Influence of the macromolecular form of a B cell epitope on the expression of antibody variable and constant region structure

Trade-offs in antibody repertoires to complex antigens

Pathogens vary in their antigenic complexity. While some pathogens such as measles present a few relatively invariant targets to the immune system, others such as malaria display considerable antigenic diversity. How the immune response copes in the presence of multiple antigens, and whether a trade-off exists between the breadth and efficacy of antibody (Ab)-mediated immune responses, are unsolved problems. We present a theoretical model of affinity maturation of B-cell receptors (BCRs) during a primary infection and examine how variation in the number of accessible antigenic sites alters the Ab repertoire. Naive B cells with randomly generated receptor sequences initiate the germinal centre (GC) reaction. The binding affinity of a BCR to an antigen is quantified via a genotype-phenotype map, based on a random energy landscape, that combines local and distant interactions between residues. In the presence of numerous antigens or epitopes, B-cell clones with different specificities compete for stimulation during rounds of mutation within GCs. We find that the availability of many epitopes reduces the affinity and relative breadth of the Ab repertoire. Despite the stochasticity of somatic hypermutation, patterns of immunodominance are strongly shaped by chance selection of naive B cells with specificities for particular epitopes. Our model provides a mechanistic basis for the diversity of Ab repertoires and the evolutionary advantage of antigenically complex pathogens.

Molecular analysis of monoclonal antibodies to group variant capsular polysaccharide of Neisseria meningitidis: recurrent heavy chains and alternative light chain partners

We determined the molecular sequence of monoclonal antibodies (mAbs) to serogroups B and C capsular polysaccharides (PS) of Neisseria meningitidis. N. meningitidis infections are a leading cause of bacterial septicemia and meningitis in humans. Antibodies to PS are fundamental to host defense and diagnostics. The polysaccharide capsule of group B N. meningitidis is poorly immunogenic and thus is an important model for studying pathogen-host co-evolution through understanding the molecular basis of the host immune response. We used a modified reverse-transcriptase PCR to amplify and sequence the V-genes of murine hybridomas produced against types B and C capsular PS. Databank analysis of the sequences encoding the V-genes of type C capsular PS mAb, 4-2-C, reveal that heavy chain alleles are recurrently used to encode this specificity in mice. Interestingly, a V-gene from the same germline family also encodes the V-domain of mAbs 2-2-B, which targets the antigenically distinct serogroup B capsular PS. Somatic mutation, junctional diversity and alternative light chains collectively impart the specificity for these serologically distinct epitopes. Knowledge of the specific immunoglobulin genes used to target common bacterial virulence factors may lead to insights on pathogen-host co-evolution, and the potential use of this information in pre-symptomatic diagnosis is discussed.

Molecular characterization of a panel of murine monoclonal antibodies specific for the SARS-coronavirus

The availability of monoclonal antibodies (mAbs) specific for the SARS-coronavirus (SARS-CoV) is important for the development of both diagnostic tools and treatment of infection. A molecular characterization of nine monoclonal antibodies raised in immune mice, using highly purified, inactivated SARS-CoV as the inoculating antigen, is presented in this report. These antibodies are specific for numerous viral protein targets, and six of them are able to effectively neutralize SARS-CoV in vitro, including one with a neutralizing titre of 0.075 nM. A phylogenetic analysis of the heavy and light chain sequences reveals that the mAbs share considerable homology. The majority of the heavy chains belong to a single Ig germline V-gene family, while considerably more sequence variation is evident in the light chain sequences. These analyses demonstrate that neutralization ability can be correlated with specific murine V_H-gene alleles. For instance, one evident trend is high sequence conservation in the V_H chains of the neutralizing mAbs, particularly in CDR-1 and CDR-2. The results suggest that optimization of murine mAbs for neutralization of SARS-CoV infection will likely be possible, and will aid in the development of diagnostic tools and passive treatments for SARS-CoV infection.

