The CD4 protein plays a critical role in the development and function of the immune system. To gain more insight into the mechanism of expression of the human CD4 gene, we cloned 42.2 kbp of genomic sequences comprising the CD4 gene and its surrounding sequences. Studies with transgenic mice revealed that a 12.6-kbp fragment of the human CD4 gene (comprising 2.6 kbp of 5' sequences upstream of the transcription initiation site, the first two exons and introns, and part of exon 3) contains the sequences required to support the appropriate expression in murine mature CD4+ CD8-T cells and macrophages but not in immature double-positive CD4+ CD8+ T cells. Expression in CD4+ CD8+ T cells was found to require additional regulatory elements present in a T-cell enhancer fragment recently identified for the murine CD4 gene (S.Sawada and D. R. Littman, Mol. Cell. Biol. 11:5506-5515, 1991). These results suggest that expression of CD4 in mature and immature T-cell subsets may be controlled by distinct and independent regulatory elements. Alternatively, specific regulatory elements may control the expression of CD4 at different levels in mature and immature T-cell subsets. Our data also indicate that mouse macrophages contain the regulatory factors necessary to transcribe the human CD4 gene.The CD4 gene encodes a transmembrane glycoprotein which is a member of the immunoglobulin gene superfamily (for a review, see reference (19). It is coexpressed with the CD8 molecule on immature thymocytes, early in thymic development, giving rise to double-positive CD4+ CD8+ T cells. The regulation of CD4 and CD8 expression is tightly controlled and tied to the maturation of CD4+ CD8+ T cells. Indeed, in the peripheral immune system, the CD4 and CD8 molecules are expressed on mutually exclusive subsets of mature T cells. CD4 is largely expressed on helper T cells that utilize class II major histocompatibility complex (MHC) proteins as restriction elements for antigen recognition, while CD8 is expressed on cytotoxic and suppressor T cells and interacts with targets expressing class I MHC proteins (19).The CD4 molecule is essential for normal T-cell functions and plays an important role in T-cell development and activation. It is closely associated with the CD3 T-cell receptor (TcR) complex (29) required for antigen recognition by T cells and for the activation of T cells (2,29). CD4 is also a signal-transducing molecule which is physically associated with the tyrosine kinase p56'ck protein (37). Upon interaction with a specific antigen in the context of MHC class II molecules, CD4+ T lymphocytes produce several lymphokines which induce the proliferation and differentiation of other effector cells of the immune response (19). Therefore, the expression of CD4 on T cells and its interaction with class II MHC proteins is crucial for the development of the cellular immune response.

The Cas-Br-E murine leukemia virus induces a spongiform myeloencephalopathy in susceptible mice. We constructed transgenic mice harboring either the viral genome (in a replication-defective form) or only its env gene. Low levels of expression of either transgene resulted in mild neuropathology and/or signs of neurological disease in more than half of these mice. These results indicate that the disease can occur in the absence of virus replication and strongly suggest that the env gp7O/pl5E complex is sufficient to induce disease.The Cas-Br-E murine leukemia virus (MuLV) induces a lower motor neuron disease in susceptible mice (reviewed in refs. 1 and 2), resulting in vacuolization, neuronal loss and intense gliosis within the grey matter, and gliosis with some demyelination in the white matter. These are found predominantly in the brainstem and in the anterior horn of the spinal cord (3, 4). Three determinants of neuropathogenicity have been identified on this viral genome. The most important has been mapped within the env gp7O sequences and is responsible for the induction of the specific spongiform lesions (5-7). The other determinants, localized within the long terminal repeat (LTR) or the R-U5-5' leader sequences, influence the incidence, severity, and the central nervous system (CNS) location of the lesions (8,9) or the latency of the disease (7, 10), respectively. However, the mechanism by which the Cas-Br-E MuLV causes spongiform lesions remains unknown. Since the gp7O harbors an important determinant of pathogenicity, we have proposed that the disease is receptormediated (2, 5, 6). To better understand the pathogenesis of this disease, we constructed transgenic (Tg) mice carrying either the coding viral sequences of Cas-Br-E MuLV or only its env sequences, both under the transcriptional control of MuLV LTR. MATERIALS AND METHODSPreparation of DNA for MicroiDJection. The transgenes were constructed essentially as before (11), using the clone pNE-8 of Cas-Br-E MuLV (12) (Fig. 1).Construction of Tg Mice. Tg mice were generated essentially as described before using one cell (C57BL/6 x C3H)F2 embryos for microinjection (11,13). The (C57BL/6 x C3H)F1, CD-1, CFW/D, and C3H mice were purchased from Charles River Breeding Laboratories. Southern hybridization analysis of tail DNA was performed with a 32P-labeled Taq I-BamHI Cas-Br-E MuLV env-specific DNA probe (14). Positive mice were first bred to C3H and then to CFW/D outbred mice. Breeding to CFW/D was carried out because this strain was found to be susceptible to 15). For assessment of neuropathology, Tg mice were bred into C3H mice for one to three generations and subsequently into CFW/D mice for zero to three generations. For the evaluation of balance and muscle strength, Tg mice were crossed into C3H mice for three to six generations and subsequently into CFW/D mice for one to three generations. Control mice, housed in the same room, were age-matched CFW/D, (C57BL/6 x C3H)F1, C3H, and non-Tg littermates.Detection of Transgene Expression. In situ hyb...

