Pentamerization of single-domain antibodies from phage libraries: a novel strategy for the rapid generation of high-avidity antibody reagents

We describe a novel type of molecule in which single-domain antibodies (sdAbs) isolated from a nai;ve llama single domain antibody library are linked to an oligomerization domain to generate high-avidity, antigen-binding reagents. An sdAb is fused to the B-subunit of Escherichia coli verotoxin, or shiga-like toxin, which self-assembles to form a homopentamer and results in simultaneous sdAb pentamerization and introduction of avidity. Molecular modeling indicated that this fusion protein (PDB: 1OJF), termed pentabody, has structural flexibility for binding to surface-presented antigen. In the instance of an sdAb specific for a peptide antigen, pentamerization resulted in a dramatic increase in functional affinity for immobilized antigen. The pentabody was expressed in high yield in E.coli in a non-aggregated state, and exhibited excellent thermostability and protease resistance. This technology provides a relatively rapid means of generating novel antigen-binding molecules that bind strongly to immobilized antigen. It is expected that pentavalent sdAbs will have general applicability in proteomics, immunochemical staining, cancer diagnosis and other applications in which antigens are presented multivalently.     

癌胚抗原单链抗体在细菌表面的展示及其应用

癌胚抗原(carcinoembryonic antigen, CEA)作为肿瘤标志物,在临床肿瘤诊断及肿瘤靶向治疗等方面具有重要应用价值。癌胚抗原单链抗体（CEA-specific single chain antibody fragments , CEA-scFv）能够特异性结合癌胚抗原。本研究将癌胚抗原单链抗体展示于大肠杆菌细胞表面,分析其作为癌胚抗原检测平台和细菌靶向载体的可行性。首先将CEA-scFv基因克隆到表面展示载体pBAD-OmpA-mCherry 中,经酶切和测序证实,成功构建了重组质粒pBAD-OmpA-mCherry-CEA。重组菌经阿拉伯糖诱导后可检测到红色荧光,荧光强度在胰酶的作用下降低,全菌ELISA检测呈阳性反应,提示融合蛋白和目的蛋白质在重组菌表面展示成功。由Western印迹分析可知,融合蛋白的相对分子质量约为85 kD,符合预期设计。进一步的研究提示,重组菌在大肠杆菌表面展示的癌胚抗原单链抗体具有生物活性,能够有效结合A549细胞裂解液的癌胚抗原。在细菌侵染细胞实验中,与对照组相比,重组菌孵育A549细胞后细胞内可明显观察到点状红色荧光,证明重组菌能够靶向侵入癌胚抗原阳性肿瘤细胞。本研究为癌胚抗原相关的快速诊断和基于细菌载体的肿瘤靶向治疗奠定了实验基础。     

Carcinoembryonic antigen (CEA) mimicry by an anti-idiotypic scFv isolated from anti-Id 6.C4 hybridoma

Since carcinoembryonic antigen (CEA) is expressed during embryonic life, it is not immunogenic in humans. The use of anti-idiotypic (Id) antibodies as a surrogate of antigen in the immunization has been considered a promising strategy for breaking tolerance to some tumor associated antigens. We have described an anti-Id monoclonal antibody (MAb), designated 6.C4, which is able to mimic CEA functionally. The anti-Id MAb 6.C4 was shown to elicit antibodies that recognized CEA in vitro and in vivo. In the present study, we sought to verify whether a single chain (scFv) antibody obtained, the scFv 6.C4, would retain the ability to mimic CEA. Two scFv containing the variable heavy and light chain domains of 6.C4 were constructed with a 15-amino acid linker: one with and another without signal peptide. DNA immunization of mice with both forms of scFv individually elicited antibodies able to recognize CEA.     

Discovery of T cell antigens by high-throughput screening of synthetic minigene libraries

The identification of novel T cell antigens is central to basic and translational research in autoimmunity, tumor immunology, transplant immunology, and vaccine design for infectious disease. However, current methods for T cell antigen discovery are low throughput, and fail to explore a wide range of potential antigen-receptor interactions. To overcome these limitations, we developed a method in which programmable microarrays are used to cost-effectively synthesize complex libraries of thousands of minigenes that collectively encode the content of hundreds of candidate protein targets. Minigene-derived mRNA are transfected into autologous antigen presenting cells and used to challenge complex populations of purified peripheral blood CD8+ T cells in multiplex, parallel ELISPOT assays. In this proof-of-concept study, we apply synthetic minigene screening to identify two novel pancreatic islet autoantigens targeted in a patient with Type I Diabetes. To our knowledge, this is the first successful screen of a highly complex, synthetic minigene library for identification of a T cell antigen. In principle, responses against the full protein complement of any tissue or pathogen can be assayed by this approach, suggesting that further optimization of synthetic libraries holds promise for high throughput antigen discovery.     

