Friend Virus-induced Leukemia Research Articles

Novel cytotoxic exhibition mode of antimicrobial peptide anoplin in MEL cells, the cell line of murine Friend leukemia virus‐induced leukemic cells

Anoplin is a recently discovered antimicrobial peptide (AMP) isolated from the venom sac of the spider wasp Anoplius samariensis, and it is one of the shortest α-helical AMP found naturally to date consisting of only ten amino acids. Previous results showed that anoplin exhibits potent antimicrobial activity but little hemolytic activity. In this study, we synthesized anoplin, studied its cytotoxicity in Friend virus-induced leukemia cells [murine erythroleukemia (MEL) cells], and proposed its possible mechanism. Our results showed that anoplin could inhibit the proliferation of MEL cells in a dose-dependent and time-dependent manner via disrupting the integrity of cell membrane, which indicated that anoplin exerts its cytotoxicity efficacy. In addition, the cell cycle distribution of MEL cells was arrested in the G₀/G₁ phase significantly. However, anoplin could not induce obvious apoptosis in MEL cells, as well as anoplin could not induce visible changes on morphology and quantity in the bone marrow cells isolated from normal mice. All of these results indicate that anoplin, as generally believed, is a selective AMP, a value characteristic in the design of safe therapeutic agents. The cytotoxicity of anoplin on MEL cells was mainly attributable to the plasma membrane perturbation and also to the intracellular events such as the arrest of cell cycle. Although this is an initial study that explored the activity of anoplin in vitro rather than in vivo, with the increasing resistance of conventional chemotherapy, there is no doubt that anoplin has desirable feature to be developed as a novel and selective anticancer agent.

Preinfection treatment of resistant mice with CpG oligodeoxynucleotides renders them susceptible to friend retrovirus-induced leukemia.

We recently reported that immunostimulatory oligodeoxynucleotides (CpG oligodeoxynucleotides [CpG-ODN]) were effective in postexposure treatment of retrovirus-induced disease (A. R. M. Olbrich et al., J. Virol. 76:11397-11404, 2002). We now show that the timing of treatment is a critical factor in treatment efficacy. In stark contrast to the success of postexposure treatments, we found that CpG treatment of susceptible mice prior to Friend retrovirus infection accelerated the development of virus-induced erythroleukemia. Furthermore, 70.8% of mice that were resistant to Friend virus-induced leukemia developed disease after inoculation of CpG-ODN before infection. The CpG pretreatment of these mice enhanced viral loads in their spleens and blood compared to controls that received ODN without CpG motifs. The main target cells of Friend virus, erythroid precursor cells and B cells, proliferated after CpG-ODN inoculation and provided an enlarged target cell population for viral infection. Our present findings together with our previous report demonstrate that CpG-ODN treatment of viral infections may be a double-edged sword that can result in an effective therapy but also in an acceleration of disease progression depending on the time point of treatment.

Effective postexposure treatment of retrovirus-induced disease with immunostimulatory DNA containing CpG motifs.

Therapeutic strategies for the treatment of acute retroviral infections have relied mainly on antiviral drugs. In this study we used the Friend virus model system to demonstrate that enhancement of the immune system can also have dramatic therapeutic effects. Since resistance to Friend virus-induced leukemia in mice is associated with T helper cell type 1 (Th1) immune responses, we enhanced these responses in susceptible mice by treatment with synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODN). Treatments begun at 4 days postinfection increased recovery from 6% in the control group to 74% in the CpG-treated group. CpG-mediated recovery was associated with a significant reduction of viral loads in the blood and spleens of treated mice compared to those of control animals. The treatment promoted Th1-type cytokine production by splenocytes of Friend virus-infected mice and augmented Friend virus-specific cytotoxic T-cell responses, but no influence on the virus-specific neutralizing antibody response was observed. Friend virus-specific CD8(+) T cells were critical for effective treatment with CpG-ODN, since in vivo depletion of these cells from treated mice prevented their recovery. Our results demonstrate that CpG-ODN therapy can significantly enhance virus-specific cellular immune responses and prevent retrovirus-induced disease. These findings may have implications for antiviral therapy in general.

