Protein arrays: issues to be addressed

Protein arrays are fast becoming established as a means to monitor protein expression levels and investigate protein interactions and function. They present particular technical demands that will need to be solved in order to achieve the maximum capability of efficient and sensitive protein analysis in the high throughput setting of functional genomics. The following resumé of some major issues around this new technology was made as the chairperson’s introduction to the workshop session on peptide and protein chips.

Autoantibodies to alpha-fodrin in primary Sjögren's syndrome and SLE detected by an in vitro transcription and translation assay

OBJECTIVE

To investigate the prevalence of alpha-fodrin autoantibodies in primary Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE) with and without secondary SS, using an in vitro transcription and translation assay (ITT).

METHODS

cDNA encoding JS-1, the amino-terminal portion of alpha-fodrin, was used for ITT. Immunoprecipitation was performed with sera from 56 primary SS patients and 67 SLE patients, 14 with and 53 without secondary SS. Correlations to RF, ANA, anti-dsDNA, anti-SS-A and anti-SS-B antibodies, hypergammaglobulinemia, labial salivary gland biopsy grade, extraglandular manifestations and a modified SLE disease activity index (mSLEDAI) were made.

RESULTS

Autoantibodies against alpha-fodrin were detected in 16/56 (29%) of primary SS patients and in 25/53 (47%) of sera from SLE patients without secondary SS. In SLE patients with secondary SS the prevalence was 3/14 (21%). None of the blood donors showed alpha-fodrin reactivity. Correlations were found to RF, ANA, anti-dsDNA antibodies and a positive mSLEDAI score.

CONCLUSION

The frequency of alpha-fodrin autoantibodies detected by this method is similar in sera from primary SS patients and SLE patients with or without secondary SS. The presence of alpha-fodrin autoantibodies seems to reflect non-organ-specific autoimmunity in primary SS and SLE and to be of limited discriminating value.

Single step generation of protein arrays from DNA by cell-free expression and in situ immobilisation (PISA method)

We describe a format for production of protein arrays termed 'protein in situ array' (PISA). A PISA is rapidly generated in one step directly from PCR-generated DNA fragments by cell-free protein expression and in situ immobilisation at a surface. The template for expression is DNA encoding individual proteins or domains, which is produced by PCR using primers designed from information in DNA databases. Coupled transcription and translation is carried out on a surface to which the tagged protein adheres as soon as it is synthesised. Because proteins generated by cell-free synthesis are usually soluble and functional, this method can overcome problems of insolubility or degradation associated with bacterial expression of recombinant proteins. Moreover, the use of PCR-generated DNA enables rapid production of proteins or domains based on genome information alone and will be particularly useful where cloned material is not available. Here we show that human single-chain antibody fragments (three domain, V(H)/K form) and an enzyme (luciferase) can be functionally arrayed by the PISA method.

Complex between Peptostreptococcus magnus protein L and a human antibody reveals structural convergence in the interaction modes of Fab binding proteins

BACKGROUND

Peptostreptococcus magnus protein L (PpL) is a multidomain, bacterial surface protein whose presence correlates with virulence. It consists of up to five homologous immunoglobulin binding domains that interact with the variable (VL) regions of kappa light chains found on two thirds of mammalian antibodies.

RESULTS

We refined the crystal structure of the complex between a human antibody Fab fragment (2A2) and a single PpL domain (61 residues) to 2.7 A. The asymmetric unit contains two Fab molecules sandwiching a single PpL domain, which contacts similar VL framework regions of two light chains via independent interfaces. The residues contacted on VL are remote from the hypervariable loops. One PpL-Vkappa interface agrees with previous biochemical data, while the second is novel. Site-directed mutagenesis and analytical-centrifugation studies suggest that the two PpL binding sites have markedly different affinities for VL. The PpL residues in both interactions are well conserved among different Peptostreptococcus magnus strains. The Fab contact positions identified in the complex explain the high specificity of PpL for antibodies with kappa rather than lambda chains.

CONCLUSIONS

The PpL-Fab complex shows the first interaction of a bacterial virulence factor with a Fab light chain outside the conventional combining site. Structural comparison with two other bacterial proteins interacting with the Fab heavy chain shows that PpL, structurally homologous to streptococcal SpG domains, shares with the latter a similar binding mode. These two bacterial surface proteins interact with their respective immunoglobulin regions through a similar beta zipper interaction.

Crystal structure of alkaline phosphatase from human placenta at 1.8 A resolution. Implication for a substrate specificity

Human placental alkaline phosphatase (PLAP) is one of three tissue-specific human APs extensively studied because of its ectopic expression in tumors. The crystal structure, determined at 1.8-A resolution, reveals that during evolution, only the overall features of the enzyme have been conserved with respect to Escherichia coli. The surface is deeply mutated with 8% residues in common, and in the active site, only residues strictly necessary to perform the catalysis have been preserved. Additional structural elements aid an understanding of the allosteric property that is specific for the mammalian enzyme (Hoylaerts, M. F., Manes, T., and Millán, J. L. (1997) J. Biol. Chem. 272, 22781-22787). Allostery is probably favored by the quality of the dimer interface, by a long N-terminal alpha-helix from one monomer that embraces the other one, and similarly by the exchange of a residue from one monomer in the active site of the other. In the neighborhood of the catalytic serine, the orientation of Glu-429, a residue unique to PLAP, and the presence of a hydrophobic pocket close to the phosphate product, account for the specific uncompetitive inhibition of PLAP by l-amino acids, consistent with the acquisition of substrate specificity. The location of the active site at the bottom of a large valley flanked by an interfacial crown-shaped domain and a domain containing an extra metal ion on the other side suggest that the substrate of PLAP could be a specific phosphorylated protein.

