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Human cytomegalovirus infection of cells of hematopoietic origin: HCMV-induced immunosuppression, immune evasion, and latency

2006; Elsevier BV; Volume: 34; Issue: 5 Linguagem: Inglês

10.1016/j.exphem.2005.11.012

1873-2399

Mariana G. Bego, Stephen St. Jeor,

Viral-associated cancers and disorders

Herpesviruses are among the most successful pathogens. Three major subfamilies of herpesviruses have been identified (α, β, and γ) based on growth characteristics, genomic structure, and tissue preference. After primary infection, herpesviruses become latent in either ganglia (α) or blood mononuclear cells (β and γ). Human cytomegalovirus (HCMV) is the prototype β-herpesvirus. It has a linear 230-kbp double-stranded DNA genome [1Chee M.S. Bankier A.T. Beck S. et al.Analysis of the protein-coding content of the sequence of human cytomegalovirus strain AD169.Curr Top Microbiol Immunol. 1990; 154: 125-169Crossref PubMed Google Scholar] capable of encoding over 200 predicted open reading frames (ORFs) [2Dunn W. Chou C. Li H. et al.Functional profiling of a human cytomegalovirus genome.Proc Natl Acad Sci U S A. 2003; 100: 14223-14228Crossref PubMed Scopus (275) Google Scholar, 3Murphy E. Rigoutsos I. Shibuya T. Shenk T.E. Reevaluation of human cytomegalovirus coding potential.Proc Natl Acad Sci U S A. 2003; 100: 13585-13590Crossref PubMed Scopus (83) Google Scholar, 4Yu D. Silva M.C. Shenk T. Functional map of human cytomegalovirus AD169 defined by global mutational analysis.Proc Natl Acad Sci U S A. 2003; 100: 12396-12401Crossref PubMed Scopus (191) Google Scholar]. The HCMV virion is composed of an icosahedral capsid that contains the DNA genome, an outer layer of proteins referred to as the tegument, and a cellular lipid layer containing viral glycoproteins. According to a recent study, the virion is composed of 71 viral ORF and over 70 cellular proteins [5Varnum S.M. Streblow D.N. Monroe M.E. et al.Identification of proteins in human cytomegalovirus (HCMV) particles: the HCMV proteome.J Virol. 2004; 78: 10960-10966Crossref PubMed Scopus (255) Google Scholar]. After initial infection, HCMV becomes latent in its host and can be activated to produce acute diseases. Although there may be more than one site of latency, it has been established that bone marrow–derived myeloid progenitor cells are a key site. HCMV infections are very common worldwide, with a 60 to 70% seroprevalence rate in Western societies and up to 100% in some parts of Africa and Asia. In healthy individuals, it can cause a generally asymptomatic disease, followed by a latent infection. It is clear that the immune status of the host is important in the regulation of latent HCMV since viral reactivation is associated with immune suppression. For this reason, immunologically immature and immunocompromised patients are the main target of HCMV disease. HCMV is the leading viral agent causing both birth defects and congenital disease. Despite several treatment options, its incidence has increased in the past decades as a result of organ allografting, immunosuppressive treatment, and human immunodeficiency virus (HIV)-infected patients [6Griffiths P.D. Walter S. Cytomegalovirus.Curr Opin Infect Dis. 2005; 18: 241-245Crossref PubMed Google Scholar, 7Rowshani A.T. Bemelman F.J. van Leeuwen E.M. van Lier R.A. ten Berge I.J. Clinical and immunologic aspects of cytomegalovirus infection in solid organ transplant recipients.Transplantation. 