A Bispecific γδ T-cell Engager Targeting EGFR Activates a Potent Vγ9Vδ2 T cell-Mediated Immune Response against EGFR-Expressing Tumors

Vγ9Vδ2 T cells are effector cells with proven antitumor efficacy against a broad range of cancers. This study aimed to assess the antitumor activity and safety of a bispecific antibody directing Vγ9Vδ2 T cells to EGFR-expressing tumors. An EGFR-Vδ2 bispecific T-cell engager (bsTCE) was generated, and its capacity to activate Vγ9Vδ2 T cells and trigger antitumor activity was tested in multiple in vitro, in vivo, and ex vivo models. Studies to explore safety were conducted using cross-reactive surrogate engagers in nonhuman primates (NHP). We found that Vγ9Vδ2 T cells from peripheral blood and tumor specimens of patients with EGFR+ cancers had a distinct immune checkpoint expression profile characterized by low levels of PD-1, LAG-3, and TIM-3. Vγ9Vδ2 T cells could be activated by EGFR-Vδ2 bsTCEs to mediate lysis of various EGFR+ patient-derived tumor samples, and substantial tumor growth inhibition and improved survival were observed in in vivo xenograft mouse models using peripheral blood mononuclear cells (PBMC) as effector cells. EGFR-Vδ2 bsTCEs exerted preferential activity toward EGFR+ tumor cells and induced downstream activation of CD4+ and CD8+ T cells and natural killer (NK) cells without concomitant activation of suppressive regulatory T cells observed with EGFR-CD3 bsTCEs. Administration of fully cross-reactive and half-life extended surrogate engagers to NHPs did not trigger signals in the safety parameters that were assessed. Considering the effector and immune-activating properties of Vγ9Vδ2 T cells, the preclinical efficacy data and acceptable safety profile reported here provide a solid basis for testing EGFR-Vδ2 bsTCEs in patients with EGFR+ malignancies.

A bispecific T cell engager recruits both type 1 NKT and Vγ9Vδ2-T cells for the treatment of CD1d-expressing hematological malignancies

Bispecific T cell engagers (bsTCEs) hold great promise for cancer treatment but face challenges due to the induction of cytokine release syndrome (CRS), on-target off-tumor toxicity, and the engagement of immunosuppressive regulatory T cells that limit efficacy. The development of Vγ9Vδ2-T cell engagers may overcome these challenges by combining high therapeutic efficacy with limited toxicity. By linking a CD1d-specific single-domain antibody (VHH) to a Vδ2-TCR-specific VHH, we create a bsTCE with trispecific properties, which engages not only Vγ9Vδ2-T cells but also type 1 NKT cells to CD1d+ tumors and triggers robust proinflammatory cytokine production, effector cell expansion, and target cell lysis in vitro. We show that CD1d is expressed by the majority of patient MM, (myelo)monocytic AML, and CLL cells and that the bsTCE triggers type 1 NKT and Vγ9Vδ2-T cell-mediated antitumor activity against these patient tumor cells and improves survival in in vivo AML, MM, and T-ALL mouse models. Evaluation of a surrogate CD1d-γδ bsTCE in NHPs shows Vγ9Vδ2-T cell engagement and excellent tolerability. Based on these results, CD1d-Vδ2 bsTCE (LAVA-051) is now evaluated in a phase 1/2a study in patients with therapy refractory CLL, MM, or AML.

Directing HIV-1 for degradation by non-target cells, using bi-specific single-chain llama antibodies

While vaccination against HIV-1 has been so far unsuccessful, recently broadly neutralizing antibodies (bNAbs) against HIV-1 envelope glycoprotein were shown to induce long-term suppression in the absence of antiretroviral therapy in patients with antibody-sensitive viral reservoirs. The requirement of neutralizing antibodies indicates that the antibody mediated removal (clearance) of HIV-1 in itself is not efficient enough in these immune compromised patients. Here we present a novel, alternative approach that is independent of a functional immune system to clear HIV-1, by capturing the virus and redirecting it to non-target cells where it is internalized and degraded. We use bispecific antibodies with domains derived from small single chain Llama antibodies (VHHs). These bind with one domain to HIV-1 envelope proteins and with the other domain direct the virus to cells expressing epidermal growth factor receptor (EGFR), a receptor that is ubiquitously expressed in the body. We show that HIV envelope proteins, virus-like particles and HIV-1 viruses (representing HIV-1 subtypes A, B and C) are efficiently recruited to EGFR, internalized and degraded in the lysosomal pathway at low nM concentrations of bispecific VHHs. This directed degradation in non-target cells may provide a clearance platform for the removal of viruses and other unwanted agents from the circulation, including toxins, and may thus provide a novel method for curing.

Functional patient-derived organoid screenings identify MCLA-158 as a therapeutic EGFR × LGR5 bispecific antibody with efficacy in epithelial tumors

Patient-derived organoids (PDOs) recapitulate tumor architecture, contain cancer stem cells and have predictive value supporting personalized medicine. Here we describe a large-scale functional screen of dual-targeting bispecific antibodies (bAbs) on a heterogeneous colorectal cancer PDO biobank and paired healthy colonic mucosa samples. More than 500 therapeutic bAbs generated against Wingless-related integration site (WNT) and receptor tyrosine kinase (RTK) targets were functionally evaluated by high-content imaging to capture the complexity of PDO responses. Our drug discovery strategy resulted in the generation of MCLA-158, a bAb that specifically triggers epidermal growth factor receptor degradation in leucine-rich repeat-containing G-protein-coupled receptor 5-positive (LGR5+) cancer stem cells but shows minimal toxicity toward healthy LGR5+ colon stem cells. MCLA-158 exhibits therapeutic properties such as growth inhibition of KRAS-mutant colorectal cancers, blockade of metastasis initiation and suppression of tumor outgrowth in preclinical models for several epithelial cancer types.

Abstract 1855: Potent anti-tumor activity against patient CLL, MM and AML by LAVA-051, a bispecific Vγ9Vδ2-T and type 1 NKT cell engager targeting CD1d

Bispecific antibodies that target tumors by engaging innate-like semi-invariant T cell subsets with inherent anti-tumor activity, such as Vγ9Vδ2-T cells and type 1 natural killer T (NKT) cells, may combine high therapeutic efficacy with limited off-tumor toxicity. Type 1 NKT cells respond to self and foreign (glyco)lipid antigens presented in the context of the MHC class I like molecule CD1d which is expressed on various (hematological) malignancies. Vγ9Vδ2-T cells respond to intracellular accumulation of phosphoantigens in cancer cells by sensing conformational alterations in the butyrophilin (BTN)2A1-3A1 complex. By linking a CD1d-specific single domain antibody (VHH) to a Vδ2-TCR specific VHH, we created a bispecific VHH, termed LAVA-051. Besides targeting Vγ9Vδ2-T cells to CD1d-expressing tumor cells, LAVA-051 has the unique ability to also trigger strong activation of type 1 NKT cells in a CD1d-restricted fashion, resulting in potent killing of CD1d-expressing tumor cells by engagement of Vγ9Vδ2-T cells and/or type 1 NKT cells (EC50 1 pM for Vγ9Vδ2-T cells and 216 pM for type 1 NKT cells; >85% target cell lysis in overnight assays at low effector:target ratios of 1:2). LAVA-051 triggered strong activation, pro-inflammatory cytokine production, and proliferation of type 1 NKT and Vγ9Vδ2-T cells and exerted substantial antitumor activity against human primary acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and multiple myeloma (MM) cells that express CD1d. Moreover, improved survival was observed in in vivo AML and MM mouse models. Due to lack of cross-reactivity of LAVA-051 with non-human primate (NHP) CD1d and Vγ9Vδ2-T cells, a cross-reactive surrogate bispecific engager was generated to assess safety, pharmacokinetic (PK) and pharmacodynamic (PD) parameters. Multiple dose studies in NHP (7 daily doses up to 1 mg/kg i.v.) showed clear signs of Vγ9Vδ2-T cell engagement but no clinical, laboratory, or histopathological toxicity. Reflecting the low molecular size of this bispecific engager, PK studies revealed a short plasma half-life which was however compensated for by the prolonged (up to 5 days) binding of the bispecific engager to peripheral blood Vγ9Vδ2-T cells allowing intermittent dosing. Although VHHs have low inherent immunogenicity LAVA-051 was humanized and engineered to avoid binding to pre-existing anti-VHH antibodies that have been reported in ±15% of human individuals and that may limit therapeutic activity. Based on the here reported strong activity against CD1d-expresssing tumors in in vitro, ex vivo and in vivo models and the favorable safety profile of the surrogate engager in NHP, LAVA-051 has been selected for a first-in-human clinical phase 1-2 study in patients with CD1d-expressing, therapy refractory, CLL, MM, or AML. Initiation of this clinical program is scheduled for Q1 2021.

