibits fertilization in vitro and in vivo by inhibiting sperm-zona pellucida interaction. Anti-idiotype antibodies to M42.15 mAb (anti-Id M42) were isolated by affinity chromatography on immobilized M42.15 mAb. Binding specificity of anti-Id M42.15 was demonstrated in a solid-phase radioimmune binding assay and by specific immunoprecipitation of soluble M42.15 mAb. Anti-Id M42 competitively inhibited M42.15 mAb, but not P220.2 mAb, binding to mouse sperm, confirming that the anti-Id preparation contained antibodies directed against idiotopes within or adjacent to the antigen-binding site of the mAb. At equimolar concentrations, anti-Id M42 inhibited binding of 125I-labeled M42.15 mAb to sperm by greater than 80%. These results showed that anti-Id M42 efficiently blocked antibody binding to sperm and suggested that anti-Id M42 could be used to neutralize the anti-fertility activity of the M42.15 mAb. When tested in in vitro fertilization assays, anti-Id M42, but not rabbit immunoglobulin, prevented M42.15 mAb-induced inhibition of fertilization. Together, these results show that the inhibitory activity of anti-sperm antibodies capable of interfering with gamete interaction can be neutralized by anti-Id that recognize determinants close to the antigen-combining site of the anti-sperm antibody.     

Kinetic analysis of engineered antibody-antigen interactions

Molecular engineering antibodies has made it possible to produce specific domains of the antibody molecule and combine them with other protein domains to achieve new properties. Using site directed mutagenesis, amino acid residues can be exchanged within the binding site; and, by analysis of crystal structures, the positions of these amino acids can be determined in three dimensions at atomic resolution. In addition, gene libraries and phage selection technology can be used to generate new antibody fragments directly from a gene pool. Both mutagenesis and selection from libraries offer opportunities to identify antibody-derived molecules with altered and useful antigen recognition properties. The detailed analysis both kinetic and equilibrium binding affinity are therefore essential to understand the activity of the molecules resulting from antibody engineering and to guide the progress of their further design. The paper reviews recently evolving techniques for the binging analysis of antibodies, their functional domains and antibody chimerae.     

A Combinatorial Histidine Scanning Library Approach to Engineer Highly pH-Dependent Protein Switches

There is growing interest in the development of protein switches, which are proteins whose function, such as binding a target molecule, can be modulated through environmental triggers. Efforts to engineer highly pH sensitive protein–protein interactions typically rely on the rational introduction of ionizable groups in the protein interface. Such experiments are typically time intensive and often sacrifice the protein's affinity at the permissive pH. The underlying thermodynamics of proton‐linkage dictate that the presence of multiple ionizable groups, which undergo a pK_a change on protein binding, are necessary to result in highly pH‐dependent binding. To test this hypothesis, a novel combinatorial histidine library was developed where every possible combination of histidine and wild‐type residue is sampled throughout the interface of a model anti‐RNase A single domain VHH antibody. Antibodies were coselected for high‐affinity binding and pH‐sensitivity using an in vitro, dual‐function selection strategy. The resulting antibodies retained near wild‐type affinity yet became highly sensitive to small decreases in pH, drastically decreasing their binding affinity, due to the incorporation of multiple histidine groups. Several trends were observed, such as histidine “hot‐spots,” which will help enhance the development of pH switch proteins as well as increase our understanding of the role of ionizable residues in protein interfaces. Overall, the combinatorial approach is rapid, general, and robust and should be capable of producing highly pH‐sensitive protein affinity reagents for a number of different applications.     

Enhancement and destruction of antibody function by somatic mutation: unequal occurrence is controlled by V gene combinatorial associations.

