Protection against retrovirus pathogenesis by SR protein inhibitors

Indole derivatives compounds (IDC) are a new class of splicing inhibitors that have a selective action on exonic splicing enhancers (ESE)-dependent activity of individual serine-arginine-rich (SR) proteins. Some of these molecules have been shown to compromise assembly of HIV infectious particles in cell cultures by interfering with the activity of the SR protein SF2/ASF and by subsequently suppressing production of splicing-dependent retroviral accessory proteins. For all replication-competent retroviruses, a limiting requirement for infection and pathogenesis is the expression of the envelope glycoprotein which strictly depends on the host splicing machinery. Here, we have evaluated the efficiency of IDC on an animal model of retroviral pathogenesis using a fully replication-competent retrovirus. In this model, all newborn mice infected with a fully replicative murine leukemia virus (MLV) develop erythroleukemia within 6 to 8 weeks of age. We tested several IDC for their ability to interfere ex vivo with MLV splicing and virus spreading as well as for their protective effect in vivo. We show here that two of these IDC, IDC13 and IDC78, selectively altered splicing-dependent production of the retroviral envelope gene, thus inhibiting early viral replication in vivo, sufficiently to protect mice from MLV-induced pathogenesis. The apparent specificity and clinical safety observed here for both IDC13 and IDC78 strongly support further assessment of inhibitors of SR protein splicing factors as a new class of antiretroviral therapeutic agents.

Tumor gangliosides inhibit the tumor-specific immune response

Tumor gangliosides are highly immunosuppressive membrane glycosphingolipids that are shed into the tumor cell microenvironment. We directly tested the impact of shed gangliosides on the in vivo antitumor immune response in a syngeneic fully autochthonous system (FBL-3 erythroleukemia cells, C57BL/6 mice, and highly purified FBL-3 cell gangliosides). The major FBL-3 ganglioside was identified as GM1b by mass spectrometry. Substantial ganglioside shedding (90 pmol/108 cells/h), a requisite for their inhibition of the immune function of tumor-infiltrating leukocytes, was detected. Immunosuppression by FBL-3 gangliosides was potent; 5-20 microM inhibited the tumor-specific secondary proliferative response (80-100%) and suppressed the generation of tumor-specific CTLs (97% reduction of FBL-3 cell lysis at an E:T ratio of 100:1). In vivo, coinjection of 10 nmol of FBL-3 gangliosides with a primary FBL-3 cell immunization led to a reduced response to a secondary challenge (the increase in the draining popliteal lymph node mass, cell number, and lymphocyte thymidine incorporation were lowered by 70, 69, and 72%, respectively). Coinjection of gangliosides with a secondary tumor challenge led to a 61, 74, and 42% reduction of the increase in lymph node mass, cell number, and thymidine uptake and a 63-74% inhibition of the increase of draining lymph node T cells (CD3+), B cells (CD19+), and dendritic cells/macrophages (Mac-3+). Overall, the clear conclusion that tumor-derived gangliosides inhibit syngeneic antitumor immune responses implicates these molecules as a potent factor in promoting tumor formation and progression.

Granulocyte-macrophage colony-stimulating factor induces the differentiation of murine erythroleukaemia cells into dendritic cells

Dendritic cells (DC) are professional antigen-presenting cells (APC) within the immune system and antigen-pulsed DC can be used as an effective vaccine for active immunotherapy of cancer. Granulocyte-macrophage colony-stimulating factor (GM-CSF) plays an important role in the generation of DC. We previously showed that GM-CSF can induce murine erythroleukaemia cells (FBL-3) to differentiate into monocyte-like cells. To develop a new vaccinating method to stimulate the host immune response to leukaemia, we further investigate whether FBL-3 cells induced by GM-CSF can differentiate into DC in the present study. After being treated with GM-CSF, FBL-3 cells expressed high levels of 33D1 and NLDC-145, which are the specific markers of DC. The expression of MHC-II, B7-1, B7-2 and vascular cell adhesion molecule-1 (VCAM-1) was up-regulated markedly; the typical morphology of DC were also observed by electron microscopy. Functionally, the GM-CSF-induced FBL-3 cells could apparently stimulate the proliferation of naive allogeneic and autologous T lymphocytes and induce the generation of specific CTL more efficiently than the wild-type FBL-3 cells. Mice immunized with GM-CSF-induced FBL-3 cells could resist the subsequent challenge with the wild-type FBL-3 cells. Collectively, these data indicate that GM-CSF differentiates murine erythroleukaemia cells into DC phenotypically, morphologically and functionally. FBL-3-derived DC can be used as a new type of vaccine. Our results may have important implications for the immunotherapy of leukaemia.

