Rabbit immunoglobulin allotypes: structure, immunology, and genetics

Sequencing of VDJ genes in Lepus americanus confirms a correlation between VHn expression and the leporid species continent of origin

Leporid VH genes used in the generation of their primary antibody repertoire exhibit highly divergent lineages. For the European rabbit (Oryctolagus cuniculus) four VHa lineages have been described, the a1, a2, a3 and a4. Hares (Lepus spp.) and cottontail (Sylvilagus floridanus) express one VHa lineage each, the a2L and the a5, respectively, along with a more ancient lineage, the Lepus spp. sL and S. floridanus sS. Both the European rabbit and the Lepus europaeus use a third lineage, VHn, in a low proportion of their VDJ rearrangements. The VHn genes are a conserved ancestral polymorphism that is being maintained in the leporid genome.Their usage in a low proportion of VDJ rearrangements by both European rabbit and L. europaeus but not S. floridanus has been argued to be a remnant of an ancient European leporid immunologic response to pathogens. To address this hypothesis, in this study we sequenced VDJ rearranged genes for another North American leporid, L. americanus. Our results show that L. americanus expressed these genes less frequently and in a highly modified fashion compared to the European Lepus species. Our results suggest that the American leporid species use a different VH repertoire than the European species which may be related with an immune adaptation to different environmental conditions, such as different pathogenic agents.

An overview of the lagomorph immune system and its genetic diversity

Our knowledge of the lagomorph immune system remains largely based upon studies of the European rabbit (Oryctolagus cuniculus), a major model for studies of immunology. Two important and devastating viral diseases, rabbit hemorrhagic disease and myxomatosis, are affecting European rabbit populations. In this context, we discuss the genetic diversity of the European rabbit immune system and extend to available information about other lagomorphs. Regarding innate immunity, we review the most recent advances in identifying interleukins, chemokines and chemokine receptors, Toll-like receptors, antiviral proteins (RIG-I and Trim5), and the genes encoding fucosyltransferases that are utilized by rabbit hemorrhagic disease virus as a portal for invading host respiratory and gut epithelial cells. Evolutionary studies showed that several genes of innate immunity are evolving by strong natural selection. Studies of the leporid CCR5 gene revealed a very dramatic change unique in mammals at the second extracellular loop of CCR5 resulting from a gene conversion event with the paralogous CCR2. For the adaptive immune system, we review genetic diversity at the loci encoding antibody variable and constant regions, the major histocompatibility complex (RLA) and T cells. Studies of IGHV and IGKC genes expressed in leporids are two of the few examples of trans-species polymorphism observed outside of the major histocompatibility complex. In addition, we review some endogenous viruses of lagomorph genomes, the importance of the European rabbit as a model for human disease studies, and the anticipated role of next-generation sequencing in extending knowledge of lagomorph immune systems and their evolution.

Sequencing of Sylvilagus VDJ genes reveals a new VHa allelic lineage and shows that ancient VH lineages were retained differently in leporids

Antigen recognition by immunoglobulins depends upon initial rearrangements of heavy chain V, D, and J genes. In leporids, a unique system exists for the VH genes usage that exhibit highly divergent lineages: the VHa allotypes, the Lepus sL lineage and the VHn genes. For the European rabbit (Oryctolagus cuniculus), four VHa lineages have been described, the a1, a2, a3 and a4. For hares (Lepus sp.), one VHa lineage was described, the a2L, as well as a more ancient sL lineage. Both genera use the VHn genes in a low frequency of their VDJ rearrangements. To address the hypothesis that the VH specificities could be associated with different environments, we sequenced VDJ genes from a third leporid genus, Sylvilagus. We found a fifth and equally divergent VHa lineage, the a5, and an ancient lineage, the sS, related to the hares' sL, but failed to obtain VHn genes. These results show that the studied leporids employ different VH lineages in the generation of the antibody repertoire, suggesting that the leporid VH genes are subject to strong selective pressure likely imposed by specific pathogens.

Sequencing of modern Lepus VDJ genes shows that the usage of VHn genes has been retained in both Oryctolagus and Lepus that diverged 12 million years ago

Among mammals, the European rabbit (Oryctolagus cuniculus) has a unique mechanism of generating the primary antibody repertoire. Despite having over 200 VH genes, the VH1 gene, the most D-proximal VH gene, is used in 80-90 % of VDJ rearrangements, while the remaining 10-20 % is encoded by the VHn genes that map at least 100 Kb upstream of VH1. The maintenance of the VHn genes usage in low frequency in VDJ rearrangements has been suggested to represent a relic of an ancestral immunologic response to pathogens. To address this question, we sequenced VDJ genes for another leporid, genus Lepus, which separated from European rabbit 12 million years ago. Approximately 25 VDJ gene sequences were obtained for each one of three Lepus europaeus individuals. We found that Lepus also uses the VHn genes in 5-10 % of its VDJ rearrangements. Our results show that the VHn genes are a conserved ancestral polymorphism that has been maintained in the leporids genome and is being used for the generation of VDJ rearrangements by both modern Lepus and Oryctolagus.

