H-2 associated resistance to leukaemia transplantation: natural killing in vivo

Protein kinase C expression links natural antibody binding with surveillance of activated and preneoplastic cells

Extensive evidence supports a role for natural antibody (NAb) acting against small tumour foci in vivo. Ras-transformation of murine C3H 10T 1/2 fibroblasts, known to partially activate and down-regulate endogenous PKC-alpha, increased their serum NAb-binding capacity consistent with the requirements for natural immune surveillance. Now a rat PKC-beta1-overexpressing 10T 1/2 clone, PKC-4, with an 11-fold increase in PKC activity and an activated, partially transformed phenotype, links higher susceptibility to transformation through v-Ha-ras infection with an 80% increase in NAb binding assayed by flow cytometry. H7 and E-64d inhibition and phorbol ester depletion of PKC reduced NAb binding. PKC-beta1 expression and NAb binding exhibited a similar temporal recovery from TPA treatment. Thus, expression of NAb-binding structures appears to be elevated by constitutive increases in the basal activation of PKC in both the ras-transformation and the PKC-beta1-preneoplasia models. This, coupled with corresponding decreases in membrane PKC-alpha and NAb binding in confluent 10T 1/2 cells raises the possibility that in general, cells activated through PKC are NAb sensitive. Together with the increased in vivo elimination of the high NAb-binding PKC-4 cells, the data extend the support for a role for NAb in immune surveillance, to resistance against preneoplastic cells, and argue for NAb contributing to homeostasis of the organism.

Allorecognition by NK cells: nonself or no self?

The issue of antigen recognition by NK cells is complex, fascinating and, as yet, unresolved. This article reviews recent research on the repertoire of human NK cell clones for the recognition of different allogeneic cells, and summarizes the studies, most of which have been performed in mice, that implicate the MHC in NK cell recognition. It goes on to provide a common conceptual framework within which these different systems may be understood.

Alteration of the natural killer repertoire in H-2 transgenic mice: specificity of rapid lymphoma cell clearance determined by the H-2 phenotype of the target

The mechanism behind natural tumor resistance conveyed by a H-2Dd transgene to C57Bl/6 (B6) mice was investigated. Transgenic D8 mice were more efficient than control mice in natural killer (NK) cell mediated rapid elimination of intravenously inoculated radiolabeled lymphoma cells of B6 origin, such as RBL-5. There was no difference between D8 and B6 mice when elimination of YAC-1 targets was monitored. The effect of the transgene on the NK repertoire was related to the H-2 phenotype of the target: the differential elimination of RBL-5 lymphoma cells in D8 and B6 mice was not seen when a H-2 deficient variant of this line was used (efficiently eliminated in both genotypes), nor was it seen with a H-2Dd transfectant (surviving in both genotypes). The data show that a MHC class I transgene can directly control natural killing in vivo by altering the repertoire rather than the general levels of NK activity. Since the NK mediated elimination seen after introduction of a novel gene in the host was neutralized by introducing the same gene (H-2Dd), but not an unrelated class I gene (H-2Dp), in the tumor, the data support the concept of NK surveillance against missing self. This combined transgenic/transfectant system may serve as a tool for a molecular dissection of the interactions between NK cells and their targets in vivo.

Relation of H-2 expression on murine RCT(+) sarcoma cells to lung colonization and sensitivity to NK cells

Murine RCT(+) sarcoma cells were sorted using a fluorescence-activated cell sorter with regard to the expression of H-2 antigens and then an increased H-2-expressing subclone was established, and named RCT(+)H-2+. The experimental metastasis of RCT(+) cells was compared with that of RCT(+)H-2+ cells by counting pulmonary colonies on the 21st day after i.v. inoculation of tumor cells (5-10 x 10(4)/mouse). When mice were inoculated with RCT(+) cells, mean numbers of pulmonary colonies were 2.1(range 0-6), 2.8(range 0-7) using 5 x 10(4) and 1 x 10(5) cells, respectively. On the other hand, in the mice inoculated with RCT(+)H-2+ cells, figures obtained were 7.0(range 4-16), 31.9(range 13-79), using 5 x 10(4) and 1 x 10(5) cells, respectively. The survival rate of RCT(+)H-2+ cells was higher than that of RCT(+) cells, when this was assayed in the early stage after i.v. injection of 51Cr-labeled cells (1 x 10(5) cells/mouse). In addition, RCT(+)H-2+ cells were more resistant than RCT(+) cells to lysis mediated by natural killer cells. These data suggest that an increase in metastatic ability was paralleled by an increase in the H-2 antigen expression and a decrease in sensitivity to the natural killer cells.