Anti-anti-idiotypic (Ab3) antibodies that bind progesterone-11alpha-bovine serum albumin differ in their combining sites from antibodies raised directly against the antigen

Polyclonal rabbit anti-idiotypic (Ab2) antibodies raised against the antiprogesterone mAb DB3 (Ab1) were used to induce an Ab3 antiprogesterone response in BALB/c mice. While the affinity of Ab3 sera for progesterone was 10-50-times lower than that of DB3, their steroid-binding specificity showed considerable similarity to DB3. Two immunoglobulin M (IgM) Ab3 monoclonal antibodies (mAbs), 1A4 and 3B11, were obtained, both of which bound progesterone conjugated to bovine serum albumin (progesterone-BSA). 1A4 also bound free progesterone, although with low affinity and very broad cross-reactivity. Like DB3, 1A4 is encoded by a heavy-chain variable region (VH) gene segment from the small VGAM3.8 family, a restriction that is characteristic of antibodies raised against progesterone-11alpha-BSA. In contrast, 3B11 binds progesterone-11alpha-BSA but not free progesterone and is encoded by an unrelated VH gene from the J558 family. The light chain variable region (VL) of 1A4 lacks the intradomain disulphide bridge owing to replacement of CysL23 by Tyr. Both the 1A4 and 3B11 heavy chains have extremely short complementarity determining region (CDR) H3 loops, comprising three and four amino acids, respectively. Modelling of the combining site of 1A4 from the X-ray crystallographic structure of DB3 indicates that the short H3 loop is a major factor in the loss of affinity and specificity for steroid.

Crystal structure of a Staphylococcus aureus protein A domain complexed with the Fab fragment of a human IgM antibody: structural basis for recognition of B-cell receptors and superantigen activity

Staphylococcus aureus produces a virulence factor, protein A (SpA), that contains five homologous Ig-binding domains. The interactions of SpA with the Fab region of membrane-anchored Igs can stimulate a large fraction of B cells, contributing to lymphocyte clonal selection. To understand the molecular basis for this activity, we have solved the crystal structure of the complex between domain D of SpA and the Fab fragment of a human IgM antibody to 2.7-A resolution. In the complex, helices II and III of domain D interact with the variable region of the Fab heavy chain (V(H)) through framework residues, without the involvement of the hypervariable regions implicated in antigen recognition. The contact residues are highly conserved in human V(H)3 antibodies but not in other families. The contact residues from domain D also are conserved among all SpA Ig-binding domains, suggesting that each could bind in a similar manner. Features of this interaction parallel those reported for staphylococcal enterotoxins that are superantigens for many T cells. The structural homology between Ig V(H) regions and the T-cell receptor V(beta) regions facilitates their comparison, and both types of interactions involve lymphocyte receptor surface remote from the antigen binding site. However, T-cell superantigens reportedly interact through hydrogen bonds with T-cell receptor V(beta) backbone atoms in a primary sequence-independent manner, whereas SpA relies on a sequence-restricted conformational binding with residue side chains, suggesting that this common bacterial pathogen has adopted distinct molecular recognition strategies for affecting large sets of B and T lymphocytes.

Antibody repertoires of four- and five-feature translocus mice carrying human immunoglobulin heavy chain and kappa and lambda light chain yeast artificial chromosomes

We have produced mice that carry the human Ig heavy (IgH) and both kappa and lambda light chain transloci in a background in which the endogenous IgH and kappa loci have been inactivated. The B lymphocyte population in these translocus mice is restored to about one-third of normal levels, with preferential (3:1) expression of human lambda over human kappa. Human IgM is found in the serum at levels between 50 and 400 microg/ml and is elevated following immunization. This primary human Ab repertoire is sufficient to yield diverse Ag-specific responses as judged by analysis of mAbs. The use of DH and J segments is similar to that seen in human B cells, with an analogous pattern of N nucleotide insertion. Maturation of the response is accompanied by somatic hypermutation, which is particularly effective in the light chain transloci. These mice therefore allow the production of Ag-specific repertoires of both IgM,kappa and IgM,lambda Abs and should prove useful for the production of human mAbs for clinical use.

Selection of a human anti-progesterone antibody fragment from a transgenic mouse library by ARM ribosome display

In antibody-ribosome-mRNA complex (ARM) ribosome display, stable complexes of nascent protein, mRNA and ribosomes are produced in a eukaryotic in vitro expression system, through coupled transcription and translation of DNA lacking a 3' stop codon. Selection of the protein simultaneously captures the relevant mRNA, which is recovered as DNA by coupled reverse transcription-polymerase chain reaction (RT-PCR) performed on the intact complexes. Here, we describe the use of ARM display to select a specific human antibody fragment from a transgenic mouse library. The mice carry unrearranged gene segments of the human heavy (H) and kappa light (L) chain loci, while the endogenous murine H and kappa loci are functionally silenced; they respond to immunisation by production of fully human IgM antibodies. A library encoding human single-chain (sc) antibody (V(H)/K) fragments, in which V(H) domains and kappa light chains were combined at random by PCR, was prepared from spleen cells of transgenic mice immunised with progesterone-bovine serum albumin (BSA). Library diversity was demonstrated by sequencing. Progesterone-binding fragments were selected over five cycles of ARM display and the selected DNA cloned and expressed in Escherichia coli. Soluble V(H)/K fragments obtained in periplasmic extracts had the same specificity as ribosome-bound V(H)/K, supporting the view that folding and specificity of the displayed and soluble proteins are equivalent. The affinity of the expressed V(H)/K was approximately 10(-8) M. Sequencing showed that ARM display selected a single V(H)/V(L) combination (V(H)1-2, Vkappa4-1) and rearrangement, with a few mutational differences between clones. Monoclonal antibodies against progesterone-BSA obtained from hybridomas were encoded by the same V(H) and V(L) segments and had similar properties to the fragments obtained in vitro. The combination of ribosome display and transgenic mouse technologies is a rapid means of generating fully human antibody fragments in vitro for expression and further manipulation.

