Regulation of maternal-fetal virus transmission in immunologically reconstituted SCID mice infected with lactate dehydrogenase-elevating virus

Viral Infections of Laboratory Mice

Viral infections of laboratory mice have considerable impact on research results, and prevention of such infections is therefore of crucial importance. This chapter covers infections of mice with the following viruses: herpesviruses, mousepox virus, murine adenoviruses, polyomaviruses, parvoviruses, lactate dehydrogenase-elevating virus, lymphocytic choriomeningitis virus, mammalian orthoreovirus serotype 3, murine hepatitis virus, murine norovirus, murine pneumonia virus, murine rotavirus, Sendai virus, and Theiler’s murine encephalomyelitis virus. For each virus, there is a description of the agent, epizootiology, clinical symptoms, pathology, methods of diagnosis and control, and its impact on research.

Transplacental murine cytomegalovirus infection in the brain of SCID mice

BACKGROUND

Congenital cytomegalovirus (CMV) infection is the most common congenital viral infection in humans and the major nonhereditary cause of central nervous system (CNS) developmental disorders. Previous attempts to develop a murine CMV (MCMV) model of natural congenital human CMV (HCMV) infection have failed because MCMV does not cross the placenta in immunocompetent mice.

RESULTS

In marked contrast with immunocompetent mice, C.B-17 SCID (severe combined immunodeficient) mice were found to be highly susceptible to natural MCMV transplacental transmission and congenital infection. Timed-pregnant SCID mice were intraperitoneally (IP) injected with MCMV at embryonic (E) stages E0-E7, and vertical MCMV transmission was evaluated using nested polymerase chain reaction (nPCR), in situ hybridization (ISH) and immunohistochemical (IHC) assays. SCID mouse dams IP injected at E0 with 102 PFU of MCMV died or resorbed their fetuses by E18. Viable fetuses collected at E18 from SCID mice IP injected with 102-104 PFU of MCMV at E7 did not demonstrate vertical MCMV transmission. Notably, transplacental MCMV transmission was confirmed in E18 fetuses from SCID mice IP injected with 103 PFU of MCMV at stages E3-E5. The maximum rate of transplacental MCMV transmission (53%) at E18 occurred when SCID mouse dams were IP injected with 103 PFU of MCMV at E4. Congenital infection was confirmed by IHC immunostaining of MCMV antigens in 26% of the MCMV nPCR positive E18 fetuses. Transplacental MCMV transmission was associated with intrauterine growth retardation and microcephaly. Additionally, E18 fetuses with MCMV nPCR positive brains had cerebral interleukin-1alpha (IL-1alpha) expression significantly upregulated and cerebral IL-1 receptor II (IL-1RII) transcription significantly downregulated. However, MCMV-induced changes in cerebral cytokine expression were not associated with any histological signs of MCMV infection or inflammation in the brain.

CONCLUSION

Severe T- and B-cell immunodeficiencies in SCID mice significantly enhance the rate of natural MCMV transplacental transmission and congenital infection. During gestation MCMV exhibits a tissue tropism for the developing brain, and vertical MCMV transmission is correlated with fetal growth retardation and abnormal cerebral proinflammatory cytokine expression. These data confirm that natural vertical MCMV infection in SCID mice constitutes a useful new experimental rodent model of congenital HCMV infection.

Glucocorticoid regulation of lactate dehydrogenase-elevating virus replication in macrophages

Lactate dehydrogenase-elevating virus (LDV) is a macrophage-tropic arterivirus which generally causes a persistent viremic infection in mice. LDV replication in vivo seems to be primarily regulated by the extent and dynamics of a virus-permissive macrophage population. Previous studies have shown that glucocorticoid treatment of chronically LDV-infected mice transiently increases viremia 10-100-fold, apparently by increasing the productive infection of macrophages. We have further investigated this phenomenon by comparing the effect of dexamethasone on the in vivo and in vitro replication of two LDV quasispecies that differ in sensitivity to immune control by the host. The single neutralizing epitope of LDV-P is flanked by two N-glycans that impair its immunogenicity and render LDV-P resistant to antibody neutralization. In contrast, replication of the neuropathogenic mutant LDV-C is suppressed by antibody neutralization because its epitope lacks the two protective N-glycans. Dexamethasone treatment of mice 16 h prior to LDV-P infection, or of chronically LDV-P infected mice, stimulated viremia 10-100-fold, which correlated with an increase of LDV permissive macrophages in the peritoneum and increased LDV infected cells in the spleen, respectively. The increase in viremia occurred in the absence of changes in total anti-LDV and neutralizing antibodies. The results indicate that increased viremia was due to increased availability of LDV permissive macrophages, and that during a chronic LDV-P infection virus replication is strictly limited by the rate of regeneration of permissive macrophages. In contrast, dexamethasone treatment had no significant effect on the level of viremia in chronically LDV-C infected mice, consistent with the view that LDV-C replication is primarily restricted by antibody neutralization and not by a lack of permissive macrophages. beta-Glucan, the receptor of which is induced on macrophages by dexamethasone treatment, had no effect on the LDV permissiveness of macrophages.

