Structural and computational investigations of the conformation of antigenic peptide fragments of human polymorphic epithelial mucin

Glycosylation of MUC1 influences the binding of a therapeutic antibody by altering the conformational equilibrium of the antigen

In cancer cells, the glycoprotein Mucin 1 (MUC1) undergoes abnormal, truncated glycosylation. The truncated glycosylation exposes cryptic peptide epitopes that can be recognized by antibodies. Since these immunogenic regions are cancer specific, they represent ideal targets for therapeutic antibodies. We investigated the role of tumor-specific glycosylation on antigen recognition by the therapeutic antibody AR20.5. We explored the affinity of AR20.5 to a synthetic cancer-specific MUC1 glycopeptide and peptide. The antibody bound to the glycopeptide with an order of magnitude stronger affinity than the naked peptide. Given these results, we postulated that AR20.5 must specifically bind the carbohydrate as well as the peptide. Using X-ray crystallography, we examined this hypothesis by determining the structure of AR20.5 in complex with both peptide and glycopeptide. Surprisingly, the structure revealed that the carbohydrate did not form any specific polar contacts with the antibody. The high affinity of AR20.5 for the glycopeptide and the lack of specific binding contacts support a hypothesis that glycosylation of MUC1 stabilizes an extended bioactive conformation of the peptide recognized by the antibody. Since high affinity binding of AR20.5 to the MUC1 glycopeptide may not driven by specific antibody-antigen contacts, but rather evidence suggests that glycosylation alters the conformational equilibrium of the antigen, which allows the antibody to select the correct conformation. This study suggests a novel mechanism of antibody-antigen interaction and also suggests that glycosylation of MUC1 is important for the generation of high affinity therapeutic antibodies.

Principles of mucin structure: implications for the rational design of cancer vaccines derived from MUC1-glycopeptides

Cancer is currently one of the world's most serious public health problems. Significant efforts are being made to develop new strategies that can eradicate tumours selectively without detrimental effects to healthy cells. One promising approach is focused on the design of vaccines that contain partially glycosylated mucins in their formulation. Although some of these vaccines are in clinical trials, a lack of knowledge about the molecular basis that governs the antigen presentation, and the interactions between antigens and the elicited antibodies has limited their success thus far. This review focuses on the most significant milestones achieved to date in the conformational analysis of tumour-associated MUC1 derivatives both in solution and bound to antibodies. The effect that the carbohydrate scaffold has on the peptide backbone structure and the role of the sugar in molecular recognition by antibodies are emphasised. The outcomes summarised in this review may be a useful guide to develop new antigens for the design of cancer vaccines in the near future.

A Synthetic Glycopeptide Vaccine for the Induction of a Monoclonal Antibody that Differentiates between Normal and Tumor Mammary Cells and Enables the Diagnosis of Human Pancreatic Cancer

In studies within the realm of cancer immunotherapy, the synthesis of exactly specified tumor-associated glycopeptide antigens is shown to be a key strategy for obtaining a highly selective biological reagent, that is, a monoclonal antibody that completely differentiates between tumor and normal epithelial cells and specifically marks the tumor cells in pancreas tumors. Mucin MUC1, which is overexpressed in many prevalent cancers, was identified as a promising target for this strategy. Tumor-associated MUC1 differs significantly from that expressed by normal cells, in particular by altered glycosylation. Structurally defined tumor-associated MUC1 cannot be isolated from tumor cells. We synthesized MUC1-glycopeptide vaccines and analyzed their structure-activity relationships in immunizations; a monoclonal antibody that specifically distinguishes between human normal and tumor epithelial cells was thus generated.

Enhanced immunogenicity of multivalent MUC1 glycopeptide antitumour vaccines based on hyperbranched polymers

A fully synthetic multivalent MUC1 glycopeptide vaccine based on a hyperbranched polyglycerol core induced IgG antibodies which strongly recognise epithelial tumour cells.