Citation Format: Roeland Lameris, Jurjen M. Ruben, Iris de Weerdt, Rob C. Roovers, Arnon P. Kater, Thilo A. Riedl, Victoria Iglesias, Ton Adang, Tanja D. de Gruijl, Paul W.H.I Parren, Hans J. van der Vliet. Potent anti-tumor activity against patient CLL, MM and AML by LAVA-051, a bispecific Vγ9Vδ2-T and type 1 NKT cell engager targeting CD1d [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1855.

A Bispecific Antibody Antagonizes Prosurvival CD40 Signaling and Promotes Vγ9Vδ2 T cell-Mediated Antitumor Responses in Human B-cell Malignancies

Novel T cell-based therapies for the treatment of B-cell malignancies, such as chronic lymphocytic leukemia (CLL) and multiple myeloma (MM), are thought to have strong potential. Progress, however, has been hampered by low efficacy and high toxicity. Tumor targeting by Vγ9Vδ2 T cells, a conserved T-cell subset with potent intrinsic antitumor properties, mediated by a bispecific antibody represents a novel approach promising high efficacy with limited toxicity. Here, we describe the generation of a bispecific Vγ9Vδ2 T-cell engager directed against CD40, which, due to its overexpression and biological footprint in malignant B cells, represents an attractive target. The CD40-targeting moiety of the bispecific antibody was selected because it can prevent CD40L-induced prosurvival signaling and reduce CD40-mediated resistance of CLL cells to venetoclax. Selective activation of Vγ9Vδ2 T cells in the presence of CD40⁺ tumor cells induced potent Vγ9Vδ2 T-cell degranulation, cytotoxicity against CLL and MM cells in vitro, and in vivo control of MM in a xenograft model. The CD40-bispecific γδ T-cell engager demonstrated lysis of leukemic cells by autologous Vγ9Vδ2 T cells present in patient-derived samples. Taken together, our CD40 bispecific γδ T-cell engager increased the sensitivity of leukemic cells to apoptosis and induced a potent Vγ9Vδ2 T cell-dependent antileukemic response. It may, therefore, represent a potential candidate for the development of novel treatments for B-cell malignancies.

MCLA-117, a CLEC12AxCD3 bispecific antibody targeting a leukaemic stem cell antigen, induces T cell-mediated AML blast lysis

Objective: We report the characterization of MCLA-117, a novel T cell-redirecting antibody for acute myeloid leukaemia (AML) treatment targeting CD3 on T cells and CLEC12A on leukaemic cells. In AML, CLEC12A is expressed on blasts and leukaemic stem cells. Methods: The functional capacity of MCLA-117 to redirect resting T cells to eradicate CLEC12A^POS tumor cells was studied using human samples, including primary AML samples. Results: Within the normal hematopoietic compartment, MCLA-117 binds to cells expressing CD3 and CLEC12A but not to early myeloid progenitors or hematopoietic stem cells. MCLA-117 induces T cell activation (EC₅₀ = 44 ng/mL), T cell proliferation, mild pro-inflammatory cytokine release, and redirects T cells to lyse CLEC12A^POS target cells (EC₅₀ = 68 ng/mL). MCLA-117-induced targeting of normal CD34^POS cells co-cultured with T cells spares erythrocyte and megakaryocyte differentiation as well as preserves mono-myelocytic lineage development. In primary AML patient samples with autologous T cells, MCLA-117 robustly induced AML blast killing (23-98%) at low effector-to-target ratios (1:3-1:97). Conclusion: These findings demonstrate that MCLA-117 efficiently redirects T cells to kill tumour cells while sparing the potential of the bone marrow to develop the full hematological compartment and support further clinical evaluation as a potentially potent treatment option for AML.

The architecture of EGFR's basal complexes reveals autoinhibition mechanisms in dimers and oligomers

Our current understanding of epidermal growth factor receptor (EGFR) autoinhibition is based on X-ray structural data of monomer and dimer receptor fragments and does not explain how mutations achieve ligand-independent phosphorylation. Using a repertoire of imaging technologies and simulations we reveal an extracellular head-to-head interaction through which ligand-free receptor polymer chains of various lengths assemble. The architecture of the head-to-head interaction prevents kinase-mediated dimerisation. The latter, afforded by mutation or intracellular treatments, splits the autoinhibited head-to-head polymers to form stalk-to-stalk flexible non-extended dimers structurally coupled across the plasma membrane to active asymmetric tyrosine kinase dimers, and extended dimers coupled to inactive symmetric kinase dimers. Contrary to the previously proposed main autoinhibitory function of the inactive symmetric kinase dimer, our data suggest that only dysregulated species bear populations of symmetric and asymmetric kinase dimers that coexist in equilibrium at the plasma membrane under the modulation of the C-terminal domain.

A bispecific nanobody approach to leverage the potent and widely applicable tumor cytolytic capacity of Vγ9Vδ2-T cells

Though Vγ9Vδ2-T cells constitute only a small fraction of the total T cell population in human peripheral blood, they play a vital role in tumor defense and are therefore of major interest to explore for cancer immunotherapy. Vγ9Vδ2-T cell-based cancer immunotherapeutic approaches developed so far have been generally well tolerated and were able to induce significant clinical responses. However, overall results were inconsistent, possibly due to the fact that these strategies induced systemic activation of Vγ9Vδ2-T cells without preferential accumulation and targeted activation in the tumor. Here we show that a novel bispecific nanobody-based construct targeting both Vγ9Vδ2-T cells and EGFR induced potent Vγ9Vδ2-T cell activation and subsequent tumor cell lysis both in vitro and in an in vivo mouse xenograft model. Tumor cell lysis was independent of KRAS and BRAF tumor mutation status and common Vγ9Vδ2-T cell receptor sequence variations. In combination with the conserved monomorphic nature of the Vγ9Vδ2-TCR and the facile replacement of the tumor-specific nanobody, this immunotherapeutic approach can be applied to a large group of cancer patients.

