The envelope gene and long terminal repeat sequences contribute to the pathogenic phenotype of helper-independent Friend viruses.

Friend murine leukemia virus (F-MuLV) and Friend mink cell focus-inducing virus (Fr-MCF) are helper-independent murine retroviruses which induce a rapidly fatal erytholeukemia in NIH Swiss mice. Amphotropic clone 4070 (Ampho) is a murine retrovirus which does not cause leukemia in these animals. Mice inoculated with Ampho, an Fr-MCF/Ampho pseudotype, or F-MuLV developed leukemia in 0, 50, and 100% of animals, respectively. To identify the F-MuLV and Fr-MCF sequences responsible for leukemia, we constructed hybrid viral genomes between these viruses and Ampho, using subgenomic fragments of molecularly cloned viral DNA. Transfection of these hybrid viral DNAs into fibroblasts produces recombinant retroviruses. These new viruses are assayed in vivo for their ability to cause leukemia. Recombinant viruses constructed between the Ampho genome and the Fr-MCF envelope gene do not cause leukemia. Similarly, viruses constructed by using either the Fr-MCF long terminal repeat U3 region or the F-MuLV long terminal repeat U3 region and the remainder of the Ampho genome do not cause leukemia. However, if the Fr-MCF envelope gene plus the Fr-MCF U3 region are joined to Ampho, the resulting virus causes erythroleukemia in 14% of mice. Recombinant viruses made between the Fr-MCF envelope gene, the F-MuLV U3 region, and the remainder of the Ampho genome cause erythroleukemia in 38% of mice. This study demonstrates that both the envelope gene of Fr-MCF and the U3 regions of Fr-MCF and F-MuLV contain sequences which contribute to the leukemic phenotype of helper-independent Friend viruses.     

Induction of self-reactive T cells after murine coronavirus infection.

We studied the mechanism of in vitro spontaneous lymphokine production by spleen cells from mice injected intraperitoneally with murine coronavirus stain JHM 1 month after infection, when infectious virus had already been cleared from the spleens. Removal of either CD4+ T cells or Ia+ antigen-presenting cells (APC) from the spleen cells abrogated interleukin-2 (IL-2) production. Addition of anti-CD4 or anti-Iad monoclonal antibodies to the culture suppressed IL-2 production. These results suggest that the response involved typical receptor-mediated activation of T cells. Surprisingly, reciprocal mixing experiments with a coculture of T cells from infected mice and APC from either infected or naive mice resulted in the production of IL-2. The absence of viral antigens in spleen cells 1 month after infection, as indicated by their inability to induce the proliferation of T-cell clones specific for the viral antigens, suggest that the T cells from mice 1 month after infection were not responding to the viral antigens. The inoculum components other than the virus did not induce this immune response. We also found that the frequency of self-reactive but not alloreactive IL-2-producing T cells in the spleens of infected mice was 3- to 10-fold higher than that in naive mice. These findings suggest that an increased frequency of self-reactive T cells which secrete IL-2 occurs following murine coronavirus infection. This may have important implications in the development of autoimmunelike phenomena following murine coronavirus infection.     

An increase in disease latency is associated with a host-dependent selection for recombinant murine leukemia viruses with substitutions in the p15E (TM) gene.

The genomes of recombinant murine leukemia viruses recovered from HRS/J (type I env recombinants) and CWD (type II env recombinants) mice have distinct envelope gene structures. To better understand the biologic significance of these differences, we examined the differences in the responses of HRS/J and CWD mice to inoculation with an oncogenic type II env recombinant. The CWD recombinant accelerated the onset of lymphoma in both strains, but the disease latency in the HRS/J mice was about 2 months longer. Analysis of the recombinant viruses in the HRS/J tumors revealed that the injected type II env recombinant had recombined in vivo with the endogenous ecotropic viruses to generate secondary recombinants with type I envelope genes. In another set of experiments, comparison of complete or partial DNA sequences of the envelope genes from six recombinant proviruses confirmed that the origins of the sequences that encode an amino-terminal region of the TM envelope protein, p15E, distinguish type I envelope genes from type II. Taken together with the results of previous studies, these observations suggest that the differences in the responses of HRS/J and CWD mice to the oncogenic type II env recombinant resulted from an interaction between the viral TM protein and a host factor expressed in HRS/J mice.     

Presence of transplantable T-lymphoid cells in C57BL/6 mice infected with murine AIDS virus.

The Duplan strain of murine leukemia virus induces murine AIDS in C57BL/6 mice. When spleen cells from C57BL/6 mice infected with the virus were transplanted into nude mice, subcutaneous solid tumors at the transplanted sites were formed and splenomegaly and lymphadenopathy were induced. These transplantable cells were Thy-1- CD4+ alpha-beta T-cell receptor-positive T cells and integrated with the pathogenic defective viral genome. These results indicate that neoplastic cells of T-cell lineage were induced by infecting C57BL/6 mice with murine AIDS virus.     

