Studies on Murine Sarcoma Virus: antigenic characterization of Murine Sarcoma Virus induced tumor cells

Moloney leukemia virus-induced cell surface antigen mimicry by monoclonal antibodies

We have investigated antigen-independent modulation of immune responses by monoclonal antibodies directed against both viral and nonviral antigens. BALB/c mice were immunized with monoclonal IgM (i.e. Ab1) specific for either Moloney murine leukemia virus-induced cell surface antigen (MCSA) or the hapten 2,4-dinitrophenyl (DNP). Injection with either Ab1 activated a functional idiotypic (Id) network as evidenced by production of both anti-Id (Ab2) antibodies and anti-anti-Id (Ab3) antibodies. A subset of induced Ab3 (designated Ab1'), exhibited specificity for antigen (virus or DNP). In mice immunized with anti-Id antibodies (Ab2), production of Ab3 and Ab1' was also observed. In the MCSA system, antibody-induced Ab1' responses were effective in protecting mice from tumor development upon subsequent challenge with live virus. Furthermore, antigen-independent modulation of immunity to both viral and nonviral antigens was found to be thymus-dependent. Similar findings in other viral systems suggest that antibody-induced activation of Id networks may prove a viable alternative vaccine strategy that can elicit antigen-specific responses, and in some cases protection, in the apparent absence of exposure to antigen.

Immune response to polyoma tumor cells in mice--III. Stimulation of tumor cell growth in vitro by spleen cells from immunized animals

We have evaluated the stimulation of target cell growth in vitro by spleen cells from mice which were immunized with polyoma-transformed cells and other tumor and non-tumor antigens. Stimulation was particularly seen under conditions of immunization that were suboptimal for the production of specific cytotoxicity. Significant stimulation of polyoma target cell growth was observed by lymphocytes from mice immunized against 10(5) Py 4198 tumor cells. This stimulation of target cell growth was not confined to polyoma-transformed cells only. Cells transformed by SV40, H-MuSV and non-transformed cells like 3T3 and embryo fibroblasts were also stimulated. Immunization of mice with syngeneic embryo fibroblasts also resulted in stimulation of tumor cell growth by the spleen cells from the immunized mice. However, the growth stimulation was less consistent and did not occur in all target cells tested. The specificity of immunostimulation was further studied with the Moloney sarcoma virus (M-MuSV) system; an antigenically distinct tumor system. Spleen cells from M-MuSV tumor-bearing mice stimulated cell growth in vitro not only against MuSV-transformed cells but also with SV40-transformed and polyoma-transformed cells as targets. Significant stimulation of target cell growth was also observed by spleen cells from mice that were immunized against 'non-pertinent' antigens, e.g. sheep red blood cells and allogeneic (C57B 1/6) spleen cells.

Antibody-dependent cell-mediated cytotoxicity in the Moloney sarcoma virus system: differential activity of IgG and IgM with different subpopulations of lymphocytes

Antibody-dependent cell-mediated cytotoxicity in the Moloney sarcoma virus (MSV) system was evaluated in terms of the differential ability of IgG and IgM from MSV regressor animals to induce cytotoxicity by lymphocytes from lymph node, spleen, and thymus. The cell-mediated cytotoxicity induced by both IgM and IgG was specific for target possessing the appropriate virally determined cell surface antigen(s). IgM induced cytotoxicity by lymphocytes from all the organs tested. However, differences in magnitude and efficiency were revealed. Lymph node cells and thymocytes were most efficient against IgM-coated target cells. Against IgG-sensitized target cells, spleen and lymph node cells were about equally active, but thymocytes were inactive. Cortisone treatment of the donors of effector cells revealed that the cortisone resistant subpopulation of thymocytes, 2 days after cortisone injection, exhibited an increased cytotoxicity against target cells treated with unfractionated antiserum and its IgM fraction. This subpopulation of thymocytes was also cytotoxic against IgG-coated target cells. At 12 days after cortisone injection, the repopulated thymus showed little change in activity, compared to control thymus, against antibody-coated target cells.

