Discovery of human antibodies to cell surface antigens by capture lift screening of phage-expressed antibody libraries

An assay for the rapid identification and cloning of antibody fragments (Fabs) reactive with cell surface antigens was established and used to identify Fabs selectively reactive with tumor cell surface antigens. The Fabs were produced by a phage expression system and screened by a modified plaque lift approach in which nitrocellulose filters were coated with an anti-immunoglobulin reagent and blocked with bovine serum albumin prior to application to the phage-infected bacterial lawn. Subsequently, capture lifts were incubated with biotinylated antigen and reactive Fabs were identified with streptavidin conjugates. This screening method, termed capture lift, results in the immobilization of greater quantities of Fab and decreases the binding of unrelated host proteins, resulting in a more sensitive plaque lift assay. The capture lift permits the simultaneous analysis of thousands of antibody clones and, more importantly, can be used with crude detergent-solubilized cell extracts, permitting the discovery of Fabs which bind integral membrane proteins present in heterogeneous mixtures of antigens. Optimal conditions were identified utilizing phage-expressed BR96 Fab and a horseradish peroxidase conjugate of Lewis Y, a soluble cross-reactive antigen. Subsequently, it was demonstrated that the assay was functional with postnuclear detergent extracts isolated from surface-biotinylated tumor cells expressing the BR96 tumor antigen. Purification of the target antigen was not required. To demonstrate the application of the capture lift assay for the discovery of Fabs reactive with novel cell surface antigens a phage-expressed human antibody library constructed from tumor-infiltrating B lymphocytes was screened. Multiple antibody clones which reacted with detergent-solubilized biotinylated surface antigens were identified. Upon further characterization a portion of these displayed selectivity for tumor cells, as demonstrated by the binding of Fab to fixed and live tumor cells but not normal fibroblasts.

[Neutralizing human antibody specific for human cytomegalovirus in E. coli expression system]

We isolated neutralizing human Fab fragment specific for human cytomegalovirus (HCMV) by phage display system. Fab libraries were constructed from peripheral lymphocyte of healthy individual. In several clones reacted for HCMV-infected HEL cells, one clone, designated 13-3, stained HCMV infected cells at 96 hrs post infection. It didn't react to cells at 6 hrs post infection or infected cells in the presence of AraC, meaning that 13-3 recognized protein synthesized at late times of infection. It also neutralized HCMV, Towne strain, efficiently. This neutralizing activity was specific for HCMV and no effect for HSV-1 and 2.

Effects of substitutions in the binding surface of an antibody on antigen affinity

The interactions between the Fab and single-chain Fv (scFv) fragments of an antibody (NC10) and its antigen, influenza virus neuraminidase, were analysed in the crystal structures of the Fab-neuraminidase and scFv-neuraminidase complexes. To investigate the contribution to binding made by cavities, salt links and hydrogen bonds in the antibody-antigen interface, 14 single amino acid replacements were made at six contact residues in the scFv fragment by site-directed mutagenesis. The binding affinity of each mutant scFv antibody for neuraminidase was determined with a BIAcore optical biosensor. Four of the mutations resulted in large changes in the free energy of binding to neuraminidase (deltadeltaG > 1 kcal/mol) and together may account for approximately 70% of the free energy of binding. Hence these data support the theory that a small number of residues form the 'functional epitope' and are most important for binding of NC10 to neuraminidase. The salt link between antibody residue (Asp)H56 and (Lys)N432 from neuraminidase was demonstrated to be important for affinity, since substitution of (Asp)H56 with Asn caused a large reduction in the free energy of binding (deltadeltaG = +2.8 kcal/mol). Hydrogen bonds provided by (Tyr)L32 and (Asp)H56 were also important for binding: mutation of (Tyr)L32 to Phe resulted in a significant reduction in binding affinity (deltadeltaG = +1.7 kcal/mol). Disruption of hydrophobic interactions (van der Waals contacts) led to significant reductions in affinity also ((Tyr)H99 to Ala, deltadeltaG = +1.5 kcal/mol; (Leu)L94 to Ala, deltadeltaG > +3.0 kcal/mol). An attempt to increase binding affinity by filling a cavity in the interface with a larger antibody side chain was unsuccessful, as the free energy gained by new antibody-antigen interactions did not compensate for the removal of cavity-bound water molecules.

