The genesis, tutelage and exodus of B cells in the ileal Peyer's patch of sheep.

The ileal Peyer's patch (PP) is a prominent lymphoid organ that extends 1-2 meters along the terminal small intestine of sheep. It is comprised of rapidly proliferating B cells that make major contributions to the animals total B cell system. The characteristics of this tissue in sheep have enabled a variety of novel approaches to studying both the B cell system and the contribution of PP to the mucosal immunity. The sheep ileal PP has characteristics that place it in a category similar to that of the thymus, bone marrow and the avian bursa of Fabricius. The ileal PP develops before birth and involutes while the sheep is still young. It produces B cells that populate the immune system but most of the large numbers of newly-formed B cells are rapidly destroyed by apoptosis. It has been concluded that this death is related to a selection event that examines each newly-formed cell. Antibody diversity in sheep is a post-rearrangement event, generated by the process of somatic hypermutation. A comparison of the lambda light chain gene from surviving and dying B cells indicates that when the PP is at its greatest size the dying cells have the characteristics of cells with high affinity receptors for a selecting ligand/antigen. It is proposed that if B cells proliferate in the continual presence of selecting ligand/antigen that the B cell receptor might develop a sufficient affinity to trigger apoptosis. This process might contribute to elimination of clones with high affinity for either self antigens or continually present environmental antigens.