Characterization of the gene for the membrane and secretory form of the IgM heavy-chain constant region gene (C mu) of the cow (Bos taurus)

The Unusual Genetics and Biochemistry of Bovine Immunoglobulins

Antibodies are the key circulating molecules that have evolved to fight infection by the adaptive immune system of vertebrates. Typical antibodies of most species contain six complementarity-determining regions (CDRs), where the third CDR of the heavy chain (CDR H3) has the greatest diversity and often makes the most significant contact with antigen. Generally, the process of V(D)J recombination produces a vast repertoire of antibodies; multiple V, D, and J gene segments recombine with additional junctional diversity at the V-D and D-J joints, and additional combinatorial possibilities occur through heavy- and light-chain pairing. Despite these processes, the overall structure of the resulting antibody is largely conserved, and binding to antigen occurs predominantly through the CDR loops of the immunoglobulin V domains. Bovines have deviated from this general paradigm by having few VH regions and thus little germline combinatorial diversity, but their antibodies contain long CDR H3 regions, with substantial diversity generated through somatic hypermutation. A subset of the repertoire comprises antibodies with ultralong CDR H3s, which can reach over 70 amino acids in length. Structurally, these unusual antibodies form a β-ribbon "stalk" and disulfide-bonded "knob" that protrude far from the antibody surface. These long CDR H3s allow cows to mount a particularly robust immune response when immunized with viral antigens, particularly to broadly neutralizing epitopes on a stabilized HIV gp140 trimer, which has been a challenge for other species. The unusual genetics and structural biology of cows provide for a unique paradigm for creation of immune diversity and could enable generation of antibodies against especially challenging targets and epitopes.

Internal Duplications of DH, JH, and C Region Genes Create an Unusual IgH Gene Locus in Cattle

It has been suspected for many years that cattle possess two functional IgH gene loci, located on Bos taurus autosome (BTA) 21 and BTA11, respectively. In this study, based on fluorescence in situ hybridization and additional experiments, we showed that all functional bovine IgH genes were located on BTA21, and only a truncated μCH2 exon was present on BTA11. By sequencing of seven bacterial artificial chromosome clones screened from a Hostein cow bacterial artificial chromosome library, we generated a 678-kb continuous genomic sequence covering the bovine IGHV, IGHD, IGHJ, and IGHC genes, which are organized as IGHVn-IGHDn-IGHJn-IGHM1-(IGHDP-IGHV3-IGHDn)3-IGHJn-IGHM2-IGHD-IGHG3-IGHG1-IGHG2-IGHE-IGHA. Although both of two functional IGHM genes, IGHM1 and IGHM2, can be expressed via independent VDJ recombinations, the IGHM2 can also be expressed through class switch recombination. Likely because more IGHD segments can be involved in the expression of IGHM2, the IGHM2 gene was shown to be dominantly expressed in most tissues throughout different developmental stages. Based on the length and identity of the coding sequence, the 23 IGHD segments identified in the locus could be divided into nine subgroups (termed IGHD1 to IGHD9). Except two members of IGHD9 (14 nt in size), all other functional IGHD segments are longer than 30 nt, with the IGHD8 gene (149 bp) to be the longest. These remarkably long germline IGHD segments play a pivotal role in generating the exceptionally great H chain CDR 3 length variability in cattle.

Genetic variations in immunoglobulin G3 and association with staphylococcal intra-mammary infections in cattle and buffaloes

Animals (n = 152) suffering with mastitis were used to study association between immunoglobulin G3 (IgG3) genotypes and staphylococcal mastitis. Thus, animals (affected and unaffected) were evaluated using PCR-RFLP. Restriction digestion of amplicons of IgG3 using BstYI showed allele A and, genotypes AC, AB and AA predominated in Karan Fries, Sahiwal and Murrah, respectively. HphI digestion revealed allele A and, genotypes AC and AB in higher frequency in animals of first group of all the breeds. Additionally, genotypes associated with mastitic infection showed predominance of AB (BstYI) in unaffected animals of Sahiwal and Murrah; whereas AC and AA were observed in affected group only. Genotype AB (HphI) was prevalent in unaffected and AC in affected animals of Karan Fries and Sahiwal. In Murrah, AC was common in affected and unaffected animal; while AB remained in affected category. Identified genotypes associated with determinants of SpA gene of S. aureus strains revealed the significant outcome. For example AB (BstYI) was found to be correalted with SpA ≤ 7R; whereas with SpA > 7R in Karan Fries. Genotypes AA and AB were more favorably associated with SpA ≤ 7R and AB with the SpA > 7R in Sahiwal cattle. The genotype AB seemed influenced (100%) with SpA > 7R and AC in SpA ≤ 7R in cases of Murrah. Similarly, AA (HphI) in Karan Fries was more likely to be correlated with SpA ≤ 7R, while AC with SpA > 7R. Overall, the molecular analysis revealed that IgG3 gene could be use for selection of animals against mastitis. However, further investigations on IgG3 needed to aid in identify disease- resistant animal.