2005; 79: 381-386Crossref PubMed Scopus (102) Google Scholar, 8Cheung T.W. Teich S.A. Cytomegalovirus infection in patients with HIV infection.Mt Sinai J Med. 1999; 66: 113-124PubMed Google Scholar]. During natural infection, it is thought that HCMV can replicate productively in a number of cell types including fibroblasts; epithelial, endothelial, smooth muscle, neuronal, and mesenchymal cells; hepatocytes; granulocytes; and monocyte-derived macrophages [9Schrier R.D. Nelson J.A. Oldstone M.B. Detection of human cytomegalovirus in peripheral blood lymphocytes in a natural infection.Science. 1985; 230: 1048-1051Crossref PubMed Google Scholar, 10Boeckh M. Hoy C. Torok-Storb B. Occult cytomegalovirus infection of marrow stroma.Clin Infect Dis. 1998; 26: 209-210Crossref PubMed Google Scholar, 11Kahl M. Siegel-Axel D. Stenglein S. Jahn G. Sinzger C. Efficient lytic infection of human arterial endothelial cells by human cytomegalovirus strains.J Virol. 2000; 74: 7628-7635Crossref PubMed Scopus (51) Google Scholar, 12Plachter B. Sinzger C. Jahn G. Cell types involved in replication and distribution of human cytomegalovirus.Adv Virus Res. 1996; 46: 195-261Crossref PubMed Google Scholar, 13Sinzger C. Grefte A. Plachter B. Gouw A.S. The T.H. Jahn G. Fibroblasts, epithelial cells, endothelial cells and smooth muscle cells are major targets of human cytomegalovirus infection in lung and gastrointestinal tissues.J Gen Virol. 1995; 76: 741-750Crossref PubMed Google Scholar, 14Soderberg C. Larsson S. Bergstedt-Lindqvist S. Moller E. Definition of a subset of human peripheral blood mononuclear cells that are permissive to human cytomegalovirus infection.J Virol. 1993; 67: 3166-3175PubMed Google Scholar]. In addition, HCMV infects and presumably remains latent in CD34+ hematopoietic progenitor populations in the bone marrow [15Goodrum F.D. Jordan C.T. High K. Shenk T. Human cytomegalovirus gene expression during infection of primary hematopoietic progenitor cells: a model for latency.Proc Natl Acad Sci U S A. 2002; 99: 16255-16260Crossref PubMed Scopus (86) Google Scholar, 16Khaiboullina S.F. Maciejewski J.P. Crapnell K. et al.Human cytomegalovirus persists in myeloid progenitors and is passed to the myeloid progeny in a latent form.Br J Haematol. 2004; 126: 410-417Crossref PubMed Scopus (28) Google Scholar, 17Maciejewski J.P. Bruening E.E. Donahue R.E. Mocarski E.S. Young N.S. St Jeor S.C. Infection of hematopoietic progenitor cells by human cytomegalovirus.Blood. 1992; 80: 170-178PubMed Google Scholar, 18Mendelson M. Monard S. Sissons P. Sinclair J. Detection of endogenous human cytomegalovirus in CD34+ bone marrow progenitors.J Gen Virol. 1996; 77: 3099-3102Crossref PubMed Google Scholar, 19Movassagh M. Gozlan J. Senechal B. Baillou C. Petit J.C. Lemoine F.M. Direct infection of CD34+ progenitor cells by human cytomegalovirus: evidence for inhibition of hematopoiesis and viral replication.Blood. 1996; 88: 1277-1283PubMed Google Scholar], suggesting that these or an even more primitive cell population can serve as a renewable primary reservoir for latent virus. Hematopoietic cells of the myeloid lineage have been shown to harbor latent virus [20Taylor-Wiedeman J. Sissons J.G. Borysiewicz L.K. Sinclair J.H. Monocytes are a major site of persistence of human cytomegalovirus in peripheral blood mononuclear cells.J Gen Virol. 1991; 72: 2059-2064Crossref PubMed Google Scholar, 21Taylor-Wiedeman J. Sissons P. Sinclair J. Induction of endogenous human cytomegalovirus gene expression after differentiation of monocytes from healthy carriers.J Virol. 1994; 68: 1597-1604PubMed Google Scholar] and following terminal differentiation of the cells into macrophages, virus can reactivate and, in an immune-compromised host, an acute infection can occur [14Soderberg C. Larsson S. Bergstedt-Lindqvist S. Moller E. Definition of a subset of human peripheral blood mononuclear cells that are permissive to human cytomegalovirus infection.J Virol. 