Immune response to mouse mammary tumor virus in mice lacking the alpha/beta interferon or the gamma interferon receptor

Interferon-gamma plasma levels and presence of mouse mammary tumor virus-like env gene: Implications on the pathogenesis of breast cancer

Mouse mammary tumor virus (MMTV) is a retrovirus that has been associated with the development of breast cancer (BC) in mice. The identification of a 95% homologous gene sequence to MMTV in human BC samples has increased interest in this hypothesis. This virus in humans received the name of mouse mammary tumor virus-like (MMTV-like). Several cytokines may be involved in the interactions between MMTV and the immune system, such as interferon-gamma (IFN-γ), which can enhance Th1-mediated antitumor immune response but it can also play a protumorigenic role by transmitting antiapoptotic and proliferative signals. Little is known about the antiviral immune response in a microenvironment with the presence of MMTV-like in BC patients. Therefore, the purpose of the present study was to quantify the plasma levels of IFN-γ in the peripheral blood of 123 neoplasia-free donors and 98 BC patients of different molecular subtypes, by enzyme-linked immunosorbent assay (ELISA), and evaluate the association of these plasma levels with the detection of the MMTV-like env gene in tumor tissue. Correlation analyzes involving IFN-γ plasma levels and clinical-pathological parameters were performed by Kendall Tau-c test. In our study, a decrease in IFN-γ levels was observed in the group of BC patients (30.85 ± 57.49 pg/ml) compared to the control group (115.00 ± 176.80 pg/ml) (p < 0.0001). In the analysis by stratified BC molecular subtypes, Luminal-A (30.79 ± 61.04 pg/ml; p < 0.0001), Luminal-B (24.74 ± 25.78 pg/ml; p = 0.0188) and triple-negative (23.95 ± 40.45 pg/ml; p = 0.0005) had a lower plasma level compared to control group. There was no significant difference between IFN-γ plasma levels of MMTV-like DNA positive samples compared to MMTV-negative samples (p = 0.2056). In general BC, patients with larger tumor size had higher IFN-γ plasma levels (Tau-c = 0.202; p = 0.019). By analyzing the MMTV-like env negative samples, we could identify that IFN-γ plasma levels were higher in larger tumor size (Tau-c = 0.222; p = 0.020) and with greater lymph node involvement (Tau-c = 0.258; p = 0.042). Also, higher IFN-γ plasma levels were observed in patients with higher histopathological grades (Tau-c = 0.384; p = 0.019) in MMTV-like env positive samples. For the first time, we assessed the association between plasma levels of IFN-γ and the presence of the MMTV-like env gene in BC samples. However, more studies are needed to clarify whether the high levels of IFN-γ in MMTV-like env positive samples are reflecting a possible antiviral immune response or whether this cytokine is promoting tumor growth.

Interferon gamma modulation of disease manifestation and the local antibody response to alphavirus encephalomyelitis

Infection of mice with Sindbis virus (SINV) produces encephalomyelitis and provides a model for examination of the central nervous system (CNS) immune response to alphavirus infection. Clearance of infectious virus is accomplished through a cooperative effort between SINV-specific antibody and IFN-γ, but the regulatory interactions are poorly understood. To determine the effects of IFN-γ on clinical disease and the antiviral immune response, C57BL/6 mice lacking IFN-γ (Ifng-/-) or IFN-γ receptor (Ifngr1-/-) were studied in comparison to WT mice. Maximum production of Ifng mRNA and IFN-γ protein in the CNS of WT and Ifngr1-/- mice occurred 5-7 days after infection, with higher levels of IFN-γ in Ifngr1-/- mice. Onset of clinical disease was earlier in mice with impaired IFN-γ signalling, although Ifngr1-/- mice recovered more rapidly. Ifng-/- and Ifngr1-/- mice maintained body weight better than WT mice, associated with better food intake and lower brain levels of inflammatory cytokines. Clearance of infectious virus from the spinal cords was slower, and CNS, but not serum, levels of SINV-specific IgM, IgG2a and IgG2b were lower in Ifngr1-/- and Ifng-/- mice compared to WT mice. Decreased CNS antiviral antibody was associated with lower expression of mRNAs for B-cell attracting chemokines CXCL9, CXCL10 and CXCL13 and fewer B cells in the CNS. Therefore, IFN-γ signalling increases levels of CNS pro-inflammatory cytokines, leading to clinical disease, but synergistically clears virus with SINV-specific antibody at least in part by increasing chemokine production important for infiltration of antibody-secreting B cells into the CNS.

