B cell leukemia 11a (Bcl11a) is required for normal B and T cell development and may function as a tumor suppressor

B cell lymphoma/leukemia 11a (Bcl11a) gene encodes a zinc finger protein that is expressed in normal B and T lymphocytes, monocytes, or megakaryocytes, and physically interacts with the human B cell proto-oncogene, BCL6. Bcl11a is activated by fusion of the Ig locus in human B cell lymphomas carrying the t(2;14)(p13:g32.3) translocation. To determine the function of Bcl11a in normal hematopoiesis and proliferation of leukemic cells, we used a Bcl11a knockout mouse model. This showed that Bcl11a is essential for postnatal development and normal lymphopoiesis. While Bcl11a+/- mice are viable, Bcl11a-/- die a few hours after birth from unknown causes. To evaluate the role of Bcl11a in hematopoiesis, fetal liver (FL) cells were analyzed by FACS for the expression of lineage specific cell surface markers. B cell maturation was blocked prior to the development of pro-B cells (B220+/CD43+), while myeloid and erythroid lineages were normal. The Bcl11a -/- FL also showed defects in T cell maturation (reduced SP-CD4 + and increased SP-CD8+ fetal thymocytes). Lethally irradiated mice transplanted with Bcl11a-/- FL cells showed normal myeloid and erythroid development, while B and T cell development was blocked. Thus, defects in lymphoid development were intrinsic to the hematopoietic stem cells (HSC). We established that this defect in lymphopoiesis was not due to homing to the thymus, as shown by failure of T cell repopulation when Bcl11a-/- FL cells were injected into the thymus. Greater than 80% of the mice transplanted with Bcl11a-/- FL cells died between 15 and 27 weeks, while 100% mice transplanted with Bcl11a+/- survived. Histopathology of moribund mice showed enlarged thymus with lymphoblasts infiltrating the bone marrow, liver, lung, heart, and kidney. Leukemic cells were tumorogenic when injected into NOD-SCID mice, and were immortalized by co-culturing on OP9 stromal cells. Immortalized leukemic cell lines showed high expression of T cell markers CD3&egr;+, SP-CD4+, SP-CD8+, immature DP-CD4+CD8+, early ProB cell markers B220+/CD43+, and primitive stem cell marker c-Kit+. In addition, overexpression of Bcl11a in normal HSC increased B cell differentiation at all stages of B cell development in the stem cell-OP9 co-culture system and in vivo. Also, it enhanced development of DP-CD4+CD8 + T cells and perturbed the SPCD4+ and SP-CD8 + ratio in the thymus indicating a role in T cell maturation in vivo. Finally, overexpression of Bcl11a increased myeloid cell proliferation in vivo and promoted macrophage differentiation of bipotential myeloid progenitors in vitro. Thus, these data show that Bcl11a is required for B and T cell differentiation and may also function as a T cell tumor suppressor gene. Furthermore, these results are the first indication of a possible regulatory role for Bcl11a during normal myeloid lineage development and differentiation and corroborate the role of Bcl11a during myeloid leukemia development. This project has been funded in part from NCI, NIH Contract # NO1-CO-56000, with SAIC-Fredrick, Inc.