Impairment of systemic concomitant antitumor immunity by excess soluble tumor antigen

Summary Animals bearing a 3-methylcholanthrene induced sarcoma called MC1 rejected substantial numbers of a suspension of the same tumor cells injected IV in comparison with normal rats. The factors that protected the host against lung metastases were impaired by the administration of tumor antigen in the form of irradiated tumor cells or soluble tumor antigen. Animals bearing an MC1 tumor which received either unrelated MC11 irradiated tumor cells or soluble tumor antigen had more lung metastasis than the animals not given any tumor products. However, a statistically significant increase in the number of lung tumor nodules was observed in the rats treated with MC1, compared with those treated with MC11 tumor antigen (soluble tumor antigen or irradiated tumor cells) or no tumor antigen. The increase in the outgrowth of lung tumor nodules in the tumor-bearing host given an excess of tumor materials was produced by a dual mechanism of inhibition of the concomitant immune resistance and nonspecific resistance. The present study shows that soluble tumor antigen similar to material shed from a primary tumor is able to impair concomitant immune resistance to tumor cells within the lungs.     

Protection against metastasis by immunization with an allogeneic lymphocyte antigen

Q5 antigens are expressed on the surface of various experimental murine tumor cells. They share partially common antigenicity with Qa-2 alloantigens expressed on normal lymphocytes. For that reason we tested the immunoprotection by anti-Qa-2 immunization of mice against a Q5+ tumor. Nerve fibrosarcoma (NSFA) tumor, which specifically develops metastasis in the lung, has been reported to be poorly immunogenic. However, expression of the Q5 antigen was evident on the surface of NFSA cells. After immunizing (C3H/He x B6.K1)F1 (Qa-2-) mice with B6 (Qa-2+) lymphocytes, the protection against the proliferation of the semi-syngeneic NFSA tumor was examined First, immunization of normal mice induced resistance to NFSA cell transplants. Second, when the tumor cells were transplanted to the hind foot of a mouse and the resulting tumor was removed by amputating the leg, the mice were protected against the development of lung metastasis after immunization by intraperitoneal inoculation of B6 cells 3 days after tumor removal. Immunization with attenuated NFSA cells in this system failed to protect the mice from lung metastasis. On the other hand, inoculation of the mice with B6 cells without removal of the original tumor on the foot showed little effect on the progression of the tumor. Thus, cytotoxic T lymphocytes (CTL), which seemed to be present in an inactive form in the mice from which the tumor had not been removed, were induced in the mice after the removal of the major tumor followed by immunization with B6 lymphocytes. The induction of CTL by the immunization was suppressed in mice bearing large tumors. Cells stimulated by the tumor antigen seemed to be involved in the suppression. It was also shown that the Q5 antigen is the direct recognition target of the CTL since the activity of Q5-specific CTL clones in lysing tumor cells was inhibited by a monoclonal antibody specific for the Q5 antigen. In contrast to immunization with attenuated tumor cells, our novel allogeneic lymphocyte immunization procedure offers high CTL activation, by-passing the induction of T cell unresponsiveness.     

Release of immature cells from the thymus during solid tumor growth: identification by assay of TdT activity.

In vivo anti-tumor activity of spleen cells from C3H/eb mice bearing a syngeneic fibrosarcoma was shown previously to decline to an undetectable level and be replaced by tumor-enhancing activity as tumor growth proceeds. In the light of our findings that thymocytes in the early stages of thymic processing can bring about tumor enhancement, we postulated that premature release of thymocytes and their accumulation in the spleen might account for the loss of the anti-tumor response. In the present experiments an injection of thymocytes did in fact cancel the anti-tumor response of reactive splenocytes from tumor-bearing mice. In order to determine whether premature thymocyte release occurs naturally in the tumor-bearing animals, we assayed activity of the enzyme TdT (as a marker for thymus cells) in the spleens of these mice during progressive tumor growth. Cells with TdT activity were clearly evident in the spleens of the tumor-bearing animals, were derived from the thymus, and accumulated in parallel to the loss of anti-tumour reactivity.     

