Murine peritoneal macrophages support murine cytomegalovirus replication

Mck2-dependent infection of alveolar macrophages promotes replication of MCMV in nodular inflammatory foci of the neonatal lung

Infection with cytomegalovirus (CMV) shows a worldwide high prevalence with only immunocompromised individuals or newborns to become symptomatic. The host's constitution and the pathogen's virulence determine whether disease occurs after infection. Mouse CMV (MCMV) is an appreciated pathogen for in vivo investigation of host-pathogen interactions. It has recently been reported that a single base pair deletion can spontaneously occur in the open reading frame of MCMV-encoded chemokine 2 (MCK2), preventing the expression of the full-length gene product. To study the consequences of this mutation, we compared the Mck2-defective reporter virus MCMV-3D with the newly generated repaired Mck2(+) mutant MCMV-3DR. Compared with MCMV-3D, neonatal mice infected with MCMV-3DR showed severe viral disease after lung infection. Viral disease coincided with high viral activity in multiple organs and increased virus replication in previously described nodular inflammatory foci (NIF) in the lung. Notably, MCMV-3DR showed tropism for alveolar macrophages in vitro and in vivo, whereas MCMV-3D did not infect this cell type. Moreover, in vivo depletion of alveolar macrophages reduced MCMV-3DR replication in the lung. We proposed an Mck2-mediated mechanism by which MCMV exploits alveolar macrophages to increase replication upon first encounter with the host's lung mucosa.

The early monocytic response to cytomegalovirus infection is MyD88 dependent but occurs independently of common inflammatory cytokine signals

Cytomegalovirus latently infects myeloid cells; however, the acute effects of the virus on this cell subset are poorly characterised. We demonstrate that systemic cytomegalovirus infection induced rapid activation of monocytes in the bone marrow, characterised by upregulation of CD69, CD11c, Ly6C and M-CSF receptor. Activated bone marrow monocytes were more sensitive to M-CSF and less sensitive to granulocyte-monocyte colony stimulating factor in vitro, resulting in the generation of more macrophages and fewer dendritic cells, respectively. Monocyte activation was also observed in the periphery and resulted in significant accumulation of monocytes in the spleen. MyD88 expression was required within the haematopoietic compartment to initiate monocyte activation and recruitment. However, monocytes lacking MyD88 were activated and recruited in the presence of MyD88-sufficient cells in mixed bone marrow chimeras, indicating that once initiated, the process was MyD88 independent. Interestingly, we found that monocyte activation occurred in the absence of the common inflammatory cytokines, namely type I interferons (IFNs), IL-6, TNF-α and IL-1 as well as the NLRP3 inflammasome adaptor protein, ASC. We also excluded a role for the chemokine-like protein MCK-2 (m131/129) expressed by murine CMV. Taken together, these results challenge the notion that a single inflammatory cytokine mediates activation and recruitment of monocytes in response to infection.

Specific history of heterologous virus infections determines anti-viral immunity and immunopathology in the lung

Having previously shown that previous immunity to one virus can influence the host response to a subsequent unrelated virus, we questioned whether the outcome to a given virus infection would be altered in similar or different ways by previous immunity to different viruses, and whether immunity to a given virus would have similar effects on all subsequent infections. In mouse models of respiratory viral infections, immunity to lymphocytic choriomeningitis virus (LCMV), murine cytomegalovirus (MCMV), or influenza A virus enhanced both Th1-type cytokine responses and viral clearance in the lung on vaccinia virus infection. A common pathological feature was the presence of chronic mononuclear infiltrates instead of the acute polymorphonuclear response seen in the infected nonimmune mice, but some pathologies such as enhanced bronchus-associated lymphoid tissue and bronchiolitis obliterans were unique for the immunizing virus, LCMV. Immunity to influenza virus influenced subsequent infections diversely, inhibiting vaccinia virus but enhancing LCMV and MCMV titers and completely altering cytokine profiles. Influenza virus immunity enhanced the mild mononuclear responses usually observed during acute infections with MCMV or LCMV in nonimmune mice, but unique features such as enhanced bronchiolization and mononuclear consolidation occurred during MCMV infection of influenza virus-immune mice. Heterologous immunity induced two patterns of disease outcome dependent on the specific virus infection sequence: improved, if the acute response switched from a neutrophilic to a lymphocytic response or worsened, if it switched from a mild to a severe lymphocytic response. Heterologous immunity thus occurs between many viruses, resulting in altered protective immunity and lung immunopathology, and this is influenced by the specific virus infection sequence.