Endogenous MMTV proviruses induce susceptibility to both viral and bacterial pathogens

Most inbred mice carry germline proviruses of the retrovirus, mouse mammary tumor virus (MMTV) (called Mtvs), which have multiple replication defects. A BALB/c congenic mouse strain lacking all endogenous Mtvs (Mtv-null) was resistant to MMTV oral and intraperitoneal infection and tumorigenesis compared to wild-type BALB/c mice. Infection of Mtv-null mice with an MMTV-related retrovirus, type B leukemogenic virus, also resulted in severely reduced viral loads and failure to induce T-cell lymphomas, indicating that resistance is not dependent on expression of a superantigen (Sag) encoded by exogenous MMTV. Resistance to MMTV in Mtv-null animals was not due to neutralizing antibodies. Further, Mtv-null mice were resistant to rapid mortality induced by intragastric inoculation of the Gram-negative bacterium, Vibrio cholerae, but susceptibility to Salmonella typhimurium was not significantly different from BALB/c mice. Susceptibility to both MMTV and V. cholerae was reconstituted by the presence of any one of three endogenous Mtvs located on different chromosomes and was associated with increased pathogen load. One of these endogenous proviruses is known to encode only Sag. Therefore, Mtv-encoded Sag appears to provide a unique genetic susceptibility to specific viruses and bacteria. Since human endogenous retroviruses also encode Sags, these studies have broad implications for pathogen-induced responses in mice and humans.

The role of IL-12, IL-23 and IFN-gamma in immunity to viruses

IL-12, IL-23 and IFN-γ form a loop and have been thought to play a crucial role against infectious viruses, which are the prototype of “intracellular” pathogens. In the last 10 years, the generation of knock-out (KO) mice for genes that control IL-12/IL-23-dependent IFN-γ-dependent mediated immunity (STAT1, IFN-γR1, IFNγR2, IL-12p40 and IL-12Rβ1) and the identification of patients with spontaneous germline mutations in these genes has led to a re-examination of the role of these cytokines in anti-viral immunity. We here review viral infections in mice and humans with genetic defects in the IL-12/IL-23-IFN-γ axis. A comparison of the phenotypes observed in KO mice and deficient patients suggests that the human IL-12/IL-23-IFN-γ axis plays a redundant role in immunity to most viruses, whereas its mouse counterparts play a more important role against several viruses.

Impact of aging upon DBA/2J B cells

The influence of age on B lymphocyte phenotype and function in DBA/2J mice was examined. The B cells of this strain express the endogenous minor lymphocyte stimulatory (Mls) retroviral superantigen (SAg) Mls-1a permitting assessment of age-related changes in cognate B cell-T cell interaction. Relative to young DBA/2J mice (< 8 months), old mice (> 17 months) had greater numbers of B cells expressing high levels of IgM and low levels of the CD11b and CD5 antigens characteristic of B-1 B cells. As measured by the T cell proliferative response to Mls, the B cells from old DBA/2J mice had reduced ability to present SAg. Upon interaction with Mls-activated T cells, old B cells secreted more IgM while young B cells made more IgG1, IgG3, and IgG2a. DBA/2J BCL functioned poorly as Mls APCs and made considerably less serum Ig. T cells from old mice exhibited a lower response to SAg and were less capable of promoting B cell differentiation. These results indicate that aging influences the cellular collaboration necessary for humoral immunity.

Interleukin-4 up-regulates mouse mammary tumor virus expression yet is not required for in vivo virus spread

The mouse mammary tumor virus (MMTV) superantigen induces T-cell production of cytokines, such as interleukin-4, which in turn increase MMTV transcription. However, interleukin-4 is not required for in vivo virus spread, because mice lacking interleukin-4 or the STAT6 transcription factor showed wild-type infection of lymphoid and mammary tissue. In spite of this, mammary tumor incidence was decreased in STAT6 null mice.

IFN-gamma-dependent and -independent initiation of switch recombination by NK cells

We have examined the effect of IL-2-propagated NK or NK-T cells on each of the steps required for B cell switch recombination leading to IgG2a production. The results indicate that NK cells, on their own and in the absence of IFN-gamma, can induce germline transcription in resting, IgG(-) B lymphocytes from the gamma2a locus as well as mRNA for activation-induced cytidine deaminase (AID) via a process that requires cell-cell interactions. The results also show that, in contrast to induction by T cells, activation by NK cells does not involve CD40-CD40 ligand interactions and does not extend to the induction of Igamma1 transcription. Furthermore, in contrast to stimulation by LPS and IFN-gamma or by T cells, the activation events initiated by NK cells do not result in significant synthesis of functional gamma2a mRNA in resting B lymphocytes even in the presence of IFN-gamma. Thus, induction of germline and AID transcripts are necessary but not sufficient events for functional switching to IgG2a. These experiments, showing that NK cells themselves cannot induce IgG2a production but can polyclonally program B lymphocytes so that they preferentially switch to this isotype may explain how activated NK cells can skew the Ag-specific immune response toward IgG2a. The findings also provide further demonstration of the definitive yet limited extent of how a non-Ag-specific component of the innate system can modulate the direction of the adaptive immune response.

