Polymerase chain reaction facilitates the cloning, CDR-grafting, and rapid expression of a murine monoclonal antibody directed against the CD18 component of leukocyte integrins

Phage display derived human monoclonal antibodies isolated by binding to the surface of live primary breast cancer cells recognize GRP78

Clinical trials using monoclonal antibodies (mAb) against cell-surface markers have yielded encouraging therapeutic results in several cancer types. Generally, however, anticancer antibodies are only efficient against a subpopulation of cancers, and there is a strong need for identification of novel targets and human antibodies against them. We have isolated single-chain human mAbs from a large naïve antibody phage display library by panning on a single-cell suspension of freshly isolated live cancer cells from a human breast cancer specimen, and these antibodies were shown to specifically recognize cancer-associated cell-surface proteins. One of the isolated human antibody fragments, Ab39, recognizes a cell-surface antigen expressed on a subpopulation of cancer cell lines of different origins. Immunohistochemical analysis of a large panel of cancerous and normal tissues showed that Ab39 bound strongly to several cancers, including 45% breast carcinomas, 35% lung cancers, and 86% melanomas, but showed no or weak binding to normal tissues. A yeast two-hybrid screen of a large human testis cDNA library identified the glucose-regulated protein of 78 kDa (GRP78) as the antigen recognized by Ab39. The interaction was confirmed by colocalization studies and antibody competition experiments that also mapped the epitope recognized by Ab39 to the COOH terminus of GRP78. The expression of GRP78 on the surface of cancer cells, but not normal cells, makes it an attractive target for cancer therapies including mAb-based immunotherapy. Our results suggest that the human antibody Ab39 may be a useful starting point for further genetic optimization that could render it a useful diagnostic and therapeutic reagent for a variety of cancers.

Humanization of chicken monoclonal antibody using phage-display system

We describe a simple method for humanizing chicken monoclonal antibody (mAb). Humanization of mAbs by simple CDR-grafting often results in loss of affinity because certain framework residues of the antibody variable regions can participate in antigen-antibody interaction. In this study, humanization of chicken mAbs was achieved by CDR-grafting, followed by framework fine-tuning using a chicken phage-displayed mAb, phAb4-31, as a model antibody. In order to fine-tune the framework, we used the phage-displayed combinatorial library with permutation of important framework residues. After panning the humanized library, the "most humanized" variants were selected and analyzed for antigen-binding activity. All of these clones retained affinity comparable to the parental chicken mAb. These results suggest that chicken mAbs can easily be humanized, and thus humanized chicken mAbs may be practically applied as therapeutic agents.

Laminin modulates morphogenic properties of the collagen XVIII endostatin domain

We have shown previously that the oligomeric endostatin domain of collagen XVIII (NC1) functioned as a motility-inducing factor regulating the extracellular matrix-dependent morphogenesis of endothelial cells. This motogenic activity gave rise to structures resembling filipodia and lamellipodia and was dependent on Rac, Cdc42, and mitogen-activated protein kinase. Here, we demonstrate that these properties of endostatin are primarily mediated by laminin in the basement membrane and heparan sulfates on the cell surface. The sites of interaction between laminin and oligomeric endostain include the N-terminal regions of all three laminin chains (amino acids 204-1243 of the alpha chain, 932-1161 of the beta chain, and 150-965 of the gamma chain). A monoclonal antibody that blocks the interactions between endostatin and laminin was utilized to inhibit the motogenic activity of endostatin. In parallel, we have engineered selective point mutations and produced recombinant forms that lack binding to heparan sulfates on the cell surface. Our data are consistent with a model of endostatin with two binding sites: one mainly to laminin in the basement membrane and the other to heparan sulfates on the cell surface. The two binding domains on endostatin appear to be separate with the possibility of some overlap between the two sites.

Phage-display selection of a human single-chain fv antibody highly specific for melanoma and breast cancer cells using a chemoenzymatically synthesized G(M3)-carbohydrate antigen

Overexpression of the cell-surface glycosphingolipid G(M3) is associated with a number of different cancers, including those of the skin, colon, breast, and lung. Antibodies against the G(M3) epitope have potential application as therapeutic agents in the treatment of these cancers. We describe the chemoenzymatic synthesis of two G(M3)-derived reagents and their use in the panning of a phage-displayed human single-chain Fv (scFv) antibody library derived from the blood of cancer patients. Three scFv-phage clones, GM3A6, GM3A8, and GM3A15, were selected for recombinant expression and were characterized using BIAcore and flow cytometry. BIAcore measurements using the purified, soluble scFvs yielded dissociation constants (K(d)) ranging from 4.2 x 10(-7) to 2.1 x 10(-5) M. Flow cytometry was used to evaluate the ability of each scFv to discriminate between normal human cells (human dermal fibroblast, HDFa), melanoma cells (HMV-1, M21, and C-8161), and breast cancer cells (BCM-1, BCM-2, and BMS). GM3A6 displayed cross-reactivity with normal cells, as well as tumor cells, and GM3A15 possessed little or no binding activity toward any of the cell lines tested. However, GM3A8 bound to five of the six tumor cell lines and showed no measurable reactivity against the HDFa cells. Hence, we have demonstrated that a synthetic G(M3) panning reagent can be used to isolate a fully human scFv that is highly specific for native G(M3) on the surface of tumor cells. The result is a significant step toward effective immunotherapies for the treatment of cancer.

