Friend Virus Complex Research Articles

Bacterial lipopolysaccharides as helper factors for Friend spleen focus-forming virus in mice.

Lipopolysaccharide (LPS) from several gram-negative bacteria significantly increased the spleen focus-forming efficiency of N-topic Friend virus complex in mice by different mechanisms. One effect was associated with an increase in the number of availability of potential target cells for spleen focus-forming virus (SFFV) in fully susceptible mouse strains. This enhancing effect was optimal when LPS was injected 5 days before SFFV but was nil when LPS and SFFV were given at the same time. In contrast, in mice genetically resistant to the native helper virus of SFFV, the helper effect of LPS was optical when it was injected with SFFV and oil when given 5 days before or after the virus. LPS did not affect helper virus expression in a standard cell culture (XC) assay, but it did increase helper virus replication in mice. Mice lacking T cells or complete endogenous murine leukemia virus genomes were just as sensitive to the helper effects of LPS on SFFV expression as were control animals. Most of the helper activity of LPS is associated with the lipid A component. The mechanism of the helper effect of lipid A is still unknown, but hypotheses must take into account that this effect did not occur in fully susceptible hosts, but only in hosts carrying resistance alleles at either the FV-1 or the FV-2 locus.

Continuous replication of Friend virus complex (spleen focus-forming virus-lymphatic leukemia-inducing virus) in mouse embryo fibroblasts. Retention of leukemogenicity and loss of immunosuppressive properties.

Exposure of NIH Swiss mouse embryo fibroblasts (MEF) to infectious Friend virus (FV) complex [containing defective spleen focus-forming virus (SFFV) and endogenous NB-tropic leukemia-inducing helper virus (LLV-F)] resulted in the productive infection of these cells by both SFFV and LLV-F. Stocks of SFFV derived after extensive growth in this Swiss MEF cell culture system are fully leukemogenic in adult mice for the induction of erythroleukemia and spleen foci. In addition, in vitro-derived LLV-F, when isolated free of SFFV, is fully leukemogenic for the induction of lymphatic leukemia when inoculated into susceptible newborn BALB/c mice. The host range of in vitro-derived FV complex (i.e., FV-TC) for focus formation in vivo is NB-tropic. Unlike in vivo-derived FV complex, FV-TC does not suppress the responsiveness of murine thymocytes to concanavalin A (Con A) in vitro. Rather, FV-TC acts as a mitogen to nonspecifically stimulate the proliferation of BALB/c thymocytes. The mitogenicity of in vitro-derived FV complex is directly associated with the presence of type-C virus particles, is a heat-labile and UV-sensitive property of the virus, and may be primarily due to LLV since equivalent amounts of LLV with or without SFFV present are equally mitogenic. One in vivo passage of FV-TC resulted in the total loss of this mitogenic property with the reappearance of full immunosuppressive properties. This result demonstrates a clear association between in vivo growth of FV and its ability to suppress mouse thymocytes, and suggests that FV complex (SFFV-LLV) is not inherently immunosuppressive for these cells. While the mechanism of this interconversion between immunostimulatory and fully suppressive virus is unknown, both virus markers appear to be dependent upon the presence of infectious FV.

Properties of cell lines derived from tumors induced by Friend virus in BALB/c and BALB/c-H-2b mice.

Cell lines have been established in culture from Friend virus-induced tumors of BALB/c (H-2d) and congenic BALB/c-H-2b (BALB.B) origin. Spleens from virus-infected hosts in the terminal stages of erythroleukemic disease provided tissues for the establishment of subcutaneously transplantable tumors of both strains. Subsequently cells of these tumors were introduced into culture and passed serially. Complete, infectious Friend virus (FV) has been routinely recovered from culture supernates of BALB.B tumor cells (HFL/b) throughout its 2-yr passage history. However, after only a few transfer generations in culture BALB/c tumor cells (HFL/d) became nonproducers of virus detectable in either the spleen focus assay in vivo or the XC assay in vitro. Nonproducer HFL/d cells possessed the complete genomes of the components of the FV complex, since FV could be recovered from them either by cocultivation with helper virus-infected syngeneic embryo fibroblasts or by serial passage in the ascitic form in normal, syngeneic adult hosts.

Induction of endogenous and of spleen focus-forming viruses during dimethylsulfoxide-induced differentiation of mouse erythroleukemia cells transformed by spleen focus-forming virus.