Preparation and characterization of the F (ab)2 fragments of an aromatase activity-suppressing monoclonal antibody

The preparation and characterization of the Fab and F(ab')2 fragments of a murine monoclonal antibody specific for aromatase cytochrome P-450 and which is suppressive of estrogen biosynthesis are described. This monoclonal antibody, MAb3-2C2, was purified from murine ascites using protein A affinity chromatography and digested with immobilized papain to produce antibody fragments. The Fab and F(ab')2 fragments were then purified using protein A affinity chromatography and S-200 HR size exclusion chromatography. The Fab fragment was further purified using S-100 HR size exclusion chromatography. Both the Fab and F(ab')2 fragments of the MAb3-2C2 suppressed aromatase activity in a dose-dependent manner. While the F(ab')2 fragment (110 kDa) maintained potent suppressive activity, the Fab fragment (42 kDa) required a higher concentration to suppress aromatase activity as compared to the IgG.

Evidence that agrin directly influences presynaptic differentiation at neuromuscular junctions in vitro

The synaptic protein agrin is required for aspects of both pre- and postsynaptic differentiation at neuromuscular junctions. Although a direct effect of agrin on postsynaptic differentiation, presumably through the MuSK receptor, is established, it is not clear whether agrin directly affects the presynaptic nerve. To provide evidence on this point, we used anti-agrin IgG to disrupt agrin function in chick ciliary ganglion (CG) neuron/myotube cocultures. In cocultures grown in the presence of 200 microg/ml anti-agrin IgG, clustering of acetylcholine receptors (AChRs), extracellular matrix proteins, and the synaptic vesicle protein synaptotagmin (syt) at nerve-muscle contacts was inhibited. Syt clustering was still inhibited in the presence of 100 microg/ml blocking antibody, while the postsynaptic clustering of AChRs, heparan sulphate proteoglycan, and s-laminin was retained. Additionally, in CG neurons cultured with COS cells expressing agrin A0B0, which lacks the ability to signal postsynaptic differentiation, syt clustering was induced and this clustering was also blocked by anti-agrin IgG. Our results demonstrate that agrin function is acutely required for pre- and postsynaptic differentiation in vitro, and strongly suggest that agrin is directly involved in the induction of presynaptic differentiation.

Quantitative comparison on the refinement of horse antivenom by salt fractionation and ion-exchange chromatography

A quantitative comparison was made on the fractionation of pepsin-digested horse antivenoms by ammonium sulfate (AS) fractional precipitation and ion-exchange chromatography on Q-Sepharose. In the precipitation process, pepsin digested horse anti-Naja kaouthia serum was precipitated by 30% saturated AS followed by 50% saturated AS. The recovery of antibody activity [as measured by an enzyme-linked immunosorbent assay (ELISA) against the cobra postsynaptic neurotoxin 3] from the 30-50% saturated AS precipitate was 53% with a 1.93-fold purification. For the chromatographic process, the behavior of the horse antitoxin antibody and its F(ab')2 fragments was first studied. The pepsin digested horse serum was then desalted on a Bio-gel P-2 column followed by chromatography on Q-Sepharose using a linear gradient (20 mM Tris-HCl, pH 8.0 containing 0.0 to 0.5 M NaCl) A peak containing primarily the F(ab')2 antibody could be obtained. This peak constituted 73% of the total antivenom activity with 2.08-fold purification. The total recovery of antibody activity by the chromatographic process was 90%. The yield of antibody activity was about 2-fold higher than that reported previously with other fractionation procedures. The implications of these results for the refining of horse therapeutic antivenoms are discussed.

