TUMOR INDUCTION BY MURINE SARCOMA VIRUS IN AKR AND C58 MICE : Reduction of Tumor Regression Associated with Appearance of Gross Leukemia Virus Pseudotypes

Adult AKR and C58 mice injected intramuscularly with murine sarcoma virus, Moloney isolate (M-MSV), developed high incidence of nonregressing local tumors. Histologically, these tumors revealed the typical pleomorphism of M-MSV sarcomas; in some cases, however, neoplastic tissue showed a nodular or diffuse growth of monomorphic myoblastlike cells, reminiscent of clonal aggregates. No depression of immune reactivity was found in M-MSV-injected mice as evaluated by direct hemolytic plaque-forming cells against SRBC and by virus-neutralizing antibody production. The MSV recovered from the induced tumors proved to be, by neutralization assay, a Gross (G)-MSV pseudotype. Moreover, tumor cell suspensions absorbed out cytotoxic antibody directed against G-cell surface antigens. Therefore, the conclusion was drawn that MSV with envelope characteristics of endogenous G leukemia virus had formed in vivo through a phenotypic mixing phenomenon. The failure of tumors to regress has been interpreted as mainly due to the partial unresponsiveness of host immune reactivity towards G-MuLV specified antigens. Since MSV-tumors arose in AKR mice after a very long latent period, the possibility was considered that this relative resistance might depend on immunologic mechanisms. In fact, M-MSV-injected AKR mice immunodepressed by goat antimouse lymphocyte serum or rendered partially tolerant by neonatal M-MuLV inoculation developed sarcomas with higher incidence and with a shorter latency. Furthermore, the MSV recovered from these early tumors proved to be the original Moloney pseudotype

Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques

A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic1–8. For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in non-human primates. Fifty-two rhesus macaques (Macaca mulatta) were immunized with Ad26 vectors that encoded S variants or sham control, and then challenged with SARS-CoV-2 by the intranasal and intratracheal routes9,10. The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs after SARS-CoV-2 challenge. Titres of vaccine-elicited neutralizing antibodies correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in non-human primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials

DNA vaccine protection against SARS-CoV-2 in rhesus macaques

The global coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has made the development of a vaccine a top biomedical priority. In this study, we developed a series of DNA vaccine candidates expressing different forms of the SARS-CoV-2 spike (S) protein and evaluated them in 35 rhesus macaques. Vaccinated animals developed humoral and cellular immune responses, including neutralizing antibody titers at levels comparable to those found in convalescent humans and macaques infected with SARS-CoV-2. After vaccination, all animals were challenged with SARS-CoV-2, and the vaccine encoding the full-length S protein resulted in >3.1 and >3.7 log10 reductions in median viral loads in bronchoalveolar lavage and nasal mucosa, respectively, as compared with viral loads in sham controls. Vaccine-elicited neutralizing antibody titers correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate vaccine protection against SARS-CoV-2 in nonhuman primates

Replication of Radiation-Induced Murine Leukemia Virus in Normal and Transformed Mouse Cells

Autonomous radiation-induced leukemia virus (RadLV) replication could be detected in mouse 3T3 cells by the development of interference with murine sarcoma virus (MSV), the appearance of covert helper activity for defective MSV, and by the induction of cytopathic effect type foci in MSV-transformed, leukemia virus-negative (S+L−) cells. A chronic infection of either 3T3 or S+L− cells with RadLV could be established. Both RadLV infectivity and helper activity were demonstrated in the same peak at a buoyant density of 1.16 g/cm(3). Additionally a soluble inhibitor of MSV focus formation was found which could be separated from infectious RadLV. Examination of cell clones derived from chronically infected 3T3 cells showed that essentially every cell was infected and produced both infectious RadLV and low levels of inhibitor. Quantitative comparisons of autonomously replicating RadLV in normal 3T3 and S+L− cells suggested that RadLV may consist of several populations of virus of varying replicative potential. Apparently 99% of RadLV can be assayed only as helper units in normal cells or as replicative units in S+L− cells. To explain the atypical results, a model for RadLV deficiency is proposed

Effect of Helper Virus on the Number of Murine Sarcoma Virus DNA Copies in Infected Mammalian Cells

Cell lines of four mammalian species were each examined for the number of Moloney murine sarcoma virus (M-MSV) DNA copies in total cellular DNA after M-MSV transformation. Sarcoma-positive, leukemia-negative (S+L−) M-MSV-transformed cells were compared to M-MSV-transformed cells infected with a replicating leukemia virus. Both unfractionated M-MSV complementary DNA and complementary DNA representing the MSV-specific and the MSV-murine leukemia virus-common regions of the M-MSV genome were hybridized to total cellular DNA of various species. DNAs of mouse, cat, dog, and human S+L−cells contained from less than one to a few proviral M-MSV DNA copies per haploid genome. In contrast, helper virus-coinfected, M-MSV-producing cells of each species showed a 3- to 10-fold increase in M-MSV proviral DNA over that found in corresponding S+L− cells. MSV-specific and MSV-murine leukemia virus-common nucleotide sequences were each increased to a similar degree. A corresponding examination of cellular DNA of leukemia virus-infected normal or S+L− mammalian cells was performed to establish the resulting number of leukemia proviral DNA copies. The infection of normal or S+L− mammalian cells with several leukemia-type viruses that did not have nucleotide sequences closely related to the cell before infection resulted in the appearance of one to three corresponding leukemia proviral DNA copies

Expression of Murine Leukemia Virus Structural Antigens on the Surface of Chemically Induced Murine Sarcomas

Cultured cells of different chemically-induced C57BL/6N murine sarcomas produced variable amounts of infectious murine leukemia virus (MuLV) and contained proportional amounts of MuLV structural components as determined by radioimmunoassay. Monospecific antisera directed against the major MuLV glycoprotein (gp71), the major internal antigen (p30), and the ribonucleoprotein (p10) were capable of mediating tumor cell lysis in the presence of complement, suggesting that these viral structural components were localized at least in part to the cell surface. Membrane immunofluorescence studies with MuLV p30 antiserum confirmed surface localization. Addition of MuLV p30 polypeptide to normal cells and tumor cells enhanced the cytotoxicity of MuLV p30 antiserum. Studies are presented which suggest that the presence of MuLV structural components on cell surfaces can be independent of virus production and cellular transformation