JOURNAL
OF
VIROLOGY, Oct. 1994, p. 6243-6253
Vol. 68, No. 10
0022-538X/94/$04.00+0 Copyright © 1994, American Society for Microbiology
Peripheral Blood Mononuclear Phagocytes Mediate Dissemination of Murine Cytomegalovirus CHERYL A. STODDART,t RHONDA D. CARDIN, JESSICA M. BONAME, WILLIAM C. MANNING,t GERARDO B. ABENES, AND EDWARD S. MOCARSKI* Department of Microbiology and Immunology, Stanford University School
of Medicine, Stanford, California 94305-5402 Received 14 February 1994/Accepted 25 June 1994
Cytomegalovirus is transmitted with blood and organs from seropositive individuals, although the particular leukocyte population harboring latent or persistent virus remains poorly characterized. Murine cytomegalovirus, tagged with the Escherichia coli lacZ gene, was used to identify cells in which virus replicates during acute infection of immunocompetent mice. Recombinant murine cytomegaloviruses, RM461, RM460, and RM427, were constructed to express ,1-galactosidase under control of the human cytomegalovirus iellie2 promoter/ enhancer. The lacZ gene was inserted between the ie2 and sggl genes in RM461 and RM460, disrupting a 0.85-kb late transcript that was found to be dispensable for replication in cultured cells as well as for infection of mice. In BALB/c mice, lacZ-tagged and wild-type viruses exhibited a similar 50%o lethal dose and all had the capacity to latently infect the spleen. Peripheral blood mononuclear phagocytes were the major infected leukocyte cell type, as demonstrated by the ability of infected cells to adhere to glass and to phagocytize latex beads; however, these cells did not exhibit typical monocyte markers. Plaque assay for virus and 5-bromo-4chloro-3-indolyl-,1-D-galactopyranoside (X-Gal) staining of frozen sections of organs from infected mice revealed that the major target organs included the spleen, adrenal glands, liver, and salivary glands, although tissues as diverse as brown fat and lungs were also involved. Individual blue-staining cells were readily identified in all infected tissues. These studies identified a mononuclear phagocyte, possibly a macrophage or dendritic cell precursor, as the vehicle of virus dissemination during acute infection, and demonstrate the utility of using lacZ-tagged murine cytomegalovirus for tropism, pathogenesis, and latency studies. Human cytomegalovirus (CMV) is a significant viral pathoof immunocompromised adults and developing fetuses (1). The pathogenesis, tissue tropism, and latency characteristics of this virus are still poorly understood. Strict species specificity has limited the use of animal models (45), and therefore much of our current knowledge derives from studies of symptomatic CMV infection of immunocompromised individuals in whom cell-associated viremia involves polymorphonuclear, mononuclear, and endothelial cells (7, 12, 46, 51, 64). Much less is known about the interaction of human CMV with immunocompetent individuals. Blood leukocytes may be a source of virus spread in blood transfusion from healthy CMV-seropositive carriers (66), but whether a viremia ensues during primary infection remains unknown. Monocytes have been best documented as sites of persistence of human CMV DNA (61, 62); however, the role of these cells or other blood cell types in viral dissemination or persistence in the normal host has not been fully evaluated. The difficulty of studying human CMV directly in humans has led to the use of murine, guinea pig, and rat CMV models. Investigations with murine CMV have reinforced biological similarities to human CMV and furthered an understanding of the tissue tropism (35, 54), pathogenesis (49, 52, 55), latency (25, 38), and immunology (15, 26, 31, 48, 53) of this group of
viruses. Intraperitoneal (i.p.) inoculation of susceptible strains of mice, such as outbred Swiss-Webster or inbred BALB/c mice, leads to dissemination, with viral lesions in mouse spleen, liver, adrenal glands, and brown fat (36, 42, 43), which are all generally cleared of virus by an aggressive CD8+ T-cellmediated immune response beginning at 7 to 10 days postinoculation (p.i.) (15, 26). Sustained, high-level growth in salivary glands continues for over 1 month (35), and clearance of virus from this organ may be mediated by helper CD4+ T cells and cytokines (20, 31). Dissemination of murine CMV from sites of initial replication in the peritoneal cavity to various organs occurs via the blood. Virus associates with leukocytes during acute infection (2, 35) and is transmissible by blood transfusion during latent or persistent infection (6). Although peripheral blood mononuclear cells have been suspected of playing a crucial role in dissemination and macrophages support viral replication in the peritoneal cavity (3, 21, 41, 63), the blood cell type responsible for dissemination remains poorly characterized. Because of the importance of the spleen as a site for studying viral latency, spleen cells infected with murine CMV have been investigated by in situ hybridization-antibody colocalization methods (38). These studies have suggested that endothelial or sinusoidal lining cells act as the primary sites of replication in the spleen, extending earlier observations in which polyclonal antisera were used to localize murine CMV during acute infection (42). The salivary gland represents the site of sustained high-level replication of murine CMV, and infection in this gland is localized largely to serous acinar epithelial cells (36, 43). Over the years, many different methods that have been used to identify virus-infected cells, ranging from the occurrence of characteristic cytopathology to the more sensitive and specific
gen
* Corresponding author. Mailing address: Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305-5402. Phone: (415) 723-6435. Fax: (415) 723-9180. Electronic mail address:
[email protected].