ω-Turn: a novel β-turn mimic in globular proteins stabilized by main-chain to side-chain C−H···O interaction

Mimicry of structural motifs is a common feature in proteins. The 10-membered hydrogen-bonded ring involving the main-chain C − O in a β-turn can be formed using a side-chain carbonyl group leading to Asx-turn. We show that the N − H component of hydrogen bond can be replaced by a C(γ) -H group in the side chain, culminating in a nonconventional C − H···O interaction. Because of its shape this β-turn mimic is designated as ω-turn, which is found to occur ∼ three times per 100 residues. Three residues (i to i + 2) constitute the turn with the C − H···O interaction occurring between the terminal residues, constraining the torsion angles ϕi + 1, ψi + 1, ϕi + 2 and χ'1(i + 2) (using the interacting C(γ) atom). Based on these angles there are two types of ω-turns, each of which can be further divided into two groups. C(β) -branched side-chains, and Met and Gln have high propensities to occur at i + 2; for the last two residues the carbonyl oxygen may participate in an additional interaction involving the S and amino group, respectively. With Cys occupying the i + 1 position, such turns are found in the metal-binding sites. N-linked glycosylation occurs at the consensus pattern Asn-Xaa-Ser/Thr; with Thr at i + 2, the sequence can adopt the secondary structure of a ω-turn, which may be the recognition site for protein modification. Location between two β-strands is the most common occurrence in protein tertiary structure, and being generally exposed ω-turn may constitute the antigenic determinant site. It is a stable scaffold and may be used in protein engineering and peptide design.

Water-soluble polymers coupled with glycopeptide antigens and T-cell epitopes as potential antitumor vaccines

Highly decorated: Tumor-associated MUC1 glycopeptide and tetanus toxoid T-cell epitope P2 can be attached to water-soluble poly(N-(2-hydroxypropyl)methacrylamide) carriers by orthogonal ligation techniques. Fully synthetic vaccine A with additional nanostructure-promoting domains induced antibodies that exhibit high affinity to tumor cells.

Synthesis and structural model of an alpha(2,6)-sialyl-t glycosylated MUC1 eicosapeptide under physiological conditions

To study the effect of O-glycosylation on the conformational propensities of a peptide backbone, a 20-residue peptide (GSTAPPAHGVTSAPDTRPAP) representing the full length tandem repeat sequence of the human mucin MUC1 and its analogue glycosylated with the (2,6)-sialyl-T antigen on Thr11, were prepared and investigated by NMR and molecular modeling. The peptides contain both the GVTSAP sequence, which is an effective substrate for GalNAc transferases, and the PDTRP fragment, a known epitope recognized by several anti-MUC1 monoclonal antibodies. It has been shown that glycosylation of threonine in the GVTSAP sequence is a prerequisite for subsequent glycosylation of the serine at GVTSAP. Furthermore, carbohydrates serve as additional epitopes for MUC1 antibodies. Investigation of the solution structure of the sialyl-T glycoeicosapeptide in a H(2)O/D(2)O mixture (9:1) under physiological conditions (25 degrees C and pH 6.5) revealed that the attachment of the saccharide side-chain affects the conformational equilibrium of the peptide backbone near the glycosylated Thr11 residue. For the GVTSA region, an extended, rod-like secondary structure was found by restrained molecular dynamics simulation. The APDTR region formed a turn structure which is more flexibly organized. Taken together, the joined sequence GVTSAPDTR represents the largest structural model of MUC1 derived glycopeptides analyzed so far.

Subtle sequence differences in a tumour-associated peptide epitope translate into major changes in antigenicity

Antigenicity, the ability to bind to members of repertoire of diverse immune receptors, is a concept that is poorly characterised with respect to its defining parameters. To learn more about its makeup, we have investigated the ability of two peptides with highly related sequences, derived from the tumour-associated antigen mucin-1, to recruit in vitro members from a large naïve repertoire of synthetic human antibody fragments. One of the peptides represents the epitope that is immunodominant in mice. We now demonstrate that the other peptide, which differs from the first only by a very conservative aspartate-threonine to glutamate-serine change, is much less antigenic than the first peptide. This is so despite the fact that there is no observable difference in the tendency of the two peptides to adopt a structure in solution. Furthermore, the peptides differ in their immunodominant parts and the less antigenic peptide selects for antibody fragments targeting residues outside of the epitope considered to be immunodominant in mice. We conclude that subtle sequence changes greatly, affect antigenicity and immunodominance of epitopes in this important tumour-associated antigen.

