Screening and kinetic analysis of recombinant anti-CEA antibody fragments

Scalable photonic crystal chips for high sensitivity protein detection

Scalable microfabrication technology has enabled semiconductor and microelectronics industries, among other fields. Meanwhile, rapid and sensitive bio-molecule detection is increasingly important for drug discovery and biomedical diagnostics. In this work, we designed and demonstrated that photonic crystal sensor chips have high sensitivity for protein detection and can be mass-produced with scalable deep-UV lithography. We demonstrated label-free detection of carcinoembryonic antigen from pg/mL to μg/mL, with high quality factor photonic crystal nanobeam cavities.

Development of optical immunosensors for detection of proteins in serum

The detection of proteins in biological samples such as blood, serum or plasma by biosensors is very challenging due to the complex nature of the matrix, which contains a high level of many interfering compounds. Here we show the application of a novel polymeric immobilisation matrix that helps in the detection of specific protein analytes in biological samples by surface plasmon resonance (SPR) immunosensors. This polymer matrix contains thioacetal functional groups included in the network, and these groups do not require any further activation in order to react with proteins, making it attractive for sensor fabrication. The protein prostate specific antigen (PSA) was selected as a model target analyte. A sandwich format with two primary antibodies recognising different parts (epitopes) of the analyte was used for the detection of PSA in serum. The efficiency of the reduction of non-specific binding achieved with novel polymer was compared with those of other techniques such as coating of sensor surface with polyethylene glycol (PEG), use of charged hydrophilic aspartic acid and surfactants such as Tween20. The detection limit of the polymer based immunosensor was 0.1 ng ml(-1) for free form PSA (f-PSA) in buffer and 5 ng ml(-1) in 20% serum. This is an improvement compared with similar devices reported on literature, indicating the potential of the immunosensor developed here for the analysis of real samples.

Carrier-specificity of a phosphorylcholine-binding antibody requires the presence of the constant domains and is not dependent on the unique VH49 glycine or VH30 threonine residues

Bacterially-produced antibody fragments, such as single-chain Fv (scFv) which comprises the variable regions of the light (VL) and heavy (VH) chains joined together by a short flexible linker, are useful as diagnostic and therapeutic agents. We previously constructed a scFv fragment from a hybridoma antibody (Mab2) but it unexpectedly lacked the unique carrier specificity of the native antibody. Thus, it bound indiscriminately to various phosphorylcholine (PC)-associated antigens, whereas the hybridoma antibody recognized the PC epitope only in the context of the immunizing antigen. Here, we investigated whether the problem was linker-related by changing the linker composition or by deleting it, but these attempts proved futile. Instead, we have constructed a recombinant Fab fragment of the antibody in bacteria that was carrier-specific. This suggests that constant regions are required for the carrier specificity, which presumably helps to mould the fine structure of the antibody combining site or in stabilizing such a structure. Consistent with this global effect is the finding that replacing specific residues in VH with germ-line residues, namely, VH49 glycine and VH30 threonine, both thought previously to be important for the carrier specificity, had no effect on the carrier specificity of the recombinant Fab.

A bivalent single-chain Fv fragment against CD47 induces apoptosis for leukemic cells

We constructed a single-chain antibody fragment (scFv) of murine monoclonal antibody, MABL, which specifically bound to human CD47 (hCD47) and induced apoptosis of the leukemic cells. The scFv of MABL antibody with a 15-residue linker (MABL scFv-15) formed both dimer (Mr 50 kDa) and monomer (Mr 25 kDa). Both MABL scFv-15 dimer and monomer had binding activity for hCD47. MABL scFv-15 dimer strongly induced apoptosis of hCD47-introduced mouse leukemic cells in vitro and exhibited anti-tumor effect in a myeloma transplanted mice model. However, MABL scFv-15 monomer scarcely exhibited these activities. These results strongly demonstrate that the ligation of CD47 antigen by two antigen-binding sites of MABL dimer is needed for inducing apoptosis. The parent MABL antibody caused hemagglutination due to the CD47 expressed on erythrocytes. Interestingly, MABL scFv-15 dimer did not cause hemagglutination. This apoptosis-inducing dimer appears to be a lead candidate for novel leukemic therapy.