t Present address: Shaman Pharmaceuticals, Inc., South San Fran-
cisco, CA 94080. t Present address: Department of Pediatrics, Stanford University School of Medicine, Stanford, CA 94305. 6243
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STODDART ET AL.
methods of immunofluorescence and in situ hybridization. Histological preservation of tissue integrity is highly variable and limits the use of these conventional approaches to identify rare infected cells present in the blood or organs. The localization of herpes simplex virus in tissues during acute and latent infection has been facilitated by the use of lacZ-tagged recombinant viruses whose presence can be readily detected in tissues by convenient and sensitive in situ staining methods (13, 14). By using lacZ-tagged murine CMV, we have achieved a highly sensitive means of detecting the presence of viruses with excellent preservation of tissue integrity and have identified the monocyte as a site of viral replication in the peripheral blood of immunocompetent BALB/c mice. (A preliminary report of this work was presented at the 16th International Herpesvirus Workshop, Asilomar Conference Center, Pacific Grove, Calif., in 1991.) MATERIALS AND METHODS
Viruses and cell culture. The Smith-K181 strain of murine CMV (obtained from M. Colin Jordan) was originally obtained as a salivary gland-passaged stock and, following plaque purification three times in our laboratory and evaluation for growth and virulence for BALB/c mice, was designated K181+. Parental K181+ and recombinant viruses were grown and plaque assayed in NIH 3T3 cells (ATCC CRL1658) in Dulbecco's modified Eagle's medium (Gibco-Life Technologies, Bethesda, Md.) supplemented with 10% NuSerum (Collaborative Research Inc., Waltham, Mass.), 100 U of penicillin per ml, 100 mg of streptomycin sulfate per ml, 0.66 mM arginine, 1.48 mM glutamine, and 0.24 mM asparagine. Prior to infection or transfection, cells were seeded into 6- or 12-well cell culture dishes (Falcon, Becton Dickinson) containing the same medium. Infections for the preparation of virus stocks and viral DNA were performed at a multiplicity of infection (MOI) of 0.0001 PFU per cell, and infections for the preparation of viral RNA were performed at an MOI of 10 PFU per cell. Tissue culture-propagated virus stocks were made by sonicating infected cells in a mixture of 50% medium and 50% autoclaved skim milk (35). Submaxillary salivary gland-propagated virus stocks were harvested 14 days after i.p. inoculation of 104 PFU of salivary gland-propagated RM461 and, after sonication, were stored as either a 10% or 25% (wt/vol) suspension in medium-skim milk. The inoculum for mock-infected control mice was prepared in a similar manner from uninfected salivary gland tissue. All virus stocks were stored at -80°C. Animals, virus inoculation, and in situ staining. Threeweek-old male BALB/c ByJ mice (Jackson Laboratories, Bar Harbor, Maine) were weight-matched prior to use between 24 and 28 days of age. Four-week old male SCID (C.B17 scid/scid) mice were bred locally at Stanford University School of Medicine, and nude (BALB/c nu/nu) were bred and used at the University of Western Australia. All animals received water and food ad libitum, were inoculated i. . with either 106 PFU of tissue culture-propagated virus or 10 to 105 PFU of salivary gland-propagated virus, and were sacrificed by CO2 asphyxiation, and organs were harvested for virus titer determinations as described previously (35), except that the lower limit of virus detection in this report was 10 PFU. Organs taken for cryosection were snap-frozen in polypropylene tubes containing OCT freezing medium (Miles, Indianapolis, Ind.) in a dry ice-ethanol bath and stored at -80°C. Sections (5 to 12 ,Lm thick) were cut on a Bright Instrument cryostat, fixed in 0.5% glutaraldehyde, and stained with 300 ,ug of 5-bromo-4-chloro-
3-indolyl-p-D-galactopyranoside (X-Gal) per ml to detect p-galactosidase (P-gal)-positive cells as described previously (13).