Efficient production of a functional mouse/human chimeric Fab' against human urokinase-type plasminogen activator by Bacillus brevis

Expression/secretion vectors for the production of Fab' and single-chain (sc) Fab' by Bacillus brevis have been constructed. For the production of Fab', the cDNAs encoding the L chain and Fd' fragment (Fd with the hinge region) of a mouse-human chimeric Fab' against human urokinase-type plasminogen activator were fused directly with the translation-start and signal-peptide-encoding regions of the mwp gene, the gene for one of the major cell-wall proteins of Bacillus brevis. The two fused genes were placed tandemly downstream from the promoter of the cell-wall protein gene operon (cwp) of B. brevis. For the production of scFab', the two cDNAs were linked with a synthetic oligonucleotide encoding a flexible peptide linker of 17 or 24 amino acids, and fused with the translation start and signal-peptide-encoding regions of the mwp gene. Fab' was efficiently produced by B. brevis, being accumulated at a level of 100 mg/l in the culture medium in a simple shake-flask culture, which is the highest level obtained so far for a gram-positive bacterium. On the other hand, the scFab' remained at a level of a few milligrams per liter in the culture medium. The Fab' produced by B. brevis showed comparable antigen-binding activity to that of the parental antibody. The L chain and Fd' fragment, constituting the Fab', had the correct N-terminal amino acid sequences. These results indicate that B. brevis is a very promising host for the production of native Ig fragments.

Diverse Fab specific for acetylcholine receptor epitopes from a myasthenia gravis thymus combinatorial library

The muscle weakness in myasthenia gravis (MG) is caused by heterogeneous high-affinity IgG autoantibodies to the nicotinic acetylcholine receptor (AChR), a complex ion channel glycoprotein. These antibodies are clearly responsible for reducing AChR numbers at the neuromuscular junction in myasthenia; however, the origins, diversity, specificity and pathogenicity of individual antibodies have not yet been established. We have cloned and characterized four different AChR-specific Fab from an MG patient's thymus by screening an IgG1/kappa gene combinatorial lambda phage library with soluble human AChR labeled with [125I] alpha-bungarotoxin. Unlike most previously cloned human antibodies, all four Fab immunoprecipitated soluble human muscle AChR. Two Fab strongly inhibited binding of mAb to the main immunogenic region on the alpha subunits and one Fab bound to an epitope on the fetal-specific gamma subunit. In sensitivity and fine specificity, these Fab resembled the anti-AChR antibodies found in many MG patients, including the donor. The closest germline counterparts for their heavy chains were in VH families 1, 3 and 4; however, there were many differences consistent with an antigen-driven response of diverse B cell clones. The combinatorial approach holds promise for further analysis of human autoantibodies.

The epitopic "fingerprint" of thyroid peroxidase-specific Fab isolated from a patient's thyroid gland by the combinatorial library approach resembles that of autoantibodies in the donor's serum

A new thyroid peroxidase (TPO)-specific Fab (KM1) was obtained from an immunoglobulin gene combinatorial library of patient KM containing L chain genes amplified with a single "promiscuous" V kappa oligonucleotide primer. The KM1 L chain is encoded by a mutated B3 gene (V kappa IV family). Another mutated B3 L chain had been identified previously in a TPO-specific Fab (WR1.223) isolated from a different patient (WR). In contrast to patient KM, the WR L chains were amplified with a panel of V kappa family-specific primers. Both KM1 and WR1.223 bind TPO with high affinity (approximately 1 x 10(-9) M) and interact with an epitope in the B domain of the TPO immunodominant region. TPO-specific Fab previously isolated from a WR combinatorial library constructed with the promiscuous V kappa primer recognised the TPO A domain and none used a B3-like L chain. Remarkably, for both patients, Fab isolated from L chains generated with the promiscuous V kappa primer had epitopic profiles similar to autoantibodies in the donor's serum (KM-B domain; WR-A domain). Our data indicate that the promiscuous primer preferentially amplifies the dominant L chain present in vivo. However, to obtain a relatively rare Fab (such as the B domain Fab from WR), family-specific kappa primers are required. These findings provide insight into the relationship between TPO autoantibody gene usage, epitopic recognition, and the effectiveness of the combinatorial library approach.