Antigenic specificities of the cell-mediated anti-tumor reactions in the MSV system studied by the secondary chromium release test

The SCRT and the inhibition test have been used to determine the specificity of cell surfacr antigens reacting with anti-M-MSV cytolytic lymphocytes. The method provides very sensitive and specific results. Some discrepancies exist between the results of SCRT and those obtained by the in vitro inhibition by tumor cells of cell-mediated immune cytolysis. The main point is that allogeneic cells are stimulatory in SCRT, whereas they are not reactive in the inhibition test. Several hypotheses are discussed to explain these discrepancies. In all the experiments, a strong secondary stimulation of cytotoxic lymphocytes was obtained in vitro when FMRGi (+) cells were used as stimulators, whatever the nature and the histocompatibility antigens of these cells. This suggests that an antigen of the "FMRGi system" is regularly involved in the cell-mediated anti-MSV reaction. However, other antigenic specificities of different natures are probably also concerned due to the antigenic complexity of these tumors.

Comparative in vitro sensitivity of twom methylcholanthrene-induced murine sarcoma lines to humoral and cellular immune cytotoxicity

MC57M and G are the tissue-culture derivatives of two methylcholanthrene-induced murine C57Bl sarcomas. Their sensitivity to immune cytoxic3, cytostatic or cytolytic spleen cells and sera was compared in parallel in vitro assays. The level of cross-reactivity displayed by the two lines was found to depend on the nature of the immune effector rather than on the assay which was used. It was high with immune spleen lymphocytes, alone or in the presence of decomplemented antisera, and low with antisera in the presence of rabbit complement. MC57G cells were more sensitive than MC57M cells to both effectors. Both cell lines were insensitive to antibody-dependent cell-mediated cytotoxicity. Preliminary evidence is presented, suggesting a probable involvement of embryonic and Moloney leukemia virus-induced cell surface antigens in the vitro sensitization of the two tumor lines to immune sera.

IgM-induced tumor cell cytotoxicity mediated by normal thymocytes

In summary, we have found that IgM, from mice which have undergone regression of primary MSV tumors, will induce cytotoxicity against the appropriate target cells by normal splenocytes and normal thymocytes. The thymocyte-induced cytotoxicity i

Antigens of tumours induced by naturally occurring murine sarcoma virus (MSV-FBJ). II. Detection of cell-surface antigens by indirect membrane immunofluorescence

Cell surface antigens expressed by cells transformed in vivo by FBJ virus, a wild type murine sarcoma virus (MSV) complex derived from a spontaneously arising sarcoma in a CF1 mouse, have been studied by indirect membrane immunofluorescence (MIF). Using mouse antisera raised by immunization of syngeneic CBA mice with transplanted FBJ sarcomata an antigen common to all FBJ tumours was detected which was also present on Gross (G) antigen positive tissues, viz. leukaemic and preleukaemic AKR lymphoid cells, but absent from the tissues of mice of G negative strains. Failure to demonstrate antigenic cross-reactivity in reciprocal MIF tests using FBJ immune sera and antisera to MSV-H (Harvey), an MSV isolate of Friend-Moloney-Rauscher (FMR) sub-group specificity, established the virus type-specificity of antigens expressed by sarcoma cells transformed by the respective MSV.The presence of a cellular antigen with G specificity on FBJ sarcoma cells was confirmed in tests with aged exbreeding C57B1 antisera containing naturally occurring G antibody lacking significant virus neutralizing activity. However, evidence for a "sarcoma-non-leukaemia" antigen on cells transformed by MSV-FBJ was not obtained since absorption studies failed to reveal any specificity on FBJ sarcoma cells which was not also present on AKR leukaemic tissues.It is suggested that the major humoral component of the immune response to FBJ sarcoma cells is evoked against antigens specified by the associated non-pathogenic leukaemia virus (MLV-FBJ) and the relationship of antigens demonstrated by MIF to those detected previously by complement fixation (CF) and tumour rejection tests is discussed.

Antigens of tumours induced by naturally occurring murine sarcoma virus (MSV-FBJ). I. Detection of group and type specific antigens by complement fixation