1993; 67: 3166-3175PubMed Google Scholar, 20Taylor-Wiedeman J. Sissons J.G. Borysiewicz L.K. Sinclair J.H. Monocytes are a major site of persistence of human cytomegalovirus in peripheral blood mononuclear cells.J Gen Virol. 1991; 72: 2059-2064Crossref PubMed Google Scholar, 21Taylor-Wiedeman J. Sissons P. Sinclair J. Induction of endogenous human cytomegalovirus gene expression after differentiation of monocytes from healthy carriers.J Virol. 1994; 68: 1597-1604PubMed Google Scholar, 22Maciejewski J.P. Bruening E.E. Donahue R.E. et al.Infection of mononucleated phagocytes with human cytomegalovirus.Virology. 1993; 195: 327-336Crossref PubMed Scopus (30) Google Scholar]. Although it is accepted that the hematopoietic compartment is an important reservoir for latent HCMV, it is unknown what viral and cellular factors are involved in regulating the switch between lytic and latent infections. Also, even when hematopoietic multipotent precursors can be infected, not all the possible lineages derived from them were found to harbor HCMV. There is a controversy whether the virus prefers to infect certain hematopoietic lineage in vivo (and not a multipotent precursor) or whether the virus has an influence upon normal cellular differentiation pathways, favoring some lineages while suppressing others. Suppression of hematopoiesis is readily observed in the patients with disseminated HCMV infection [23Sing G.K. Ruscetti F.W. Preferential suppression of myelopoiesis in normal human bone marrow cells after in vitro challenge with human cytomegalovirus.Blood. 1990; 75: 1965-1973PubMed Google Scholar, 24Chesney P.J. Taher A. Gilbert E.M. Shahidi N.T. Intranuclear inclusions in megakaryocytes in congenital cytomegalovirus infection.J Pediatr. 1978; 92: 957-958Abstract Full Text PDF PubMed Google Scholar, 25Foster K.M. Jack I. A prospective study of the role of cytomegalovirus in post-transfusion mononucleosis.N Engl J Med. 1969; 280: 1311-1316Crossref PubMed Google Scholar, 26Risdall R.J. McKenna R.W. Nesbit M.E. et al.Virus-associated hemophagocytic syndrome: a benign histiocytic proliferation distinct from malignant histiocytosis.Cancer. 1979; 44: 993-1002Crossref PubMed Google Scholar, 27Verdonck L.F. van Heugten H. de Gast G.C. Delay in platelet recovery after bone marrow transplantation: impact of cytomegalovirus infection.Blood. 1985; 66: 921-925PubMed Google Scholar, 28Maciejewski J.P. St Jeor S.C. Human cytomegalovirus infection of human hematopoietic progenitor cells.Leuk Lymphoma. 1999; 33: 1-13PubMed Google Scholar]. HCMV infection can also be associated with a variety of blood disorders including thrombocytopenia, neutropenia, and anemia. In nonimmunocompromised adults, HCMV can cause severe hemolytic anemia often accompanied by thrombocytopenia [29Gavazzi G. Leclercq P. Bouchard O. Bosseray A. Morand P. Micoud M. Association between primary cytomegalovirus infection and severe hemolytic anemia in an immunocompetent adult.Eur J Clin Microbiol Infect Dis. 1999; 18: 299-301Crossref PubMed Scopus (5) Google Scholar, 30van Spronsen D.J. Breed W.P. Cytomegalovirus-induced thrombocytopenia and haemolysis in an immunocompetent adult.Br J Haematol. 1996; 92: 218-220Crossref PubMed Google Scholar]. Severe HCMV infection–associated thrombocytopenia in immunocompetent adults is uncommon, and only 13 cases can be found in the literature [30van Spronsen D.J. Breed W.P. Cytomegalovirus-induced thrombocytopenia and haemolysis in an immunocompetent adult.Br J Haematol. 1996; 92: 218-220Crossref PubMed Google Scholar, 31Wright J.G. Severe thrombocytopenia secondary to asymptomatic cytomegalovirus infection in an immunocompetent host.J Clin Pathol. 