A humanized monoclonal antibody constructed from intronless expression vectors targets human hepatocellular carcinoma cells

An anti-human hepatocellular carcinoma (HCC) monoclonal antibody, hHP-1, was genetically humanized from a murine monoclonal antibody. In this study, a concept of positional template approach was applied to design the amino acid sequence of hHP-1's variable region, and synthetic DNA fragments for protein expression were produced through overlapping PCR from single strand oligonucleotides. Synthetic DNA fragments and human antibody constant region cDNA were used to construct two CMV promotor-based expression vectors for the antibody light and heavy chains, in which the variable region was connected directly to the constant region without an intron sequence. Completely assembled humanized antibody was successfully expressed in mammalian cells as IgG1 kappa molecules and purified using protein A affinity column. The immunogenicity of the hHP1 was estimated by the amino acid sequence and determined through a HAMA (human anti-murine antibody) serum reaction assay. Results indicated that the immunogenicity of hHP-1 was significantly reduced. In vitro binding activity assay showed that the hHP-1 had retained its binding function to a human HCC SMMC-7721 cell-line, without cross binding to other human normal tissues. Immunofluorescence staining showed that hHP-1 had a strong binding activity to SMMC cells. A competitive binding assay showed that the relative binding activity of hHP-1 was approximately 25% binding activity of the original murine antibody. Our results indicate that a humanized antibody could be produced using intronless vectors and expressed as a complete IgG1 kappa antibody. Hence we believe that hHP-1 could be a potential candidate for HCC treatment.

A single backmutation in the human kIV framework of a previously unsuccessfully humanized antibody restores the binding activity and increases the secretion in cos cells

Humanization of rodent mAbs by CDR-grafting (also called "reshaping") is now a standard procedure for reducing immunogenicity and recruiting human effector functions. However, the design of the humanized mAb can sometimes prove circuitous. Attempts were made to humanize L-25, a mouse antibody against the human alpha-4 integrin subunit using the usual protocols. Despite reaching eight backmutations in the light chain, it was not possible to recover the binding activity to the level of the chimeric. In an effort to restore the binding activity, an analysis of the human kappa IV acceptor frameworks was undertaken. This analysis highlighted the Asp at position 9 in framework 1, which although a common amino acid in human kappa IV frameworks, was an unusual residue in mouse kappa frameworks. Backmutating this position to the mouse amino acid completely restored the binding of the humanized antibody and as a by-product also increased the secretion levels in cos cells. Mutating position 9 to the consensus residue for human kappa I also restored the binding and secretion levels although not to the same extent. The resulting humanized antibody had a light chain with only a single backmutation to the mouse sequence.

Synthesis, cloning and expression of the single-chain Fv gene of the HPr-specific monoclonal antibody, Jel42. Determination of binding constants with wild-type and mutant HPrs

The monoclonal antibody Jel42 is specific for the Escherichia coli histidine-containing protein, HPr, which is an 85 amino acid phosphocarrier protein of the phosphoenolpyruvate:sugar phosphotransferase system. The binding domain (Fv) has been produced as a single chain Fv (scFv). The scFv gene was synthesized in vitro and coded for pelB leader peptide-heavy chain-linker-light chain-(His)(5) tail. The linker is three repeats from the C-terminal repetitive sequence of eukaryotic RNA polymerase II. This linker acts as a tag; it is the antigen for the monoclonal antibody Jel352. The codon usage was maximized for E.coli expression, and many unique restriction endonuclease sites were incorporated. The scFv gene incorporated into pT7-7 was highly expressed, yielding 10-30% of the cell protein as the scFv, which was found in inclusion bodies with the leader peptide cleaved. Jel42 scFv was purified by denaturation/renaturation yielding preparations with K(d) values from 20 to 175 nM. However, based upon an assessment of the amount of active refolded scFv, the binding dissociation constant was estimated to be 2.7 +/- 2.0 nM compared with 2.8 +/- 1.6 and 3.7 +/- 0.3 nM previously determined for the Jel42 antibody and Fab fragment respectively. The effect of mutation of the antigen HPr on the binding constant of the scFv was very similar to the properties determined for the antibody and the Fab fragment. It was concluded that the small percentage ( approximately 6%) of refolded scFv is a true mimic of the Jel42 binding domain and that the incorrectly folded scFv cannot be detected in the binding assay.

Phage-display library selection of high-affinity human single-chain antibodies to tumor-associated carbohydrate antigens sialyl Lewisx and Lewisx

mAbs against tumor-associated carbohydrate antigens have the potential to play a prominent role in cancer immunotherapy. However, it has not been possible to fully exploit the clinical utility of such antibodies primarily, because those of adequate affinity could be derived only from murine sources. To address this problem, we prepared a single-chain Fv (scFv) antibody library from the peripheral blood lymphocytes of 20 patients with various cancer diseases. Completely human high-affinity scFv antibodies were then selected by using synthetic sialyl Lewisx and Lewisx BSA conjugates. These human scFv antibodies were specific for sialyl Lewisx and Lewisx, as demonstrated by ELISA, BIAcore, and flow cytometry binding to the cell surface of pancreatic adenocarcinoma cells. Nucleotide sequencing revealed that at least four unique scFv genes were obtained. The Kd values ranged from 1.1 to 6.2 x 10(-7) M that were comparable to the affinities of mAbs derived from the secondary immune response. These antibodies could be valuable reagents for probing the structure and function of carbohydrate antigens and in the treatment of human tumor diseases.

Antibody engineering: comparison of bacterial, yeast, insect and mammalian expression systems

Engineered antibody molecules, and their fragments, are being increasingly exploited as scientific and clinical tools. However, one factor that can limit the applicability of this technology is the ability to express large amounts of active protein. In this review we describe the relative advantages and disadvantages of bacterial, yeast, insect and mammalian expression systems, and discuss some of the problems that can be encountered when using them. There is no 'universal' expression system, that can guarantee high yields of recombinant product, as every antibody-based molecule will pose its own problems in terms of expression. As a result the choice of system will depend on many factors, including the molecular species being expressed, the precise sequence of the individual antibody and the preferences of the individual investigator. However, there are general rules with regards to the design of expression vectors and systems which will help the investigator to make informed choices as to which strategy might be appropriate for their application.

