Unique lymphoid cell subset target to infection and proliferation induced in vitro by a murine leukemia virus.

The mechanisms by which non-oncogene bearing, slowly transforming T-cell-tropic retroviruses induce leukemia is not well understood. Viruses such as the murine radiation leukemia virus (RadLV) induce oncogenic transformation of T-cells in the thymus only in vivo and after a long latency. The capacity of RadLV to induce proliferation of lymphoid cells in vitro has been analysed here as a first attempt at mapping oncogenic transformation. Autonomously replicating cell lines have been isolated following exposure of splenic lymphocytes to two different isolates of RadLV, following in vitro culture in the presence of T-cell growth factors. Cells of similar precursor lymphoid morphology and phenotype have been isolated and cloned from cultures established from different animals. These cell lines all grow independently of exogenous growth factors in vitro, but are not tumorigenic in mice. Exposure to RadLV under the culture conditions provided has allowed integration of a new retroviral genome into each cell line, but no active replication of virus has been detected in any of the cell lines analysed. A common cell type resembling a lymphoid precursor has been induced to proliferate. These cell lines express cell surface markers attesting to their bone marrow origin, such as CD44 (Pgp-1), Gr-1, B220 and NK1.1, but they do not show the characteristics of T cells which have undergone differentiation within the thymus. They do not express the Thy-1 marker, nor show rearrangement involving any of the T-cell receptor (TCR) alpha, beta gamma or sigma genes. These cells bind several antibodies specific for the CD3-epsilon and TCR-alpha beta structures, and there appears to be aberrant expression of TCR proteins in cells bearing fully rearranged TCR genes. Precursor lymphoid cells and not mature T-cells in spleen, appear to be appropriate targets for RadLV-induced proliferation/immortalisation in vitro. Oncogenic transformation induced by RadLV in vivo may occur within precursor lymphoid cells and must be a complex process dependent on both the differentiation events which occur within the thymus, as well as the thymic environment of stromal cells.

Cyclo-oxygenase type 2-dependent prostaglandin E2 secretion is involved in retrovirus-induced T-cell dysfunction in mice.

MAIDS (murine AIDS) is caused by infection with the murine leukaemia retrovirus RadLV-Rs and is characterized by a severe immunodeficiency and T-cell anergy combined with a lymphoproliferative disease affecting both B- and T-cells. Hyperactivation of the cAMP-protein kinase A pathway is involved in the T-cell dysfunction of MAIDS and HIV by inhibiting T-cell activation through the T-cell receptor. In the present study, we show that MAIDS involves a strong and selective up-regulation of cyclo-oxygenase type 2 in the CD11b+ subpopulation of T- and B-cells of the lymph nodes, leading to increased levels of PGE2 (prostaglandin E2). PGE2 activates the cAMP pathway through G-protein-coupled receptors. Treatment with cyclo-oxygenase type 2 inhibitors reduces the level of PGE2 and thereby reverses the T-cell anergy, restores the T-cell immune function and ameliorates the lymphoproliferative disease.

Growth regulation of a T-cell lymphoma via the T-cell receptor.

Autocrine stimulation is described for a Radiation leukaemia virus (RadLV)-induced T-cell lymphoma, C6VL/1. The proliferation of this tumour cell line can be regulated by several agents, including interleukin-2 (IL-2), antibodies to the IL-2 receptor and the T-cell antigen-specific receptor (TCR), as well as RadLV retrovirus particles produced by the cell itself. This information has been gained using various procedures to slow or arrest C6VL/1 proliferation, including the addition of gamma interferon (gamma-IFN) and cell culture at low density. All data suggest that these cells can receive growth stimulation via the T-cell receptor (TCR) and IL-2 receptor, implicating autocrine stimulation of growth involving IL-2 and retroviral gene products.

Radiation leukemia virus common integration at the Kis2 locus: simultaneous overexpression of a novel noncoding RNA and of the proximal Phf6 gene.

Retroviral tagging has been used extensively and successfully to identify genes implicated in cancer pathways. In order to find oncogenes implicated in T-cell leukemia, we used the highly leukemogenic radiation leukemia retrovirus VL3 (RadLV/VL3). We applied the inverted PCR technique to isolate and analyze sequences flanking proviral integrations in RadLV/VL3-induced T lymphomas. We found retroviral integrations in c-myc and Pim1 as already reported but we also identified for the first time Notch1 as a RadLV common integration site. More interestingly, we found a new RadLV common integration site that is situated on mouse chromosome X (XA4 region, bp 45091000). This site has also been reported as an SL3-3 and Moloney murine leukemia virus integration site, which strengthens its implication in murine leukemia virus-induced T lymphomas. This locus, named Kis2 (Kaplan Integration Site 2), was found rearranged in 11% of the tumors analyzed. In this article, we report not only the alteration of the Kis2 gene located nearby in response to RadLV integration but also the induction of the expression of Phf6, situated about 250 kbp from the integration site. The Kis2 gene encodes five different alternatively spliced noncoding RNAs and the Phf6 gene codes for a 365-amino-acid protein which contains two plant homology domain fingers, recently implicated in the Börjeson-Forssman-Lehmann syndrome in humans. With the recent release of the mouse genome sequence, high-throughput retroviral tagging emerges as a powerful tool in the quest for oncogenes. It also allows the analysis of large DNA regions surrounding the integration locus.