Protection against metastasis by immunization with an allogeneic lymphocyte antigen

Q5 antigens are expressed on the surface of various experimental murine tumor cells. They share partially common antigenicity with Qa-2 alloantigens expressed on normal lymphocytes. For that reason we tested the immunoprotection by anti-Qa-2 immunization of mice against a Q5+ tumor. Nerve fibrosarcoma (NSFA) tumor, which specifically develops metastasis in the lung, has been reported to be poorly immunogenic. However, expression of the Q5 antigen was evident on the surface of NFSA cells. After immunizing (C3H/He x B6.K1)F1 (Qa-2-) mice with B6 (Qa-2+) lymphocytes, the protection against the proliferation of the semi-syngeneic NFSA tumor was examined First, immunization of normal mice induced resistance to NFSA cell transplants. Second, when the tumor cells were transplanted to the hind foot of a mouse and the resulting tumor was removed by amputating the leg, the mice were protected against the development of lung metastasis after immunization by intraperitoneal inoculation of B6 cells 3 days after tumor removal. Immunization with attenuated NFSA cells in this system failed to protect the mice from lung metastasis. On the other hand, inoculation of the mice with B6 cells without removal of the original tumor on the foot showed little effect on the progression of the tumor. Thus, cytotoxic T lymphocytes (CTL), which seemed to be present in an inactive form in the mice from which the tumor had not been removed, were induced in the mice after the removal of the major tumor followed by immunization with B6 lymphocytes. The induction of CTL by the immunization was suppressed in mice bearing large tumors. Cells stimulated by the tumor antigen seemed to be involved in the suppression. It was also shown that the Q5 antigen is the direct recognition target of the CTL since the activity of Q5-specific CTL clones in lysing tumor cells was inhibited by a monoclonal antibody specific for the Q5 antigen. In contrast to immunization with attenuated tumor cells, our novel allogeneic lymphocyte immunization procedure offers high CTL activation, by-passing the induction of T cell unresponsiveness.     

The antimetastatic function of concomitant antitumor immunity. II. Evidence that the generation of Ly-1+2+ effector T cells temporarily causes the destruction of already disseminated tumor cells

Progressive growth of the P815 mastocytoma in an immunocompetent host evokes the generation of an antitumor immune response that can be measured in terms of the production of cytolytic Ly-1+2+ T cells in the draining lymph node and spleen. This immunity, designated concomitant immunity, is present on day 6 of tumor growth, peaks on day 9, and decays progressively thereafter. It fails to develop in mice made T cell deficient by thymectomy and lethal whole-body gamma-radiation, and reconstituted with syngeneic bone marrow cells (TXB mice). Employment of a mouse survival assay, capable of enumerating metastatic P815 cells in cell suspensions, showed that the P815 tumor metastasizes to the draining lymph node and spleen at the same rate in normal and TXB mice for the first 6 days of growth of an intradermal P815 tumor. By day 6 of tumor growth there were approximately 10(3) P815 cells in the draining lymph node in both types of mice. However, during the generation of concomitant immunity between days 6 and 9, the number of metastatic P815 cells in the draining lymph nodes and spleens of normal tumor-bearing mice declined by nearly 90%. After day 12, however, the number of tumor cells in the nodes and spleens increased concordantly with the decay of concomitant immunity. These findings, together with the demonstration that T cell-deficient mice failed to restrain the number of metastatic P815 cells in the draining lymph node and spleen, suggest that concomitant immunity is an important defense mechanism against the development of systemic disease. Additional evidence consistent with this interpretation was provided by studies which showed that adoptive immunization with spleen cells from concomitant immune donors significantly prolonged the median survival time of TXB tumor-bearing mice by destroying a substantial proportion of P815 tumor cells already seeded in the draining lymph node. Adoptive immunization also delayed the appearance of metastatic tumor cells in the spleen.     

K cell cytotoxicity against antibody-coated chicken erythrocytes in tumor-bearing mice: its development with progressively growing tumor and the effect of immunization against the tumor.

Antibody-dependent (K cell) cytotoxic activity of spleen cells from mice bearing a chemically induced fibrosarcoma has been studied by using antibody-coated chicken erythrocytes as target cells. Spleen cells from tumor-bearing animals caused a significantly greater degree of target cells destruction than did those from control animals. The elevated cytotoxic activity in tumor-bearing animals increased with time after the tumor inoculation and correlated directly with the size of the tumor. The development of increased cytotoxic activity could be circumvented by surgical removal of the tumor. Mice that received x-irradiated tumor cells of x-irradiated tumor cells followed by a live challenge did not show a tumor growth and also failed to show increased K cell cytotoxic activity. It has been concluded that the increased K cell activity results directly from the active growth of tumor. The role of K cells in immunosurveillance has been discussed.     

