|Ph.D Student||Diamant Eran|
|Subject||Developmental Regulation of Positive Selection and|
Maturation in Bone Marrow B Cells
|Department||Department of Medicine||Supervisor||Professor Doron Melamed|
Autonomous "tonic" signaling capacity of the B-cell antigen receptor (BCR) confers an important positive selection checkpoint for immature B cells. Although inappropriate signals impose developmental arrest of immature B cells, the fate of these cells has not been investigated.
The lack of CD19 results in inappropriate signaling, which impairs positive selection and stimulates receptor editing in immunoglobulin (Ig) transgenic (Tg) mice. VAV proteins are thought to function in the CD19 signal transduction cascade. Immature B cells deficient of VAV1 and VAV2 fail to complete their maturation.
Here, we studied CD19-regulated positive selection of nontransgenic, as well as VAV2-regulated selection of 3-83Tg B-lymphopoiesis, using our bone marrow culture system. We found that the lack of CD19 resulted in elevated tonic signaling and impaired maturation, as revealed by surface marker expression and by functional assays. Immature CD19-/- B cells underwent intensive receptor editing attempts in culture. In vivo analyses validated our in vitro observations. However, 3-83Tg VAV2-/- B cells imposed allelic exclusion in cultures, and appropriately migrated upon chemokine induction. In addition, they were subjected to developmental arrest and receptor editing in the presence of self antigen in vitro and in vivo, not different relative to the control 3-83Tg.
Our results obtained in nontransgenic mice lacking CD19 suggest that inappropriate signaling not only blocks positive selection, but also stimulates intensive secondary light-chain recombination and receptor editing. Thus, receptor editing, which is a main mechanism in negative selection, is an important salvage mechanism for immature B cells that fail positive selection.
CD19 role can be inefficiently compensated. In contrast, intact positive and negative selection in B cells deficient of VAV2 takes place presumably due to the compensational effect of VAV1 and VAV3 isoforms.