Role of natural killer cells in resistance against friend retrovirus-induced leukemia.

We have previously shown that immunization with a synthetic peptide that contains a single CD4(+) T-cell epitope protects mice against immunosuppressive Friend retrovirus infection. Cells producing infectious Friend virus were rapidly eliminated from the spleens of mice that had been immunized with the single-epitope peptide. However, actual effector mechanisms induced through T-helper-cell responses after Friend virus inoculation were unknown. When cytotoxic effector cells detected in the early phase of Friend retrovirus infection were separated based on their expression of cell surface markers, those lacking CD4 and CD8 but expressing natural killer cell markers were found to constitute the majority of effector cells that lysed Friend virus-induced leukemia cells. Depletion of natural killer cells by injecting anti-asialo-ganglio-N-tetraosylceramide antibody did not affect the number of CD4(+) or CD8(+) T cells in the spleen, virus antigen-specific proliferative responses of CD4(+) T cells, or cytotoxic activity against Friend virus-induced leukemia cells exerted by CD8(+) effector cells. However, the same treatment markedly reduced the killing activity of CD4(-) CD8(-) effector cells and completely abolished the effect of peptide immunization. Although the above enhancement of natural killer cell activity in the early stage of Friend virus infection was also observed in mice given no peptide, these results have demonstrated the importance and requirement of natural killer cells in vaccine-induced resistance against the retroviral infection.

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.

Rejection of an IA+ variant line of FBL-3 leukemia by cytotoxic T lymphocytes with CD4+ and CD4-CD8- T cell receptor-alpha beta phenotypes generated in CD8-depleted C57BL/6 mice.

FBL-3N is an MHC class II Ag+ variant line that was obtained spontaneously during maintenance of Friend virus-induced leukemia FBL-3 in an athymic C57BL/6 (B6) mouse. Inocula of FBL-3N, but not the parental FBL-3 tumor, regressed after initial growth in CD8-depleted, syngeneic B6 mice. The cellular mechanisms by which FBL-3N was rejected in these mice were investigated in this study. We demonstrated that CTL with both CD4+ and CD4-CD8-TCR-alpha beta phenotypes were generated in mixed lymphocyte tumor cell culture spleen cells obtained from CD8-depleted B6 mice that had rejected FBL-3N by in vitro stimulation with mitomycin C-treated FBL-3N. After adoptive transfer of these CTL that were generated in vitro into athymic B6 mice, challenge with the FBL-3N tumor resulted in tumor regression after its initial growth. Thus, CD4+ and CD4-CD8-TCR-alpha beta CTL mediated rejection of the FBL-3N tumor in CD8-depleted B6 mice. Furthermore, the findings that depletion of B6 mice of CD4+ cells in addition to CD8+ cells abrogated the rejection of FBL-3N and generation of CTL in mixed lymphocyte tumor cell culture spleen cells suggest that CD4+ cells were required not only as a source of CD4+ CTL, but also as helper cells for generation of CD4-CD8-TCR-alpha beta CTL. Tumor Ag recognition of CD4-CD8-TCR-alpha beta CTL was restricted to Db, like that of classical CD8+ CTL, but the restriction appeared to be less obligatory than that of CD8+ CTL.

Role and specificity of T-cell subsets in spontaneous recovery from Friend virus-induced leukemia in mice.

Spontaneous recovery from Friend virus complex-induced leukemic splenomegaly in H-2Db/b mice correlated with the appearance of Friend virus complex-specific cytotoxic T lymphocytes (CTL) detectable directly in spleen cell populations. By testing CTL on target cells containing expression vectors encoding individual retroviral structural proteins, the main viral protein recognized was shown to be the Friend murine leukemia helper virus envelope glycoprotein. In vivo depletion of CD8-positive T cells drastically reduced the incidence of recovery, providing direct evidence for the role of CD8-positive CTL in the spontaneous recovery process. In vivo depletion of CD4-positive cells had little effect on the early stages of recovery but did cause a marked reduction in the final incidence of recovery at 60 to 90 days. Thus, CD8-positive cells were required for the initiation of the recovery process, whereas CD4-positive cells appeared to be required for maintenance of the recovered status.

