The murine sarcoma virus-induced tumor: exception or general model in tumor immunology?

Resistance to Moloney Murine Sarcoma Virus (M-MuSV) Tumor Induction is Associated with Natural Antibody Production to «Endogenous» Moloney Leukemia Virus (M-MuLV) in Balb/Mo Mice

BALB/Mo mice are characterized by early expression of « endogenized » M-MuLV and are resistant to M-MuSV tumor induction. Furthermore, compared to normal BALB/c mice, sera from BALB/Mo mice exhibit a significant reactivity which is specific for M-MuLV when tested in a 125I-labelled Staphylococcus protein A binding assay. The possible significance of this reactivity in conferring tumor resistance is explored.

Role of Infectious Virus Expression and Immune Response in Retrovirus-Induced Oncogenesis

Some of the parameters involved in retrovirus-induced oncogenesis were analyzed in newborn mice injected with Moloney-murine leukemia virus (M-MuLV) as well as in adults who received the virus by the intraperitoneal (i.p.) or intrathymic (i.t.) route. The neonatally injected mice were permissive for both viral replication and virus-induced cell-surface antigen expression on thymus cells, peripheral T and B lymphocytes, and macrophages, whereas the M-MuLV was not present in the adult i.p. injected mice. Instead, in i.t. injected mice, the virus was expressed in thymus and peripheral T cells only, but was not detected in tail extracts as assessed by means of the UV-XC plaque assay. Lack of virus spread in adult-treated animals correlated with a prompt humoral and cellular immune response, whereas the highly viremic newborn inoculated mice showed negligible virus-specific antibody production and an extremely low frequency of splenic cytotoxic T lymphocyte precursors. Moreover, immune response in both groups of adult-treated mice efficiently prevented tumor induction by Moloney-murine sarcoma virus (M-MuSV), which has the same antigenic determinants as M-MuLV, its natural helper. In contrast, M-MuSV sarcomas grew progressively in newborn inoculated mice and killed the host. Finally, 80 % of neonatally injected mice developed lymphomas, whereas all treated adults remained free of disease for more than 15 months. These findings imply that the immune response may, in fact, prevent retrovirus-induced oncogenesis through restriction of virus replication and/or destruction of virus-infected cells.

Tumour antigens recognized by T lymphocytes: at the core of cancer immunotherapy

In this Timeline, we describe the characteristics of tumour antigens that are recognized by spontaneous T cell responses in cancer patients and the paths that led to their identification. We explain on what genetic basis most, but not all, of these antigens are tumour specific: that is, present on tumour cells but not on normal cells. We also discuss how strategies that target these tumour-specific antigens can lead either to tumour-specific or to crossreactive T cell responses, which is an issue that has important safety implications in immunotherapy. These safety issues are even more of a concern for strategies targeting antigens that are not known to induce spontaneous T cell responses in patients.

Evaluation of the therapeutic efficacy of tripeptide tyroserleutide (YSL) for human hepatocarcinoma by in vivo hollow fiber assay

Tyroserleutide (YSL), extracted from spleen of pigs, is a tripeptide that has shown therapeutic efficacy in an experimental BEL-7402 human hepatocarcinoma model. The hollow fiber assay (HFA) is a solid tumor model for large-scale screening of potential anticancer compounds that minimizes expenditures of materials, time, and money. Tumor cells are cultivated within biocompatible, semipermeable hollow fibers, which are implanted in immunosuppressed mice. In this study, the HFA was used to investigate the therapeutic efficacy of YSL for human hepatocarcinoma. In vitro effects of YSL on human hepatocarcinoma cell lines BEL-7402, SMMC-7721, Hep3B, HepG2, and SK-HEP-1 were assayed by the MTS method. In vivo effects of YSL on the five human hepatocarcinoma cell lines were assayed by HFA. Mice implanted with tumor cells in hollow fibers were treated with YSL, and the effects of YSL on tumor cell populations were assessed by MTT assay. YSL significantly inhibited the proliferation of the five human hepatocarcinoma, both in vitro and in vivo (P < 0.05). The HFA is a rapid, accurate, and economical method for evaluating the inhibitory effects of drugs on different tumor cells in vivo. These results support the clinical application of YSL for treatment of human hepatocarcinoma.

Infection and cancer: the common vein

The role of infectious agents in the development of cancer is well documented. The pathogenesis of various human neoplasms ranging from non-Hodgkin lymphoma (NHL) to cervical carcinoma frequently involves a chronic, most often viral, infection. At the same time, there is compelling evidence that certain acute infections result in the inhibition of neoplastic growth. The basis for this phenomenon is often thought to be concomitant anti-tumor immunity. Yet, experimental data supporting this hypothesis are scarce, and other non-immune anti-tumor factors could be involved. For instance, since virtually all aggressive tumors outstrip their blood supply, development of new vessels, or angiogenesis, is a limiting factor during neoplastic growth. In this review, we will discuss recent studies that implicate anti-angiogenesis in infection-mediated tumor suppression and suggest that this mechanism could also complement cytotoxic immunity arising from the use of cancer vaccines.

CD28 is not required for rejection of unmanipulated syngeneic and autologous tumors

To gain a better understanding of the requirement of CD28 co-stimulation in different types of T cell-dependent tumor rejection responses, we performed a series of syngeneic and autologous tumor rejection experiments on CD28 knockout mice. In a preimmunization-challenge model, virally-induced ALC lymphoma and methylcholanthrene-induced MC57X fibrosarcoma transplants were rejected similarly by syngeneic CD28 knockout and immunocompetent controls. ALC-specific cytotoxic T lymphocytes (CTL) and MC57X-specific tumor necrosis factor (TNF) release were induced in CD28 knockouts, although at a reduced level in the latter case. Secondly, the spontaneous regression of Moloney murine sarcoma virus (MMSV)-induced primary tumors in the autologous hosts occurred equally in CD28 knockouts and in immunocompetent control mice. A comparable virus-specific CTL response was generated in both, as revealed in cytolytic assays against RBL-5 targets. Thirdly, the spontaneous rejection of the B7-transfected EL-4 lymphoma by immunocompetent hosts was abrogated in CD28 knockout mice, since more than 82% CD28 knockouts developed tumors after inoculation with B7-transfected EL-4 cells. Our results therefore show that CD28 co-stimulatory molecules are not required for the rejection of unmanipulated syngeneic tumors in hyperimmunized hosts and the regression of MMSV-induced sarcoma in autochthonous hosts.

