Generation of precursor, immature, and mature murine B1-cell lines from c-myc/bcl-xL-overexpressing pre-BI cells.

Deregulated expression of c-myc and bcl-xL is long known to generate transformed B cells in humans and mice. We overexpressed these genes to induce in vitro and in vivo differentiation of fetal liver-derived mouse pre-BI cells to B1-lineage pre-BII-like, immature and mature B-cell lines, and to Ig-secreting cells. In vitro, doxycycline-controlled c-myc/bcl-xL-overexpressing CD19+ CD93+ c-kikt+ IgM- pre-BI cells differentiate to and survive as CD19+ CD93+ c-kit- IgM+ immature B1 cells. Timed CpG stimulation of these oncogene-overexpressing pre-B or immature B1 cells generates either CD19+ CD93low c-kit- IgM- SLC- pre-BII-like or IgM+ MHCII+ CD73+ CD80+ CD40+ mature B1-cell lines and IgM-secreting B1 cells in vitro and fixes their state of differentiation. All cell lines are clonable, but a majority of immature and mature B1-cell clones eventually reach a nonproliferating, surviving G0 -state. Transplanted in vivo, c-myc/bcl-xL-overexpressing pre-B cells expand to mature B1 cells, and to IgM- and IgA-secreting plasmablasts and plasma cells. Within 2 months, plasmablasts have expanded most prominently in BM and spleen, indicating that the host selectively expanded development of these transformed plasma cells. The sIgM+ B1-cell lines and clones offer the possibility to study their roles in the development of B1-Ab repertoires, of B1-cell-mediated autoimmune diseases and of B1-cell malignancies.

cDNA-library testing identifies transforming genes cooperating with c-myc in mouse pre-B cells.

While c-myc often contributes to the generation of B cell transformation, its transgenic overexpression alone does not lead to full transformation of B-lineage cells. Synergistically acting second genes must cooperate. Here, we constructed doxycycline-inducible cDNA-libraries from pre-B cell mRNA. These libraries were retrovirally transduced as single copies into single cells and overexpressed in fetal-liver-derived c-myc-overexpressing pre-B cell lines. We scored transformation by survival and/or expansion of differentiating B-lineage cells in vitro and in vivo. Only one double c-myc/cDNA-library-expressing cell line was found in less than 5 × 106 library-transduced pre-B cells surviving and expressing a cDNA-library-derived transcript in vitro. This transcript was identified as a shortened form of the Exosc1 gene, encoding the RNA exosome complex component CSL4. Transplantations of double c-myc/Exosc1 short-form- or c-myc/Exosc1 full-length-transgenic cells into Rag1-/- mice resulted in survival, differentiation to CD19+ CD93- sIgM+ CD5low/- CD11b+ mature B1 cells and, surprisingly, also vigorous expansion in vivo. Strikingly, after transplantations of c-myc/cDNA-library pre-BI cells the frequencies of double-transgenic pre-B cells and their differentiated progeny, expanding in vivo to heterogeneous phenotypes, was at least tenfold higher than in vitro. In a first analysis Ptprcap, Cacybp, Ndufs7, Rpl18a, and Rpl35a were identified. This suggests a strong influence of the host on B-cell transformation.

miR-221 redirects precursor B cells to the BM and regulates their residence.

Pluripotent hematopoietic stem cells and multipotent myeloid/lymphoid progenitors express miR-221 and miR-222. When Pax5 expression commits these progenitors to monopotent pre-B lymphocytes the two microRNAs (miRNAs) are downregulated. Upon transplantation, stem cells and progenitors can reside in the BM, while pre-B cells, after their commitment, no longer do so. Retrovirally transduced, doxycycline-induced overexpression of either miR-221 or miR-222 in pre-B-I cells does not revert their monopotency to multipotency. However, upon transplantation miR-221, but not miR-222, transduced pre-B-I cells regain the capacity to reside in the BM. Upon subsequent termination of miR-221-expression by removal of doxycycline, the transplanted cells leave the BM again. Microarray analyses identified 25 downregulated miR-221-target genes, which could function to localize phases of B-lymphocyte development in BM before and after commitment.