A nucleolar protein that affects mating efficiency in Saccharomyces cerevisiae by altering the morphological response to pheromone

SSF1 and SSF2 are redundant essential yeast genes that, when overexpressed, increase the mating efficiency of cells containing a defective Ste4p Gbeta subunit. To identify the precise function of these genes in mating, different responses to pheromone were assayed in cells that either lacked or overexpressed SSF gene products. Cells containing null alleles of both SSF1 and SSF2 displayed the normal transcriptional induction response to pheromone but were unable to form mating projections. Overexpression of SSF1 conferred the ability to form mating projections on cells containing a temperature-sensitive STE4 allele, but had only a small effect on transcriptional induction. SSF1 overexpression preferentially increased the mating efficiency of a strain containing a null allele of SPA2, a gene that functions specifically in cell morphology. To investigate whether Ssf1p plays a direct physical role in mating projection formation, its subcellular location was determined. An Ssf1p-GFP fusion was found to localize to the nucleolus, implying that the role of SSF gene products in projection formation is indirect. The region of Ssf1p-GFP localization in cells undergoing projection formation was larger and more diffuse, and was often present in a specific orientation with respect to the projection. Although the function of Ssf1p appears to originate in the nucleus, it is likely that it ultimately acts on one or more of the proteins that is directly involved in the morphological response to pheromone. Because many of the proteins required for projection formation during mating are also required for bud formation during vegetative growth, regulation of the activity or amount of one or more of these proteins by Ssf1p could explain its role in both mating and dividing cells.

Display of functional thrombin inhibitor hirudin on the surface of phage M13

A synthetic gene for hirudin was ligated into phagemid pCANTAB5E. This construct allows production of either soluble hirudin or phage having hirudin displayed on the surface. Similarly, hirudin variants with extensions either at their N- or C-terminus were generated. The genes were expressed in their soluble form in a non-suppressor strain of E. coli. Periplasmatic fractions were evaluated in standard thrombin inhibition assays. Extending hirudin by a single Gln residue at the N-terminus reduces the activity by two orders of magnitude. This suggests that either the terminal amine group makes an important interaction or that steric constraints do not allow additional amino acids here. Only C-terminal extensions maintain most of the thrombin inhibitor activity of r-hirudin. The r-hirudin gene was also expressed on the tips of filamentous phage as a fusion protein with protein III (pIII). The hirudin-pIII fusion protein was detected with anti-hirudin antibody and with anti-E-tag antibody by Western blot analysis. Recombinant phages were shown to bind to immobilized thrombin in a dose-dependent manner. Upon addition of soluble thrombin, recombinant hirudin phages could be eluted specifically. Finally, purified phages carrying displayed r-hirudin were shown to inhibit thrombin in a standard amidolytic assay for thrombin inhibitor activity. These results demonstrate that hirudin can be C-terminally extended without diminishing the antithrombic activity. Beyond that, active hirudin can be displayed on the surface of M13 phage. As a conclusion, applied molecular evolution, i.e. the selection of hirudin-based thrombin inhibitor variants with tailored properties from (partially) randomized peptide pools should now be possible.

Maspin is an intracellular serpin that partitions into secretory vesicles and is present at the cell surface

The tumor suppressor maspin (mammary serpin) was originally identified as a component of human mammary epithelial cells that is downregulated as mammary tumor cells progress from the benign to the invasive and metastatic states. Maspin inhibits cellular invasion, motility, and proliferation, but its mechanism of action is currently unknown. Because the cellular machinery responsible for these processes is cytoplasmic, we have reexamined the tissue distribution and subcellular localization of maspin. We find that maspin, or a maspin-like protein, is present in many human organs, in which it localizes to epithelia. In cultured human mammary myoepithelial cells, maspin is predominantly a soluble cytoplasmic protein that associates with secretory vesicles and is present at the cell surface. In vitro assays show that the vesicle association is due to the existence of an uncleaved facultative secretion signal that allows small amounts of maspin to partition into the endoplasmic reticulum. These results demonstrate that maspin is more widespread than previously believed. The subcellular localization studies indicate that soluble intracellular and vesicle-associated maspin probably play an important role in controlling the invasion, motility, and proliferation of cells expressing it, whereas extracellular maspin may also regulate these processes in adjacent cells.

The in vivo and in vitro characterisation of an engineered human antibody to E-selectin

BACKGROUND

E-selectin is an endothelial cell specific adhesion molecule that is believed to play an important role in the early stages of leukocyte extravasation.

OBJECTIVES

Here we describe the construction and evaluation of an engineered human monoclonal antibody that blocks E-selectin function.

RESULTS

SPLAT-1 is an engineered human monoclonal antibody that has a very similar affinity for E-selectin as its murine parent antibody. In vitro SPLAT-1 blocks the binding of human leukocytes to E-selectin and does not mediate antibody-dependent cellular cytotoxicity (ADCC) or complement-mediated lysis of endothelial cells. In vivo, SPLAT-1 inhibits the recruitment of leukocytes to cytokine-inflamed human skin grafted on to SCID mice and has a long circulating half-life in primates. It does not appear to provoke an immune response in primates even on repeat administration.

CONCLUSIONS

SPLAT-1 has the characteristics of a antibody suitable for human therapy studies.

Antibody engineering

The development of recombinant techniques for the rapid cloning, expression, and characterization of cDNAs encoding antibody (Ab) subunits has revolutionized the field of antibody engineering. By fusion to heterologous protein domains, chain shuffling, and inclusion of self-assembly motifs, novel molecules such as bispecific Abs can now be generated which possess the subset of functional properties designed to fit the intended application. Rapid technological developments in phage display of peptides and proteins have led to a plethora of applications directed towards immunology and antibody engineering. Many of the problems associated with the therapeutic use of Abs are being addressed by the application of these new techniques.