GFP标记的肿瘤生长和转移的整体荧光成像

Fugene 6脂质体介导pEGFP-C1转染人源肺癌细胞(SPC-A1),经G418抗性筛选和96孔板有限稀释获得稳定高表达GFP的单克隆细胞株SPC-A1-EGFP。裸鼠腹腔注射SPC-A1-EGFP细胞建立自发转移模型;裸鼠尾静脉注射SPC-A1-EGFP细胞建立实验转移模型。利用整体光学成像系统(wllole-body optical imaging system)对荷瘤鼠整体荧光成像。结果表明,整体光学成像系统可实时非侵入监测腹腔肿瘤生长和扩散过程,通过胸腔皮瓣窗chest—wall skin-flap window)可低侵入检测肺转移。该研究为在体监测原位移植瘤的自发转移和发现抗肿瘤新药物提供了良好实验平台。     

Sublethal,whole-body ionizing irradiation can be tumor promotive or tumor destructive depending on the stage of development of underlying antitumor immunity

Summary It was shown that sublethal (500 rads), whole-body -irradiation of mice bearing an established i.d. immunogenic tumor can result, after several days delay, in complete tumor regression and long-term survival, but only if radiation is given after the tumor is established and growing progressively. Exposing mice to the same dose of radiation several hours after tumor cells were implanted resulted, in contrast, in enhanced growth of the primary tumor and in earlier death from systemic disease. Irradiation-induced tumor regression failed to occur in mice that were incapable of generating antitumor immunity, because of having been made T cell deficient by thymectomy and irradiation. Again, irradiation-induced tumor regression could be blocked by infusion of spleen cells from donor mice bearing a well-established tumor. These and previously published results support the view that sublethal, whole-body ionizing irradiation causes tumor regression by preferentially destroying radiosensitive suppressor T cells, thereby enabling the host to generate a therapeutic level of concomitant immunity. It is suggested that the preferential destruction of suppressor cells by irradiation depends on the acquisition, during immunologic induction, of radioresistance by antigen-activated effector T cells, and that this is the reason irradiation causes regression only of established tumors. Not all tumors tested were immunogenic enough to undergo regression in response to -irradiation.This study was supported by Grants CA-16642 and CA-27794 from the National Cancer Institute, Grant RR-05705 from the Division of Research Resources, NIH; and a Grant-in-aid from RJR Nabisco     

γ-Irradiation facilitates the expression of adoptive immunity against established tumors by eliminating suppressor T cells

It was found that sublethal (550 rad) whole-body gamma-irradiation of mice bearing established immunogenic tumors enabled tumor-sensitized spleen cells infused intravenously 1 h later to cause complete tumor regression in all mice. In contrast, gamma-irradiation alone caused only a temporary halt in tumor growth, and immune cells gave practically no therapeutic effect at all. This result was obtained with the SA1 sarcoma, Meth A fibrosarcoma, P815 mastocytoma, and P388 lymphoma. Additional experiments with the Meth A fibrosarcoma revealed that the spleen cells from tumor-immune donors that caused tumor regression in gamma-irradiated recipients were T cells, as evidenced by their functional elimination by treatment with anti-Thy-1.2 antibody and complement. It was shown next that adoptive T-cell-mediated regression of tumors in gamma-irradiated recipients was inhibited by an intravenous infusion of spleen cells from donors with established tumors, but not by spleen cells from normal donors. The spleen cells that suppressed the expression of adoptive immunity were functionally eliminated by treatment with anti-Thy-1.2 antibody and complement. Moreover, they were destroyed by exposing the tumor-bearing donors to 500 rad of gamma-radiation 24 h before harvesting their spleen cells. The results are consistent with the interpretation that gamma-radiation facilitates the expression of adoptive T-cell-mediated immunity against established tumors by eliminating a population of tumor-induced suppressor T cells from the tumor-bearing recipient.     