1992; 45: 1037-1038Crossref PubMed Google Scholar, 32Sahud M.A. Bachelor M.M. Cytomegalovirus-induced thrombocytopenia. An unusual case report.Arch Intern Med. 1978; 138: 1573-1575Crossref PubMed Google Scholar, 33Shimm D.S. Logue G.L. Rosse W.F. Recurrent thrombocytopenia following idiopathic thrombocytopenic purpura. The importance of platelet-bound IgG in establishing cause.Arch Intern Med. 1980; 140: 855-857Crossref PubMed Google Scholar, 34Fiala M. Payne J.E. Berne T.V. et al.Epidemiology of cytomegalovirus infection after transplantation and immunosuppression.J Infect Dis. 1975; 132: 421-433Crossref PubMed Google Scholar, 35Gural A. Gillis S. Gafanovich A. et al.Massive intracranial bleeding requiring emergency splenectomy in a patient with CMV-associated thrombocytopenia.Haemostasis. 1998; 28: 250-255PubMed Google Scholar, 36Arruda V.R. Rossi C.L. Nogueira E. Annicchino-Bizzacchi J.M. Costa F.F. Costa S.C. Cytomegalovirus infection as cause of severe thrombocytopenia in a nonimmunosuppressed patient.Acta Haematol. 1997; 98: 228-230Crossref PubMed Google Scholar, 37Muntendam H. [Thrombocytopenia caused by cytomegalovirus infection].Ned Tijdschr Geneeskd. 1975; 119: 15-17PubMed Google Scholar, 38Chanarin I. Walford D.M. Thrombocytopenic purpura in cytomegalovirus mononucleosis.Lancet. 1973; 2: 238-239Abstract PubMed Google Scholar, 39Harris D.J. Immune complications associated with chronic transfusion.J Infus Nurs. 2002; 25: 316-319Crossref PubMed Google Scholar, 40Ichiche M. Fontaine C. Lacor P. Severe thrombocytopenia secondary to cytomegalovirus infection in an immunocompetent adult.Eur J Intern Med. 2003; 14: 56-59Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar, 41Miyahara M. Shimamoto Y. Yamada H. Shibata K. Matsuzaki M. Ono K. Cytomegalovirus-associated myelodysplasia and thrombocytopenia in an immunocompetent adult.Ann Hematol. 1997; 74: 99-101Crossref PubMed Scopus (23) Google Scholar]. Diagnosis of HCMV-induced anemia is done using serologic tests for HCMV in combination with antiglobulin tests, which detect immunoglobulin or complement reacting against the patient's own erythrocytes [29Gavazzi G. Leclercq P. Bouchard O. Bosseray A. Morand P. Micoud M. Association between primary cytomegalovirus infection and severe hemolytic anemia in an immunocompetent adult.Eur J Clin Microbiol Infect Dis. 1999; 18: 299-301Crossref PubMed Scopus (5) Google Scholar, 42Veldhuis W. Janssen M. Kortlandt W. van Houte A. van de Ree M. Coombs-negative severe haemolytic anaemia in an immunocompetent adult following cytomegalovirus infection.Eur J Clin Microbiol Infect Dis. 2004; 23: 844-847Crossref PubMed Scopus (5) Google Scholar]. Antiglobulin tests were first introduced in 1945 by R. Coombs, who showed that they could be used to detect nonagglutinating red cell antibodies (indirect antiglobulin test, IAT) or sensitized red cells (direct antiglobulin test, DAT). Antiglobulin or "Coombs" reagent preparations contain a mix of monoclonal antibodies directed against human IgG and complement component C3. It is important to identify HCMV as responsible for this autoimmune anemia, since treatment with gancyclovir, gamma globulins, and/or steroids is available for this particular setting [30van Spronsen D.J. Breed W.P. Cytomegalovirus-induced thrombocytopenia and haemolysis in an immunocompetent adult.Br J Haematol. 1996; 92: 218-220Crossref PubMed Google Scholar, 43Juneja S.K. Phillips K.A. Speed B. Januszewicz E.H. High-dose γ-globulin responsive haemolysis due to cytomegalovirus in an immunocompetent adult.Br J Haematol. 