Development of Accessory Cells in B-Cell Compartments Is Retarted in B-Cell-Depleted Fetal Sheep

Accessory-cell populations in the lymphoid tissues of fetal sheep were investigated following depletion of B cells. An intraperitoneal injection of an anti-IgM antibody early in gestation resulted in a marked depletion of IgM(+) cells in lymphoid tissues. Immune and enzyme histochemical techniques were used to identify accessory-cell populations in the ileal Peyer's patch, spleen, and lymph nodes of B-cell-depleted fetal sheep. The rudimentary follicles in the ileal Peyer's patch showed strong enzyme reactivity for 5′ nucleotidase, indicating the presence of follicular dendritic cells (FDCs). Enzyme reactivities for FDCs in primary follicles of the spleen and lymph nodes were absent, as were reactivities for metallophilic macrophages in the marginal zone of the spleen. MgATPase reactivity associated with dendritic-cell populations in the gut-associated lymphoid tissues was detected. A monoclonal antibody against complement receptor-2 (CD21) reacted with FDCs in the rudimentary follicles of the ileal Peyer's patch and immature FDCs in lymph nodes. The results suggest that the development of accessory-cell populations in B-cell compartments of peripheral but not central lymphoid tissues is dependent on the presence of B cells.