Transplacental lactate dehydrogenase-elevating virus (LDV) transmission: immune inhibition of umbilical cord infection, and correlation of fetal virus susceptibility with development of F4/80 antigen expression

Maternal-to-fetal transmission of the murine lactate dehydrogenase-elevating virus (LDV) has been previously shown to be regulated by maternal immunity as well as gestational age. For the present study, the role of maternal immunity in placental and umbilical cord virus protection was studied, and virus targeting of umbilical cord and fetal macrophages was correlated with expression of the F4/80 macrophage phenotypic marker. The results showed that LDV-infected macrophages appeared in umbilical cord by 24 h post-infection of pregnant mice, and some LDV-infected macrophages displayed the F4/80 phenotype. This potential reservoir of virus for the fetus was inhibited by passive immunization of pregnant mice with IgG anti-LDV antibodies, which rapidly concentrated in the placenta and umbilical cord. Probing of umbilical cord cells with antibodies directed at MHC genetic markers demonstrated the presence of both maternal and fetal cells in umbilical cords. A strong developmental correlation was observed between fetal F4/80 expression and LDV susceptibility, at about 13.6 days of gestation. These results demonstrate immune suppression of free and cell-associated virus in umbilical cord, thus defining a potentially important mechanism for immune protection of the fetus from transplacental virus infection. The results also clarify the developmental basis for fetal susceptibility to LDV infection.

Regulation of immune complexes during infection of mice with lactate dehydrogenase-elevating virus: studies with interferon-gamma gene knockout and tolerant mice

Mice persistently infected with lactate dehydrogenase-elevating virus (LDV) develop circulating IgG-containing hydrophobic immune complexes, with a molecular mass of 150 to 300 kd, which bind to the surfaces of high-capacity enzyme-linked immunosorbent assay (ELISA) plates. LDV infection also stimulates polyclonal B-cell activation and autoimmunity. For this study, interferon-gamma gene knockout (GKO) mice were utilized to study circulating immune complexes and other parameters of LDV infection. The kinetics of LDV viremia, formation of plasma IgG anti-LDV antibodies, and LDV replication in the spleen and liver were essentially normal in GKO mice. Polyclonal activation of B cells, as reflected by increased total plasma IgG concentration during LDV infection, was found to be intact in GKO mice, although at a lower magnitude than in control mice. The plasma concentration of IgG-containing hydrophobic immune complexes was reduced about 75% in LDV-infected GKO mice relative to normal LDV-infected controls. Allogeneic tissue responses were also found to be reduced in LDV-infected GKO mice relative to those in normal LDV-infected controls. These results dissociate specific anti-LDV immunity from formation of hydrophobic immune complexes, show that the IgG anti-LDV response as well as LDV replication in the spleen and liver are insensitive to physiological levels of interferon (IFN)-gamma, and suggest that IgG-containing immune complexes stimulated by LDV infection are a marker for autoimmunity.

Inhibition of virus-induced age-dependent poliomyelitis by interferon-gamma

Age-dependent poliomyelitis (ADPM) is a neuroparolytic disease which results from combined infection of susceptible mice with lactate dehydrogenase-elevating virus (LDV) and murine leukemia virus (MuLV). The present study examined the effects of interferon-gamma (IFN-gamma) treatment on the incidence of ADPM, replication of LDV and MuLV and anti-LDV immunity. IFN-gamma treatment of ADPM-susceptible C58/M mice protected them from paralytic disease, but had no detectable effect on the IgG anti-LDV response or LDV viremia. IFN-gamma-mediated protection from ADPM correlated with reduced expression of LDV RNA, but not MuLV RNA, in the spinal cords of C58/M mice. These results confirm that spinal cord LDV replication is the determinant of ADPM and demonstrate that cytokine-mediated inhibition of LDV replication in the central nervous system prevents neuroparalytic disease.

Regulation of transplacental virus infection by developmental and immunological factors: studies with lactate dehydrogenase-elevating virus

Placental and fetal infections with lactate dehydrogenase-elevating virus (LDV) were determined by virus titration, indirect fluorescence antibody (IFA), and in situ hybridization with cDNA probes. Experiments were designed to determine the effects of gestational age, timing of maternal LDV infection, and immunological (antibody and cytokine) factors on mouse placental and fetal LDV infection. Virus infection of the placenta was detected at high levels (almost all placentas infected) within 24 h post-maternal infection (p.m.i.), whereas fetal LDV infection was detected only at a low level by 24 h p.m.i. The percentage of fetuses becoming LDV infected progressively increased between 24 and 72 h p.m.i. When fetal infection was studied at 72 h p.m.i., earlier gestational ages (9-11 days) were associated with fetal resistance to infection, whereas between 12.5 and 15 days of gestation, virus infection was detected in 50-71% of fetuses. Maternal treatment with interferon-gamma (IFN-gamma) or anti-LDV monoclonal antibodies was associated with reduced rates of fetal, but not placental, LDV infection. These results demonstrate that both developmental and immunological factors are important in the regulation of transplacental LDV infection.

Cytokine regulation of lactate dehydrogenase-elevating virus: inhibition of viral replication by interferon-gamma

The mechanisms which regulate the replication of lactate dehydrogenase-elevating virus (LDV), a persistent murine model virus which infects macrophages, are unclear. For this study, the effects of murine recombinant interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on LDV replication were examined. LDV permissiveness was reduced in macrophages obtained from uninfected mice treated with IFN-gamma prior to cell harvest and in vitro LDV infection. Virus inhibition by IFN-gamma was also observed when neonatal LDV-infected mice were injected with this cytokine prior to macrophage harvest and analysis of LDV replication-positive cells. Persistently LDV-infected mice demonstrated an increase in viremia levels following treatment with TNF-alpha. Neither IFN-gamma nor TNF-alpha had any direct in vitro effect on LDV replication in cultured macrophages, suggesting that the actions of these cytokines required secondary or accessory in vivo events. These results provide evidence for cytokine-mediated regulation of LDV infection and support a role for the immune system in the LDV-host relationship.