Molecular bases of genetic diversity and evolution of the immunoglobulin heavy chain variable region (IGHV) gene locus in leporids

The rabbit has long been a model for studies of the immune system. Work using rabbits contributed both to the battle against infectious diseases such as rabies and syphilis, and to our knowledge, of antibodies' structure, function, and regulated expression. With the description of rabbit Ig allotypes, the discovery of different gene segments encoding immunoglobulins became possible. This challenged the "one gene-one protein" dogma. The observation that rabbit allotypic specificities of the variable regions were present on IgM and IgG molecules also led to the hypothesis of Ig class switching. Rabbit allotypes contributed to the documentation of phenomena such as allelic exclusion and imbalance in production of allelic gene products. During the last 30 years, the rabbit Ig allotypes revealed a number of unique features, setting them apart from mice, humans, and other mammals. Here, we review the most relevant findings concerning the rabbit IGHV. Among these are the preferential usage of one VH gene in VDJ rearrangements, the existence of trans-species polymorphism in the IGHV locus revealed by serology and confirmed by sequencing IGHV genes in Lepus, the unusually large genetic distances between allelic lineages and the fact that the antibody repertoire is diversified in this species only after birth. The whole genome sequence of a rabbit, plus re-sequencing of additional strains and related genera, will allow further evolutionary investigations of antibody variation. Continued research will help define the roles that genetic, allelic, and population diversity at antibody loci may play in host-parasite interactions.

Quantitation of cytokine mRNA by real-time RT-PCR during a vaccination trial in a rabbit model of fascioliasis

Use of the rabbit as disease model has long been hampered by a lack of immunological assays specific to this species. In the present study we developed a SYBR Green-based, real-time RT-PCR protocol to quantitate cytokine mRNA in freshly harvested rabbit peripheral mononuclear cells. The method was validated in the course of a vaccination trial in which animals vaccinated with the recombinant antigen FhSAP2 were challenged with Fasciola hepatica metacercariae. Changes in the levels of rabbit interleukin (IL)-2, IL-4, IL-6, IL-10, tumor necrosis factor-alpha (TNFalpha), and interferon-gamma (IFNgamma) mRNA were determined. Messenger RNA from the universally expressed housekeeping gene GAPDH was used as an amplification control and allowed for correction of variations in the efficiencies of RNA extraction and reverse transcription. Rabbits vaccinated with FhSAP2 showed an 83.3% reduction in liver fluke burden after challenge infection when compared to non-vaccinated controls. All cytokine mRNAs were found at detectable levels; however, the levels of IFNgamma, TNFalpha, IL-2 and IL-10 were significantly higher in the vaccinated group compared to the non-vaccinated group. These results suggest that protection conferred by FhSAP2 protein could be associated with a mixed Th1/Th2 immune response in which Th1 cytokines are dominant. The real-time RT-PCR method described herein can be a useful tool for monitoring changes in basic immune functions in the rabbit model of fascioliasis and may also aid in studies of human diseases for which the rabbit is an important experimental model.

Cross-reaction of a minor variant of the a1 allotypic specificity with anti-a2 antibodies

Certain samples of hare IgG can combine with cross-linked anti-a2 antisera prepared in the a3/a3 rabbit. This cross-reaction permitted the isolation, on hare IgG immunoadsorbent, of anti-a2 cross-reacting antibodies (anti-a2(Lv) antibodies). The binding of labeled rabbit a2 IgG to insolubilized anti-a2(Lv) antibodies is inhibited by a1 IgG, demonstrating a cross-reactivity with a2. The percentage of a1 IgG cross-reacting with anti-a2 antiserum (a1(2) IgG) is about 0.5% of total a1 IgG. The a1(2) molecules represent another variant (or set of variants) of the a1 specificity. Demonstration of this variant of a1 IgG brings to seven the minimum number of described a1 variants, and its low concentration among a1 IgG (0.5%) is in favor of a larger number of variants of allotypic specificities. Arguments in favor of common ancestor genes for allotypy in lagomorphs are given.

Quantitation of rabbit cytokine mRNA by real-time RT-PCR

Fundamental understanding of rabbit immunology and the use of the rabbit as a disease model have long been hindered by the lack of immunological assays specific to this species. In the present study, we sought to develop a method to quantitate cytokine expression in rabbit cells and tissues. We report the development of a quantitative real-time RT-PCR method for measuring the relative levels of rabbit IFN-gamma, IL-2, IL-4, IL-10 and TNF-alpha mRNA. Quantitation was accomplished by comparison to a standard curve generated using plasmid DNA containing partial sequences of the relevant cytokines. Experimental studies demonstrate applicability of this assay to quantitate cytokine mRNA levels from rabbit spleen cells following mitogen stimulation. We have further utilized this assay to also examine cytokine expression in rabbit tissues during experimental syphilis infection.

A genetic marker of rabbit immunoglobulin VHa region: a109

A "new" rabbit immunoglobulin allotype belonging to the VHa series (a109) that seems particular to the Iberian O. cuniculus has been detected in a Portuguese wild rabbit population. This allotype was found in the wild population of Spain but not in those of France and Zembra island (Tunisia). Using antisera and monoclonal antibodies raised against already known VHa allotypes, we found that a 109 presents cross-reactivity with a1, a3, a100, a101, a107 and a108 allotypes. On the basis of studies of these cross-reactivities, we show that a109 is constituted of at least three variants (or subfamilies of variants) possessing determinants common with either a1 and a109, a3 and a109 or a1, a3 and a109 allotypes. These results are discussed according to recent data relative to generation of antibody diversity in rabbit.