The Role of B Cells for in Vivo T Cell Responses to a Friend Virus-Induced Leukemia

B cells can function as antigen-presenting cells and accessory cells for T cell responses. This study evaluated the role of B cells in the induction of protective T cell immunity to a Friend murine leukemia virus (F-MuLV)-induced leukemia (FBL). B cell-deficient mice exhibited significantly reduced tumor-specific CD4+ helper and CD8+ cytotoxic T cell responses after priming with FBL or a recombinant vaccinia virus containing F-MuLV antigens. Moreover, these mice had diminished T cell responses to the vaccinia viral antigens. Tumor-primed T cells transferred into B cell-deficient mice effectively eradicated disseminated FBL. Thus, B cells appear necessary for efficient priming but not expression of tumor and viral T cell immunity.

Adoptively transferred antigen-specific T cells can be grown and maintained in large numbers in vivo for extended periods of time by intermittent restimulation with specific antigen plus IL-2.

The aim of the current study was to determine whether cultured tumor Ag-specific T cells could be induced to grow and maintained functional in large numbers in vivo by intermittent restimulation in vivo with specific Ag plus IL-2. T cells derived from spleens of B6 mice (Thy-1.2) immune to FBL-3, a Friend virus-induced leukemia, were activated by in vitro stimulation with irradiated FBL-3 and expanded by culture for 14 days with low concentrations of IL-2. The resultant FBL-3-specific T cell lines were adoptively transferred into cyclophosphamide pretreated congenic hosts (B6/Thy-1.1), and restimulated every 14 days by an injection of irradiated FBL-3 plus a 7-day course of IL-2. Donor T cells residing in the host were identified and quantified by use of antibody to the Thy-1.2 allele. The results confirmed that stimulation with FBL-3 on the day of transfer (day 0) plus IL-2 on days 0 to 6 induced rapid growth of donor T cells to approximately an 11-fold increase in total donor T cell number recoverable from host ascites and spleen by day 7. However, prolonging the course of IL-2 administration to 35 days did not maintain the number or the specific cytolytic function of donor T cells. By contrast, intermittent restimulation with specific Ag plus IL-2 induced intermittent regrowth of donor T cells in vivo, maintained the number of donor T cells in vivo at greater than the number input for longer than 1 mo, and allowed detection of substantially augmented donor T cell-mediated specific antitumor function over that period of time.

Studies of the mechanisms of action of the antiretroviral agents hypericin and pseudohypericin.

Administration of the aromatic polycyclic dione compounds hypericin or pseudohypericin to experimental animals provides protection from disease induced by retroviruses that give rise to acute, as well as slowly progressive, diseases. For example, survival from Friend virus-induced leukemia is significantly prolonged by both compounds, with hypericin showing the greater potency. Viremia induced by LP-BM5 murine immunodeficiency virus is markedly suppressed after infrequent dosage of either substance. These compounds affect the retroviral infection and replication cycle at least at two different points: (i) Assembly or processing of intact virions from infected cells was shown to be affected by hypericin. Electron microscopy of hypericin-treated, virus-producing cells revealed the production of particles containing immature or abnormally assembled cores, suggesting the compounds may interfere with processing of gag-encoded precursor polyproteins. The released virions contain no detectable activity of reverse transcriptase. (ii) Hypericin and pseudohypericin also directly inactivate mature and properly assembled retroviruses as determined by assays for reverse transcriptase and infectivity. Accumulating data from our laboratories suggest that these compounds inhibit retroviruses by unconventional mechanisms and that the potential therapeutic value of hypericin and pseudohypericin should be explored in diseases such as AIDS.

Tumor-specific T-lymphocyte cytotoxicity enhanced by low dose of C. parvum.

