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 microenvironment specialized for B cell maturation. Therefore, chicken can be used as a model to study specifically the molecules and interactions which control the development of the B cell compartment. In this work, we studied expression, localization and function of beta 1 integrins on maturing B cells and bursal stroma. The expression of beta 1 integrins on B cells increases during the embryonic development and beta 1 integrin-positive cells can be found both in the medulla and the cortex throughout the bursal development. The binding assays show that the attachment of B cells to stroma is mediated by beta 1 integrins. Binding to 10-day-old embryonic stroma is fibronectin-independent, whereas fibronectin-mediated binding takes place in an increasing manner during further embryonic maturation. After hatching fibronectin appears to be the main binding site for B cells. However, the epitope of beta 1 integrin which takes part in the adhesion of B cells to stroma and to fibronectin is not crucial for the homing of cells into the bursa in a cell transfer model. Our results indicate that the interaction of beta 1 integrins with their ligands is developmentally regulated in the bursa and suggest that B cell maturation may be partially controlled by this interaction.