Restricted utilization of germ-line VH genes in rabbits: implications for inheritance of VH allotypes and generation of antibody diversity

The presence of inherited VH region allotypic specificities, a1, a2 or a3, on nearly all rabbit immunoglobulins has presented a paradox. We know the germline contains hundreds of VH genes, and if we assume that most of these are used in the generation of antibody diversity, then we must ask how have the a allotype-encoding regions been maintained over time? On the other hand, if we assume that only one (or a small number) of these VH gene(s) is (are) used in VDJ gene rearrangements, then, how is antibody diversity generated? To address these questions, we have cloned and determined the nucleotide sequence of the 3'-most germline VH genes from the a1, a2 and a3 chromosomes and shown in each case that the 3'-most H gene, VH1-a1, VH1-a2, or VH1-a3, encodes an a1, a2 or a3 VH region, respectively. Analysis of rearranged VDJ genes from leukemic B cells showed that VH1 was utilized in these rearrangements. Based on these data, we propose that the allelic inheritance of the VH allotypes is explained by the preferential usage of the VH1 gene in VDJ rearrangements. Support for this hypothesis was obtained from analysis of the mutant rabbit Alicia in which most serum Ig molecules do not have VHa allotypic specificities, but instead have so-called VHa-negative Ig molecules. In this rabbit, VH1 is not expressed as it has been deleted. Analysis of cDNA clones from spleen of Alicia rabbits suggests that the expressed VHa-negative molecules also are encoded by a single germline VH gene. Thus, we suggest that nearly all rabbit VH regions are encoded by one to two germline VH genes and that antibody diversity is generated primarily by somatic hypermutation and gene conversion.

Macrophage-Fc-receptor affinity: role in cellular mediation of antibody initiated glomerulonephritis

The role of immunoglobulin (Ig) Fc-macrophage cell surface receptor affinity (macrophage Fc-affinity) in determining the mediation systems responsible for immune glomerular injury has been studied. Glomerular injury was initiated in rabbits by the passive administration of immunoglobulin preparations directed against a "planted" glomerular antigen. The mediation systems inducing injury initiated by antibody pools of high and low macrophage Fc-affinity were compared. In this model of glomerulonephritis macrophage Fc-affinity was the only variable. IgFc-macrophage affinity was quantitated in fluid-phase and solid-phase assay systems (high affinity pool KA = 7.05 +/- 1.11 x 10(5) L/M; low affinity pool KA = 0.79 +/- 0.13 x 10(5) L/M). Comparative antibody binding studies demonstrated that renal injury occurred with high affinity antibody at kidney-fixed-antibody (KFA) binding levels significantly below the KFA levels required for glomerular injury with administration of low affinity antibody [KFA's: 25.1 +/- 2.1 micrograms antibody globulin/g kidney cortex (micrograms/g) and 60.5 +/- 2.4 micrograms/g, respectively, P less than 0.01]. Furthermore, antibody with high macrophage Fc-receptor affinity induced a macrophage-mediated, complement-independent glomerular injury while antibody with low macrophage Fc-receptor affinity induced renal injury via complement and neutrophil dependent mechanisms. IgFc-receptor affinity determined the degree of macrophage recruitment into glomeruli via immune adherence mechanisms, and therefore is an important determinant of which inflammatory mediator system is ultimately responsible for antibody-initiated glomerular injury.

A role for mature B cells in bone marrow transplantation

The proliferative potential of membrane Ig (mIg)-bearing B lymphocytes was assessed in an adoptive transfer system based on the use of non-inbred rabbits matched for major histocompatibility (MHC) antigens and mismatched for immunoglobulin (Ig) allotypes. Cell suspensions made from spleens (SP), mesenteric lymph nodes (LN), or bone marrow (BM) of allotype b4b5 rabbits were deprived of B cells with mIg of the b4 type by adherence to plastic dishes coated with affinity-purified anti-b4. When such b4-depleted cell populations were injected into newborn hosts of allotype b6b6, stable and lasting chimerism promptly resulted, in which donor-derived products were almost entirely of the b5 allotype. Chimeras formed by transfer of unfractionated cells from b4b5 donors, on the other hand, exhibited a predominance of the b4 allotype, as seen in the living donors. BM but not SP or LN contained precursors capable of differentiating into mIg+ lymphocytes in culture, but no evidence was obtained for engraftment and differentiation by such B-cell precursors or more primitive stem cells in vivo. These studies suggest a potentially significant role for mature B cells in reconstituting the immune system of human transplant recipients.

Mab. 198: a monoclonal antibody recognizing the complement type 3 receptor (CR3) in the rabbit

A mouse monoclonal (Mab.198, IgG1) was generated that recognizes an epitope expressed on rabbit peripheral phagocytes and tissue macrophages. The membrane antigen recognized by Mab.198 consisted of two polypeptide chains of 165,000 and 95,000 molecular weight (MW). This Mab efficiently inhibited complement receptor-mediated granulocyte functions such as phagocytosis of complement-opsonized particulate antigens and zymosan-induced chemiluminescent responses. Fc receptor-mediated phagocyte functions were, on the contrary, barely affected by Mab.198. The cellular distribution, molecular structure and functional characteristics of the membrane antigen defined by Mab.198 suggested that this antibody recognizes the complement type 3 receptor (CR3). We found that Mab.M1/70, specific for mouse CR3 (i.e. CD 11 or Mac-1 antigen), also binds on rabbit phagocytes. However, this Mab recognizes a different CR3 epitope than Mab.198 as shown by cross-blocking experiments. This anti-rabbit CR3 Mab will be useful in the characterization of rabbit complement receptors and their involvement in immune reactions.