Humanization of agonistic anti-human 4-1BB monoclonal antibody using a phage-displayed combinatorial library

Given the key role 4-1BB plays in the stimulation of T cells, humanization of agonistic anti-human 4-1BB monoclonal antibody (mAb) may have important clinical applications. In this paper, we present the humanization of agonistic anti-human 4-1BB mAb, BBK-4, using a phage display library. We first prepared the combinatorial library by incorporating murine and human alternative at positions representing buried residues that might affect the structural integrity of the antigen binding site. Six humanized single chain Fv (scFv) fragments were selected from the combinatorial library expressing phage-displayed humanized scFv. They were found to retain the epitope specificity of the original mAb but had affinities of lower than 1/10 of the original. In spite of the lower affinity, the humanized scFv coated on the surface expanded human peripheral blood mononuclear cells (PBMCs) in MLR similarly to the original mAb in the presence of anti-CD3 mAb. These results suggest that humanized anti-human 4-1BB scFvs can be used as a valuable reagent for clinical application.

A single-chain-Fv-based immunofluorometric assay specific for the CEA variant NCA-2

Carcinoembryonic antigen (CEA) is a member of the immunoglobulin gene superfamily. In meconium, a C-terminal truncated form of CEA, denominated NCA-2, is predominant. This molecular variant is present also in sera from adult cancer patients, but no NCA-2-specific assay is yet available, and its possible clinical significance is largely unknown. We have used phage display technology to produce a single-chain antibody (scFv) suitable for an NCA-2-specific assay and compared the NCA-2 results with CEA values in patient sera. A phagemid library with a diversity of 10(7) was constructed from splenic mRNA obtained from a mouse immunized with NCA-2 purified from human meconium. Following phage rescue and three rounds of panning on solid phases coated with NCA-2, several clones were isolated, which displayed high specificity for NCA-2. These were sub-cloned into an Escherichia coli expression vector for high-level expression of soluble scFv. From BIAcore studies a single scFv with low k(off) and a K(D) of 10(-10) mol/l was selected for europium labelling and an immunofluorometric assay with a sensitivity of 0.2 microg/l was established. The assay detects a protein in serum with a molecular weight significantly lower than CEA. For comparison, a routine assay for CEA, which measures both CEA and NCA-2 equivalently, and a CEA-specific assay based on the T84.66 antibody were employed. The difference between serum values measured with the two CEA assays corresponded to the values determined with the NCA-2-specific assay.

Single-chain Fv fragment lacks carrier specificity of the native antibody

A single-chain antibody fragment (scFv) was constructed from a hybridoma antibody that binds to phosphorylcholine (PC) only when this hapten is presented in the form of the immunizing antigen (derived from Trichinella) but not when it is presented on other carriers (as found, for example, in pneumococcal capsules). The scFv derivative was found to lack this carrier specificity as it bound indiscriminately, but specifically, to the various PC-associated antigens, and exhibits a two-fold lower affinity (3.5x10(5)M(-1)) for nitrophenyl-PC than the native antibody. The findings suggest that the scFv combining site is different in fine structure from that of the native antibody.

Construction of dimeric F(ab) useful in blood group serology

BACKGROUND

When expressed in Escherichia coli, recombinant F(ab) contain a heavy-chain Fd fragment and a complete light-chain fragment. Because these F(ab) are monovalent, their avidity is significantly lower than that of a corresponding bivalent IgG antibody. In addition, when monovalent F(ab) are used in hemagglutination assays, antiglobulin reagents are required. Therefore, it would be useful to develop a system that expresses recombinant bivalent F(ab) in E. coli.

