Interaction of abl and raf with IL-7 signaling pathway and transformation of pre-B cells from resistant mice.

After in vivo inoculation with abl/myc- and raf/myc-containing retroviruses, BALB/c mice predominantly develop late stage B cell tumors (plasmacytomas) and less frequently develop earlier B-lineage tumors while DBA/2 mice do not develop B-lineage tumors. We have investigated the in vitro tumorigenic potential of these viruses using cultured normal pre-B cell lymphocytes from both BALB/c and DBA/2 mice. Interestingly, both viruses infect cultured pre-B lymphocytes from both mouse strains. Following infection, IL-7 dependent pre-B cells become independent of normal in vitro growth requirements within 24 h and can rapidly form in vivo pre-B lymphomas in both mouse strains. Mechanisms mediating loss of IL-7 dependence are different depending on whether the raf or abl gene is present in myc-containing viruses. IL-7 JAK-STAT signaling is constitutively active in abl/myc induced pre-B cell tumors. In contrast, IL-7 JAK-STAT signaling is not constitutive in raf/myc induced pre-B cell tumors, demonstrating that subversion of this component of IL-7 signal transduction is not obligatory for pre-B cell transformation or loss of IL-7 dependence.

Modulated expression of the epidermal growth factor-like homeotic protein dlk influences stromal-cell-pre-B-cell interactions, stromal cell adipogenesis, and pre-B-cell interleukin-7 requirements.

A close relationship exists between adipocyte differentiation of stromal cells and their capacity to support hematopoiesis. The molecular basis for this is unknown. We have studied whether dlk, an epidermal growth factor-like molecule that intervenes in adipogenesis and fetal liver hematopoiesis, affects both stromal cell adipogenesis and B-cell lymphopoiesis in an established pre-B-cell culture system. Pre-B-cell cultures require both soluble interleukin-7 (IL-7) and interactions with stromal cells to promote cell growth and prevent B-cell maturation or apoptosis. We found that BALB/c 3T3 fibroblasts express dlk and function as stromal cells. Transfection of these cells with antisense dlk decreased dlk expression and increased insulin-induced adipocytic differentiation. When antisense transfectants were used as stroma, IL-7 was no longer required to support the growth of pre-B cells and prevent maturation or apoptosis. Antisense dlk transfectants of S10 stromal cells also promoted pre-B-cell growth in the absence of IL-7. These results show that modulation of dlk on stromal cells can influence their adipogenesis and the IL-7 requirements of the pre-B cells growing in contact with them. These results indicate that dlk influences differentiation signals directed both to the stromal cells and to the lymphocyte precursors, suggesting that dlk may play an important role in the bone marrow hematopoietic environment.

A unique series of lymphomas related to the Ly-1+ lineage of B lymphocyte differentiation.

Six new B lineage lymphomas of NFS mice established in primary tissue culture were examined for a number of phenotypic, functional, virologic, and molecular genetic characteristics. Two of the tumors and their cloned derivatives bore surface markers characteristic of B cells, whereas four tumors resembled pre-B cells. One of the B cell and two of the pre-B cell lymphomas had rearrangements of both heavy and light chain immunoglobulin genes, confirming their designation as B-lineage lymphomas. All the tumors but one were Ly-1+, indicating that Ly-1 may be expressed by some pre-B cells as well as some B cells. In addition, one pre-B cell lymphoma was Mac-1+. MCF murine leukemia viruses obtained from two of the tumors did not accelerate development of B-lineage lymphomas in NFS mice.