Natural resistance against lymphoma grafts conveyed by H-2Dd transgene to C57BL mice

The H-2Dd transgenic strain D8 on C57BL background was more resistant to subcutaneous challenge of RBL-5 lymphoma cells than B6 controls. The direct role of the H-2Dd antigen was investigated by the use of (D8 x B6)F1 crosses and (D8 B6) x B6 backcrosses. The latter showed cosegregation with regard to Dd antigen expression and lymphoma resistance, both of which were inherited in a pattern consistent with control by a single dominant gene. The rejection potential in (D8 x B6)F1 mice appeared as strong as that seen in crosses between B6 and MHC congenic mice (on B10 background) carrying H-2Dd. The lymphoma resistance could be abrogated by treatment with anti-asialo GM1 antiserum or anti-NK 1.1 mAb, indicating a role for NK cells.

The effect of bearing tumours on the ability of mice to reject bone marrow transplants

To examine the possible relationship between the cells mediating resistance to tumour cells and those mediating rejection of foreign bone marrow transplants (BMT), the effect of tumour-bearing on BMT rejection has been measured by means of the spleen colony assay. Moderate doses (less than 5.0 X 10(6] of bone marrow cells from DBA/2 strain mice, which normally produce few haemopoietic spleen colonies in gamma-irradiated (950R) CBA/J strain mice, gave numerous (confluent) colonies when given soon after injection of Ehrlich ascites tumour (EAT) cells. Transfusion of [3H]UdR-labelled DBA/2 bone marrow cells demonstrated that the increased spleen colony formation in tumour-bearing mice was not due simply to changes in the total number of injected cells homing to the spleen. Injection of EAT ascites fluid alone, given to CBA/J mice before 950R + BMT, transiently reduced spleen colony development, the effect being more marked when fluid from older tumors was used. Supernatant fluid from EAT cells grown in vitro also depressed growth of BMT in vivo. The results reveal two processes in progress in mice bearing an ascites tumour: (1) an early reduction in the natural resistance to BMT allowing successful grafting and spleen colony formation, and (2) a progressive production by the tumour cells of short-acting soluble factors tending to suppress the proliferation of colony forming bone marrow cells in the transplant. The effect of the tumour-bearing state in weakening the natural resistance to foreign BMT strongly suggests that both tumour and foreign marrow graft resistance are mediated by the same or closely related effector cells.

Selective acceptance of MHC class I-deficient tumor grafts in the brain

H-2-deficient (H-2-) tumor variants were accepted equally well compared with H-2+ wild-type cells in the brain of syngeneic mice, while the H-2- cells were selectively eliminated when inoculated extracranially. This indicates a specific absence or suppression of the defense against MHC class I-deficient cells in the brain, suggested to be mediated by NK cells. In contrast, T cell-mediated immune reactions could clearly be detected in the brain under the same experimental conditions. This was shown in control experiments where H-2+ tumor cells were rejected from the brain of preimmunized or allogeneic mice. The present findings may be important for the understanding of neurotropic virus infections, immunology and immunotherapy of brain tumors, as well as for the growing interest in tissue grafting within the central nervous system.

Natural killer cell suppression of Friend virus-induced preleukemic hemopoietic stem cells

To determine whether hemopoietic cells infected with Friend polycythemia-inducing spleen focus-forming virus (SFFVp) are conserved or suppressed via natural surveillance in leukemia-resistant adult mice, we engrafted C57BL/6 recipients with isologous transgenic (donor origin marker) or natural killer (NK) cell-deficient B6 beige marrow cells exposed to SFFVp in vitro. Both groups of primary recipients were viremic and nonleukemic. Spleen cells from primary SFFVp-infected chimeras were engrafted into irradiated leukemia-susceptible secondary recipients to reveal dormant leukemia and grew as tumors of donor origin in 8 of 38 (21%) and 33 of 47 (70%) instances, respectively. Treatment of marrow donors and recipients with anti-asialo GM1 serum resulted in the depression of NK cell activity and the rapid development of dormant leukemia. We conclude that NK cells are an effective surveillance mechanism able to suppress SFFVp-induced preleukemic stem cells.