Rapid appearance of M cells after microbial challenge is restricted at the periphery of the follicle-associated epithelium of Peyer's patch

M cells within the follicle-associated epithelium (FAE) of the gut play a central role in the initiation of mucosal immune responses by transporting antigens to the intestinal lymphoid tissue. We have previously demonstrated that the instillation into the gut of a nonenteric microorganism, Streptococcus pneumoniae R36a, is an excellent experimental model to investigate the highly dynamic nature of the FAE in response to microbial challenge. In the present study, S. pneumoniae was introduced into rabbit ileal loops, each one containing a Peyer's patch (PP), and the number of M cells was assessed by morphological and functional characteristics in different areas of the FAE after a short time (1-3 hours). We report that a marked increase in the number of M cells was detected in the periphery, but not in the apical area, of the FAE as early as 1 hour after exposure to S. pneumoniae. Furthermore, a variant of this experiment enabled us to establish that the increased numbers of M cells led to an improved capability of the FAE to transport latex fluorescent microspheres (0.5 microm), highly specific to rabbit M cells, from the gut lumen to the intestinal lymphatic system. In these animals the cisterna chyli was cannulated, and the microparticles were introduced into the intestinal loops after stimulation with pneumococci. The microparticles reaching the lymph were then counted by flow cytometer. We interpreted these results as showing that only enterocytes located within the periphery of the FAE are converted to fully operational M cells by certain microbial interaction and the ability of enterocytes to undergo this conversion may depend on their stage of differentiation.

Up-regulation of microsphere transport across the follicle-associated epithelium of Peyer's patch by exposure to Streptococcus pneumoniae R36a

Transport of antigens through the follicle-associated epithelium (FAE) of Peyer's patch (PP) is the critical first step in the induction of mucosal immune responses. We have previously described that short-term exposure to Streptococcus pneumoniae R36a induced dramatic morphological alterations of the FAE in rabbit PP. These results prompted us to investigate whether the pneumococci-induced modifications were accompanied by enhanced ability of the FAE to transport antigens. We addressed this problem by evaluating the ability of the FAE to bind, internalize, and transport fluorescent polystyrene microparticles, highly specific to rabbit M cells, after exposure to S. pneumoniae. Quantitative study revealed a marked increase in the number of microspheres in PP tissues exposed to S. pneumoniae compared to tissues exposed to either phosphate-buffered saline or Escherichia coli DH5alpha as controls. No sign of bacterially induced damage to the epithelial barrier was observed. Further confocal microscopy analysis of the FAE surface showed that a significant increase in the number of cells that showed both morphological and functional features of M cells took place within pneumococci-treated PP tissues. These data provide the first direct evidence that the FAE-specific antigen sampling function may be manipulated to improve antigen and drug delivery to the intestinal immune system.

The structure of a human rheumatoid factor bound to IgG Fc

This is the first crystal structure analysis of a complex between an autoantibody and its autoantigen, and it reveals a mode of interaction never before seen in an antibody-antigen complex. Not only are there relatively few antibody contact residues, contributing perhaps to its very low affinity, but these residues are to be found on only one side of the potential combining site surface. Indeed, so many CDR residues are not involved in Fc binding, including those in the central region of the combining site, that it is easy to envisage that this RF may have another, entirely different, specificity. The antibody may therefore have originated in response to another, as yet unidentified, antigen, and the reactivity with IgG Fc may be an unfortunate cross-reactivity. Certainly some of the CDR residues which do interact with IgG Fc are germline encoded, but significantly one of only two residues in the light chain, Pro56, which makes many contacts with Fc, is a somatic mutation. Since this mutation would appear to make a significant contribution to the binding affinity, it is therefore evidence for an antigen driven response to the IgG Fc in the generation of this autoantibody. The Fc epitope recognised by RF-AN is strikingly similar to the binding sites for the bacterial binding proteins A and G, but the significance of this is not clear. What is clear however is that the epitope does not include any part of the Fc carbohydrate residues, although the structure of the complex does reveal that there is an alteration in the carbohydrate conformation when the galactose residues are absent. Loss of the interaction between the terminal galactose residue on the alpha (1-6) linked branch and the C gamma 2 domain appears to allow the carbohydrate chains to become mobile, at the same time exposing a predominantly hydrophobic patch on the C gamma 2 surface. Accessibility to either the agalactosyl carbohydrate chains or the newly exposed residues may account for the enhanced reactivity for G0-IgG that has been reported for certain RFs, and such an epitope need not be very different to that recognised by RF-AN. In order to understand more completely the effect of the presence or absence of the terminal galactose residue, the fully galactosylated glycoform of Fc must be studied for comparison; this work is underway. It is also important now to study a RF which is known to sense this difference in oligosaccharide composition, and also to study RFs of higher affinity, of the IgG class, and from the synovium. RF-AN was the first RF to be immortalised as a cell line, and in many ways it is a typical RF (in terms of specificity, relationship to germline sequence and affinity), but we must now establish whether the novel structural features revealed in this analysis are indeed typical of other RFs. Only when comparisons can be made between RFs of different origin and with contrasting functional properties will we begin to understand what constitutes a pathogenic RF, and the mechanism by which such auto-reactive antibodies are generated.

Antibody-ribosome-mRNA (ARM) complexes as efficient selection particles for in vitro display and evolution of antibody combining sites

We describe a rapid, eukaryotic, in vitro method for selection and evolution of antibody combining sites using antibody-ribosome-mRNA (ARM) complexes as selection particles. ARMs carrying single-chain (VH/K) binding fragments specific for progesterone were selected using antigen-coupled magnetic beads; selection simultaneously captured the genetic information as mRNA, making it possible to generate and amplify cDNA by single-step RT-PCR on the ribosome-bound mRNA for further manipulation. Using mutant libraries, antigen-binding ARMs were enriched by a factor of 10(4)-10(5)-fold in a single cycle, with further enrichment in repeated cycles. While demonstrated here for antibodies, the method has the potential to be applied equally for selection of receptors or peptides from libraries.

Recognition of porcine growth hormone by a panel of monoclonal antibodies

A panel of murine monoclonal antibodies (MAbs) against porcine growth hormone (pGH) has been raised from BALB/c mice. MAbs were characterized for binding to growth hormones (GH), prolactins (PRL), and placental lactogen (PL) from different species and to the N-terminal peptides of GH. From their patterns of cross-reactivity MAbs were assigned into nine specificity groups. The sharing of pGH epitopes among hormones of different species was related to the sequence similarity to pGH, i.e., overlap was greatest for equine, ruminant, and rodent GHs and least for human GH, ovine, and porcine PRLs, and human PL. Partial epitope mapping was carried out by relating hormone cross-reactivity patterns with amino acid sequences. Two epitopes were localized to interhelical loops, around valine-73 and glycine-130, respectively. Direct mapping with synthetic peptides localized other epitopes (Groups 7, 8, and 9) to the N-terminal region of the GH molecule. Selected MAbs were studied for the enhancement of the somatogenic activity of pGH in the dwarf mouse bioassay, measuring weight gain and sulphate incorporation into costal cartilage. Only those antibodies with specificities for GHs and not PRL or PL showed significant enhancement in this assay.