1996; 95: 433-435PubMed Google Scholar]. In this review, we will address the significance of hematopoietic cells as a reservoir for latent HCMV and the mechanism by which the virus enters and remains in a latent state, as well how virus is reactivated from latency. Although a great deal of progress has been made in treating active infections, the ultimate control of HCMV infections is dependent on our ability to eliminate latent virus. We will give emphasis to possible differentiation alterations, and changes in gene expression patterns, as well as the significance of HCMV immune evasion. Early posttransfusion studies indicated that HCMV could be transmitted by blood components, but it was not well understood why the virus could not be directly isolated from blood cells [34Fiala M. Payne J.E. Berne T.V. et al.Epidemiology of cytomegalovirus infection after transplantation and immunosuppression.J Infect Dis. 1975; 132: 421-433Crossref PubMed Google Scholar, 44Schrier R.D. Nelson J.A. Oldstone M.B. Detection of human cytomegalovirus in peripheral blood lymphocytes in a natural infection.Science. 1985; 230: 1048-1051Crossref PubMed Google Scholar]. Immunohistochemistry demonstrated that a number of cell types can be infected by HCMV in vitro, including endothelial cells, fibroblasts, adipocytes, granulocytes, and macrophages [45Apperley J.F. Dowding C. Hibbin J. et al.The effect of cytomegalovirus on hemopoiesis: in vitro evidence for selective infection of marrow stromal cells.Exp Hematol. 1989; 17: 38-45PubMed Google Scholar, 46Reiser H. Kuhn J. Doerr H.W. Kirchner H. Munk K. Braun R. Human cytomegalovirus replicates in primary human bone marrow cells.J Gen Virol. 1986; 67: 2595-2604Crossref PubMed Google Scholar, 47Simmons P. Kaushansky K. Torok-Storb B. Mechanisms of cytomegalovirus-mediated myelosuppression: perturbation of stromal cell function versus direct infection of myeloid cells.Proc Natl Acad Sci U S A. 1990; 87: 1386-1390Crossref PubMed Google Scholar]. In 1977 we reported that human lymphoblasts and peripheral leukocytes were susceptible to in vitro HCMV infection, but in a persistent rather that a productive mode [48St. Jeor S. Weisser A. Persistence of cytomegalovirus in human lymphoblasts and peripheral leukocyte cultures.Infect Immun. 1977; 15: 402-409PubMed Google Scholar]. In 1979, HCMV intranuclear inclusions were found in bone marrow biopsy from a previously seronegative immunosuppressed individual recovering from renal transplant. Increasing antibody titers indicated an acute posttransplant infection, and showed for the first time that human bone marrow was susceptible to viral infection in vivo [49Penchansky L. Krause J.R. Identification of cytomegalovirus in bone marrow biopsy.South Med J. 1979; 72: 500-501Crossref PubMed Google Scholar]. A few years later, in vitro infection of primary bone marrow demonstrated that both bone marrow fibroblasts and a subset of bone marrow stem cells supported a productive virus infection. The authors noted that infection of early stem cells could explain the presence of HCMV in peripheral blood leukocytes [46Reiser H. Kuhn J. Doerr H.W. Kirchner H. Munk K. Braun R. Human cytomegalovirus replicates in primary human bone marrow cells.J Gen Virol. 1986; 67: 2595-2604Crossref PubMed Google Scholar]. Dankner et al. studied the in vivo interaction of HCMV with leukocytes from a group of immunosuppressed patients, using specific subgenomic DNA probes and in situ hybridization. Monocytes were found occasionally positive by hybridization and in contrast, lymphocytes were rarely positive [50Dankner W.M. McCutchan J.A. Richman D.D. Hirata K. Spector S.A. Localization of human cytomegalovirus in peripheral blood leukocytes by in situ hybridization.J Infect Dis. 1990; 161: 31-36Crossref PubMed Google Scholar]. Using PCR it was shown in 1991 that healthy seropositive donors harbored HCMV DNA predominantly in CD14+ monocytes [20Taylor-Wiedeman J. Sissons J.G. Borysiewicz L.K. Sinclair J.H. Monocytes are a major site of persistence of human cytomegalovirus in peripheral blood mononuclear cells.J Gen Virol. 