The thyroid-specific enhancer-binding protein (T/ebp) gene was disrupted by homologous recombination in embryonic stem cells to generate mice lacking T/EBP expression. Heterozygous animals developed normally, whereas mice homozygous for the disrupted gene were born dead and lacked the lung parenchyma. Instead, they had a rudimentary bronchial tree associated with an abnormal epithelium in their pleural cavities. Furthermore, the homozygous mice had no thyroid gland but had a normal parathyroid. In addition, extensive defects were found in the brain of the homozygous mice, especially in the ventral region of the forebrain. The entire pituitary, including the anterior, intermediate, and posterior pituitary, was also missing. In situ hybridization showed that the T/ebp gene is expressed in the normal thyroid, lung bronchial epithelium, and specific areas of the forebrain during early embryogenesis. These results establish that the expression of T/EBP, a transcription factor known to control thyroid-specific gene transcription, is also essential for organogenesis of the thyroid, lung, ventral forebrain, and pituitary.[Key Words: T/EBP; gene targeting; thyroid; lung; ventral forebrain; pituitary] Received August 22, 1995; revised version accepted November 10, 1995.Thyroid-specific enhancer-binding protein (T/EBP) binds to an enhancer element located -5.5 kb upstream of the human thyroid peroxidase gene transcription start site and regulates thyroid-specific gene expression (Kikkawa et al. 1990;Mizuno et al. 1991). T/EBP, also named thyroid-specific transcription factor 1 (TTF-1)or Nkx-2.1, was originally described to govern thyroid-specific expression of the rat thyroglobulin gene (Civitareale et al. 1989). Several studies have established the role of T/EBP in expression of genes encoding thyroid peroxidase (Kikkawa et al. 1990;Mizuno et al. 1991;Abramowicz et al. 1992;Francis-Lang et al. 1992), thyroglobulin (Civitareale et al. 1989), and the thyrotropin (TSH) receptor tCivitareale et al. 1993;Shimura et al. 1994). All three proteins are essential for thyroid hormone biosynthesis (DeGroot and Niepomniszcze 1977). T/EBP is also expressed in the lung (Guazzi et al. 1990;Mizuno et al. 1991), and it has been recently demonstrated that the expression of genes encoding the lung surfactant proteins A and B is regulated by this DNA-binding protein (Bohinski et al. 1994;Bruno et al. 1995). T/ebp ) is the first member of the mouse Nkx-2 gene family that is closely related to Drosophila NK-2 in their homeo domain sequences (68%-95% similarity) (Kim and Nirenberg 1989;Guazzi et al. 1990;Price et al. 1992;Lints et al. 1993). Members of the Nkx-2 family also share a highly conserved 17-aminoacid motif that is located on the carboxyl-terminal side of the homeo domain. From the six members of this family characterized to date, the expression patterns of three genes, T/ebp (Nkx-2.1), Nkx-2.2, and Nkx-2.5, have been studied. Lazzaro et al. (1991) have established T/ebp(Nkx-2.1) gene expression at -10.5 days postcoitum (El0.5) in the ...

Primary infection with the human immunodeficiency virus (HIV) is generally followed by a burst of viraemia with or without clinical symptoms. This in turn is followed by a prolonged period of clinical latency. During this period there is little, if any, detectable viraemia, the numbers of infected cells in the blood are very low, and it is extremely difficult to demonstrate virus expression in these cells. We have analysed viral burden and levels of virus replication simultaneously in the blood and lymphoid organs of the same individuals at various stages of HIV disease. Here we report that in early-stage disease there is a dichotomy between the levels of viral burden and virus replication in peripheral blood versus lymphoid organs. HIV disease is active in the lymphoid tissue throughout the period of clinical latency, even at times when minimal viral activity is demonstrated in blood.

No abstract

The source of increasing viremia that characterizes the latter stages of human immunodeficiency virus (HIV) disease has remained a paradox because it occurs at a time when lymphoid tissue is quantitatively and qualitatively impaired, and the patients' CD4 T lymphocytes are steadily declining. Here, macrophages, both infected and uninfected with common opportunistic pathogens of HIV disease such as Mycobacterium avium complex and Pneumocystis carinii, were identified as highly productive sources of HIV in coinfected lymph nodes. These observations indicate that tissue macrophages are not only infected with HIV, but that common pathogens of HIV disease can dramatically increase their production of virus. Thus, prevention or successful treatment of opportunistic coinfections, or both, potentially benefits the patient twofold by limiting the pathology caused by opportunistic infection and by controlling induction of HIV replication.