STUDY DESIGN AND METHODS

Three modified vectors were constructed. Each contained cDNA sequences encoding a peptide linked to the C terminus of a heavy-chain CH1 region: an IgG1 hinge region (Hinge), a leucine zipper (Zip), or a peptide containing the Hinge and Zip sequences in tandem (HingeZip). The vectors were used to express two cloned F(ab) recognizing human antigens M and N: NNA7 (anti-N) and 425/2B (anti-M). The recombinant proteins were expressed in E. coli and were purified and evaluated by ELISA and hemagglutination.

RESULTS

By gel filtration chromatography, 35, 90, and 70 percent of the purified F(ab) expressing the Hinge, Zip, and HingeZip tails, respectively, were dimers. By ELISA, the avidity of F(ab) containing the Zip or HingeZip tails was six to eight times higher than that of the corresponding monovalent F(ab). In addition, the dimeric F(ab) directly agglutinated RBCs in concentrations similar to those of corresponding bivalent IgG antibodies.

CONCLUSIONS

An introduction of dimer-inducing peptides allowed the isolation of bacterially produced, bivalent F(ab). This approach could be useful for obtaining inexpensive, serologic reagents that may replace or complement conventional MoAbs produced by mammalian tissue culture methods.

A human and a mouse anti-idiotypic antibody specific for human T14(+) anti-DNA antibodies reconstructed by phage display

Little is known about human anti-idiotypic antibodies. Phage display methodology was used to reconstruct these antibodies from lupus patients, which recognize a subset (T14(+)) of anti-DNA antibodies. Antigen-specific B cells were isolated from the blood using a peptide based on a complementarity determining region (V(H)CDR3) of the prototypic T14(+) antibody. cDNA fragments of the V(H) and V(L) genes prepared from the cells were expressed as phage displayed single chain Fv (scFv) fragments using the pCANTAB-5E phagemid vector. From a reactive clone obtained, the Ig genes used were identified to be V(H)3, D5-D3, J(H)4b, V(kappa)I and J(kappa)2. The heavy chain was highly mutated, especially in CDR3, which bears mutations mostly of the replacement type; this region is also unusual in being extremely long due to a D-D fusion. In contrast, a mouse hybridoma antibody, made to the same T14(+) peptide and transformed as a scFv fragment, uses a short V(H)CDR3 comprising five amino acids, three of which are tyrosines. Tyrosines may be important for antigen binding because two of these also exist in the human V(H)CDR3. The light chains of both antibodies may also contribute to the specificity of the protein, because their V(L) segments, including the CDRs, are highly homologous to each other.

Reduced toxicity of expression, in Escherichia coli, of antipollutant antibody fragments and their use as sensitive diagnostic molecules

Single-chain antibody fragments (scAb), specific for the chlorophenoxy acid herbicide mecoprop, have been expressed and purified from the bacterium Escherichia coli. Co-expression with the colE1-compatible, arabinose-inducible, skp expression vector pHELP1 prevented bacterial lysis and significantly increased both total and functional expression yield. The periplasmic protein, SKP, may have a role as a generic detoxification protein. Surface plasmon resonance (BIAcore 2000) analysis confirmed that the purified scAb retained similar binding kinetics to the monoclonal antibody (Mab) from which it was cloned. In competition ELISA, the bacterial scAb showed the same specificity for mecoprop and a related herbicide, MCPA, as the Mab but an increase in sensitivity for free antigen in all ELISA formats. Bacterially expressed antibody fragments provide a simple, sensitive and cost-effective alternative to the traditional production of diagnostic Mabs via tissue culture.