1991; 72: 2059-2064Crossref PubMed Google Scholar]. The same year, using in vitro–infected human long-term marrow culture, HCMV-infected mononuclear cells were detected in the nonadherent fraction, and infectious virus was recovered. HCMV was also shown to cause destruction of the stromal layer and early differentiation of nonadherent cells into a homogenous population of macrophage-like cells. It was proposed that the human bone marrow is an important site for HCMV replication during acute infection and latent HCMV infection [51Preiksaitis J.K. Janowska-Wieczorek A. Persistence of cytomegalovirus in human long-term bone marrow culture: relationship to hemopoiesis.J Med Virol. 1991; 35: 76-84Crossref PubMed Google Scholar]. Following this initial study, it was reported that even when HCMV DNA is present in polymorphonuclear leukocytes (PMNL) during the viremic phase, these cells were not the site of viral persistence [52Taylor-Wiedeman J. Hayhurst G.P. Sissons J.G. Sinclair J.H. Polymorphonuclear cells are not sites of persistence of human cytomegalovirus in healthy individuals.J Gen Virol. 1993; 74: 265-268Crossref PubMed Google Scholar]. In 1994, using PCR the presence of HCMV DNA was reported in bone marrow progenitor cells from healthy seropositive donors [53Minton E.J. Tysoe C. Sinclair J.H. Sissons J.G. Human cytomegalovirus infection of the monocyte/macrophage lineage in bone marrow.J Virol. 1994; 68: 4017-4021PubMed Google Scholar]. In the same study the absence of immediate-early (IE) expression by indirect immunofluorescence was reported [53Minton E.J. Tysoe C. Sinclair J.H. Sissons J.G. Human cytomegalovirus infection of the monocyte/macrophage lineage in bone marrow.J Virol. 1994; 68: 4017-4021PubMed Google Scholar]. That same year, a model for HCMV latency was established by Kondo et al., using cultured primary granulocyte-macrophage progenitors. They propagated progenitor cells from human fetal liver or bone marrow (phenotype CD14+, CD15+, CD33+) in suspension cultures. They reported that exposure to HCMV did not reduce growth or alter the phenotype of these cells, and that 2 to 5% of latently infected cells expressed transcripts from IE region. They also demonstrated that latent virus could be reactivated from their system [54Kondo K. Kaneshima H. Mocarski E.S. Human cytomegalovirus latent infection of granulocyte-macrophage progenitors.Proc Natl Acad Sci U S A. 1994; 91: 11879-11883Crossref PubMed Scopus (230) Google Scholar]. Furthermore, they reported that HCMV DNA was detected by PCR from CD34+ sorted population from bone marrow and mobilized peripheral blood. Interestingly, HCMV DNA was amplified from immunosuppressed patients but not healthy seropositive donors [55von Laer D. Meyer-Koenig U. Serr A. et al.Detection of cytomegalovirus DNA in CD34+ cells from blood and bone marrow.Blood. 1995; 86: 4086-4090PubMed Google Scholar]. Presently, detection of HCMV DNA from healthy seropositive donors is possible due to many optimizations and improvements of modern molecular biology techniques. Unexpectedly, a few years after the initial PCR studies, Larsson et al. showed that HCMV DNA could be amplified from monocyte-enriched peripheral blood mononuclear cells (PBMC) from a considerable percentage of seronegative donors [56Larsson S. Soderberg-Naucler C. Wang F.Z. Moller E. Cytomegalovirus DNA can be detected in peripheral blood mononuclear cells from all seropositive and most seronegative healthy blood donors over time.Transfusion. 