Humanization of an antibody recognizing a breast cancer specific epitope by CDR-grafting

BACKGROUND

Muc1-H23 is a cell surface mucin that is expressed on normal breast luminal epithelial cells and over-expressed in most breast tumors. In addition, Muc-1 expressed by malignant cells is glycosylated differently than Muc-1 expressed by normal cells. This difference in glycosylation exposes a peptide epitope on malignant cells which is not exposed on normal cells. Murine monoclonal antibody H23 recognizes this epitope and stains 91% of breast cancers, but only 1/56 non-malignant breast tissue samples.

OBJECTIVE

To create a human antibody that was equivalent to H23 for potential uses in imaging and/or the therapy of breast cancer.

STUDY DESIGN

We decided to humanize H23 by CDR-grafting using overlap PCR, and to this end, designed and constructed a bacterial expression vector that would allow V-regions, cloned via unique restriction sites, to be expressed as Fab fragments. In this way, we hoped to be able to rapidly evaluate Fab constructs for binding to Muc-1 and to cells and tissue sections that expressed the antigen.

RESULTS

A fully humanized Fab fragment was able to bind Muc-1 peptide, as well as breast cancer cells known to express the epitope and tissue sections, generally showing the same reactivity as the native antibody. In addition, an analysis of sFab expressed with a [His]6 tag preceded by a factor Xa proteolytic cleavage site suggested that E. coli periplasmic signal peptidase was able to cleave the factor Xa site, thereby removing the [His]6 tag.

CONCLUSION

We have generated a human antibody that is capable of recognizing a tumor specific epitope expressed by 91% of breast cancers.

Loss of sustained Fus3p kinase activity and the G1 arrest response in cells expressing an inappropriate pheromone receptor

The yeast pheromone response pathway is mediated by two G protein-linked receptors, each of which is expressed only in its specific cell type. The STE3DAF mutation results in inappropriate expression of the a-factor receptor in MATa cells. Expression of this receptor in the inappropriate cell type confers resistance to pheromone-induced G1 arrest, a phenomenon that we have termed receptor inhibition. The ability of STE3DAF cells to cycle in the presence of pheromone was found to correlate with reduced phosphorylation of the cyclin-dependent kinase inhibitor Far1p. Measurement of Fus3p mitogen-activated protein (MAP) kinase activity in wild-type and STE3DAF cells showed that induction of Fus3p activity was the same in both strains at times of up to 1 h after pheromone treatment. However, after 2 or more hours, Fus3p activity declined in STE3DAF cells but remained high in wild-type cells. The level of inducible FUS1 RNA paralleled the changes seen in Fus3p activity. Short-term activation of the Fus3p MAP kinase is therefore sufficient for the early transcriptional induction response to pheromone, but sustained activation is required for cell cycle arrest. Escape from the cell cycle arrest response was not seen in wild-type cells treated with low doses of pheromone, indicating that receptor inhibition is not simply a result of weak signaling but rather acts selectively at late times during the response. STE3DAF was found to inhibit the pheromone response pathway at a step between the G beta subunit and Ste5p, the scaffolding protein that binds the components of the MAP kinase phosphorylation cascade. Overexpression of Ste20p, a kinase thought to act between the G protein and the MAP kinase cascade, suppressed the STE3DAF phenotype. These findings are consistent with a model in which receptor inhibition acts by blocking the signaling pathway downstream of G protein dissociation and upstream of MAP kinase cascade activation, at a step that could directly involve Ste20p.

An interspecies hybrid RNA virus is significantly more virulent than either parental virus

Cucumber mosaic cucumovirus (CMV) infects a very wide range of plant species (>1000 species). We recently demonstrated that a previously undescribed gene (2b) encoded by RNA 2 of the tripartite RNA genome of CMV is required for systemic virus spread and disease induction in its hosts. Herein we report that when this CMV gene is replaced by its homologue from tomato aspermy cucumovirus (TAV), the resultant hybrid virus is significantly more virulent, induces earlier onset of systemic symptoms, and accumulates to a higher level in seven host species from three families than either of the parents. Our results indicate that CMV and the TAV 2b protein interact synergistically despite the fact that no synergism occurs in double infections with the two parental viruses. To our knowledge, this is the first example of an interspecific hybrid made from plant or animal RNA viruses that is more efficient in systemic infection of a number of hosts than the naturally occurring parents. As CMV and the hybrid virus accumulated to a similar level in the infected tobacco protoplasts, the observed synergistic responses most likely resulted from an increased efficacy of the hybrid virus in systemic spread in host plants provided by the TAV 2b protein. The relevance of our finding to the application of pathogen-derived resistance is discussed.

Cloning, expression, and characterization of the human eosinophil eotaxin receptor

Although there is a mounting body of evidence that eosinophils are recruited to sites of allergic inflammation by a number of beta-chemokines, particularly eotaxin and RANTES, the receptor that mediates these actions has not been identified. We have now cloned a G protein-coupled receptor, CC CKR3, from human eosinophils which, when stably expressed in AML14.3D10 cells bound eotaxin, MCP-3 and RANTES with Kds of 0.1, 2.7 and 3.1 nM, respectively. CC CKR3 also bound MCP-1 with lower affinity, but did not bind MIP-1 alpha or MIP-1 beta. Eotaxin, RANTES, and to a lessor extent MCP-3, but not the other chemokines, activated CC CKR3 as determined by their ability to stimulate a Ca(2+) -flux. Competition binding studies on primary eosinophils gave binding affinities for the different chemokines which were indistinguishable from those measured with CC CKR3. Since CC CKR3 is prominently expressed in eosinophils we conclude that CC CKR3 is the eosinophil eotaxin receptor. Eosinophils also express a much lower level of a second chemokine receptor, CC CKR1, which appears to be responsible for the effects of MIP-1 alpha.