Regulation of tumor development: the biphasic effects of silica and of lipopolysaccharide on natural resistance

The impact on tumor development of the BRM silica and LPS was assessed through analysis of changes in NR parameters in vivo and in vitro. Although injection of the fumed silica Cab-o-sil 3 days before a threshold s.c. inoculum of L5178Y-F9 cells increased the tumor frequency in syngeneic DBA/2 mice, tumors recovered from silica-treated animals exhibited an augmented resistance to NAb and to in vivo NR. Cab-o-sil increased in vivo NR and induced a biphasic modulation of anti-tumor NAb and NK activities. The appearance of more autonomous tumors in Cab-o-sil-treated mice corresponding with a stimulation of NR parameters, suggests that the adjuvant activity of silica also contributes to its co-carcinogenic effect by accelerating tumor development. While injection of LPS 2-3 days before a threshold tumor inoculum lowered the tumor incidence, the survival of tumor cells injected within 1 day of LPS was increased. A corresponding early decrease in NAb activity occurred, in contrast with increases in NK cell and NAb levels previously observed after 5 days. This biphasic effect of LPS on NR effectors assayed in vitro was also seen on in vivo NR. Although their frequency was higher, tumors initiated during the period of LPS-induced NR abrogation exhibited greater reductions in NAb binding and sensitivity to NR than tumors from control mice. These data extend the support for NAb acting against tumor cells in vivo and reveal the dual nature of NR in tumor development, defending against small tumor foci and driving the progression of the surviving neoplasm.

Regulation of hematopoiesis by T lymphocytes and natural killer cells

T lymphocytes and natural killer (NK) cells exert both stimulatory and suppressive effects that regulate growth and differentiation of hematopoietic cells. Activated T and NK cells have been demonstrated in different pathological states of bone marrow failure and are proposed to play a role in the pathogenesis of the disease. T and NK cells have also been shown to be responsible for bone marrow graft rejection in both allogeneic and syngeneic donor/recipient combinations. Lymphocytes can regulate hematopoietic cell growth by direct cellular contact or by releasing soluble factors, such as colony-stimulating factors, immune interferon, lymphotoxin, and tumor necrosis factor, active on hematopoietic precursor cells.

Selective rejection of H-2-deficient lymphoma variants suggests alternative immune defence strategy

Metazoan organisms may discriminate between self and non-self not only by the presence of foreign antigens but also by the absence of normal self markers. Mammalian adaptive immune responses use the first strategy, with the additional requirement that foreign antigens are recognized in the context of self-major histocompatibility complex (MHC) products at the cell surface. Aberrant cells which fail to express MHC products adequately can therefore avoid detection. A more primitive but complementary defence system, eliminating such cells on the basis of absent self-markers, is suggested by a re-interpretation of phenomena associated with metastasis and natural resistance. We now show that murine lymphoma cells selected for loss of H-2 expression are less malignant after low-dose inoculation in syngeneic hosts than are wild-type cells, and that the rejection of such cells is non-adaptive. On the basis of our data, we suggest that natural killer cells are effector cells in a defence system geared to detect the deleted or reduced expression of self-MHC.

Lung colonization and metastasis by disseminated B16 melanoma cells: H-2 associated control at the level of the host and the tumor cell

We have studied the experimental metastasis of H-2+ and H-2- melanoma sublines in H-2b/b and H-2a/b hosts by enumerating pulmonary colonies 20-50 days after i.v. inoculation of tumor cells. In H-2b/b hosts, the H-2+ "B16-S" cells gave rise to a moderate number of metastatic colonies (mean: 6.3 +/- 6). The "BL16-L" sublines that had lost the expression of MHC class-I antigens, according to FACS-analysis and quantitative absorption tests, gave no metastases under the same conditions. Pretreatment of the H-2+ met+ B16-S with interferons (beta or alpha + beta) increased their H-2 antigen expression and the number of metastatic colonies (mean: 25 +/- 16). Interferon pretreatment of B16-L cells partially restored their H-2b expression and induced them to form a small number of metastatic colonies. The reduction in pulmonary colonization by the H-2 negative B16-L cells could be attributed to their rapid elimination by natural killer cells, already observed within 24 hr of inoculation of radiolabelled cells. H-2- B16-L cells were more susceptible than H-2+ B16-S cells to in vitro lysis by poly I:C-treated splenocytes, and they acquired full metastatic abilities if the hosts were treated with anti-asialo GM-I serum. In H-2a/b heterozygous hosts, the H-2+ B16-S cells also failed to metastasize. Reduced pulmonary colonization was evident by 24 hr after injection in comparison with H-2b/b hosts, and could be reversed by anti-asialo GM-I treatment of the hosts. In vitro, H-2a/b splenocytes were more cytotoxic to the B16 cells than syngeneic effectors. The results are discussed in relation to a recent hypothesis on a surveillance mechanism for elimination of cells on the basis of their lack (or insufficient expression) of host MHC genes.