Biphenotypic B-lymphoid/myeloid cells expressing low levels of Pax5: potential targets of BAL development.

The expression of Pax5 commits common lymphoid progenitor cells to B-lymphoid lineage differentiation. Little is known of possible variations in the levels of Pax5 expression and their influences on hematopoietic development. We have developed a retroviral transduction system that allows for the study of possible intermediate stages of this commitment by controlling the levels of Pax5 expressed in Pax5-deficient progenitors in vitro and in vivo. Retroviral transduction of Pax5-deficient pro-/pre-B cell lines with a doxycycline-inducible (TetON) form of the human Pax5 (huPax5) gene yielded cell clones that could be induced to different levels of huPax5 expression. Clones inducible to high levels developed B220(+)/CD19(+)/IgM(+) B cells, while clones with low levels differentiated to B220(+)/CD19(-)/CD11b(+)/Gr-1(-) B-lymphoid/myeloid biphenotypic cells in vitro and in vivo. Microarray analyses of genes expressed at these lower levels of huPax5 identified C/ebpα, C/ebpδ, Pu.1, Csf1r, Csf2r, and Gata-3 as myeloid-related genes selectively expressed in the pro-/pre-B cells that can develop under myeloid/lymphoid conditions to biphenotypic cells. Therefore, reduced expression of huPax5 during the induction of early lymphoid progenitors to B-lineage-committed cells can fix this cellular development at a stage that has previously been seen during embryonic development and in acute lymphoblastic lymphoma-like biphenotypic acute leukemias.

Pim1 and Myc reversibly transform murine precursor B lymphocytes but not mature B lymphocytes.

The proto-oncogenes Myc and Pim1, which are deregulated in many types of cancers, are known to cooperate in B lymphoma development. Here we show that overexpression of retrovirally transduced, doxycycline-inducible Myc alone in IL-7-deprived, growth-arrested pre-B cells enhanced cell cycle entry without impairing apoptosis. Overexpression of Pim1 decreased apoptosis, but had no effect on cell cycle entry. Co-expression of Pim1 and Myc inhibited apoptosis and led to IL-7-independent proliferation of the transduced pre-B cells in vitro, while blocking their differentiation to IgM(+) immature cells. Transplantation of Pim1/Myc overexpressing pre-BI cells into B-cell-deficient mice expanded the pre-B-cell compartments up to 100-fold within 4-8 weeks. Transformation remained dependent on the expression of both oncogenes, as removal of doxycycline in vitro and in vivo terminated proliferation and induced differentiation to IgM(+) B cells. In contrast, Pim1/Myc-transduced mature B cells that developed from the oncogene-transduced pre-BI cells in the absence of oncogene overexpression in vivo were not capable of long-term proliferation after induction of Pim and Myc overexpression, neither in vivo nor in vitro, neither with nor without stimulation by polyclonal activators.

Reprint of: Environments of B cell development.

B lymphocyte development in the mouse begins with the generation of long-term reconstituting, pluripotent hematopoietic stem cells, over multipotent myeloid/lymphoid progenitors and common lymphoid progenitors to B-lineage committed pro/pre B and pre B cells, which first express pre B cell receptors and then immunoglobulins, B cell receptors, to generate the repertoires of peripheral B cells. This development is influenced and guided by cells of non-hematopoietic and hematopoietic origins. We review here some of the recent developments, and our contributions in this fascinating field of developmental immunology.

Environments of B cell development.

B lymphocyte development in the mouse begins with the generation of long-term reconstituting, pluripotent hematopoietic stem cells, over multipotent myeloid/lymphoid progenitors and common lymphoid progenitors to B-lineage committed pro/pre B and pre B cells, which first express pre B cell receptors and then immunoglobulins, B cell receptors, to generate the repertoires of peripheral B cells. This development is influenced and guided by cells of non-hematopoietic and hematopoietic origins. We review here some of the recent developments, and our contributions in this fascinating field of developmental immunology.