The influence of cyclophosphamide on antitumor immunity in mice bearing late-stage tumors

Spleen cells from mice bearing late-stage methylcholanthrene-induced tumor did not show any tumor activity when mixed with tumor cells in Winn's assay. Treatment of these mice with cyclophosphamide (CY) induced a tumor-inhibitory activity in spleen, occurring on day 7 after treatment, reaching its maximum on day 11 and disappearing by day 21. This antitumor activity could not be induced in control, tumor-free or T-deficient tumor-bearing mice. CY-induced tumor-inhibitory activity was immunologically specific, and mediated by Thy-1⁺, L3T4^–, Ly-2⁺ cells. Contrary to spleen cells from untreated tumor-bearing mice, spleen cells from CY-treated tumor-bearing mice did not suppress the antitumor activity of immune spleen cells in Winn's assay. However, in contrast to immune spleen cells, CY-induced tumor-inhibitory cells did not manifest antitumor activity when transferred systemically (i. v.) into T-cell-deficient tumor-bearing mice. Even more, spleen cells from CY-pretreated mice, harvested 7–15 days after the drug administration, partially suppressed the antitumor activity of concomitantly transferred spleen cells from specifically immune mice. Nevertheless, CY-pretreated mice manifested concomitant immunity, i.e. these mice exhibited higher resistance to a second inoculum of the same tumor than did nontreated mice or even mice with excised primary tumor.     

Influence of immune status of host on immunogenicity of tumors induced with two doses of methylcholanthrene

Summary Previous studies by Prehn demonstrated a direct correlation between the dose of carcinogen used for tumor induction and the immunogenicity of the resulting tumors. The purpose of the present study was to determine the role of the host's immune response and the influence of the carcinogen on immune function in this relationship. For that reason, a comparison was made of the immunogenicities of tumors induced with two doses of carcinogen in immunologically normal mice and in mice immunodepressed by adult thymectomy and irradiation. If the direct relationship between dose and immunogenicity demonstrated in normal mice was due to the degree of immunosuppression produced by the carcinogen, this correlation should not be apparent in mice already immunosuppressed. Although there was some increase in the immunogenicity of tumors induced in the immunosuppressed mice, the same relationship between carcinogen dose and immunogenicity was observed in both groups of mice. These results indicate that the degree of immunogenicity of tumors induced with both high and low doses of carcinogen was influenced by immunoselection, but in addition another, non-immunologic factor was significant in the relationship between carcinogen dose and immunogenicity. Abbreviations used: B6, C57BL/6J; CBy, BALB/cByJ; IP, intraperitoneal; IR, Immunogenicity Ratio; IV, intravenous; LP, latent period; MCA, 3-methylcholanthrene; SC, subcutaneous; TXB, adult thymectomized, lethally irradiated, and injected with 2×10⁶ syngeneic bone marrow cells; TXR, thymectomized and sublethally irradiated; WBI, whole-body irradiation     

Z-100, a polysaccharide-rich preparation extracted from the human typeMycobacterium tuberculosis, improves the resistance of Meth-A tumor-bearing mice to endogenous septic infection

The effect of Z-100, an immunomodulatory arabinomannan extracted fromMycobacterium tuberculosis, on cecal ligation and puncture (CLP)-induced sepsis in mice bearing Meth-A fibrosarcoma was investigated. When normal BALB/c mice were subjected to the CLP procedure, their mortality rate was 17%. On the other hand, an increased mortality was observed in tumor-bearing mice subjected to CLP 10 days after tumor inoculation, and then all mice died when tumor- bearing mice were subjected to CLP 20 days after tumor inoculation. However, the increased percent mortality was decreased by 50% when these mice were injected intraperitoneally with a 10 mg/kg dose of Z-100. When splenocytes (5 × 10⁷ cells), obtained from Meth-A tumor-bearing mice 20 days after tumor inoculation, were transferred intravenously to normal mice (recipient mice), mortality of these recipient mice were increased by 62% as compared with that of the control (22%). However, no increased mortality (25%) was observed in recipient mice which were transferred with splenocytes from tumor-bearing mice injected intraperitoneally with Z-100 (10 mg/kg). In addition, suppressor cell activity was demonstrated in splenocytes from Meth-A tumor-bearing mice at 20 days after tumor inoculation using one-way mixed lymphocyte reaction. However, the suppressor cell activity was significantly decreased by the intraperitoneal administration of a 10 mg/kg dose of Z-100 (p<0.01). The increase of mortality in recipient mice by adoptive transfer of mononuclear cells (MNCs) from tumor-bearing mice was not detected when these MNCs were treated with anti-Thy 1.2 monoclonal antibody (mAb), anti-Lyt 2.2 mAb or anti-CD11b mAb, but an increase was seen with anti-Lyt 1.2 mAb or anti-immunoglobulin antiserum treated MNCs. These results suggest that the suppressor cells affect the mortality of CLP-induced sepsis and Z-100 may have a therapeutic activity against opportunistic infections in immunocompromised hosts through the regulation of suppressor T-cells.     

