B cell development in the chicken.

A central feature of the vertebrate humoral immune system is that an organism must have a vast repertoire of antibodies to protect it against foreign pathogens. Chickens create a diverse immunological repertoire by intrachromosomal gene conversion of the single variable gene segments of the Ig heavy and light chain genes. This diversification process has been shown to require the bursa of Fabricius. Immature cells commit to the B cell lineage by rearranging their Ig genes prior to migration to the bursa. Recent work has suggested that the ability of a developing B cell to migrate to the bursa may depend on the expression of the carbohydrate structure sialyl Lewis x. Developing B cells in the spleen with the ability to migrate to the bursa have been shown to express sialyl Lewis x. Cells expressing sialyl Lewis x begin appearing in the bursa anlage between embryonic Days 10 and 12. These sialyl Lewis x-positive cells appear to form the nascent bursal follicles and are induced to proliferate. Coincident with the time that B cells initiate the gene conversion process, cells cease expressing sialyl Lewis x and begin expressing the related surface epitope Lewis x. As cells mature further, they undergo another phenotypic change and switch from expressing high levels of Lewis x to become Lewis x-low. At the same time that Lewis x-low cells accumulate in the bursa, cells with this phenotype begin to appear in the spleen. These phenotypic markers may be useful in identifying chicken B cells at different developmental stages.