Structural and dynamic consequences of increasing repeats in a MUC1 peptide tumor antigen

MUC1 mucin is a large transmembrane glycoprotein whose extracelluler domain is composed of repeating units of a 20 amino acid sequence. In the cancer associated state, this protein expression becomes upregulated and underglycosylated. Previous studies, which show an enhanced binding of a 5-repeat over a 1-repeat MUC1 peptide to a panel of anti-MUC1 antibodies, have led us to investigate the structural and dynamic consequences of increasing repeat number. Two MUC1 peptides were studied: a 16mer corresponding to slightly less than one full repeat of the MUC1 tandem repeat sequence (GVTSAPDTRPAPGSTA) and a 40mer corresponding to two full repeats of the MUC1 sequence (VTSAPDTRPAPGSTAPPAHG)2. Isotopically labeled versions of these MUC1 peptides were cloned, expressed, purified, and evaluated structurally and dynamically using 15N- and 13C-edited NMR approaches. The data show that MUC1 structure, dynamics, and antibody binding affinity are invariant with increasing repeat number. In light of these results, we conclude that the enhanced antibody affinity of the 5-repeat over the 1-repeat MUC1 peptide is due to multivalency effects, and not due to the development of higher order structure in the longer length peptides. The implications of these results are discussed within the context of a multiple repeat MUC1 breast cancer vaccine design.

MUC1 gene-derived glycoprotein assays for monitoring breast cancer (CA 15-3, CA 27.29, BR): are they measuring the same antigen?

CONTEXT

There are 2 general types of assays measuring MUC1 gene-derived glycoprotein: assays for cancer antigen (CA) 15-3, which are sandwich assays, and assays for CA 27.29, which are competitive assays. These 2 types of assays measure slightly different parts of this tandem-repeat molecule. Across-method assay differences hinder the exchange of patient test values among integrated health care networks and among countries.

OBJECTIVE

This report evaluates the method differences among these assays to determine if the differences between these assays are mainly related to variations in calibration or differences in analyte specificity.

DESIGN

Data from 22 College of American Pathologists survey challenges were analyzed to compare 10 commercial assay methods for these 2 related analytes. In addition, data from 58 patient samples were analyzed to compare 3 of these assays.

RESULTS

The linear correlation coefficients comparing the within-method medians of these proficiency test distributions were very high (>0.99) for all of the methods; however, the regression slopes varied from 0.836 to 1.095. The regression slopes for the patient specimens varied similarly, but the correlation coefficients were lower.

CONCLUSIONS

This study indicates that many of the test value differences for these measurements are due to differences in assay calibration rather than differences in the specificity of the assay measurement systems. Survey test data potentially could be used to help harmonize these assay differences.

Probing the conformational and dynamical effects of O-glycosylation within the immunodominant region of a MUC1 peptide tumor antigen

MUC1 mucin is a large transmembrane glycoprotein, the extracellular domain of which is formed by a repeating 20 amino acid sequence, GVTSAPDTRPAPGSTAPPAH. In normal breast epithelial cells, the extracellular domain is densely covered with highly branched complex carbohydrate structures. However, in neoplastic breast tissue, the extracellular domain is under-glycosylated, resulting in the exposure of a highly immunogenic core peptide epitope (PDTRP in bold above), as well as in the exposure of normally cryptic core Tn (GalNAc), STn (sialyl alpha2-6 GalNAc) and TF (Gal beta1-3 GalNAc) carbohydrates. Here, we report the results of 1H NMR structural studies, natural abundance 13C NMR relaxation measurements and distance-restrained MD simulations designed to probe the structural and dynamical effects of Tn-glycosylation within the PDTRP core peptide epitope. Two synthetic peptides were studied: a nine-residue MUC1 peptide of the sequence, Thr1-Ser2-Ala3-Pro4-Asp5-Thr6-Arg7-Pro8-Ala9, and a Tn-glycosylated version of this peptide, Thr1-Ser2-Ala3-Pro4-Asp5-Thr6(alphaGalNAc)-Arg7-Pro8-Ala9. The results of these studies show that a type I beta-turn conformation is adopted by residues PDTR within the PDTRP region of the unglycosylated MUC1 sequence. The existence of a similar beta-turn within the PDTRP core peptide epitope of the under-glycosylated cancer-associated MUC1 mucin protein might explain the immunodominance of this region in vivo, as the presence of defined secondary structure within peptide epitope regions has been correlated with increased immunogenicity in other systems. Our results have also shown that Tn glycosylation at the central threonine within the PDTRP core epitope region shifts the conformational equilibrium away from the type I beta-turn conformation and toward a more rigid and extended state. The significance of these results are discussed in relation to the possible roles that peptide epitope secondary structure and glycosylation state may play in MUC1 tumor immunogenicity.