The effect of combination therapy of radiation and Z-100, an arabinomannan on tumor growth in mice

The effect of radiation combined with intraperitoneal administration of Z-100, an immunomodulatory arabinomannan extracted fromMycobacterium tuberculosis, was studied using Meth A fibrosarcoma in BALB/c mice and metastasis of Lewis lung carcinoma, 3LL, in C57BL/6 mice.In mice bearing Meth A fibrosarcoma, a moderate degree of growth inhibition was observed in the group of single therapy with Z-100 or radiation (10 Gy). When radiation was combined with Z-100, the tumor growth was significantly inhibited. In mice bearing 3LL, slight inhibition of pulmonary metastasis was observed in the group of single therapy, while significant degrees of inhibition of primary tumor growth and pulmonary metastasis were observed in the combination group. This suggests the usefulness of combined use of Z-100 in radiation therapy.Abbreviations 3LL Lewis lung carcinoma - IL-3 Interleukin-3 - BRM(s) Biological response modifier(s) - N-CWS Nocardia-Cell wall skeleton - CSF Colony stimulating factor     

CD11b+/Gr-1+ immature myeloid cells mediate suppression of T cells in mice bearing tumors of IL-1beta-secreting cells

Tumor cells secreting IL-1beta are invasive and metastatic, more than the parental line or control mock-transfected cells, and concomitantly induce in mice general immune suppression of T cell responses. Suppression strongly correlates with accumulation in the peripheral blood and spleen of CD11b+/Gr-1+ immature myeloid cells and hematological alterations, such as splenomegaly, leukocytosis, and anemia. Resection of large tumors of IL-1beta-secreting cells restored immune reactivity and hematological alterations within 7-10 days. Treatment of tumor-bearing mice with the physiological inhibitor of IL-1, the IL-1R antagonist, reduced tumor growth and attenuated the hematological alterations. Depletion of CD11b+/Gr-1+ immature myeloid cells from splenocytes of tumor-bearing mice abrogated suppression. Despite tumor-mediated suppression, resection of large tumors of IL-1beta-secreting cells, followed by a challenge with the wild-type parental cells, induced resistance in mice; protection was not observed in mice bearing tumors of mock-transfected fibrosarcoma cells. Altogether, we show in this study that tumor-derived IL-1beta, in addition to its proinflammatory effects on tumor invasiveness, induces in the host hematological alterations and tumor-mediated suppression. Furthermore, the antitumor effectiveness of the IL-1R antagonist was also shown to encompass restoration of hematological alterations, in addition to its favorable effects on tumor invasiveness and angiogenesis that have previously been described by us.     

Cell contacts and surface features of three murine tumors grown as multicellular spheroids

Ultrastructural observations using scanning and transmission electron microscopy were made on three murine tumors, line 1 lung carcinoma, fibrosarcoma (FSA) and mammary carcinoma MCa-11, grown in vitro as multicellular tumor spheroids (MTS). The cytology of these MTS revealed the presence of characteristic cellular organelles as well as varying amounts of intracisternal type-A viral particles. In line 1 and FSA, the occurrence of gap junctions in the outer shells of these MTS was correlated with the growth behavior of these spheroids. In FSA, extracellular collagen bundles were identified next to tumor cells and represent synthetic activity by these cells under these conditions. No specific cytological correlations were made with the slow growing MCa-11 spheroid.     

Changes in the homing properties of labeled lymphoid cells caused by solid tumor growth

The effect of a progressively growing fibrosarcoma upon the distribution of ⁵¹Cr-labeled cells from the lymph nodes, spleen, thymus, bone marrow and Peyer's Patches was measured in tumor-bearing recipient mice. Tumor presence caused a uniform depression of migration of labeled cells to the bone marrow. In most cases increased homing of cells to the spleen was also observed. Labeled cells prepared from lymph nodes and Peyer's Patches were generally unaffected by the presence of a growing tumor. Migration of labeled cells from tumor bearing donors into normal syngeneic recipients suggests depletion or incapacitation of parts of the T-cell population of the spleen. These results emphasize the important relationship between splenic function and tumor progression.     