A recombinant PCR approach requiring only three non-chimeric primers to generate a minigene of interest

The recombinant PCR allows construction of chimeric molecules. Here we describe this approach utilizing non-chimeric primers. Unlike previous recombinant PCR methods, this approach eliminates the need of multiple sets of primers and multiple rounds of PCR making it an economical and expeditious alternative. We have used this approach to generate an FGF-1 minigene.

Preparation and preclinical evaluation of humanised A33 immunoconjugates for radioimmunotherapy

A humanised IgG1/k version of A33 (hA33) has been constructed and expressed with yields up to 700 mg l-1 in mouse myeloma NS0 cells in suspension culture. The equilibrium dissociation constant of hA33 (KD = 1.3 nM) was shown to be equivalent to that of the murine antibody in a cell-binding assay. hA33 labelled with yttrium-90 using the macrocyclic chelator 12N4 (DOTA) was shown to localise very effectively to human colon tumour xenografts in nude mice, with tumour levels increasing as blood concentration fell up to 144 h. A Fab' variant of hA33 with a single hinge thiol group to facilitate chemical cross-linking has also been constructed and expressed with yields of 500 mg l-1. Trimaleimide cross-linkers have been used to produce a trivalent Fab fragment (hA33 TFM) that binds antigen on tumour cells with greater avidity than hA33 IgG. Cross-linkers incorporating 12N4 or 9N3 macrocycles have been used to produce hA33 TFM labelled stably and site specifically with yttrium-90 or indium-111 respectively. These molecules have been used to demonstrate that hA33 TFM is cleared more rapidly than hA33 IgG from the circulation of animals but does not lead to accumulation of these metallic radionuclides in the kidney. 90Y-labelled hA33 TFM therefore appears to be the optimal form of the antibody for radioimmunotherapy of colorectal carcinoma.

Construction and characterization of a humanized, internalizing, B-cell (CD22)-specific, leukemia/lymphoma antibody, LL2

The murine monoclonal antibody, LL2, is a B-cell (CD22)-specific IgG2a which has been demonstrated to be clinically significant in the radioimmunodetection of non-Hodgkin's B-cell lymphoma. The antibody carries a variable region-appended glycosylation site in the light chain and is rapidly internalized upon binding to Raji target cells. Humanization of LL2 was carried out in order to develop LL2 as a diagnostic and immunotherapeutic suitable for repeated administration. Based on the extent of sequence homology, and with the aid of computer modeling, we selected the EU framework regions (FR) 1, 2 and 3, and the NEWM FR4 as the scaffold for grafting the heavy chain complementarity determining regions (CDRs), and REI FRs for that of light chains. The light chain glycosylation site, however, was not included. Construction of the CDR-grafted variable regions was accomplished by a rapid and simplified method that involved long DNA oligonucleotide synthesis and the polymerase chain reaction (PCR). The humanized LL2 (hLL2), lacking light chain variable region glycosylation, exhibited immunoreactivities that were comparable to that of chimeric LL2 (cLL2), which was shown previously to have antigen-binding properties similar to its murine counterpart, suggesting that the VK-appended oligosaccharides found in mLL2 are not necessary for antigen binding. Moreover, the hLL2 retained its ability to be internalized into Raji cells at a rate similar to its murine and chimeric counterparts.

Generation, characterization, and in vivo studies of humanized anticarcinoma antibody CC49

Monoclonal antibody (MAb) CC49 reacts with tumor-associated glycoprotein (TAG)-72, a human pancarcinoma antigen. In clinical trials, radiolabeled CC49 has shown excellent tumor localization; however, many of the patients receiving MAb CC49 develop a human antimouse antibody response. In an attempt to prevent this antiimmunoglobulin response, we have developed a humanized CC49 (HuCC49) by grafting the MAb CC49 hypervariable regions onto the variable light (VL) and variable heavy (VH) frameworks of the human MAbs LEN and 21/28' CL, respectively, while retaining those murine framework residues that may be required for the integrity of the antigen combining-site structure. The HuCC49 MAb was compared with native murine CC49 (nCC49) and chimeric CC49 (cCC49), using a variety of assays. SDS-PAGE analysis under nonreducing conditions showed that the HuCC49 MAb has virtually identical mobility to that of cCC49. Under reducing conditions, the HuCC49 yielded two bands of approximately 25-28 and approximately 50-55 kDa, characteristic of heavy and light immunoglobulin chains. In competition radioimmunoassays, HuCC49 completely inhibited the binding of 125I-labeled nCC49 to TAG-72, although 23- to 30-fold more HuCC49 was required to achieve a level of competition similar to those of cCC49 and nCC49. The relative affinity of HuCC49 was 2- to 3-fold less than those of the cCC49 and nCC49 MAbs, respectively. The plasma clearance in mice of HuCC49 was virtually identical to that of cCC49. Biodistribution studies demonstrated equivalent tumor-targeting of HuCC49 and cCC49 to human colon carcinoma xenografts. These studies thus suggest that HuCC49 and genetically modified molecules, such as sFv and domain-deleted immunoglobulins developed by using the HuCC49 variable region as a cassette, may be potentially useful in both diagnostic and therapeutic clinical trials in patients with TAG-72-positive tumors.