Identification of genes coding for tumor antigens recognized by cytolytic T lymphocytes

Strategies have been developed to characterize tumor antigens recognized by cytolytic T lymphocytes (CTL). We use a genetic approach based on the transfection of HLA genes and cDNA libraries in COS cells to isolate the gene producing the antigenic peptide. The tumor-specific expression of this gene can be evaluated by cDNA synthesis and quantitative PCR amplification. Transfection of fragments of the isolated gene allows the identification of the region encoding the antigenic peptide. Peptides are synthesized and tested for their ability to sensitize target cells to lysis by the CTL.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Decreased tumor surveillance in perforin-deficient mice

Immune surveillance against tumors usually depends on T cell recognition of tumor antigens presented by major histocompatibility complex (MHC) molecules, whereas MHC class I- tumors may be controlled by natural killer (NK) cells. Perforin-dependent cytotoxicity is a major effector function of CD8+ MHC class I-restricted T cells and of NK cells. Here, we used perforin-deficient C57BL/6 (PKO) mice to study involvement of perforin and Fas ligand in tumor surveillance in vivo. We induced tumors in PKO and normal C57BL/6 mice by (a) injection of different syngeneic tumor cell lines of different tissue origin in naive and primed mice; (b) administration of the chemical carcinogens methylcholanthrene (MCA) or 12-O-tetradecanoylphorbol-13-acetate (TPA) plus 7,12-dimethylbenzanthracene (DMBA), or (c) by injection of acutely oncogenic Moloney sarcoma virus. The first set of models analyzes the defense against a tumor load given at once, whereas the last two sets give information on immune defense against tumors at the very moment of their generation. Most of the tumor cell lines tested were eliminated 10-100-fold better by C57BL/6 mice in an unprimed situation; after priming, the differences were more pronounced. Lymphoma cells transfected with Fas were controlled 10-fold better by PKO and C57BL/6 mice when compared to untransfected control cells, indicating some role for FasL in tumor control. MCA-induced tumors arose more rapidly and with a higher incidence in PKO mice compared to C57BL/6 or CD8-deficient mice. DMBA+TPA-induced skin papillomas arose with similar high incidence and comparable kinetics in both mouse strains. C57BL/6 and PKO mice have a similar incidence of Moloney murine sarcoma and leukemia virus-induced sarcomas, but tumors are larger and regression is retarded in PKO mice. Thus, perforin-dependent cytotoxicity is not only a crucial mechanism of both cytotoxic T lymphocyte- and NK-dependent resistance to injected tumor cell lines, but also operates during viral and chemical carcinogenesis in vivo. Experiments addressing the role of Fas-dependent cytotoxicity by studying resistance to tumor cell lines that were stably transfected with Fas neither provided evidence for a major role of Fas nor excluded a minor contribution of Fas in tumor surveillance.

On immunological memory

Immunological memory is a hallmark of the immune system. Evolution can teach us which effector arms of immunological memory are biologically relevant against which virus. Antibodies appear to be the critical protective mechanism against cytopathic viruses. Since these viruses cause cell damage and disease directly, particularly in the absence of an immune response, mothers protect their offspring during a critical immunoincompetent period (a consequence of MHC- restricted T cell recognition) by passive transfer of neutralizing antibodies. In contrast, CTL appear to be the crucial effector mechanism against noncytopathic viruses. Since MHC polymorphism has made vertical transmission of T cell memory impossible, immunoincompetent offspring are not, and need not be, protected against such noncytopathic viruses. During the primary response and again during secondary infection, the most important function of CTL is to eliminate noncytopathic viruses, which may otherwise cause lethal immunopathology. Increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody; B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a noncytopathic virus, CTL have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is available yet of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Identification of an H-2 Kb-presented Moloney murine leukemia virus cytotoxic T-lymphocyte epitope that displays enhanced recognition in H-2 Db mutant bm13 mice

Upon infection with the Moloney murine sarcoma virus-murine leukemia virus (MuLV) complex, H-2b C57BL/6 (B6) mice respond with a class I Db-restricted cytotoxic T-lymphocyte (CTL) response, which protects against virus-induced tumorigenesis. In the B6-derived Db mutant B6.CH-2bm13 (bm13) strain, part of the class I Db antigen-presenting groove is shaped by a class I Kb-encoded sequence. Like B6 mice, bm13 mice reject Moloney virus-induced tumors, but the protective CTL response is Kb restricted. In this study we show enhanced levels of Moloney MuLV-specific CTLp with a restriction for Kb in bm13 mice. Through the use of CTL clones from Moloney virus-immunized bm13 mice, the class I Kb-presented CTL epitope was identified. The epitope is located in the Moloney virus gp70 envelope protein region (Moloney envelope, amino acids 189 to 196 [Mol env (189-196)]), SSWDFITV and has the Kb allele-specific binding motif. The Dbm13 molecule does not present the env(189 to 196) epitope to Kb-restricted bm13 CTL. In B6 mice, Mol env(189-196)-specific CTL could be induced by peptide vaccination. B6 mice thus have CTL precursors specific for this epitope but at considerably lower levels than do bm13 mice. We hypothesize that additional positive selection of Kb-restricted CTL on the Dbm13 molecule in bm13 mice explains this difference in precursor frequencies. We examined related strains of MuLV for the presence of Mol env(189-196) sequence equivalents. Rauscher, Friend, and AKV MuLV-encoded Mol env(189-196) epitope equivalents were properly recognized in cytotoxicity assays, both as synthetic and as endogenously expressed (Rauscher MuLV) peptides. In contrast, the mink cell focus-forming virus MuLV-encoded epitope equivalent, lacking a Kb anchor residue, was not presented for CTL recognition and hence can be excluded as an important CTL epitope for mink cell focus-forming viruses.