High affinity nanobodies against human epidermal growth factor receptor selected on cells by E. coli display

Most therapeutic antibodies (Abs) target cell surface proteins on tumor and immune cells. Cloning of Ab gene libraries in E. coli and their display on bacteriophages is commonly used to select novel therapeutic Abs binding target antigens, either purified or expressed on cells. However, the sticky nature of bacteriophages renders phage display selections on cells challenging. We previously reported an E. coli display system for expression of VHHs (i.e., nanobodies, Nbs) on the surface of bacteria and selection of high-affinity clones by magnetic cell sorting (MACS). Here, we demonstrate that E. coli display is also an attractive method for isolation of Nbs against cell surface antigens, such as the epidermal growth factor receptor (EGFR), upon direct selection and screening of Ab libraries on live cells. We employ a whole cell-based strategy using a VHH library obtained by immunization with human tumor cells over-expressing EGFR (i.e., A431), and selection of bacterial clones bound to murine fibroblast NIH-3T3 cells transfected with human EGFR, after depletion of non-specific clones on untransfected cells. This strategy resulted in the isolation of high-affinity Nbs binding distinct epitopes of EGFR, including Nbs competing with the ligand, EGF, as characterized by flow cytometry of bacteria displaying the Nbs and binding assays with purified Nbs using surface plasmon resonance. Hence, our study demonstrates that E. coli display of VHH libraries and selection on cells enables efficient isolation and characterization of high-affinity Nbs against cell surface antigens.

Highly specific and potently activating Vγ9Vδ2-T cell specific nanobodies for diagnostic and therapeutic applications

Vγ9Vδ2-T cells constitute the predominant subset of γδ-T cells in human peripheral blood and have been shown to play an important role in antimicrobial and antitumor immune responses. Several efforts have been initiated to exploit these cells for cancer immunotherapy, e.g. by using phosphoantigens, adoptive cell transfer, and by a bispecific monoclonal antibody based approach. Here, we report the generation of a novel set of Vγ9Vδ2-T cell specific VHH (or nanobody). VHH have several advantages compared to conventional antibodies related to their small size, stability, ease of generating multispecific molecules and low immunogenicity. With high specificity and affinity, the anti-Vγ9Vδ2-T cell receptor VHHs are shown to be useful for FACS, MACS and immunocytochemistry. In addition, some VHH were found to specifically activate Vγ9Vδ2-T cells. Besides being of possible immunotherapeutic value, these single domain antibodies will be of great value in the further study of this important immune effector cell subset.

Camelid Ig V genes reveal significant human homology not seen in therapeutic target genes, providing for a powerful therapeutic antibody platform

Camelid immunoglobulin variable (IGV) regions were found homologous to their human counterparts; however, the germline V repertoires of camelid heavy and light chains are still incomplete and their therapeutic potential is only beginning to be appreciated. We therefore leveraged the publicly available HTG and WGS databases of Lama pacos and Camelus ferus to retrieve the germline repertoire of V genes using human IGV genes as reference. In addition, we amplified IGKV and IGLV genes to uncover the V germline repertoire of Lama glama and sequenced BAC clones covering part of the Lama pacos IGK and IGL loci. Our in silico analysis showed that camelid counterparts of all human IGKV and IGLV families and most IGHV families could be identified, based on canonical structure and sequence homology. Interestingly, this sequence homology seemed largely restricted to the Ig V genes and was far less apparent in other genes: 6 therapeutically relevant target genes differed significantly from their human orthologs. This contributed to efficient immunization of llamas with the human proteins CD70, MET, interleukin (IL)-1β and IL-6, resulting in large panels of functional antibodies. The in silico predicted human-homologous canonical folds of camelid-derived antibodies were confirmed by X-ray crystallography solving the structure of 2 selected camelid anti-CD70 and anti-MET antibodies. These antibodies showed identical fold combinations as found in the corresponding human germline V families, yielding binding site structures closely similar to those occurring in human antibodies. In conclusion, our results indicate that active immunization of camelids can be a powerful therapeutic antibody platform.

Site-specific conjugation of single domain antibodies to liposomes enhances photosensitizer uptake and photodynamic therapy efficacy

Photodynamic therapy for therapy-resistant cancers will greatly benefit from targeted delivery of tumor photosensitizing agents. In this study, a strategy for the site-specific conjugation of single domain antibodies onto liposomes containing the photosensitizer zinc phthalocyanine was developed and tested.

Combining somatic mutations present in different in vivo affinity-matured antibodies isolated from immunized Lama glama yields ultra-potent antibody therapeutics

Highly potent human antibodies are required to therapeutically neutralize cytokines such as interleukin-6 (IL-6) that is involved in many inflammatory diseases and malignancies. Although a number of mutagenesis approaches exist to perform antibody affinity maturation, these may cause antibody instability and production issues. Thus, a robust and easy antibody affinity maturation strategy to increase antibody potency remains highly desirable. By immunizing llama, cloning the 'immune' antibody repertoire and using phage display, we selected a diverse set of IL-6 antagonistic Fabs. Heavy chain shuffling was performed on the Fab with lowest off-rate, resulting in a panel of variants with even lower off-rate. Structural analysis of the Fab:IL-6 complex suggests that the increased affinity was partly due to a serine to tyrosine switch in HCDR2. This translated into neutralizing capacity in an in vivo model of IL-6 induced SAA production. Finally, a novel Fab library was designed, encoding all variations found in the natural repertoire of VH genes identified after heavy chain shuffling. High stringency selections resulted in identification of a Fab with 250-fold increased potency when re-formatted into IgG1. Compared with a heavily engineered anti-IL-6 monoclonal antibody currently in clinical development, this IgG was at least equally potent, showing the engineering process to have had led to a highly potent anti-IL-6 antibody.

Endocytosis of EGFR requires its kinase activity and N-terminal transmembrane dimerization motif

EGFR signaling is attenuated by endocytosis and degradation of receptor-ligand complexes in lysosomes. Endocytosis of EGFR is known to be regulated by multiple post-translational modifications. The observation that prevention of these modifications does not block endocytosis completely, suggests the involvement of other mechanism(s). Recently, receptor clustering has been suggested to induce internalization of multiple types of membrane receptors. However, the mechanism of clustering-induced internalization remains unknown. We have used biparatopic antibody fragments from llama (VHHs) to induce EGFR clustering without stimulating tyrosine kinase activity. Using this approach, we have found an essential role for the N-terminal GG4-like dimerization motif in the transmembrane domain (TMD) for clustering-induced internalization. Moreover, conventional EGF-induced receptor internalization depends exclusively on this TMD dimerization and kinase activity. Mutations in this dimerization motif eventually lead to reduced EGFR degradation and sustained signaling. We propose a novel role for the TMD dimerization motif in the negative-feedback control of EGFR. The widely conserved nature of GG4-like dimerization motifs in transmembrane proteins suggests a general role for these motifs in clustering-induced internalization.

Structural evaluation of EGFR inhibition mechanisms for nanobodies/VHH domains

The epidermal growth factor receptor (EGFR) is implicated in human cancers and is the target of several classes of therapeutic agents, including antibody-based drugs. Here, we describe X-ray crystal structures of the extracellular region of EGFR in complex with three inhibitory nanobodies, the variable domains of heavy chain only antibodies (VHH). VHH domains, the smallest natural antigen-binding modules, are readily engineered for diagnostic and therapeutic applications. All three VHH domains prevent ligand-induced EGFR activation, but use two distinct mechanisms. 7D12 sterically blocks ligand binding to EGFR in a manner similar to that of cetuximab. EgA1 and 9G8 bind an epitope near the EGFR domain II/III junction, preventing receptor conformational changes required for high-affinity ligand binding and dimerization. This epitope is accessible to the convex VHH paratope but inaccessible to the flatter paratope of monoclonal antibodies. Appreciating the modes of binding and inhibition of these VHH domains will aid in developing them for tumor imaging and/or cancer therapy.