Inherited polymorphism of the human T-cell antigen receptor detected by a monoclonal antibody

Three different murine monoclonal antibodies to the human clonotypic T-cell antigen receptor immunoprecipitate the alpha-beta chain heterodimer; induce comodulation of the clonotypic molecule with the T3 molecular complex; stain small populations of normal polyclonal T cells, suggesting that they react with variable or joining region determinants of the clonotypic receptor; and induce proliferation of resting T cells. While two of these antibodies detect the clonotypic receptor in all individuals studied, the third antibody (OT145), described herein, does not detect the T-cell antigen receptor on T cells of all individuals. By indirect immunofluorescence, three groups can be distinguished within a population of individuals (n = 138) by OT145. Individuals lacking T cells reactive with OT145 have a homozygous OT145-phenotype. T cells from such individuals fail to proliferate in the presence of OT145 in contrast to T cells from OT145+ individuals. Individuals with a relatively large percentage of OT145+ T cells, 4.5 +/- 1.54% (mean +/- 2 SEM) are homozygous OT145+, while those with an intermediate percentage, 2.04 +/- 0.9%, have a heterozygous phenotype. Family studies suggest autosomal codominant inheritance of the OT145 phenotype. The distribution of the three OT145-defined phenotypes varies considerably in populations of different ethnic background. Taken together these data suggest that the polymorphism detected by OT145 may represent a variable or joining region allotypic system of the human T-cell antigen receptor. In addition, our results indicate that allelic exclusion governs the expression of the clonotypic receptor by human T cells.

Evolution of the immunoglobulin kappa light chain locus in the rabbit: evidence for differential gene conversion events

The rabbit kappa light chain gene family is characterized by the presence of two constant region (C kappa) genes; the C kappa 1 gene encodes the constant region of the principal rabbit immunoglobulin light chain, the C kappa 2 gene being not or very poorly expressed in domestic rabbits. There exist four major K1 alleles (b4, b5, b6, and b9), which are unequally expressed in heterozygous rabbits at the K1 locus. Here, we compare the nucleotide sequences of the joining (J) clusters of the kappa light chain gene (J kappa) linked to the b4K2 locus and to the b4 and b9 alleles at the K1 locus. As for C kappa genes, there is evidence for intergenic conversion between the J kappa 1 and J kappa 2 clusters as well as maximum divergence in the expressed J segments. The b9 J kappa 1 cluster differs from its b4 counterpart in that two out of the five J kappa segments (J1 and J2) are expressed instead of only one. This implies that preferential expression of the b4 allele as compared to the b9 allele is not only correlated to the number of available J kappa pieces. The b9 J2 segment is functional in spite of the presence of a termination codon immediately upstream of its coding region. Two major structural differences were observed between the J-C intron sequences of the b9 and b4 alleles; namely a 160-base-pair deletion of an A + T-rich sequence in b9 (which also occurs in the K2 locus) and a 10-base-pair deletion plus some substitutions in the region corresponding to the mouse kappa intron activating element. These differences could underlie the lower transcriptional rate of the b9 allele.

Mutation affecting the expression of immunoglobulin variable regions in the rabbit

We have found a variant of the allotype allele a2 in the rabbit, which presumably arose by mutation, that segregates as expected for an allele at the a locus. This allele is called "ali" and the corresponding rabbit strain is called "Alicia." In heterozygous animals (ali/a1 and ali/a3) the concentration of a2 molecules is lower by a factor of 1000 than in standard a2/a2 homozygotes. In homozygous ali/ali individuals the a2 concentration varies with age--i.e., very low in young rabbits and higher in older ones--but it never reaches normal levels. The low level of a2 is compensated by increased amounts of a-negative molecules. Southern blot analysis did not reveal any gross changes in the intron between JH and C mu (joining region of immunoglobulin heavy chain and constant region of immunoglobulin mu chain) or in the number of VH gene segments encoding a locus specificities. We suggest that the ali phenotype is due to a mutation in a control element.

Location of antigenic epitopes on antibody molecules

Using X-ray crystallographic co-ordinates of immunoglobulins, surface regions accessible to a large spherical probe, comparable in size to an antibody domain, were computed. Locations of these exposed regions were compared with those of experimentally determined antigenic sites, i.e. idiotypic, allotypic and isotypic serological markers. In all cases, an excellent agreement was found. The most prominent computed epitopes correspond to convex parts of antibody surface made by reverse turn segments of the polypeptide chain. The computed epitopes occur in homologous positions in all the immunoglobulin domains, and most of the beta-sheet surfaces on the domains are poorly antigenic. The CH2 domain (Fc fragment) has many more antigenic sites than the Fab fragments (antigen-binding fragments). Variable domain epitopes (idiotypes) involve both hypervariable and framework residues, and only about 25% of the hypervariable residues are strongly antigenic. The results indicate that, in a vertebrate body, each antibody molecule may be recognized, and its concentration regulated, by at least 40 complementary anti-immunoglobulin antibodies; therefore, a possibility of an "immune network" with much higher connectivity than is generally assumed should be seriously contemplated.

Isolation and partial characterization of a fragment corresponding to the dimeric form of the CH2 domain of rabbit IgG

A fragment corresponding to the intact dimeric form of the CH2 domain of rabbit IgG, including the hinge region disulfide linkage, was obtained by plasmin digestion of crystalline Fc derived from IgG by the action of papain. Identification and assessment of purity of the fragment was established by SDS-PAGE, amino acid composition analysis, N-terminus sequence and C-terminus amino acid analysis and SDS-urea-PAGE of the reduced fragment. The fragment retains serologic reactivity with anti-Fc specific antisera. Comparison of deglycosylation by endoglycosidase F indicates a more open special relationship between the two CH2 domains in the fragment than in Fc.

