In vivo generation of antigenic variants of murine retroviruses

Response of aged mice to primary virus infections

Aging is associated with an increased morbidity to virus infections as well as a delay in clearance of symptoms after infection. Studies of sublethal virus infections of aged mice closely mirror the human situation: there is a delay in clearance of virus. The delay in virus clearance is accompanied by a delay and a decrease in T-cell response, particularly of CD8(+) T cells. Intrinsic alterations of T cells of aged mice contribute to this decrease in virus-specific T-cell response; however, evidence suggests that environmental or innate components of the aged host also influence this age-associated decline in clearance of virus. While the changes in the adaptive immune response have been carefully described, the early events in the generation of the T-cell response after virus infection have received limited attention. Importantly, age-associated changes in the innate response to virus infection, particularly production of and response to interferon (IFN)-alpha/beta, cytotoxicity and IFN-gamma production by natural killer cells, interleukin-12 induction, and depletion of non-specific T cells early during virus infection need further evaluation.

Cuttine edge: T cells from aged mice are resistant to depletion early during virus infection

Aging is associated with decreased expansion of T cells upon stimulation. In young mice, infection induces a transient T cell depletion followed by the development of an Ag-specific T cell response that controls the infection. We found that T cells were depleted early after infection with E55 + murine leukemia retrovirus in young, but not aged, mice. Adoptive transfer experiments showed donor T cells of young, but not aged, mice were depleted due to apoptosis in various tissues of young recipients. However, T cells of neither young nor aged donors were depleted in aged recipients. These results indicate that both environmental and intrinsic cellular properties limit depletion of T cells of aged mice and suggest a novel explanation for the decreased T cell response associated with aging.

Decreased IL-2, IFN-gamma, and IL-10 production by aged mice during the acute phase of E55+ retrovirus infection

We previously reported that aged mice demonstrated a 12-week delay in virus clearance compared to young mice after infection with E55+ murine leukemia retrovirus (E55+MuLV). The current study demonstrates that both the levels of IL-2, IFN-gamma, and IL-10 and the number of cells producing IL-2 and IFN-gamma were lower at 2 and 4 weeks postinfection (p.i.) in aged compared to young mice after virus-specific stimulation of spleen cells in vitro. In both age groups, IL-2 and IL-10 were produced by CD4(+) T and B cells, respectively. IFN-gamma was produced mainly by CD4(+) T cells at 2 weeks p.i. and by CD4(+) and CD8(+) T cells at 4 weeks p.i. in young, but primarily by CD8(+) T cells, in aged mice. Therefore, delayed virus clearance is associated with age-related decreases in type 1 and type 2 cytokines and a shift in the primary source of at least one cytokine.

Prolonged E55+ retrovirus expression in aged mice is associated with a decline in the anti-virus immune response

E55+ murine leukemia retrovirus (E55+ MuLV) infection of young and aged C57BL/6 (B6) mice was used to investigate the relationship between increased incidences of infection and decreased immune responsiveness of elderly individuals. Young mice decreased E55+ MuLV burden to below detectable levels by 8 weeks postinfection (p.i.). In contrast, virus burden in aged mice did not reach undetectable levels until 20 weeks p.i. A significant T cell proliferative response to E55+ MuLV was detected from 2 to 12 weeks p.i. in young mice, but was never observed in aged mice. Both age groups demonstrated significant E55+ MuLV-specific T-cell-mediated cytotoxic responses at 3 and 4 weeks p.i. and virus neutralizing antibody titers at 2, 4, 8, and 12 weeks p.i. In both cases, responses were consistently higher in young mice (P < 0.04 and P < 0.02, respectively). These results demonstrate that the observed delay in E55+ MuLV clearance by aged mice is associated with an age-related decrease in the immune response to the virus.