Three routes of immunotherapy with Corynebacterium parvum (CP) on an ascitic Friend virus-induced leukemia were evaluated. Only the intraperitoneal route, which provided optimal contact between CP and tumor cells, showed prolonged mean survival time. Greatest effectiveness was obtained with multiple injections of CP at weekly intervals and with small initial tumor load. Of particular interest was that lower dosages of CP (5 and 25 micrograms) gave longer protection than dosages of 50 and 250 micrograms. Using in vitro 125I-iododeoxyuridine release assay, these lower dosages were shown to selectively enhance the cytotoxicity associated with T lymphocytes, whereas higher dosages appeared to primarily augment the activity of phagocytes. Moderate natural killer cell activity was observed with both the lower and higher dosages of CP. Data from this study indicate that route of administration, dosage of CP, and size of tumor burden are crucial variables determining optimal response to immunotherapy.

Studies of the mechanisms for the induction of in vivo tumor immunity. VII. Development of specific antitumor immunity in progressors and regressors

The present study was aimed at comparing the development of specific antitumor immunity between hosts with progressively growing tumors and hosts with regressing tumors. The experiments were performed with a Friend virus-induced leukemia FBL-3 in syngeneic C57BL/6 mice. The specific antitumor immunity was determined by in vitro cell-mediated cytotoxicity assay and in vivo tumor neutralization test. Both the systemic immunity (demonstrated in spleen) and immunity developed at tumor site were examined. For progressors, the tumor site was in the peritoneal cavity. For regressors, it was in a subcutaneous site of both flanks. Testing by the cell-mediated cytotoxicity assay showed that immune hosts and regressors had higher levels of systemic immunity than the progressors. However, when lymphocytes isolated from tumor sites were assayed, it was found that there was no remarkable difference between lymphocytes from progressor tumors (PTL) and lymphocytes from regressor tumors (RTL). Both lymphocyte populations were similar in profile analysis; they were characterized as T cells and possessed the same antigenic specificity. Nevertheless, when in vivo tumor transplantation experiments were performed, RTL were found to give protection against FBL-3 challenge whereas PTL consistently failed to do so. On cytomorphological examination, the PTL were seen to contain large amounts of macrophages. The presence of macrophages in PTL appeared to have an inverse relationship to the in vivo protective effect. After removal of macrophages from PTL by Petri dish adherence, the nonadherent PTL were found to give in vivo protection. Furthermore, thymocytes from progressors and macrophages isolated from the progressor tumors were found to suppress the in vivo T-cell-mediated immunity. These findings demonstrated that suppressor T cells and suppressor macrophages were present in tumor-bearing hosts. These suppressor cells could interfere with the function of immune T cells at the efferent arm of the immune response.

Use of monoclonal anti-gp70 antibodies to mimic the effects of the Rfv-3 gene in mice with Friend virus-induced leukemia.

During the course of progressive Friend virus-induced leukemia in Rfv-3r/s mice, antiviral antibody caused a marked reduction in the frequency of leukemic spleen cells releasing infectious virus. We investigated the mechanism of this antibody-induced alteration of leukemia cell phenotype in a series of passive transfer experiments using monoclonal antiviral antibodies. Our results indicated that two IgG2a anti-gp70 cytotoxic antibodies could reduce the frequency of virus-producing cells within the leukemic spleen as well as maintain the virus-nonproducing phenotype once it was established. IgG2a and IgG2b monoclonal anti-p 15 antibodies, IgM, and IgA anti-gp70 antibodies, and an IgG3 anti-p 15(E) antibody were not effective. The mechanism of this phenotypic alteration appeared to involve an antibody-mediated cytostasis of virus-producing leukemia cells with the subsequent over-growth of virus-nonproducing cells. The maintenance of the virus-nonproducing phenotype was dependent on the presence of anti-gp70 antibodies capable of neutralizing and clearing infectious F-MuLV in vivo. The presence of these neutralizing antibodies appeared to prevent reinfection of virus-nonproducing cells and therefore interfered with reversion to the virus-producing phenotype.

Augmentation of the anti-tumor therapeutic efficacy of long-term cultured T lymphocytes by in vivo administration of purified interleukin 2.

