In vitro cytotoxicity studies of murine sarcoma virus-induced immunity in mice

Structural and functional neuroimaging of late-life depression: a coordinate-based meta-analysis

Several neuroimaging studies have investigated localized aberrations in brain structure, function or connectivity in late-life depression, but the ensuing results are equivocal and often conflicting. Here, we provide a quantitative consolidation of neuroimaging in late-life depression using coordinate-based meta-analysis by searching multiple databases up to March 2020. Our search revealed 3252 unique records, among which we identified 32 eligible whole-brain neuroimaging publications comparing 674 patients with 568 controls. The peak coordinates of group comparisons between the patients and the controls were extracted and then analyzed using activation likelihood estimation method. Our sufficiently powered analysis on all the experiments, and more homogenous subsections of the data (patients > controls, controls > patients, and functional imaging experiments) revealed no significant convergent regional abnormality in late-life depression. This inconsistency might be due to clinical and biological heterogeneity of LLD, as well as experimental (e.g., choice of tasks, image modalities) and analytic flexibility (e.g., preprocessing and analytic parameters), and distributed patterns of neural abnormalities. Our findings highlight the importance of clinical/biological heterogeneity of late-life depression, in addition to the need for more reproducible research by using pre-registered and standardized protocols on more homogenous populations to identify potential consistent brain abnormalities in late-life depression.

Viruses Teaching Immunology: Role of LCMV Model and Human Viral Infections in Immunological Discoveries

Virology has played an essential role in deciphering many immunological phenomena, thus shaping our current understanding of the immune system. Animal models of viral infection and human viral infections were both important tools for immunological discoveries. This review discusses two immunological breakthroughs originally identified with the help of the lymphocytic choriomeningitis virus (LCMV) model; immunological restriction by major histocompatibility complex and immunotherapy using checkpoint blockade. In addition, we discuss related discoveries such as development of tetramers, viral escape mutation, and the phenomenon of T-cell exhaustion.

Immune recognition of tumor cells in mice infected with Pichinde virus

Pichinde virus (PV), a member of the Arenaviridae family, protects mice from a lethal inoculation with the sarcoma 180 (S180) tumor cell line. Virus replication, which is required for protection, occurs primarily in the spleen and tumor. During the first 4 days, elevated natural killer (NK) cell activity parallels an increase in serum interferon in PV-infected mice. On day 7 after infection virus-specific cytotoxic T cells (CTLs) are found in the mouse. This strong response peaks on day 13 and gradually declines over the next 17 days. The tumor-specific CTL response appears more slowly and is less intense than the virus-specific response, especially in the uninfected mouse. However, CTLs from either type of mouse recognize PV-infected tissue culture S180 target cells better than uninfected ones. Even though the primary tumor-specific immune response appears weak, mice that have cleared both virus and tumor are refractory to a subsequent challenge with S180 cells and rapidly produce tumor-specific CTLs. Thus, our data indicate a number of ways in which virus infection could lead to immune elimination of tumors: (1) Virus-induced interferon stimulates NK-cell activity, which in turn could control tumor load until a specific response is mounted against the S180 cells; (2) early onset of the tumor-specific T-cell response could be brought about by viral-enhanced tumor antigen presentation to the immune system; and (3) the tumor-specific T-cell response could be augmented through a "bystander' phenomenon involving factors associated with T cells responding specifically and vigorously to the virus itself.

Suppression of cell-mediated immunity to challenge with P 815 mastocytoma in concanavalin A-treated mice

C57Bl/6 (B6) mice allogeneic to the P 815 mastocytoma tumor cell line when treated with concanavalin A prior to and at frequent intervals following challenge intraperitoneally with 10(7) tumor cells showed a significant suppression of their cell-mediated immune response at 9-10 days when compared with untreated animals. Suppression of the immune response of mice syngeneic (DBA/2) or hybrid (BDF1) to the tumor was also evidenced by increased mortality rates in concanavalin A-treated animals. The suppression of cell-mediated cytotoxicity observed in B6 mice treated with concanavalin A could be reversed by pretreatment with 20 mg silica injected intraperitoneally 7 days prior to challenge. These results suggest that macrophages play a significant role in the concanavalin A-induced immune suppression observed in this in vivo tumor-host system.

