Origin of the bursal secretory dendritic cell.

The origin of vimentin-positive secretory dendritic cells of the bursa of Fabricius was studied by chick-quail chimera, parabiosis and immunohistochemistry using species-specific monoclonal antibodies. Quail bursal primordia of different ages were transferred to coelomic cavity of 3-day-old chicken embryos and further incubated for 18 days. In transplanted quail bursas the secretory dendritic cells of chicken and quail origin were detected by double staining of vimentin plus 74.3 and vimentin plus QCPN monoclonal antibodies, respectively. In bursal primordia of 5- and 6-day-old quail embryos both dendritic cells and B cells were of host, i.e. chicken origin. Mixed dendritic cell population of quail and chick origin emerged in chimeric birds of 6.5 days of age. In quail embryos transplanted at 7 and 8 days of age both dendritic cells and B cells were mixed i.e. of chicken and quail origin. Bursal secretory dendritic cells and medullary epithelial cells create "dendro-epithelial tissue" to receive pre-B cells. Colonization of dendro-epithelial tissue by pre-B cells initiates at day 7, thus the colonization of bursal anlage by blood-borne cells is a two-step process; entering of dendritic cells at day 6.5 is followed by that of B cells at day 7 and afterwards. It is discussed that bursal secretory dendritic cells and their product are key elements of bursal function therefore the mammalian bursa equivalent organ might be represented by a cell, which is analogous with the bursal secretory dendritic cell.