1998; 38: 271-278Crossref PubMed Google Scholar]. Evidence of viral spread to tissues came in 1996. Monoclonal antibodies against viral and cellular antigens were used to identify viral infected cells from tissue sections from various infected organs. Presence of viral proteins showed that macrophages and polymorphonuclear cells are in fact targets for HCMV infection in different tissues. Macrophages were shown to be permissive for HCMV infection and support a complete viral replication cycle, whereas polymorphonuclear cells only expressed immediate-early genes, possibly indicating an abortive infection [57Sinzger C. Plachter B. Grefte A. The T.H. Jahn G. Tissue macrophages are infected by human cytomegalovirus in vivo.J Infect Dis. 1996; 173: 240-245Crossref PubMed Google Scholar]. Although there was ample evidence suggesting peripheral blood monocytes as a site of latency in healthy carriers, it is unlikely that monocytes represent the primary site of HCMV infection [18Mendelson M. Monard S. Sissons P. Sinclair J. Detection of endogenous human cytomegalovirus in CD34+ bone marrow progenitors.J Gen Virol. 1996; 77: 3099-3102Crossref PubMed Google Scholar]. In 1996, the presence of endogenous HCMV within bone marrow progenitors (CD34+) in the absence of HCMV lytic gene expression in healthy carriers was demonstrated for the first time [18Mendelson M. Monard S. Sissons P. Sinclair J. Detection of endogenous human cytomegalovirus in CD34+ bone marrow progenitors.J Gen Virol. 1996; 77: 3099-3102Crossref PubMed Google Scholar]. The same group also suggested that the permissiveness of myeloid cells for HCMV is critically dependent on the differentiation state of the cell [18Mendelson M. Monard S. Sissons P. Sinclair J. Detection of endogenous human cytomegalovirus in CD34+ bone marrow progenitors.J Gen Virol. 1996; 77: 3099-3102Crossref PubMed Google Scholar]. Our laboratory reported infection of purified CD34+ cells with a clinical isolate and with the recombinant laboratory strain Towne/lox containing the Escherichia coli β-galactosidase (β-gal) gene regulated by the IE HCMV promoter. Polymerase chain reaction analysis showed that HCMV persisted in infected BM CD34+ cells and their progeny for up to 4 weeks. However, IE and late gene products were detected only late in the course of infection and their expression correlated with terminal macrophage differentiation of the CD34+-derived progeny. We also demonstrated that infectious virus could be recovered from the terminally differentiated cultures. It was stated that BM progenitor cells may serve as a reservoir of the latent virus with limited transcription [58Zhuravskaya T. Maciejewski J.P. Netski D.M. Bruening E. Mackintosh F.R. St Jeor S. Spread of human cytomegalovirus (HCMV) after infection of human hematopoietic progenitor cells: model of HCMV latency.Blood. 1997; 90: 2482-2491PubMed Google Scholar]. In a more recent publication it was noted that HCMV DNA can be harvested from seropositive donors' CD14+ cells in a circular conformation, not linearized or integrated, analogous to other herpesvirus [59Bolovan-Fritts C.A. Mocarski E.S. Wiedeman J.A. Peripheral blood CD14+ cells from healthy subjects carry a circular conformation of latent cytomegalovirus genome.Blood. 1999; 93: 394-398PubMed Google Scholar]. Recently, Goodrum and colleagues studied HCMV infection in different subsets of CD34+ cells. They reported that hematopoiesis was inhibited in HCMV-infected CD34+/CD38− populations but not CD34+/c-kit+ cells. The CD34+/CD38− population also expressed a large number of HCMV genes until 10 days after infection. CD34+/c-kit+ cells or cells expressing a more mature cell-surface phenotype appeared to develop an abortive infection. Furthermore, the only subpopulation that virus could be reactivated from was CD34+/CD38−. The data presented indicated that CD34+/CD38− cells support a HCMV latent infection in vitro [60Goodrum F. Jordan C.T. Terhune S.S. High K. Shenk T. Differential outcomes of human cytomegalovirus infection in primitive hematopoietic cell subpopulations.Blood. 