A bidirectional phage display vector for the selection and mass transfer of polyclonal antibody libraries

An approach to the creation of antigen-specific polyclonal libraries of intact antibodies is presented. A polyclonal library of Fab antibody fragments would be expressed using a phage display vector, and selected for reactivity with an antigen or group of antigens. For conversion into a sublibrary of intact polyclonal antibodies, the selected heavy (H) and light (L) chain variable (V) region gene combinations would be transferred in mass, as linked pairs, to a eukaryotic expression vector which provides immunoglobulin (Ig) constant (C) region genes. To enable this selection and transfer, a bidirectional phage display vector was generated, in which the V region gene pairs are linked head to head in opposite transcriptional orientations. The functionality of this vector was demonstrated by the selection, transfer and expression of linked V region gene pairs derived from an A/J mouse that had been immunized with p-azophenylarsonate (Ars)-coupled keyhole limpet hemocyanin (KLH). As expected, the expressed IgG2b anti-Ars antibodies with selected V region gene pairs were shown to have V region sequences and Ars-binding characteristics similar to those of anti-Ars hybridoma antibodies. The technology presented here has potential for many diagnostic and therapeutic applications. These include the generation of polyclonal antibody libraries against multiple epitopes on infectious agents or cancer cells, and of polyclonal libraries encoding chimeric molecules composed of antibody V regions and T cell receptor C regions.

Immune response of lambs to vaccination with Ostertagia circumcincta surface antigens eliciting bile antibody responses

Immune responses to surface antigens of infective larvae of Ostertagia circumcincta recognized by bile antibodies of sheep immune to challenge were studied in 5-month-old Finn-Dorset male lambs. The sheep were vaccinated subcutaneously with 2 doses of 25 micrograms/kg body weight of surface proteins immunoprecipitated by bile antibodies derived from protected lambs. These antigens were purified from immune complexes by affinity chromatography and then injected with beryllium hydroxide as an adjuvant. The immunized lambs were challenged with 5 x 10(4) L3 and the worm burdens evaluated on day 21 post challenge. These were significantly (P < 0.01) lower in the vaccinated group than in the challenged controls (72% protection). The mucosal and bile IgM, recognizing the L3 surface, showed significantly higher levels in the vaccinated lambs compared to the challenge controls. Mucosal and bile IgA antibody levels against the same antigens were low and no significant differences were observed between vaccinated and control lambs.

Lack of connectivity between the induced and autoimmune repertoires of lpr/lpr mice

It has been proposed that the autoantibody-secreting cells active during autoimmune diseases are derived from B cells initially responding to environmental antigens. In order to test the relationship between the antigen-induced and autoimmune repertoires, we monitored the fate of antigen-activated idiotypically defined B cells present in mice that developed the systemic lupus erythematosus (SLE)-like syndrome associated with the lpr mutation. Mice homozygous for both the A/J-derived Igh and Ig kappa region haplotypes and the lpr mutation were bred. Immunization of these mice with p-azophenylarsonate (Ars)-protein conjugates elicited the idiotypic components (IdCR) characteristic of the A/J anti-Ars response and did not interfere with the spontaneous development of the lpr-mediated autoimmune disease. These Id/lpr mice provided an ideal system for studying the relationship between the exogenously and endogenously induced responses because: (1) VHIdCR antibodies have been shown to bind autoantigens in vitro; and (2) serological and molecular reagents exist which can identify and monitor VHIdCR antibody production as disease progresses. Serum samples and hybridoma cell lines derived from non-immune as well as Ars-keyhole limpet haemocyanin (KLH)-immunized Id/lpr mice were monitored for idiotype expression as well as Ars and ssDNA reactivity at various stages of disease progression. We found that antibodies utilizing the VHIdCR gene segment did not preferentially contribute to the autoantibody pool. Moreover, even when IdCR B-cell clones were expanded by deliberate immunization with Ars-KLH, Ars non-binding variants were only rarely detected among the activated B-cell populations of diseased mice. These results indicate that there is only minimal overlap between the VHIdCR conventional and autoimmune repertoires.

Defining the structural correlates responsible for loss of arsonate affinity in an IDCR antibody isolated from an autoimmune mouse

Immunization of the autoimmune mouse strain (M x A) Id/lpr with Ars-KLH, has been shown to elicit a prolonged anti-Ars IdCR response similar to that found in A/J mice. Cell fusion of splenocytes from a diseased mouse previously immunized with Ars-KLH resulted in a monoclonal antibody, 1-52.30, that was found to express the strain A major cross-reactive idiotype, but failed to bind Ars. Nucleotide sequence analysis demonstrated that 1-52.30: (a) used the "canonical" combination of gene segments associated with this idiotype, and (b) exhibited a pattern of somatic mutation consistent with selection for high affinity Ars binding. Two amino acids, VL 91 and 93, were mutated in 36-65, the germline equivalent of the IdCR antibodies, to 1-52.30-like residues (91G-->D, 93T-->M). The results of the mutagenesis showed that changing a single light chain residue, VL 91, from glycine to aspartic acid, resulted in a dramatic loss of Ars binding activity.

