Upregulation of endogenous p53 and induction of in vivo apoptosis in B-lineage lymphomas of E(mu)-myc transgenic mice by deregulated c-myc transgene.

E(mu)-myc transgenic mice carry a constitutively overexpressed c-myc oncogene and develop B-lineage lymphomas. Previous studies have shown that c-myc overexpression can lead to in vitro apoptosis. Here, we investigated the in vivo effects of altered c-myc expression on cell proliferation versus death in spontaneously arising E(mu)-myc tumors. E(mu)-myc tumors display extensive in vivo apoptosis confined to small clusters of cells with greatly increased expression of both the c-myc transgene and the endogenous p53 gene as compared with that in normal, pretumor, or surrounding tumor tissue. This restricted overexpression of both the c-myc transgene and the endogenous p53 gene in small clusters of apoptotic tumor cells indicates that overexpression of these genes and apoptosis are not obligatory or uniform during tumor development and suggests that further somatic mutations or microenvironmental influences may be responsible for these properties. Nevertheless, the clear ability of tumor cells to undergo apoptosis in vivo may be exploitable for therapeutic purposes.

Loss of surface immunoglobulin expression precedes B cell death by apoptosis in the bursa of Fabricius.

The vast majority of lymphocytes generated daily in the chicken bursa of Fabricius do not emigrate to the periphery but die in situ. Apoptotic cells in the bursa can be readily detected by the presence of fragmented DNA and by the large numbers of condensed cellular nuclei observed by electron microscopy. Consequently, most newly generated lymphocytes die by programmed cell death. We show that bursal cells divide rapidly and apoptotic cells are derived from rapidly dividing precursors. Analysis of the phenotype of bursal cells undergoing apoptosis demonstrated that cell death does not occur in the most mature bursal cell population and is therefore not random. High levels of surface Ig are expressed on bursal cells entering S phase of the cell cycle. In contrast, bursal cells in the early stages of apoptosis in vivo express very low to undetectable levels of surface Ig but were unequivocally confirmed as being of the B lineage by polymerase chain reaction (PCR) detection of rearranged Ig genes. Bursal cells induced to undergo apoptosis in vitro express high levels of surface Ig demonstrating that induction of apoptosis does not in itself induce a loss of surface Ig expression. Consequently, loss of surface Ig expression precedes bursal cell death by apoptosis in vivo, suggesting that maintenance of a threshold level of surface Ig may be a requirement for the continued progression of chicken B lymphocyte development in the bursa.

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 clonal pre-B- or B-cell tumors. The preneoplastic period is characterized by increased polyclonal proliferation of pro-B and pre-B cells in the bone marrow (BM) associated with a reduced number of B cells, suggesting 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 pretumorous E mu-myc transgenic mice by in vivo radiolabeling and electron microscope radioautography. Large mitotic B220(+)-labeled cells form prominent clusters in the extravascular compartment of the BM. Some B220+ small lymphocytes, as well as large lymphoid cells, enter BM sinusoids. However, in addition, large numbers of B220+ cells exhibit nuclear chromatin condensation, fragmentation, and other morphologic features characteristic of apoptotic cell death. Propidium iodide staining and flow cytometry of BM cells from pretumorous E mu-myc transgenic mice, as well as agarose gel electrophoresis of DNA, confirm extensive apoptosis. Many B220+ apoptotic cells are closely associated with the extensive processes of prominent macrophages that contain numerous B220+ apoptotic bodies and complex lysosomal systems. These results suggest that the constitutive expression of c-myc oncogene in BM B-lineage 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 protective mechanisms and permit surviving c-myc-dysregulated cells to leave the BM and to initiate tumorigenesis.

Rearrangement of immunoglobulin genes in chicken B cell development.

Immunoglobulin gene rearrangement in the chicken has evolved not to generate antibody diversity per se but to generate an immunoglobulin variable region which can be diversified by subsequent somatic gene conversion events. While the molecular mechanism of V(D)J recombination in chickens cannot be distinguished from that seen in other species, the way in which this recombination is regulated during chicken B lymphocyte development does differ from the more widely known models of gene rearrangement in humans and rodents. In this review we focus on these differences, relating V(D)J recombination to the progression of chicken B cell development in the bursa of Fabricius.