USAGE OF BETA-1 INTEGRIN LIGANDS BY B-CELLS IS DEVELOPMENTALLY-REGULATED IN AVIAN BURSA

Chicken B cell development takes place in a separate organ, the bursa of Fabricius, which provides the blood-borne stem cells, a microenviro nment specialized for B cell maturation. Therefore, chicken can be use d as a model to study specifically the molecules and interactions whic h control the development of the B cell compartment. In this work, we studied expression, localization and function of beta 1 integrins on m aturing B cells and bursal stroma. The expression of beta 1 integrins on B cells increases during the embryonic development and beta 1 integ rin-positive cells can be found both in the medulla and the cortex thr oughout the bursal development. The binding assays show that the attac hment of B cells to stroma is mediated by pi integrins. Binding to 10- day-old embryonic stroma is fibronectin-independent, whereas fibronect in-mediated binding takes place in an increasing manner during further embryonic maturation. After hatching fibronectin appears to be the ma in binding site for B cells. However, the epitope of beta 1 integrin w hich takes part in the adhesion of B cells to stroma and to fibronecti n is not crucial for the homing of cells into the bursa in a cell tran sfer model. Our results indicate that the interaction of beta 1 integr ins with their ligands is developmentally regulated in the bursa and s uggest that B cell maturation may be partially controlled by this inte raction.