Production of monoclonal antibodies in COS and CHO cells

Recent advances in the generation of genetically engineered monoclonal antibodies have enhanced the importance of COS cells as expression systems for rapidly producing sufficient quantities of these proteins for preliminary biochemical and biophysical analysis. In order to meet the demand for clinical supplies, a gradual increase has occurred in the usage of dihydrofolate reductase negative (DHFR-) Chinese hamster ovary (CHO) cells for large-scale antibody production. Using a variety of mammalian expression vectors and selection/amplification protocols, CHO cell lines capable of producing monoclonal antibodies at levels exceeding 1 gl-1 can now be obtained in an almost routine fashion. For the applications of monoclonal antibodies to expand into additional therapeutic areas, however, a 5-10-fold increase over current highest expression levels may still need to be achieved.

Comprehensive pharmacokinetics of a humanized antibody and analysis of residual anti-idiotypic responses

A murine antibody to human tumour necrosis factor-alpha (TNF-alpha) (CB0010) was complementarity-determining region (CDR)-grafted using human IgG4 heavy and kappa light chain constant regions. In cynomolgus monkeys, the grafted antibody (CDP571) was eliminated with a half-life of 40-90 hr, two to three times longer than CB0010, and immunogenicity was reduced by > 90%. Responses to the constant regions were almost entirely eliminated and responses to the CDR loop (anti-idiotype) were lowered. CDP571 was given to 24 human volunteers in doses from 0.1 to 10.0 mg/kg. It was well tolerated, with a half-life of approximately 13 days. Anti-CDP571 antibodies were low or undetectable at higher doses. At lower doses, anti-CDP571 peaked at 2 weeks and then declined. The response was primarily IgM (in contrast to the cynomolgus monkey, where by 5 weeks IgG predominated) and was against a conformational epitope comprising heavy and light chain CDR loops. No antibodies were detected against the gamma 4/kappa domains or frameworks. The response had little or no effect on CDP571 binding to TNF-alpha or on plasma clearance.

PCR-mediated recombination and mutagenesis. SOEing together tailor-made genes

Gene Splicing by Overlap Extension (gene SOEing) is a sequence-independent method for site-directed mutagenesis and/or recombination of DNA molecules. It is based on the idea that a PCR product can be engineered by adding or changing sequences at its ends so that the product can itself be used to prime DNA synthesis in a subsequent overlap-extension reaction to create mutant or recombinant molecules. As the engineered genes are created in vitro without reliance on host organisms or restriction sites, gene SOEing provides a powerful and versatile tool for genetic investigation and engineering.

Intrinsic ligand binding properties of the human and bovine alpha-interferon receptors

The Type I interferon receptor (IFN-alpha R) interacts with all IFN-alpha s, IFN-beta and IFN-omega, and seems to be a multisubunit receptor. To investigate the role of a cloned receptor subunit (IFN-alpha R1), we have examined the intrinsic ligand binding properties of the bovine and human IFN-alpha R1 polypeptides expressed in Xenopus laevis oocytes. Albeit with different efficiencies, Xenopus oocytes expressing either the human or bovine IFN-alpha R1 polypeptide exhibit significant binding and formation of crosslinked complexes with human IFN-alpha A and IFN-alpha B. Thus, the IFN-alpha R1 polypeptide most likely plays a direct role in ligand binding.

The genetic engineering of monoclonal antibodies

A number of recent technological developments have greatly facilitated the genetic engineering of immunoglobulins. The use of PCR has permitted the variable regions to be rapidly cloned either from a specific hybridoma source or as a gene library from non-immunised cells. The conversion of the rodent antibody into a humanized version is now well established. To develop these antibodies for clinical use has required the development of high level expression systems. For the expression of large multimeric glycoproteins, mammalian cell systems generally provide the highest levels of secreted product and therefore are the methods of choice for producing whole recombinant antibodies. Novel antigen-binding units have been developed by joining the two variable domains of an antibody into single-chain polypeptides. Such fragments can be produced in high yield by secretion from E. coli raising the prospect of bulk preparation of these antibody fragments for the development of low-cost immunopurification and assay reagents. Finally, the ability to screen for antigen binding by displaying immunoglobulin variable regions on the surface of filamentous bacteriaphages has opened up the possibility of bypassing the immune system to generate novel antibody specificities in vitro.

Humanization of murine monoclonal antibodies through variable domain resurfacing

Two murine monoclonal antibodies, N901 (anti-CD56) and anti-B4 (anti-CD19), were humanized by a process we call "resurfacing." A systematic analysis of known antibody structures has been used to determine the relative solvent accessibility distributions of amino acid residues in murine and human antibody variable (Fv) regions and has shown that the sequence alignment positions of surface amino acids for human and murine variable region heavy (VH) and light (VL) chains are conserved with 98% fidelity across species. While the amino acid usage at these surface positions creates surface residue patterns that are conserved within species, there are no identical patterns across species. However, surprisingly few amino acid changes need to be made to convert a murine Fv surface pattern to that characteristic of a human surface. Resurfacing was used to change the patterns of surface accessible residues in the Fv regions of the N901 and anti-B4 antibodies to resemble those found on the Fv regions of human antibody sequences. Two different procedures for selecting a human sequence were compared. For anti-B4, a data base of clonally derived human VL-VH sequence pairs was used, while for N901, sequences for VL and VH were independently selected from the Kabat et al. data base [Kabat, E. A., Wu, T. T., Reid-Miller, M., Perry, H. M. & Gottesman, K. S. (1991) Sequences of Proteins of Immunological Interest (DHHS, Washington, DC), 5th Ed.]. Resurfaced N901 and anti-B4 antibodies had apparent affinities for their cell surface ligands that were identical to those of their respective parent murine antibodies. These data provide evidence that, despite the differences in the surfaces of mouse and human Fv regions, it is possible to substitute one for the other while retaining full antigen binding affinity.