Selective suppression of allotype expression induced in vitro: maintenance of suppression following adoptive transfer

The question of the ultimate fate of lymphocytes subjected to treatment with anti-allotype antibody (Ab) has been investigated by means of an adoptive transfer system that uses noninbred rabbits matched for major histocompatibility antigens and mismatched for allotype. Suppression of b4 immunoglobulin (Ig) production was induced by incubating lymphocytes from b4b5 rabbits with anti-b4 in culture. Transfer of b4-suppressed cells to newborn recipients of allotype b6b6 resulted in stable chimerism of mixed donor-recipient allotypes, in which b4 Ig production remained suppressed. In recipients of non-Ab-treated cells, b4 Ig production predominated over b5, as had been the case in the intact donor. No evidence for stimulation of b4 Ig synthesis was seen, even when lymphocytes and serum from 1-week-old recipients were examined. When lymphoid cells of antigen-primed b4b5 donors were treated with anti-b4 in vitro and transferred, Ab production of the b4 type was specifically suppressed, with compensatory over-production by Ab-forming cells of the b5 type. The results reported here indicate that although anti-allotype Ab is not directly cytotoxic, a significant proportion of the b4-committed cells were irreversibly inactivated as a result of Ab pulse treatment.

Acquired resistance and expression of a protective humoral immune response in guinea pigs infected with Treponema pallidum Nichols

Resistance to cutaneous syphilitic reinfection in strain 2 and strain 13 guinea pigs developed gradually 3 to 7 months after primary infection and reached maximum levels at 6 to 7 months after the induction of primary cutaneous disease. Associated with this acquired resistance was the occurrence of Arthus reactions and anamnestic-type antibody responses. Passive transfer of immune serum containing high-titered treponemal antibody into normal strain 2 guinea pigs significantly delayed the appearance and markedly diminished the severity and duration of skin lesions that developed after these recipients were challenged with treponemes but did not prevent the dissemination of organisms to the draining lymph nodes. These findings provide direct evidence that syphilitic infection elicits the formation of serum factors that are, at least, partially protective against symptomatic disease.

Impaired lymphocyte functions in heterozygous rabbits recovering from neonatal allotype suppression

Lymphocytes from heterozygous rabbits suppressed for an allotypic determinant on kappa light chains by exposure to maternally derived antibodies specific for the paternal gene product were analyzed for their capacity to express membrane-bound and secreted immunoglobulin (Ig). Individual cells displaying allotypic membrane Ig (mIg) were enumerated by a rosette test, while Ig-secreting cells were assessed by means of a hemolytic plaque assay. In a group of suppressed rabbits varying in age from 3 to 19 months, the proportion of cells with mIg of the paternal type was markedly higher than that of cells secreting that type of Ig. The same high proportion of lymphocytes displaying mIg of the suppressed type was observed whether lymphocytes from blood, spleen, or lymph nodes of suppressed rabbits were examined. In contrast, similar analyses performed with cells of normal heterozygous rabbits showed no discrepancy between mIg expression and secretion of either allotype. Lymphocytes synthesizing Ig of the paternal type were also defective in responses to lipopolysaccharide (LPS), which stimulates differentiation to Ig secretion in normal B lymphocytes. These results support the idea that B lymphocytes capable of synthesizing the suppressed type of Ig have functional impairments affecting secretion and responses to environmental stimuli.

Studies on human low serum IgD phenotype and Gm markers

The human "low serum IgD phenotype" was studied by simultaneous Gm typing and IgD immunoassay of several populations. An association between Gm (f+b+) haplotype and low human IgD was confirmed and extended to the "low serum IgD phenotype"--as defined from population distribution and genetic studies by Dunnette et al. 1978. Further, it was shown that Black American sera determined by Gm haplotype, had a similar percentage of "low serum IgD phenotype" samples (16%) although they lacked the "associated" Gm(f+b+) haplotype of White American samples. Sardinian sera showed a low incidence of the "low serum IgD phenotype" which was not correlated with Gm haplotype distribution. Familial aggregation of the "low serum IgD phenotype" was observed. No association was found between "low serum IgD phenotype" and serum IgE values. Age related abiotrophy of IgD could not be attributed to selective survival of "low serum IgD phenotype" persons.

Tetraparental rabbits chimeric for their lymphoid system--I. Allotype expression

We constructed tetraparental rabbits by immunosurgically prepared inner-cell-mass transplantation. The two constitutive parental strains were chosen so that they not only differed by their coat colour (brown and white rabbits) but also by the allotypic specificities of their immunoglobulins. It was thus possible to distinguish among the rabbits born from composite blastocysts those which were chimeric for their lymphoid system and to unambiguously quantitate the respective contribution of each parental strain to this chimerism. In this peculiar situation, the possible perturbation of the expression of immunoglobulin allotypic specificities could be tested. Thus further analysis of the tetraparental rabbit's immunoglobulins was performed to look at possible anomalies of their constitution. In only one out of the 20 tetraparental rabbits studied, we found IgG having heavy chains with the allotypic specificities of one parental strain and light chains with the allotypic specificities of the other parental strain. We did not observe, in any case, the expression of immunoglobulin allotypic specificities other than the expected ones. We always observed a marked decrease with time in the expression of the immunoglobulins produced by the lymphocytes of one parental strain accompanied by a concomitant increase in the expression of the immunoglobulins produced by the lymphocytes of the other parental strain. Two tetraparental rabbits were found to be germ-line chimeras. Analysis of the immunoglobulin allotypic specificities of the progeny of our tetraparental rabbits including these two did not reveal any perturbation in their hereditary transmission according to the usual Mendelian laws.

