Immunoglobulin diversity: analysis of the germ-line VH gene repertoire of the murine anti-GAT response

Antigen binding and cytotoxic properties of a recombinant immunotoxin incorporating the lytic peptide, melittin

BACKGROUND

The majority of immunotoxins studied to date incorporate toxins that act in the cytosol and thus need to be endocytosed by the target cell. An alternative strategy for immunotoxin development is the use of membrane active toxins, such as the pore-forming proteins. Melittin, a 26 amino acid cytolytic peptide from bee venom, is such a protein.

OBJECTIVES

We report here the construction, production and functional analysis of a recombinant immunotoxin obtained by fusion of genes which encode an antibody fragment (scFv) with an oligonucleotide encoding melittin.

STUDY DESIGN

The antibody fragment was derived from a murine monoclonal antibody, K121, which recognises a specific epitope (KMA) expressed on the surface of human kappa myeloma and lymphoma cells, and on human free kappa Bence Jones protein (BJP). Melittin is a 26-amino acid, membrane-lytic peptide which is a major component of bee venom. The scFv of K121 was constructed by PCR to link VH and VL genes via an oligonucleotide which encodes a flexible, hydrophilic peptide. An oligonucleotide encoding melittin and the peptide marker sequence FLAG was fused to the scFv construct using a similar linker peptide. The gene construct (scFv-mel) was inserted into the secretion vector pPOW and expressed in Escherichia coli (TOPP2).

RESULTS

Expression of the recombinant scFv-mel gene and purification of the protein product was monitored by Western blot analysis. Following purification by anti-FLAG affinity chromatography, the recombinant immunotoxin (scFv-mel) was assessed for antigen binding and for cytotoxic activity by flow cytometry using antigen-expressing and non-expressing cell targets. The scFv-mel was found to exhibit binding and killing properties consistent with the specificity of the original K121 antibody. Moreover, the cytolytic activity of the scFv-mel was significantly greater on a molar basis than that of native melittin alone.

CONCLUSION

The data presented here constitute the first report of a melittin-based recombinant immunotoxin and demonstrate that such a membrane active immunotoxin can be synthesised in a bacterial expression. Linking of melittin to an antibody fragment overcame the non-specific toxicity of melittin as the recombinant immunotoxin exhibited specific toxicity towards antigen-bearing target cells. The observation that the immunotoxin exhibited enhanced cytotoxic activity over the free toxin indicates the potential of this approach for the development of an effective therapeutic agent.

Public and private V beta T cell receptor repertoires against hen egg white lysozyme (HEL) in nontransgenic versus HEL transgenic mice

We have previously produced a transgenic mouse line for hen egg lysozyme (HEL), an experimental model for analyzing tolerance to self-antigens at the peptide level. We have now characterized transgenic mice with HEL blood levels below 2 ng/ml, where significant T cell proliferative responses to HEL and its immunodominant peptide were observed. This HEL-low transgenic model was chosen because it mimics physiological conditions in which autoreactive T lymphocytes, recognizing self-components expressed at very low levels, persist without inducing a break in tolerance. Furthermore, in H-2d mice, HEL-specific T lymphocytes are triggered by a single immunodominant region, allowing us to compare the HEL-specific T cell V beta repertoires of transgenic and nontransgenic animals against a single peptide presented as self or foreign, respectively. We found that a V beta 8.2-D beta 1-J beta 1.5 rearrangement is found in response to HEL in all nontransgenic mice, whereas this V beta-restricted response is absent in HEL-low transgenic animals. At the nucleotide level, this rearrangement results from the trimming of the genomic segments during VDJ or DJ joining, without N additions, suggesting that the dominant rearrangement is selected early during fetal or neonatal life, before the expression of terminal deoxynucleotidyl transferase. In HEL-low transgenic mice, no dominant rearrangements are found as alternatives to the one observed in normal mice. Instead, each transgenic animal uses a different set of V beta-J beta combinations in its response to the immunodominant HEL peptide. In nontransgenic mice, besides the dominant V beta 8.2-D beta 1-J beta 1.5 combination, minor V beta repertoires were found which differed in each animal and were distinct from the rearrangements used by individual transgenic mice. These findings suggest that the T cell response to an immunodominant peptide involves a "public" V beta repertoire found in all animals and a "private" one which is specific to each individual.