J. VIROL.
In some cases, organs were fixed in 4% paraformaldehyde, stained overnight with X-Gal, embedded in paraffin, and sectioned (this was performed at the Diagnostic Pathology Laboratory, Stanford Medical Center). Rabbit polyclonal antihuman von Willebrand factor (Dakopatts, Glostup, Denmark) was used in indirect-immunofluorescence assays on acetonefixed cells or tissue sections with rhodamine-conjugated goat anti-rabbit immunoglobulin G F(ab')2 fragments (Tago, Burlingame, Calif.). The 50% lethal dose (LD50) was determined as previously described (35). For isolation of peripheral blood leukocytes, heart blood was collected into phosphate-buffered saline (PBS)-8 mM EDTA and erythrocytes were lysed by the addition of 150 mM ammonium chloride, after which cells were washed twice in cold PBS and suspended at 106 cells per ml. Leukocytes were either prepared by cytocentrifugation (Shandon Southern, Sewickley, Pa.) or allowed to incubate at 37°C on eight-well Labtek chamber slides (Miles Scientific, Elkhart, Ind.). Exposure of adherent cells to 5 x 106 3-,um-diameter latex beads (Polysciences, Warrington, Pa.) for 4 h was done as previously described (60). Slides were rinsed in PBS, fixed with 0.5% glutaraldehyde, and stained with X-Gal (13) or fixed with methanol and stained with May-Grunwald-Giemsa. Recombinant plasmid and virus construction. pON432 carried a HpaI-EcoRI fragment isolated from the right end of the EcoRI E fragment (pON400), which was cloned from the murine CMV genome (35) into pMT11 (56). A 3.9-kbp XhoI-BamHI fragment from pON283 (5), consisting of the human CMV iellie2 promoter-enhancer (-219 to -19 relative to the transcription start site) fused to the lacZ gene and a simian virus 40 early polyadenylation signal (56), was inserted in either of two orientations into the unique HindIII site in pON432, after the fragment ends were filled in with Klenow DNA polymerase, thus generating pON460 and pON461. pON460 places the human CMV enhancer on the side closest to the murine CMV ie2 gene, and pON461 places the human CMV enhancer on the side closest to the sggl gene (Fig. 1). A HindIII site was reconstituted at the site upstream of the enhancer in each clone. Recombinant viruses RM460 and RM461 were constructed by cotransfection of PstI-linearized pON460 or pON461 and intact murine CMV DNA into NIH 3T3 cells approximately 24 h after seeding at 50% cell density by using previously described methods (35). Before purification of individual recombinant viruses, the pool of virus from the cotransfection was used to inoculate mice i.p., and salivary gland-passaged virus stocks were prepared after 14 days. This step was included to subject candidate recombinants to the selective pressure of replicating in normal mouse tissues, to encourage the isolation of viruses capable of replicating in salivary glands, and to limit the number of recombinant viruses carrying adventitious mutations. lacZ+ recombinants were isolated from this stock by triple plaque purification of bluestaining plaques on NIH 3T3 cells after X-Gal overlay (34, 57). For RM460 and RM461, five and seven plaque-purified recombinants, respectively, were isolated and examined by HindIII enzyme digestion, and all but one isolate in each set appeared to carry the insertion without any adventitious deletions elsewhere in the viral genome. Isolates with no unexpected alterations and with growth and latency characteristics of wild-type virus were chosen, and these were denoted RM460 and RM461, respectively (Fig. 1). The construction of RM427, a viral recombinant that carries mutations in the ie2 and sggl genes as well as a lacZ insertion under control of the same human CMV iellie2 promoter-enhancer fragment as RM460 and RM461, has been described previously (35). RNA, DNA, and 0-gal analyses. Protocols for isolation of
CMV DISSEMINATION AND TISSUE TROPISM
VOL. 68, 1994
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Murine CMV (K1 81 ) Hindill Map QMP
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FIG. 1. Map position of mutations in RM427, RM461, and RM460. The HindIII map of the murine CMV (K181+ strain) genome is shown on the top line. The HindIII J, K, and L fragments have been expanded to show the region containing the major a (immediate-early) iel, ie2, and ie3 genes (23, 24, 39, 40), the sggl gene (35), and additional genes in the HindIII J region (65). The hatched box depicts the iellie2lie3 enhancer. The recombinant viruses are depicted in the lower portion. RM427, RM461, and RM460 each cariy an insertion of lacZ under control of the human CMV iellie2 promoter-enhancer. In RM427 the insertion replaces a 79-bp fragment between two HpaI sites in the ie2 promoter, and this virus carries a spontaneous 323-bp deletion in the sggl gene (35). In RM461 and RM460, the insertion is in the depicted orientation within the HindIIl L/J site. The positions of HindIll sites within and flanking each insertion are indicated.