A mouse monoclonal antibody against rabbit VH allotype shares the predominant idiotype with a rabbit antibody of the same specificity

A monoclonal antibody, termed 3-2F1, is specific for the rabbit VH al allotypic marker. This antibody appears to recognize a common al determinant since it reacts with all of the a1 immunoglobulin (Ig) that is bound by polyclonal rabbit anti-a1, and completely inhibits the binding of polyclonal anti-a1 to a1 Ig. When tested for the predominant IdX-a1 idiotype previously shown to be expressed on virtually all rabbit anti-a1 antibody, it was found that 3-2F1 also bears this idiotype. In fact, sequential absorption and inhibition assays indicate that 3-2F1 displays the entire set of rabbit IdX-a1 determinants. The results are consistent with the concept that the anti-IdX reagent bears a latent internal image of the common VH a1 epitope.

Nucleotide sequence of a cDNA encoding the constant region of a rabbit immunoglobulin light chain of the lambda type

A cDNA library has been constructed in the plasmid pBR322 using as template 12 S poly(A)-RNA isolated from spleen cells of a hyperimmunized Basilea rabbit. One of these cDNA-containing clones was used to determine the nucleotide sequence coding for the lambda light chain constant (C) region. The deduced amino acid sequence of this cDNA was found in good agreement with a Basilea rabbit C lambda region amino acid sequence previously determined. The nucleotide sequence of the rabbit C lambda-coding region was compared with man, mouse and chicken C lambda sequences and showed 78%, 72% and 66% homology, respectively. Southern blot hybridization analyses of liver DNA from various rabbits were carried out. The comparison of the restriction patterns suggests that a few C lambda-related genes occur in the rabbit genome. In addition, discrete differences in the restriction patterns may exist between rabbits of different genetic backgrounds.

Kappa-chain allotypes and isotypes in the rabbit: cDNA sequences of clones encoding b9 suggest an evolutionary pathway and possible role of the interdomain disulfide bond in quantitative allotype expression

The constant regions of rabbit kappa light chains are unusual because the sequences of the allotypic forms can differ more from each other than do some variable regions with which they associate. We report the nucleic acid sequence of a full-length cDNA clone of b9 allotype and show comparisons to available sequences of the rabbit kappa allotypes b4, b5, and bas-N4. Our analyses suggest that the primordial rabbit kappa gene encoded a bas-like sequence. They also reveal a surprising difference in the position of the variable region cysteine that forms the interdomain disulfide bond that is unique to most rabbit kappa chains. One b9 cDNA sequence lacks the usual cysteine-80 and instead encodes cysteine-108, which in three-dimensional models appears capable of forming the interdomain disulfide bond with cysteine-171 in the constant region. A partial sequence of a second b9 clone encodes both cysteine-80 and cysteine-108; the translation product of this clone could have a free reactive sulfhydryl group that might lead to an unstable nonfunctional Ig molecule. The fact that pre-B cells with b9 kappa chains do not differentiate and expand into productive Ig-producing cells with frequencies comparable to the other allotypes may be explained if a substantial proportion of the gene products have a free sulfhydryl group. Our sequence results suggest that in cells differentiating to produce kappa light chains of b9 allotype the number and location of the cysteines influence immunoglobulin expression.

Cell surface glycoproteins of rabbit lymphocytes: characterization with monoclonal antibodies

The structural characteristics of antigens recognized by a panel of monoclonal antibodies prepared against a rabbit T-lymphocyte cell line have been investigated. Those antigens which could be isolated using immunoadsorbents prepared from the monoclonal antibodies had mol. wts of 42,000, 90,000 and 120,000. The 42,000 mol. wt molecule is similar or identical to a rabbit class I major histocompatibility complex antigen and its characterization has been reported elsewhere. Three different 90,000 mol. wt proteins can be distinguished by their reactivity with lectins and by sequential immunoprecipitation. The 120,000 mol. wt protein is a very abundant surface glycoprotein that appears to be a specific marker for T-cells in the rabbit. It is the immunodominant antigen in a lentil lectin bound glycoprotein pool. Over half of the antibodies were directed against this antigen. All antigens detected by the panel of monoclonal antibodies have been detected on normal lymphoid cells.

Genetics and expression of kappa-type light chains in Basilea rabbits

In contrast to rabbits of b4, b5, b6, and b9 allotypes whose serum immunoglobulins (Igs) are predominantly composed of kappa-type light chains, rabbits of the mutant Basilea strain have serum Igs that are largely of lambda type. We prepared several antisera that recognized a minor K2 (bas) light chain that is produced by Basilea rabbits. With these antisera we identified the K2 (bas) isotype in the serum of the original b9/b9 male rabbit whose offspring displayed the Basilea mutant phenotype. It was present in one half of his nonmutant offspring which inherited b9 from him and another b allotype from their mothers. Breeding was conducted both in Basel and at the NIH to develop and maintain colonies of mutant Basilea strain rabbits. The data obtained during colony development confirm that the trait of expression of the bas allotype maps to the same genetic region (b locus) that is known to control the allelic b allotypes b4, b5, b6 and b9. Homozygotes or heterozygotes of b4, b5 or b6 allotype (bb/bb) were mated with homozygous bbas / bbas rabbits to produce F1s , and then F2s as well as progeny of backcrosses to both homozygous parental types (bb/bb and bbas / bbas ) were produced. The bas allotype segregates as an allele (or pseudoallele ) at the b locus although there was a deficiency in recovery of homozygous bas offspring in both the F2 and backcross matings to bbas / bbas parental type in the NIH colony. This selective deficiency may reflect a deleterious effect on survival of homozygous bas progeny.