Idiotypic cross-reactivity of anti-GAT and anti-alprenolol antibodies: an approach to the structural correlates of the pGAT idiotypic specificity

Most anti-GAT antibodies in the BALB/c strain express a public idiotypic specificity (pGAT), which is encoded by specific germline genes (VH10, VK5.1 and VK1A5). One or both of these germline genes, referred to as "GAT-specific genes", are also used by four anti-alprenolol antibodies. Anti-Alp and anti-GAT antibodies show no cross-reactivity for the antigens. The light chain of one anti-Alp antibody, 22C4, is encoded as the anti-GAT antibodies by a VK5.1-J2 combination and expresses part of the pGAT idiotopes, whereas the heavy chain is not "GAT"-related. Two anti-Alp using the VH10-VK5.1-J1 association do not express any of the pGAT idiotopes. Sequence comparison of the various CDR sequences points to the predominant role of the VH-CDR2 and VL-CDR3 for the constitution of the pGAT specificity. Regarding VL-CDR3, a drastic change in idiotypic determinants appears to be linked to V-J junctional diversity.

The T-cell independent antigen, NP-ficoll, primes for a high affinity IgM anti-NP response

In a number of different strains of inbred mice, immunization with a hapten coupled to a protein carrier results in production of homogeneous serum antibodies. At the genetic level this corresponds to the use of a very limited set of variable region genes in the actively secreting B-cells. In contrast, immunization with the same hapten coupled to a T-cell independent (TI) carrier produces a heterogeneous antibody response. Here we show that successive immunizations of C57BL/6 mice, first with the hapten NP coupled to ficoll, a TI carrier, and then one month later with a subliminal dose of the same hapten coupled to a protein carrier, generate a novel set of hybridomas. These hybridomas produce antibodies which are of the IgM isotope and which lack somatic mutation. Some of these antibodies have a much higher affinity for NP than do antibodies which use the prototypical gene combination (VH186.2-lamda 1) of the strain specific response in C57BL/6 mice.

High connectivity, natural antibodies preferentially use 7183 and QUPC 52 VH families

Twenty-seven B cell hybridomas derived from spleen cells of nonimmunized, 6-day-old BALB/c mice and previously characterized serologically were screened for VH-gene expression: hybridomas expressing the two most D-proximal VH-gene families, 7183 and QUPC 52 were over-represented in this sample. Strikingly, clones serologically characterized by high degree of degenerate "specificity" and high idiotypic connectivity were found to use almost exclusively these two VH-gene families. As these serological properties are a unique feature of "natural" antibody repertoires, the results may reflect a functional relationship between expression of these VH genes and activation of B cells in the internal environment, particularly in the context of idiotypic interactions.

Paucity of phosphorylcholine-specific clones in B cells expressing the VHT15 gene product

The aim of this work was to study the cellular basis of the phenomenon of clonal dominance. To this end we analyzed two collections of BALB/c and C.B20 hybridomas that we selected on the basis of the expression of the VHT15 gene product independently from their antigen specificity. Our study demonstrates that none of the 28 BALB/c and only 2 of the 29 C.B20 hybridomas obtained have variable regions that bind PC. We conclude therefore that the domination of the immune response to PC by particular variable regions cannot be due to the establishment of clonal dominance prior to immunization.