Multiplicity of virus-encoded helper T-cell epitopes expressed on FBL-3 tumor cells

To identify retroviral antigenic determinants recognized by CD4+ T helper cells during tumor rejection, we established four noncytolytic, helper-type, CD4+ T-cell clones by limiting dilution cultures of mixed lymphocyte-tumor cultures from mice immune to a Friend virus-induced tumor, FBL-3. Among these, three T helper cell clones were isolated from C57BL/6 mice and the fourth was isolated from a (BALB/c x C57BL/6)F1 mouse. All these clones proliferated in response to the immunizing FBL-3 tumor cells in a major histocompatibility complex class II-restricted manner. Each clone expressed a distinct T-cell receptor with a characteristic combination of alpha and beta chains. The localization of helper T-cell determinants on viral proteins was analyzed with recombinant vaccinia viruses expressing Friend murine leukemia virus (F-MuLV) gag or env genes or shorter fragments of the env gene. Epitopes recognized by these T-cell clones were mapped to at least two distinct portions in the env region of the F-MuLV genome. These epitopes were identified more precisely with synthetic peptides derived from the F-MuLV envelope protein sequence. One of these epitopes was common to Friend and Moloney MuLVs and was located in the N-terminal region of the gp70 glycoprotein at amino acids 122 to 141. The second epitope, which was recognized in the context of hybrid I-Eb/d major histocompatibility complex class II molecule, was located close to the C-terminal end of gp70 at amino acids 462 to 479. In addition, a possible third epitope was located in the N-terminal half of the gp70 sequence and differed from the first epitope in that it was not cross-reactive with the Moloney MuLV envelope protein.

Expression of the MuLV-tumor-associated antigen is restricted to MuLV-transformed cells

Immunization of mice with Moloney murine leukemia virus (MoMuLV) induces the generation of a population of CTL which recognizes a non-viral, tumor-associated antigen (TAA) expressed on MuLV-induced tumors. To determine whether this TAA could be used as a pre-leukemic or leukemic cell marker, CTL clones directed against Moloney viral and TAA antigen were used to analyze viral and TAA antigen expression on chronically infected and leukemic lymphoid cells obtained from mice inoculated neonatally with MoMuLV. Although both sets of cells could be recognized and lysed by viral antigen specific CTL, they are not recognized by TAA-specific CTL. Only after transformed cell lines were established from leukemic spleen cells could susceptibility to TAA-specific CTL be observed. Thus, the appearance of the MoMuLV-TAA was restricted to MoMuLV-transformed cells.

Transfection and expression of a gene coding for a human melanoma antigen recognized by autologous cytolytic T lymphocytes

Human melanoma line MZ2-MEL expresses several antigens recognized by autologous cytolytic T lymphocytes (CTL). As a first step towards the cloning of the gene coding for one of these antigens, we tried to obtain transfectants expressing the antigen. The DNA recipient cell was a variant of MZ2-MEL which had been selected with a CTL clone for the loss of antigen E. It was cotransfected with genomic DNA of the original melanoma line and with selective plasmid pSVtkneo beta. Geneticin-resistant transfectants were obtained at a frequency of 2 x 10(-4). These transfectants were then screened for their ability to stimulate the production of tumor necrosis factor by the anti-E CTL clone. One transfectant expressing antigen E was identified among 70,000 drug-resistant transfectants. Its sensitivity to lysis by the anti-E CTL was equal to that of the original melanoma cell line. When this transfectant was submitted to immunoselection with the anti-E CTL clone, the resulting antigen-loss variants were found to have lost several of the transfected pSVtkneo beta sequences. This indicated that the gene coding for the antigen had been integrated in the vicinity of pSVtkneo beta sequences, as expected for cotransfected DNA.

The gene coding for a major tumor rejection antigen of tumor P815 is identical to the normal gene of syngeneic DBA/2 mice

We showed previously that mouse mastocytoma P815 expresses several distinct antigens that are recognized by cytolytic T lymphocytes (CTL) of syngeneic DBA/2 mice. Antigens P815A and P815B are usually lost jointly and are targets for immune rejection responses in vivo. We used a cosmid library and a CTL stimulation assay to obtain transfectants expressing tumor rejection antigen P815A. From these transfectants we retrieved gene P1A which transferred the expression of both P815A and B. This gene is unrelated to three previously isolated genes coding for tum-antigens. It encodes a putative protein of 224 amino acids which contains two highly acidic domains showing homology with similar regions of nuclear proteins. The P1A gene expressed by tumor P815 is completely identical to the gene present in normal DBA/2 cells. Expression of the gene was tested by Northern blots. Cells from liver, spleen, and a number of mast cell lines were negative, but mast cell line L138.8A produced a high level of P1A message and was lysed by CTL directed against antigens P815A and B. We conclude that major tumor rejection antigens of P815 are encoded by a gene showing little or no expression in most normal cells of adult mice.