Purification and stability studies of immunoglobulins from Lachesis muta muta antivenom

Immunoglobulins were isolated from hyperimmune horse plasma against Lachesis muta muta venom by ammonium sulfate precipitation and immunoaffinity technique (Sepharose-venom L. m. muta column). When necessary, limited proteolysis with pepsin was used to generate a bivalent antigen-binding fragment (F(ab')2). Solutions with immunoglobulins or F(ab')2 fragments were fractionated by molecular filtration chromatography (Superose 12) and the expected mol. wt species were observed. The L. m. muta venom shows caseinolytic and haemorrhagic activities. Incubation of the venom with these purified antibodies resulted in a decrease of both activities. High temperatures promote aggregation and the formation of protein precipitates. Sorbitol (1.0 M) was used as an osmolyte (a natural substance or an organic compound capable of stabilizing proteins) and decreased the formation of protein precipitates in solutions of antibodies, as judged by a spectrophotometric assay (280 nm), by nephelometry or when tested by enzyme-linked immunosorbent assay. Circular dichroism was used to study the spectra of antibodies in the presence of phosphate-buffered saline or sorbitol. Up to an osmolyte concentration of 1.0 M, there was no significant perturbation of the F(ab')2 fragments spectra in the amide region. However, whole immunoglobulins in the presence of 1.0 M sorbitol presented a small spectral perturbation, suggesting that the beta-structure was reinforced. The effect of osmolyte on the affinity of antibodies was observed by enzyme-linked immunosorbent assay. There was no significant difference in the results when the antibodies were previously incubated with venom in phosphate-buffered saline or in the presence of 1.0 M sorbitol. In conclusion, an osmolyte (sorbitol) was shown to be capable of stabilizing antibodies at high temperatures, with no significant perturbation in the secondary structure or affinity to L. m. muta venom. These results point to the possibility of using sorbitol, or other osmolytes, as stabilizers of immunoglobulin preparations.

Differential inhibitory effect of L-lysine on renal accumulation of 67Cu-labelled F(ab')2 fragments in mice

The basic amino acid L-lysine was administered to mice in an attempt to circumvent unwanted renal accumulation of 67Cu-labelled F(ab')2 fragments derived from the anti-NCAM IgG1, SEN7 and anti-CEA IgG1 monoclonal antibody (MAb)35. In control experiments, significant renal uptake of both 67Cu-labelled F(ab')2 fragments was observed, radiolabel being primarily localised to proximal tubules in the renal cortex. Following optimised L-lysine dosing protocols, renal uptake of 67Cu-MAb35 F(ab')2 was inhibited by up to 42%. Surprisingly, little inhibition (< 10%) of 67Cu-SEN7 F(ab')2 uptake was observed. Experiments to investigate this differential inhibition indicated that inhibition of MAb35 F(ab')2 uptake was relatively short-lived (approx. 6 hr), whilst no apparent differences were found in blood clearance rates between either 67Cu-F(ab')2 fragment. L-lysine administration caused a significant diuresis with high levels of intact 67Cu-labelled SEN7 and MAb35 F(ab')2 appearing in the urine, possibly due to blockade of renal uptake and lysine-induced increases in glomerular membrane permeability. Iso-electric focusing studies failed to identify any charge differences between the 67Cu-labelled F(ab')2 fragments, although a cathodal migration of all 67Cu-labelled samples, presumably due to the net positive charge conferred by addition of 67Cu2+ ions, was observed. Our results demonstrate that in addition to net charge, other unidentified characteristics may influence renal accumulation of radiometal-labelled F(ab')2 fragments and their inhibition by L-lysine.

Noninvasive detection of atherosclerotic lesions by 99mTc-based immunoscintigraphic targeting of proliferating smooth muscle cells

BACKGROUND

Mouse/human chimeric antibody Z2D3 identifies an antigen produced exclusively by proliferating smooth muscle cells of human atheroma, and also cross reacts with experimentally induced atherosclerotic lesions in rabbits. Fab' fragments of Z2D3 antibody were labeled with (99m)Tc using glucaric acid as a weak transchelator. The potential role of (99m)Tc-labeled Z2D3 scintigraphy was explored for noninvasive imaging of experimental atherosclerotic lesions.