Fv-1 host cell restriction of friend leukemia virus: microinjection of unintegrated viral DNA

The murine gene Fv-1 has been shown to exert a major influence over the replication of ecotropic murine leukemia viruses. Studies of the replication of Friend murine leukemia virus have shown that the restriction of viral replication occurs intracellularly after the initiation of viral DNA synthesis. The precise mechanism of the block imposed by the Fv-1 gene product is not completely understood. Our studies of Fv-1 restrictive infection have shown a variable decrease in the accumulation of intracellular unintegrated form I viral DNA. Analysis by microinjection of the viral DNA formed in nonpermissively infected BALB/c cells indicates that this DNA is infectious. These studies indicate that the form I DNA accumulated in nonpermissively infected BALB/c cells contains the complete viral sequences necessary for the production of viral progeny, and therefore, they suggest that the Fv-1 host restrictive mechanism recognizes viral factors other than form I DNA alone. These results support the possibility that Fv-1 host restriction occurs after formation of infectious viral DNA, perhaps at the integration step itself.     

Mechanism of selection of class II recombinant murine leukemia viruses in the highly leukemic strain CWD

The development of spontaneous lymphomas in CWD mice is associated with the expression of endogenous ecotropic murine leukemia viruses (MuLV) and the formation of recombinant viruses. However, the pattern of substitution of nonecotropic sequences within the envelope genes of the CWD class II recombinant viruses differs from that seen in class I recombinant MuLVs of AKR, C58, and HRS mice. To determine how CWD host genes might influence the envelope gene structure of the recombinant viruses, we characterized the responses of these mice to two different types of exogenous MuLVs. Neonatal mice injected the HRS class I recombinant PTV-1 became infected and developed T-cell lymphomas more rapidly than controls did. The inoculation of CWD mice with the leukemogenic AKR ecotropic virus SL3-3 led to the formation of recombinant MuLVs with a novel genetic structure and class II-like envelope genes, although SL3-3 generates class I recombinants in other strains. These results suggest that the absence of class I recombinant MuLVs in CWD mice is not related to the restriction of the replication or oncogenicity of class I viruses or to the absence of an appropriate ecotropic virus that can generate class I recombinants. More likely, the genes of CWD mice that direct the formation or selection of class II recombinant viruses affect the process of recombination between the ecotropic and nonecotropic envelope gene sequences.     

Feline leukemia virus subgroup B uses the same cell surface receptor as gibbon ape leukemia virus.

Pseudotypes of gibbon ape leukemia virus/simian sarcoma-associated virus (GALV/SSAV) and feline leukemia virus subgroup B (FeLV-B) have been constructed by rescuing a Moloney murine leukemia virus vector genome with wild-type GALV/SSAV or FeLV-B. The resulting recombinant viruses utilized core and envelope proteins from the wild-type virus and conferred resistance to growth in L-histidinol upon infected cells by virtue of the HisD gene encoded by the vector genome. They displayed the host range specificity of the rescuing viruses and could be neutralized by virus-specific antisera. Receptor cross-interference was observed when the GALV/SSAV or FeLV-B pseudotypes were used to superinfect cells productively infected with either GALV/SSAV or FeLV-B. Although murine cells are resistant to FeLV-B infection, murine cells expressing the human gene for the GALV/SSAV receptor became susceptible to FeLV-B infection. Therefore GALV/SSAV and FeLV-B utilize the same cell surface receptor.     

Host restriction of friend leukemia virus; fate of input virion RNA

Host restriction of oncogenesis by RNA tumor viruses may be studied in vitro by measuring the replication of the lymphatic leukemia component of the Friend Virus Complex (LLV-F) in either NIH-Swiss or Balb/C mouse embryo cells. These cells derive from mice differing at the Fv-1 locus, which controls the replication of all murine RNA leukemia viruses. Studies of early events in the replication of LLV-F were carried out by following the infection of permissive and restrictive mouse embryo cells by ³²P labeled LLV-F. ³²P labeled viral genome RNA rapidly becomes associated with cell nuclei and may be found integrated to the same extent with high molecular weight host DNA of either permissive or restrictive cells. These results suggest that Fv-1 mediated host restriction of LLV-F occurs at a step following integration of viral genome RNA into host DNA.Two other conclusions are suggested by these data. The nucleus appears to be the site of activation and synthesis of DNA of the infecting virus; and the “provirus”, at least transiently, is represented as an RNA-DNA hybrid molecule covalently integrated with host cell DNA.