C3H mouse mammary tumor virus superantigen function requires a splice donor site in the envelope gene

Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is required for efficient milk-borne transmission of virus from mothers to offspring. The mRNA used for Sag expression is controversial, and at least four different promoters (two in the long terminal repeat and two in the envelope gene) for sag mRNA have been reported. To determine which RNA is responsible for Sag function during milk-borne MMTV transmission, we mutated a splice donor site unique to a spliced sag RNA from the 5' envelope promoter. The splice donor mutation in an infectious provirus was transfected into XC cells and injected into BALB/c mice. Mice injected with wild-type provirus showed Sag activity by the deletion of Sag-specific T cells and induction of mammary tumors in 100% of injected animals. However, mice injected with the splice donor mutant gave sporadic and delayed T-cell deletion and a low percentage of mammary tumors with a long latency, suggesting that the resulting tumors were due to the generation of recombinants with endogenous MMTVs. Third-litter offspring of mice injected with wild-type provirus showed Sag-specific T-cell deletion and developed mammary tumors with kinetics similar to those for mice infected by nursing on MMTV-infected mothers, whereas the third-litter offspring of the splice donor mutant-injected mice did not. One of the fifth-litter progeny of splice donor mutant-injected mice showed C3H Sag activity and had recombinants that repaired the splice donor mutation, thus confirming the necessity for the splice donor site for Sag function. These experiments are the first to show that the spliced sag mRNA from the 5' envelope promoter is required for efficient milk-borne transmission of C3H MMTV.

Using genetics to probe host-virus interactions; the mouse mammary tumor virus model

It is clear that there is genetic variation among different individuals in their susceptibility to infection by viruses and other pathogens. Identification of the genes involved in conferring resistance or susceptibility to viral infection will allow us to understand both mechanisms of infection and pathogenesis and to develop reagents for treating or preventing them. Because of the large number of genetically well-characterized inbred mouse strains and the ability to generate targeted germ line mutations, this species is particularly well-suited for such analysis. This review focuses on how the use of genetics to study the retrovirus mouse mammary tumor virus allowed the dissection of both the viral infection pathway and the response of the host to this infection.

Biologic functions of the IFN-gamma receptors

Interferon-gamma (IFN-gamma) is a cytokine that plays an important role in inducing and modulating an array of immune responses. Cellular responses to IFN-gamma are mediated by its heterodimeric cell-surface receptor (IFN-gammaR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. In order to study the role of IFN-gamma in a number of immune responses and pathways, researchers have generated mice with altered patterns of IFN-gammaR gene expression. These studies, together with analyses of naturally occurring mutations of the IFN-gammaR in man, have been instrumental in elucidating the diverse functions of IFN-gamma, and are the subject of this review.

Interplays between mouse mammary tumor virus and the cellular and humoral immune response

Mouse mammary tumor virus has developed strategies to exploit the immune response. It requires vigorous immune stimulation to achieve efficient infection. The infected antigen-presenting cells present a viral superantigen on the cell surface which stimulates strong CD4-mediated T-cell help but CD8 T-cell responses are undetectable. Despite the high frequency of superantigen-reactive T cells, the superantigen-induced immune response is comparable to classical antigen responses in terms of T-cell priming, T-cell-B-cell collaboration as well as follicular and extra-follicular B-cell differentiation. Induction of systemic anergy is observed, similar to classical antigen responses where antigen is administered systemically but does not influence the role of the superantigen-reactive T cells in the maintenance of the chronic germinal center reaction. So far we have been unable to detect a cytotoxic T-cell response to mouse mammary tumor virus peptide antigens or to the superantigen. This might yet represent another step in the viral infection strategy.

The IL-4 Rapidly Produced in BALB/c Mice After Infection with Leishmania major Down-Regulates IL-12 Receptor β2-Chain Expression on CD4+ T Cells Resulting in a State of Unresponsiveness to IL-12

Within 1 day of infection with Leishmania major, susceptible BALB/c mice produce a burst of IL-4 in their draining lymph nodes, resulting in a state of unresponsiveness to IL-12 in parasite-specific CD4+ T cells within 48 h. In this report we examined the molecular mechanism underlying this IL-12 unresponsiveness. Extinction of IL-12 signaling in BALB/c mice is due to a rapid down-regulation of IL-12R β2-chain mRNA expression in CD4+ T cells. In contrast, IL-12R β2-chain mRNA expression was maintained on CD4+ T cells from resistant C57BL/6 mice. The down-regulation of the IL-12R β2-chain mRNA expression in BALB/c CD4+ T cells is a consequence of the early IL-4 production. In this murine model of infection, a strict correlation is shown in vivo between expression of the IL-12R β2-chain in CD4+ T cells and the development of a Th1 response and down-regulation of the mRNA β2-chain expression and the maturation of a Th2 response. Treatment of BALB/c mice with IFN-γ, even when IL-4 has been produced for 48 h, resulted in maintenance of IL-12R β2-chain mRNA expression and IL-12 responsiveness. The data presented here support the hypothesis that the genetically determined susceptibility of BALB/c mice to infection with L. major is primarily based on an up-regulation of IL-4 production, which secondarily induces extinction of IL-12 signaling.