METHODS AND RESULTS

(99m)Tc-Z2D3 Fab' was utilized for noninvasive imaging in four rabbits with experimentally induced atherosclerotic lesions and in one control rabbit. In addition, (99m)Tc-labeled nonspecific 103D2 Fab' was used for comparison in four other rabbits with atherosclerotic lesions. The atherosclerotic lesions were induced by balloon de-endothelialization of the infradiaphragmatic abdominal aorta and 12 weeks of hyperlipidemic diet. An aliquot of 15 mCi (550 mBq) of (99m)Tc pertechnetate was incubated with 6.25 mg of glucaric acid for 30 min followed by incubation of (99m)Tc glucarate with 375 microg of Z2D3 Fab' or 103D2 Fab' for an additional 30 min. Instant thin-layer chromatography demonstrated almost complete radiolabeling. (99m)Tc-Z2D3 was administered IV and gamma imaging was performed at the time of injection, 3, 6, 9, and 12 h, followed by ex vivo imaging of the excised aorta, and biodistribution was performed. Unequivocal visualization of atherosclerotic lesions was possible in all four animals at 9 to 12 h with Z2D3 Fab'. Quantitative uptake, as represented by mean lesion-to-liver count density ratio, was 0.6+/-0.05. Imaging with nonspecific 103D2 Fab' did not show any localization in the abdominal aorta (lesion-to-liver ratio, 0.45+/-0.02, p=0.02). Ex vivo lesion-to-normal aortic segment ratio was 4.3+/-0.9 for Z2D3 and 1.04+/-0.08 for nonspecific 103D2 Fab' (p=0.01). Biodistribution studies demonstrated 0.03+/-0.003% injected Z2D3 dose per gram in the atherosclerotic lesions as compared with 0.01+/-0.003% in the nondenuded thoracic aorta of atherosclerotic rabbits (p=0.008). However, only 0.008+/-0.002% of the mean injected dose per gram was obtained in the atherosclerotic lesions (p=0.001) as compared with 0.005+/-0.003% in the normal aortic segments with 103D2. No Z2D3 uptake in normal rabbits was observed on either the in vivo or ex vivo images.

CONCLUSIONS

The present study demonstrates that (99m)Tc-based immunoimaging of the vascular lesions may be feasible by the use of smaller antibody fragments. Earlier visualization is possible at the expense of a lower absolute antibody uptake in the lesions as compared to the use of intact antibody or larger fragments with longer circulating time.

Anti-acetylcholine receptor Fab fragments isolated from thymus-derived phage display libraries from myasthenia gravis patients reflect predominant specificities in serum and block the action of pathogenic serum antibodies

Myasthenia gravis (MG) is a prototype antibody-mediated autoimmune disease in which antibodies against nicotinic acetylcholine receptors (AChR) induce loss of functional receptors at the neuromuscular junction. Germinal centers present in MG hyperplastic thymus contain activated B-cells spontaneously producing anti-human AChR (huAChR) Ab in vitro. In order to access the anti-huAChR repertoire phage display Fab libraries of thymic lymphocytes were constructed from two MG patients. A total of four Fabs highly specific for huAChR were isolated that bind to determinants in or near the main immunogenic region (MIR). These anti-huAChR Fabs showed evidence of significant somatic mutations supporting the notion that the anti-huAChR Ab response in MG patients is driven by antigen. A total of two Fabs were able to inhibit up to 90% of donor serum anti-huAChR antibodies. Competition with serum anti-huAChR Ab was also observed in unrelated MG patients and indicate that anti-huAChR Fabs bind to epitopes on huAChR recognized by the majority of MG patients. In vitro antigenic modulation studies demonstrated that anti-huAChR Fabs were able to induce AChR loss when cross-linked by an anti-Fab antibody but not as monovalent Fab. Moreover, anti-huAChR Fabs were able to protect against AChR loss by antigenic modulation induced by MG serum antibodies suggesting a potential therapeutic role for these recombinant Fabs in patients with a myasthenic crisis.

Separation method of IgG fragments using protein L

Protein L (IgG kappa-chain-binding bacterial protein) showed a precipitate line(pseudo-immuno-reaction) with IgG and F(ab')2 fragment, but did not show any line with the Fab fragment, the Fc fragment and free kappa-chains in the micro-Ouchterlony method. The IgG and Fab fraction obtained from pa-pain-digested IgG (from the sera of patients with chronic thyroiditis), followed by Protein A-Sepharose, were separated by Protein L-Sepharose affinity chromatography. The unbound fraction (UF) consisted of IgG(lambda) or Fab(lambda) and the bound fraction (BF) consisted of IgG(kappa) or Fab(kappa) were obtained. Anti-thyroglobulin and anti-thyroid peroxidase antibody activities were found equally in both the UF and the BF. When Fab(kappa) was reduced with dithiothreitol (DTT), the Fd fragment in the UF could be separated from the free kappa-chain and the unreduced Fab(kappa) in the BF with a Protein L-Sepharose column. A separation method of human IgG fragments such as free kappa-chain, combined forms of kappa-chain [Fab or F(ab')2], and the Fd region, using Protein L, is described.

