Reassortment of cell populations within the lymphoid apparatus of the sheep

Histophysiology of follicular structures and germinal centres in relation to B cell differentiation

A historical review of germinal centres since their original description in 1885 by Flemming is given, followed by a description of the germinal centre reaction. Microenvironmental aspects are discussed, special attention being paid to the role of antigen in the induction and further regulation of germinal centre activity. Germinal centres give rise to a new population of lymphocytes (germinal centre-derived cells). Their immune capacities were studied using the rabbit appendix as an experimental model. The possible properties of the cell from which a germinal centre develops (germinal centre precursor cell) were investigated using germinal centre cell suspensions from the rabbit appendix and bone marrow and thoracic duct lymphocytes in the rat in adoptive transfer systems. From these experiments a working hypothesis was formulated for the possible role of germinal centres in B cell differentiation. Essentially, germinal centres are thought not only to be sites of selective amplification of antigen-reactive but still immature B cells, resulting in the generation of memory cells, but also to contribute to the overall pool of B cells by polyclonal amplification of B cells of specificities unrelated to the antigen inducing the germinal centre.

Differential migration of T and B cells during an acute inflammatory response

Inflammation in the liver and mammary glands of sheep caused by challenge infection with Taenia hydatigena or infusion of killed Staphylococcus aureus, respectively were characterized by the recruitment of both T and B cells. The patterns of migration of these two major lymphocyte subpopulations were distinctly different. While T cells seemed to migrate out of existing, flat endothelium-lined blood vessels resulting in a diffuse distribution at the sites of inflammation, B cells were characteristically present as clusters of tightly packed cells at restricted sites in the inflamed tissue. Within these B cell clusters distinct capillary vessels lined with plumb endothelial cells were always present suggesting that they were the sites of intense migration of B cells originating from the draining lymph nodes. These results indicate differential regulation of adhesion molecules on B and T cells and/or their ligands on endothelium during acute inflammatory reactions.