B lymphocytes are generated throughout life from hematopoietic stem cells by a complex program of cell differentiation, in the liver before birth and in the bone marrow afterward. In bone marrow, there are cells at various stages of differentiation, from the earliest progenitors to mature B cells. My laboratory focuses on determining the signals that promote the proliferation and differentiation of B lymphocyte precursors, fostering their eventual development into mature functional subpopulations, including follicular, marginal zone and B1/CD5+ B cells.
We employ multiparameter flow cytometry to resolve cells from mouse bone marrow into fractions representing intermediate developmental stages, based on differential expression of cell surface proteins. These phenotypic subsets show important molecular and functional distinctions: cells in the earliest CD19-negative stages possess multilineage (myeloid, T, NK. B cell) potential which becomes progressively restricted to the B lineage in later stages; B-lineage restricted stages, expressing CD19, progress through a critical pre-BCR proliferative expansion, wherein newly rearranged heavy chain VDJ segments are selected for capacity to pair with light chain, based on interaction with surrogate light chain; finally, newly generated B cells are selected through several transitional stages, and, depending on BCR-signaling and other microenvironmental interactions, either die or enter one of several functionally distinct mature B cell populations.
In our current work we use this fractionation scheme as a framework for investigating several important issues in lymphocyte development: 1) relationship between B cell development and Ig rearrangement; 2) differences between fetal and adult B cell development, particularly in the generation of autoreactive malignant-prone CD5+ B cells; and 3) mechanism of B lineage specification during the earliest stages of B cell development. This work has implications for human disease. The disregulation of normal development at an early stage, the pre-B cell, can generate pre-B leukemias; the most common childhood cancer is a pre-B leukemia and recent studies suggest that alteration of a component in a particular pre-B cell signaling pathway is responsible for a significant number of these. Thus further elucidation of the normal regulation of B cell development is likely to lead to insights into the origins of B cell leukemias and lymphomas, potentially allowing the design of rational therapies.