Migration and invasion of NSCLC suppressed by the downregulation of Src/focal adhesion kinase using single,double and tetra domain anti- CEACAM6 antibodies

Carcinoembryonic antigen-related cell adhesion molecules 6 (CEACAM6) is a cell adhesion receptor. Expression of CEACAM6 in non-small cell lung cancer (NSCLC) associated with tumor progression and metastatic condition via Src/FAK signaling pathway. We established three anti-CEACAM6 antibodies with valences, which were designed to be monomeric sdAb, bivalent sdAb (2Ab), and tetravalent sdAb (4Ab). The anti-CEACAM6 antibodies can be used to target CEACAM6 overexpressing NSCLC. Anti-CEACAM6 antibodies, sdAb, 2Ab and 4Ab, were modified with different valency via protein engineering. sdAb and multivalent sdAbs (2Ab & 4Ab) were expressed and purified from E.coli and CHO cells, respectively. We compared the effect of anti-CEACAM6 antibodies with doxorubicin in NSCLC cell line both in vitro and in vivo. The 4Ab showed significant effect on cell viability. In addition, A549 cells treated with 2Ab and 4Ab inhibited the invasion and migration. In western blot, the 2Ab and 4Ab showed significant inhibition of phospho FAK domain Ty397 that is essential for activation of Src kinase family. Meanwhile, overall protein analysis revealed that 2Ab and 4Ab potently inhibited the phosphorylation of pSRC, pERK, pFAK, pAKT, MMP-2, MMP-9 and N-cadherin. Anti-tumor effect was observed in an A549 NSCLC xenograft model treated with 2Ab or 4Ab compared with doxorubicin. Confocal analysis showed higher targeting ability of 4Ab than that of 2Ab at 4 h incubation. Our data suggests that 2Ab and 4Ab inhibits EMT-mediated migration and invasion via suppression of Src/FAK signaling, which exhibits therapeutic efficiency for NSCLC treatment.     

Monoclonal antibodies for carcinoembryonic antigen (CEA) as a model system: identification of two novel CEA-related antigens in meconium and colorectal carcinoma tissue by Western blots and differential immunoaffinity chromatography

In a previous study, five monoclonal antibodies against the carcinoembryonic antigen (CEA) with different epitope specificities were delineated. One of these antibodies which exhibits a high affinity for CEA binds to different carcinoma tissues, to liver tissue, and to granulocytes. This antibody was selected for the immunoaffinity purification of CEA and related antigens from colorectal carcinoma tissue, from spleen tissues, from bile, and from meconium. After elution from the immunosorbent, the antigens were separated by SDS-PAGE, were transferred to nitrocellulose, and were incubated with the five different antibodies. Antibody T84.1 bound to the following antigens: 177 kD and 128 kD from colonic carcinoma, 81 kD from bile, 49 kD from spleen, as well as 165 kD and 100 kD from meconium. Two additional antibodies showed a similar binding pattern. The fourth antibody (CEA.11) bound to the 165 kD meconium antigen and to the two colorectal carcinoma antigens. The fifth antibody (T84.66) showed a strong reaction with the 177 kD colorectal carcinoma antigen and a faint reaction with a 183 kD antigen in meconium. As judged from m.w. and immunochemical properties, the 128 kD colorectal carcinoma antigen and the 100 kD meconium antigen are two novel CEA-related antigens. Because antibody CEA.11 did not bind to the 100 kD meconium antigen in Western blots, the 165 kD antigen could be eluted from a CEA.11 immunosorbent without contamination by the 100 kD antigen. Similarly, as predicted from the binding pattern in the Western blots, the two colorectal carcinoma antigens were separated from each other by a T84.66 immunosorbent.     