Molecular interactions of Porphyromonas gingivalis fimbriae with host proteins: kinetic analyses based on surface plasmon resonance

Fimbriae of Porphyromonas gingivalis are thought to play an important role in the colonization and invasion of periodontal tissues. In this study, we analyzed the interactions of P. gingivalis fimbriae with human hemoglobin, fibrinogen, and salivary components (i.e., proline-rich protein [PRP], proline-rich glycoprotein [PRG], and statherin) based on surface plasmon resonance (SPR) spectroscopy with a biomolecular interaction analyzing system (BIAcore). The real-time observation showed that the fimbriae interacted more quickly with hemoglobin and PRG than with other proteins and more intensely with fibrinogen. The significant association constant (ka) values obtained by BIAcore demonstrated that the interactions between fimbriae and these host proteins are specific. These estimated Ka values were not too different; however, the Ka values for hemoglobin (2.43 x 10(6)) and fibrinogen (2.16 x 10(6)) were statistically greater than those for the salivary proteins (1.48 x 10(6) to 1.63 x 10(6)). The Ka value of anti-fimbriae immunoglobulin G for fimbriae was estimated to be 1. 22 x 10(7), which was 6.55-fold higher than the mean Ka value of the host proteins. Peptide PRP-C, a potent inhibitor of PRP-fimbriae interaction, dramatically inhibited fimbrial association to PRP and PRG and was also inhibitory against other host proteins by BIAcore. The binding of fimbriae to these proteins was also evaluated by other methods with hydroxyapatite beads or polystyrene microtiter plates. The estimated binding abilities differed considerably, depending on the assay method that was used. It was noted that the binding capacity of PRP was strongly diminished by immobilization on a polystyrene surface. Taken together, these findings suggest that P. gingivalis fimbriae possess a strong ability to interact with the host proteins which promote bacterial adherence to the oral cavity and that SPR spectroscopy is a useful method for analyzing specific protein-fimbriae interactions.

A molecular approach for isolating high-affinity Fab fragments that are useful in blood group serology

BACKGROUND

Multiple mouse hybridoma antibodies recognize the antigens of the MNS blood group system. The Fab fragments of several of these antibodies were expressed on bacteriophage and as soluble proteins. The parental N92 anti-N IgG monoclonal antibody (parental N92 MoAb), but not its monovalent, soluble Fab fragment (N92 Fab fragment), agglutinated antigen-positive red cells by an antiglobulin method. Light-chain shuffling was used to isolate mutant N92 Fab fragments with higher affinity that would function by agglutination.

STUDY DESIGN AND METHODS

Light-chain cDNA libraries, constructed from mice immunized with N-type glycophorin A, were inserted into a recombinant pComb3H vector containing the N92 Fd fragment. The N92 Fd fragment:light-chain libraries were panned on N-type glycophorin A or NN red cells, and antigen-binding clones were isolated. Purified parental N92 MoAb and the Fab fragments were evaluated by enzyme-linked immunosorbent assay and agglutination.

RESULTS

The novel NNA7, C1, and G11 Fab fragments all bound to N-type glycophorin A with higher affinity than did the N92 Fab fragment. The affinity of the library-derived clones was equivalent to that of the parental N92 MoAb. Although their fine specificity differed slightly from the parental N92 MoAb, the clones functioned equivalently by agglutination using an antiglobulin method.

CONCLUSIONS

Light-chain shuffling allowed the isolation of bacterially produced, high-affinity, soluble, monovalent recombinant anti-N Fab fragments that functioned well by agglutination. This approach is useful in obtaining inexpensive serologic reagents that may replace conventional MoAbs produced by tissue culture methods.

Influence of antibody valency in a displacement immunoassay for the quantitation of 2,4-dichlorophenoxyacetic acid

The influence of antibody valency in a displacement immunoassay was investigated by comparing the whole antibody molecule with the corresponding Fab-fragment. The displacement immunoassay for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) takes advantage of the cross-reactivity of monoclonal anti-2,4-D antibodies and the Fab-fragments toward immobilized 2-methyl-4-chlorophenoxyacetic acid (MCPA). Due to the low affinity of the antibodies toward MCPA (cross-reactivity of approximately 30%), the addition of 2,4-D resulted in a displacement of the antibodies or the fragments. The detection limits obtained with whole anti-2,4-D antibodies and Fab-fragments were 0.1 microg/l and 0.01 microg/l 2,4-D, respectively. The whole antibodies and the Fab-fragments show similarities, such as the cross-reactivity toward MCPA (26% and 33%), and some characteristics of the calibration curve, for example the large detection range and the sensitivity. In contrast to the bivalent antibodies, however, increasing the hapten/protein ratios of the immobilized MCPA-BSA conjugates did not affect the detection limit when using the Fab-fragments. Moreover, kinetic experiments reveal a faster displacement reaction with the Fab-fragments. A disadvantage of using the Fab-fragments is the generation of lower absorbance values in the ELISA.