Spleen focus-forming virus-transformed erythroleukemic cell clones, which have been established by infection of N type mice with NB trophic Friend virus, continue to release biologically active tfriend virus of NB host range. Dimethylsulfoxide induces erythroid differentiation and a 10- to 100-fold increase in the release of biologically active Friend virus. The increase of Friend virus release is a function of the differentiating erythroleukemic cell. The induced Friend viurs is not the NB tropic Friend virus complex, but shows N host range. The induction of the Friend virus complex is due to simultaneous induction of both spleen focus-forming and endogenous viruses.

Induction of endogenous virus and of thymidine kinase by bromodeoxyuridine in cell cultures transformed by Friend virus.

Thymidine kinase positive (TK(+)) N type cell lines that had been transformed by spleen focus-forming virus were established by transformation with NB tropic Friend virus complex. Thymidine kinase deficient (TK(-)) cell clones were isolated. Some of these cell clones release 1000- to 100,000-fold reduced amounts of Friend virus complex as compared to the TK(+) parental cell clone. TK(-) clones were grown in medium without BrdUrd. Some of these TK(-) clones can be induced to release endogenous helper virus and transforming spleen focus-forming virus on reexposure to 10(-6)-10(-4) M BrdUrd. The induced Friend virus complex is of N host range as expected with induced endogenous virus in N-type cells. Before the induction of the endogenous virus spleen focus-forming virus complex, an induction of thymidine kinase (ATP:thymidine 5'-phosphotransferase, EC 2.7.1.75) activity is observed. The latter is possibly a prerequisite for the induction of endogenous virus in TK(-) cells. Induction of thymidine kinase activity and of endogenous virus is transient and always correlated. The role of BrdUrd and another thymidine analogue, azidothymidine, in interfering with C-type virus release in virus positive cells is discussed. Azidothymidine is unable to induce endogenous virus. Induction of endogenous virus by BrdUrd and inhibition of virus release in virus positive cells is apparently not caused by the same mechanism.

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.

Isolation of a myelogenous leukemia-inducing virus from mice infected with the Friend virus complex.

AbstractA myelogenous leukemia‐inducing virus (MyLV) has been isolated from a chloroleukemic C57Bl mouse, injected neonatally with the lymphatic leukemia virus (LLV)‐containing helper component of the Friend virus (FV) complex. MyLV, free of detectable spleen focus‐forming virus (SFFV), consistently induced only myelogenous leukemia in neonatally infected C57Bl, Ha/ICR, C3H and BALB/c mice, within 5 months (including chloroleukemias in C57Bl and Ha/ICR). The leukemia involved chiefly the spleen, liver, lymph nodes and bone marrow, and tumor cells could be recovered from the marrow to grow in the solid (subcutaneous) or ascites (intraperitoneal) form. MyLV preparations had ample helper activity for SFFV in restrictive hosts, and could be used to prepare a MyLV pseudotype of SFFV which was antigenically indistinguishable from native FV complex. MyLV could have arisen by selection of a pre‐existing variant in the LLV population, by a genetic interaction between LLV and the C57Bl chloroleukemic host cell, or could have arisen from the original host C57Bl mice as a result of activation following the infection with LLV.

B-tropic friend virus: A host-range pseudotype of spleen focus-forming virus (SFFV)

The N-tropic strain of Friend virus (FV) complex includes two different infectious entities: defective SFFV and helper virus, the latter of which is responsible for the N-tropism of the complex. The N-tropic helper, highly infectious in homozygous Fv-1 n mice, is poorly infectious in Fv-1 b mice. B/T-L virus, a leukemogenic virus free of SFFV, includes a B-tropic helper virus which is much more infectious in Fv-1 b than in in Fv-1 n mice. N-tropic FV complex to which B/T-L virus has been added (i.e., SFFV with its associated N-tropic helper, plus B-tropic helper) induced FV disease in young Fv-1 b mice and gave rise to a strain of virus which could be passaged serially through Fv-1 b hosts. Analysis of the host range properties of the new virus strain indicated that the N-tropic helper component of FV complex had been rapidly eliminated during the first few passage generations of the virus, leaving a B-tropic pseudotype of SFFV, i.e., SFFV associated with B-tropic helper virus. Susceptibility to this B-tropic pseudotype was influenced in the expected manner by the Fv-1 and Fv-2 genes, but host range studies indicated that one or more previously unidentified genes also influenced the host response to this virus. Serial forced passage of the B-tropic FV through Fv-1 -restrictive hosts resulted in the recovery of an NB-tropic variant of the pseudotype.

Helper-dependent properties of Friend spleen focus-forming virus: effect of the Fv-1 gene on the late stages in virus synthesis.

