Page 532 - Veterinary Immunology, 10th Edition

to the generation of a stem and knob structure, where the knob
VetBooks.ir shows significant structural heterogeneity as a result of somatic

mutation. This knob can fold into multiple mini-domains and can
bind diverse antigens (see Fig. 16.17).

The bovine IGL locus contains 25 V genes, of which 17 are
functional, organized in three subclusters 5′ to four J-C genes. The
predominantly expressed IGLV1 genes are found in two 5′
subclusters, whereas the rarely expressed IGLV2 and IGLV3 genes

are proximal to the J-C genes. Cattle have more than one IGLJ gene,
but only one is expressed. Many of the pseudogenes are fused to
IGLJ in the germline. Cattle also have four IGLC genes. Two of these
(IGLC2 and IGLC3) are functional, whereas the other two (IGLC1

and IGLC4) are pseudogenes. IGLC3 is preferentially expressed.
Over 90% of bovine antibodies use lambda light chains.

Sheep

Sheep also use both recombination and conversion. Immature B
cells first diversify their V (D) and J genes in lymphoid tissues such

as the spleen or bone marrow. The immature cells then migrate to
follicles in the ileal Peyer's patches where somatic mutation occurs
(Fig. 17.12). Sheep light chain genes have more than 90 IGLV genes

and a single IGLJ gene, so these are diversified by recombination.
On the other hand, sheep have only 7 functional IGHV genes and
therefore use conversion to diversify their heavy chains. They have
6 IGHJ genes, 2 of which are pseudogenes. One of the active genes,
IGHJ1, is used in 90% of heavy chains, suggesting that

recombination is minimal. More than 98% of all rearrangement
events are in frame, and there are few N- or P-nucleotides. Unlike
rabbit, human, or mouse, stimulation by the intestinal microbiota is

not absolutely necessary for V gene diversification in sheep.

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