Transcriptional analysis of the murine cytomegalovirus HindIII-I region: identification of a novel immediate-early gene region

Cytomegaloviruses likely encode numerous gene products involved in regulating virus-host cell interactions and pathogenesis. We previously identified a region of murine cytomegalovirus (MCMV) within HindIII-J and -I that regulates pathogenesis of the virus [open reading frames (ORFs) M139-M141] or is likely required for MCMV replication (ORFs m142 and m143). As a prerequisite for further studies on the structure and function of this gene region, we mapped the transcripts encoded within MCMV HindIII-I. Probes for ORFs M140 and M141 hybridized to 5.4- and 7.0-kb RNA, respectively, which were transcribed with early kinetics and were 3' coterminal with HindIII-J ORF M139. Probes representing ORFs m142, m143, or m144 hybridized to 3' coterminal transcripts of 1.8, 3.8, and 5.1 kb, respectively. ORFs m142 and m143 were transcribed with immediate-early kinetics but were most abundantly expressed at early times. Probes for the rightmost end of HindIII-I hybridized to a 5. 1-kb early/late RNA corresponding to m144 and to a 1.8-kb early RNA transcribed from m145. All of the major transcripts were polyadenylated and therefore are likely coding. Additional minor transcripts of intermediate sizes were also detected. ORFs M139-m143 showed homology to the betaherpesvirus-specific HCMV US22 gene family. Because deletion of these viral genes results in attenuated or helper-dependent phenotypes, this conserved region of US22 family genes may have a role in virus replication as well as in the pathogenesis of betaherpesviruses in their natural hosts.

Murine cytomegalovirus infection inhibits IFN gamma-induced MHC class II expression on macrophages: the role of type I interferon

Macrophage (M phi) activation, as measured by cell surface expression of MHC class II, was examined during infection of immunocompetent and immunocompromised mice with murine cytomegalovirus (MCMV). Intraperitoneal infection of CB17 SCID mice with 10(6) PFU of MCMV elicited a large population of M phi which expressed low levels of MHC class II. This was surprising since infection of SCID mice with lower doses (e.g., 10(4) PFU) of MCMV elicits M phi expressing high levels of MHC class II (M. T. Heise and H. W. Virgin, J. Virol. (1995) 69, 904-909). In vivo administration of recombinant mouse IFN gamma resulted in high levels of MHC class II expression on M phi from control but not MCMV-infected SCID mice, suggesting that MCMV infection generates a state in which IFN gamma is not effective at activating M phi. The effect of MCMV infection was MHC class II specific, since MHC class I and ICAM-1 levels were increased on M phi expressing low levels of MHC class II. Interference with IFN gamma action was not due to productive or abortive infection of M phi. This suggested that MCMV infection induces a soluble factor that alters M phi responsiveness to IFN gamma. Infection of SCID mice with 10(6) PFU of MCMV induced higher levels of serum IFN alpha beta (one candidate for inhibition of IFN gamma induction of MHC class II expression) than infection with 10(4) PFU. We therefore evaluated the role of MCMV-induced IFN alpha beta on IFN gamma responses of bone marrow-derived (BMM phi) or thioglycollate-elicited M phi in vitro. Infection of normal M phi with MCMV at a low m.o.i. (0.1 to 0.2) impaired IFN gamma-mediated induction of M phi MHC class II expression, but not MHC class I expression. Inhibition of IFN gamma responses was not observed in M phi from mice with a null mutation in the IFN alpha beta receptor (IFN alpha beta R-/-). To test the in vivo relevance of virus-induced IFN alpha beta to IFN gamma-mediated responses, the kinetics of MHC class II induction during MCMV infection of IFN alpha beta R-/- mice was evaluated. MCMV-infected IFN alpha beta R-/- mice mounted an earlier M phi MHC class II response than normal mice. We conclude that MCMV infection specifically impairs IFN gamma-mediated MHC class II expression on M phi and that induction of IFN alpha beta is one mechanism by which this inhibition occurs.