Primary structure of IgE monoclonal antibodies expressing an intrastrain crossreactive idiotype

We have obtained amino acid sequences (by mRNA and amino acid sequencing) for two IgE kappa mAb that have specificity for the Ars hapten group and are related to the major idiotypic family, CRIA (crossreactive idiotype A), in the A strain of mouse. One mAb, SE20.2, fully expresses CRIA; the other, SE1.3, possesses some but not all of the characteristic idiotopes. Both IgE proteins contain VH and V kappa segments that are closely related to those associated with CRIA. The D segment of SE20.2 is also typical of CRIA+ mAb, but that of SE1.3 is one amino acid residue longer. Chain recombination experiments indicated that the L chain of SE1.3 is fully capable of supporting CRIA expression. Its deficiency with respect to idiotopes of CRIA was attributed to the extra amino acid in the D region and/or substitutions in the VH segment. A major objective was to ascertain the frequency of somatic mutations in IgE. For the VH segment (amino acids 1-98) of SE20.2, there are only three nucleotide differences and one uncertainty with respect to the nucleotide sequence of the germline gene associated with CRIA. A somewhat higher frequency of substitutions is present in the VH segment of SE1.3. The VK amino acid sequences of the IgE proteins are nearly identical to those of a prototype of the CRIA family, mAb R16.7. The results are discussed with reference to the mechanism of the IgM to IgE switch.

Genetic basis for expression of the idiotypic network. One unique Ig VH germline gene accounts for the major family of Ab1 and Ab3 (Ab1') antibodies of the GAT system

Ig germline genes have been isolated from recombinant clones prepared in separate libraries constructed from adult BALB/c liver DNA either in pBR328 plasmid or in EMBL 3 phage. Three clones that gave a very strong positive hybridization signal with a VH anti-GAT-specific probe were completely characterized and sequenced. All three were greater than 95% homologous, with the exception of the 5' noncoding region, which was only 85% homologous but contained characteristic regulatory signals. One of these genes, H10, had a sequence that was completely identical to that of a cDNA derived from a GAT-specific BALB/c hybridoma. Southern blot analysis using Eco RI-digested DNA from rearranged GAT-specific hybridomas revealed that the same gene was used for other GAT-specific VH regions, including one differing from the H10 sequence by 12 nucleotides, which must have been generated by a somatic mechanism. The same H10 germline gene was also used, in most cases without any nucleotide substitution, in hybridomas of the Ab1' set of the GAT idiotypic cascade, suggesting that immunization with Ab2 (antiidiotypic) antibodies preferentially stimulates the direct expression of VH germline genes. Finally, the previous hypothesis that NPa and GAT VH genes were derived from the same germline gene was definitively confirmed, both from sequence data and Southern blot analysis.

The T-cell-independent immune response to the hapten NP uses a large repertoire of heavy chain genes

Hybridomas generated from C57BL/6 mice immunized with the hapten NP coupled to ficoll, a T-cell-independent carrier, produce monoclonal antibodies that use a large repertoire of VH regions and light chains. This contrasts with the homogeneity of the strain-specific response to NP observed with T-cell-dependent carriers, where most of the antibodies use a single VH region, V186.2, in combination with the lambda-1 light chain. There is no evidence for somatic mutation in any of the sequenced regions of the antibodies generated by NP-ficoll. Thus T cell participation is required for the homogeneity of the strain-specific hapten response, and probably for somatic mutation as well.

Gene repertoire of the anti-poly(Glu60Ala30Tyr10) (GAT) immune response: comparison of VH, V kappa, and D regions used by anti-GAT antibodies and monoclonal antibodies produced after anti-idiotypic immunization