Structural effects of O-glycosylation on a 15-residue peptide from the mucin (MUC1) core protein

To study the effect of O-glycosylation on the conformational propensities of a peptide backbone, the 15-residue peptide PPAHGVTSAPDTRPA (PPA15) from the MUC1 protein core and its analogue PPA15(T7), glycosylated with alpha-N-acetylgalactosamine on Thr7, were prepared and investigated by NMR spectroscopy. The peptide contains both the GVTSAP sequence, which is an effective substrate for GalNAc-T1 and -T3 transferases, and the PDTRP fragment, which is a well-known immunodominant epitope recognized by several anti-MUC1 monoclonal antibodies. Useful structural results were obtained in water upon decreasing the temperature to 5-10 degrees C. The sugar attachment slightly affected the conformational equilibrium of the peptide backbone near the glycosylated Thr7 residue. The clustering of low-energy conformations for both PPA15 and PPA15(T7) within the GVTSAP and APDTRP fragments revealed structural similarities between glycosylated and nonglycosylated peptides. For the GVTSAP region, minor but distinct clusters formed by either PPA15 or PPA15(T7) conformers showed distinct structural propensities of the peptide backbone specific for either the nonglycosylated or the glycosylated peptide. The peptide backbone of the APDTRP fragment, which is a well-known immunodominant region, resembled an S-shaped bend. A similar structural motif was found in the GVTSAP fragment. The S-shaped structure of the peptide backbone is formed by consecutive inverse gamma-turn conformations partially stabilized by hydrogen bonding. A comparison of the solution structure of the APDTRP fragment with a crystal structure of the MUC1 peptide antigen bound to the breast tumor-specific antibody SM3 demonstrated significant structural similarities in the general shape.

Structural analysis of peptide substrates for mucin-type O-glycosylation

The structures of three nine-residue peptide substrates that show differential kinetics of O-linked glycosylation catalyzed by distinct recombinant uridine diphosphate-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc transferases) were investigated by NMR spectroscopy. A combined use of NMR data, molecular modeling techniques, and kinetic data may explain some structural features required for O-glycosylation of these substrates by two GalNAc transferases, GalNAc-T1 and GalNAc-T3. In the proposed model, the formation of an extended backbone structure at the threonine residue to be glycosylated is likely to enhance the O-glycosylation process. The segment of extended structure includes the reactive residue in a beta-like or an inverse gamma-turn conformation and flanking residues in a beta-strand conformation. The hydroxyl group of the threonine to be glycosylated is exposed to solvent, and both the amide proton and carbonyl oxygen of the peptide backbone are exposed to solvent. The exchange rate of the amide proton for the reactive threonine correlated well with substrate efficiency, leading us to hypothesize that this proton may serve as a donor for hydrogen bonding with the active site of the enzyme. The oxygens of the residue to be glycosylated and several flanking residues may also be involved in a set of hydrogen bonds with the GalNAc-T1 and -T3 transferases.

Effects of glycosylation on fragments of tumour associated human epithelial mucin MUC1

The glycodecapeptide AcPAPGS(alpha GalNAc)T(alpha GalNAc)APPA and the C-terminal glycohexapeptide AcS(alpha GalNAc)T(alpha GalNAc)APPA have been synthesized by applying the N-terminal Fmoc group in combination with the heptyl ester cleavable by lipase-catalyzed hydrolysis at pH 7. The solution conformation of these MUC1-related synthetic glycopeptides and the control, non-glycosylated decapeptide AcPAPGSTAPPA have been investigated using NMR spectroscopy. The structural studies indicate that the glycohexapeptide has a folded structure in solution. For this molecule, unrestrained molecular dynamics has been used to confirm the presence of the observed solution through-space connections. The results indicate that the non-globular nature of MUC1 is due to both protein core sequence and the effect of carbohydrate.

