Characterization of pBOR-Il-3 transgenic mice : effects of the pBOR-Il-3 transgene on normal hematopoiesis and growth of azacytidine-induced thymic lymphomas

The pBOR-Il-3 transgenic mice were developed by the pronuclear microinjection method to study how activation of transcriptional expression driven by a retroviral long terminal repeat occurs and especially how insults by both chemical and physical agents modulate transcription driven from the LTR, and to study the biological functions of interleukin-3. Our studies showed that pBOR-Il-3 mice that were not treated with any chemical or physical agents were completely normal, and did not develop any particular spontaneous tumors, relative to the FVB/N control group. Different hematological/cellular immunological techniques revealed no major differences between the hematopoietic systems of pBOR-Il-3 and the FVB/N mice. Only minor differences were observed, such as a higher number of monocytes in 2-months-old pBOR-Il-3 mice, and a higher number of neutrophils in 5-month-old pBOR-Il-3 mice, relative to FVB/N control mice. The fact that the hematopoietic subpopulations in the pBOR-Il-3 mice were not different from the ones from FVB/N controls, suggested that 11-3 was not being expressed in transgenic mice at levels high enough to disturb hematopoiesis. Two methods were used to perturb the hematopoietic system of the mice: the first was the use of a physical agent, x rays. Both pBOR-Il-3 mice, and FVB/N mice were irradiated with 2 Gy of x ray, and the recovery of the different hematopoietic subpopulations was asessed on days 1 and 8 after irradiation. There were no differences in the radiation recovery of the different hematopoietic subpopulations between 11-3 and FVB/N mice. The other method used to perturb the hematopoietic system of the mice was the injection of azacytidine, a demethylating agent traditionally used to activate the LTR-driven expression of retroviral genomes. Upon azacytidine treatment, both the pBOR-Il-3 mice and the FVB/N controls developed thymic lymphomas, the transgenic mice developed the tumors at a higher frequency and relatively faster than the FVB/N controls. This result suggested that 11-3 expression is not a necessary step to the development of thymic lymphomas, but that 11-3 accelerates the growth of the thymic lymphoma cells. The thymic lymphoma cells were of a T-cell origin, as determined by the T-cell receptor gene rearrangement analysis, and, in most cases, were of a monoclonal origin. According to the flow cytometric analysis of thymic lymphoma cells, labelled with florescet-labelled T-cell-specific antibodies, the thymic lymphoma cells did not lose the ability to differentiate, but the differentiation process was aberrant. Studies on the CD4/CD8 T-cell surface markers revealed that in pBOR-Il-3 thymic lymphomas the predominant differentiation pathway is one in which the CD4⁺CD8⁺ subpopulation differentiates toward the CD8⁺CD4^- subpopulation; conversely, in the FVB/N group, the predominant differentiation pathway is the CD4⁺CD8⁺ subpopulation differentiating toward the CD4⁺CD8^- subpopulation.