Phage library-derived human anti-TETA and anti-DOTA ScFv for pretargeting RIT

Pretargeting techniques have promise for radioimmunotherapy (RIT) in cancer because of the potential for markedly increasing the therapeutic ratio. Human antibody fragments can be retrieved from phage libraries and used to realize this potential. The library can be used to select the genetic material required to generate molecules with binding sites for both radiochelates and tumor antigens. In this study, human anti-chelate scFvs (single-chain fragments) for two metal chelates (Cu-TETA and Y-DOTA) were selected from a large, naive human scFv library. These anti-chelate scFvs were intended to serve as one arm of bispecific pretargeting molecules and to bind radiochelates given subsequently as Cu-67-TETA or Y-90-DOTA. Phage that displayed the anti-chelate scFv were selected by absorption to antibody (Lym-1) bound Cu-TETA or Y-DOTA. Enzyme-linked immunosorbent assays (ELISA) were performed to assess the intensity and specificity of phage binding to the specific chelate. Ninety-six clones demonstrating metal chelate binding seven times greater than to Lym-1 alone were chosen for diversity analysis. BstN I restriction digests were performed on DNA from these clones. Twenty-three and 43 different DNA fingerprint patterns were identified for anti-TETA and anti-DOTA clones, respectively. DNA sequencing of 39 anti-TETA clones for 23 different BstN I fingerprint patterns revealed 22 distinct sequences. Eleven of the anti-TETA clones were selected for further study. Five hundred to 1000 microg (100 to 320 microg per liter of culture) of purified scFv was produced from each of the 11 anti-TETA clones. Preliminary studies by BIAcore demonstrated evidence of 25- to 200-nM affinities. Comparable examination of the anti-DOTA clones is in progress. This study provides evidence that human scFv against unique synthetic targets can be readily selected from a large, naive human immunoglobulin phage library. Selections against metal chelated antibodies provided a wealth of scFvs with diverse binding affinities useful for engineering molecules for pretargeting RIT.

Comparative sensitivity of immunoassays for haptens using monomeric and dimeric antibody fragments

A single-chain anti-atrazine antibody fragment, scAb (single-chain Fv with a CK domain), was expressed in Escherichia coli, and monomeric and dimeric species were preferentially purified from periplasmic extracts by chromatography upon nickel chelate immunosorbent columns or by immunoaffinity purification using a constant domain (CK) tag. Recombinant monomeric and dimeric antibody fragments, Fab, and intact monoclonal antibodies were compared in assays by competition between free atrazine in solution and (a) immobilized atrazine-bovine serum albumin conjugate (indirect assay) or (b) atrazine-alkaline phosphatase (direct assay). Recombinant antibody fragments provided a lower detection limit than either Fab or intact monoclonal antibody in both assay formats. Monomeric fragments displayed a sensitivity of detection down to 0.1 ppb, compared to 1.0 ppb for dimeric fragments and the parental monoclonal.

Surface plasmon resonance biosensors as a tool in antibody engineering

Modern gene technology combined with efficient microbial expression systems provides tools to produce antibodies with reduced functional size and improved binding properties as well as antibody fusions or novel antibodies. Surface plasmon resonance based biosensors, which measure antigen-antibody interactions in real-time, can be used for a diverse characterization of the modified antibodies. To date, the majority of published work originates from real-time biospecific interaction analysis based on the BIAcore instruments. This article describes the range of applications in antibody engineering in which BIAcore has been applied.