Genetic regulation of long-term nonprogression in E-55+ murine leukemia virus infection in mice

Certain inbred mouse strains display progression to lymphoma development after infection with E-55+ murine leukemia virus (E-55+ MuLV), while others demonstrate long-term nonprogression. This difference in disease progression occurs despite the fact that E-55+ MuLV causes persistent infection in both immunocompetent BALB/c-H-2(k) (BALB.K) progressor (P) and C57BL/10-H-2(k) (B10.BR) long-term nonprogressor (LTNP) mice. In contrast to immunocompetent mice, immunosuppressed mice from both P and LTNP strains develop lymphomas about 2 months after infection, indicating that the LTNP phenotype is determined by the immune response of the infected mouse. In this study, we used bone marrow chimeras to demonstrate that the LTNP phenotype is associated with the genotype of donor bone marrow and not the recipient microenvironment. In addition, we have mapped a genetic locus that may be responsible for the LTNP trait. Microsatellite-based linkage analysis demonstrated that a non-major histocompatibility complex gene on chromosome 15 regulates long-term survival and is located in the same region as the Rfv3 gene. Rfv3 is involved in recovery from Friend virus-induced leukemia and has been demonstrated to regulate neutralizing virus antibody titers. In our studies, however, both P and LTNP strains produce similar titers of neutralizing and cytotoxic anti-E-55+ MuLV. Therefore, while it is possible that Rfv3 influences the course of E-55+ MuLV infection, it is more likely that the LTNP phenotype in E-55+ MuLV-infected mice is regulated by a different, closely linked gene.

Differences in the Immune Response During the Acute Phase of E-55+ Murine Leukemia Virus Infection in Progressor BALB and Long Term Nonprogressor C57BL Mice

E-55+ murine leukemia virus infection of both progressor (BALB) and long term nonprogressor (C57BL) mouse strains is characterized by an acute and a persistent phase of infection. During the acute phase, progressor strains require CD8+ T cells to decrease virus burden, whereas the long term nonprogressor strains do not. In the present studies the immune response in BALB and C57BL mice during the acute phase of E-55+ murine leukemia virus infection was examined. The results demonstrate that BALB mice produce both IL-4 and IFN-γ, in contrast to C57BL mice, which produce only IFN-γ. In BALB mice, IL-4 production results in the absolute requirement for CD8+ T cells to reduce the virus burden during the acute phase of infection. The anti-virus immune response in these mice is IFN-γ dependent. On the other hand, C57BL mice do not produce IL-4 and, in the absence of both CD8+ T cells and IFN-γ, still generate an effective anti-virus immune response. Genetic studies suggest that these distinct immune responses are regulated by more than one non-MHC-linked gene. Two candidate regions that may encode this gene(s), located on chromosomes 7 and 19, respectively, were identified by recombinant inbred strain linkage analysis.

Role of T-cell subsets in acute and persistent E-55+ murine leukemia virus infection in susceptible progressor and resistant long-term nonprogressor mouse strains. Women and Infants Transmission Study

Previous studies from this laboratory have demonstrated that E-55+MuLV-infected BALB/c-H-2k (BALB.K) mice progress to develop thymic lymphoma about 7 months after infection whereas infected C57BL/10-H-2k (B10.BR) mice are long-term nonprogressors that fail to develop disease even after 2 years of infection. Both resistant long-term nonprogressor (B10.BR) and progressor (BALB.K) mice generate an early immune response that results in a dramatic decrease in the number of virus-infected cells. Despite this early immune response, mice from both strains become persistently infected. However, resistant B10.BR mice also demonstrate a late T-cell-mediated response that may be causally related to long-term nonprogression whereas susceptible BALB.K mice fail to demonstrate this late T-cell response. In the present studies, the T-cell subsets involved in the effective early immune response in both B10.BR and BALB.K mice as well as the late T-cell response in B10.BR mice were determined by in vivo antibody-mediated depletion. Results from these studies demonstrate that during the early acute phase of infection, elimination of CD4+ T cells ablated the ability of both BALB.K and B10.BR mice to decrease the burden of virus-infected cells. However, elimination of CD8+ T cells ablated this result in BALB.K but not B10.BR mice. Thus, despite the fact that both immunocompetent B10.BR and BALB.K mice are able to decrease the number of virus-infected cells during the early acute phase of infection, there is a difference in the T-cell subsets that mediate this effect in these strains of mice. In addition, characterization of the late immune response that keeps virus at very low levels during the persistent stage of virus infection in resistant B10.BR mice demonstrated that simultaneous elimination of both CD4+ and CD8+ T cells allowed the emergence of virus-infected cells whereas the elimination of either subset alone showed no effect compared to untreated control mice that are immunologically intact. Since B10.BR and BALB.K are identical with respect to their H-2k-haplotypes, it appears that the differences between these strains with respect to the generation of effective early and late anti-virus immune responses are regulated by a non-H-2-linked gene(s).

