Cross‐linking of the surface immunoglobulin on lymphocytes from the bursa of Fabricius results in second messenger generation

The bursa of Fabricius represents the major site of B lymphocyte development in avianspecies. Although the vast majority of bursal lymphocytes express cell surface immunoglobulin (sIg), it is generally considered that the bursa does not represent a significant site of antigen‐induced B cell maturation to Ig secretion. However, the question as to whether antigen, either exogenous or self, can induce positive or negative selection of bursal lymphocytes in such a way as to modify the peripheral B cell repertoire remains open. Clearly, such intrabursal selection would require that bursal lymphocyte sIg have the molecular machinery to transduce signals into the cell as a consequence of its interaction with antigen. In this report we demonstrate that exposure of bursal lymphocytes to antibodies directed against sIg induced a rapid increase in cytosolic free calcium ion concentrations [Ca2+]i. Furthermore, such antibodies also induced a rapid increase in intracellular phosphatidic acid concentrations followed by a rise in intracellular phosphatidylinositol. Increased [Ca2+]i, phosphatidic acid and phosphatidylinositol levels required the cross‐linking of sIg and were not induced by antibodies to other bursal cell surface antigens. Thus, cross‐linkingof the sIg on bursal lymphocytes results in second messenger generation, demonstrating that bursal sIg is a functional signal transduction element.