Characterization of tumor progression from threshold tumor inocula: evidence for natural resistance

An examination of the variant generation and selection hypothesis of tumor progression was undertaken using the NK-sensitive, NAb-sensitive SL2-5 lymphoma in the threshold inoculum model of tumor progression. Tumor cells obtained from the i.p. injection site expressed increased heterogeneity for sensitivity to syngeneic NAb and to NR measured in the 131IUdR-labelled tumor elimination assay. Cells retrieved from the s.c. injection site exhibited reductions in sensitivity to NR which correlated with decreases in sensitivity to syngeneic NAb and NK cells in vitro. These data confirm and extend our previous observations with the NK-resistant L5178Y-F9 lymphoma and further substantiate the evidence for the participation of NAb and NK cells in host-mediated anti-tumor resistance. Characterization of the model revealed that the selection for reduced sensitivity to NR in thymus-depleted AT x BM mice and normal animals could not be distinguished, suggesting that thymus-independent mechanisms may be major contributors to the surveillance of nascent tumors. The decreases in susceptibility to NR occurred in a stepwise and time-dependent manner in accord with the sequential multistage concept of progression. Furthermore, the selection for tumor cells which exhibited reductions in sensitivity to NR correlated with selection for increased tumorigenicity, in keeping with the idea that progression is associated with development towards an increasingly autonomous tumor.

Adaptive immune defects and delayed rejection of allogeneic tumor cells in beige mice

The effect of the beige (bg) mutation on adaptive allogeneic tumor rejection was examined by monitoring tumor cell survival in vivo using [131I]iododeoxyuridine-prelabeled cells. Accelerated elimination of allogeneic tumor cells normally begins 8 days after ip injection and is due to active immune responses. Two independent mutations to beige on two different inbred backgrounds (C57BL/6J bgJ and DBA/2JCo bg8J) were tested, and bg/bg mice showed a 1-day delay in immune elimination of allogeneic cells. This delayed rejection was not due to a defect in clearing label from dead cells, nor to an inability to effect antibody-induced killing in vivo. Both humoral and cell-mediated responses against the allogeneic tumor cells were significantly lower in bg/bg than in +/bg mice.

Variant generation and selection: an in vitro model of tumor progression

Evidence for a new in vitro model of tumor progression was sought on the basis of the variant generation and selection hypothesis. The stability of a cloned murine tumor was examined during growth in standard tissue culture or in media containing the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Analysis of subclones from the appropriate tumor populations revealed that growth of the L5178Y-F9 clone in 100 ng/ml TPA and 0.1% dimethyl sulphoxide (DMSO) for 2 days yielded a tumor which exhibited increased cellular heterogeneity for susceptibility to both syngeneic and allogeneic natural antibodies (NAb). Subsequent exposure of TPA- and DMSO-treated cells to two cycles of syngeneic NAb-mediated cytolysis resulted in tumor populations which expressed a reduced sensitivity to syngeneic NAb. Thus the elements of tumor variant generation and selection were demonstrated by means of this approach, and repeated cycles of the TPA treatment and NAb cytolysis produced tumor cells with a reduced susceptibility not only to NAb in vitro but also to anti-tumor natural resistance (NR) measured in a tumor elimination assay in vivo. These observations extend the support for the notion that tumor progression can proceed through variant generation and selection. Furthermore, the association of tumor variant generation with exposure to the combination of TPA and DMSO, both non-mutagens, offers a model for studying non-mutagenic mechanisms of tumor development.