Binding characteristics of anti-atrazine monoclonal antibodies and their fragments synthesised in bacteria and plants

Single-chain antibody fragments (scAb), specific for the herbicide atrazine, have been expressed in the bacterium Escherichia coli and in transgenic tobacco plants. The scAb could be purified as a monomer (monovalent) via a hexa-histidine tail or as a dimer (divalent) by antibody affinity chromatography. In competition ELISA, the bacterial scAb showed the same specificity for atrazine and related triazine herbicides as the parental mAb cell line, but both plant and bacterial monomeric scAbs showed increased sensitivity to free atrazine. Surface plasmon resonance (BIAcore 2000) analysis confirmed that purified scAb, derived from plant or bacteria, retained similar association rates as the mAb. However, the monomeric plant and bacterial scAbs showed a lower affinity for immobilised antigen, than the equivalent dimeric scAbs or mAb. This decrease in affinity was due to a 10 fold slower dissociation rate and is likely due to loss of the avidity contribution of dimeric molecules.

Regulation and expression of human Fabs under the control of the Escherichia coli arabinose promoter, PBAD

BACKGROUND

The L-arabinose operon from E. coli contains an inducible promoter PBAD which has been extensively studied for the control of gene expression. PBAD has a number of potential advantages over Plac, and has been used successfully to promote high level expression of recombinant proteins.

OBJECTIVES

The aim of this study was to investigate PBAD as an alternative system to Plac for the bacterial expression of recombinant Fabs.

STUDY DESIGN

The promoter PBAD from the E. coli arabinose operon araBAD and the gene encoding the regulator of this promoter, were cloned into the phagemid expression vector MCO1. Expression of human recombinant tetanus toxoid (TT) and c-erbB2 Fabs under the control of PBAD was compared at two induction temperatures with the same Fabs produced under the control of Plac.

RESULTS

Expression of TT and c-erbB2 Fabs under the control of PBAD was comparable to Fab expression from Plac. However, highly expressed TT Fab under the control of PBAD was localised to the soluble periplasmic fraction whereas under the control of Plac, there was greater leakage of Fab into the culture supernatant. In addition, Fab expression from PBAD could be more tightly repressed than from Plac.

CONCLUSION

PBAD is a useful and cheaply inducible alternative to the more commonly used Plac for the rapid expression of soluble recombinant human antibody fragments.

Preparation, characterisation and tumour targeting of cross-linked divalent and trivalent anti-tumour Fab' fragments

The monoclonal anti-CEA antibody, A5B7, has previously been administered to patients for radioimmunotherapy (RIT). Long circulation time and the formation of an immune response have limited therapeutic success in the clinic. Antibody fragments can be used to reduce the in vivo circulation time, but the best combination of fragment and radioisotope to use for therapy is far from clear. In this study we have compared the biodistribution of A5B7 IgG and F(ab')2 with chemically cross-linked divalent (DFM) and trivalent (TFM) A5B7 Fab' fragments in nude mice bearing human colorectal tumour xenografts. The cross-linkers were designed to allow site-specific labelling using yttrium 90 (90Y), a high-energy beta-emitter. We have also compared the above antibody forms conjugated to both 131I and 90Y. Both DFM and TFM were fully immunoreactive and remained intact after radiolabelling and incubation in serum at 37 degrees C for 24 h. Biodistribution results showed similar tumour uptake levels and an identical blood clearance pattern for F(ab')2 and DFM with high tumour-blood ratios generated in each case. However, unacceptably high kidney accumulation for both F(ab')2 and DFM and elevated splenic uptake of DFM labelled with 90Y was observed. Kinetic analysis of antigen binding revealed that DFM had the fastest association rate (kass = 1.6 x 10(5) Ms-1) of the antibody forms, perhaps owing to increased flexibility of the cross-linker. This advantage implies that DFM may be more suitable than F(ab')2 radiolabelled with 131I for RIT. TFM cleared from the blood significantly faster than A5B7 IgG when labelled with both 131I and 90Y, producing an improved therapeutic tumour-blood ratio. Kidney accumulation was not observed for [90Y]TFM, but a slightly higher splenic uptake was observed that may indicate reticuloendothelial system (RES) uptake. Overall, tumour uptake was higher for 90Y-labelled antibodies than for 131I-labelled antibodies. Because of the faster clearance, it should be possible to administer a higher total dose of 90Y-labelled TFM than IgG, which is attractive for RIT. Both A5B7 DFM and TFM, therefore, show favourable properties compared with their parent antibody forms.