Spleen cells from C57BL/6 mice immunized in vivo with a syngeneic Friend virus-induced leukemia, FBL-3, were specifically activated by culture for 7 d with FBL-3, then nonspecifically induced to proliferate in vitro for 12 d by addition of supernatants from concanavalin A-stimulated lymphocytes containing interleukin 2 (IL-2). Such long-term cultured T lymphocytes have previously been shown to specifically lyse FBL-3 and to mediate specific adoptive therapy of advanced disseminated FBL-3 when used as an adjunct to cyclophosphamide (CY) in adoptive chemoimmunotherapy. Because the cultured cells are dependent upon IL-2 for proliferation and survival in vitro, their efficacy in vivo is potentially limited by the availability of endogenous IL-2. Thus, the aim of the current study was to determine whether exogenously administered purified IL-2 could augment the in vivo efficacy of long-term cultured T lymphocytes. Purified IL-2 alone or as an adjunct to CY as ineffective in tumor therapy. However, IL-2 was extremely effective in augmenting the efficacy of IL-2-dependent long-term cultured T lymphocytes in adoptive chemoimmunotherapy. The mechanism by which IL-2 functions in vivo is presumably by promoting in vivo growth and/or survival of adoptively transferred cells. This assumption was supported by the findings that IL-2 did not enhance the modest therapeutic efficacy of irradiated long-term cultured cells that were incapable of proliferating in the host and was ineffective in augmenting the in vivo efficacy of noncultured immune cells that are not immediately dependent upon exogenous IL-2 for survival.

Eradication of disseminated murine leukemia by chemoimmunotherapy with cyclophosphamide and adoptively transferred immune syngeneic Lyt-1+2- lymphocytes.

The phenotype of T cells therapeutically effective in immunotherapy of advanced Friend virus-induced (FBL) leukemia in vivo and cytotoxic to FBL in vitro was determined. Mice bearing disseminated FBL leukemia were successfully treated by a combination of cyclophosphamide and adoptive transfer of syngeneic immune lymphocytes. Therapeutic efficacy was largely dependent on the presence of Lyt-1+2- T cells in the transferred cells, whereas cells cytotoxic to FBL tumor in vitro were derived from the Lyt-1+2+ and Lyt-1-2+ subsets. Thus, the predominate cell required to eradicate tumor in adoptive chemoimmunotherapy was not cytolytic to tumor in vitro. Potentially, the Lyt-1+2- cell may operate in vivo as an amplifier cell rather than by a direct anti-tumor effect. Elimination of the Lyt-1+ population with alpha-Lyt-1 and complement prevented the generation of significant cytotoxic responses during both primary in vitro sensitization to alloantigens and in vitro sensitization of tumour-primed cells. The capacity of Lyt-1+ cell-depleted population to generate cytotoxic responses was partially reconstituted by addition, at the initiation of culture, of interluekin 2, a T cell growth factor derived from Lyt-1+2- cells, which contain the CTL and CTL precursors, were nearly as effective in vitro as unseparated immune cells. If the remaining effector cells (i.e., Lyt-1+2- T cells) function in vivo predominantly as amplifier cells, than the tumour-bearing host must be capable of making a positive contribution to the outcome of therapy.

Chemoimmunotherapy of a Friend Leukemia With Cells Secondarily Sensitized In Vitro: Effect of Culture Duration on Therapeutic Efficacy<xref ref-type="fn" rid="FN2">2</xref><xref ref-type="fn" rid="FN3">3</xref>

Although in vivo-primed cells are known to be rendered more effective in adoptive tumor therapy by secondary sensitization in vitro, they were tested only at the time of maximum in vitro cytolytic reactivity. Since the requirements for in vitro cytotoxicity and tumor therapy differ, the present study was designed to evaluate and compare the influence of culture duration on these two effector functions. Spleen cells from inbred C57BL/6 mice primed in vivo with the Friend virus-induced leukemia FBL-3 were secondarily sensitized in vitro by culture with tumor and tested for cytotoxicity in vitro in a 4-hour 51Cr release assay and for therapeutic efficacy in vivo against established tumor. In vivo-primed cells tested directly without prior culture were effective in therapy but were not cytotoxic in vitro. Culture of primed cells for 3 days rendered them cytotoxic as measured in vitro. This cytotoxicity increased through day 5, then it plateaued. Cultured duration modified the in vivo efficacy of primed cells differently. Cells cultured for 3 days with secondary sensitization by tumor became more effective in tumor therapy than noncultured primed cells. Increasing the duration of in vitro sensitization from 3 to 5 days did not enhance in vivo therapeutic efficacy, despite a concurrent increase in in vitro cytotoxicity. However, further lengthening of the culture duration to 7 days rendered cells more effective in vivo. These discrepancies presumably reflect different effector cell requirements and/or regulation operative for tumor lysis during a short-term in vitro assay and for tumor eradication following adoptive transfer of immune cells in vivo.