In vitro activation of bovine macrophage and peripheral blood mononuclear cells by Nocardia rubra cell wall skeleton (N-CWS)

Nocardia rubra cell wall skeleton (N-CWS) was used for immunotherapy to bovine leukemia virus (BLV)-positive cattle with enlarged subcutaneous lymphatic nodules. Electron microscopical observations showed the infiltration of macrophage and T cells around N-CWS treated lesions. Antitumor effect induced by N-CWS was examined in vitro. The maximum cytolytic activity of macrophage was observed, when the cells were incubated with 10 micrograms/ml of N-CWS. Chemiluminescence response of peripheral blood mononuclear cells (PBMC) using N-CWS as stimulant was observed with a high level of activity for a long period, 5 hr. Mitogenic effect of N-CWS to PBMC was observed in the presence of macrophages but not without macrophages. Furthermore, interleukin 2 activity was recognized in supernatant of PBMC cultured with N-CWS. Maximum cytotoxic T lymphocyte response was induced when PBMC were cultured with 0.1 micrograms/ml of N-CWS.

Modulation of immune response against tumor cells by the in vivo administration of an autoreactive Th clone

The immunization of C3H mice with irradiated syngeneic MM48 tumor cells induced specific tumor-neutralizing cells (TNC). The TNC activity was mediated by the L3T4+, Ly-2- T cell population, and the generation of TNC coincided with the appearance of delayed-type hypersensitivity response against MM48 antigen. The administration of an auto-I-Ak reactive T helper cell clone MS202 to normal C3H mice resulted in the facilitation of growth of MM48 tumor due to the induction of T suppressor (Ts) cells. Splenic T cells from animals given this T cell clone inhibited the TNC activity of immunized mice resulting in the escape of MM48 from the TNC effect. The surface phenotype of the Ts cells was L3T4+, Ly-2-. The Ts cells induced by the clone MS202 were totally antigen-nonspecific, and were able to suppress both the effector and inductive processes of TNC. The results suggest the presence of a physiological MHC-restricted T cell circuit that regulates immune responses against the growing tumors.

Enhancement and suppression of DTH reactivity to Rauscher murine leukaemia virus induced tumour cell lines

Delayed-type hypersensitivity (DTH) to Rauscher murine leukaemia virus (R-MuLV) encoded or induced determinants was induced in mice by three syngeneic R-MuLV-induced tumour cell lines, i.e. a myeloid tumour, RMB-1, an erythroid tumour, RED-1, and a lymphoid tumour, RLD-1. DTH to subcutaneously (s.c.) administered RMB-1 cells appeared on day 4, with a maximum DTH response on day 6 or 7. The induction of DTH could be prevented by intravenous (i.v.) pre-immunisation with R-MuLV-induced tumour cells several days before the s.c. immunisation. The three R-MuLV-induced tumour cell lines showed cross-reactivity in the DTH assay, whereas no cross-reactivity was found with syngeneic WEHI-3 cells. This indicates that the three R-MuLV-induced tumour cell lines share a virally encoded or induced antigenic determinant, which activates T-cells. When the RMB-1 cells used for immunisation had been cultured in medium supplemented with interferon-gamma (IFN-gamma), the subsequent DTH response was increased. This coincided with an increased expression of the R-MuLV-specific antigenic determinants on RMB-1 cells as demonstrated by Scatchard analysis. Furthermore, IFN-gamma increased the MHC class I antigen expression on RMB-1 cells, whereas the class II antigen expression remained undetectable.

Role of endogenous alpha/beta interferon in the selection of virus nonproducer Friend leukemia cells after serial intraperitoneal passages in syngeneic mice

Serial intraperitoneal passage of interferon (IFN)-sensitive Friend leukemia cells (FLCs) and L1210-S and RBL-5 tumor cells in syngeneic mice resulted in the selection of tumor cells exhibiting a marked decrease in the capacity to release reverse transcriptase (RT) activity. The virus nonproducer phenotype was a stable characteristic of clones derived from in vivo-passaged IFN-sensitive 745 FLCs. In contrast, in vivo passages of IFN-resistant 3Cl-8 FLCs and L1210-R cells did not result in any significant decrease in the capacity of these tumor cells to release in vitro RT activity. Although in vitro treatment of IFN-sensitive FLCs with mouse alpha/beta IFN (IFN-alpha/beta) for 1 or 10 passages resulted in a marked inhibition in the release of RT activity, these effects were completely reversible after removal of IFN from the culture medium. In addition, in vitro treatment of 745 FLCs with IFN resulted in a marked increase in the expression of H-2 (class I) and gp70 Friend virus antigens on the cell membrane. These effects were not observed in IFN-resistant 3Cl-8 cells. To investigate the possible role of endogenous IFN in the in vivo selection of virus nonproducer tumor cells, IFN-sensitive virus producer FLCs were serially passaged intraperitoneally in mice treated with antibodies to IFN-alpha/beta and in control mice, and the recovered tumor cells were cloned in vitro. Most (83 to 91%) of the clones derived from 745 cells recovered from control mice did not produce any detectable RT activity in the culture supernatants. In contrast, 96% of the clones (26 of 27) derived from 745 cells recovered from mice serially treated with antibodies to IFN-alpha/beta were still capable of releasing high levels of RT activity in the culture medium, indicating that endogenous IFN-alpha/beta was indeed an important host component for the in vivo selection of virus nonproducer tumor cell variants. The results reported in this article indicate that both direct effects of IFN on tumor cells and host-mediated effects are involved in this phenomenon.