Restriction of Friend virus-induced erythroid cell proliferation by CD4+ T-lymphocytes that recognize a single gp70 epitope

Friend murine retrovirus complex induces acute and fatal erythroleukemia when inoculated into immunocompetent adult mice. The development of leukemia after inoculation of Friend virus complex is controlled by several host genes. Some of the host genes influence immune responses against the viral antigens. Both CD4-positive T helper cells and CD8-positive cytotoxic T-lymphocytes specific for Friend viral antigens are required for spontaneous resistance against the virally induced leukemia. We have identified two separate T helper cell epitopes in the gp70 envelope glycoprotein encoded by the helper component of Friend virus complex. Immunization of mice with a synthetic peptide that represented one of the two T helper cell epitopes by a single injection with an adjuvant induced potent protective immunity against Friend virus-induced leukemia, even in the absence of CD8-positive T lymphocytes. In the immunized mice, virus-infected erythroid progenitor cells were rapidly eliminated from the spleen within two weeks after inoculation of the Friend virus. These data indicate unexpected importance and efficacy of CD4-positive T helper cells in immunity against retrovirus infections.

Generation of a novel Fli-1 protein by gene targeting leads to a defect in thymus development and a delay in Friend virus-induced erythroleukemia

The proto-oncogene Fli-1 is a member of the ets family of transcription factor genes. Its activation by either chromosomal translocation or proviral insertion leads to Ewing's sarcoma in humans or erythroleukemia in mice, respectively, Fli-1 is preferentially expressed in hematopoietic and endothelial cells. This expression pattern resembled that of c-ets-1, another ets gene closely related and physically linked to Fli-1. We also generated a germ line mutation in Fli-1 by homologous recombination in embryonic stem cells. Homozygous mutant mice exhibit thymic hypocellularity which is not related to a defect in a specific subpopulation of thymocytes or to increased apoptosis, suggesting that Fli-1 is an important regulator of a prethymic T-cell progenitor. This phenotype was corrected by crossing the Fli-1 deficient mice expressing Fli-1 cDNA. Homozygous mutant mice remained susceptible to erythroleukemia induction by Friend murine leukemia virus, although the latency period was significantly increased. Surprisingly, the mutant Fli-1 allele was still a target for Friend murine leukemia virus integration, and leukemic spleens with a rearranged Fli-1 gene expressed a truncated Fli-1 protein that appears to arise from an internal translation initiation site and alternative splicing around the neo cassette used in the gene targeting. The fortuitous discovery of the mutant Fli-1 protein, revealed only as the result of the clonal expansion of leukemic cells harboring a rearranged Fli-1 gene, suggests caution in the interpretation of gene-targeting experiments that result in either no or only a subtle phenotypic alteration.