The T-cell-independent role of gamma interferon and tumor necrosis factor alpha in macrophage activation during murine cytomegalovirus and herpes simplex virus infections

We defined the normal and innate (without functional B or T cells) inflammatory response to infection with mouse cytomegalovirus (MCMV) or herpes simplex virus (HSV). Intraperitoneal infection with MCMV or HSV induced an inflammatory infiltrate consisting largely of macrophages (M phi) in both normal CB17 and severe combined immunodeficient (SCID) mice (lacking functional B or T cells). M phi from infected mice were activated as shown by (i) spread morphology, (ii) increased expression of major histocompatibility complex (MHC) class II, MHC class I, and intercellular adhesion molecule-1 molecules, and (iii) downregulation of M phi-specific cell surface protein F4/80. In vivo administration of neutralizing antibodies specific for gamma interferon (IFN gamma) or tumor necrosis factor alpha (TNF alpha) inhibited MHC class II induction on infiltrating M phi in both normal and CB17 SCID mice. Anti-TNF alpha decreased the number of M phi in virus-induced inflammatory exudates. The MCMV titer increased in the spleen and liver of IFN gamma-depleted SCID mice, while TNF alpha depletion increased only splenic titers. MCMV-induced pathology was also increased in spleens of IFN gamma- and TNF alpha-depleted SCID mice. We conclude that (i) M phi activation is a prominent part of inflammatory responses to herpesvirus infection and (ii) IFN gamma and TNF alpha play a critical role in both virus-induced M phi activation and control of herpesvirus growth independent of T and B cells. This suggests that IFN gamma- and TNF alpha-mediated M phi activation is an important aspect of innate immunity to viral infection. As the M phi may be involved in MCMV latency, IFN gamma- and TNF alpha-dependent M phi activation during primary infection may be relevant to establishment of viral latency.

Animal cytomegaloviruses

Cytomegaloviruses are agents that infect a variety of animals. Human cytomegalovirus is associated with infections that may be inapparent or may result in severe body malformation. More recently, human cytomegalovirus infections have been recognized as causing severe complications in immunosuppressed individuals. In other animals, cytomegaloviruses are often associated with infections having relatively mild sequelae. Many of these sequelae parallel symptoms associated with human cytomegalovirus infections. Recent advances in biotechnology have permitted the study of many of the animal cytomegaloviruses in vitro. Consequently, animal cytomegaloviruses can be used as model systems for studying the pathogenesis, immunobiology, and molecular biology of cytomegalovirus-host and cytomegalovirus-cell interactions.

Cytomegalovirus-infected adherent cells interact synergistically and antagonistically with Staphylococcus aureus protein A in vitro

Cytomegalovirus (CMV) has been shown to exert suppressive effects on the immune response but also to have mitogenic properties. A bacterial product, protein A from Staphylococcus aureus (SpA) was chosen to study possible interactions in vitro between bacterial products and adherent cells incubated with infectious CMV and ultraviolet light (UV)-inactivated CMV. Small amounts of infectious CMV potentiated SpA-induced DNA synthesis and Ig secretion measured by induction of plaque-forming cells (PFC). The reason for this may be that CMV in small amounts may act in synergism with the non-specific mitogen SpA. UV-inactivated CMV did not influence these responses except for a markedly enhanced PFC induction with SpA in lymphocytes from seronegative individuals. This remarkable synergism with SpA was also seen in enriched B cells. No synergism was seen in lymphocytes from seropositive donors. Large amounts of infectious CMV markedly reduced SpA-induced immune responses. Preliminary data suggest that the immunosuppressive effects are mediated by an interleukin 1 inhibitory factor. CMV was not shown to be a polyclonal B-cell activator but may, possibly in small amounts, act as such together with bacterial products, which would explain certain autoimmune phenomena. To conclude, CMV could in interaction with a bacterial product generate both synergistic and suppressive effects on immune response.

Immunopathological study of spleen during Japanese encephalitis virus infection in mice

Following intraperitoneal inoculation, Japanese encephalitis virus replicated in peritoneal macrophages, appeared on day 3 in the splenic macrophages of the perifollicular region and later in cells of the periarteriolar lymphoid sheath (PALS) as shown by indirect immunofluorescence. Productive JEV infection was observed both in macrophages and T-cells. Morphological study of spleen during JEV infection revealed proliferative changes, with increased number of macrophages from day 3 p.i. in the perifollicular region followed by accumulation of polymorphonuclear leucocytes which reached a maximum on day 9 p.i. The T dependent areas were considerably enlarged by day 9 and gradually reduced in size by week 3. At later periods germinal centres appeared in the T independent area and were prominent by day 15. The cells containing virus antigen disappeared with the appearance of germinal centres, thus indicating the role of the latter also in virus clearance.