Amphiregulin enhances regulatory T cell-suppressive function via the epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) is known to be critically involved in tissue development and homeostasis as well as in the pathogenesis of cancer. Here we showed that Foxp3(+) regulatory T (Treg) cells express EGFR under inflammatory conditions. Stimulation with the EGF-like growth factor Amphiregulin (AREG) markedly enhanced Treg cell function in vitro, and in a colitis and tumor vaccination model we showed that AREG was critical for efficient Treg cell function in vivo. In addition, mast cell-derived AREG fully restored optimal Treg cell function. These findings reveal EGFR as a component in the regulation of local immune responses and establish a link between mast cells and Treg cells. Targeting of this immune regulatory mechanism may contribute to the therapeutic successes of EGFR-targeting treatments in cancer patients.

Amphiregulin enhances regulatory T cell-suppressive function via the epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) is known to be critically involved in tissue development and homeostasis as well as in the pathogenesis of cancer. Here we showed that Foxp3(+) regulatory T (Treg) cells express EGFR under inflammatory conditions. Stimulation with the EGF-like growth factor Amphiregulin (AREG) markedly enhanced Treg cell function in vitro, and in a colitis and tumor vaccination model we showed that AREG was critical for efficient Treg cell function in vivo. In addition, mast cell-derived AREG fully restored optimal Treg cell function. These findings reveal EGFR as a component in the regulation of local immune responses and establish a link between mast cells and Treg cells. Targeting of this immune regulatory mechanism may contribute to the therapeutic successes of EGFR-targeting treatments in cancer patients.

Rapid visualization of human tumor xenografts through optical imaging with a near-infrared fluorescent anti-epidermal growth factor receptor nanobody

Given that overexpression of the epidermal growth factor receptor (EGFR) is found in many types of human epithelial cancers, noninvasive molecular imaging of this receptor is of great interest. A number of studies have employed monoclonal antibodies as probes; however, their characteristic long half-life in the bloodstream has encouraged the development of smaller probes. In this study, an anti-EGFR nanobody-based probe was developed and tested in comparison with cetuximab for application in optical molecular imaging. To this aim, the anti-EGFR nanobody 7D12 and cetuximab were conjugated to the near-infrared fluorophore IRDye800CW. 7D12-IR allowed the visualization of tumors as early as 30 minutes postinjection, whereas with cetuximab-IR, no signal above background was observed at the tumor site. Quantification of the IR-conjugated proteins in the tumors revealed ≈ 17% of injected dose per gram 2 hours after injection of 7D12-IR, which was significantly higher than the tumor uptake obtained 24 hours after injection of cetuximab-IR. This difference is associated with the superior penetration and distribution of 7D12-IR within the tumor. These results demonstrate that this anti-EGFR nanobody conjugated to the NIR fluorophore has excellent properties for rapid preclinical optical imaging, which holds promise for its future use as a complementary diagnostic tool in humans.

Tumor-targeted Nanobullets: Anti-EGFR nanobody-liposomes loaded with anti-IGF-1R kinase inhibitor for cancer treatment

The epidermal growth factor receptor (EGFR) is a validated target for anti-cancer therapy and several EGFR inhibitors are used in the clinic. Over the years, an increasing number of studies have reported on the crosstalk between EGFR and other receptors that can contribute to accelerated cancer development or even acquisition of resistance to anti-EGFR therapies. Combined targeting of EGFR and insulin-like growth factor 1 receptor (IGF-1R) is a rational strategy to potentiate anti-cancer treatment and possibly retard resistance development. In the present study, we have pursued this by encapsulating the kinase inhibitor AG538 in anti-EGFR nanobody-liposomes. The thus developed dual-active nanobody-liposomes associated with EGFR-(over)expressing cells in an EGFR-specific manner and blocked both EGFR and IGF-1R activation, due to the presence of the EGFR-blocking nanobody EGa1 and the anti-IGF-1R kinase inhibitor AG538 respectively. AG538-loaded nanobody-liposomes induced a strong inhibition of tumor cell proliferation even upon short-term exposure followed by a drug-free wash-out period. Therefore, AG538-loaded nanobody-liposomes are a promising anti-cancer formulation due to efficient intracellular delivery of AG538 in combination with antagonistic and downregulating properties of the EGa1 nanobody-liposomes.

ErbB1 dimerization is promoted by domain co-confinement and stabilized by ligand binding

The extent to which ligand occupancy and dimerization contribute to erbB1 signaling is controversial. To examine this, we utilized two-color Quantum Dot tracking for visualization of erbB1 homodimerization and quantification of the dimer off rate (k_off) on living cells. Kinetic parameters were extracted using a 3-state Hidden Markov Model to identify transition rates between free, co-confined, and dimerized states. We report that dimers composed of 2 ligand-bound receptors are long-lived and their k_off is independent of kinase activity. By comparison, unliganded dimers have >4-fold faster k_off. Transient co-confinement of receptors promotes repeated encounters and enhances dimer formation. Mobility decreases >6-fold when ligand-bound receptors dimerize. Blockade of erbB1 kinase activity or disruption of actin networks results in faster diffusion of receptor dimers. These results implicate both signal propagation and the cortical cytoskeleton in reduced mobility of signaling-competent erbB1 dimers.

A biparatopic anti-EGFR nanobody efficiently inhibits solid tumour growth

The epidermal growth factor receptor (EGFR) has been shown to be a valid cancer target for antibody-based therapy. At present, several anti-EGFR monoclonal antibodies have been successfully used, such as cetuximab and matuzumab. X-ray crystallography data show that these antibodies bind to different epitopes on the ecto-domain of EGFR, providing a rationale for the combined use of these two antibody specificities. We have previously reported on the successful isolation of antagonistic anti-EGFR nanobodies. In our study, we aimed to improve the efficacy of these molecules by combining nanobodies with specificities similar to both cetuximab and matuzumab into a single biparatopic molecule. Carefully designed phage nanobody selections resulted in two sets of nanobodies that specifically blocked the binding of either matuzumab or cetuximab to EGFR and that did not compete for each others' binding. A combination of nanobodies from both epitope groups into the biparatopic nanobody CONAN-1 was shown to block EGFR activation more efficiently than monovalent or bivalent (monospecific) nanobodies. In addition, this biparatopic nanobody potently inhibited EGF-dependent cell proliferation. Importantly, in an in vivo model of athymic mice bearing A431 xenografts, CONAN-1 inhibited tumour outgrowth with an almost similar potency as the whole mAb cetuximab, despite the fact that CONAN-1 is devoid of an Fc portion that could mediate immune effector functions. Compared to therapy using bivalent, monospecific nanobodies, CONAN-1 was clearly more potent in tumour growth inhibition. These results show that the rational design of biparatopic nanobody-based anticancer therapeutics may yield potent lead molecules for further development.

Facile labelling of an anti-epidermal growth factor receptor Nanobody with 68Ga via a novel bifunctional desferal chelate for immuno-PET

Purpose

The ∼15 kDa variable domains of camelid heavy-chain-only antibodies (called Nanobodies®) have the flexibility to be formatted as monovalent, monospecific, multivalent or multispecific single chain proteins with either fast or slow pharmacokinetics. We report the evaluation of the fast kinetic anti-epidermal growth factor receptor (EGFR) Nanobody 7D12, labelled with ⁶⁸Ga via the novel bifunctional chelate (BFC) p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS). Df-Bz-NCS has recently been introduced as the chelate of choice for ⁸⁹Zr immuno-positron emission tomography (PET).