2004; 104: 687-695Crossref PubMed Scopus (56) Google Scholar]. Our laboratory demonstrated that when hematopoietic progenitor cells from HCMV-seronegative donors were infected in vitro, viral DNA was localized in the nucleus. Although no viral production could be detected throughout the culture, the comparison of the numbers of latently infected cells after culture suggests that proliferation of hematopoietic progenitor cells may lead to the expansion of latently infected cells [16Khaiboullina S.F. Maciejewski J.P. Crapnell K. et al.Human cytomegalovirus persists in myeloid progenitors and is passed to the myeloid progeny in a latent form.Br J Haematol. 2004; 126: 410-417Crossref PubMed Scopus (28) Google Scholar]. A recent report using myeloid dendritic cell (DC) progenitors isolated from healthy HCMV carriers demonstrated that ex vivo differentiation of mature DC is also linked with reactivation of infectious virus. Chromatin remodeling of the viral major immediate-early promoter, a differentiation-dependent mechanism, is necessary for the reactivation [61Reeves M.B. MacAry P.A. Lehner P.J. Sissons J.G. Sinclair J.H. Latency, chromatin remodeling, and reactivation of human cytomegalovirus in the dendritic cells of healthy carriers.Proc Natl Acad Sci U S A. 2005; 102: 4140-4145Crossref PubMed Scopus (129) Google Scholar]. Monocytes had been identified as the predominant latently infected cell type among peripheral blood mononuclear cell populations from HCMV-seropositive donors [20Taylor-Wiedeman J. Sissons J.G. Borysiewicz L.K. Sinclair J.H. Monocytes are a major site of persistence of human cytomegalovirus in peripheral blood mononuclear cells.J Gen Virol. 1991; 72: 2059-2064Crossref PubMed Google Scholar]. However, HCMV is unable to generate a productive infection in monocytes and viral gene expression is restricted to early events [62Ibanez C.E. Schrier R. Ghazal P. Wiley C. Nelson J.A. Human cytomegalovirus productively infects primary differentiated macrophages.J Virol. 1991; 65: 6581-6588PubMed Google Scholar]. Thus, macrophage differentiation is believed to be a prerequisite for productive HCMV infection in cells from the myeloid lineage, since extensive viral gene expression is normally found in tissue macrophages after viral reactivation [19Movassagh M. Gozlan J. Senechal B. Baillou C. Petit J.C. Lemoine F.M. Direct infection of CD34+ progenitor cells by human cytomegalovirus: evidence for inhibition of hematopoiesis and viral replication.Blood. 1996; 88: 1277-1283PubMed Google Scholar, 53Minton E.J. Tysoe C. Sinclair J.H. Sissons J.G. Human cytomegalovirus infection of the monocyte/macrophage lineage in bone marrow.J Virol. 1994; 68: 4017-4021PubMed Google Scholar, 57Sinzger C. Plachter B. Grefte A. The T.H. Jahn G. Tissue macrophages are infected by human cytomegalovirus in vivo.J Infect Dis. 1996; 173: 240-245Crossref PubMed Google Scholar, 63Gnann Jr., J.W. Ahlmen J. Svalander C. Olding L. Oldstone M.B. Nelson J.A. Inflammatory cells in transplanted kidneys are infected by human cytomegalovirus.Am J Pathol. 1988; 132: 239-248PubMed Google Scholar]. In vitro infection of PBMC pointed out CD14+ monocytes and a small population of CD3/CD8+ large granular lymphocytes as the primary viral targets [14Soderberg C. Larsson S. Bergstedt-Lindqvist S. Moller E. Definition of a subset of human peripheral blood mononuclear cells that are permissive to human cytomegalovirus infection.J Virol. 1993; 67: 3166-3175PubMed Google Scholar]. Early in 1988 it was shown that when a monocytic cell line, THP-1, was induced to differentiate with a phorbol ester (12-0-tetradecanoylphorbol-13-acetate; PMA) it was susceptible to a productive human cytomegalovirus infection [64Weinshenker B.G. Wilton S. Rice G.P. Phorbol ester–induced differentiation permits productive human cytomegalovirus