Natural resistance of mice pretreated with 5-(3,3'-dimethyl-1-triazeno)-imidazole-4-carboxamide (DTIC) + cyclophosphamide (Cy) against virus-induced lymphoma cells

Virus-induced leukemia was inoculated into histocompatible or allogeneic hosts pretreated with 5-(3,3'-dimethyl-1-triazeno)-imidazole-4-carboxamide (DTIC) + Cyclophosphamide (Cy), which abrogate endogenous cell proliferation and T-dependent graft responses, but not selected "natural resistance" (NR) activities. Marked impairment of lymphoma cell growth occurred mainly in the spleen of allogeneic mice with respect to that of histocompatible controls. Tumor inhibition was still present when lymphoma challenge was performed on day + 3 after Cy administration. Parallel studies on "natural killer" (NK) activity in vitro or in vivo showed that complete abrogation of the NK function was detectable on day + 3 or + 6 after Cy treatment. It was concluded that in vivo inhibition of lymphoma growth in mice pretreated with DTIC + Cy could be a drug-resistant NR at least in part distinguishable from the NK function.

A genetic analysis of natural resistance to nonsyngeneic cells: the role of H-2

The genetic control of natural resistance in vivo to four natural killer (NK) cell-resistant H-2 homozygous lymphoid tumor cell lines was investigated by following the survival and organ distribution of cells prelabeled with radioactive iododeoxyuridine. Backcross mice derived from DBA/2J and CBA/J parents were injected with H-2d tumor cells and tumor cell elimination was lowest in H-2d homozygotes. Natural killer cell activity was also reduced in mice with the H-2d haplotype, but no direct correlation between NK cell levels against YAC-1 or SL2-5 lymphoma cells and natural resistance in vivo was demonstrable. Analysis of 23 BXD recombinant inbred strains indicated that natural resistance to H-2d tumors was restricted to H-2b strains. There was no direct association of NK cell activity with H-2 type in the BXD strains and NK cell levels did not correlate with tumor survival in vivo. By comparing natural resistance to H-2d and H-2b tumors in DBA/2, C57BL/6, B6D2F1, and B10.D2 mice we found that H-2 nonidentity between the tumor and the host, rather than the host H-2 haplotype, determined whether natural resistance occurred. Again, NK cell activity against YAC-1 cells was not predictive of tumor survival in these strains. These results provide genetic evidence that NK cells alone cannot account for natural resistance to H-2 nonidentical cells of hemopoietic origin.

In vivo generation and selection of variants with altered sensitivity to natural resistance (NR): a model of tumor progression

The stability of a cloned murine tumor for sensitivity to NR was examined following growth in vivo in order to test the hypothesis that tumor progression proceeds through the generation and selection of variants. Clonal sensitivity to the [131I]-dUrd elimination assay of NR was assessed for the L5178Y-F9 tumor grown in syngeneic DBA/2 mice or maintained solely in tissue culture. Subclones derived from a tumor obtained from the injection site 3 1/2 weeks after the s.c. inoculation of 25 cells were less sensitive to NR in comparison with subclones derived from cells grown only in vitro. Subclones from the cells grown in vivo exhibited increased heterogeneity in sensitivity to NR in addition to their expanded range of susceptibility to complement-mediated lysis by CBA serum natural antibodies. The extent of the heterogeneity argues against tumor "adaptation" forming the basis for the phenotypic alteration while chromosomal studies eliminate the possibility that a new tumor was induced. These data support the hypothesis that tumor progression proceeds through the random generation of variants and host-mediated selection for the proliferation of clones with an increased ability to survive.

Hybrid resistance against a natural killer (NK) cell-resistant lymphoma (YWA) is mediated by a T cell-dependent mechanism

Rejection of the Moloney virus-induced YAC lymphoma of strain A origin by semisynegeneic F1 hybrids has previously been shown to correlate with the levels of natural killer (NK) cell activity in the same F1 hybrids against this target cell line in vitro. In the present study, YAC and another Moloney virus-induced lymphoma,, YWA, derived from the A congenic A.SW strain, were tested for F1 hybrid resistance after s.c. inoculation of small numbers of cells into syngeneic and semisyngeneic F1 mice. While YAC cells invariably grew progressively once they formed a palpable tumor, regression of YWA tumors was frequently observed in both susceptible and resistant genotypes. The hybrid resistance pattern for YAC and YWA differed in one important respect: outcross of the syngeneic host to the A-congenic A.BY strain introduced a strong H-2b-associated resistance factor against YWA, but not against YAC. Compared to YAC, which is highly NK-sensitive and rapidly eliminated from mice with high NK activity, YWA was insensitive to NK-mediated lysis in vitro and [125I] UdR-labelled YWA cells were not eliminated more efficiently from the highly resistant (A.SW X A.BY) F1 then from the parental strain in short-term (4-18h) in vivo rejection assays. It was therefore concluded that the H-2b-associated resistance against YWA was independent of NK cells or other rapidly acting effector mechanisms. Moreover, thymectomy, followed by irradiation and fetal liver reconstitution, completely abolished the resistance against YWA but left the resistance against YAC virtually intact. These data suggest that two lymphomas induced by the same agent can be rejected by different effectors. The NK-resistant YWA lymphoma is rejected by a T-dependent mechanism, while the resistance against the inoculation of the highly NK-sensitive YAC line is T-independent and, in all probability, mediated by NK cells.

