Page 535 - Veterinary Immunology, 10th Edition

domestic herbivores, develop their B cell antibody repertoire in two
VetBooks.ir stages. The first diversification involves rearrangements of a small

number of V, D, and J genes. These early B cells then migrate to the
intestinal lymphoid tissue, where they greatly increase their

numbers of B cells as well as the diversity of their B cell repertoire.
This second phase of B cell diversification takes place in intestinal
lymphoid organs in direct contact with the intestinal microbiota.
The importance of the microbiota is supported by the failure of

germ-free pigs to develop significant B cell diversity. Intestinal
bacteria play an especially critical role in this process. For example,
in rabbits, normal intestinal lymphoid tissue development can take
place in the presence of both Bacteroides fragilis and Bacillus subtilis

but not with either alone. Other bacterial combinations are also
effective, suggesting that some form of bacterial interaction is
needed for optimal effect.
Analysis of the expansion of intestinal B cells by commensal

bacteria also shows that it tends to affect B cells with certain V H
domains. This expansion is not simply a specific response to
microbial antigens but rather a polyclonal, non–antigen-specific

response. It may be directed through the toll-like receptors or be a
result of microbial superantigen binding to the BCR, or some
combination thereof.

535