The envelopes of two ecotropic murine leukemia viruses display distinct efficiencies in retroviral vaccination by interference

In cell cultures infected with a retrovirus, the expression of the viral envelope interferes with superinfection by retroviruses which recognize the same receptor. We have previously demonstrated that vaccination of susceptible strains of mice (of the Mus musculus species) with the attenuated ecotropic Friend murine leukemia virus (F-MuLV) B3 efficiently protects against the early hemolytic anemia and the erythroleukemia induced by a challenge with the virulent F-MuLV 57 through a similar in vivo mechanism of interference to superinfection (A. Corbin and M. Sitbon, J. Virol. 67, 5146-5152, 1993). Vaccination with the heterologous ecotropic Moloney-MuLV (M-MuLV) efficiently protects against the early hemolytic anemia but has a weak protective effect on the F-MuLV 57-induced erythroleukemia. Furthermore, vaccination with the attenuated F-MuLV B3 had only a transient protective effect on M-MuLV-induced thymomas. These different efficiencies of F- and M-MuLV to confer protection in this model of vaccination by interference were mostly due to envelope sequences, indicative of distinct in vivo interference properties of the two ecotropic envelopes.

Protection against retroviral diseases after vaccination is conferred by interference to superinfection with attenuated murine leukemia viruses

Cell cultures expressing a retroviral envelope are relatively resistant to superinfection by retroviruses which bear envelopes using the same receptor. We tested whether this phenomenon, known as interference to superinfection, might confer protection against retroviral diseases. Newborn mice first inoculated with the attenuated strain B3 of Friend murine leukemia virus (F-MuLV) were protected against severe early hemolytic anemia and nonacute anemiant erythroleukemia induced by the virulent strain 57 of F-MuLV. Vaccinated animals were also protected as adults against acute polycythemic erythroleukemia induced upon inoculation with the viral complex containing the defective spleen focus-forming virus and F-MuLV 57 as helper virus. Animals were inoculated as newborns, which is known to induce immune tolerance in mice, and the rapid kinetics of protection, incompatible with the delay necessary for the immune response to develop, indicated that protection was not due to an immune mechanism but rather was due to the rapid and long-lasting phenomenon of interference. This result was confirmed by combining parental and envelope chimeric MuLV from different interference groups as vaccinal and challenge viruses. Although efficient protection could be provided by vaccination by interference, we observed that attenuated replication-competent retroviruses from heterologous interference groups might exert deleterious synergistic effects.

Inhibition of retrovirus-induced disease in mice by camptothecin

We have previously shown that noncytotoxic doses of camptothecin (CPT), a topoisomerase I-specific antagonist, inhibit retrovirus replication in acutely and chronically infected cells. To evaluate the efficacy of CPT as an antiretroviral drug in vivo, we injected newborn BALB/c mice with Moloney murine leukemia virus and adult NFS mice with Friend spleen focus-forming virus. The Moloney murine leukemia virus-injected mice developed lymphoma, and the Friend spleen focus-forming virus-injected mice developed erythroleukemia. CPT, administrated together with the virus or 1 or 2 days after virus injection, prevented the onset of the disease in both cases. We showed that repeated CPT treatments increased the effectiveness of the drug when administrated 3 days after virus injection. This ability of CPT to inhibit retrovirus-induced disease in vivo without causing any apparent toxic side effects suggests its application as a legitimate remedy for the treatment of retroviral diseases.

Antitumor effect of thymosin alpha 1/interleukin-2 or thymosin alpha 1/interferon alpha,beta following cyclophosphamide in mice injected with highly metastatic Friend erythroleukemia cells

We investigated the effects of the systemic administration of thymosin alpha 1 plus relatively low doses of human recombinant interleukin-2 or very low doses of interferon alpha,beta in untreated and cyclophosphamide (CY)-treated DBA/2 mice challenged either subcutaneously or intravenously (i.v.) with Friend erythroleukemia cells (FLC). Both treatments resulted in the complete regression of subcutaneous tumor and cured a significative percentage of mice. They also increased the survival time of mice i.v. injected with large numbers of FLC. Neither immunotherapy alone nor CY, alone or in combination with single cytokines, produced similar effects. The antitumor action of these combined chemoimmunotherapy protocols seems to involve activation of the immune response since (a) a synergistic increase of the cytotoxicity of spleen cells was demonstrated in treated mice; (b) selective in vivo depletion of asialo-GM1, CD4, or CD8-positive cells abrogated this antitumor activity; and (c) a high lymphoid cell infiltration was found at the tumor site and in the livers of treated mice.