We examined the positive and negative effects of somatic mutation on antibody function using saturation mutagenesis in vitro to mimic the potential of the in vivo process to diversify antibodies. Identical mutations were introduced into the second complementarity determining region of two anti-phosphocholine antibodies, T15 and D16, which share the same germline VH gene sequence. T15 predominates in primary responses and does not undergo affinity maturation. D16 is representative of antibodies that co-dominate in memory responses and do undergo affinity maturation. We previously reported that > 50% of T15 mutants had decreased antigen binding capacity. To test if this high frequency of binding loss was unique to T15 or a consequence of random point mutations applicable to other combining sites, we analyzed the same mutations in D16. We show that D16 suffers a similar loss of function, indicating an equally high potential for B-cell wastage. However, only D16 displayed the capacity for somatic mutation to improve antigen binding, which should enhance its persistence in memory responses. Mutation of residues contacting the haptenic group, as determined by molecular modeling, did not improve binding. Instead, productive mutations occurred in residues that either contacted carrier protein or were distant from the antigen binding site, possibly increasing binding site flexibility through long-range effects. Targeting such residues for mutation should aid in the rational design of improved antibodies.     

A bispecific antibody targeting CD47 and CD20 selectively binds and eliminates dual antigen expressing lymphoma cells

Agents that block the anti-phagocytic signal CD47 can synergize with pro-phagocytic anti-tumor antigen antibodies to potently eliminate tumors. While CD47 is overexpressed on cancer cells, its expression in many normal tissues may create an ‘antigen sink’ that could minimize the therapeutic efficacy of CD47 blocking agents. Here, we report development of bispecific antibodies (BsAbs) that co-target CD47 and CD20, a therapeutic target for non-Hodgkin lymphoma (NHL), that have reduced affinity for CD47 relative to the parental antibody, but retain strong binding to CD20. These characteristics facilitate selective binding of BsAbs to tumor cells, leading to phagocytosis. Treatment of human NHL-engrafted mice with BsAbs reduced lymphoma burden and extended survival while recapitulating the synergistic efficacy of anti-CD47 and anti-CD20 combination therapy. These findings serve as proof of principle for BsAb targeting of CD47 with tumor-associated antigens as a viable strategy to induce selective phagocytosis of tumor cells and recapitulate the synergy of combination antibody therapy. This approach may be broadly applied to cancer to add a CD47 blocking component to existing antibody therapies.     

Quantitative specificity‐based display library screening identifies determinants of antibody‐epitope binding specificity

Despite the critical importance of molecular specificity in bimolecular systems, in vitro display technologies have been applied extensively for affinity maturation of peptides and antibodies without explicitly measuring the specificity of the desired interaction. We devised a general strategy to measure, screen, and evolve specificity of protein ligand interactions analogous to widely used affinity maturation strategies. The specificity of binding to target and nontarget antibodies labeled with spectrally distinct fluorophores was measured simultaneously in protein mixtures via multiparameter flow cytometry, thereby enabling screening for high target antibody specificity. Isolated antibody specific ligands exhibited varying specificity, revealing critical amino acid determinants for target recognition and nontarget avoidance in complex mixtures. Molecular specificity in the mixture was further enhanced by quantitative directed evolution, yielding a family of epitopes exhibiting improved specificities equivalent, or superior to, the native peptide antigen to which the antibody was raised. Specificity screening simultaneously favored affinity, yielding ligands with three‐fold improved affinity relative to the parent epitope. Quantitative specificity screening will be useful to screen, evolve, and characterize the specificity of protein and peptide interactions for molecular recognition applications.     

Antigen-antibody interaction. Synthetic peptides define linear antigenic determinants recognized by monoclonal antibodies directed to the cytoplasmic carboxyl terminus of rhodopsin

The specificities of four monoclonal antibodies rho 1D4, 1C5, 3A6, and 3D6 prepared by immunization of rod outer segments containing rhodopsin have been defined using synthetic peptides. All of these antibodies interact within the 18 residues at the COOH terminus of rhodopsin and recognize linear antigenic determinants of 4-11 residues. Twenty-seven synthetic peptide analogs of varying lengths of native sequence or containing single amino acid substitutions at each position of the COOH-terminal 18 residues have provided some insight into the mechanism of antigen-antibody binding. Our results clearly demonstrate that antibodies can be highly specific at key positions as shown by the loss of binding on single amino acid substitutions in the binding site. In contrast single amino acid substitutions at other positions in the binding site only affect affinity for some antibodies. Ionic interactions can dominate immunogenic determinants. Immunogenic determinants are not restricted to highly charged hydrophilic regions on the surface of a protein and may be dominated by hydrophobic interactions. Although certain side chains can dominate the interaction of the antigen with antibody, our results are in agreement with the interpretation that the free energies of all the contact points are additive and a certain free energy must be present to achieve binding. Antibodies with different specificities directed to the same region of the protein antigen can be produced in an immune response. Peptide antigens representing regions of a protein antigen bind best to the anti-protein antibody when the sequence is shortened to contain only those residues binding to the specificity site in the antibody. Cross-reactivity between protein antigens can be explained by conservation of the critical residues in the combining site.     