Immune responses of sheep to surface antigens of infective larvae of Ostertagia circumcincta

Immune responses to surface antigens of infective larvae of Ostertagia circumcincta were studied in 5-month old Finn-Dorset male lambs. The sheep were vaccinated subcutaneously with 2 doses of 25 micrograms/kg body weight of infective larvae surface or somatic extracts and Freund's or beryllium hydroxide adjuvants. It was found that only in lambs vaccinated with L3 surface extracts and beryllium hydroxide as an adjuvant did worm burdens differ significantly (P less than 0.01) from those of challenge controls (71.7% protection). Sheep vaccinated with the same antigenic preparation but administered with Freund's adjuvant or with beryllium adjuvant and somatic extracts demonstrated rather poor protection (32.4 and 30.5% respectively). All the vaccinated sheep showed a high IgG response to L3 surface antigens while significant levels of serum and bile IgA reacting with the surface extracts were only detected in those lambs vaccinated with L3 surface extracts and beryllium hydroxide. In immunofluorescence tests serum IgA of this group reacted with the whole surface of exsheathed larvae or with the site of opening of excretory pore while IgG antibodies reacted strongly with the anterior and posterior parts of the infective larvae. Both IgA and IgG responses to surface antigens were stage specific.

Regulation of the immune response to peptide antigens: differential induction of immediate-type hypersensitivity and T cell proliferation due to changes in either peptide structure or major histocompatibility complex haplotype

The immunodominant CD4 T cell epitope of the bacteriophage lambda cI repressor protein in several inbred mouse strains can be represented by a peptide encompassing amino acids 12-26. Here, we show that this peptide, and a variety of its sequence variants, can induce immediate-type hypersensitivity in mice. 12-26 variants that differ by as little as single amino acid residues deviate greatly in their ability to induce hypersensitivity. Further, differences in major histocompatibility complex class II alleles appear to be as influential as changes in peptide structure in determining whether hypersensitivity is developed. The ability of a given peptide-class II combination to induce hypersensitivity correlates with production of peptide-specific antibody, but not with ability or inability to induce a T cell proliferative response. Administration of anti-interleukin 4 (IL-4) mAb prevents the development of hypersensitivity, and analysis of cytokine production by T cell hybridomas derived from peptide-immunized mice suggests that whether a given peptide-class II combination can induce hypersensitivity depends on its ability to induce IL-4 production. The data demonstrate that changes in the nature of the epitope(s) recognized by the CD4 T cell population can result in qualitative differences in the response elicited in this population, ultimately leading to dramatic quantitative and qualitative variations in the effector phase of the immune response.

Maturation of the antibody response to a protein-coupled form of the organophosphorus toxin soman

We have analysed the serum antibody response of BALB/c mice to the organophosphorus toxin soman coupled to the protein carrier keyhole limpet haemocyanin (So-KLH) and compared the specificity of the serum antibodies to that of hybridomas described previously. The relative inhibitory capacities of various soman analogues for serum antibodies correlated with those for the monoclonal antibodies. Our results also demonstrate that immune memory to this organophosphorus hapten is stable for greater than 1 year. Interestingly, maturation of the serum antibody response is accompanied by fine specificity changes resulting in increased binding to soman-protein conjugates but not in significant changes in binding to free hapten analogues of soman. This finding suggests that contributions made by the protein carrier or bridge structure, including those made by amino acid side chains involved in the linkage, may play a significant role in the maturation process of antibodies recognizing protein-coupled organophosphorus haptens such as So-KLH. Structurally related but charge-dissimilar organophosphate haptens such as nitrophenylphosphocholine were poorly recognized, even when conjugated to protein with the same diazophenyl linkage used to conjugate soman. This is consistent with maintenance of high specificity in the memory immune response to soman-coupled protein.

