Page 531 - Veterinary Immunology, 10th Edition

VetBooks.ir Species Differences

Antigen receptor diversity is generated in different ways
depending on species. Some mammals rely on gene recombination

followed by somatic mutation. In these species, immunoglobulin
diversity is continuously generated from B cell precursors
throughout an animal's life. Other mammals, in contrast, use gene
conversion for a short period early in life. After initial B cell
diversity is generated, this pool of B cells expands by a self-

renewing mechanism with little somatic mutation.

Horses

The equine IGH locus contains 40 D genes, 8 J genes and 52 V genes
as well as 11 C genes. The IGK locus contains a single C gene, 5 J

genes and 60 V genes. The IGL locus contains 7 C genes, each
preceded by a single J gene and 144 V genes located downstream of
the J-C cluster. Horses therefore predominantly employ gene
recombination. Lambda light chains constitute 92% of the antibody

repertoire in horses.

Cattle

Cattle likely employ recombination for their light chains and a
combination of recombination and conversion for their heavy

chains. Initial diversification occurs in lymphoid organs followed
by somatic mutation in ileal Peyer's patches. They have at least 36
IGHV genes divided into three subgroups. Subgroup IGHV1
contains at least 11 and perhaps as many as 20 functional genes.
Subgroups IGHV2 and IGHV3 consist entirely of pseudogenes (see

Table 17.1). Heavy chain diversity in older cattle is generated
primarily by somatic mutation. IGHJ contains six genes of which
two are functional. IGHD contains 14 long and short genes. As a

result, their heavy chain CDR3 regions range from 31 to 154
nucleotides. In addition, conserved short nucleotide sequences of 13
to 18 nucleotides may be inserted at V-D junctions encoding IgM, so
that the CDR3 loop is exceptionally long and may contain as many
as 61 amino acids. As pointed out in the previous chapter, this leads

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