The sheep and cattle Peyer's patch as a site of B-cell development

In sheep and cattle, the ileal Peyer's patch (PP), which extends one-two meters along the terminal small intestine, is a primary lymphoid organ of B-cell development. B-cell diversity in the ileal PP is thought to develop by combinatorial mechanisms, gene conversion and/or point mutation. These species also have jejunal PP that function more like secondary lymphoid tissues concerned with mucosal immune reactions. These two types of PP differ significantly in their histology, ontogeny and the extent of lymphocyte traffic. The prenatal development of follicles in the PP begins first in the jejunum during the middle of gestation and then in the ileum during late gestation. B-cells proliferate rapidly in the ileal PP follicle; up to five percent of these cells survive while the majority dies by apoptosis, perhaps driven by the influence of environmental antigen and/or self-antigen. The surviving cells migrate from the ileal PP and populate the peripheral B-cell compartment. By adolescence, the ileal PP has involuted but the function of jejunal PP, compatible with a role as secondary lymphoid organ, continues throughout life. In this review, we focus on the development of PP as a site of B-cell repertoire generation, positive and negative B-cell selection, and the differences between ileal PP and jejunal PP.

The bovine antibody repertoire

Cattle are able to produce a full range of Ig classes including the long-elusive IgD through rearrangement of their germline genes. Several IgL groupings have been reported but as in several other livestock species (e.g. sheep, rabbits, chickens), rearrangement per se fails to generate significant IgH diversity. This is largely because of the modest number of bovine VH segments that participate in rearrangement and their conserved sequences. Perhaps in compensation, bovine Ig heavy chains carry CDR3 sequences of exceptional length. Processes that operate post-rearrangement to generate diversity remain ill defined as are the location, timing and triggers to these events. Reagents are needed to understand better the maturation of B lymphocytes, their responses to antigens and cytokines, and to provide standards for the quantitation of Ig responses in cattle; recombinant methods may help meet this need as Ab engineering technologies become more widely used.

Physical mapping of the bovine immunoglobulin heavy chain constant region gene locus

Bovine antibodies have recently attracted increasing attention, as they have been shown to exhibit prophylactic and therapeutic properties in selected infectious diseases in humans. In the present study, we have isolated bacterial artificial chromosomes and cosmid clones containing the bovine JH, mu, delta, gamma 1, gamma 2, gamma 3, epsilon, and alpha genes, which allowed us to make a contig of the genes within the bovine IGHC locus. The genes are arranged in a 5'-JH-7 kb-mu-5 kb-delta-33 kb-gamma 3-20 kb-gamma 1-34 kb-gamma 2-20 kb-epsilon- 13 kb-alpha-3' order, spanning approximately 150 kb DNA. Examination of the bovine germline JH locus revealed six JH segments, two of which, JH1 and JH2, were shown to be functional although there was a strong preference for expression of the former. Sequence alignment of the bovine 5' E mu enhancer core region with those of other mammals, demonstrated an absence of the mu E3 motif and a shortened spacer between the mu A and mu B sites within the bovine E mu enhancer core region. Furthermore, the essential sequence element for class switching, switch mu, spanning approximately 3-kb repetitive sequence and abundant in the switch region motifs CTGGG (187 repeats) and CTGAG (127 repeats), was identified immediately upstream of the mu gene. A further sequence comparison revealed that the bovine IGHC genes display an extensive polymorphism leading to expression of multiple antibody allotypes.

Cloning of the complete rat immunoglobulin delta gene: evolutionary implications

The recent discovery of a Cdelta encoding gene in artiodactyls has raised questions regarding the evolution of the gene. In the present study, we have analysed the complete rat Cdelta gene both at the cDNA and genomic levels, showing that the rat Cdelta gene is structurally similar to the corresponding mouse gene. Analysis of the rat immunoglobulin D heavy chain cDNA tail sequences, revealed two transcripts for the secreted form with varying sizes of their 3' untranslated region (UTR), resulting from usage of two different poly(A) addition signals. Furthermore, a membrane-bound form encoding transcript, possessing a long 3' UTR, was also observed. Phylogenetic analysis supports that the Cdelta gene appeared early in the evolution of vertebrates, and it was probably duplicated from the C micro gene more than 400 million years ago.