Cellular site and mode of Fv-2 gene action. II. Conditional protection of Fv-2ss cells by admixture with Fv-2rr cells

Fv-2ss marrow cells are protected in vivo from Friend virus-induced erythroleukemia by admixture with a preponderance of Fv-2rr marrow cells. This was demonstrated both in allophenic (mosaic) mice and in bone marrow chimeras constructed from C57BL/6 strains differing at Fv-2 and an enzyme marker (glucose phosphate isomerase). The bone marrow chimeras were constructed by injection of marrow cells from Fv-2ss mice into unirradiated Fv-2rr mice. Bone marrow chimeras derived from this procedure produced 1%-2% donor (Fv-2ss) erythrocytes; this level of chimerism was maintained indefinitely. All the bone marrow chimeras as well as allophenic mice with less than 20% Fv-2ss red cells failed to develop any of the symptoms of Friend disease after infection with the polycythemic strain of Friend virus. The Fv-2rr-mediated protection of Fv-2ss marrow cells could be reversed by pretreatment of the two types of chimeras with either monoclonal or polyclonal antithymocyte antisera. Chimeras treated with either reagent and infected with Friend virus developed symptoms of Friend disease and experienced a shift in their erythrocyte mosaic composition favoring cells of the susceptible genotype. These results are consistent with the notion that a functioning immune system plays a role in the Fv-2rr-mediated protection of Fv-2ss Friend virus target cells. Furthermore, these studies establish conditions whereby a small population of sensitive strain cells in an overwhelming background of resistant strain cells can be selectively expanded. Such conditions could be useful in efforts to clone the Fv-2 gene.

T-lymphocyte priming and protection against Friend leukemia by vaccinia-retrovirus env gene recombinant

The current prevalence of the acquired immune deficiency syndrome in humans has provoked renewed interest in methods of protective immunization against retrovirus-induced diseases. In this study, a vaccinia-retrovirus recombinant vector was constructed to study mechanisms of immune protection against Friend virus leukemia in mice. The envelope (env) gene from Friend murine leukemia virus (F-MuLV) was inserted into the genome of a vaccinia virus expression vector. Infected cells synthesized gp85, the glycosylated primary product of the env gene. Processing to gp70 and p15E, and cell surface localization, were similar to that occurring in cells infected with F-MuLV. Mice inoculated with live recombinant vaccinia virus had an envelope-specific T-cell proliferative response and, after challenge with Friend virus complex, developed neutralizing antibody and cytotoxic T cells (CTL) and were protected against leukemia. In contrast, unimmunized and control groups developed a delayed neutralizing antibody response, but no detectable CTL, and succumbed to leukemia. Genes of the major histocompatibility complex influenced protection induced by the vaccinia recombinant but not that induced by attenuated N-tropic Friend virus.

Immunoprevention of Friend leukaemia virus-induced erythroleukaemia by vaccination with aggregated gp70

Vaccines prepared from Friend leukaemia virus envelope and core polypeptides were compared for their efficiency in preventing erythroleukaemia in mice. High doses (100 micrograms) of gp85, the micellar complex of the envelope polypeptides gp70 and p15E, completely protected STU mice. The same dose of purified gp70 still protected about 80% of the animals, while p15E did not affect the cumulative mortality. The internal viral polypeptide p30 was ineffective. Serological examination indicated that immunity against death from leukaemia was mediated by specific antibodies. These leukaemia-preventing antibodies were predominantly induced by immunization with the gp70 env gene product, since p15E showed only minor protection. Glycoprotein gp70, however, was more effective when given as the gp85 micellar complex. An even more potent vaccine was obtained when gp70 was coupled to keyhole limpet haemocyanin (KLH) by glutaraldehyde. Ten micrograms gp70 coupled to KLH was enough to save more than 90% of Friend leukaemia virus-infected mice from erythroleukaemia. KLH may also be a suitable experimental carrier for subunits of gp70 or synthetic oligopeptides for viral vaccines.