Production of human antibody repertoires in transgenic mice

Transgenic mice have been created that carry human immunoglobulin heavy and light chain genes in germline configuration and that have the corresponding endogenous genes silenced. The transgenes are either minigene constructs or large, almost authentic, transloci on yeast artificial chromosomes and undergo B-cell-specific DNA rearrangement and hypermutation in the mouse lymphoid tissue. Monoclonal antibodies with good affinities for human antigens have been obtained after immunisation. These mice may be a future source of human antibodies for therapy.

Structure of human IgM rheumatoid factor Fab bound to its autoantigen IgG Fc reveals a novel topology of antibody-antigen interaction

Rheumatoid factors are the characteristic autoantibodies of rheumatoid arthritis, which bind to the Fc regions of IgG molecules. Here we report the crystal structure of the Fab fragment of a patient-derived IgM rheumatoid factor (RF-AN) complexed with human IgG4 Fc, at 3.2 A resolution. This is the first structure of an autoantibody-autoantigen complex. The epitope recognised in IgG Fc includes the C gamma 2/C gamma 3 cleft region, and overlaps the binding sites of bacterial Fc-binding proteins. The antibody residues involved in autorecognition are all located at the edge of the conventional combining site surface, leaving much of the latter available, potentially, for recognition of a different antigen. Since an important contact residue is somatic mutation, the structure implicates antigen-driven selection, following somatic mutation of germline genes, in the production of pathogenic rheumatoid factors.

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.

Crystallization of a complex between the Fab fragment of a human immunoglobulin M (IgM) rheumatoid factor (RF-AN) and the Fc fragment of human IgG4

Rheumatoid factors (RF) are the characteristic autoantibodies found in patients with rheumatoid arthritis. They recognize epitopes in the Fc region of immunoglobulin G (IgG) and are often of the IgM isotype. In order to analyse the nature of RF-Fc interactions, we have crystallized a complex between the Fab fragment of a human monoclonal IgM rheumatoid factor (RF-AN) and the Fc fragment of human IgG4. The stoichiometry of the complex within the crystals was found to be 2:1 Fab:Fc. The crystals diffracted X-rays to 0.3 nm resolution, and the space group was C2, with cell dimensions a = 16.03 nm, b = 8.19 nm, c = 6.42 nm, beta = 98.3 degrees. We have also determined the sequence of the variable region of the RF-AN light chain, not hitherto reported. This belongs to the V lambda III-a subgroup and is closely related to the germline gene Humlv318, from which it differs in three amino acid residues. This is the first reported crystallized complex between a human autoantibody and its autoantigen.

Characterization of a progesterone-binding, three-domain antibody fragment (VH/K) expressed in Escherichia coli

The heavy chain variable region (VH) and the kappa light chain of the anti-progesterone monoclonal antibody (mAb) DB3, have been expressed as a single-chain three-domain polypeptide, designated VH/K, and secreted into the periplasmic space of Escherichia coli (E. coli). The linker sequence was derived from the VH-CH1 elbow region. The C kappa domain provides a sensitive detection tail for Western blotting and enzyme-linked immunosorbent assay (ELISA). Periplasmic extracts of transformed E. coli contained material that bound progesterone and related steroids with similar specificity and affinity to DB3, and displayed the DB3 idiotype and kappa chain epitopes. Reference to the crystal structure of DB3 suggests that all the characteristics of the combining site interaction with steroids are retained in the bacterially expressed material. Western blotting demonstrated material with a molecular weight equivalent to three domains after reduction, but six domains in the unreduced state, suggesting that the VH/K polypeptide is assembled in the periplasm as a disulphide-bridged dimer. The VH/K construct provides a novel route to expression of antibody combining sites in E. coli for antibody engineering.

Structural analysis of antibody specificity. Detailed comparison of five Fab'-steroid complexes

Structures of the Fab' fragment of the anti-progesterone antibody DB3 in complex with five cross-reactive steroids (aetiocholanolone, 5 beta-androstane-3,17-dione, 5 alpha-pregnane-20-one-3 beta-ol-hemisuccinate, progesterone-11 alpha-ol-hemisuccinate and progesterone) have been determined by X-ray crystallography to a maximum resolution of 2.7 A. These different steroids compete with progesterone binding with affinities in the nanomolar range despite substantial differences in their three-dimensional structures. Comparison of the unliganded DB3 Fab' and these five steroid-Fab' complexes reveals that all the steroid ligands bind to an "open" conformation of the Fab' as defined by the orientation of the indole side-chain of TrpH100, whereas in the unliganded or "closed" form the binding site is occluded by TrpH100. Small but significant conformational changes take place in the antibody to maximize the physical and chemical complementarity with each ligand. The various cross-reactive ligands are accommodated in the binding site in two distinct orientations. We term these binding modes syn and anti, as they are defined by the orientation of the steroid beta face relative to TrpH50. In all cases, the steroid D ring is inserted into a hydrophobic cavity formed mainly by TrpH50, TyrH97, TrpH100 and PheH100b; a hydrogen bond interaction with AsnH35 to the keto group at position C17 or C20 orients the steroid in the pocket. The AsnH35 hydrogen bond and the interaction with TrpH50 account for the restricted heavy chain response to immunization with progesterone-like steroids derivatized at the 11 alpha position. Cross-reactivity of the antibody with different steroids is explained by alternative binding pockets for the A ring, which generates different ligand orientations in the binding site. This study suggests which factors are most likely to contribute to the observed antibody specificity, such as linker position and the paucity of functional groups on the immunogenic hapten.