Different epitope structures select distinct mutant forms of an antibody variable region for expression during the immune response

Antibody variable (V) regions that initially differ from one another by only single amino acid residues at VH-D and D-JH segment junctions (termed canonical V regions) can be elicited in strain A/J mice by three different haptens. Among such V regions an amino acid substitution due to somatic mutation is recurrently observed at VH CDR2 position 58, regardless of which of these haptens is used for immunization. This substitution confers upon a canonical V region a generic increase in affinity for all the haptens. Conversely, the type of amino acid substitution at VH position 59 resulting from somatic mutation that is recurrently observed among such V regions changes with the eliciting hapten, in a manner that correlates directly with the cognate affinity increases (or decreases) for hapten conferred by the observed substitutions. This small subregion of VH CDR2 therefore plays a major role in determining both affinity and specificity for antigen. The data confirm that affinity for antigen is of pivotal importance in determining the degree of selection of different mutant forms of a V region. Moreover, during an immune response a sufficiently diverse mutant repertoire can be generated from a single canonical V region to allow adaptation to increase affinity for three different epitopes.

Limits on heavy chain junctional diversity contribute to the recurrence of an antibody variable region

Antibody V region structural diversity in the mouse is generated, in part, by the combinatorial joining of different gene segments, as well as by the "imprecision" of these joining events. The same two gene segments can be joined at different locations, and nucleotides can be deleted or added de novo to the segment junction. While it is clear that such junctional processes are a major contributor to V region diversity, the mechanisms that generate this diversity are poorly understood. Here I present sequences in the VH-D-JH region of 34 VH genes that are composed of the same three VH gene segments. In combination with a single V kappa-J kappa pair, these VH genes encode a family of V regions that are recurrently expressed in the immune response of A/J mice to p-azophenylarsonate (Ars). The germline sequences of the three constituent gene segments for these VH genes are known, making it possible to determine the origin of the nucleotides in junctional regions. An examination of the frequency and type of nucleotides present in these regions provides insight into the properties of the segment joining mechanism. In addition, the data suggest that recurrent expression of the anti-Ars V regions which these VH genes partially encode is due not only to antigenic selection, but to the high probability with which these VH genes are formed during B cell differentiation.

Isotype responses of infected, virus-vaccinated and peptide-vaccinated cattle to foot-and-mouth disease virus

An ELISA to measure bovine serum immunoglobulin isotypes (IgG1, IgG2, IgM and IgA) specific for foot-and-mouth disease virus (FMDV) or for synthetic FMDV peptides is described. Sera from cattle infected by FMDV, vaccinated with conventional inactivated virus vaccines or vaccinated with synthetic peptides were examined using this assay. Generally IgG subclasses dominated the antibody responses of all groups after an early IgM response had waned. An exception to this pattern was seen in the case of a group of immature calves given multiple or high doses of synthetic peptide and in which levels of IgM continued to rise until the end of the experimental period. Both infected animals and those vaccinated with inactivated virus mounted antibody responses in which IgG1 titres tended to predominate over those of IgG2. In some infected animals, an early IgG2 response was evident but resolution of lesions and clinical recovery did not occur until IgG1 antibody appeared in the serum some days later. In synthetic-peptide immunized animals the response was more variable but IgG1:IgG2 ratios at 21 days postvaccination were significantly lower than those of virus-vaccinated animals. It is proposed that differences in the isotype profiles induced by conventional FMD vaccines and those resulting from vaccination of cattle with synthetic FMDV peptides may in part account for the lower protective index of peptide-induced antibodies.

The NK model of rugged fitness landscapes and its application to maturation of the immune response