Methods

Nanobody 7D12 was premodified with Df-Bz-NCS at pH 9. Radiolabelling with purified ⁶⁸Ga was performed at pH 5.0–6.5 for 5 min at room temperature. For in vitro stability measurements in storage buffer (0.25 M NaOAc with 5 mg ml⁻¹ gentisic acid, pH 5.5) at 4°C or in human serum at 37°C, a mixture of ⁶⁷Ga and ⁶⁸Ga was used. Biodistribution and immuno-PET studies of ⁶⁸Ga-Df-Bz-NCS-7D12 were performed in nude mice bearing A431 xenografts using ⁸⁹Zr-Df-Bz-NCS-7D12 as the reference conjugate.

Results

The Df-Bz-NCS chelate was conjugated to Nanobody 7D12 with a chelate to Nanobody molar substitution ratio of 0.2:1. The overall ⁶⁸Ga radiochemical yield was 55–70% (not corrected for decay); specific activity was 100–500 MBq/mg. Radiochemical purity of the conjugate was >96%, while the integrity and immunoreactivity were preserved. ^68/67Ga-Df-Bz-NCS-7D12 was stable in storage buffer as well as in human serum during a 5-h incubation period (<2% radioactivity loss). In biodistribution studies the ⁶⁸Ga-labelled Nanobody 7D12 showed high uptake in A431 tumours (ranging from 6.1 ± 1.3 to 7.2 ± 1.5%ID/g at 1–3 h after injection) and high tumour to blood ratios, which increased from 8.2 to 14.4 and 25.7 at 1, 2 and 3 h after injection, respectively. High uptake was also observed in the kidneys. Biodistribution was similar to that of the reference conjugate ⁸⁹Zr-Df-Bz-NCS-7D12. Tumours were clearly visualized in a PET imaging study.

Conclusion

Via a rapid procedure under mild conditions a ⁶⁸Ga-Nanobody was obtained that exhibited high tumour uptake and tumour to normal tissue ratios in nude mice bearing A431 xenografts. Fast kinetic ⁶⁸Ga-Nanobody conjugates can be promising tools for tumour detection and imaging of target expression.

Downregulation of EGFR by a novel multivalent nanobody-liposome platform

The epidermal growth factor receptor (EGFR) is a recognized target for tumor therapy and monoclonal antibodies (mAbs, e.g. cetuximab) have been developed to inhibit receptor activation. Besides blocking ligand (e.g. EGF) binding to the receptor, reports have shown that mAbs promote slow receptor internalization and degradation in lysosomes, i.e. downregulation. The efficacy of receptor downregulation was recently shown to depend on the size of receptor clusters formed at the cell surface. In this study, a multivalent platform is presented, consisting of nanobodies recognizing the ectodomain of EGFR (EGa1) coupled to PEG-liposomes, and the in vitro and in vivo effects of this system on EGFR internalization and downregulation were investigated. Nanobodies are the smallest functional antigen-binding immunoglobulin fragments and the EGa1 nanobody has been described as an EGFR-antagonist. EGa1-liposomes (EGa1-L) induced a more than 90% removal of EGFR from the cell surface, as a result of receptor internalization. Furthermore, this massive sequestration of EGFR mediated by EGa1-L lead to receptor degradation, while no degradation was detected with the monovalent nanobody. The downregulatory capacity here reported was found to be independent of the epitope on EGFR recognized by the grafted nanobody, and exclusive to the nanobody-liposomes, as anti-EGFR single chain variable fragments (scFv) coupled to liposomes were unable to induce this effect. Importantly, EGa1-L induced a significant inhibition of tumor cell proliferation, in vitro, an effect likely mediated by the combination of receptor downregulation and receptor antagonism. Also in vivo, EGFR downregulation was observed in tumors of mice intravenously injected with EGa1-L, indicating that this multivalent platform blocks ligand binding to the receptor and simultaneously induces the downregulation of EGFR.

Crosstalk between epidermal growth factor receptor- and insulin-like growth factor-1 receptor signaling: implications for cancer therapy

Both the epidermal growth factor receptor (EGFR) and the insulin-like growth factor-1 receptor (IGF-1R) can contribute to tumor development and -progression through their effects on cell proliferation, inhibition of apoptosis, angiogenesis, anchorage-independent growth and tumor-associated inflammation. EGFR-targeting monoclonal antibodies and small molecule tyrosine kinase inhibitors are currently in clinical use for the treatment of several types of cancer. However, primary and acquired resistance to these agents often occurs and thereby limits the clinical efficacy of mono-specific targeted therapy. Results from both in vitro and in vivo studies indicate that cross-talk between EGFR and IGF-1R can lead to acquired resistance against EGFR-targeted drugs. This review describes the interface between the EGFR and IGF-1R signaling networks and the implications of the extensive cross-talk between these two receptor systems for cancer therapy. EGFR and IGF-1R interact on multiple levels, either through a direct association between the two receptors, by mediating the availability of each others ligands, or indirectly, via common interaction partners such as G protein coupled receptors (GPCR) or downstream signaling molecules. This multi-layered cross-talk and its involvement in the induction of resistance to targeted therapies provide a clear rationale for dual targeting of EGFR and IGF-1R. We discuss several (potential) strategies to simultaneously inhibit EGFR and IGF-1R signaling as promising novel therapeutic approaches.

Selection and characterization of KDEL-specific VHH antibody fragments and their application in the study of ER resident protein expression

Several diseases are caused by defects in the protein secretory pathway of the cell, particularly in the endoplasmic reticulum (ER). These defects are manifested by the activation of the unfolded protein response (UPR) that involves the transcriptional up-regulation of several ER resident proteins, the down-regulation of protein translation and up-regulation of ER associated degradation (ERAD). Although this transcriptional up-regulation of ER resident proteins during ER stress has been well described, data on differential protein expression of these same proteins are hardly available. Tools that would enable the simultaneous analysis of this set of proteins would be of high importance. Since the C-terminal KDEL sequence is a conserved epitope present in a large set of ER resident proteins, an antibody directed against this sequence would be such a tool. Using a carefully designed selection strategy, VHH antibody fragments from a non-immune phage display library were isolated that recognize the KDEL sequence at the C-terminus of proteins, irrespective of the protein context. In an accepted in vitro model for ER stress, this antibody was shown to be an excellent tool to study differences in ER resident protein expression. Furthermore, the application of this antibody showed differences in ER resident protein levels during replicative senescence of human umbilical vein endothelial cells (HUVECs), underlining its significance in biological research. The selection strategy used to obtain these KDEL-specific antibodies opens up ways to select antibodies to other conserved epitopes, such as the nuclear localization signal (NLS) or the peroxisomal targeting sequence, permitting the simultaneous analysis of specific groups of proteins.

EGF induces coalescence of different lipid rafts

The suggestion that microdomains may function as signaling platforms arose from the presence of growth factor receptors, such as the EGFR, in biochemically isolated lipid raft fractions. To investigate the role of EGFR activation in the organization of lipid rafts we have performed FLIM analyses using putative lipid raft markers such as ganglioside GM1 and glycosylphosphatidylinositol (GPI)-anchored GFP (GPI-GFP). The EGFR was labeled using single domain antibodies from Llama glama that specifically bind the EGFR without stimulating its kinase activity. Our FLIM analyses demonstrate a cholesterol-independent colocalization of GM1 with EGFR, which was not observed for the transferrin receptor. By contrast, a cholesterol-dependent colocalization was observed for GM1 with GPI-GFP. In the resting state no colocalization was observed between EGFR and GPI-GFP, but stimulation of the cell with EGF resulted in the colocalization at the nanoscale level of EGFR and GPI-GFP. Moreover, EGF induced the enrichment of GPI-GFP in a detergent-free lipid raft fraction. Our results suggest that EGF induces the coalescence of the two types of GM1-containing microdomains that might lead to the formation of signaling platforms.