Rabbit leukocyte surface antigens defined by monoclonal antibodies

Several monoclonal antibodies (mAb) against rabbit leukocytes were characterized in binding and functional studies. mAb 1.24 stains thymocytes, bone marrow cells, peripheral T and B cells and blood monocytes. T cells express more 1.24 antigen than B cells. In the absence of added complement (C), mAb 1.24 inhibits alloantigen-, concanavalin A (Con A)-, and phytohemagglutinin (PHA)-, but not pokeweed mitogen (PWM)- or anti-immunoglobulin (Ig)-induced cell proliferation. It also strongly blocks anti-sheep erythrocyte plaque-forming cell responses. A second mAb, designated 4.B9, binds to 20% of thymocytes and to most, if not all, peripheral T cells and in vitro-activated T cell blasts. A third one, 10.B3, is reactive with the nearly entire thymocyte and a major peripheral T cell population. Two-color membrane immunofluorescence reveals the presence of a small population of peripheral blood leukocytes which bear surface Ig and are weakly stained by mAb 4.B9 and 10.B3. Without C, both 4.B9 and 10.B3 inhibit Con A- and PHA-induced mitogenesis, but have no effect on PWM-, antigen-, or alloantigen-induced cell proliferation. Depletion of 4.B9+ cells by panning or complement lysis completely abrogates proliferative responsiveness to antigen and alloantigen, significantly reduces responsiveness to the T cell mitogens Con A and PHA, but enhances that to the B cell mitogen anti-Ig. A fourth mAb, 12.C7, binds to 60% of thymocytes and to 10-30% of peripheral T lymphocytes at high-level fluorescence. T cell blasts obtained in mixed leukocyte reactions are partially stained by mAb 12.C7, while those obtained after Con A or PHA activation are not. In addition, mAb 12.C7 is completely unreactive with B cells or monocytes. Without complement, it does not seem to interfere with any of the in vitro functions tested. All antigens studied here do not appear to be expressed in nonleukon tissues, as they do not bind to erythrocytes and are absent from brain, heart, liver and kidney as shown by quantitative absorption analysis.

Mouse monoclonal antibodies against rabbit b6 allotype and their use for characterization of light-chain subpopulations

Mouse anti-allotypic hybridomas directed against different antigenic determinants of the b6 rabbit allotype have been raised. The fine specificity of these monoclonal antibodies (mAbs) was determined by radioimmunoassay and it was possible to classify them into three groups, each directed against distinct epitopes of the b6 allotype. Hare "b6" IgG were tested with anti-b6 mAb and no reaction was found indicating that the number of allotopes present on b6 molecules is greater than the three detected by the mAb. Comparative analysis by precipitation in gel of IgG from homozygous b6/b6 rabbits using mouse mAb and rabbit anti-b6 antibodies suggested that at least two categories of molecules can be discriminated. The observation of b6 subpopulations was confirmed by isolation of a minor subpopulation of IgG on a mouse monoclonal immunoadsorbent.

Circulating immune complexes in experimental syphilis: identification of treponemal antigens and specific antibodies to treponemal antigens in isolated complexes

As a prelude to characterization of the host and treponemal antigens present in purified immune complexes from the sera of rabbits with disseminated syphilis, autoradiographic and immunoenzymatic analyses of solubilized extracts of Treponema pallidum, Treponema phagedenis biotype Reiter, and Treponema refringens were performed on electroblots of polypeptides first separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Electroblots of purified immune complexes were developed with the same panel of antisera so that protein profiles could be compared. Eight treponemal antigens were consistently present in isolated complexes; four of these cross-reacted with antisera prepared against avirulent treponemes. The average molecular weights of these antigens were 87,000, 76,000, 66,000, and 45,000. Antibodies dissociated from isolated immune complexes, when used for the development of T. pallidum electroblots, reacted with four antigens of comparable molecular weight. Antibodies to those polypeptides were also present in the sera of animals immunized with immune complexes. The demonstration of treponemal antigens in purified immune complexes convincingly argues that their occurrence in experimental syphilis is not merely due to tissue destruction and responses to endogenous host antigens.

The sequences of rabbit kappa light chains of b4 and b5 allotypes differ more in their constant regions than in their 3' untranslated regions

We report the sequence of a cDNA clone encoding the entire variable and constant regions of a rabbit kappa light chain of b5 allotype. The deduced amino acid sequence of the variable region (positions 1-95) is 86% homologous to that of a b4 light chain protein [BS-1) (1) but the b4 and b5 constant regions are only 74% homologous. Comparison of this DNA sequence to that of a cDNA clone encoding a b4 constant region shows that the kappa allotypes b4 and b5 have diverged significantly more in their coding region than in the 3' untranslated regions (86% vs 96% nucleotide sequence homologies). This implies either a function for the 3' untranslated region with evolutionary pressures to conserve or an accelerated divergence of the coding regions.