Monoclonal antibodies to lipocortin-1 as probes for biological function.

We have developed two monoclonal antibodies to human lipocortin-1 (103 and 105) as reagents for quantitating the protein in biological systems and neutralizing its activity. Lipo 105 is a high affinity antibody that is functional in ELISA and Western blot formats. The antibody recognizes a site between amino acids 30 and 55 in the lipocortin-1 sequence and can be used on native or denatured protein. Lipo 103 is an antibody that neutralizes the phospholipase A2 inhibitory activity of lipocortin-1 by blocking binding of the protein to phospholipid surfaces. The antibody is specific for native human lipocortin-1. Lipo 103 was recently shown to block lipocortin-1-dependent differentiation of a squamous carcinoma cell line, demonstrating its usefulness as a probe for function.     

Generation of epitope-specific antibodies to rat GHBP in the sheep using an interspecies switching strategy involving site-directed mutagenesis of ovine GHBP.

Site-directed antibodies to the growth hormone receptor could be potentially useful as growth hormone mimics but, in previous attempts, we found that antisera generated using peptides derived from growth hormone receptor sequences failed to recognize the intact protein. As an alternative approach to this problem, we have now adopted a strategy of epitope-switching between rat and ovine growth hormone receptors to produce rat epitopes in the correct structural context. Using site-directed mutagenesis, we altered the two dominant linear epitopes in the ovine growth hormone binding protein to the analogous sequences in rat growth hormone binding protein. Site A, between Thr28 and Leu34, is equivalent to epitope 1 in ovine growth hormone binding protein and site B, between Ser121 and Asp124, corresponds to epitope 5. The wild-type ovine growth hormone binding protein and the two mutant proteins were bacterially expressed, refolded and, following purification by metal-chelate affinity chromatography, used to raise antisera in sheep. We showed using RIA, in which wild-type ovine growth hormone binding protein acted as a competitor for the binding of rat growth hormone binding protein, that only the site A mutant protein elicited a specific anti-rat growth hormone binding protein response. This was confirmed in subsequent RIA studies using the antiserum to the site A mutant protein in which only peptides corresponding to the site A sequences in mutant ovine growth hormone binding protein and rat growth hormone binding protein, but not that in wild-type ovine growth hormone binding protein, were able to act as competitors for rat growth hormone binding protein. Antibodies specific for rat growth hormone binding protein could be separated from the antiserum to the site A mutant protein by means of affinity chromatography using immobilized wild-type ovine growth hormone binding protein to remove antibodies which cross-reacted with the ovine protein. The work lays the foundations for further studies in which the biological effects of these antibody fractions will be investigated and demonstrates an approach with general applicability in the production of antibodies directed towards specific epitopes on protein molecules.     

Development of a compact anti-BAFF antibody in Escherichia coli

Recombinant antibodies, especially single-chain antibody fragment (scFv), can be applied as detection reagents and even substitute for some reagents used in immunoassays. For scFv fragments, there is no such universal system available up to now. We have constructed vectors for the convenient, rapid expression of a novel compact antibody composed of anti-B-cell-activating factor of the TNF family (BAFF) scFv and the Fc portion (the hinge region, CH2, and CH3 domains) of the human IgG1 in Escherichia coli. After expression in bacteria as inclusion bodies, the recombinant antibody was purified and refolded in vitro. The scFv-Fc antibody was demonstrated to retain high binding affinity to antigen, including membrane-bound BAFF and soluble BAFF, and to possess some human IgG crystallizable fragment domain functions, such as human complement C1q and protein A binding. Both size-exclusion high-performance liquid chromatography column analysis and Western blotting of proteins subjected to nonreducing sodium dodecyl sulfate polyacrylamide gel electrophoresis suggested that scFv-Fc antibody is homodimeric with relative molecular mass of 110 kDa. These findings suggest that the compact antibody may be useful in diagnostic application for the prediction of BAFF relevant to autoimmune diseases in human.     

