APOPTOSIS AND MACROPHAGE-MEDIATED DELETION OF PRECURSOR B-CELLS IN THE BONE-MARROW OF E-MU-MYC TRANSGENIC MICE

Transgenic mice expressing the c-myc proto-oncogene under the control of the Ig heavy chain enhancer (E mu-myc) all eventually develop clona l pre-B- or B-cell tumors. The preneoplastic period is characterized b y increased polyclonal proliferation of pro-B and pre-B cells in the b one marrow (BM) associated with a reduced number of B cells, suggestin g a high degree of B-cell loss. To examine the mechanisms of this cell loss, we have identified B220(+) B-lineage cells within the BM of pre tumorous E mu-myc transgenic mice by in vivo radiolabeling and electro n microscope radioautography. Large mitotic B220(+)-labeled cells form prominent clusters in the extravascular compartment of the BM. Some B 220(+) Small lymphocytes, as well as large lymphoid cells, enter BM si nusoids. However, in addition, large numbers of B220(+) cells exhibit nuclear chromatin condensation, fragmentation, and other morphologic f eatures characteristic of apoptotic cell death. Propidium iodide stain ing and flow cytometry of BM cells from pretumorous E mu-myc transgeni c mice, as well as agarose gel electrophoresis of DNA, confirm extensi ve apoptosis. Many B220(+) apoptotic cells are closely associated with the extensive processes of prominent macrophages that contain numerou s B220(+) apoptotic bodies and complex lysosomal systems. These result s suggest that the constitutive expression of c-myc oncogene in BM B-l ineage cells, which increases the proliferation of precursor B cells, also leads to increased apoptotic cell death and rapid elimination by resident macrophages. Further mutations may be needed to block these p rotective mechanisms and permit surviving c-myc-dysregulated cells to leave the BM and to initiate tumorigenesis. (C) 1994 by The American S ociety of Hematology.