Antigens associated with cells transformed in vivo by FBJ virus, a wild type murine sarcoma virus (MSV) complex originating from a spontaneously arising osteosarcoma in a CF1 mouse, have been partially characterized by complement fixation (CF). Using rat antisera against antigens specified by Gross leukaemia virus (GLV) the group specific (gs) antigen of C-type RNA murine tumour viruses was demonstrated in FBJ tumours as well as in GLV rat leukaemias, AKR lymphomata and sarcomata induced by MSV-H (Harvey), an MSV isolate of Friend-Moloney-Rauscher (FMR) subgroup specificity. Using mouse antisera against antigens present in FBJ cells the Gross (G) or wild type specificity of FBJ tumours was demonstrated by cross reactivity with antigens expressed on normal AKR lymphoid tissues and leukaemias. These antigens were absent from MSV-H induced sarcomata and in reciprocal tests mouse antisera to MSV-H failed to react with antigens present in FBJ tumour cells. No distinction between cellular and virion antigens expressed by FBJ cells was possible by CF although evidence for a cellular antigen with G specificity was obtained in tests using aged C57B1 antiserum containing a naturally occurring G antibody lacking significant virus neutralizing capacity. However, the likelihood that mouse FBJ antisera contain antibodies to type specific viral envelope antigens (VEA) as well as cellular antigen is discussed.

Prevention of murine sarcoma virus oncogenesis in offspring of immunized female mice

BALB/c mice born to and nursed by females immunized against MSV-M showed a reduced tumour incidence and a high tumour regression rate following MSV-M injection at 7-14 days of age. Females immunized long before mating could also confer protection to their offspring whereas females immunized after parturition could not. A reduced number of tumours was observed in 3 out of 14 MSV-M injected litters whose mothers had been previously exposed to the virus while nursing infected offspring. Sera from suckling mice born to and nursed by immunized mothers contained MSV-M neutralizing antibody as shown by an in vitro focus reduction assay. Cell-free extracts from mice which developed leukaemia after MSV-M inoculation were tested for oncogenic activity in 1-week old mice. Out of 6 extracts, 4 induced typical MSV-M tumours and 2 caused leukaemias.

The lymphocyte response to primary Moloney sarcoma virus tumors in BALB-c mice. Definition of the active subpopulations at different times after infection

Adult BALB/c mice were injected with Moloney sarcoma virus (MSV) after which the animals' lymphocytes were examined for activity against Moloney leukemia virus (MLV) antigen-bearing target cells at 5-day intervals for 30 days. Lymphocytes from these animals and appropriately matched controls were fractionated into B cell-deficient (primarily T cells) and T cell-deficient (primarily B cells) subpopulations. Macrophages were removed using iron powder and magnetism. The unfractionated lymphocytes, T cells, and non-T cells were then tested in microcytotoxicity tests. Antigen-specific activity was found in the unfractionated lymphocytes from animals that had not yet developed palpable tumors and from regressor animals. The T cells were active just before tumor development and just after regression; however, by day 30 after virus infection (8-10 days after regression) the T cell subpopulation was much less active. The non-T cell subpopulation was also active before tumor development and soon after regression. However, this activity continued to rise after regression and was highest at 30 days. At day 15 (peak tumor size) neither subpopulation was active. The activity was demonstrated to be specific for the MLV-determined cell surface antigen by testing on control target cells that were MLV antigen negative and by comparison of the inhibitory effects with lymphocytes immune to a nonpertinent antigen as well as normal lymphocytes. The non-T cells were tested for activity before and after removal of macrophages with iron powder and magnetism. Such cells were significantly more active after removal of the macrophages. These data demonstrate specific T cell and non-T cell activity in microcytotoxicity tests with a tumor-specific system and strongly suggest that the non-T cell activity described herein is a B cell function.

Tumour-associated transplantation antigens of neoplasms induced by a naturally occurring murine sarcoma virus (FBJ-MSV)

FBJ osteosarcoma virus (FBJ-MSV) isolated originally from a spontaneously arising osteosarcoma in a CF1 mouse is the only known naturally occurring murine sarcoma virus (MSV). It is unique among strains of MSV in producing primarily sarcomata in mice. The capacity of tumour cells transformed in vivo by this agent to elicit specific transplantation immunity in syngeneic hosts was investigated. A low level of resistance (10(4)-10(5) cells) was consistently induced by implantation of x-irradiated (15,000 rad) tumours or surgical excision of developing subcutaneous grafts. By contrast intraperitoneal inoculation of virus containing cellfree extracts of FBJ-MSV sarcomata was a far less effective immunization procedure. Confirmatory evidence for the antigenicity of these neoplasms was obtained in tests in which preincubation of tumour cells with lymphoid cells from specifically immune donors inhibited in vivo outgrowth of the FBJ-MSV cells in untreated syngeneic recipients. The induction of host resistance to FBJ-MSV cells by immunization with identical and independently-induced FBJ-MSV tumours established that FBJ-MSV cells possess common cell surface antigenic specificities in a manner analogous to those of experimental neoplasms induced by other oncogenic DNA and RNA viruses. Since FBJ-MSV cells release infectious virus it was not possible in this system to establish whether the tumour-rejection antigen was cellular or virion in nature. The antigenic weakness of FBJ-MSV cells in syngeneic hosts is comparable with that of virus-induced murine leukaemias of the Gross (G) or "wild" type subgroup to which category FBJ-MSV also belongs. These features suggest that FBJ-MSV exemplifies naturally occurring sarcomagenic viruses more closely than those of the Friend-Moloney-Rauscher-Graffi (FMRGr) subgroup which in general induce highly antigenic neoplasms.