Co-infection of neonatal BALB/c mice with Friend virus (FV) complex (containing defective spleen focus-forming virus [SFFV] and endogenous N-tropic leukemia-inducing helper virus [LLV-F]) and B-tropic Tennant leukemia virus (TenLV) resulted in the inhibition of LLV-F by the Fv-1(b) gene and recovery of a TenLV pseudotype of SFFV, abbreviated SFFV(TenLV). The host range of this pseudotype was B-tropic, since SFFV(TenLV) was 10 to 100 times more infectious for B-type (Fv-1(bb)) than for N-type (Fv-1(nn)) mice. The similar patterns of neutralization of N-tropic and B-tropic SFFV by type-specific murine antisera suggested that the difference in infectivity between these two SFFV preparations did not reside in envelope determinants. Rather, helper control of SFFV's host range was only apparent and dependent upon the ability of associated virus to provide a helper function for late stages in SFFV synthesis. Early stages in SFFV's infectious cycle were shown to be helper independent. The Fv-1 gene did not act at the level of the cell membrane to effectively restrict SFFV infection, since SFFV-induced transformed cells could be detected in the absence of spleen focus formation and SFFV synthesis. Further, the generation of these transformed cells by SFFV followed a one-hit, dose-response pattern, suggesting that SFFV-induced cell transformation is helper independent. Finally, restriction of helper function by Fv-1 may be an intracellular event, because both SFFV and its associated LLV-F helper share common envelope determinants and presumably adsorb onto and penetrate target cells with equal efficiency.

Development and regression of murine leukemia induced by friend virus complex.

SummaryInjection of a large dose (5000 FFU) of Friend virus complex (FV) lead to rapid development of leukemia and to death in A/J mice. Leukemia developed at a slower rate in A/J mice inoculated with 5 FFU of FV. A very small dose (0.5 FFU) of the same virus caused slow development of leukemia that later regressed in most A/J mice (73%). In (A/J X C57BL/6J) F1 and their reciprocal hybrids, leukemia developed rapidly after the injection of 500 FFU, but regressed in 97% of the mice.The authors thank Drs. W. D. Sawyer, S. Sirisinha and L. C. Olson for their helpful advice.

A CLASSIFICATION OF THE MURINE LEUKEMIA VIRUSES

Coinfection of neonatal BALB/c mice with helper-dependent Friend spleen focus-forming virus (SFFV), as contained in the Friend virus (FV) complex, and antigenically distinct Moloney leukemia virus (MolLV) resulted in the recovery of a MolLV pseudotype of SFFV, abbreviated SFFV(MolLV). The antigenic alteration of SFFV was observed by following its neutralization kinetics in vitro by specific Friend or Moloney typing antiserum. Effective pseudotype production was accomplished only when N-tropic LLV-F (the natural helper virus in the FV complex) was inhibited in B-type mice coinfected with an NB-tropic MolLV or other murine leukemia virus (MuLV) preparation. SFFV pseudotypes could not be prepared by using murine viruses other than leukemia viruses. SFFV prepared after two serial passages in the presence of MolLV was effectively neutralized by Moloney antiserum, but not by Friend typing antiserum; therefore, the envelope of the pseudotype virus, SFFV(MolLV), is homogeneous. Pseudotype virus was antigenically stable in the absence of continued mixed infection of BALB/c mice with SFFV(MolLV) and MolLV. However, SFFV(MolLV) was easily converted back to the LLV-F type after only one passage in BALB/c mice coinfected with NB-tropic LLV-F. The antigenic interconversion between LLV-F and MolLV types demonstrated that SFFV is defective with respect to the expression of neutralizable envelope antigens. Analysis of the neutralizable envelope antigens of nine SFFV(MuLV) pseudotypes by a panel of seven typing antisera made possible a "type-specific" SFFV(MuLV) envelope classification. Two major categories have been identified which correspond to the Gross (G) and Friend-Moloney-Rauscher (FMR) subgroups. Further, the FMR subgroup was divided into four types on the basis of distinct neutralization patterns. These results indicated that the specificity observed by cytotoxic G vs. FMR antisera is different from that observed by neutralization kinetics. We therefore suggest that the specific antigens revealed by virus neutralization tests be referred to as type specific.

Helper Activity of Human Leukemic Tissue Extracts for Leukemia Virus Expression in Mice

Extracts of cultured human leukemic tissues increased the spleen focus-forming activity of Friend leukemia virus preparations in BALB/c and other partially resistant mice. Such mice carry the Fv-1(b) gene, which inhibits the expression of helper virus indigenous to the Friend virus complex, and allows co-infecting leukemia viruses of mice, cats, or chickens to substitute for the inhibited helper virus and increase the expression of the spleen focus-forming virus. The helper activity of extracts from human leukemic tissues shared several important properties with that associated with known leukemogenic viruses of animals.