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Unusually long germline DH genes contribute to large sized CDR3H in bovine antibodies

Farbod Shojaei1, Surinder S Saini, Azad K Kaushik

  • 1Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

Molecular Immunology
|August 12, 2003
PubMed

Insights

Cattle possess diverse germline diversity genes (D(H)) that contribute to the exceptionally long heavy chain third complementarity-determining region (CDR3H) in IgM antibodies. This genetic variation explains antibody diversity beyond recombination mechanisms.

Area of Science:

  • Immunogenetics
  • Molecular Biology
  • Comparative Genomics

Background:

  • Cattle IgM antibodies exhibit unusually long heavy chain third complementarity-determining regions (CDR3H), sometimes exceeding 61 amino acids and containing multiple cysteine residues.
  • The genetic basis for this CDR3H length variation in cattle remained largely unexplored.

Purpose of the Study:

  • To investigate the germline diversity gene (D(H)) origins contributing to the extensive CDR3H length heterogeneity observed in bovine IgM antibodies.
  • To characterize the bovine germline D(H) gene segments and their potential role in encoding the unique CDR3H sequences.

Main Methods:

  • Isolation and sequencing of a 2.3kb genomic DNA fragment containing putative bovine D(H) gene sequences.
  • Identification and analysis of germline D(H) gene segments based on nucleotide sequence.
  • Phylogenetic analysis to determine evolutionary relationships of bovine D(H) genes.

Main Results:

  • Three bovine germline D(H) gene segments were identified, with lengths of 42bp, 58bp, and 148bp.
  • Two D(H) genes contained repetitive GGT and TAT codons, which are characteristic of fetal VDJ rearrangements.
  • These D(H) genes are preferentially expressed in a specific reading frame, encoding glycine and tyrosine residues in the CDR3H region.
  • Phylogenetic analysis indicated that bovine D(H) genes are most closely related to those of rabbits and chickens.

Conclusions:

  • Cattle possess both short and long germline D(H) genes.
  • These germline D(H) genes directly contribute to the observed size heterogeneity in the CDR3H region, including exceptionally long variants.
  • The identified germline D(H) diversity provides a genetic explanation for extensive CDR3H length variation in bovine IgM, independent of recombination mechanisms.

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