Sequence, specificity and crystallization of an oestrone-3-glucuronide antibody (3910)

We describe the specificity profile and V region sequences of a high-affinity monoclonal antibody (mAb), 3910, directed against oestrone-3-glucuronide (E3G). Inhibition studies show that the D-ring is critical for steroid specificity, while the glucuronic acid attached to the A ring is required for high binding affinity, suggesting that both 'ends' of the E3G ligand are recognized. The VH domain is encoded by a gene from the VH7183 family, while VL appears to be encoded by the Vk5.1 gene (kappa II subgroup) with a deletion of six residues from complementarity-determining region-1 (CDR1). The VH CDR3 is 10 amino acid residues in length, of which D/N contributes five residues. Comparison of VH CDR of 3910 with those of mAb against progesterone (DB3) and digoxin (26-10, 40-50), for which crystal structures have been determined, suggests that aromatic side chains are important for E3G binding and that tyrosine residues H50, H97 and H100 may interact with the ligand. The Fab fragment of 3910 has been crystallized in its native and steroid (E3G and oestriol-3-glucuronide) complexed forms. An X-ray diffraction data set to 3 A resolution has been collected for the native Fab.

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.

An optimized method for cell-based phage display panning

BACKGROUND

Traditional methods of phage display panning, bind purified antigen to plates or other solid phases to which libraries are then applied, followed by vigorous washings in detergent-supplemented buffers to select for specific phage Fab. These methods are not directly applicable to antigens in their native environment on cell surfaces or in settings where the target antigen is unknown.

OBJECTIVES

To develop a model antigen system employing whole cells rather than purified protein immobilized on a substrate; to optimize methods for phage display panning using a cell-based system.

RESULTS

Specificity of binding of phage Fab to antigen on cells was demonstrated by output titer and by flow cytometry. Output titers showed a plateau and binding advantage, after four washes, corresponding to removal of most non-specifically bound phage. Enrichment advantage was independent of input phage number. Longer incubation times, to cell tolerance, improved specific binding. Temperature had modes impact as a variable in the panning and washing. An increase in output titers paralleled enrichment for specific phage Fab.

CONCLUSION

An optimized method applied to whole cells can productively enrich specific phage Fab in mixtures with large excesses of non-specific phage Fab over several rounds of panning.

Human anti-nicotinic acetylcholine receptor recombinant Fab fragments isolated from thymus-derived phage display libraries from myasthenia gravis patients reflect predominant specificities in serum and block the action of pathogenic serum antibodies

Myasthenia gravis (MG) is a prototype Ab-mediated autoimmune disease in which Abs against nicotinic acetylcholine receptors (AChR) induce loss of functional receptors at the neuromuscular junction. Germinal centers present in MG hyperplastic thymus contain activated B cells spontaneously producing anti-human AChR (anti-huAChR) Ab in vitro. To access the anti-huAChR repertoire, phage display Fab libraries of thymic lymphocytes were constructed from two MG patients. Four Fabs highly specific for huAChR were isolated that bind to determinants in or near the main immunogenic region. These anti-huAChR Fabs showed evidence of significant somatic mutations, supporting the idea that the anti-huAChR Ab response in MG patients is driven by Ag. Two Fabs were able to inhibit up to 90% of donor serum anti-huAChR Abs. Competition with serum anti-huAChR Ab was also observed in unrelated MG patients and indicate that anti-huAChR Fabs bind to epitopes on huAChR recognized by the majority of MG patients. In vitro antigenic modulation studies demonstrated that anti-huAChR Fabs were able to induce AChR loss when cross-linked by an anti-Fab Ab but not as monovalent Fab. Moreover, anti-huAChR Fabs were able to protect against AChR loss by antigenic modulation induced by MG serum Abs, suggesting a potential therapeutic role for these recombinant Fabs in patients with a myasthenic crisis.