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4-CD8-and CD4+CD8+ T cells and was shut off in mature CD4+CD8-and CD4-CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+CD8-and CD4-CD8+ T cells and support a model of differentiation of CD4+CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.DMBA-LV leukemia virus is a type B replication-competent thymotropic retrovirus which is highly related to mouse mammary tumor virus (MMTV) (5, 19). However, in contrast to MMTV, known to induce mammary carcinomas (36), DMBA-LV MMTV (MMTVD) induces a high incidence of T-cell leukemias (thymomas) after a short latent period (6). Sequence analysis of the MMTVD long terminal repeat (LTR) revealed that in addition to several point mutations, the U3 region has undergone structural alterations compared with the C3H MMTV LTR (7). These modifications introduce two additional copies of the distal glucocorticoid regulatory element and truncate the 3' end of the open reading frame known to encode a protein exhibiting the characteristics of a superantigen (1, 15). However, the biological significance of these sequence alterations is still unclear. In another class of retroviruses, murine leukemia virus, it has clearly been established that the LTR, and specifically the U3 LTR, harbors the sequences determining the tissue (11, 21, 43) and disease (12,13,20,48) specificity of these viruses.The majority of the DMBA-LV-induced thymomas were found to be of an immature T-cell phenotype, Thy-1.2+, CD4+, CD8+ (Lyt2+), and CD5+ (Lytl+) (37,38 is regulated so that the cells expressing class II-restricted T-cell receptor (TcR) are CD4+CD8-, while those expressing class I-restricted TcR are CD4-CD8+.To determine whether the tissue specificity of DMBA-LV virus was determined by its LTR, and to identify the T-cell subpopulation in which this LTR was most active, we constructed transgenic mouse lines by using the MMTVD LTR as a promoter to drive expression of two independent reporter genes, the murine c-myc proto-oncogene and the human CD4 cDNA. Transgenic mice expressing the MMTVD/c-myc transgene consistently developed clonal or oligoclonal CD4+CD8+ T-cell lymphomas (thymomas). In addition, in mice bearing the MMTVD/CD4 transgene, the human CD4 protein was preferentially expressed in CD4+CD8+ thymic T cells and their progenitors, and its expression was turned off in both CD4+CD8-and CD4-CD8+ mature T cells. These experiments support a model of differentiation of CD4+CD8+ thymic T cells through common signals. MATERMILS AND METHODSConstruction of the transgenes. ...

Transgenic mice containing the complete human immunodeficiency virus (HIV) coding sequences fused to the mouse mammary tumor virus long terminal repeat were generated. They were found to produce high levels of authentic gag and env HIV proteins in several tissues known to support mouse mammary tumor virus-driven transcription. HIV proteins were also detected in serum and in body fluids (milk and epididymal secretions) known to be natural sites of retrovirus, and specifically of HIV, production. These results indicate that primary mouse cells from different tissues have the capacity to produce HIV proteins. These mice represent a novel animal model for HIV infection.

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.