Humanization of an anti-mucin antibody for breast and ovarian cancer therapy

Antibody-drug conjugates utilize the targetting potential of antibodies to improve the potential of cytostatic or cytocidal drugs. One such murine monoclonal antibody, CTM01 (mCTM01), which recognizes an epitope on breast epithelial mucin, has potential for the treatment of breast and ovarian cancers. We examine in this paper the comparative properties of mCTM01 against a number of other anti-mucin antibodies. We then describe the humanization and high level re-expression of humanized CTM01 (hCTM01), a process designed to avoid the immune response to administered murine antibodies in human patients and to produce sufficient material for clinical studies. We show that the humanized form has properties superior to mCTM01 in terms of binding affinity to antigen presented on tumour cells.

Vectors for the expression of PCR-amplified immunoglobulin variable domains with human constant regions

Cassette vectors have been constructed for mammalian expression of complete immunoglobulin heavy and light chain genes whose variable regions are produced by the polymerase chain reaction (PCR). The light and heavy chain vectors have promoter, leader, partial intron, enhancer and constant region segments within modified pSV2-gpt and pSV2-neo plasmids, respectively. Variable (V) regions are obtained by PCR using a two step process: 1) the V gene is amplified from genomic or cDNA, cloned into an intermediate vector and sequenced; 2) the first PCR product serves as the template for a second amplification in which restriction enzyme recognition sites and limited flanking intron sequence are added. The second PCR product is inserted into the expression vector, which is then transfected into mouse myeloma cells. These vectors contain human constant regions and may be used to express chimeric, humanized or human Ig genes. This report describes the design of these vectors and their application for the expression of chimeric 60.3, an anti-CD18 antibody.

Two domains of ISGF3 gamma that mediate protein-DNA and protein-protein interactions during transcription factor assembly contribute to DNA-binding specificity

Alpha interferon (IFN-alpha) induces the transcription of a large set of genes through activation of multimeric transcription factor ISGF3. This factor can be dissociated into two protein components, termed ISGF3 gamma and ISGF3 alpha. ISGF3 gamma is a 48-kDa protein related at the amino terminus to members of the IFN-regulatory factor (IRF) and Myb families of DNA-binding proteins; ISGF3 alpha consists of three polypeptides of 84, 91, and 113 kDa that self-assemble to form an activated component in response to IFN-alpha. DNA-binding studies indicated that ISGF3 gamma binds DNA alone, recognizing the IFN-stimulated response element, while the ISGF3 alpha polypeptides alone display no specific interactions with DNA. A complex between ISGF3 gamma and activated ISGF3 alpha binds the IFN-stimulated response element with much greater affinity than does the 48-kDa ISGF3 gamma protein alone. The DNA-binding domain of ISGF3 gamma and regions responsible for protein-protein interaction with ISGF3 alpha were identified by using deleted forms of ISGF3 gamma expressed in vitro. The amino-terminal region of ISGF3 gamma homologous to the IRF and Myb proteins was sufficient for interaction with DNA and displayed the binding specificity of the intact protein; phosphorylation of this region was necessary for activity. A second region of 160 amino acids separated from the DNA-binding domain by over 100 amino acids contained a domain capable of associating with ISGF3 alpha and was sufficient to confer specific ISGF3 alpha interaction to a heterologous protein. Interaction of the ISGF3 alpha component with the protein interaction domain of ISGF3 gamma altered the DNA-binding specificity of the resulting complex, suggesting that one or more of the ISGF3 alpha polypeptides make base-specific contacts with DNA. This interaction defines a mechanism through which IRF-like proteins complexed with regulatory components can display novel DNA-binding specificities.

Two domains of ISGF3 gamma that mediate protein-DNA and protein-protein interactions during transcription factor assembly contribute to DNA-binding specificity

Alpha interferon (IFN-alpha) induces the transcription of a large set of genes through activation of multimeric transcription factor ISGF3. This factor can be dissociated into two protein components, termed ISGF3 gamma and ISGF3 alpha. ISGF3 gamma is a 48-kDa protein related at the amino terminus to members of the IFN-regulatory factor (IRF) and Myb families of DNA-binding proteins; ISGF3 alpha consists of three polypeptides of 84, 91, and 113 kDa that self-assemble to form an activated component in response to IFN-alpha. DNA-binding studies indicated that ISGF3 gamma binds DNA alone, recognizing the IFN-stimulated response element, while the ISGF3 alpha polypeptides alone display no specific interactions with DNA. A complex between ISGF3 gamma and activated ISGF3 alpha binds the IFN-stimulated response element with much greater affinity than does the 48-kDa ISGF3 gamma protein alone. The DNA-binding domain of ISGF3 gamma and regions responsible for protein-protein interaction with ISGF3 alpha were identified by using deleted forms of ISGF3 gamma expressed in vitro. The amino-terminal region of ISGF3 gamma homologous to the IRF and Myb proteins was sufficient for interaction with DNA and displayed the binding specificity of the intact protein; phosphorylation of this region was necessary for activity. A second region of 160 amino acids separated from the DNA-binding domain by over 100 amino acids contained a domain capable of associating with ISGF3 alpha and was sufficient to confer specific ISGF3 alpha interaction to a heterologous protein. Interaction of the ISGF3 alpha component with the protein interaction domain of ISGF3 gamma altered the DNA-binding specificity of the resulting complex, suggesting that one or more of the ISGF3 alpha polypeptides make base-specific contacts with DNA. This interaction defines a mechanism through which IRF-like proteins complexed with regulatory components can display novel DNA-binding specificities.