Isolation of anti-mesothelin antibodies from a phage display library

Phage display has been used to select single-chain Fvs (scFvs) against mesothelin, a surface antigen present on mesothelial cells as well as mesotheliomas and non-mucinous ovarian cancers. Several attempts to produce anti-mesothelin hybridomas from spleen cells of mice immunized with recombinant mesothelin were unsuccessful. This report describes the isolation of anti-mesothelin scFvs from a phage display library made from the mRNA of the same spleens. Panning on recombinant antigen produced in E. coli or on antigen positive cells was employed. Several scFvs which bind specifically to mesothelin were isolated. Panning on recombinant antigen yielded five different scFvs. Panning on cells selected two different scFvs which also differ from the scFvs selected by recombinant antigen. The heavy chains of the scFvs selected on recombinant antigen are derived from four different heavy chain gene families and the scFvs selected on cells are derived from two of these families. In contrast, the light chains of all of these scFvs are derived from family XI. Moreover, the light chains of the two scFvs selected on cells are very similar to the light chains of two of the scFvs selected by panning on recombinant mesothelin except for a few point mutations. One of these scFvs which have been studied in detail has been shown to bind specifically to mesothelin positive transfected cells.

Inverse targeting of peritoneal tumors: selective alteration of the disposition of methotrexate through the use of anti-methotrexate antibodies and antibody fragments

We have hypothesized that antidrug antibodies (ADAb) may be employed to impart site-specific alterations in the disposition of drug molecules, potentially allowing for targeted drug therapy. We are specifically interested in minimizing systemic exposure to free drug and systemic toxicities resultant from regional chemotherapy through the intravenous administration of ADAb. In this report, we present the production and purification of anti-methotrexate Fab fragments, and we present investigations of the effects of anti-methotrexate Fab and anti-methotrexate immunoglobulin G on the disposition of methotrexate in the rat. Pharmacokinetic studies revealed that intravenous anti-methotrexate immunoglobulin G (anti-MTX IgG) and anti-methotrexate Fab (anti-MTX Fab) administration produced dramatic alterations in the plasma pharmacokinetics of methotrexate (MTX), following intraperitoneal MTX administration (area under the total MTX concentration vs time curve for anti-MTX IgG relative to control, 420 +/- 90 (p < 0.05); for anti-MTX Fab relative to control, 46 +/- 6.1 (p < 0.05); area under the free MTX concentration vs time curve for anti-MTX IgG relative to control, 0.64 +/- 0.16; for anti-MTX Fab relative to control, 0.45 +/- 0.20 (p < 0.05)). Additional studies conducted in anesthetized rats revealed no significant alterations in the area under the total peritoneal MTX concentration vs time curves, free MTX peritoneal concentration vs time curves, or peritoneal exit rate of MTX in anti-MTX Fab treated animals relative to controls. Therefore, our pharmacokinetic studies demonstrate that ADAb may produce site-specific alterations in drug pharmacokinetics, potentially enhancing the site specificity of drug distribution and drug action following regional chemotherapy.

Cloning and expression of a murine Fab fragment recognizing a defined linear epitope of Chironomus thummi thummi major allergen Chi t 1-9

We have cloned and expressed in bacteria the genes coding for the Fab fragment of the monoclonal antibody (MoAb) M3. M3 is a murine IgG1 antibody reactive with the major allergen Chi t 1-9 of Chironomus thummi thummi. The major allergen Chi t 1-9 is known to be an aggressive inhalant allergen and causes type-I allergy. The immunoglobulin (Ig) fragment genes were cloned as a synthetic dicistronic operon. In this operon each gene is preceded by a bacterial signal sequence to direct the recombinant protein to the periplasmic space of the bacteria. The cloned genes were expressed in Escherichia coli using the strong T7-RNA-polymerase-based system. Sequence analysis revealed that the light and heavy chains of MoAb M3 belong to the V kappa II and V kappa IIC group of the Ig family, respectively. Genes of the V kappa II and V kappa IIC group are known to be used in response to haptens. The apparent affinity constants of the parent antibody M3 and the recombinant Fab fragment are nearly equivalent.