Structural and functional insight into how the Plasmodium falciparum VAR2CSA protein mediates binding to chondroitin sulfate A in placental malaria

Malaria is a major global health problem. Pregnant women are susceptible to infection regardless of previously acquired immunity. Placental malaria is caused by parasites capable of sequestering in the placenta. This is mediated by VAR2CSA, a parasite antigen that interacts with chondroitin sulfate A (CSA). One vaccine strategy is to block this interaction with VAR2CSA-specific antibodies. It is a priority to define a small VAR2CSA fragment that can be used in an adhesion blocking vaccine. In this, the obvious approach is to define regions of VAR2CSA involved in receptor binding. It has been shown that full-length recombinant VAR2CSA binds specifically to CSA with nanomolar affinity, and that the CSA-binding site lies in the N-terminal part of the protein. In this study we define the minimal binding region by truncating VAR2CSA and analyzing CSA binding using biosensor technology. We show that the core CSA-binding site lies within the DBL2X domain and parts of the flanking interdomain regions. This is in contrast to the idea that single domains do not possess the structural requirements for specific CSA binding. Small-angle x-ray scattering measurements enabled modeling of VAR2CSA and showed that the CSA-binding DBL2X domain is situated in the center of the structure. Mutating classic sulfate-binding sites in VAR2CSA, along with testing dependence of ionic interactions, suggest that the CSA binding is not solely dependent on the sulfated CSA structure. Based on these novel PfEMP1 structure-function studies, we have constructed a small VAR2CSA antigen that has the capacity to induce highly adhesion-blocking antibodies.     

Use of thiol-terminal silanes and heterobifunctional crosslinkers for immobilization of antibodies on silica surfaces

A procedure for covalent immobilization of functional proteins on silica substrates was developed using thiol-terminal silanes and heterobifunctional cross-linkers. Using this procedure, a high density of functional antibodies was bound to glass cover slips and silica fibers. The amount of anti-IgG antibody immobilized was determined to be in the range of 0.66 to 0.96 ng/mm2 using radiolabeled antibody. The relative amount of IgG antigen bound by the immobilized antibody (0.37 to 0.55 mol antigen/mol antibody) was three to five times greater than other investigators have reported. In addition, the amount of protein nonspecifically adsorbed to the antibody-coated surface was further reduced by the addition of blocking agents so that nonspecific adsorption of protein antigens represented only 2-6% of the total antigen binding. With this low background, IgG antigen binding could be measured at levels as low as 150 fmol when an antigen concentration of 3 pmol/ml was applied. The process for antibody immobilization is straightforward, easy to perform, and adaptable for modifying mass quantities of biosensor components.     

High affinity antigen recognition of the dual specific variants of herceptin is entropy-driven in spite of structural plasticity

The antigen-binding site of Herceptin, an anti-human Epidermal Growth Factor Receptor 2 (HER2) antibody, was engineered to add a second specificity toward Vascular Endothelial Growth Factor (VEGF) to create a high affinity two-in-one antibody bH1. Crystal structures of bH1 in complex with either antigen showed that, in comparison to Herceptin, this antibody exhibited greater conformational variability, also called "structural plasticity". Here, we analyzed the biophysical and thermodynamic properties of the dual specific variants of Herceptin to understand how a single antibody binds two unrelated protein antigens. We showed that while bH1 and the affinity-improved bH1-44, in particular, maintained many properties of Herceptin including binding affinity, kinetics and the use of residues for antigen recognition, they differed in the binding thermodynamics. The interactions of bH1 and its variants with both antigens were characterized by large favorable entropy changes whereas the Herceptin/HER2 interaction involved a large favorable enthalpy change. By dissecting the total entropy change and the energy barrier for dual interaction, we determined that the significant structural plasticity of the bH1 antibodies demanded by the dual specificity did not translate into the expected increase of entropic penalty relative to Herceptin. Clearly, dual antigen recognition of the Herceptin variants involves divergent antibody conformations of nearly equivalent energetic states. Hence, increasing the structural plasticity of an antigen-binding site without increasing the entropic cost may play a role for antibodies to evolve multi-specificity. Our report represents the first comprehensive biophysical analysis of a high affinity dual specific antibody binding two unrelated protein antigens, furthering our understanding of the thermodynamics that drive the vast antigen recognition capacity of the antibody repertoire.     