A shift in the requirement for CD4+ T cells in the generation of AKR/Gross MuLV-specific CTL in AKR.H-2b:Fv-1b mice occurs prior to the onset of age-dependent CTL nonresponsiveness

In the current study, the elimination of CD4+ T cells from B6 mice, by treatment with anti-CD4 monoclonal antibody, had little effect on their ability to mount an AKR/Gross (MuLV)-specific CTL response. In contrast, for AKR.H-2b:Fv-1b mice, there was a shift as the mice aged from 5 to 7 weeks to a requirement for CD4+ T cells for AKR/Gross MuLV-specific CTL generation. When CD4+ T-cell-depleted AKR.H-2b:Fv-1b responder mice were immunized at 5 weeks of age they were able to elicit a strong anti-AKR/Gross MuLV CTL response. However, if the CD4+ T-cell depletion was done at 6 weeks and then the mice were primed in vivo, their antiviral CTL responsiveness was markedly decreased. Following CD4+ T-cell depletion at 7 weeks the mice were totally incapable of generating anti-AKR/Gross MuLV-specific CTL. AKR/Gross MuLV-specific CTL isolated from AKR.H-2b:Fv-1b mice recognized the class I-restricted immunodominant epitope (KSPWFTTL) and three subdominant epitopes, previously identified as CTL epitopes for B6 mice. Analysis of IL-2, IFN-gamma, IL-4, and IL-10 lymphokine profiles in supernates harvested from MLTC wells, and the results of supernate transfer experiments, suggested that the age-dependent shift to CD4+ T-cell dependence in AKR.H-2b:Fv-1b mice does not correlate with an obvious change in the in vitro lymphokine profiles. Experiments in which exogenous IL-2 was used to supplement in vitro cultures containing CD4+ T-cell-depleted 7-week responder mice suggested that the CD4+ T-cell requirement was at the in vivo priming stage of antiviral CTL generation. These data suggested a fundamental change in virus-specific CTL which correlates with slight aging in the AKR.H-2b:Fv-1b mouse strain. To our knowledge, this is the first report of a shift in the requirement for CD4+ T lymphocytes for the generation of virus-specific CTL over such a short period of time. Moreover, it is of interest that this shift in CD4+ T-cell-dependence by antiviral CTL occurs just prior to the onset of CTL nonresponsiveness in the AKR.H-2b:Fv-1b mouse strain.

Interactions between Exogenous and Endogenous Retroviruses

Retroviruses are distinguished from other viruses by several features. Notably, some retroviruses are present as normal elements in the genomes of virtually all vertebrates (endogenous proviruses). Others are exogenous, i.e. horizontally transmitted agents, many of which cause fatal diseases. The endogenous retroviruses are genetically transmitted and to a large extent their significance is uncertain. However, there is evidence suggesting that they contribute to the development of diseases in several animal species. Most importantly, some endogenous retroviruses are capable of interacting with exogenous counterparts through a variety of different mechanisms with serious consequences to the host. Conversely, others are advantageous in that they protect against exogenous retroviruses. In this review various types of interactions between endogenous and exogenous retroviruses are discussed, including receptor interference, recombination, phenotypic mixing, immunological interactions and heterologous trans-activation. Copyright 1997 S. Karger AG, Basel