RNA (10) and DNA (35), for treating cells with cycloheximide (34) or phosphonoformate (30), and for RNA blot hybridization (30) have all been published. The ie2-specific probe was a 0.9-kbp EcoRV-NheI fragment from pON401 (35), the sgglspecific probe was an HindIII-EcoRI fragment from pON446 (35), the -yO.85-specific probe was a 0.26-kbp SphI-DraIII fragment from pON467 located between the 3' end of the ie2 gene and the HindIII L/J site (4), and the lacZ-specific probes were either a 1.16-kbp HpaI fragment from pON405 (34) or a 3.5-kbp HindIII-EcoRI fragment from pON1 (56). Probes were isolated from an agarose gel and then radiolabeled with [a-32P]dCTP (Amersham, Arlington Heights, Ill.) by the random-primed method as described previously (9). ,3-Gal levels were measured by replacing normal growth medium with medium supplemented with 4-methylumbelliferyl-13-D-galactoside (MUG) at a concentration of 150 mg/ml. This substrate freely diffuses into cells and is cleaved by 1-gal, and the accumulated cleavage product diffuses into the medium and is detected at hourly intervals by a fluorometric assay as de-
scribed previously (11). 13-Gal activity was expressed as fold activation by calculating the ratio of MUG fluorescence level in replicate wells after infection with lacZ-tagged recombinant virus divided by the fluorescence after infection with parental K181+ performed in parallel. Latent infections and explant reactivation. Mice were inoculated i.p. with 106 PFU of tissue culture-propagated virus, and after 4 months, the animals were sacrificed by CO2 asphyxiation. The spleen and salivary glands were removed from each animal. Salivary glands were sonicated and plated onto NIH 3T3 cell monolayers to detect actively replicating virus. The spleen was divided into three sections, and each was minced and plated with medium in three wells of a six-well multiwell dish. Virus was detected in the supernatant of the explant
cultures by plaque assay on monolayer cultures of NIH 3T3 cells at weekly intervals through 6 weeks postexplant. Under these conditions, virus is only occasionally (1 in 15 animals) detected persistently replicating in the spleen. Consistent with the establishment of latency, explanted spleens did not yield detectable virus in the supernatant at 7 days but did so only after 14 days or longer in culture. Thus, animals used in these studies adhered to the operational criterion established for latent infection (59). At 4 months p.i., spleen explant culture (21) yielded virus reactivation in 90 to 100% of animals, and this level was observed whether tissue culture-propagated virus (106 PFU inoculated i.p.) or salivary gland-propagated virus (105 PFU inoculated i.p.) was used as inoculum. RESULTS Construction and characterization of RM460 and RM461. The two genes known to flank the HindIII site between the L and J fragments of the CMV genome, ie2 and sggl, have each been found to be completely dispensable for growth in cultured fibroblast cells (34, 35). RNA blot hybridization had not detected any a or 1 transcripts other than ie2 and sggl in this region, and neither of these transcripts crossed this HindIII site (33, 35, 40). We therefore assessed the use of this HindIlI site for the insertion of a lacZ marker gene to construct a virus that would be free of intentional disruptions of known viral genes. A chimeric lacZ gene under transcriptional control of the human CMV iel/ie2 promoter-enhancer that had been previously incorporated into RM427 (35) was inserted in both orientations at this site. The resulting viruses, RM460 and RM461, were isolated from pools of candidates that had been subjected to passage through mice and isolated as a salivary gland stock collected 14 days after i.p. inoculation. This step
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J. VIROL.
STODDART ET AL. INX
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