Humanized antibodies: enhancing therapeutic utility through antibody engineering

The promise of antibody therapeutics has been greatly expanded by the development of monoclonal antibody technology and more recently antibody humanization. By transferring the mouse antibody binding site into a human antibody gene, we can engineer a "human antibody" which retains the specificity and biological effects of the original mouse antibody but has the potential to be nonimmunogenic in humans. Additionally, antibody effector functions can be improved through manipulation of the antibody constant region genes. We have produced a humanized version of OKT3 with human IgG4 and kappa constant regions. This antibody retains all of the in vitro characteristics of murine OKT3 including induction of cytokine release and T cell activation markers. Humanized OKT3 has an affinity of 1.4 x 10(9) M-1 relative to a 1.2 x 10(9) M-1 affinity of murine OKT3. Substitution of a glutamic acid for leucine at residue 235 in the antibody constant region abrogates FcR I binding and causes a marked reduction of T cell activation. The humanized FcR mutant of OKT3 has potential to be an improved therapeutic for transplantation and may have applications in autoimmune disease treatment.

Engineering antibodies for therapy

Success in the generation of an antibody-based therapeutic requires careful consideration of the binding site, to achieve specificity and high affinity; of the effector, to produce the desired therapeutic effect; of the means of attachment of the effector to the binding site; production of the end product; and the response made by the patient to the administered compound. Each of these areas is receiving attention by antibody-engineering techniques. The number of potentially useful monoclonal antibodies developed over the last 10 years, and currently in clinical trials or preregistration, is now being increased by these engineered newcomers. It will be interesting to see over the next few years how many of these antibodies, and of which kind, emerge as products.

Antibodies to soluble human T cell receptor beta chain recognize multiple epitopes on cell surface TcR

The T cell receptor (TcR) is an integral membrane protein occurring as a disulfide linked heterodimer, non-covalently associated with CD3 on the surface of T lymphocytes. Antibodies to the TcR have been shown to be effective for treating autoimmune disorders in animals. We describe here a method for producing antibodies to cell surface determinants of the human TcR, using a soluble form of the receptor as antigen. Soluble V alpha 1.2, V beta 8.1, V beta 11 TcR chains are expressed from a construct in which the extracellular domains of the TcR are fused to the mouse gamma 2a heavy chain constant region lacking the CH1 domain. These chimeric molecules contain both immunoglobulin and TcR determinants, as revealed by antibody probes. Amino-terminal sequence analysis of a chimeric V beta 8.1 molecule indicates that the TcR leader peptide is correctly processed from the soluble form. Antibodies raised against the soluble human V beta 8.1 molecule recognize the native determinants on Jurkat cells, and on natural T cells derived from resting human peripheral blood lymphocytes. Epitope mapping studies using competitive binding assays suggest that the anti-V beta 8 antibodies produced using soluble antigen recognize multiple overlapping determinants on the cell surface form of the TcR.

New recombinant DNA methodology for protein engineering

Over the past year considerable progress has been made in the application of recombinant DNA technology to protein engineering. A number of new methods for gene synthesis and mutagenesis have been reported that simplify the construction of novel coding sequences. The polymerase chain reaction plays an increasingly important role in these methods. Amino acid diversity has been extended by the incorporation of unnatural amino acids via coupled in vitro transcription-translation methods. Novel random mutagenesis strategies have been developed that substitute amino acids with a desired chemical character at a given position, thereby generating a sophisticated library of protein variants.

Crystal structure of a chimeric Fab' fragment of an antibody binding tumour cells

The crystal structure of a chimeric Fab' fragment of a monoclonal antibody is presented. The Fab' comprises the murine light chain and heavy chain variable domains of the carcinoma-binding antibody B72.3 fused to the constant domain of human kappa, and the first constant domain and hinge domain of human gamma 4, respectively. A model for the Fab' has been determined by molecular replacement and refined to a resolution of 3.1 A with an R-factor of 17.6%. The additional residues that distinguish a Fab' from a Fab fragment are seen to be disordered in the crystals. The H3 hypervariable loop is short and adopts a sharp hairpin turn in a conformation that results from an interaction between the lysine side-chain of H93 and the main-chain carbonyl group of H96. The remaining hypervariable loops display conformations similar to those predicted from the canonical structures approach, although loop H2 is apparently displaced by a salt-bridge formed between H55 Asp and the neighbouring H73 Lys. These and other features of the structure likely to be important in grafting the hypervariable loops to an otherwise human framework are discussed.

Direct and crossover PCR amplification to facilitate Tn5supF-based sequencing of lambda phage clones

The 264 bp mini-transposon Tn5supF was constructed to sequence DNAs cloned in phage lambda without extensive shotgun subcloning or primer walking. Unique sequences near each transposon end serve as primer binding sites, and a supF gene is used to select transposition to lambda. We describe here PCR methods that facilitate Tn5supF-based sequencing. In a first pass, insertions are mapped relative to the ends of the cloned fragment using pairs of primers specific for vector DNA next to the cloning site and for a Tn5supF end. Most insertions not mapped in this step are near the center of the cloned fragment or in the vector arms, and are then mapped relative to the two innermost insertions by 'crossover' PCR. This involves amplification from primers on different DNA molecules, and generates hybrid DNA products whose lengths correspond to the distances between the two insertions. We routinely amplified more than 6 kb in direct PCR and 3 kb in crossover PCR; at the limit we amplified up to approximately 10 kb in direct PCR and approximately 6 kb in crossover PCR, but not reproducibly. Crossover PCR products were also obtained with insertions separated by only 200 bp, indicating that no rare sites are needed to switch templates. PCR products were purified by adsorption and then elution from glass slurry, and sequenced directly. Ladders of more than 400 bp were obtained from primer sites on each DNA strand; 2 kb was read from crossover PCR products, and showed that they were amplified with fidelity. In conclusion, direct and crossover PCR methods expedite transposon insertion mapping, and yield templates for accurate sequencing of both DNA strands.