Nanobodies in therapeutic applications

Over the years, many antibodies have been successfully generated to treat patients with life-threatening diseases, most notably cancer. While the first generation of antibodies, originating from mice, caused severe side effects and were relatively inefficient, technological advances have made it possible to obtain fully human antibodies for therapeutic use. 'Heavy-chain only' antibodies have recently been discovered in the blood of camelids. Because of their size, the antigen-binding units of these antibodies comprising only a single Ig fold are called Nanobodies. These antibody fragments have several remarkable features that make them ideal candidates as next-generation cancer therapeutics. Particularly appealing is their ability to simultaneously inhibit various crucial growth factor receptors or their ligands with a single molecule. In addition, they are easy to clone and express on the tip of filamentous phage, which opens the possibility to select for Nanobodies inducing particular biological effects. Nanobodies have potential to become important cancer therapeutics in the near future, displaying unequalled and unprecedented efficacies in treatment.

Efficient inhibition of EGFR signaling and of tumour growth by antagonistic anti-EFGR Nanobodies

The development of a number of different solid tumours is associated with over-expression of ErbB1, or the epidermal growth factor receptor (EGFR), and this over-expression is often correlated with poor prognosis of patients. Therefore, this receptor tyrosine kinase is considered to be an attractive target for antibody-based therapy. Indeed, antibodies to the EGFR have already proven their value for the treatment of several solid tumours, especially in combination with chemotherapeutic treatment regimens. Variable domains of camelid heavy chain-only antibodies (called Nanobodies) have superior properties compared with classical antibodies in that they are small, very stable, easy to produce in large quantities and easy to re-format into multi-valent or multi-specific proteins. Furthermore, they can specifically be selected for a desired function by phage antibody display. In this report, we describe the successful selection and the characterisation of antagonistic anti-EGFR Nanobodies. By using a functional selection strategy, Nanobodies that specifically competed for EGF binding to the EGFR were isolated from "immune" phage Nanobody repertoires. The selected antibody fragments were found to efficiently inhibit EGF binding to the EGFR without acting as receptor agonists themselves. In addition, they blocked EGF-mediated signalling and EGF-induced cell proliferation. In an in vivo murine xenograft model, the Nanobodies were effective in delaying the outgrowth of A431-derived solid tumours. This is the first report describing the successful use of untagged Nanobodies for the in vivo treatment of solid tumours. The results show that functional phage antibody selection, coupled to the rational design of Nanobodies, permits the rapid development of novel anti-cancer antibody-based therapeutics.

Ubiquilin recruits Eps15 into ubiquitin-rich cytoplasmic aggregates via a UIM-UBL interaction

Eps15 and its related protein Eps15R are key components of the clathrin-mediated endocytic pathway. We searched for new binding partners of Eps15 using a yeast two-hybrid screen. We report here that ubiquilin (hPLIC1), a type-2 ubiquitin-like protein containing a ubiquitin-like domain (UBL) and a ubiquitin-associated domain (UBA), interacts with both Eps15 and Eps15R. Using glutathione-S-transferase pull-down experiments, we show that the first ubiquitin-interacting motif of Eps15 (UIM1) interacts directly with the UBL domain of ubiquilin, whereas it does not bind to ubiquitinated proteins. The second UIM of Eps15 (UIM2) binds poorly to the UBL domain but does bind to ubiquitinated proteins. Two other UIM-containing endocytic proteins, Hrs and Hbp, also interact with ubiquilin in a UIM-dependent manner, whereas epsin does not. Immunofluorescence analysis showed that endogenous Eps15 and Hrs, but not epsin, colocalize with green-fluorescent-protein-fused ubiquilin in cytoplasmic aggregates that are not endocytic compartments. We have characterized these green-fluorescent-protein-fused-ubiquilin aggregates as ubiquitin-rich intracytoplasmic inclusions that are recruited to aggresomes upon proteasome inhibition. Moreover, we show that endogenous Eps15 and endogenous ubiquilin colocalize to cytoplasmic aggregates and aggresomes. Finally, we show that the recruitment of Eps15 into ubiquilin-positive aggregates is UIM dependent. Altogether, our data identify ubiquilin as the first common UIM-binding partner of a subset of UIM-containing endocytic proteins. We propose that this UIM/UBL-based interaction is responsible for the sequestration of certain UIM-containing endocytic proteins into cytoplasmic ubiquitin-rich protein aggregates.

Regulation of Tiam1-Rac signalling

The GTPases of the Rho family are molecular switches that play an important role in a wide range of cellular processes and are increasingly implicated in tumourigenesis. Unlike what was found for the Ras oncogenes in tumours, hardly any activating mutations have been found in the genes encoding Rho proteins. In the past, we have identified Tiam1 (T-lymphoma invasion and metastasis) as a specific activator for the Rho-like GTPase Rac. In vivo, Tiam1 deficiency protects against Ras-induced skin carcinogenesis, underscoring the consequences of deregulated signalling for the onset and progression of tumours. Thus, an important level of regulation of signalling via the Rho-like GTPases comes from the specific control of their activators. In this paper, we review what is known on the specific regulation of Tiam1 signalling towards Rac.

Evidence for a bias toward intracellular antigens in the local humoral anti-tumor immune response of a colorectal cancer patient revealed by phage display

Many patients with colorectal carcinoma (CRC) mount a cellular as well as a humoral immune response to the tumor. To investigate the nature and specificity of the humoral immune response in a CRC patient, lymphocytes infiltrating the primary colorectal tumor and lymph nodes draining the tumor were used as antibody variable (V)-gene pools for the construction of phage antibody repertoires. These libraries were first validated by selection on the antigen tetanus toxoid and shown to contain antibodies that were probably derived from both naive and memory B cells. The repertoires were then screened for the presence of antibodies directed to CRC by selection on the cell line CaCo2. For comparison, the same selections were performed with a phage antibody repertoire made from B cells of healthy donors. Striking differences were observed in the panel of specificities selected from these different repertoires: although a large panel of antibodies reactive with patient-derived primary tumors was obtained from the immune repertoires, none of these discriminated between normal colonic epithelium and colon cancer and none were reactive with cell-surface antigens. However, selections using the non-immune library did result in numerous antibodies that recognized cell surface markers on CaCo2. These data suggest a bias in the local humoral immune response in this CRC patient, directed primarily toward intracellular epithelial-cell specific target antigens.

Identification of colon tumour-associated antigens by phage antibody selections on primary colorectal carcinoma

Immunotargeting of solid tumours using antibodies has become a valuable tool for the detection of cancer metastases and the treatment of minimal residual disease. However, only few tumour antigens useful for targeting have been described to date. To identify cell-surface targets on colorectal carcinoma (CRC), we selected a large, human phage antibody repertoire on freshly isolated colon tumour cells. Two antibodies were identified that reacted with epithelial cell-restricted cell-surface antigens, whereas one clone preferentially reacted with stromal cells. These antigens are tumour-associated antigens, as shown by their uniform expression in tumours of different patients and of different differentiation stages and by their limited expression on normal tissues. The pattern of reactivity in immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA) suggests that these antigens are different from previously identified tumour-associated antigens (e.g. Ep-CAM or c-ERB-2). This phage antibody-based method may lead to the cloning of novel tumour antigens that are useful for the immunotargeting of solid tumours.