Combined effects of natural resistance and chemotherapy against a murine lymphoma in lethally-irradiated mice

Lymphoma EL-4 of B6 origin was inoculated into lethally-irradiated syngeneic B6 or resistant BD2F1 hybrid recipients. Marked impairment of lymphoma cell proliferation (i.e. hybrid resistance, HR) occurred in the spleen but not in the liver or lung of BD2F1 hosts. Treatment with DTIC at the optimal dose of 80 mg/kg i.p. produced inhibitory effects in spleen, liver and lung of young BD2F1 mice, that were much greater than those observed in syngeneic B6 hosts. Increased anti-lymphoma effects for HR and DTIC chemotherapy were not detectable in the liver or lung of old BD2F1 mice or in the spleen, liver or lung of young hybrid recipients depressed for HR by pretreatment with cyclophosphamide. This is the first report on combined antileukemic effects of the host's anti-tumor natural resistance and chemotherapy.

Murine natural anti-tumor antibodies. II. The contribution of natural antibodies to tumor surveillance

The objective of this study was to investigate whether natural anti-tumor antibodies (NAb) contributed to the surveillance of small inocula of syngeneic tumors. Experiments were designed to distinguish between NAb and NK-mediated host resistance. Four approaches were used: (1) the isolation of tumor variants differing in their tumorigenicity and susceptibility to these mechanisms, and to activated macrophages; (2) a comparison of the effects of adjuvants on the modification of both host resistance and the activity of these effectors; (3) the relationship between the ontogeny of the natural resistance mechanisms and tumorigenicity in aged mice; and (4) the use of a Winn-type assay. These studies provided support for a role in natural resistance correlated with both the ability of tumors to bind NAb, and the production of NAb in adjuvant-stimulated mice. Furthermore, the frequency of tumors observed after tumor challenge more closely correlated with the ontogeny of natural antibody than NK cells, and tumors coated with NAb were less tumorigenic than controls. The reduced tumorigenicity of an NK-sensitive tumor, when compared to an NK-resistant variant of the same line, provided evidence for NK-cell-mediated natural resistance in young adult mice. It was concluded that natural resistance to tumors is a complex phenomenon dependent on the tumor phenotype, as well as the activity of several effector mechanisms, and that natural anti-tumor antibody must be considered an important component of host resistance.

Transplantation resistance of drug-treated allogeneic mice against murine lymphomas--II. Studies with various tumor-host combinations

Previous studies showed that treatment of mice with 5(3,3'-dimethyl-1-triazeno)-amidazole-4-carboxamide (DTIC) plus cyclophosphamide (Cy) produce profound depression of classical allograft responses and impairment of endogenous cell proliferation similar to that detectable in lethally-irradiated mice. However efficient localized graft resistance was found in the spleen of drug-treated hybrid or allogeneic mice challenged with lymphoma cells. The present report describes the genetic patterns of this type of natural resistance [hereafter called drug-resistant inhibition of tumors (DRIT) in various tumor-host combinations DRIT was evaluated measuring the extent of 125I-5-iodo-2'-deoxyuridine (125IUdR) uptake in the spleen and liver of leukemic hosts. The results of the experiments performed with two H-2d (i.e. L1210 and LSTRA), one H-2b (i.e. L5MF-22) and one H-2a (i.e. LAF-17) lymphomas inoculated into drug-treated recipients pointed out that: (a) tumor cell proliferation was markedly inhibited in the spleen and weakly or not impaired in the liver of D end Hh-1-incompatible euthymic or nude mice responder for the hh system; (b) no resistance was found in the spleen and liver of Hh-1-compatible B10.A (2R) mice against L5MF-22 lymphoma or of SJL recipients genetically non-responder for the Hh system, against LSTRA cells; (c) splenic resistance against L1210 leukemia was detectable in Hh-compatible B10.A or B10.A (5R) mice; (d) splenic and liver resistance was found in Hh-incompatible but genetically Hh non-responder SJL or C3H mice against L5MF-22 or LSTRA lymphomas, respectively. These results showed that the genetic patterns of the DRIT system parallels the Hh-type immunity in certain tumor-host combinations [(a) and (b)] but not in others [(a) and (d)], as previously detected in lethally-irradiated mice. It is concluded that genetically-controlled lymphoma graft resistance can be retained by mice treated with high doses of antitumor drugs, capable of abrogating classical T-dependent transplantation immunity.

