Role of environmental antigen in the development of IgG+ cells in the bursa of fabricius.

IgG+ cells were detected in the bursa of Fabricius after hatching by immunofluorescence staining of single cells and immunohistologic studies using mAb anti-Ig gamma-heavy chain. The frequency of IgG+ cells in the bursa increased rapidly immediately after hatching. In histologic studies, most of the IgG+ cells were found in the medullary areas of the bursal follicle. Isolation of the bursa from the gut by bursal duct ligation before hatching suppressed the development of IgG+ cells in the bursa after hatching. In addition, administration of Ags into the bursal lumen just before bursal duct ligation caused a significant increase of IgG+ cells in the bursa compared with the isolated bursa. However, parenteral administration of Ags had no effect on the frequency of IgG+ cells in the isolated and normal bursa. These results suggest that Ags in the bursa are a prerequisite for the development of IgG+ cells in the bursa. Although the majority of IgG+ bursal cells were IgM+ IgG+ double-positive cells, sorted Bu1+ bursal cells, which contained 99.9% of IgG+ cells but not plasma cells, synthesized de novo IgM but little or no IgG. Therefore, it is likely that surface IgG on the bursal cells is not produced locally, but is trapped by IgM+ bursal cells. We speculate that Ags derived from the bursal lumen form immune complexes within the bursa, which are subsequently trapped on the surface of IgM+ bursal cells mediated either by Fc-receptors, C3 receptors, or surface Ig.