Determination of active single chain antibody concentrations in crude periplasmic fractions

We have measured active single chain antibody (scFv) concentrations under mass transfer limitation conditions using surface plasmon resonance on the BIAcore. For the creation of a standard curve scFv 4Dwt, derived from monoclonal antibody (mAb) 4D, directed against human chorionic gonadotropin (hCG), was purified by positive affinity chromatography. Determination of the active antibody fraction after purification was performed using anti-FLAG, reactive against a tag sequence C-terminally fused to the scFv. Two independent experiments showed that the activity remaining represented over 75% of the total amount of purified protein. Calibration curves on high density antigen surfaces showed a linear relationship between antibody concentration and binding rates. Periplasmic fractions of six mutant scFvs, also derived from mAb 4D, revealed a clear difference in the amount of soluble active scFv expressed in the periplasm of E. coli compared with the total amount of antibody present, indicating the necessity of measuring active antibody concentrations. This rapid concentration determination method will be particularly useful for accurately comparing affinity constants, using antibody concentrations determined with the BIAcore, of the many different scFv fragments routinely isolated from phage display libraries.

An enzyme-linked oligonucleotide assay

The recent development of in vitro methods to select high-affinity ligands by combinatorial chemistry methodologies promises unique and theoretically unlimited supplies of novel therapeutic and diagnostic reagents. One such combinatorial chemistry process, systematic evolution of ligands by exponential enrichment (SELEX), allows rapid identification, from large random sequence pools, of the few oligonucleotide sequences that bind to a desired target molecule with high affinity and specificity. We describe an enzyme-linked sandwich assay that uses a SELEX-derived oligonucleotide. This assay demonstrates that these oligonucleotides can be effective and useful analytical reagents.

Tumor localization of anti-CEA single-chain Fvs: improved targeting by non-covalent dimers

BACKGROUND

Genetic engineering can produce novel antibody fragments with improved properties for applications such as tumor targeting in vivo.

OBJECTIVES

To produce stable monomeric (27 kDa) and dimeric (55 kDa) forms of a single-chain Fv (scFv) from the anti-carcinoembryonic antigen (anti-CEA) antibody T84.66, and assess the targeting and biodistribution properties in an animal model.

STUDY DESIGN

ScFv were constructed with either a 28 or 14 amino acid connecting peptide and expressed by secretion from E. coli. Following affinity purification, proteins were characterized by gel electrophoresis and mass spectrometry. Binding properties were assessed by size exclusion HPLC after incubation with antigen, and affinities determined by surface plasmon resonance. The shorter linker favored formation of dimers (and higher multimers) which showed unusual stability. ScFv were radiolabeled with 125I for tumor targeting and biodistribution studies of monomeric or dimeric forms were conducted in athymic mice bearing LS174T human colorectal carcinoma xenografts.

RESULTS

125I-scFv monomers and dimers targeted exhibited rapid clearance kinetics in tumor-bearing mice. Nevertheless, the anti-CEA scFvs targeted very well to xenografts, leading to high tumor: normal organ ratios (greater than 20:1 at 24 h) for both forms. Tumor localization of the non-covalent dimers was much higher than monomers, reaching 10-15% injected dose per gram at 1 h.

CONCLUSION

Non-covalent dimers of scFv (also known as diabodies) are stable, easy to produce and show excellent targeting as compared to monomeric scFv, probably due to increased mass and valency.