Double labeling immunohistological study of African swine fever virus-infected spleen and lymph nodes

To identify cells in situ in which African swine fever (ASF) virus is present, a double immunohistological labeling technique was used on sections of ASF-infected spleen and lymph nodes. Cells were identified by an indirect immunoalkaline phosphatase technique using monoclonal antibodies (MoAb) reactive against different leukocyte subsets. ASF virus, detected by a direct immunoperoxidase method using swine immunoglobulin G (IgG) anti-ASF virus antigens, was not present in T helper or in T cytotoxic/suppressor lymphocytes, whereas it was detected in tissue macrophages that reacted with different MoAb (74-22-15, C4, A7, and F2). A large number of cells strongly reactive with MoAb 74-12-4 (T helper lymphocytes) were found in the marginal zone in infected spleen. In infected lymph nodes, these intensely stained cells were found in small numbers. Cells reactive with MoAb 76-2-11 (T cytotoxic/suppressor lymphocytes) were less stained in infected spleen and lymph nodes than in non-infected organs.

Pathogenesis of murine cytomegalovirus infection: identification of infected cells in the spleen during acute and latent infections

Spleen cells which replicate murine cytomegalovirus (MCMV) during acute infection in vivo were identified by electron microscopy and combined immunocytochemical staining and in situ cytohybridization. Most infected cells, as defined by in situ hybridization for viral RNA with MCMV-specific probes, were shown to be positive for factor VIII-related antigen and negative for Ia, Thy-1, and F4/80 antigens. Electron microscopic ultrastructural observations indicated that the infected cells in the spleen are predominantly sinusoidal-lining cells. We also studied reactivation of MCMV from latently infected mice by cocultivation of spleen cells with mouse embryo fibroblasts. Virus was only recovered from cells in preparations of stromal (or reticular) fragments, and not from spleen cell suspensions. Neither removal of immunoglobulin-bearing cells from the stromal fragments by panning nor depletion of Thy-1- and Ia-bearing stromal cells by treatment with monoclonal antibodies and complement reduced the frequency of reactivation of MCMV. These data suggest that T lymphocytes, mature B lymphocytes, and other Ia-bearing cells are not predominant reservoirs of latent MCMV.

The role of NK cell activity in age-dependent resistance of mice to murine cytomegalovirus infection

The resistance of mice to cell culture passaged murine cytomegalovirus (CC-MCMV) infection developed with age. In parallel with this finding, augmentation of the splenic NK cell activity in older mice was always higher than that of younger mice. The splenic NK cell activity reached the maximum level at 6 day post infection (PI) in 2-4-week-old mice while in 6-8-week-old mice it peaked at 4 days PI. When the dose of CC-MCMV was increased, the NK cell activity was potentiated accordingly. However, it was decreased on the infection with increased doses of the salivary gland passaged MCMV (SG-MCMV). NK cells augmented by MCMV infection actually inhibited in vitro replication of MCMV when they were added to mouse embryonic fibroblast (MEF) monolayers infected with CC-MCMV. Splenic and peritoneal macrophages inhibited in vitro replication of MCMV, but their activities were less potent than those of NK cells.

Studies of latent cytomegalovirus infection: the macrophage as a virus-harboring cell

During chronic infection of mice with mouse cytomegalovirus (MCMV), the virus was isolated from various tissues by cocultivation with allogeneic mouse embryonic fibroblasts (MEF). Infectious virus was recovered from over 15% of the pancreases, salivary glands, kidneys, lacrimal glands, and spleens. When activated macrophages were obtained by intraperitoneal injection of peptone into mice infected 3 months earlier, they harbored MCMV. Macrophages or lymphocytes were infected with MCMV in vitro and injected into normal mice intravenously. The peritoneal cavities of these mice were then stimulated by peptone injection 3 months after the transfer, and peritoneal or splenic macrophages and lymphocytes were cocultured with allogeneic MEF. MCMV was recovered from the peritoneal and splenic macrophages and not from the lymphocytes.