Anti-tumor immunity in B-lymphocyte-deprived mice. III. Immunity to primary Moloney sarcoma virus-induced tumors

Tumor induction and immunity to tumors were studied following the injection of Moloney sarcoma virus (MSV) into mice whose B-lymphocyte functions had been suppressed by the chronic administration of anti-IgM antibodies. Two preparations of MSV were used; one which gives rise to tumors which uniformly regress in normal adult mice, and another which elicits progressively growing tumors in the majority of recipients. The tumor incidence, mean tumor size, and tempo of regression were not modified by treatment with anti-IgM. However, whereas tumors induced by the regressor virus were all rejected in 19 NRG-treated and 29 untreated recipients, continued growth was obtained in 2 of 23 B-lymphocyte-deprived mice. Furthermore, in 9 additional mice from this group, apparent rejection was followed by tumor recurrence at the site of the initial tumor. Continued growth was accompanied by widespread metastasis. These tumors were freely transplantable to normal syngeneic recipients. Metastasis and transplantability were also detected in 7 of 24 anti-IgM-treated mice given progressor virus, but were not seen in the control animals. Recurrence and metastasis was obtained despite the presence of high levels of specific cytotoxic T lymphocytes in the spleen. It is concluded that B lymphocytes or their products play an essential role in host protection against MSV-induced tumors.

Direct cell-mediated cytotoxicity in experimental allergic encephalomyelitis

Splenocytes from guinea pigs with experimental allergic encephalomyelitis were assayed in a short-term 51Cr release assay for cytolytic activity against target cells coated with myelin basic protein. The basic protein was attached to target cells via a concanavalin A bridge which had been complexed to the basic protein with glutaraldehyde. Cytolytic activity was detected in symptomatic animals and the activity could be boosted by culture in vitro with additional basic protein. Histone was used as a control antigen.

Immunization against primary, transplanted and spontaneous murine leukaemia using a live Moloney sarcoma virus vaccine

The purpose of this study was to use an immunization protocol with Moloney sarcoma virus (MSV-M) as active immunogen against exogenous and endogenous leukaemia. The s.c. route was chosen since it offered advantages over the i.m. route: the primary sarcomas were smaller, the regression faster, there were fewer recurrences and there was good persistent immunity. Strong protection was obtained against primary leukaemias induced by Friend leukaemia virus (FLV), Moloney leukaemia virus (MLV), Rauscher leukaemia virus (RLV), Precerutti-Law leukaemia virus (PLLV/T2), and H179A leukaemia virus. It was not possible to protect against leukaemia induced by Gross leukaemia virus (GLV). With transplantable leukaemias the results varied: partial protection was observed against H110 leukaemia (induced with human material) and R14 leukaemia (induced by X-irradiation) whilst no protection was obtained against P277 leukaemia (induced by Moloney leukaemia virus). As for spontaneous leukaemias, immunized BALB/c mice showed an increased incidence over the controls, while in F1 (Swiss x AKR) mice the incidence was similar but the latent period was shorter. Furthermore, in long-term observations the MSV-M-immunized mice showed an increased mortaltiy, which could be related to (1) new phenotypic mixtures between MSV-M and leukaemia viruses; (2) reactivation of MSV-M sarcoma-genesis with age, and (3) genotype susceptibility to MSV-M.