Adaptive evolution is, to a large extent, a complex combinatorial optimization process. Such processes can be characterized as "uphill walks on rugged fitness landscapes". Concrete examples of fitness landscapes include the distribution of any specific functional property such as the capacity to catalyze a specific reaction, or bind a specific ligand, in "protein space". In particular, the property might be the affinity of all possible antibody molecules for a specific antigenic determinant. That affinity landscape presumably plays a critical role in maturation of the immune response. In this process, hypermutation and clonal selection act to select antibody V region mutant variants with successively higher affinity for the immunizing antigen. The actual statistical structure of affinity landscapes, although knowable, is currently unknown. Here, we analyze a class of mathematical models we call NK models. We show that these models capture significant features of the maturation of the immune response, which is currently thought to share features with general protein evolution. The NK models have the important property that, as the parameter K increases, the "ruggedness" of the NK landscape varies from a single peaked "Fujiyama" landscape to a multi-peaked "badlands" landscape. Walks to local optima on such landscapes become shorter as K increases. This fact allows us to choose a value of K that corresponds to the experimentally observed number of mutational "steps", 6-8, taken as an antibody sequence matures. If the mature antibody is taken to correspond to a local optimum in the model, tuning the model requires that K be about 40, implying that the functional contribution of each amino acid in the V region is affected by about 40 others. Given this value of K, the model then predicts several features of "antibody space" that are in qualitative agreement with experiment: (1) The fraction of fitter variants of an initial "roughed in" germ line antibody amplified by clonal selection is about 1-2%. (2) Mutations at some sites of the mature antibody hardly affect antibody function at all, but mutations at other sites dramatically decrease function. (3) The same "roughed in" antibody sequence can "walk" to many mature antibody sequences. (4) Many adaptive walks can end on the same local optimum. (5) Comparison of different mature sequences derived from the same initial V region shows evolutionary hot spots and parallel mutations. All these predictions are open to detailed testing by obtaining monoclonal antibodies early in the immune response and carrying out in vitro mutagenesis and adaptive hill climbing with respect to affinity for the immunizing antigen.

Evolution of antibody structure during the immune response. The differentiative potential of a single B lymphocyte

Changes in the structure and function of antibodies occur during the course of an immune response due to variable (V) region gene somatic mutation and isotype switch recombination. While the end products of both these processes are now well documented, their mechanisms, timing, and regulation during clonal expansion remain unclear. Here I describe the characterization of antibodies expressed by a large number of hybridomas derived from single B cell clones at an intermediate stage of an immune response. These data provide new insights into the mechanism, relative timing, and potential of V gene mutation and isotype switching. The data suggest that somatic mutation and isotype switching are completely independent processes that may, but need not, occur simultaneously during clonal expansion. In addition, the results of this analysis demonstrate that individual B cell clones are far more efficient than previously imagined at generating and fixing particular V region somatic mutations that result in increased affinity for the eliciting epitope. Models to account for this high efficiency are discussed. Taken together with previous data, the results of this analysis also suggest that the "somatic evolution" of V region structure to a single epitope takes place in two stages; the first in which particular mutations are sustained and fixed by antigen selection in the CDR regions of the V region genes expressed in a clone over a short period of clonal expansion, and the second in which these selected CDR mutations are maintained in the growing clone, deleterious mutations are lost, and selectively neutral mutations accumulate throughout the length of V genes over long periods of clonal expansion.

Molecular analysis of original antigenic sin. I. Clonal selection, somatic mutation, and isotype switching during a memory B cell response

To determine how the memory B cell population elicited to one epitope might be used in immune responses to other, structurally related epitopes, we explored the phenomenon of original antigenic sin. Strain A/J mice reproducibly respond to immunization with p-azophenylarsonate (Ars) by production of anti-Ars antibodies encoded predominantly by a single VH gene segment (VHIdCR). The structural analogue of Ars p-azophenylsulfonate (Sulf) fails alone to elicit such V regions, but can do so in A/J mice previously immunized with Ars, providing a means to specifically examine B cells capable of responding secondarily to a crossreactive antigen (i.e., memory cells). VHIdCR-expressing hybridomas were derived from the Ars-primed, Sulf-boosted original antigenic sin response of A/J mice at various times after Ars priming, and the properties of the antibodies they express and the structure of the genes encoding these antibodies were characterized. The data obtained support the following conclusions: (a) The Ars-induced memory B cell population capable of being crossreactively stimulated by Sulf is largely formed from a small fraction of all B cells participating in the anti-Ars primary response that express somatically mutated V regions; (b) the antibody repertoire and clonal composition of this population are stable over long periods of time; (c) memory B cells are capable of clonal expansion in the absence of a high rate of V gene somatic mutation; (d) the activation requirements for clonal selection of memory, versus naive B cells appear to differ; and (e) a major fraction of Ars-induced memory B cells express either IgM or IgG3 prior to and during the initial stages of the sin response.