In vitro characterisation of a monovalent and bivalent form of a fully human anti Ep-CAM phage antibody

Antibodies to tumour-associated antigens are increasingly being used as targeting vehicles for the visualisation and for therapy of human solid tumours. The epithelial cell adhesion molecule (Ep-CAM) is an antigen that is overexpressed on a variety of human solid tumours and constitutes an attractive target for immunotargeting. We set out to obtain fully human antibodies to this antigen by selecting from a large antibody repertoire displayed on bacteriophages. Two single-chain variable antibody fragments (scFv) were identified that specifically bound recombinant antigen in vitro. One of the selected antibodies (VEL-1) cross-reacted with extracellular matrix components in immunohistochemistry of colon carcinoma, whereas the other scFv (VEL-2) specifically recognised colon cancer cells. The latter antibody was further characterised with respect to epitope specificity and kinetics of antigen-binding. It showed no competition with the well-characterised anti Ep-CAM MOC-31 monoclonal antibody and had an off-rate of 5 x 10(-2) s-1. To obtain an antibody format more suitable for in vivo tumour targeting and to increase the apparent affinity through avidity, the genes of scFv VEL-2 were re-formatted by fusion to a human (gamma 1) hinge region and CH3 domain. This "minibody" was expressed in Escherichia coli, specifically bound the Ep-CAM antigen and showed a 20-fold reduced off-rate in surface plasmon resonance analysis. These results show that phage antibody selection, combined with antibody engineering, may result in fully human antibody molecules with promising characteristics for in vivo use in tumour targeting.

Design and application of diabodies, triabodies and tetrabodies for cancer targeting

Multivalent recombinant antibody fragments provide high binding avidity and unique specificity to a wide range of target antigens and haptens. This review describes the design and expression of diabodies, triabodies and tetrabodies using examples of scFv molecules that target viruses (influenza neuraminidase) and cancer (Ep-CAM; epithelial cell adhesion molecule). We discuss the preferred choice of linker length between V-domains to direct the formation of either diabodies (60 kDa), triabodies (90 kDa) or tetrabodies (120 kDa), each with size, flexibility and valency suited to different applications for in vivo imaging and therapy. The increased binding valency of these scFv multimers results in high avidity (low off-rates). A particular advantage for tumour targeting is that molecules of 60-100 kDa have increased tumour penetration and fast clearance rates compared to the parent Ig (150 kDa). We highlight a number of cancer-targeting scFv multimers that have recently successfully undergone pre-clinical trials for in vivo stability and efficacy. We also review the design of multi-specific Fv modules suited to cross-link two or more different target antigens. These bi- and tri-specific multimers can be formed by association of different scFv molecules and, in the first examples, have been designed as cross-linking reagents for T-cell recruitment into tumours (immunotherapy), viral retargeting (gene therapy) and as red blood cell agglutination reagents (immunodiagnostics).

Human anti-CD30 recombinant antibodies by guided phage antibody selection using cell panning

In various clinical studies, Hodgkin's patients have been treated with anti-CD30 immunotherapeutic agents and have shown promising responses. One of the problems that appeared from these studies is the development of an immune response against the nonhuman therapeutics, which limits repeated administration and reduces efficacy. We have set out to make a recombinant, human anti-CD30 single-chain variable fragment (scFv) antibody, which may serve as a targeting moiety with reduced immunogenicity and more rapid tumour penetration in similar clinical applications. Rather than selecting a naive phage antibody library on recombinant CD30 antigen, we used guided selection of a murine antibody in combination with panning on the CD30-positive cell line L540. The murine monoclonal antibody Ki-4 was chosen as starting antibody, because it inhibits the shedding of the extracellular part of the CD30 antigen. This makes the antibody better suited for CD30-targeting than most other anti-CD30 antibodies. We have previously isolated the murine Ki-4 scFv by selecting a mini-library of hybridoma-derived phage scFv-antibodies via panning on L540 cells. Here, we report that phage display technology was successfully used to obtain a human Ki-4 scFv version by guided selection. The murine variable heavy (VH) and light (VL) chain genes of the Ki-4 scFv were sequentially replaced by human V gene repertoires, while retaining only the major determinant for epitope-specificity: the heavy-chain complementarity determining region 3 (CDR3) of murine Ki-4. After two rounds of chain shuffling and selection by panning on L540 cells, a fully human anti-CD30 scFv was selected. It competes with the parental monoclonal antibody Ki-4 for binding to CD30, inhibits the shedding of the extracellular part of the CD30 receptor from L540 cells and is thus a promising candidate for the generation of anti-CD30 immunotherapeutics.

Guided selection of a pan carcinoma specific antibody reveals similar binding characteristics yet structural divergence between the original murine antibody and its human equivalent

Antibody engineering provides an excellent tool for the generation of human immunotherapeutics for the targeted treatment of solid tumours. We have engineered and selected a completely human antibody to epithelial glycoprotein-2 (EGP-2), a transmembrane glycoprotein present on virtually all human simple epithelia and abundantly expressed on a variety of human carcinomas. We chose to use the procedure of "guided selection" to rebuild a high-affinity murine antibody into a human antibody, using two consecutive rounds of variable domain shuffling and phage library selection. As a starting antibody, the murine antibody MOC-31 was used. After the first round of guided selection, where the V(H) of MOC-31 was combined in Fab format with a human V(L)C(L) library, a small panel of human light chains was identified, originating from a segment of the VkappaIII family, whereas the MOC-31 V(L) is more homologous to the VkappaII family. Nevertheless, one of the chimaeric Fabs, C3, displayed an off-rate similar to MOC-31 scFv. Combining the V(L) of C3 with a human V(H) library, while retaining the V(H) CDR3 of MOC-31, clones were selected using human V(H) genes originating from the rarely used V(H)7 family. The best clone, 9E, shows over 13 amino acid mutations from the germline sequence, has an off-rate comparable to the original antibody and specifically binds to the "MOC-31"-epitope on EGP-2 in specificity and competition ELISA, FACS analysis and immunohistochemistry. In both V(L) and V(H) of antibody 9E, three germline mutations were found creating the MOC-31 homologue residue. Structural modelling of both murine and human antibodies reveals that one of the germline mutations, 53Y in V(H) CDR2, is likely to be involved in antigen binding. We conclude that, although they may bind the same epitope and have similar binding affinity to the antigen as the original murine antibody, human antibodies derived by guided selection unlike CDR-grafted antibodies, may retain only some of the original key elements of the binding site chemistry. The selected human anti-EGP-2 antibody will be a suitable reagent for tumour targeting.

Model systems to study the parameters determining the success of phage antibody selections on complex antigens

Phage antibody display technology offers a powerful tool for the isolation of specific antibodies to defined target antigens. Most selection strategies described to date have relied on the availability of purified and often recombinant antigen, providing the possibility to perform selections on a well-defined antigen source. However, when the target antigen cannot be purified (e.g., an integral membrane protein), or if the antigen is unknown (e.g., when searching for novel markers on cells or tissues), panning of phage antibody libraries has to be performed on complex antigen sources such as cell surfaces or tissue sections, or even by in vivo selection methods. This provides a series of technical and experimental challenges. One focus of our research is to select antibodies directed to novel cancer-induced antigens expressed by tumours and by the tumour vasculature. To understand the parameters governing selection on complex antigen sources and to assess the efficiency of these phage library selections, we have set up two model selection systems in which both tumour cells and vascular endothelial cells serve as target "antigen". We describe a model based on phage antibodies directed to the tumour antigen epithelial glycoprotein-2, to compare phage antibody selections on a range of different antigen sources including purified and recombinant antigen, whole live cells, tissue cryosections and in vivo grown solid tumours. Secondly, we describe a model based on a phage antibody directed against the endothelial cell inducible adhesion molecule E-selectin. We compare selections on cultured cell monolayers with selections on cell suspensions immobilised on columns, to determine which selection approach is most suitable for the identification of novel tumour endothelial cell markers. Our data provide insight into the efficiency and thus potency of different selection strategies and show that there are very large differences in the recovery and enrichment of binding phage between the different methods tested. Our results further demonstrate the feasibility of phage antibody selections on whole, intact cells and show that these may sometimes compare favourably to selections on purified antigen. Selections on endothelial cells immobilised on columns compare favourably with selections on cell-monolayers; the most favourable conditions for both selection procedures are described. The implications of our data for phage antibody selections on these different complex antigen sources using either non-immune or immune phage antibody repertoires are discussed. The use of model systems such as the ones described here will help to determine optimal experimental conditions for phage library selections on complex antigens and aid in developing more powerful selection procedures for target discovery.