Monoclonal antibodies against progesterone: effect of steroid-carrier coupling position on antibody specificity

Monoclonal anti-progesterone antibodies were raised by immunizing mice with progesterone coupled through either the C3, C6 or C11 positions to protein carrier (bovine serum albumin, BSA). The specificity of four antibodies for a range of steroids related to progesterone, some carrying substitutions at various ring positions, was studied by competitive inhibition in an ELISA system. The results demonstrated that the ring coupling position has a determining effect on the cross-reactivity of the antibodies obtained. The patterns of cross-reaction were interpreted in the light of the structure of the combining site of an anti-progesterone antibody (DB3) recently determined by X-ray crystallography, and inferences drawn about the orientation of steroid in the combining sites of the antibodies studied. Specifically, in two antibodies raised against progesterone-11-BSA, the orientation of steroid resembled that of the progesterone-DB3 complex, with positions C11 and C3 exposed and C6 and C20 buried; an antibody raised against progesterone-6-BSA bound steroid in an apparently similar disposition, except that C6 was exposed and C11 buried; finally, in an antibody raised against progesterone-3-BSA, all steroid positions other than C3 were apparently buried in the steroid-antibody complex.

Molecular basis of crossreactivity and the limits of antibody-antigen complementarity

Two major unanswered questions concerning the specificity of antibodies are: how do structurally different antigens bind with high affinity to the same antibody, and what are the limits of the antibody combining site complementarity and flexibility that contribute to such crossreactivity? We report here a comparative analysis of the X-ray structures of five conformationally different steroids in complex with the Fab' fragment of an anti-progesterone antibody DB3 at 2.7 A. This antibody is unable to complement completely the shape of the hydrophobic antigen so that crossreactivity occurs with other ligands without major structural rearrangements of the binding site. Antigen specificity can be explained through conserved interactions of DB3 with the steroid D-ring, whereas some of the crossreactivity is realized through different binding orientations of the steroid skeleton that place the A-ring into alternative pockets on the antibody surface. The restricted gene usage of the VGAM3.8 family in the generation of anti-progesterone monoclonal antibodies may be explained by the specific interaction of VH hallmark residues with the steroid D-ring. This first detailed structure of steroid interactions with a protein could be applied to the understanding of general mechanisms of steroid recognition as well as in the design of specific binding sites for small hydrophobic ligands.

Three-dimensional structure of an anti-steroid Fab' and progesterone-Fab' complex

The monoclonal anti-progesterone antibody DB3 binds progesterone with nanomolar affinity (Ka approximately 10(9) M-1), suggesting high specificity. However, DB3 also cross-reacts with similar affinity with a subgroup of structurally distinct, progesterone-like steroids. Crystals of the unliganded Fab' and various steroid-Fab' complexes are isomorphous and belong to the hexagonal space group, P6(4)22, with unit cell dimensions of a = b = 135 A, c = 124 A. Structures of free and progesterone-bound Fab' have been determined by X-ray crystallography at 2.7 A resolution using molecular replacement techniques. Progesterone is bound in a hydrophobic pocket formed mainly by the interaction of three complementarity determining regions L1, H2 and H3. The orientation of the ligand in the binding site was aided by both crystallographic and biochemical analyses of substituted steroids. The indole side-chain of TrpH100 of the DB3 has two different conformations, inter-converting "open" and "closed" forms of the antibody combining site. The TrpH100 indole thus appears to be acting as an antibody-derived surrogate ligand for its own hydrophobic binding pocket. These structures provide the first atomic view of how a steroid interacts with a protein and offer a structural explanation for the restriction of the anti-progesterone response to the VGAM3.8 family of VH genes.

Characterization of germ-line genes of the VGAM3.8 VH gene family from BALB/c mice

Five germ-line genes of the VGAM3.8 VH family in BALB/c mice have been isolated from genomic libraries and sequenced. The genes are functional and three are expressed in antibodies of different specificities. Overall nucleotide sequence homologies within the family are greater than 90%, whereas homologies with other VH families are less than 70%. Southern blot hybridization and sequencing indicate a minimum family size of six genes. Differences in the coding regions are mostly confined to CDR, where there is a high replacement/silent substitution ratio, indicative of positive selection for diversification associated with Ag binding. VHVGAM3.8 sequences are highly conserved, and polymorphism in the coding regions appears to be very limited. Evidence is presented that the family has evolved, and been homogenized, by recombinatorial events.

Characterization of germ-line genes of the VGAM3.8 VH gene family from BALB/c mice

Five germ-line genes of the VGAM3.8 VH family in BALB/c mice have been isolated from genomic libraries and sequenced. The genes are functional and three are expressed in antibodies of different specificities. Overall nucleotide sequence homologies within the family are greater than 90%, whereas homologies with other VH families are less than 70%. Southern blot hybridization and sequencing indicate a minimum family size of six genes. Differences in the coding regions are mostly confined to CDR, where there is a high replacement/silent substitution ratio, indicative of positive selection for diversification associated with Ag binding. VHVGAM3.8 sequences are highly conserved, and polymorphism in the coding regions appears to be very limited. Evidence is presented that the family has evolved, and been homogenized, by recombinatorial events.

Abnormal maternal behaviour in mice previously immunized against progesterone

Anti-progesterone immunization leads to reversible infertility in mice; this can be achieved by passive immunization with a monoclonal antibody to progesterone (DB3), or by active immunization with either a progesterone-protein (bovine serum albumin; BSA) conjugate or anti-idiotype directed against DB3. Recovery of fertility in treated females varied from 39.5 to 75.5 median days after passive or active (progesterone-BSA) immunization respectively. Litter size after the first pregnancy also differed from 8.6 +/- 0.8 to 5.0 +/- 0.6 (mean +/- S.E.M.) per mother after passive or active immunization respectively. When litter size was standardized to a maximum of four pups per litter, aberrant maternal responses were observed in the first 5 days after delivery in 40-70% of the nursing mothers. These responses took the forms of cannibalism and failure to retrieve or to nurse pups and resulted in a high incidence of pup rejection (up to 40%), compared with no rejection in control mothers. When mothers were allowed to keep entire litters, an even higher incidence of pup rejection occurred (51% compared with 8% in controls). There was an apparent relation between the degree of negative maternal behaviour and the progesterone antibody concentration in the circulation during the infertile period. Whereas aberrant behaviour occurred mainly within the first 5 days of lactation, it was significantly reduced thereafter. Aberrant behaviour of the mother towards pups may be a consequence of the presence of residual progesterone antibodies in the circulation which affects the process of progesterone withdrawal at parturition that is essential for the establishment of normal maternal responses to the neonate.