Diversity in the rabbit immunoglobulin kappa chain variable regions is amplified by nucleotide deletions and insertions at the V-J junction

To analyze the rabbit immunoglobulin kappa locus, we isolated, from a phenotypically homozygous b4/b4 rabbit genomic library, a C kappa gene corresponding to the b4 var allelic form of the b4 C kappa gene. We also determined the nucleotide sequence of the J kappa cluster 3 kb upstream of the b4 var C kappa gene. Southern blot experiments with rabbit J kappa probes indicate that the bas C kappa gene is an isotype probably associated with its own J kappa segment(s). The rabbit b4 J kappa gene region contains a cluster of five J kappa segments, homologous to the human J kappa cluster, and two remnant J kappa segments approximately 5 kb upstream of the b4 C kappa gene. Nucleotide sequence analyses of the coding J kappa segments and their recombination signal sequences show only one functional J kappa segment. Comparisons with the protein data indicate that the rabbit kappa gene family, unlike its human and mouse homologs, increases kappa light chain variability by deletions and insertions at the V-J junction.

Nucleotide sequence of a rabbit IgG heavy chain from the recombinant F-I haplotype

We report the sequence of a cDNA encoding a rabbit immunoglobulin gamma heavy chain of d12 and e14 allotypes with high homology to partial cDNA sequences from rabbits of d11 and e15 allotypes. The encoded rabbit protein shows homologies with human (68-70%) and mouse (60-63%) gamma chains. The nucleotide sequence homologies of the CH domains range from 76-84% with human and 64-76% with mouse sequences. Comparison of the portion of VH encoding amino acid positions 34-112 with a previously determined VH sequence of the same allotype shows high conservation of sequences in the second and third framework segments but more marked differences both in length and encoded amino acids of the second and third complementarity-determining regions (CDRs). We also found a high degree of homology with a human genomic V-region, VH26 (77%) and a remarkable similarity between rabbit and human second CDR sequences and human genomic D minigenes. These results provide additional evidence that D minigene sequences share information with the CDR2 portion of VH regions.

Effects of anti-Ig reagents on T cell functions I. Activation of rabbit T cells by anti-allotype antibodies

Rabbit lymphocytes were analyzed by flow microfluorometry, using anti-T cell and anti-Ig reagents. Rabbit T cells and cells expressing surface Ig (B cells) appeared to belong to distinct subpopulations which could be separated on the basis of their selective adherence to nylon wool columns or to anti-Ig-coated dishes. Using flow microfluorometry, no evidence was obtained for the expression of a allotypes (VH framework) on T cells. Separated lymphocyte populations were functionally characterized using an in vitro proliferation assay. B and T cells from rabbit spleen or peripheral blood responded in a differential fashion to B and T cell-specific mitogens and to anti-Ig antibodies. Although such T cells did not respond upon stimulation with anti-Ig antibodies alone, significant proliferation could be induced by simultaneous addition of anti-Ig and T cell growth factor. In addition, activated T cells, derived from lymph nodes of immunized rabbits, generated a proliferative response upon stimulation with anti-Ig reagents alone. The above-mentioned effects on T cells could be obtained using heterologous anti-Ig antibodies or isologous anti-allotype antibodies, directed either against a allotypes (VH framework) or against b allotypes (kappa light chain). Antibodies against the Fc portion of rabbit Ig or against irrelevant allotypic specificities were ineffective in triggering T cells. Fab fragments from anti-allotype antibodies were equally stimulatory for T cells as compared to intact IgG, indicating that cross-linking of Ig-like molecules is not a necessary requirement for anti-Ig-induced T cell activation.

Heavy chain genes of rabbit IgG: isolation of a cDNA encoding gamma heavy chain and identification of two genomic C gamma genes

A cDNA library was constructed by using rabbit spleen poly(A)+RNA as template, and from this library was isolated a cDNA clone, p2a2, that encodes 179 amino acids of the heavy chain of rabbit IgG. The nucleotide sequence of p2a2 showed that it encodes the COOH-terminal eight amino acids of the CH1 domain, the hinge region, the CH2 domain, and the NH2-terminal half of the CH3 domain of C gamma. Southern blot hybridization analysis of rabbit sperm DNA showed that two EcoRI fragments hybridized strongly with the C gamma cDNA. The p2a2 cDNA was used as a probe to isolate recombinant Charon 4A phage clones containing C gamma sequences from a genomic library of rabbit liver DNA. Two distinct DNA segments were identified by restriction mapping and hybridization analysis, suggesting that the haploid rabbit genome may contain two different C gamma genes.

Messenger RNA from allotype-defined rabbits directing the cell-free synthesis of immunoglobulin heavy and light chains

In vitro synthesis studies were performed utilizing polyA(+)-RNA and lymphocytes from the spleens of rabbits hyperimmunized with Micrococcus luteus or Streptococcus pneumoniae (type III). PolyA(+)-RNA isolated after 4 M guanidinium thiocyanate extraction and oligo(dT)-selection appeared to be undegraded on CH3HgOH-agarose gel electrophoresis and demonstrated biological activity when translated in a rabbit reticulocyte cell-free system. The electrophoretic patterns of the specifically immunoprecipitated cell-free products were compared with those of Nonidet-P40 extracts (lysates) and secretions (supernatants) from rabbit spleen lymphocyte cultures and serum proteins. Kappa light chains with specific b allotypes, as well as immunoglobulin heavy chains, were identified. The efficient translation of mRNA of defined allotypes was a necessary prerequisite for production of characterized cDNA clones and identification of genomic sequences for rabbit immunoglobulin heavy and light chains.