Regulation of the In Vitro Secondary Cell‐Mediated Cytotoxic Response against Syngeneic FBL‐3 Leukemia by Macrophages

Depletion of macrophages from immune spleen cells by treatment with carbonyl iron and magnet or by in vivo treatment with carrageenan enhanced the in vitro secondary cell-mediated cytotoxic response against a syngeneic Friend virus-induced leukemia, FBL-3 cells of C57BL/6 mice. However, further depletion of macrophages by passing the carbonyl iron-treated immune spleen cells through a nylon wool column abrogated the cytotoxic response. The addition of splenic macrophage-enriched preparations from either FBL-3-immune or normal mice suppressed the cytotoxic response of immune spleen cells treated with carbonyl iron and magnet. This suppressive effect of splenic macrophages presented a marked contrast with the enhancing effect of normal peritoneal macrophages on the same cell-mediated cytotoxic response, indicating regulation of the generation of killer T cells against a syngeneic tumor by functionally distinct macrophages. The suppressed cell-mediated cytotoxic response against FBL-3 cells by immune spleen cells was augmented by the addition of indomethacin to the culture medium, and this augmentation with indomethacin was greatly decreased by depletion of phagocytic cells from the immune spleen by treatment with carbonyl iron and magnet. The mechanisms of regulation of the cell-mediated cytotoxic response with soluble factors released from macrophages are discussed.

The regulation of differentiation in murine virus-induced erythroleukemic cells.

Continuous lines of erythroleukemia cells, which originate from the spleens of mice with Friend virus-induced leukemia, provide a convenient experimental system in which the molecular control of erythrodifferentiation can be examined in some detail (Friend et al. 1966; Ikawa and Sugano 1966; Ostertag et al. 1972; Freedman and Lilly 1975). The cultures contain cells at various stages of differentiation. The majority are large, primitive erythroid cells with prominent nucleoli, but a few more mature cells recognizable as normoblasts are present. We had developed these lines with the idea of studying leukemia as a disease resulting from a block in maturation, and had hoped to find means of stimulating the cells to differentiate. To a certain extent, we achieved this goal.

Suppression of T cell‐mediated immunity by tumor cells: Immunogenicity versus immunosuppression and preliminary characterization of the suppressive factors

In studying the immunogenicity of spleen cells and tumor cells in the generation, of cytotoxic T lymphocyte (CTL) in the allogeneic mixed lymphocyte culture (MLC) or mixed lymphocyte tumor cell culture (MLTC) reactions, we have found that the tumor cells not only appear to be poorly immunogenic, but are also immunosuppressive. This was shown by the ability of the tumor cells or their cell-free extracts to suppress standard MLC reactions. This suppression was acting mainly at the induction phase of the cytotoxic response. It could not interfere with the killing activity of the fully generated CTLs. In a Friend virus-induced leukemia FBL-3 system, at least two major components could be attributed to the cause of immunosuppression; one was of viral origin and the other was of non-viral origin. The viral component was sensitive to UV-irradiation and could be pelleted after ultracentrifugation at 100,000 g. The non-viral component was UV-resistant and was retained in the supernatant fraction after ultracentrifugation. Friend virus and 12 commonly found murine viruses have been excluded as the possible candidates causing the immunosuppression. The immunosuppressive viruses are very likely of endogenous origin and are defective in replication as shown by electromicroscopy, and by the virus focus-inducing and reverse transcriptase assays. These findings indicate that probably all tumor cells possess the immunosuppressive factor(s) which may account for their apparent lack of immunogenicity and the lack of proper immune responses in the tumor-bearing hosts.

Effects of whole body hyperthermia on progression of friend virus-induced leukemia in mice

Effects of whole body hyperthermia on progression of friend virus-induced leukemia in mice