Tumors induced by progressor sarcoma virus (Moloney) in mice: growth in the presence of an immune response and isolation of autonomously growing cells

Courses of tumors, which had been induced in adult STU mice with a regressor and with a progressor strain of Moloney sarcoma virus (MSV-M) were followed and compared. All 73 tumors induced by the regressor strain of MSV-M (R-MSV-M) regressed and 181 of 183 tumors induced by a progressor strain (P-MSV-M) grew progressively and killed their hosts between 16 and 171 days after infection. Even after inoculation of about 4 FFU of P-MSV-M tumor development may occur and lead to progressively growing tumors. Both strains of MSV-M induce strong immune responses in their host, namely cytotoxic effector cells, cytotoxic antibodies and tumor resistance. Simultaneous injection of mice on separate sites with P-MSV-M on one side and R-MSV-M producing ascitic cells on the other side did not induce mutual influences on the different tumor courses. It is therefore concluded that the immune response is obviously not decisive for the failure of P-MSV-M induced tumors to regress. From seven P-MSV-M induced primary tumors two producer and five nonproducer transformants could be isolated, maintained in culture and partly held as easily transplantable tumors in adult immunocompetent mice. Both producer transformants release sarcoma virus with the capacity to induce progressively growing tumors in adult mice. All transformants induce progressively growing tumors after transplantation independent of the applied cell dose. It is concluded that the ability of the P-MSV-M to induce stably transformed cells (in contrast to R-MSV-M, WEILAND et al., Brit. J. Cancer, 1979) favours clonal development and ultimately progressive tumor course, in spite of the presence of an immune response.

Functional heterogeneity and T cell-dependent activation of macrophages from murine sarcoma virus (MSV)-induced tumors

In studies on the functional activity of macrophages isolated from murine sarcoma virus (MSV)-induced tumors, we have found that these cells may suppress immune responses as well as act as effector cells against the tumor. Previously, we reported that macrophages from the tumor could inhibit the antitumor response by suppressing proliferation-dependent immune functions. Here, we demonstrate that macrophages can also suppress the production of migration inhibition factor (MIF) and macrophage activation factor (MAF), two lymphocyte activities that are independent of cell proliferation. Conversely, we and others have found that macrophages from the tumor can exert an antitumor, cytolytic effect. In this study, using 1 g velocity sedimentation separation techniques, we have been able to identify 2-3 subpopulations of cytolytic macrophages in regressing tumors but in progressing tumors, only the smallest subpopulation of macrophages was active. T cells appeared to be required for activation of macrophages within the tumor, since MSV tumors induced in athymic, nude mice did not contain cytolytic macrophages.

Immunochemical characterization of tumor-associated surface antigens on a Moloney leukemia virus-lymphoma, MBL-2

Tumor-associated surface antigens (TASA) on a Moloney leukemia virus (M-MuLV)-induced lymphoma, MBL-2, in C57BL/6 mice (B6) were characterized. The surface proteins of MBL-2 cells were selectively radioiodinated and then extracted by Nonidet P40. The solubilized materials were then reacted with a variety of antisera: monospecific antisera to murine leukemia viral proteins (anti-gp69/71, anti-p30, anti-p15, anti-p12 and anti-p10), sera from B6 which regressed murine sarcoma tumors induced by murine sarcoma virus (anti-MSV) and a rabbit anti-MBL-2 antiserum. The resulting radioimmune precipitates were analyzed and compared in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The following results were obtained. (1) Among all anti-viral protein antisera tested only anti-gp69/71 was active and detected a protein doublet of gp69/71 and its degradation fragments of 42,000 and 35,000 daltons. (2) Radioimmune precipitates prepared with anti-MSV showed a SDS-PAGE pattern similar to that seen with anti-gp69/71. This result indicated that the surface antigen detected by the anti-MSV serum on MBL-2 tumor cell was probably a viral envelope antigen. (3) The rabbit anti-MBL-2 serum detected on the cell membrane an antigen of approximately 95,000 daltons which was tumor-associated and did not appear to be related to virion components. The anti-MBL-2-serum still reacted with the 95,000 dalton antigen after absorption with disrupted M-MuLV virus and with gp69/71 and p30 purified from the virus.