Myosin isoform expression in rat rhabdomyosarcoma induced by Moloney murine sarcoma virus

Myosin isoform expression was analyzed in experimental rhabdomyosarcoma (RMS) using monoclonal antibodies (mAbs) and immunofluorescence techniques. Tumors induced by inoculating newborn rats with Moloney murine sarcoma virus (Mo-MSV) were examined 30-90 days after birth. Nine tumors and two lymph node metastases were studied by direct, indirect, and double immunofluorescence assays using a panel of five anti-myosin mAbs. The mAb BF-45 was specifically reactive with embryonic myosin heavy chain (MHC), mAb BF-34 was specific for a neonatal MHC epitope, mAb BF-B6 was directed against an epitope present in both embryonic and neonatal MHC, and mAbs BF-F3 and BF-32 detected epitopes present in adult MHC isoforms. Anti-desmin antibodies were also used for comparison. The results of this study show that: (1) the majority of neoplastic cells stained for desmin while only a minority of neoplastic cells were labeled by anti-myosin antibodies; (2) myosin positive tumor cells contained predominantly embryonic and neonatal MHC types but rare RMS cells reacted exclusively with anti-adult myosin antibodies; and (3) adult and embryonic MHC phenotypes were occasionally detected within the same tumor cell especially in RMS with the longest latencies. Together these results would suggest that the mechanism(s) regulating MHC gene expression in skeletal muscle cells can be altered by the transforming activity of Mo-MSV.

The generation of tumor-specific in vivo protective immunity in the tumor mass from mice rendered tolerant to tumor antigens

C3H/He mice were inoculated i.v. with 10(6) heavily X-irradiated syngeneic X5563 plasmacytoma cells 3 times at 4 day intervals. When these mice received an appropriate immunization procedure consisting of i.d. inoculation of viable tumor cells plus the surgical resection of the tumor which enables i.v. nonpresensitized mice to produce anti-X5563 immunity, they failed to develop tumor-specific immunity. This was demonstrated by the abrogation in potential of spleen and lymph node cells to generate in vivo protective immunity. In contrast, the tumor mass from X5563 tumor-bearing mice which had received the i.v. presensitization contained comparable anti-X5563 tumor neutralizing activity to that obtained from the tumor mass from nonpresensitized, X5563 tumor-bearing mice. Such an in vivo protective immunity was revealed to be mediated by tumor-specific T cells. These results demonstrate the differential generation and antitumor capability of tumor infiltrating T cells and T cells in lymphoid organs from mice which are in the tumor-specific tolerant state. The results are discussed in the context of potential utilization of tumor infiltrating in vivo protective T cells to enhance the local tumor-specific immunity in tumor-specific tolerant mice.

Friend murine leukemia virus and spleen focus-forming virus expression in highly malignant interferon-sensitive and interferon-resistant Friend leukemia cells

Analysis of expression of the Friend murine leukemia virus (F-MuLV) and of the spleen focus forming virus (SFFV) has been undertaken in highly malignant interferon (IFN)-sensitive (745) and IFN-resistant (3Cl-8) Friend leukemia cells (FLC), serially passaged intraperitoneally in DBA/2 mice. In vivo passaged 745 cells, as well as the clones derived thereof, did not release Friend virus (FV). Western blot analysis of the plasma membrane fractions of the virus nonproducer 745 cells revealed the lack of gp69/70 glycoprotein expression. At least 10 intraperitoneal passages of virus producer in vitro passaged of virus producer in vitro passaged 745 cells were necessary to obtain the selection of the virus nonproducer phenotype. In contrast in vivo passaged 3Cl-8 cells continued to produce FV even after 100 in vivo passages and expressed gp69/70 antigens to a similar extent as the original in vitro passaged FLC. The expression of F-MuLV and SFFV RNAs in virus producer and virus nonproducer FLC clones has been investigated by means of Northern blot technique using probes specific for either F-MuLV or SFFV. No F-MuLV specific RNA sequences were detected in virus nonproducer 745 clones. SFFV specific RNA transcripts and gp52/55 glycoprotein production could be revealed in all the FLC tested. Southern blot analysis showed the presence of F-MuLV specific sequences in the cellular DNA of virus nonproducer 745 clones. As both in vivo passaged F-MuLV producer 3Cl-8 and F-MuLV nonproducer 745 cells were equally barely immunogenic and highly malignant when injected into syngeneic DBA/2 mice, these results indicate that F-MuLV expression does not result per se in a high immunogenic potential of tumor cells. For the time being, as a specific property of 3Cl-8 versus 745 cells is the interferon-resistant phenotype, it is tempting to speculate that the selection of virus nonproducer cell variants after in vivo passages of interferon-sensitive 745 cells could depend on the presence of low levels of endogenous interferon in normal young mice.

The nature of immunity to Snyder-Theilen fibrosarcoma virus induced tumors in cats

Snyder-Theilen fibrosarcoma virus (ST-FeSV) induced tumors evoked a vigorous immune response in adolescent cats. The response was characterized histologically by a lymphoid and histiocytic cell infiltrate beginning around the 9th day post inoculation. Hyperemia edema, hemorrhage, and necrosis of the tumors occurred shortly thereafter. Gross regression of the tumors commenced around the 15th day. Viable fibrosarcoma cells could be recovered as almost pure cultures from tumors biopsied on the 9th day. Biopsies taken between days 9 and 15 contained progressively fewer tumor cells and increasing numbers of lymphoid cells, histiocytes, giant cells, and normal fibroblasts. Tumor cells in such mixed cultures did not replicate as fast as normal and died out within 7 to 14 days. Viable tumor cells were not recovered from biopsies taken after day 15. Fibrosarcoma regression was associated with the appearance of tumor cell specific cytotoxic lymphocytes and antibodies in the blood. Cell mediated immunity, as determined by a chromium release assay, consisted of both antibody dependent and independent mechanisms. Fluorescent and complement dependent cytolytic antibodies were detected in the blood at the same time as cytotoxic lymphocytes, but persisted after regression. In a preliminary experiment, serum from tumor regressor cats was injected into susceptible kittens, and the kittens were then challenged with ST-FeSV transformed fibroblasts or whole FeSV. Immune serum did not prevent the appearance of initial growth of tumors, but did slow their subsequent growth and increased the rate of regression. Immune serum had a much more dramatic inhibitory effect on the accompanying retrovirus infection.