Artiodactyl IgD: the missing link

IgD has been suggested to be a recently developed Ig class, only present in rodents and primates. However, in this paper the cow, sheep, and pig Ig delta genes have been identified and shown to be transcriptionally active. The deduced amino acid sequences from their cDNAs show that artiodactyl IgD H chains are structurally similar to human IgD, where the cow, sheep, and pig IgD H chain constant regions all contain three domains and a hinge region, sharing homologies of 43.6, 44, and 46.8% with their human counterpart, respectively. According to a phylogenetic analysis, the Cdelta gene appears to have been duplicated from the Cmu gene >300 million yr ago. The ruminant mu CH1 exon and its upstream region was again duplicated before the speciation of the cow and sheep, approximately 20 million yr ago, inserted upstream of the delta gene hinge regions, and later modified by gene conversion. A short Sdelta (switch delta) sequence resulting from the second duplication, is located immediately upstream of the bovine Cdelta gene and directs regular mu-delta class switch recombination in the cow. The presence of Cdelta genes in artiodactyls, possibly in most mammals, suggests that IgD may have some as yet unknown biological properties, distinct from those of IgM, conferring a survival advantage.

Origin of bovine IgM structural variants

The structure of IgM determined from two cDNAs isolated from a Holstein (BLV7G1) and an Angus x Hereford cross-bred (B5D8) cow reveals high sequence similarity both at nucleotide (98.7%) and amino acid (97.9%) level and is closest to sheep (89.4%). Three bovine IgM allotypes, designated as IgMa, IgMb and IgMc, are classified based on nucleotide substitutions in all the Cmu exons resulting in amino acid replacements. Further, insertion of three in-frame codons at Cmu1 and Cmu2 junction of B5D8 IgM from the intervening intron, via cleavage of pre-mRNA at an alternate cryptic 5' splice donor site, leads to generation of additional bovine IgM variants. The C1q-binding site, involved in classical complement pathway, is identified in bovine IgM where ten amino acids are conserved across species. Interestingly, bovine IgM has the lowest number of proline residues (5) in the Cmu2 domain in comparison to other species (7-9) and this is likely to impose structural constraints on mobility of Fab arms of the bovine IgM during antigen recognition. The rigidity in the bovine IgM Cmu2 domain may, however, facilitate exposure of C1q-binding site subsequent to antigen binding and enhance its complement fixing ability. The restricted mobility of bovine IgM Fab arms may possibly favor generation of an antigen-combining site requiring an unusually long third complementarity determining region of the heavy chain (CDR3H), apart from antigen selection of variable domains. This is consistent with the fact that an exceptionally long CDR3H has not been observed in bovine IgG which bears a long and more flexible hinge region. Additional hydrophilic threonine and serine residues in the Cmu2 domain of bovine IgM, as compared to other species, however, enhance its ability to extend into the solvent. Finally, restricted fragment length polymorphism analysis of genomic DNA from four cattle breeds reveals the presence of; at least, four allelic variants of bovine Cmu gene.

Structure and diversification of the bovine immunoglobulin repertoire

Our understanding of the basis to immunoglobulin formation in cattle has benefited substantially from the application of molecular biology over the past decade. It is now established that both the lambda light chain and heavy chain repertoires are founded upon the frequent expression of single gene families and subgroups of segments which are of conserved sequence. It is likely that a functional kappa locus exists in the bovine genome but this isotype comprises as few as 5% of bovine light chains. Similarly, alternative but non-expressed V(H) gene families are present posing intriguing but unresolved questions about the regulation of immunoglobulin synthesis. The heavy chain frequently bears a third complementarity-determining region which is atypically long but the processes which expand this region of the reading frame and its contribution to the interaction with antigen remain matters of speculation. Opportunities exist to map the major immunoglobulin loci and to define the membership and sequence diversity of the gene families which dominate each repertoire. However, it is already evident that cattle cannot generate significant diversity from rearrangement and junctional imprecision alone. Elucidation of the mechanism(s), dynamics and tissue distribution of immunoglobulin diversification in cattle, thus, remain key challenges in this branch of veterinary immunology.