The availability for recognition of normal H-2 antigens by cytotoxic T lymphocytes on a Rauscher-virus-transformed cell, RBL-5A

The availability for recognition of cell surface H-2Db antigenic determinants on RBL-5A cells in comparison to normal C57BL/L cells was investigated by an in vitro immunological assay. No differences in the immunological recognition of the H-2Db antigens on RBL-5A cells compared to normal C57BL/L cells were detected. This assay system, however, readily detected changes in the H-2Kb determinant profile on target cells from the C57BL/L H-2Kb region mutant strains H(zl) and H(zl70) when compared to normal C57BL/6 mice. The result obtained with RBL-5A target cells therefore suggests that Rauscher murine leukaemia virus transformation does not induce, either genotypically or phenotypically, an immunologically recognizable alteration of the H-2Db antigen profile on this cell.

Suppression of lymphokine production by macrophages infiltrating murine virus-induced tumors

Immune spleen cells from mice injected with murine sarcoma virus (MSV) can produce migration inhibition factor (MIF) in vitro after stimulation with intact tumor cells. We have now found that this effector function can be regulated by suppressor macrophages present in the tumor. The cells from the tumor (CfT) exert a strong suppressive effect even after treatment with anti-Thy 1.2 plus complement but lose this capacity if adherent or phagocytic cells are removed, suggesting that the macrophage is the suppressor cell. The effects of the suppressor cells were exerted on a proliferation-independent function of the lymphocytes, since MIF production, as well as its suppression, occurred even after proliferation of the lymphocytes was blocked by mitomycin-C. The implications of these findings are that macrophages can suppress some early events involved in the activation of lymphocytes by antigen and that they do so through a mechanism that does not relate to the proliferation of the cells.

Association of mouse major histocompatibility and Rauscher murine leukaemia virus envelope glycoprotein antigens on leukaemia cells and their recognition by syngeneic virus-immune-cytotoxic T-lymphocytes

Physical association was measured between MLV gp70, the envelope glycoprotein of Rauscher murine leukaemia virus (R-MLV), and serologically defined H-2 antigens on the surface of R-MLV transformed C57BL/6 (H-2DbKb) mouse leukaemia cells (RBL-5A). Capping and patching with antisera against H-2Db caused specific co-capping and co-patching of R-MLV gp70 antigens as seen by fluorescence microscopy. Despite the physical proximity of R-MLV gp70 and H-2Db antigens, high titre alphaR-MLV gp70 sera had no effect in blocking syngeneic T-lymphocyte mediated cytolysis of RBL-2A cells whereas alphaH-2Db sera were effective.

Immunological study of two stocks of Moloney sarcoma virus producing regressor and progressor tumors in C57BL/6 mice

The immune response to progressor murine sarcoma virus (P-MuSV)2 and regressor murine sarcoma virus (R-MuSV) was analyzed by the radioisotopic footpad assay (FPA) for delayed hypersensitivity (DH) and by the membrane immunofluorescence assay for anti-tumor antibodies. In mice injected with P-MuSV, the footpad response remained at a low level for 7 weeks, while membrane immunofluorescence antibody titers reached their maximum levels after 5 weeks. By contrast, in mice injected with R-MuSV, the footpad response rose to a maximum by 4 weeks and membrane immunofluorescence antibody titers reached a maximum after 4 weeks. The adoptive footpad response reached its highest level in 2 weeks. A most striking difference between the P-MuSV and the R-MuSV systems was the lower immunogenicity of tumor cells induced by the P-MuSV as measured by the FPA. Considerable differences between the two stocks were also observed in oncogenic activity and tumor growth patterns. The P-MuSV contained one-tenth the amount of helper Moloney leukemia virus found in R-MuSV. Addition of more helper Moloney leukemia virus to the P-MuSV did not produce regressor tumors.