Eight monoclonal antibodies were selected from BALB/c mice immunized with two different monoclonal anti-idiotypic antibodies recognizing two discrete idiotopes characteristic of the anti-poly(Glu60Ala30Tyr10) (GAT) antibody response. These monoclonal antibodies were previously classified as Ab1 (anti-GAT-like) and Ab3 (anti-anti-idiotype) on the basis of expression of the public idiotypic specificity (p.GAT) studied with a xenogeneic serum, anti-GAT activity, and expression of various public idiotopes. All the heavy chain variable region (VH) sequences from Ab1 are nearly identical to the VH sequences of Ab1 anti-GAT monoclonal antibodies. The same type of results has been found with the Ab1 kappa light chain variable region (V kappa) sequences. Confirming our classification, Ab3 VH and V kappa sequences were found to be completely different from Ab1 VH and V kappa sequences. The Ab1 diversity (D) regions are different from one another and different from the D regions found on monoclonal anti-GAT antibodies but function similarly. These D regions are not simply derived from already described D genes. Finally, our results suggest that in the anti-GAT response VH and V kappa sequence are mainly responsible for idiotype expression.

Immune response against poly(Glu60,Ala30,Tyr10) (GAT): immunization with monoclonal anti-idiotypic antibodies leads to the predominant stimulation of idiotypically similar immunoglobulins with anti-GAT activity

Two monoclonal anti-idiotypic antibodies (HP-Id20 and HP-Id22) recognizing two different public idiotopes expressed in the anti-poly(Glu60,Ala30,Tyr10) (GAT) response were used to immunize BALB/c and C57BL/6 mice. From these animals hybridomas were isolated. From BALB/c and C57BL/6 mice eight and seven monoclonal antibodies were characterized, respectively. The reagents were classified according to the expression of the public idiotypic specificity p.GAT (recognized by a rabbit antiserum). The anti-GAT activity and the expression of the various idiotopes characterized on anti-GAT polyclonal and monoclonal antibodies were also studied. Most of the reagents are Ab1'-type of antibody resembling anti-GAT antibodies. One anti-anti-idiotypic monoclonal antibody (Ab3) was also isolated from BALB/c mice. This suggests that in this experimental model the repertoire induced after HP-Id immunization and antigen stimulation is comparable. The idiotypic analysis of a large number of anti-GAT and of Ab1' monoclonal antibodies suggests that only two public idiotopes are involved in the anti-GAT response.

A single VH-gene associated with a variety of D- and J-segments encodes for a large family of ABPC48-related antibodies induced by antiidiotypic immunization

Two series of monoclonal antibodies have been obtained from BALB/c mice immunized against two antiABPC48 antiidiotypic antibodies. They are divided into two serologically different classes. Class I antibodies bind only the immunizing antibody; class II antibodies display a broad binding capacity to various antiidiotypic antibodies, and some bind levan, as does ABPC48. Northern blot analyses and partial mRNA sequencing show that all class II antibodies express the VH-gene coding for ABPC48 and UPC10 antilevan antibodies associated with a variety of D- and J-segments. The third hypervariable region of the sequenced antibody with antilevan activity is structurally related to that of ABPC48 and UPC10 antibodies but has a different genetic origin. This study indicates that the identification of idiotype-related antibodies arising from antiidiotypic immunization may be misleading, if based on their antigen-binding properties; and it stresses the importance of structural approaches for the analysis of regulatory mechanisms ruling immune responses.

The internal image and the structural idiotypic network (Ab1, Ab2, Ab3) in the GAT system

The GAT repertoire expressed at the different levels of the classical idiotypic cascade Ag----Ab1----Ab2----Ab3 has been analysed by direct nucleotide sequencing of H- and L-enriched mRNA. Ab1 and Ab3 expressing the major public idiotypes used similar, if not identical, VH and VK genes. The VH Ab3 of the Ab1 type (Ab1') appeared highly conserved. Ab2 also use a small number of germ-line genes. The D region of Ab2 is of particular interest since it contains either a Tyr-Tyr-Glu or a Glu-Glu-Tyr sequence, characteristic of the GAT major determinants. It is therefore suggested that this D region contains the internal image of the antigen. A large number of VH germ-line genes have been isolated and sequenced. They all pertain to the VH-II family, which contains a large number of members, some of them being very close in sequence.