Antibodies, implantation and embryo survival

The diverse strategies adopted among species for the maintenance of luteal function converge to meet the indispensable requirement for this hormone particularly at the time of onset of implantation. What has emerged from immunization studies is the difference that exists between various species in the effects of progesterone depletion. The findings affirm the uterus as a primary site of progesterone action in the preimplantation period of gestation in the mouse, whereas in the hamster the positive feedback by progesterone on pituitary luteotrophin secretion is important in maintaining luteal function and hormone secretion at a level necessary for uterine preparation. The impact of progesterone in the rat seems to be established within 48 h after fertilization, whereas in the ferret the hormone's action ensures a suitable uterine environment in which the early embryo can flourish. Its effects in early pregnancy in the marmoset are ambiguous from the present studies unless it emerges that target organ responses are tuned to low concentrations of active steroid. However, the discoveries in mice of a conserved family of immunoglobulin genes used exclusively by immunogenic forms of progesterone conjugated to proteins to stimulate antibody production, and of antibody binding to the uterine epithelium, reveal systems potentially inimical for embryo survival. The endometrial expression of the proto-oncogene erb-A at a time when the embryo has not yet arrived in the uterus, and of antibody-binding to the uterine epithelium, are early maternal responses whose significance require closer examination. The present findings support the hypothesis that the primary site of action of progesterone during the preimplantation period differs between species, and that it is not only the mother that recognizes, but the embryo that initiates early changes before the onset of implantation 'in the bed of soil that nourishes it', a symbiosis that may yet prove to be a common feature of the adoption of viviparity as a preferred mode of reproduction.

Induction of anti-progesterone immunity and pregnancy blocking by anti-progesterone anti-idiotypes. Variable efficacy of polyclonal Ab2 antibodies directed against a panel of closely related Ab1 antibodies

Polyclonal rabbit anti-idiotypes (Ab2) have been raised against three mouse monoclonal antiprogesterone Ab1 antibodies (DB3, 11/32, 11/64) closely related in VH and VL sequences. The anti-idiotypes were characterized for specificity and used to immunize groups of female mice. The latter responded with production of anti-progesterone (Ab3) antibodies, confirming the ability of anti-idiotypes to mimic the immunogenicity of a steroid. The response to one of the anti-idiotypic reagents (anti-DB3-id) was 5-10 times stronger than those to the others, despite close sequence homology between the idiotypes. Moreover, immunization with anti-DB3-id led to a reduction in fertility rate from 90% (control) to 30%, whereas immunization with the other anti-idiotypes was without effect. Sequence and structural comparisons suggest that residues associated with VH CDR3 and VL CDR3 may have a key role in determining the efficiency of anti-idiotypic immunization against progesterone. The variability in outcome of using anti-idiotypic reagents against a defined panel of related antibodies is relevant to the use of anti-idiotypes as surrogate antigens.

Blot-sequencing of antibodies: application to analysis of V gene usage among anti-steroid monoclonal antibodies

Automated gas-phase protein sequencing has been used to characterize variable regions of antibody heavy and light chains separated by SDS-polyacrylamide gel electrophoresis (PAGE) and electroblotted onto Immobilon polyvinylidene difluoride membranes ('blot-sequencing'). Starting from 100 micrograms of antibody, 20 or more residues of N-terminal VH and VL sequences can regularly be obtained, which is often sufficient to assign the V region to a known family or subgroup. We have applied the blot-sequencing method to analysis of VH and VL usage among a panel of monoclonal anti-steroid antibodies, namely anti-progesterone, anti-pregnanediol, anti-estrone and anti-testosterone. The results demonstrate restricted, repetitive usage of VL subgroups and VH families related to anti-steroid specificities. VL regions of the VK1 group were particularly associated with anti-progesterone, VK21 with anti-estrone, and VK8 and VK9 with anti-pregnanediol. VH regions of anti-progesterone antibodies were all derived from the VHVGAM3.8 family; anti-estrone and anti-pregnanediol antibodies were derived from the VH7183 and VH36-60 families. The latter two families appear to characterize antibodies raised against steroids conjugated to proteins via a sugar bridge. Differences in VH/VL combination were associated with diversity of antibody specificity. In order to extend the sequence data obtained by this technique and confirm family assignments, we have shown that internal V-region sequences can be obtained by limited chemical cleavage of whole antibody with cyanogen bromide, followed by separation of individual fragments by SDS-PAGE and blot-sequencing.

Pregnancy-blocking progesterone antibody targets specifically the uterus through its progesterone-binding sites

Passive immunization with a mouse monoclonal antibody against progesterone, designated DB3, blocks pregnancy in several species. We have previously reported that DB3 localizes in the mouse uterine epithelium shortly before normal implantation. This phenomenon is pregnancy dependent and specific for the progesterone antibody. In this study we demonstrate that DB3 is present in the lumen of the uterus 36 h after an i.p. injection; this correlates with the time of maximum antibody reaction on the uterine epithelium. Incubation of DB3 with free progesterone, progesterone-hemisuccinate or progesterone-bovine serum albumin before administration prevented its localization on the epithelium, indicating that the localization requires free progesterone-binding sites and thus probably depends upon progesterone binding. In addition, studies in vitro show that DB3 can effectively bind to progesterone carried by high-affinity progesterone-binding protein purified from coypu plasma. We suggest that specific targeting of DB3 may be through progesterone associated with a progesterone-binding molecule on the membrane of the uterine epithelia. This may be an important part of the mechanism of antibody action against implantation.