Immune responses to weakly immunogenic virally induced tumors. I. Overcoming low responsiveness by priming mice with a syngeneic in vitro tumor line or allogeneic cross-reactive tumor

This report describes model systems which show low primary in vitro syngeneic cytotoxic responses to a Moloney-induced YAC tumor (syngeneic in A mice) and a Rauscher-induced RBL5 tumor (syngeneic in C57BL/6 mice) and examines different approaches to overcome these defects. Two major findings were obtained: (a) spleen cells from A mice, injected with tumor cells from an in vitro tumor line YAC-1, derived from YACL, could generate a significant syngeneic cytotoxic response. In contrast, spleen cells from A mice injected with tumor cells from the in vivo tumor line failed to generate a syngeneic cytotoxic response. Thus, tumor cells from the in vitro line were more immunogeneic that those from the in vivo line. (b) Spleen cells from A mice which were injected with the cross-reactive allogeneic tumor RBL5, could generate significant cytotoxic responses to the syngeneic tumors YAC and YAC-1. Similarly, spleen cells from C57BL/6 mice injected with the cross-reactive allogeneic tumor YAC-1, could generate a significant cytotoxic response to the syngeneic tumor RBL5. Thus, cross-reactive allogeneic tumors could stimulate syngeneic cytotoxicity. The theoretical and the practical implications of these studies are discussed.

Natural cell-mediated cytotoxicity in mice against non-lymphoid tumor cells and some normal cells

Lymphocytes from normal mice were found to have cell-mediated cytotoxicity, in a short-term 51Cr release assay, against a variety of non-lymphoid tumor cells as well as against lymphomas. Some of the non-lymphoid tumors were as susceptible to natural cytotoxicity as the standardly used lymphoid lines. Some tissue culture cell lines and in vivo passaged tumor lines were susceptible to lysis, as were some primary virus-induced lymphomas. Tumor which arose in nude mice, which have high levels of natural cytotoxic activity, were all resistant to lysis. In addition to the susceptibility of transformed cells to natural cell-mediated cytotoxicity, some untransformed cultured cells and cells from normal tissues were targets for this mechanism. Low levels of cytotoxicity were seen with normal thymus cells, bone-marrow cells, and short term cultures of macrophages, whereas normal spleen and lymph-node cells were completely resistant to lysis. These results indicate a broader spectrum for mouse natural cell-mediated cytotoxicity reactivity than has been previously recognized.

Detection of cytotoxic lymphoid spleen cells from STU-mice with Moloney sarcoma by a 3H-proline microcytotoxicity assay

A microcytoxicity assay with 3H-proline prelabeled target cells was used for the detection of sensitized lymphoid spleen cells from STU inbred mice inoculated with Moloney sarcoma virus (MSV-M) or ascitic MSV-M tumor cells. The target cell line was derived from ascitic MSV-M tumor cells. With regard to the specificity of the assay nonimmune slpeen cells displayed no or only a weak cytotoxicity against these cells, and this was also the case when 3H-proline-labeled secondary cultures of syngeneic mouse embryo cells were exposed to both sensitized and nonimmune spleen cells. The time-course pattern of the development of cytotoxic lymphoid spleen cells in STU mice inoculated intramuscularly either with MSV-M or ascitic MSV-M tumor cells was studied. At the stages of tumor development, peak tumor size, and tumor regression the lymphoid spleen cell preparations were found to have relatively strong cytotoxic activity independent of whether the tumor was induced by MSV-M inoculation or tumor cell transplantation. However, in the latter case effector cells appeared earlier and were demonstrable for a longer period than in MSV-M-inoculated mice. Anti-theta serum treatment of lymphoid spleen cells taken at the stage of peak tumor size abrogated the cytotoxic activity or diminished it considerably indicating a T-lymphocyte response.