Surface structure of human mucin using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface sensitive analytical technique that measures the binding energy of electrons in atoms and molecules on the surface of a material. XPS was used to determine the distribution of the oligosaccharide side chains in the glycoprotein, MUC1 mucin. Low-resolution XPS spectra provided elemental composition of MUC1 mucin (fully glycosylated), mucin polypeptide (nonglycosylated), and carbohydrates found in mucin. The nitrogen content of MUC1 mucin was determined to be intermediate between the mucin polypeptide and the carbohydrates. Assuming a uniform distribution of carbohydrate on MUC1 mucin, the average thickness of the carbohydrate layer was calculated to be 4.9 nm using the low-resolution N 1s signals. High-resolution XPS spectra give detailed information about the chemical bonding of the surface molecules. Calculations based on the high-resolution O 1s spectra showed a carbohydrate thickness of 6.6 nm. These experimentally determined values agree reasonably well with an estimated 5 nm of carbohydrate thickness from a simple model which assume that the core protein is a rodlike molecule approximately 5 nm in diameter. Although the carbohydrate coating on the MUC1 mucin appears to be thick enough to cover the core protein entirely, fully glycosylated breast milk MUC1 mucin is susceptible to proteolytic digestion without removal of any oligosaccharide side chain, suggesting areas of exposed core protein. A possible explanation is that the oligosaccharide side chains may form patches of carbohydrate along the core protein with regions of exposed core protein.

Effect of glycosylation of a synthetic MUC1 mucin-core-related peptide on recognition by anti-mucin antibodies

Human epithelial mucins are heterogeneously glycosylated proteins associated with breast and ovarian cancer. Several peptide-reactive anti-mucin MUC1 monoclonal antibodies are used in experimental and diagnostic assays but it is not known how glycosylation of the mucin influences antibody recognition. In this report we show that increasing glycosylation of a synthetic 25-amino acid fragment of the MUC1 core protein with N-acetylgalactosamine (GalNAc) elicits different responses in its recognition by two anti-MUC1 antibodies, C595 and HMFG1. We propose that increasing glycosylation of the synthetic mucin fragment produces an alteration in the structure of the epitope which enhances binding in C595, but not in HMFG1.

Structure of a tumor associated antigen containing a tandemly repeated immunodominant epitope

Human mucins are T or S glycosylated tandem repeat proteins. In breast cancer, mucins become under or unglycosylated. Two-dimensional nuclear magnetic resonance experiments are performed on chemically synthesized mucin tandem repeat polypeptides, (PDTRPAPGST-APPAHGVTSA)n the unglycosylated form for n=1,3 where (APDTR) constitutes the antigenic sites for the antibodies isolated form the tumors in the breast cancer patients. These studies demonstrate how the tandem repeats assemble in space giving rise to the overall tertiary structure, and the local structure and presentation of the antigenic site(APDTR) at the junction of two neighboring repeats. The NMR data reveal repeating knob-like structures connected by extended spacers. The knobs protrude away from the long-axis of Muc-1 and the predominant antigenic site (APDTR) forms the accessible tip of the knob. Multiple tandem repeats enhance the rigidity and presentation of the knob-like structures.

Structurally defined synthetic cancer vaccines: analysis of structure, glycosylation and recognition of cancer associated mucin, MUC-1 derived peptides

Translation of an immune response into therapy is probably the toughest task in designing vaccines for cancer due to the heterogeneity of the cell surface antigens which display tremendous variations in glycoforms. Consequently, a small segment (antigen) of cancer-associated mucin, in spite of generating antigen-specific immune responses, may be limited in therapeutic value. It is important that the synthetic segment resembles the native cancer-associated mucin in both structure and conformation. Synthetic cancer associated mucin derived 16 amino acid peptide GVTSAPDTRPAPGSTA and its partially glycosylated forms have demonstrated specific binding to two monoclonal antibodies, B27.29 and BCP8, raised against the native cancer associated mucin, MUC-1 and a MUC-1 derived synthetic peptide, respectively. In spite of the structural similarities at the core peptide level of both glycosylated and unglycosylated peptides, it appears that partial glycosylation does not inhibit and even slightly enhances binding to the MAb B27.29 indicating that the glycosylated synthetic peptide more closely resembles the native mucin epitope recognized by MAb B27.29. From molecular dynamic simulations using NMR derived distance constraints, both glycosylated and unglycosylated peptides have shown a type 1 beta turn involving the same amino acids in both glycosylated and unglycosylated peptides. The alpha GalNAc attached to the threonine (T3) and serine (S4) in the 16 amino acid sequence has not imposed any conformational changes to the peptide backbone nor has offered severe steric resistance to the binding of either antibody to the glycopeptides as indicated by hapten inhibition studies. Nevertheless, all peptides have displayed glycosylation dependent specificities in binding to these antibodies, i.e. the glycosylated peptides demonstrated relative higher affinities to the native mucin antibody B27.29 while the unglycosylated peptide is more specific to the MAb BCP8. Immune responses generated by these synthetic glycopeptides are highly specific in recognizing the native cancer associated mucin.