Blocking of pregnancy in mice by immunization with anti-idiotype directed against monoclonal anti-progesterone antibody

Passive transfer of monoclonal anti-progesterone antibodies shortly after mating blocks the onset of pregnancy in different species (mouse, rat, and ferret). Here we report that BALB/c mice can be actively immunized against progesterone, and hence against pregnancy, by means of rabbit anti-idiotypic antibodies specific for a mouse monoclonal anti-progesterone antibody, DB3. Some of the anti-idiotypic antibodies reacted with the steroid-combining site on the DB3 molecule. In response to repeated anti-idiotypic immunization, mice produced serum anti-progesterone antibodies (up to 100 micrograms/ml) that resembled DB3 in idiotypy, affinity, and specificity for progesterone and other steroid ligands. Thus an anti-idiotype can mimic the antigenicity of a steroid hormone with a high degree of accuracy. Compared with immunization with a progesterone-bovine serum albumin conjugate, the anti-progesterone response to anti-idiotype was considerably lower and clonally restricted. When mated after completion of the immunization course, the fertility rate of anti-idiotype-immunized mice was reduced to 30% from a control level of 91%. The anti-fertility effect was correlated with the circulating anti-progesterone concentration in individual animals and persisted for 4 or 5 estrous cycles. Active immunization with progesterone-bovine serum albumin was a highly effective means of rendering mice infertile; it reduced the fertility rate to zero over 16 or 17 estrous cycles. Our results suggest that anti-idiotypes may form the basis of contraceptive vaccines.

Immunofluorescent localization, by use of anti-idiotypic antibody, of monoclonal anti-progesterone antibody in the mouse uterus before implantation

Mouse monoclonal anti-progesterone IgG1 antibody designated DB3 has an anti-fertility effect when injected into female mice shortly after mating. In BALB/c mice, pregnancy is blocked, probably as a result of progesterone withdrawal with inhibition of implantation. Rabbit polyclonal anti-idiotype raised against DB3 has been used in an indirect immunofluorescence method on frozen tissue sections to demonstrate the presence of DB3 on the surface of uterine luminal and glandular epithelia before implantation. DB3 was only detectable 30-60 h after a single parenteral injection (9 nmol antibody per mouse i.p. or i.v. at 32 h post coitum). Immunolocalization was both pregnancy-dependent and anti-progesterone antibody-specific, as it was not seen in pseudopregnant mice or mice treated with P3 (mouse myeloma IgG1 protein, using polyclonal anti-P3 anti-idiotype as a probe) or saline. The immunofluorescent reaction was completely blocked by addition of DB3 idiotype in vitro. The results indicate that anti-progesterone antibody binds to an antigen associated with luminal and glandular epithelia which may locally inhibit the uterine uptake of progesterone and disrupt the process of implantation.

Regulation of immunoglobulin gene rearrangement and expression

The molecular genetic events leading to Ig expression and their control formed the topic of a recent EMBO workshop. This report by Michael Taussig, Martin Sims and Ulrich Krawinkel discusses contributions dealing with genes expressed in early pre-B cells, the mechanism of rearrangement, aberrant rearrangements seen in B cells of SCID mice, the feedback control of rearrangement as studied in transgenic mice, the control of Ig expression at the transcriptional and post-transcriptional levels, and class switching.

Anti-idiotypic antibody used for the localization of parenterally administered monoclonal anti-progesterone antibody in mice

Affinity-purified rabbit and sheep anti-idiotypic antisera raised against mouse monoclonal anti-progesterone IgG1 antibody (DB3) or mouse myeloma IgG1 protein P3 (MOPC 21) showed high binding specificities to the respective idiotypes used for immunization as determined by RIA or ELISA. They have been used in an indirect immunofluorescent method to demonstrate the localization of parenterally administered idiotypes in pregnant or pseudopregnant BALB/c mouse frozen tissue preparations, at known stages post coitum after a single intraperitoneal or intravenous injection of DB3 or P3. DB3 was visualized on the surface of uterine luminal and glandular epithelia of pregnant mice 36 h after treatment; the localization was DB3-specific as it was not seen in mice treated with P3 (using sheep anti-P3 anti-idiotype as a probe) or saline. The fluorescent staining reaction in oviduct was weak and only appeared on the surface of the oviducal serosa (peritoneal side). Both DB3 and P3 were also localized in liver (granules of Kupffer cells), kidney (glomerular basement membrane), spleen (on the membrane surface of mononuclear cells in the white pulp), and peritoneal exudate cells (on the membrane surface). Staining could be completely blocked by the addition of the free idiotypes against which the anti-idiotypes were made but not by the unrelated idiotype. Anti-idiotypic labelling in vivo is more specific and selective than anti-whole immunoglobulin labelling.

Inhibition of pregnancy before and after implantation in rats with monoclonal antibody against progesterone

A monoclonal antibody against progesterone completely blocked pregnancy when rats were injected intraperitoneally at 0.53 mumol/kg/day on days 1 and 2, or on day 11 of a first pregnancy. The antibody was equally effective when injected again during a second pregnancy. There was no evidence for anti-idiotypic antibody production even when the injection of a heterologous (mouse) antibody was repeated in a second pregnancy.

Efficacy and specificity of monoclonal antibodies to progesterone in preventing the establishment of pregnancy in the mouse

Anti-progesterone monoclonal antibody prevents the establishment of pregnancy in BALB/c mice by the prevention of implantation when injected i.p. 32 h after mating. To determine the specificity of this effect, mice were injected with immune and non-immune purified mouse immunoglobulins. The results show that anti-implantation efficacy was due to high-affinity antibody which bound progesterone since two further mouse immunoglobulin (Ig) G1 preparations, mouse IgA and mouse IgM which failed to bind the steroid, had no effect on pregnancy rates. From a panel of anti-progesterone monoclonal antibodies, six with a high affinity (affinity constant, 0.24-0.80 litres/nmol) and specificity for progesterone were selected for additional studies. Anti-implantation efficacy for five antibodies was similar, with a 50% effective dose within the range of 0.8-2.0 nmol. Antibody reached high concentrations in plasma within 12 h after i.p. injection, and declined with a half-life of about 80 h. Purified F(ab')2 fragments of antibody also bound progesterone, but were less effective than the native molecule in blocking pregnancy. The results show that implantation in the mouse can be blocked by a high-affinity antibody that binds progesterone and which is removed from the blood at a slow rate.

