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 stud ies using mAb anti-Ig gamma-heavy chain. The frequency of IgG(+) cells in the bursa increased rapidly immediately after hatching. In histolo gic studies, most of the IgG(+) cells were found in the medullary area s of the bursal follicle. Isolation of the bursa from the gut by bursa l duct ligation before hatching suppressed the development of IgG(+) c ells in the bursa after hatching. In addition, administration of Ags i nto the bursal lumen just before bursal duct ligation caused a signifi cant 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 resu lts suggest that Ags in the bursa are a prerequisite for the developme nt of IgG(+) cells in the bursa. Although the majority of IgG(+) bursa l cells were IgM(+) IgG(+) double-positive cells, sorted Bu1(+) bursal cells, which contained 99.9% of IgG(+) cells but not plasma cells, sy nthesized de novo IgM but little or no IgG. Therefore, it is likely th at surface IgG on the bursal cells is not produced locally, but is tra pped by IgM(+) bursal cells. We speculate that Ags derived from the bu rsal lumen form immune complexes within the bursa, which are subsequen tly trapped on the surface of IgM(+) bursal cells mediated either by F c-receptors, C3 receptors, or surface Ig.