Mixed lymphocyte/tumor-cell interaction in a murine sarcoma virus (Moloney)-induced tumor system. Comparison between lymphoproliferation and lymphocyte cytotoxicity

Studies of the MLTI were performed in a primary MSV-induced tumor system. Reactivity against mitomycin-C-treated RBL-5 cells could be detected during a limited period at about 14 days after virus injection. The degree of reactivity of MSV 14 spleen cells was quite variable; stimulation indices ranged from 2.2 to 9.4. Activity in the MLTI appeared to a great extent to be dependent on T-lymphocytes. A relatively broad spectrum of specificity was demonstrated in experiments using additional stimulating cells besides RBL-5. The kinetics of the MLTI correlated well with the kinetics of the primary cytotoxic response in the CRA against RBL-5 cells. However, positive results in the in vivo and in vitro secondary cytotoxicity assays could be detected between 21 and 100 days after MSV injection when the MLTI was negative. Transplantation resistance against challenge with RBL-5 tumor cells also persists for a long time after MSV inoculation. Our data therefore indicate that the MLTI, in contrast to the secondary cytotoxicity assays, does not correlate well with the status of in vivo immunity.

Studies on the role of the host immune response in recovery from Friend virus leukemia. I. Antiviral and antileukemia cell antibodies

The humoral immune response to Friend virus leukemia was studied in congenic F1 mice differing in their incidence of recovery from leukemia. Antiviral neutralizing antibodies rose in titer in vivo concurrently with disappearance of viremia and fall in spleen virus levels. Cytotoxic antileukemia cell antibodies also appeared at this time. Passive transfer of these antibodies could inactivate low numbers of leukemia cells in vivo; however, mice of both high and low recovery genotypes produced antibodies in equal titer and recovered from viral infection in spite of striking differences in recovery from leukemic splenomegaly. Mice lacking C57BL genes did not produce antibodies or recover from viremia except in rare instances. Recovery from splenomegaly was found to be influenced by three or more C57BL genes independent of the H-2 complex.

Studies on the role of the host immune response in recovery from Friend virus leukemia. II. Cell-mediated immunity

Congenic mouse strains differing only at genes within the H-2 complex were found to have virus-specific cytotoxic effector cells in their spleens during or after recovery from Friend leukemia virus-induced splenomegaly. These effector cells were theta-positive T lymphocytes which functioned in vitro without help or inhibition by B lymphocytes or glass-adherent cells. The antigenic specificities recognized by the effector cells were viral-induced cellular antigens apparently different from those identified by serological techniques.

Cellular immune reactivity in vitro and tumor rejection provided by tumor-associated antigens of friend-virus-induced leukemia

Cell-mediated immunity (CMI) and tumor rejection were studied in the Friend virus leukemia system of C57Bl/6 mice. Mice were immunized with Friend leukemia virus (FLV) or X-irradiated FBL-3 leukemic cells and studied temporally for the development of CMI reactivity by assays of 51Cr release lymphocyte cytotoxicity, lymphocyte transformation, migration inhibition, Winn tumor cell neutralization and transplantation rejection. High levels of specific lymphocyte cytotoxicity were observed by day 7 f0llowing FLV infection; this reactivity reached a peak between 17 and 21 days, and returned to background levels by day 36. Further, positive Winn assays were obtained with spleen cells from mice immunized with FLV at times when the mice resisted live FBL-3 tumor challenge. Positive lymphocyte transformation was obtained with spleen cells from mice immunized with FLV or FBL-3, but not with cells from normal mice or mice immune to a syngeneic methycholanthrene-induced tumor, when cultured with papain-soluble FBL-3 or RBL-5 tumor-cell extracts or mitomycin-C (MMC)-treated FBL-3 or RBL-5 cells. Positive reactivity in the lymphocyte transformation assay occurred after reactivity had peaked in the lymphocyte cytotoxicity test. Similar positive macrophage migration inhibition patterns were also obtained with peritoneal exudate cells (PEC) from FLV-immunized mice using papain-solubilized tumor-associated antigen (TAA) from FBL-3 cells. These data suggest that sequential development and modulation of CMI reactivity occurs as observed in different assays following immunization in this system.