Demonstration of a cell-surface antigen associated with murine sarcoma virus by immunoelectron microscopy

Cells transformed by murine sarcoma virus have been examined for the presence of a new virus-associated cell-surface antigen by immunoelectron microscopy. A common antigen has been detected on the surface of nonproductively transformed cells that were induced by two different strains of murine sarcoma virus, Kirsten and Moloney. This antigen shows crossreaction with cell lines transformed by murine sarcoma virus that were produced in two different mammalian species, rats and mice. Further, this antigen is distinct from previously described antigens on the surfaces of cells infected by murine leukemia virus, on the viral envelope, and on the surfaces of spontaneously transformed cell lines or cell lines transformed by x-irradiation.

Lack of distinctive surface antigen on cells transformed by murine sarcoma virus

Some murine sarcoma virus (MSV)-transformed mouse 3T3 cells contain the MSV genome in the absence of infectious helper murine leukemia virus (MuLV) and MSV production. These cells, designated S+L- (sarcoma positive, leukemia negative), were analyzed for the presence of a possible MSV-determined membrane antigen by the mixed hemadsorption test and in vitro lymphocyte cytotoxicity assay. Two different serological approaches were used: (a) isoantibody-free sera were obtained by immunizing with MSV of syngeneic origin or by allowing primary, autologous MSV sarcomas to regress, or (b) alloantisera obtained by immunizing C57BL mice with S+L- cells were absorbed with the corresponding nontransformed 3T3 cells until all activity against 3T3 had been removed. While MuLV-superinfected S+L- cells and a culture line of an MSV sarcoma known to produce both MSV and MLV were highly reactive, normal 3T3 and S+L- cells were negative. Similarly, lymph node cells from MSV immune mice or rats did not kill S+L- cells, although they were cytotoxic against target cells known to carry MuLV-associated antigens. Thus, the present study gives no positive evidence for the existence of any MSV-induced new surface antigen in the transformed target cell, known to carry the viral genome.

Antigenic properties of murine sarcoma virus-transformed BALB-3T3 nonproducer cells

The isolation of clonal lines of murine sarcoma virus-transformed, non-producer BALB/3T3 cells has provided a model system for determining whether RNA tumor virus-transformed cells possess virus-specific transplantation antigens. MSV nonproducer cells (K-234) were clonally derived from an inbred mouse cell line, BALB/3T3. A parallel virus-producing cell line was obtained by infection of the MSV nonproducer cells with Rauscher leukemia virus. K-234 was much more tumorigenic than K-234(R). Preimmunization of syngeneic mice with either K-234(R) or with UV-inactivated Rauscher leukemia virus induced transplantation resistance to subsequent challenge with K-234(R), but not with K-234. In contrast, mice preimmunized with nonproducer cells were not made resistant to subsequent challenge with the homologous cells. Antisera prepared from mice immunized with K-234(R) were specifically cytotoxic and positive by fluorescent antibody staining for K-234(R) target cells, but not to either BALB/3T3 or K-234. The results show that MSV nonproducer cells lack detectable transplantation antigens and suggest that the transplantation resistance to the producing cells is attributable to maturing virus at the cell surface.

Fluorometric quantitation of fluorescein-coupled antibodies attached to the cell membrane

A method was devised which allows the measurement, by direct and indirect fluorescence assay, of the binding of fluorescein-coupled antibodies directed against cell-membrane antigens. The systems used were: (1) species-specific antigens, (2) genetically determined isoantigens, (3) virus-determined, tumor-specific antigens, and (4) membrane-bound immunoglobulin in a Burkitt cell line. Titration of antibodies gave straight lines in the log versus log scale, the slope of the titration curve probably being dependent on the density of antigen receptors on the cell surface.