Immunity against Moloney sarcoma virus in H-2Db mutant bm14 mice. Unimpaired tumor immunity despite absence of a virus-specific cytotoxic T-cell response

It was shown previously that B6.C-H-2bm14 (bm14) mice, carrying a mutation in the H-2Db locus, are unable to generate cytotoxic T cells (CTL) against Moloney murine sarcoma virus (M-MSV). We now report an analysis of tumor induction and regression kinetics and of immunity to the virus, following the injection of graded doses of M-MSV into C57BL/6 (B6) and bm14 mice. Contrary to expectation, bm14 mice showed slightly less tumors than wildtype B6 mice. Moreover, all bm14 mice that developed a tumor were able to reject this tumor, even after injection of the highest virus dose tested. From the spleen cells of bm14 mice that had rejected tumors, no secondary in vitro CTL responses could be generated, in contrast to strong CTL responses generated from B6 spleen cells. Although bm14 mice were unable to generate virus-specific CTL, they showed normal antibody and T-cell proliferative responses against Moloney virus, suggesting an intact T-helper-cell function. It is concluded that in bm14 mice, under the conditions tested, virus-specific CTL are not generated despite excellent tumor immunity. Therefore, this CTL response is not necessary for protection against M-MSV-induced tumors. Protection is likely to be mediated by a normal T-cell proliferative response.

Tolerance to viral antigens in Mov-13 mice carrying endogenized Moloney-murine leukemia virus

In virus-positive (V+) C57BL/6 Mov-13 mice, Moloney-murine leukemia virus (M-MuLV) is integrated into the germ line and expressed early in lymphoid and nonlymphoid organs. The pattern of immune reactivity to viral antigens in these mice was studied and compared to that of their virus-negative (V-) counterparts. Using a radioimmuno-binding or -precipitation assay, V- mice showed good antibody production after challenge with Moloney-murine sarcoma virus (M-MSV), but no antibodies were detected in V+ mice. Moreover, Mov-13 V+ mice failed to generate virus-specific cytotoxic T lymphocytes (CTL) in secondary mixed-leukocyte tumor cell culture (MLTC) while V- mice showed a strong cytotoxic response. This lack of activity in mass V+ cultures was due to a very low frequency of CTL precursors which did not increase following in vivo challenge with M-MSV or in vitro stimulation with MBL-2 Moloney leukemia cell line. These findings indicate that early M-MuLV activation in Mov-13 V+ mice induces a state of tolerance to viral antigens involving both the humoral and cellular immune responses and related to the high incidence of T-cell lymphomas.

Biological behavior of tumors and associated retroviremia in cats inoculated with Snyder-Theilen fibrosarcoma virus and the phenomenon of tumor recurrence after primary regression

The fate of tumors and associated retroviremia was studied in 111 cats infected with the Snyder-Theilen strain of feline sarcoma virus (FeSV). Tumors appeared at the site of inoculation within 7 to 10 days. A retroviremia, due mainly to the associated feline leukemia virus helper virus (FeLV-helper), developed at the same time as tumors. Of the cats, 44 developed progressively growing tumors and therefore had to be killed, and 67 developed tumors that regressed. There was a strong correlation between the persistence of the accompanying retroviremia and the growth of the tumors. The 44 cats with progressively growing fibrosarcomas remained retroviremic until death. Conversely, 53 of the 67 cats with solitary, regressing tumors were only transiently retroviremic. Tumor regression in these cats paralleled the disappearance of retrovirus from the blood. The fate of tumors and retroviremia was not always the same, however. Twelve cats remained persistently retroviremic after all signs of gross tumors disappeared. Two other kittens became nonviremic within 20 days after inoculation, yet tumors continued to grow and even metastasize for another 3 to 5 weeks before regressing. Fibrosarcomas recurred 3 weeks to 8 months later in 8 of 12 persistently retroviremic cats with regressed tumors. Although the blood and bone marrow from these cats contained predominantly FeLV-helper, tumor cells yielded both FeSV and FeLV-helper. Of 53 animals, 3 developed recurrent fibrosarcomas 5 weeks to 8 months after all signs of tumors and retroviremia had disappeared. Cells cultured from these tumors appeared initially like normal fibroblasts and were virus nonproducers. After one to three passages in culture, however, cells became malignantly transformed and replicated both FeSV and FeLV-helper. Cultures of the bone marrow from these and other nonviremic cats with regressed tumors yielded only FeLV-helper.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Altered transcription of genes coding for class I histocompatibility antigens in murine tumor cells

Three murine tumors induced by Moloney murine leukemia virus (M-MLV) which exhibited loss of some or all H-2 class I antigens at the cell surface were analyzed at the DNA and RNA level with molecular probes specific of H-2 heavy chains and beta 2-microglobulin sequences. No observable difference could be detected at the DNA level between the tumors and the parent animals. However, a decrease in H-2 mRNA was observed, especially in phenotypically H-2 negative tumor, BM5R, where H-2 transcripts were at least 30-fold less abundant. These results show that an H-2-negative character may result from a general alteration in the transcription of H-2 genes, which could reflect some kind of regulatory process.

Leukocyte adherence inhibition response to murine sarcoma virus-induced tumors. II. Requirement for T-cell subpopulations and macrophages

Murine sarcoma virus (MSV)-immune T cells from C57BL/6 mice respond to intact RBL-5 tumor cells with the production of leukocyte adherence inhibition factor (LAIF), which mediates an adherence inhibition response of macrophages. LAIF is elaborated by isolated Lyt-2+ cells incubated with RBL-5 cells, whereas Lyt-1+ cells elaborate a substance that enhances macrophage adherence. Spleen macrophages or peritoneal exudate macrophages from MSV-immune mice when present at concentrations of 0.1% changed the response of Lyt-1+ cells from the formation of an adherence enhancing factor to the formation of an adherence inhibiting factor. Migration inhibition factor (MIF) was formed by Lyt-1+ cells, but not by Lyt-2+ cells under identical culture conditions. Addition of either spleen macrophages from mice with progressively growing tumors or tumor-infiltrating macrophages suppressed LAIF formation by both Lyt-1+ and Lyt-2+ cells. Tumor-infiltrating macrophages elicited an adherence enhancing factor from Lyt-2+ cells when present at high concentrations. The results suggest that the extent of macrophage adherence in vitro is the outcome of an interaction of macrophages with mediators that have opposing effects.