Primary sequence determination and molecular modelling of the variable region of an antiMUC1 mucin monoclonal antibody

Polymerase chain reaction (PCR) products representative of the DNA sequence coding for the variable heavy (VH) and the variable light (VL) chains of an antiMUC1 mucin monoclonal antibody, C595, have been produced. These products were cloned, sequenced, and the primary amino acid sequences of the VH and VL regions deduced. The hypervariable complementarity determining regions (CDRs) and framework regions in the heavy and light chains were located, and homologies with canonical forms for the CDR loops L1, L2, L3, H1 and H2 were identified by database searching. The structure for the H3 loop was calculated directly. Computational molecular modelling was accomplished using the fully automated AbM package (Oxford Molecular, Oxford, U.K.). Energy minimisation was performed using the program InsightII (Biosym, San Diego, California, U.S.A.). The investigation provides a basis for the molecular analysis of the antigen binding site of the C595 antibody with the aim to identify key residues and interactions involved in the immune recognition of the C595 antibody defined epitope, which is expressed in the majority of breast and ovarian carcinomas.

Characterisation of a recombinant Fv fragment of anti-MUC1 antibody HMFG1

A recombinant Fv (variable fragment) has been produced for the murine monoclonal antibody HMFG1. This antibody was raised against human milk fat globules and reacts with an epitope (PDTR) in the protein core of MUC1 mucins, which are up-regulated in human breast and other carcinomas. Binding specificity of the Fv fragment has been demonstrated through immunoaffinity purification, and by radioimmunoassay. The affinity constants for this Fv fragment and for the proteolytically produced Fab (antigen binding fragment) of the related humanised antibody HuHMFG1 were determined by monitoring the fluorescence quenching of the antibody fragments whilst adding aliquots of MUC1 related antigenic peptides KAPDTRPAPG and VTSAPDTRPAPG. Using these techniques it has been demonstrated that the products of these different methods of antibody fragmentation are comparable, and suitable for solution structure analysis using nuclear magnetic resonance (NMR) spectroscopy.

Induction of antibody responses to breast carcinoma associated mucins using synthetic peptide constructs as immunogens

A strategy for directing and enhancing B cell immune responses against synthetic peptide determinants has been developed in order to produce antibodies specifically against protein epitopes of clinical relevance. A peptide sequence based upon the MUC-1 mucin protein core was selected for this purpose since anti-MUC-1 antibodies have proven diagnostic application and therapeutic potential in human breast and ovarian cancer. Peptide constructs were synthesised co-linearly linking the immunodominant B cell determinant region, PDTRPAP, in the protein core of the MUC-1 mucin, to sequence 111-120 of influenza haemagglutinin A/X-31, a determinant recognised by T helper cells through association with MHC class II molecules. Induction of anti-MUC-1 antibodies to the B cell determinant region by immunisation with peptide was shown to be dependent upon both the presence and the position of the T cell determinant. In addition, haplotype mismatching with respect to the T cell determinant resulted in a significant lowering of the anti-MUC-1 antibody response in peptide construct immunised mice. These findings are relevant to the design of immunogens to produce antibodies against peptide epitopes of tumour associated proteins and glycoproteins.

Construction of a reshaped HMFG1 antibody and comparison of its fine specificity with that of the parent mouse antibody

A human antibody with milk mucin specificity was obtained by transferring the complementarity determining regions (CDR) of the mouse antibody HMFG1 onto carefully selected human framework regions. The resulting reshaped human antibody, HuHMFG1, showed no difference in relative affinity for its antigen compared with the parent mouse HMFG1. The minimum epitope recognized by both the mouse and reshaped antibodies was demonstrated by epitope mapping to be identical, and consists of the tetramer PDTR. In a replacement net analysis, in which each of the amino acids was replaced in turn with the 19 other residues, it was determined that mouse HMFG1 and HuHMFG1 reacted with this series of synthetic peptides in an equivalent manner, indicating retention of identical fine specificity in the HuHMFG1 antibody. In contrast to other published reports, this was achieved without involvement of any framework residues in the binding site transfer. These data demonstrate that if well-matching human framework regions are employed grafting the CDR only can be sufficient to confer desired specificities to human antibodies and can, indeed, provide human analogues of mouse antibodies with virtually indistinguishable affinities and fine specificities relative to the mouse parent antibodies.