A large non-immunized human Fab fragment phage library that permits rapid isolation and kinetic analysis of high affinity antibodies

We report the design, construction, and use of the first very large non-immunized phage antibody library in Fab format, which allows the rapid isolation and affinity analysis of antigen-specific human antibody fragments. Individually cloned heavy and light chain variable region libraries were combined in an efficient two-step cloning procedure, permitting the cloning of a total of 3.7 x 10(10) independent Fab clones. The performance of the library was determined by the successful selection of on average 14 different Fabs against 6 antigens tested. These include tetanus toxoid, the hapten phenyl-oxazolone, the breast cancer-associated MUC1 antigen, and three highly related glycoprotein hormones: human chorionic gonadotropin, human luteinizing hormone, and human follicle-stimulating hormone. In the latter category, a panel of either homone-specific or cross-reactive antibodies were identified. The design of the library permits the monitoring of selections with polyclonal phage preparations and to carry out large scale screening of antibody off-rates with unpurified Fab fragments on BIAcore. Antibodies with off-rates in the order of 10(-2) to 10(-4) s-1 and affinities up to 2.7 nM were recovered. The kinetics of these phage antibodies are of the same order of magnitude as antibodies associated with a secondary immune response. This new phage antibody library is set to become a valuable source of antibodies to many different targets, and to play a vital role in target discovery and validation in the area of functional genomics.

An anti-CD30 single-chain Fv selected by phage display and fused to Pseudomonas exotoxin A (Ki-4(scFv)-ETA') is a potent immunotoxin against a Hodgkin-derived cell line

The human CD30 receptor is highly overexpressed on the surface of Hodgkin Reed-Sternberg cells and has been shown to be an excellent target for selective immunotherapy using monoclonal antibody-based agents such as immunotoxins. To construct a new recombinant immunotoxin for possible clinical use in patients with Hodgkin's lymphoma, we have chosen the murine anti-CD30 hybridoma Ki-4 to generate a high-affinity Ki-4 single-chain variable fragment (scFv). Hybridoma V-genes were polymerase chain reaction-amplified, assembled, cloned and expressed as a mini-library for display on filamentous phage. Functional Ki-4 scFv were obtained by selection of binding phage on the Hodgkin lymphoma-derived, CD30-expressing cell line L540Cy. The selected recombinant Ki-4 scFv was shown to specifically bind to an overlapping epitope on the CD30 antigen with binding kinetics similar to those of the original antibody. The Ki-4 scFv was subsequently fused to a deletion mutant of Pseudomonas exotoxin A (ETA'). The resulting immunotoxin Ki-4(scFv)-ETA' specifically binds to CD30+ L540Cy cells and inhibits the protein synthesis by 50% at a concentration (IC50) of 43 pM. This recombinant immunotoxin is a promising candidate for further clinical evaluation in patients with Hodgkin's lymphoma or other CD30+ malignancies.

High-affinity recombinant phage antibodies to the pan-carcinoma marker epithelial glycoprotein-2 for tumour targeting

The tumour-associated antigen epithelial glycoprotein-2 (EGP-2) is a promising target for detection and treatment of a variety of human carcinomas. Antibodies to this antigen have been successfully used in patients for imaging of small-cell lung cancer and for adjuvant treatment of minimal residual disease of colon cancer. We describe here the isolation and complete characterization of high-affinity single-chain variable fragments (scFv) to the EGP-2 antigen. First, the binding kinetics of four murine whole antibodies directed to EGP-2 (17-1A, 323/A3, MOC-31 and MOC-161) were determined using surface plasmon resonance (SPR). The MOC-31 antibody has the lowest apparent off-rate, followed by MOC-161 and 323/A3. The V-genes of the two MOC hybridomas were cloned as scFv in a phage display vector and antigen-binding phage were selected by panning on recombinant antigen. The scFvs compete with the original hybridoma antibodies for binding to antigen and specifically bind to human carcinomas in immunohistochemistry. MOC-31 scFv has an off-rate which is better than those of the bivalent 17-1A and 323/A3 whole antibodies, providing it with an essential characteristic for tumour retention in vivo. The availability of these high-affinity anti-EGP-2 antibody fragments and of their encoding V-genes creates a variety of possibilities for their future use as tumour-targeting vehicles.

Antibody phage display technology and its applications

In recent years, the use of display vectors and in vitro selection technologies has transformed the way in which we generate ligands, such as antibodies and peptides, for a given target. Using this technology, we are now able to design repertoires of ligands from scratch and use the power of phage selection to select those ligands having the desired (biological) properties. With phage display, tailor-made antibodies may be synthesized and selected to acquire the desired affinity of binding and specificity for in vitro and in vivo diagnosis, or for immunotherapy of human disease. This review addresses recent progress in the construction of, and selection from phage antibody libraries, together with novel approaches for screening phage antibodies. As the quality of large naïve and synthetic antibody repertoires improves and libraries becomes more generally available, new and exciting applications are pioneered such as the identification of novel antigens using differential selection and the generation of receptor a(nta)gonists. A combination of the design and generation of millions to billions of different ligands, together with phage display for the isolation of binding ligands and with functional assays for identifying (and possibly selecting) bio-active ligands, will open even more challenging applications of this inspiring technology, and provide a powerful tool for drug and target discovery well into the next decade.

Construction and characterization of a bispecific diabody for retargeting T cells to human carcinomas

We describe the construction of a recombinant bispecific antibody fragment in the diabody format with specificity for both the well-established human pancarcinoma associated target antigen EGP2 (epithelial glycoprotein 2, also known as the CO17-1A antigen or KSA) and the CD3epsilon chain of human TCR/CD3 complex. The murine anti-EGP2 (MOC31) single chain variable fragment (scFv) and the humanized anti-CD3 (Ucht1v9) scFv were cast into a diabody format (designated Dia5v9) using a short 5 amino acid Gly-Ser linker between immunoglobulin heavy-chain and light-chain variable domains. Purification of the poly-histidine tagged Dia5v9 was achieved from extracts of protease deficient Escherichia coli by IMAC chromatography. The Dia5v9 diabody showed strong binding to both EGP2 and CD3 in transfected cells. The in vitro efficacy of Dia5v9 in mediating tumor cell lysis by interleukin-2 activated human T cells appeared to be similar to that of the hybrid-hybridoma-derived BsF(ab')2 Bis1 (anti-EGP2/anti-CD3) in a standard 4-hr 51Cr-release assay. This small and partially humanized recombinant bispecific antibody fragment may be valuable for T-cell-based immunotherapeutical treatment protocols, retargeting activated peripheral blood T lymphocytes to lyse various human carcinomas in vivo.