Adoptive T cell immunotherapy of MSV-induced tumours in nude mice. Part I. Biology of tumour regression and recurrence

The induction of immunity to progressively growing murine sarcoma virus (MSV) tumours in nude (nu/nu) mice by reconstitution with immune T cells from syngeneic (+/+) donors has been studied. Whole spleen cell preparations served as the source of immune T cells. Transfer of immune, but not of normal, spleen cells resulted in partial or apparently complete regression of primary tumours and a related moderate to considerable extension of survival time. The dose, the time in days between immunization and transfer, as well as timing of the spleen cells in relation to tumour cell challenge, were all factors which influenced the effectiveness of the protective inocula. An unexpected consequence of even the very effective primary immunotherapy regimens, was secondary tumour development after varying tumour-free intervals. This was most frequently manifest as tumour recurrences at the original injection site either on their own or in combination with distant metastases. Such a relatively high frequency of tumour reappearance and metastatic spread contrasts markedly with the rare instances of secondary regrowth in normal immunocompetent mice. The present reconstitution system may therefore provide a new model for studying the inhibitory or stimulatory properties of T cells with respect to tumour regression and dissemination.

Specificity in the immunosuppression induced by avian reticuloendotheliosis virus

Several parameters of the cellular and humoral immune responses of chickens infected with reticuloendotheliosis virus (REV-T), an avian defective acute leukemia virus, or with its helper virus, reticuloendotheliosis-associated virus (REV-A), were evaluated. Spleen cells from chickens infected with REV-T (REV-A) or REV-A exhibited depressed mixed lymphocyte and mitogen responses in vitro. Allograft rejection was delayed by 6 to 14 days in birds infected with REV-A. The specific antitumor cell immune response was also studied by a 51Cr-release cytotoxicity assay. Lymphocytes from chickens infected with low numbers of the REV-T-transformed cells exhibited significant levels of cytolytic reactivity against the 51Cr-labeled REV-T tumor cells in vitro. The mitogen response of lymphocytes from these injected birds was similar to that of uninjected chickens. In contrast, lymphocytes from chickens injected with higher numbers of REV-T-transformed cells exhibited suppressed mitogen reactivity and failed to develop detectable levels of cytotoxic activity directed against the REV-T tumor cells. These results suggest that the general depression of cellular immune competence which occurs during REV-T (REV-A) infection could contribute to the development of this acute leukemia by inhibiting the proliferation of cytotoxic cells directed against the tumor cell antigens. The cytotoxic effect observed after the injection of chickens with non-immunosuppressive levels of REV-T-transformed cells appears to be specific for the REV-T tumor cell antigens since cells transformed by Marek's disease virus or avian erythroblastosis virus were not lysed. In marked contrast, birds whose cellular immune responses were suppressed by infection with REV-A were capable of producing a humoral immune response to viral antigens. Detectable levels of viral antibody, however, did not appear until 12 to 15 days after REV-A infection. Since REV-T (REV-A) induced an acute leukemia resulting in death within 7 to 14 days, it appears unlikely that the ability of chickens to make antiviral antibody influences the development of lethal reticuloendotheliosis.

Genetic control of sensitivity to moloney leukemia virus in mice. VI. Involvement of virus-specific T helper cells collaborating with B cells

T cell responses to Moloney virus involve cytolytic and helper lymphocytes. In contrast to specific cytolytic T lymphocytes, few studies have been devoted to the characteristics of helper T cells for antibody production. The present experiments describe an assay for Moloney virus-specific help for B cells using dinitrophenylated virus. This method, using the Moloney virus as a carrier in a hapten-carrier system, allows to definition of the specific helper function of antibody responses. T helper cells were induced in murine sarcoma virus or inactivated Moloney murine leukemia virus-primed spleens or lymph nodes. T helper function was due to Thy-1.2, Lyt-1+2- cells and was macrophage-dependent. It was stimulated by whole virus of Moloney gp71 envelope protein but not Moloney p30 internal protein. Cross-reactive stimuli were obtained with other dinitrophenylated type C viruses. High and low responses were correlated respectively with resistance and susceptibility to Moloney leukemia virus. Cultures of helper T cells with preserved activity have been established and maintained for one month.

Immunotoxicology studies on lead: effects of exposure on tumor growth and cell-mediated tumor immunity after syngeneic or allogeneic stimulation

Chronic exposure of C57BL/6 mice to lead acetate in the drinking water enhanced the growth of primary Moloney sarcoma virus (MSV)-induced tumors. Regression of MSV-induced tumors was not prevented by lead exposure and lead-treated animals were more resistant to late sarcoma development following primary tumor regression. The primary cell-mediated cytotoxic response in the spleen or lymph nodes of MSV-tumor bearing mice was significantly augmented by lead exposure. This augmentation appeared to reflect the increased antigenic stimulation resulting from the enhanced primary tumor growth in lead-exposed animals. Using an allogeneic tumor model, under conditions of similar antigenic stimulation, little effect of lead on T cell-mediated cytotoxicity was observed. On the other hand, macrophage phagocytic activity was significantly depressed in lead-exposed mice. Coupled with a decrease in the total number of macrophages recovered from lead-exposed mice, the results suggested significant impairment of macrophages function by lead. The influence of lead-induced macrophage dysfunction on tumor growth is discussed.