Environmental lighting alters the infection process in an animal model of AIDS

In this study, we examined the effects of altered environmental lighting on the infection process of a murine leukemia virus, E-55(+), which induces a thymic lymphoma/leukemia in 100% of BALB.K mice inoculated as adults. One to two weeks after inoculation, high levels of proviral DNA are usually found. This is followed by an asymptomatic period of many weeks during which proviral DNA becomes essentially undetectable. Leukemia develops approximately 28 weeks postinoculation. In this experiment, one group of mice was exposed a consistent 10L: 14D cycle while a second was maintained in constant light (LL). A third group was exposed to a rotating cycle characterized by phase shifting a 10L: 14D cycle every three 24-h days (rLD). All cycles began 2 weeks prior to inoculation and were maintained thereafter. Animals were sacrificed at 1, 5, 10, and 15 weeks, and hematopoietic tissue was examined for proviral DNA content. At 1 week, LL- and rLD-exposed animals showed considerably less proviral DNA in bone marrow and spleen compared with controls. At 15 weeks, thymuses from controls were showing signs of infection whereas tissue from LL and rLD mice remained at background levels. We conclude that environmental lighting does alter the infective pattern displayed by this retrovirus, although whether this effect is mediated by changes in the target stem cells or through immunoenhancement has not yet been determined.

Impaired immune responsiveness is an essential component in persistent central nervous system infection with gross murine leukemia virus

Exposure of newborn mice to Gross murine leukemia virus (GMuLV) results in persistent viral infection of the central nervous system (CNS) white matter. Animals exposed to virus as neonates showed a marked depression in GMuLV-specific B lymphocyte function as evidenced by significant decreases in adult and neonatal anti-GMuLV antibody levels. Immunohistochemical analyses showed that the sites of GMuLV infection in the CNS were also devoid of major histocompatibility complex (MHC) class I and II protein expression, although transplantation of GMuLV-infected brain tissue to the kidney capsules of immunocompetent mice induced a potent mononuclear cell graft infiltrate. These results indicate that persistent GMuLV infection of the CNS is linked to both impairment of anti-GMuLV peripheral immune responses and deficient antigen-presenting cell function within the CNS.

Neonatal exposure to thymotropic gross murine leukemia virus induces virus-specific immunologic nonresponsiveness

Neonatal exposure to Gross murine leukemia virus results in a profound inhibition of the virus-specific T and B cell responses of adult animals. Animals exposed to virus as neonates exhibit a marked depression in virus-specific T cell function as measured by the virtual absence of in vivo delayed type hypersensitivity responses and in vitro proliferative responses to virally infected stimulator cells. Further, serum obtained from neonatally treated mice failed to either immunoprecipitate viral proteins or neutralize virus in an in vitro plaque assay, suggesting the concurrent induction of a state of B cell hyporesponsiveness. The specificity of this effect at the levels of both T and B cells was demonstrated by the ability of neonatally treated mice to respond normally after adult challenge with either irrelevant reovirus or influenza virus. The replication of Gross virus within both stromal and lymphocytic compartments of the neonatal thymus suggests that thymic education plays a key role in the induction of immunologic nonresponsiveness to viruses.

Viral escape by selection of cytotoxic T cell-resistant virus variants in vivo

Viruses persist in an immune population, as in the case of influenza, or in an individual, as postulated for human immunodeficiency virus, when they are able to escape existent neutralizing antibody responses by changing their antigens. It is now shown that viruses can in principle escape the immunosurveillance of virus-specific cytotoxic T cells by mutations that alter the relevant T-cell epitope.

A host gene regulates the structure of the transmembrane envelope protein of murine leukemia viruses

Heterogeneity in the structure of the envelope proteins has been observed in many human and animal retroviruses and may influence pathogenicity. However, the biological significance of this heterogeneity and the mechanisms by which it is generated are poorly understood. We have studied a mouse model in which the envelope gene structure of lymphoma-associated viruses appears to be controlled by a single host gene. The inoculation of HRS and CWD mice with a leukemogenic murine leukemia virus (MuLV) results in recombination between the injected virus and envelope gene sequences of endogenous retroviruses. The genomes of HRS (class I) env recombinants and CWD (class II) env recombinants differ in the sequences encoding the NH2-terminal portion of the transmembrane envelope protein (TM). We have shown that an HRS gene linked to the MHC on chromosome 17 mediates a dominant selection for recombinant retroviruses with the class I envelope gene structure. CBA mice, which share the H-2k haplotype with HRS, also carry the dominant allele at this locus. This system provides a useful model for studies of host factors involved in the selection of specific variants of pathogenic retroviruses.