Serological identification and bioinformatics analysis of immunogenic antigens in multiple myeloma

Identifying appropriate tumor antigens is critical to the development of successful specific cancer immunotherapy. Serological analysis of tumor antigens by a recombinant cDNA expression library (SEREX) allows the systematic cloning of tumor antigens recognized by the spontaneous autoantibody repertoire of cancer patients. We applied SEREX to the cDNA expression library of cell line HMy2, which led to the isolation of six known characterized genes and 12 novel genes. Known genes, including ring finger protein 167, KLF10, TPT1, p02 protein, cDNA FLJ46859 fis, and DNMT1, were related to the development of different tumors. Bioinformatics was performed to predict 12 novel MMSA (multiple myeloma special antigen) genes. The prediction of tumor antigens provides potential targets for the immunotherapy of patients with multiple myeloma (MM) and help in the understanding of carcinogenesis. Crude lysate ELISA methodology indicated that the optical density value of MMSA-3 and MMSA-7 were significantly higher in MM patients than in healthy donors. Furthermore, SYBR Green real-time PCR showed that MMSA-1 presented with a high number of copy messages in MM. In summary, the antigens identified in this study may be potential candidates for diagnosis and targets for immunotherapy in MM.     

High-affinity binding measurements of antibodies to cell-surface-expressed antigens

A simple method that allows affinity measurements of antibodies to integral membrane proteins is described. Kinetic Exclusion Assay was used to determine the concentration of free antibody that remains in solution after equilibrium has been established between the antibody and the cell-surface-expressed antigen, from which the equilibrium dissociation constant (Kd) was determined. It eliminates the requirement for soluble antigen and modifications such as radio-labeling or fluorescent labeling of the antibody. For one of the cell-surface-expressed antigens, it was determined that the affinity of the antibody to the cell-surface-expressed antigen was similar to that of the purified, soluble form of the antigen. In addition to the simplicity of the approach, the method provides a true measure of the affinity/avidity of the antibody to the native form of cell-surface-expressed targets, including antigens that cannot be produced in soluble forms, and to unknown cell surface antigens.     

A Novel Platform for the Potentiation of Therapeutic Antibodies Based on Antigen-Dependent Formation of IgG Hexamers at the Cell Surface

IgG antibodies can organize into ordered hexamers on cell surfaces after binding their antigen. These hexamers bind the first component of complement C1 inducing complement-dependent target cell killing. Here, we translated this natural concept into a novel technology platform (HexaBody technology) for therapeutic antibody potentiation. We identified mutations that enhanced hexamer formation and complement activation by IgG1 antibodies against a range of targets on cells from hematological and solid tumor indications. IgG1 backbones with preferred mutations E345K or E430G conveyed a strong ability to induce conditional complement-dependent cytotoxicity (CDC) of cell lines and chronic lymphocytic leukemia (CLL) patient tumor cells, while retaining regular pharmacokinetics and biopharmaceutical developability. Both mutations potently enhanced CDC- and antibody-dependent cellular cytotoxicity (ADCC) of a type II CD20 antibody that was ineffective in complement activation, while retaining its ability to induce apoptosis. The identified IgG1 Fc backbones provide a novel platform for the generation of therapeutics with enhanced effector functions that only become activated upon binding to target cell–expressed antigen.     

Convergent potency of internalized gelonin immunotoxins across varied cell lines, antigens, and targeting moieties

Gelonin-based immunotoxins vary widely in their cytotoxic potency as a function of antigen density, target cell internalization and trafficking kinetics, and conjugate properties. We have synthesized novel gelonin immunotoxins using two different binding scaffold types (single-chain antibody variable fragments and fibronectin domains) targeting two different tumor antigens (carcinoembryonic antigen and EGF receptor). Constructs were characterized using an antigen-negative cell line (HT-1080), cell lines positive for each antigen (HT-1080(CEA) for carcinoembryonic antigen and A431 for EGF receptor), and a cell line positive for both antigens (HT-29). Immunotoxins exhibited K(d) values between 8 and 15 nm and showed 20-2000-fold enhanced cytotoxicity compared with gelonin (IC(50) ～ 0.25-30 nM versus 500 nM). Using quantitative fluorescence flow cytometry, we measured internalization of gelonin (via pinocytosis) and gelonin-based immunotoxins (via antigen-dependent, receptor-mediated endocytosis). Results were matched with cytotoxicity measurements made at equivalent concentration and exposures. Unexpectedly, when matched internalization and cytotoxicity data were combined, a conserved internalized cytotoxicity curve was generated that was common across experimental conditions. Considerable variations in antigen expression, trafficking kinetics, extracellular immunotoxin concentration, and exposure time were all found to collapse to a single potency curve on the basis of internalized immunotoxin. Fifty percent cytotoxicity occurred when ～ 5 × 10(6) toxin molecules were internalized regardless of the mechanism of uptake. Cytotoxicity observed at a threshold internalization was consistent with the hypothesis that endosomal escape is a common, highly inefficient, rate-limiting step following internalization by any means tested. Methods designed to enhance endosomal escape might be utilized to improve the potency of gelonin-based immunotoxins.