Antigenic stimulation of T lymphocytes in chronic nononcogenic retrovirus infection: equine infectious anemia

Equine infectious anemia is a chronic disease of horses caused by a nononcogenic retrovirus. Studies were undertaken to determine the types of cells involved in the in vitro lymphoproliferative response to viral antigens and the dynamics of this reaction. It was observed that reactive lymphocytes were present at unpredictable times in the peripheral blood of infected horses. This reaction was shown to be specific for the interaction of equine infectious anemia virus and T lymphocytes. Enriched B-lymphocyte populations did not divide when exposed to equine infectious anemia virus. Macrophages were depleted from the reaction by two methods: adherence to Sephadex and a combination of binding to Sephadex and adherence to complement-coated erythrocytes. Both methods reduced the number of monocytes, but only the combination of Sephadex and complement-coated cells removed the accessory cells needed for lymphocyte proliferation. We conclude that during the chronic stages of equine infectious anemia the number of antigen-reactive T lymphocytes fluctuates within the peripheral blood and that these cells require a complement-binding cell for reaction. The relationship of these cells to the lymphoproliferative stages of this disease is discussed.

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.

Quantitation and clonal isolation of cytolytic T lymphocyte precursors selectively infiltrating murine sarcoma virus-induced tumors

A limiting dilution mixed leukocyte-tumor cell microculture system was used to quantitate cytolytic T lymphocytes and their precursors (CTL-P), which infiltrate tumors induced by injection of Moloney sarcoma-leukemia virus (MSV-MoLV) complex into C57BL/6 mice. Leukocyte populations obtained from tumors collected on day 10 after virus injection were found to contain significantly higher frequencies of operationally defined (tumor-specific) CTL-P than either peripheral blood leukocytes (PBL) or spleen cells from the same animals. When these frequencies were normalized according to the content of Lyt-2+ T cells in each tissue, average CTL-P frequencies were found to be 1/9 in tumor-infiltrating cells vs. 1/41 in PBL. These results directly demonstrate selective accumulation of CTL-P in the tumor mass. A number of clonal isolates obtained from tumor-infiltrating leukocyte populations were expanded and studied for cytolytic activity and specificity. Of 11 isolates, 10 were found to have high cytolytic activity, leading to 50% lysis of the syngeneic MoLV-derived tumor target cells in 3.5 h at lymphocyte:target cell ratios ranging from 0.5:1 to 3.2:1. Furthermore, five randomly selected clones showed H-2 restriction by their selective lytic activity against MoLV-derived syngeneic MBL-2 target cells and their lack of activity against either MoLV-derived allogeneic (LSTRA) tumor cells or against syngeneic (EL4) or allogeneic (P815) target cells unrelated to MoLV.

Cytotoxic T cell response against lymphoblasts infected with Moloney (Abelson) murine leukemia virus. Methodological aspects and H-2 requirements

A method for infection of lymphocytes with Moloney(Abelson) murine leukemia virus [M(A)-MuLV] is described. Only lymphoblasts obtained after stimulation of normal spleen cells by the B cell mitogen lipopolysaccharide (LPS) were satisfactory targets for virus-specific, secondary cytotoxic T lymphocytes (CTL), whereas spleen cells stimulated by the T cell mitogen concanavalin A were not. The secondary CTL response against M(A)-MuLV could be efficiently measured using M(A)-MulV-infected LPS blasts as stimulating cells for secondary in vitro restimulation and as target cells for virus-specific destruction. Cold target inhibition demonstrated virus specificity of CTL. The T cell character of the cytotoxic cells was demonstrated by their sensitivity to anti-Thy-1.2 treatment. Using syngeneic virus-infected LPS blasts as target and stimulator, CTL responses were measured with effector cells from C57BL mice of the H-2b haplotype and of recombinant haplotypes sharing either K or D alleles with H-2b. In analogy with previous studies on Moloney virus-specific CTL, it was observed that C57BL/6 (H-2b) effector cells predominantly lysed Db-compatible, virus-infected target cells; B10.A(5R), (KbDd) effector cells showed a poor CTL response against syngeneic, virus-infected target cells. The combined findings indicate the existence of an Ir gene in the H-2D region regulating the CTL response against Moloney leukemia virus.

In vitro demonstration of in situ autologous tumour-cell cytotoxicity in MSV-induced tumours in A/SN mice

Moloney sarcoma-virus (MSV)-induced tumours in A/Sn mice have been dispersed with collagenase and DNase 8-15 days after virus inoculation, and both "sarcoma" and inflammatory cells separated by sedimentation velocity and adherence techniques. The isolated "sarcoma" cells had the morphological characteristics of atypical cells (i.e. cytoplasmic blebbing, vacuolization and prominent nucleoli) and were easily adapted to in vitro growth. As few as 2 x 10(3) of these cells inoculated i.m. produced new tumours within 8 days of injection in both syngeneic and allogeneic mice. Also, cell-free supernatant from "sarcoma"-cell cultures produced tumours, indicating that the successful transplantation of the "sarcoma" cells was probably due to production of infective virus. Cells cytotoxic in vitro against the "sarcoma" cells were present within both spleen and tumour of the tumour donors, but not in the spleens of normal mice. The cytotoxicity was specific against virus-infected cells, since in a mixture of virus-positive (gp 70) and virus-negative cells, positive cells were removed while negative cells were not affected, as measured by a visual cytotoxicity assay using immunostaining. Although T cells could be isolated from the MSV-induced tumours, these cells did not appear to mediate the cytotoxicity detected against the MSV "sarcoma" cells. These results suggest that early MSV infections might be sensitive to cytotoxic mechanisms distinct from those reported with established MLV- or MSV-induced tumour lines.