Prevention of oncogenic viral infections in mice with CGP 11637, a synthetic muramyl dipeptide analog

The efficacy of N-acetyl-nor-muramyl-L-alanyl-D-isoglutamine (nor-MDP) in controlling viral oncogenesis in mice was investigated. The tumors studied were blood cell malignancies induced by Friend leukemia virus in SJL/J mice, spontaneous mammary neoplasms in RIII/Imr and C3H/OuJ mice, and spontaneous lymphocytic leukemia in AKR/J mice. A transplantable tumor, Lewis lung carcinoma, in C57BL/6J mice was used as a nonvirally induced control model. The nor-MDP was dissolved in saline and made into an emulsion with an equal volume of squalene-Arlacel A and injected subcutaneously at 1- to 2-month intervals. Test and control mice were challenged with exogenous virus or tumor transplant 2 weeks after a second injection of nor-MDP. Administration was started at around 2 months of age in mice that develop spontaneous neoplasms. Electron microscopy studies were done on neoplastic tissues of selected test and control mice. This administration of nor-MDP prevented erythroleukemia in SJL/J mice caused by low doses of Friend leukemia virus (although erythroleukemia survivors were not protected from a late-developing lymphoma) and also delayed (possibly prevented) the development of a spontaneous mammary neoplasm in RIII/Imr mice. No antitumor effects were observed on the spontaneous neoplasms of C3H/OuJ and AKR/J mice or on the Lewis lung carcinoma implanted into C57BL/6J mice. Electron microscopic examinations of the various neoplastic tissues indicated that nor-MDP administration eliminated or reduced extracellular viruses. The results suggested that under the experimental conditions used nor-MDP appears to effect the viruses and not the malignant cells per se.

Immunotherapy of murine leukemia VI. Development of immunologic memory in mice protected against Friend leukemia virus-induced disease by passive serum therapy

The resistance of DBA/2 mice to infection with a leukemogenic dose of Friend leukemia virus (FLV) as a result of the passive administration of chimpanzee anti-FLV serum is maintained for at least six months after infection and is accompanied by the continued synthesis of mouse anti-viral antibodies analogous to that previously found to be associated with short-term protection (approx. 30 days). Furthermore, the induction of immunologic memory in serum-protected mice is demonstrated by their long-term resistance to rechallenge with FLV of FLV-infected leukemic spleen cells, although cells of an established transplantable syngeneic FLV-erythroleukemia (FLC-745) are not rejected. In the course of these studies, a serum prophylaxis effect was defined in DBA/2 mice treated with the chimpanzee anti-FLV serum before, rather than after FLV infection. The prophylaxis effect is highly dependent on the time period between the last serum inoculation and virus challenge. The results indicate that the long-term resistance of therapeutically protected mice to subsequent virus or leukemic cell challenge is not due to the short-term prophylactic effect of serum treatment alone.

Suppression of extablished Friend virus leukemia by statolon: potentiation of statolon's leukemosuppressive activity by chlorite-oxidized oxyamylose

Treatment of Friend virus (FV)-infected mice, 3 days after FV inoculation, with statolon, an extract of the mold Penicillium stoloniferum, induces interferon and restores immunocompetence to viral and nonviral antigens such as sheep erythrocytes. Clinical remissions are established in 20 to 70% of the infected mice. Cholorite-oxidized oxyamylose administered intraperitoneally 24 h before, 3 h before, or 3 h after statolon enhanced interferon production, but the increased number of mice protected against FV disease was more closely related to the associated enhanced synthesis of FV cytotoxic antibody. The prolonged selective immunodepression to intraperitoneal sheep erythrocytes after intraperitoneal administration of chlorite-oxidized oxyamylose-statolon appeared to be related to a stimulation in number and erythrocyte-phagocytic capacity of peritoneal macrophages. The marked activation of macrophages in FV leukemic mice after such treatment may also have contributed to the enhanced FV leukemosuppressive effects of chlorite-oxidized oxyamylose-statolon.