Progesterone assay by inhibition of reverse passive haemagglutination

An assay for the steroid hormone progesterone is described based on the inhibition of the agglutination by progesterone-bovine serum albumin conjugate of red cells coupled with monoclonal antiprogesterone antibody (reverse passive haemagglutination). A low-affinity agglutination system produced the optimal sensitivity, capable of detecting less than 1 ng/ml steroid. This assay has the simplicity and sensitivity for a potential clinical test for placental or corpus luteum function. It also serves as a model for detection of other small ligands (drugs, hormones) by inhibition of reverse passive haemagglutination.

Analysis of an anti-progesterone antibody: variable crystal morphology of the Fab' and steroid-Fab' complexes

Anti-progesterone monoclonal antibodies are being used for structural studies of antibody-antigen interaction, for their ability to block pregnancy shortly after fertilization, and for hormone immunoassay. A mouse anti-progesterone monoclonal Fab' fragment has been crystallized in its native form and co-crystallized with seven different, but structurally related, steroids. The crystals show interesting preferences in their crystal morphology, depending on the bound steroid ligand. The X-ray crystallographic analysis of this Fab', complexed with a series of related steroid ligands, should reveal details of the chemistry of antibody-antigen union and provide insights into how steroids interact with proteins.

Idiotype-anti-idiotype interactions of VHIX-coded anti-progesterone and anti-arsonate antibodies. Comparison of passive haemagglutination and radioimmunoassays

The reactivity and specificity of polyclonal and monoclonal anti-idiotypic antibodies raised against monoclonal anti-progesterone and anti-arsonate antibodies have been studied by solid phase radioimmunoassay (RIA) with immobilized idiotype and by passive haemagglutination with idiotype-coupled red cells. The sensitivity of the two methods was comparable, though some cross-reactions were only detected by RIA. Passive haemagglutination was found to be especially suitable in screening for monoclonal anti-idiotypes in hybridoma supernatants and ascites, and had advantages over RIA in detection of syngeneic anti-idiotypes. Demonstration of binding site-associated idiotopes was possible by haemagglutination inhibition. RIA and haemagglutination were used to investigate the idiotypic relationships between BALB/c antiprogesterone and anti-arsonate monoclonal antibodies which share heavy chains encoded by VHIX variable region genes.

Recent research in organization and expression of immunoglobulin V region genes

The first Jacques Monod Conference was held in Roscoff, Brittany on 1-5 June 1987 and dealt with the topic of 'Selection of Lymphocyte Repertoires' (organizers F. W. Alt, Columbia University, New York. P.-A. Cazenave, Institut Pasteur, Paris and A. Coutinho, Institut Pasteur, Paris). Much of the meeting was concerned with the recent developments in immunoglobulin and T-cell receptor V region organization and expression. This review is based on the contributions and discussions on Ig V genes, in particular their organization, rearrangements, allelic exclusion and somatic mutation. (Names without references indicate the conference speaker on that topic.)

Anti-idiotypic sera against monoclonal anti-progesterone antibodies: production in rabbits and rats and characterization of specificity

Antisera were raised in rabbits and rats against three mouse monoclonal anti-progesterone IgG1 antibodies. Anti-idiotypic antibodies were isolated from rabbit sera by successive passage over immunoadsorbent columns of normal mouse Ig and the specific immunizing monoclonal, with elution from the latter. Radioimmunoassays for anti-idiotype and free idiotype were established, enabling detection of idiotype in sera of mice immunized with progesterone-BSA conjugate. Binding of rabbit anti-idiotype to anti-progesterone monoclonals was partially inhibitable by free progesterone-hemisuccinate or progesterone-ovalbumin conjugate. While showing considerable specificity for their respective inducing monoclonals, the anti-idiotypes also cross-reacted in varying degrees with the other anti-progesterone monoclonals, demonstrating the presence of IdI and IdX determinants. The patterns of cross-reactivity showed some correlation with the relative isoelectric points and combining-site specificities of the anti-progesterones. The specificities of rat and rabbit anti-idiotypes were similar, but not identical.

Two genes in the major histocompatibility complex control immune response

Sixty-four (57.6%) of 111 cancer patients with cerebrospinal fluid cytology positive for malignant cells had cranial computed tomographic (CT) scans within 2 weeks before or after a lumbar puncture. Twenty-two (34.3%) of the 64 had abnormal CT findings indicative of leptomeningeal metastasis: (1) sulcal-cisternal enhancement, (2) ependymal-subependymal enhancement, (3) widened irregular tentorial enhancement, or (4) communicating hydrocephalus. Thirteen (59.6%) of these 22 patients had associated parenchymal metastases. Recognition of leptomeningeal disease may alter the management of patients with parenchymal metastases. Communicating hydrocephalus in cancer patients should be considered to be related to leptomeningeal metastasis until proven otherwise.

Immunological activity of a T-hybrid line. II. Mapping H-2 genes for the SRBC-specific suppressor factor, its acceptor, and suppression of the antibody response

Studies are reported on the nature and action of a suppressor factor with specificity for sheep red cells produced by a mouse T-hybrid cell line. Absorption of the factor with anti-H-2 reagents indicated that it carries determinants coded between the I-J and D loci. The factor was bound well by spleen cells of certain strains (B10, B10.BR), but not others (B10.S); absorption studies with spleen cells of H-2 recombinant strains suggested that a gene contributing to a cellular acceptor site for the factor also mapped in the region between I-J and D. On the other hand, studies of functional suppression of the anti-SRBC response of different strains in vitro indicated that an H-2 gene in the I-A or I-B subregions controlled suppression. Thus, at least two H-2 genes seem to be involved in the suppressive action of this factor.

Antigen-specific T-cell factors

Antigen-specific T-cell factors are mediator molecules which are produced by helper and suppressor T cells and which can perform the function of those cells in an antigen-specific manner. They probably play an important part in immunoregulation. The major histocompatibility complex has a controlling influence on their structure and activity, while their antigen-recognition properties may be conferred by immunoglobulin V regions. Interest in the factors derives from three related areas of research, namely (i) the problem of T-cell recognition of antigen; (ii) the mechanisms of cellular interactions in antibody production and cell-mediated immunity; and (iii) the genetic control of immune responses. This review discusses the literature up to June 1980 on their production, structure, genetic restriction and mechanism of action.