Transplantation resistance of drug-treated hybrid or allogeneic mice against murine lymphomas. I. Immunopharmacology studies

Sequential treatment of mice with non-lethal doses of 5(3,3'-dimethyl-l-triazeno)-imidazole-4-carboxamide (DTIC) and cyclophosphamide (Cy) was found to produce long-term inhibition of endogenous cell proliferation in the spleen and profound impairment of classical allograft responses, similar to that detectable in lethally irradiated mice. Studies were carried out with drug-treated (i.e. treated with DTIC + Cy) conventional or nude mice inoculated with lymphoma cells homozygous for the H-2b or H-2d haplotype. Transplantation resistance in various tumour-host combinations was studied in terms of survival times after tumor challenge or lymphoma cell proliferation in spleen and liver, measured by the uptake of DNA precursor 125I-labelled 5-iodo-2'-deoxyuridine ([125I]dUrd). The results of in vivo transplantation immunity tests or in vitro tests of generation of cytotoxic lymphocytes confirmed that classical T-dependent allograft responses were abrogated by drug treatment of H-2-incompatible hosts. However, localized resistance against lymphoma graft, mainly at spleen level, was found in drug-treated hybrid mice, or conventional and "nude" allogeneic recipients, as judged by [123I]dUrd uptake inhibition. Resistance presumably regulated, at least in part by the Hh (hemopoietic histocompatibility) system, was abrogated by pretreatment with carrageenan, an antimacrophage agent. In addition, treatment with DTIC + Cy did not abrogate NK activity of mice when the in vitro cytotoxicity test was conducted 5 h after Cy administration, i.e. at the time used for tumor challenge in vivo. It was concluded that selected immunological functions (i.e., antilymphoma natural resistance insensitive to DTIC + Cy, called drug-resistant inhibition of tumors, DRIT) possibly of non-T origin, similar to those detectable in lethally-irradiated mice, can be retained by hosts subjected to high doses of certain anti-tumor agents.

In vitro NK-activity and in vivo resistance to leukemia: studies of beige, beige//nude and wild-type hosts on C57BL background

The bg mutation in C57BL mice causes a partial impairment of NK activity, and has therefore been proposed as a model to evaluate the in vivo function of NK cells. In the present report, we studied natural resistance against the ascitic lines of one chemically and two virally induced syngeneic leukemias in C57BL bg/bg mice and their phenotypically normal heterozygous +/bg littermates. S.c. threshold inocula of all three leukemia lines grew faster and caused death earlier in bg/bg than in +/bg mice, and two of the lines were rejected completely at a significantly higher frequency in +/bg control animals. The +/bg mice also eliminated [125I]-IdUrd-labelled leukemia cells at a faster rate than bg/bg mice, as measured by pulmonary, hepatic and splenic radioactivity retained 18-30 h after i.v. injection. Direct splenic killing of 51Cr-labelled leukemia cells was also studied in vitro, and was found to be severely depressed in bg/bg compared to +/bg. This natural killer activity was independent of adherent cells and showed a rapid, but transient, increase after inoculation of the tumor cell doses used in the transplantation tests. It was also possible to study the bg mutation in T-cell-free mice, by combining it with the nu mutation on a C57BL background. The NK activity of such beige-nude mice was found to be partially impaired compared to nude (non-beige) or wild-type animals, but higher than that of beige (non-nude) mice. Our results suggest that NK cells may be responsible for elimination of small numbers of tumor cells in the intact syngeneic host. The further use of beige and beige-nude mice in studies of transplanted and primary tumors is discussed.