Superantigenic characteristics of mouse mammary tumor viruses play a critical role in susceptibility to infection in mice

Mouse mammary tumor viruses (MMTV) are retroviruses that induce mammary carcinomas. An interesting feature of these viruses is the superantigen (SAg) encoded in an open reading frame within the 3′ long terminal repeat. The mechanism by which ingestion of milk-borne virus results in infection of the host mammary tissue remains incompletely understood. However, a working model has been proposed in which the interaction between viral SAg, T-cell receptor and MHC class II I-E facilitates viral replication and hence infectivity. In this review we summarize current studies demonstrating the role of SAg stimulation in susceptibility to MMTV infection.     

Deletion of CD4+ T cells and thymocytes by apoptosis in mouse mammary tumor virus (C4)-infected Vβ2 transgenic mice

Mouse mammary tumor virus MMTV(C4) encodes a Vβ2-specific superantigen. In Vβ2 transgenic (TG2) mice more than 98 % of peripheral T cells express Vβ2. Infection of Tg2 mice with MMTV(C4) at birth through their mothers' milk or at 6–8 weeks of age by intravenous injection resulted in massive deletion of peripheral CD4⁺ T cells and suppressed thymopoiesis. The number of peripheral CD8⁺ T cells was not affected in neonatally infected mice. In older mice injected with MMTV(C4), splenic CD8⁺ T cells were significantly elevated. Suppressed thymopoiesis was observed in both neonatally infected and older mice injected with MMTV(C4). Thymocytes which expressed high level CD3 or Vβ2 were deleted. To determine if T cells or thymocytes were deleted through apoptosis, DNA fragmentation was examined by flow cytometry and diphenylamine (DPA) binding assay. Approximately 31 % of CD4⁺ T cells from MMTV(C4)-infected Tg2 mice as compared to 6% from normal Tg2 mice contained fragmented nuclear DNA by flow-cytometric analysis. The DPA binding assay showed significantly increased total soluble DNA in lymph node cells and thymocytes from MMTV(C4)-infected mice. The kinetics of T cell and thymocyte apoptosis correspond to their deletion, supporting apoptosis as the mechanism of T cell and thymocyte deletion. CD4⁺ T cell and thymocyte deletion by MMTV(C4) in Tg2 mice provides a sensitive system for the analysis of retrovirus superantigen-induced apoptosis.     

Mouse mammary tumor virus derived from wild mice does not target Notch-4 protooncogene for the development of mammary tumors in inbred mice

The colony of wild mice, named Jyg, has been shown to express an exogenous mouse mammary tumor virus (Jyg-MMTV). This virus induces mammary tumors in its natural host at a high incidence (≈ 80%) resulting from insertion mutations in Notch-4 (43%), Wnt-1 (26%), and Fgf-3 (13%). Since the activation of Notch-4 is not common in mammary tumors of standard laboratory strains of mice infected with various MMTV strains, we examined the consequences of Jyg-MMTV infection in BALB/c and C57BL/6 mice. The results show that Jyg-MMTV induces mammary tumors in both mouse strains, but the incidence of mammary tumors in BALB/c mice is greater than in C57BL/6 mice. Surprisingly, however, none of the 75 mammary tumors, analyzed both by Southern and Northern hybridizations, showed insertion mutations in or expression of Notch-4. In contrast, both Wnt-1 and Fgf-3 were found to be involved in these tumors. Our findings may suggest, among other possibilities, the existence of a structural difference(s) between laboratory and wild mice at the Notch-4 locus that regulates the integration of Jyg-MMTV proviral DNA.     

Role of a heterologous retroviral transport element in the development of genetic complementation assay for mouse mammary tumor virus (MMTV) replication

The mouse mammary tumor virus (MMTV) is a type B retrovirus that is unique from other retroviruses in having multiple “tissue specific” and “hormone inducible” promoters. This unique feature has lead to the increasing interest in studying the biology of MMTV replication with the ultimate goal of developing MMTV based vectors for potentially targeted human gene therapy. In this report, we describe, for the first time, the establishment of an in vivo genetic complementation assay to study various aspects of MMTV replication. In the assay described here, the function of MMTV Rem/RmRE regulatory pathway has been successfully substituted by a heterologous retroviral constitutive transport element (CTE) from Mason Pfizer Monkey Virus (MPMV) for mature MMTV particle production. Our results revealed that in the absence of MPMV CTE or Rem/RmRE, RNA transcribed from MMTV Gag-Pol expression plasmids were efficiently transported to the cytoplasm. However, the presence of CTE was indispensable for Gag-Pol protein expression. In addition, we report the development of MMTV based vectors in which the packageable RNA was transcribed either from MMTV LTR or from a chimeric LTR, which could successfully be packaged and propagated by particles produced from MMTV Gag-Pol expression plasmids containing a heterologous transport element. The role of MPMV CTE in the transport of MMTV transfer vector RNA was not found to be significant. Development of such an assay should not only shed light on how MMTV regulates its gene expression, but also should provide additional molecular tools for delineating the packaging determinants for MMTV, which is imperative for the development of novel vectors for targeted and inducible gene therapy.