Development of a Functional Antibody by Using a Green Fluorescent Protein Frame as the Template

Single-chain variable fragment (scFv) antibodies are widely used as diagnostic and therapeutic agents or biosensors for a majority of human disease. However, the limitations of the present scFv antibody in terms of stability, solubility, and affinity are challenging to produce by traditional antibody screening and expression formats. We describe here a feasible strategy for creating the green fluorescent protein (GFP)-based antibody. Complementarity-determining region 3 (CDR3), which retains the antigen binding activity, was introduced into the structural loops of superfolder GFP, and the result showed that CDR3-inserted GFP displayed almost the same fluorescence intensity as wild-type GFP, and the purified proteins of CDR3 insertion showed the similar binding activity to antigen as the corresponding scFv. Among of all of the CDRs, CDR3s are responsible for antigen recognition, and only the CDR3a insertion is the best format for producing GFP-based antibody binding to specific antigen. The wide versatility of this system was further verified by introducing CDR3 from other scFvs into loop 9 of GFP. We developed a feasible method for rapidly and effectively producing a high-affinity GFP-based antibody by inserting CDR3s into GFP loops. Further, the affinity can be enhanced by specific amino acids scanning and site-directed mutagenesis. Notably, this method had better versatility for creating antibodies to various antigens using GFP as the scaffold, suggesting that a GFP-based antibody with high affinity and specificity may be useful for disease diagnosis and therapy.     

1.9 A structure of the therapeutic antibody CAMPATH-1H fab in complex with a synthetic peptide antigen.

CAMPATH-1 antibodies have a long and successful history in the treatment of leukaemia, autoimmune disease and transplant rejection. The first antibody to undergo "humanisation", CAMPATH-1H, permits treatment with limited patient anti-globulin response. It recognises the CD52 antigen which is a small glycosylphosphatidylinositol(GPI)-anchored protein expressed on lymphocytes and mediates cell depletion. We present the 1.9 A structure of the CAMPATH-1H Fab complexed [corrected] with an analogue of the antigenic determinant of CD52. Analysis of the CAMPATH-1H binding site reveals that in contrast to most antibodies CDR L3 plays a dominant role in antigen binding. Furthermore CDR H3, which is essential for effective antigen recognition in most antibodies, participates in only two main-chain interactions in CAMPATH-1H. The CAMPATH-1H binding site is highly basic; ionic interaction with the enthanolamine phosphate of the CD52 GPI anchor has long been hypothesised to be important in antigen binding. The structure reveals a number of important specific ionic interactions, including Lys53H but not Lys52bH as had previously been suggested. Prolonged treatment with CAMPATH-1H can lead to patient anti-idiotype responses which may be exacerbated by the unusually high number of basic residues in the antibody. This suggests that a strategy where redundant basic residues are replaced with neutral counterparts may be effective in further reducing the immunogenicity of this versatile and widely used antibody.     

Intracellular antibodies and cancer: New technologies offer therapeutic opportunities

Since the realisation that the antigen‐binding regions of antibodies, the variable (V) regions, can be uncoupled from the rest of the molecule to create fragments that recognise and abrogate particular protein functions in cells, the use of antibody fragments inside cells has become an important tool in bioscience. Diverse libraries of antibody fragments plus in vivo screening can be used to isolate single chain variable fragments comprising VH and VL segments or single V‐region domains. Some of these are interfering antibody fragments that compete with protein‐protein interactions, providing lead molecules for drug interactions that until now have been considered difficult or undruggable. It may be possible to deliver or express antibody fragments in target cells as macrodrugs per se. In future incarnations of intracellular antibodies, however, the structural information of the interaction interface of target and antibody fragment should facilitate development of binding site mimics as small drug‐like molecules. This is a new dawn for intracellular antibody fragments both as macrodrugs and as precursors of drugs to treat human diseases and should finally lead to the removal of the epithet of the ‘undruggable’ protein‐protein interactions.