肠道菌群对蛋氨酸脑啡肽治疗肿瘤疗效的影响

蛋氨酸脑啡肽(Methionine enkephalin,MENK)是一种内源性阿片肽,有良好的抗肿瘤作用,而肠道菌群对MENK抗肿瘤作用的影响尚不清楚.本研究建立小鼠肠道菌群紊乱模型及荷瘤小鼠模型,以16s rRNA测序法检测肠道菌群丰度、监测小鼠体重、绘制肿瘤生长曲线,以流式细胞术检测小鼠肿瘤浸润CD4+T细胞、C...     

The therapeutic significance of concomitant antitumor immunity

Summary Intravenous injection of 50 g bacterial endotoxin can cause complete regression of an established SA1 sarcoma, but not if the tumor ir growing in mice that are incapable of generating concomitant immunity because they have been made T cell-deficient by thymectomy and -radiation (TXB mice). It also was shown that endotoxin fails to cause complete regression of a tumor that is either too large or too small. Only when administered on day 9 of tumor growth, at the time of peak concomitant immunity, did endotoxin cause the tumor to undergo complete regression. Direct evidence that the antitumor effect of endotoxin is dependent on concomitant immunity consisted in the demonstration that an SA1 sarcoma growing in TXB recipients can be primed for endotoxin-induced regression by IV infusion of splenic T cells from concomitantly immune donors bearing an endotoxin-susceptible 9-day tumor. Surprisingly, the donor T cells that primed the recipient tumor for endotoxin-induced regression were of the Ly-1⁺2^– phenotype, as evidenced by their susceptibility to treatment with anti-Ly-1 antibody and complement, and their complete resistance to treatment with anti-Ly-2 antibody and complement. They were different, therefore, from the T cells that cause the regression of smaller tumors in -irradiated recipients without the aid of endotoxin. It is suggested that the antitumor function of endotoxin depends on its ability to cause intratumor macrophages to acquire and express tumoricidal function, but only after the macrophages have been activated by Ly-1⁺2^– tumor-sensitized T cells.     

Zinc depletion suppresses tumor growth in mice

The hypothesis that in tumor-bearing animals an increase of host hepatic zinc metallothionein (Zn-MT) causes a restriction of zinc in the tumor tissue was studied. Three types of tumors were induced in laboratory mice by cell transplant. Tumor growth appears to be inhibited under zinc-deficient conditions, even in cases where zinc deficiency was started after tumor cell transplant. The survival times of tumor-bearing mice were prolonged by administration of cadmium chloride, which induces the synthesis of a combined zinc-cadmium metallothionein derivative in the host liver, but not in the tumor tissue, leading to an increase of hepatic zinc in the treated animals. The uptake of⁶⁵Zn by the liver of Cd-treated, tumor bearing mice was significantly higher than that of controls whereas uptake of⁶⁵Zn by tumor cells was significantly higher in controls than in the treated animals. These results suggest that restriction of zinc intake suppresses tumor growth.     

Influence of T-lymphocyte deprivation on the antileukemic effect of bacillus calmette-guérin in vivo

Summary The effect in AKR mice of T-lymphocyte deprivation in vivo, obtained by adult thymectomy plus/minus whole-body irradiation and bone-marrow reconstitution, was studied in the growth of grafted leukemia cells obtained from spontaneous AKR leukemia. Both thymectomized mice and mice subjected to thymectomy, whole-body irradiation, and bone-marrow reconstitution (B) had a lower take-frequency of graft leukemia than conventional mice. Growth of graft leukemia was inhibited by BCG treatment both in thymectomized mice and in B mice. Concomitant with the increased inhibition of leukemia growth, an increased incidence of wasting-like disease was observed. In vitro cytotoxicity studies revealed that spleen lymphoid cells from nonleukemic mice were cytotoxic to AKR leukemia cells, to nonmalignant AKR fibroblasts, and to other nonmalignant cells with H-2^k haplotype. The activity of this self-directed cytotoxicity was most marked in AKR mice with wasting-like disease. The presence of autocytotoxic cells was frequently associated with a positive direct Coombs' test. Immunofluorescence studies showed, further, that the cytotoxic activity was independent of retrovirus antigens as tested by indirect immunofluorescence with anti-MuLV antibodies. Adult thymectomy of AKR mice confers an increased antitumoral immune potential, but also an increased risk of development of serious autoimmune disease.