Idiotypic cross-reactivity of low-affinity anti-fluorescyl monoclonal antibodies

Previous studies concerning structure-function relationships of anti-fluorescyl hybridoma proteins utilized primarily high-affinity proteins (Ka greater than 5.0 X 10(7) M-1) possessing distinct idiotypes. Low-affinity anti-fluorescyl monoclonal antibodies, predominantly IgGl or IgG2a possessing kappa light chains were analyzed. Two fusions produced 18 monoclonals, 13 binding fluorescein with a low affinity (less than or equal to 3.0 X 10(6) M-1) and five possessing high affinities (greater than or equal to 5.3 X 10(8) M-1). Solid-phase idiotype assays, utilizing rabbit anti-idiotype reagents against two low-affinity proteins (3-13 and 3-17), showed that all the low-affinity clones (except 2-9 and 2-21) were capable of inhibiting (40-100%) these two idiotype-anti-idiotype interactions while no high-affinity proteins inhibited them. The interactions with 3-13 and 3-17 were inhibited 100 and 88%, respectively, by free fluorescein. When these idiotype-anti-idiotype interactions were inhibited with increasing concns of heterologous hybridoma proteins, three clones inhibited both interactions as effectively as the homologous proteins at all concns tested and inhibition reached 100%. These three clones appeared to possess all the idiotopes that the anti-3-13 and anti-3-17 reagents detected on 3-13 and 3-17. Screening of eight high-affinity anti-fluorescyl proteins previously produced [Kranz and Voss, Molec. Immun. 18, 889-898 (1981a)] identified a single clone [20-4-4 (Ka = 5.0 X 10(7) M-1)] significantly inhibiting the 3-13 and 3-17 interactions (71.0 and 63.6%, respectively). In addition, recombination experiments utilizing H- and L-chains derived from three low-affinity and three high-affinity antibodies resulted in reformation of active sites in all six heterologous combinations when both chains were derived from low-affinity antibodies, and in only one of six combinations when both chains were derived from high-affinity molecules. Thus, the apparent lack of private idiotopes on clones 3-13 and 3-17 and the presence of these idiotopes (or cross-reactive ones) on 11 of 13 low-affinity antibodies and on one of 13 high-affinity antibodies may indicate that clones 3-13 and 3-17 are encoded by germline genes. The H- and L-chain recombination experiments indicated that the idiotype and affinity of parental molecules may be involved in H- and L-chain association.

Molecular analysis of the anti-GAT repertoire in three strains of mice

GAT-specific antibodies which express the CGAT (or pGAT) public idiotypic specificities are encoded by a paucigene system. Antibodies expressing discrete specificities, i.e. anti-GAT and anti-NP, appeared to use the same VH germ-line genes, an observation which directly supports the importance of random H-L pairing to generate a large number of distinct antibody molecules.

V kappa gene family in (Glu60 Ala30 Tyr10)n (GAT)-specific antibodies that express CGAT (or pGAT) public idiotypic specificities. Protein and mRNA sequencing of eight monoclonal V kappa chains

A large proportion of (Glu60 Ala30 Tyr10)n (GAT)-specific antibodies expresses public idiotypic specificities, termed CGAT (or pGAT), that require the presence of both the heavy and the light chains in order to be expressed. We report in this paper the complete sequence of eight V kappa regions pertaining to eight anti-GAT monoclonal antibodies derived from three strains of mice: BALB/c, DBA/2, and C57BL/6. The methodology used a combination of NH2-terminal amino acid and mRNA nucleotide sequencing. All eight sequences analyzed, although highly homologous and all pertaining to the same V kappa 1 subgroup, allowed definition of three germline genes that are likely to be present in all three strains of mice and also in NZB. It seems likely, however, that any given strain may not necessarily use all three genes for making anti-GAT antibodies. The search for structural correlates of idiotypes could not be framed in a simple picture, but our data suggest that similar idiotopes may result from different interacting primary structures, leading to structural homologies that should be visualized at three-dimensional level.