Natural cytotoxic reactivity of mouse lymphoid cells against syngeneic acid allogeneic tumors. I. Distribution of reactivity and specificity

Lymphoid cells from many normal mice of a variety of inbred strains were found to have reactivity, in a 51Cr release cytotoxicity assay, against several syngeneic and allogeneic tumors. Very high reactivity was seen with effector cells from athymic nude mice, which was consistent with other evidence that the reactivity was not T-cell dependent. Target cells susceptible to lysis included tumors induced by oncogenic type-C viruses but also tumors induced by other means and expressing endogenous type-C viruses. The levels of natural reactivity were influenced by age, with highest cytotoxicity produced by cells from 5- to 8-week-old mice. Lymph-node cells, spleen cells, peritoneal exudate cells and peripheral blood lymphocytes all had cytotoxic reactivity. The specificity of the reactions was analyzed in detail by ana inhibition assay. Evidence was obtained for natural reactivty against several different antigens, each apparently associated with expression of murine endogenous type-C viruses.

Cell-medicated cytotoxicity against ectromelia virus-infected target cells. III. Role of the H-2 gene complex

The role of the H-2 gene complex in expression of cytotoxicity exerted by specific ectromelia-immune thymus-derived (T) cells against ectromelia-infected target cells was examined. A repertoire of inbred mouse strains (some congenic) including the H-2 haplotypes k, d, b, s, q, the recombinant H-2a(k/d) and F1 hybrids (k/b and d/b) were immunized with virus and their spleen cells tested 6 days later, at the peak of the primary response, against H-2k,H-2d and H-2b target cells. Significant specific cytotoxicity occurred only when the immune cell donors and the target cells shared all or part of the same H-2 gene complex. For example, H-2a (k/d) immune cells killed both H-2k and H-2d target cells. There was no detectable effect of the non-H-2 genetic background, H-2 public specificities, or the M-locus. Target cells infected with ectromelia virus exhibited quantitative or qualitative changes (or both) in expression of normal H-2 antigens as indicated by reduced susceptibility to killing by T cells activated against H-2 antigens in mixed lymphocyte culture. These data are consistent with the hypothesis that T cells in this system are responding to virus-induced, specific changes in antigens on infected cells which are controlled by genes in the H-2 complex; these genes seem likely to be those coding for H-2 private specificities, or genes closely linked to them.

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.

Evidence of suppressor cell activity in spleens of mice bearing primary tumors induced by Moloney sarcoma virus

Spleens from Moloney sarcoma virus (MSV) tumor-bearing C57BL/6N mice contained four times the normal number of mononuclear cells and displayed a markedly elevated "spontaneous" (mitogen-independent) DNA synthesis on a per cell basis. The number of macrophages were increased three-fold while there was a slight reduction in the percentage of T lymphocytes. The phytohemagglutinin (PHA) response on a per cell basis of spleens from tumor-bearing mice was decreased about 90% when compared with normal control mice. The primary in vitro immune response to sheep red blood cells was also suppressed to levels of less than 10% of normals. The PHA response could be restored by purification of MSV spleen cells by rayon adherence columns and by removal of phagocytic cells by an iron/magnet technique. The activity of suppressor cells in MSV spleens was demonstrated in mixtures with syngeneic normal spleen cells where a marked impairment of the PHA response was observed. Spleen cells from tumor-free nude mice and normal spleen cells treated by anti-theta serum plus guinea pig complement (C'), both totally unreactive to PHA, had no such effect. The inhibitor cell in MSV spleens was shown to be insensitive to inactivation by anti-theta plus C', but could be removed by the adherence columns and the iron/magnet technique. These data suggest that this suppressor cell is a cell of the monocyte/macrophage series. Suggestive evidence was also presented that the suppressor cells belong to a proliferating population in MSV spleens. Similar suppressor cells have been previously demonstrated in spleens of mice during a variety of immune responses. Our data show, that a tumor, although stimulating the immune system, nevertheless may be suppressive on certain immune functions through the activation of suppressor cells.