CD99 in early B lymphopoiesis

Human B-lymphopoiesis essentially depends on close interactions of B cell precursors (BCP) with the bone marrow (BM) microenvironment. In vitro, normal BCP and blasts of B-lineage acute lymphoblastic leukemia (ALL) rapidly die by apoptosis, but their survival can be maintained by mesenchymal BM-stroma cells due to currently only partially defined mechanisms which involve cell adhesion. CD99 is a human 32 kD transmembrane- sialoglycoprotein expressed on many hematopoietic and non-hematopoietic cell types. The antigen is a cell-cell- adhesion receptor molecule involved in migration of leukocytes through endothelial junctions. In hematopoietic progenitors under dendritic-cell breeding conditions, CD99-ligation interferes with further differentiation, while in mature B cells CD99 appears critically involved in regulating cell cycle and maintaining differentiation. Particularly high levels of CD99 expression are found in immature T cells and Ewing tumors, and in these cells CD99 has been shown to elicit homotypic cell-aggregation phenomena as well as to deliver apoptotic signals. CD99 has therefore been considered to play a role in thymus development by regulating T lymphoid differentiation and selection. Previously we found that CD99 is very strongly expressed by the most immature BCP stages in BM, and that this maturation-linked expression pattern is conserved despite malignant transformation in BCP-ALL. We therefore speculated that CD99 may have a pivotal role also in early B-lymphopoiesis. However, functional data in this context are completely lacking to date. We plan to investigate, whether CD99 plays a role in the adhesion process of immature B cells to BM stroma which is so critical to cell-fate. Stroma-contact cultures of cell lines, of normal and leukemic primary BCP cells, CD99-ligation via epitope-defined monoclonal antibodies, as well as RNA interference (by taking advantage of an ongoing collateral study in our institute) will form the basis of our investigational set-up. In long-term B cell cultures we will also focus on whether CD99 is involved in BCP differentiation, as well as in growth by regulating proliferation and survival. By the end of this project we should be able to understand whether immature B cell lymphopoiesis critically depends on CD99, and whether normal and leukemic BCP differ in their functional requirements of CD99. Unraveling a pivotal role of CD99 in BCP could aid in developing appropriate targeted therapy for ALL.

Human B-lymphopoiesis essentially depends on close interactions of B cell precursors (BCP) with the bone marrow (BM) microenvironment. In vitro, normal BCP and blasts of B-lineage acute lymphoblastic leukemia (ALL) rapidly die by apoptosis, but their survival can be maintained by mesenchymal BM-stroma cells due to currently only partially defined mechanisms which involve cell adhesion. CD99 is a human 32 kD transmembrane- sialoglycoprotein expressed on many hematopoietic and non-hematopoietic cell types. The antigen is a cell-cell- adhesion receptor molecule involved in migration of leukocytes through endothelial junctions. In hematopoietic progenitors under dendritic-cell breeding conditions, CD99-ligation interferes with further differentiation, while in mature B cells CD99 appears critically involved in regulating cell cycle and maintaining differentiation. Particularly high levels of CD99 expression are found in immature T cells and Ewing tumors, and in these cells CD99 has been shown to elicit homotypic cell-aggregation phenomena as well as to deliver apoptotic signals. CD99 has therefore been considered to play a role in thymus development by regulating T lymphoid differentiation and selection. Previously we found that CD99 is very strongly expressed by the most immature BCP stages in BM, and that this maturation-linked expression pattern is conserved despite malignant transformation in BCP-ALL. We therefore speculated that CD99 may have a pivotal role also in early B-lymphopoiesis. However, functional data in this context are completely lacking to date. We plan to investigate, whether CD99 plays a role in the adhesion process of immature B cells to BM stroma which is so critical to cell-fate. Stroma-contact cultures of cell lines, of normal and leukemic primary BCP cells, CD99-ligation via epitope-defined monoclonal antibodies, as well as RNA interference (by taking advantage of an ongoing collateral study in our institute) will form the basis of our investigational set-up. In long-term B cell cultures we will also focus on whether CD99 is involved in BCP differentiation, as well as in growth by regulating proliferation and survival. By the end of this project we should be able to understand whether immature B cell lymphopoiesis critically depends on CD99, and whether normal and leukemic BCP differ in their functional requirements of CD99. Unraveling a pivotal role of CD99 in BCP could aid in developing appropriate targeted therapy for ALL.