Vaccination with mouse mammary adenocarcinoma cells coexpressing B7-1 (CD80) and B7-2 (CD86) discloses the dominant effect of B7-1 in the induction of antitumor immunity

Bioprofiling TS/A Murine Mammary Cancer for a Functional Precision Experimental Model

The TS/A cell line was established in 1983 from a spontaneous mammary tumor arisen in an inbred BALB/c female mouse. Its features (heterogeneity, low immunogenicity and metastatic ability) rendered the TS/A cell line suitable as a preclinical model for studies on tumor-host interactions and for gene therapy approaches. The integrated biological profile of TS/A resulting from the review of the literature could be a path towards the description of a precision experimental model of mammary cancer.

Enhancement of suboptimal CD8 cytotoxic T cell effector function in vivo using antigen-specific CD80 defective T cells

T cell upregulation of B7 molecules CD80 and CD86 limits T cell expansion in immunodeficient hosts; however, the relative roles of CD80 separate from CD86 on CD4 versus CD8 T cells in a normal immune system are not clear. To address this question, we used the parent-into-F1 (P→F1) murine model of graft-versus-host disease and transferred optimal and suboptimal doses of CD80 and/or CD86 knockout (KO) T cells into normal F1 hosts. Enhanced elimination of host B cells by KO T cells was observed only at suboptimal donor cell doses and was greatest for CD80 KO→F1 mice. Wild-type donor cells exhibited peak CD80 upregulation at day 10; CD80 KO donor cells exhibited greater peak (day 10) donor T cell proliferation and CD8 T cell effector CTL numbers versus wild-type→F1 mice. Fas or programmed cell death-1 upregulation was normal as was homeostatic contraction of CD80 KO donor cells from days 12-14. Mixing studies demonstrated that maximal host cell elimination was seen when both CD4 and CD8 T cells were CD80 deficient. These results indicate an important role for CD80 upregulation on Ag-activated CD4 and CD8 T cells in limiting expansion of CD8 CTL effectors as part of a normal immune response. Our results support further studies of therapeutic targeting of CD80 in conditions characterized by suboptimal CD8 effector responses.

Triple expression of B7-1, B7-2 and 4-1BBL enhanced antitumor immune response against mouse H22 hepatocellular carcinoma

OBJECTIVES

Costimulatory signals are essential for T-cell activation and hence play a very important role in antitumor immunity. B7 and 4-1BBL which belongs to tumor necrosis factor (TNF) family provide costimulatory interaction for T-cell activation and function. This study investigated the role of B7 and 4-1BBL in the amplification of tumor immunity by transduction of the B7-1, B7-2 and 4-1BBL into mouse hepatocellular carcinoma cell line H22.

METHODS

The tumorigenicity of H22 variants expressing either B7-1, B7-2 (H22/B7-1/B7-2) or 4-1BBL was compared with an H22 variant expressing B7-1, B7-2 and 4-1BBL (H22/B7-1/B7-2/4-1BBL). The study next investigated whether the combination of B7-1/B7-2 and 4-1BBL cell injection induced cytotoxic T lymphocyte (CTL) response and IL-2/IFN-γ secretion. The immune mechanisms underlying this combination treatment were then analyzed.

RESULTS

Syngeneic BALB/c mice injected with H22/B7-1/B7-2/4-1BBL cells that expressed elevated levels of B7-1, B7-2 and 4-1BBL showed a tumor development frequency of 50% compared with 100% in mice injected with the H22 parental line, H22/neo, H22/B7-1/B7-2 and H22/4-1BBL. Mice inoculated with H22 tumor cells expressing B7-1, B7-2 and 4-1BBL developed a strong cytotoxic T lymphocyte response and long-term immunity against wild-type tumor, suggesting a synergistic effect between the B7 and 4-1BBL costimulatory pathways. Results showed that H22/B7-1/B7-2/4-1BBL tumor vaccines probably protect the infiltrating lymphocytes from apoptosis and induce NF-κB activation to improve T-cell-mediated antitumor response.

CONCLUSIONS

In this study, the antitumor consequences of using B7-1, B7-2 and 4-1BBL gene transfer have demonstrated the therapeutic potential of gene therapy approach for hepatocellular carcinoma.

Expression of fusion IL2-B7.1(IgV+C) and effects on T lymphocytes

The search for an effective immunotherapeutic treatment for tumors is an important area of cancer research. To prepare a more effective form of the bifunctional fusion protein IL2-B7.1(IgV+C) and analyze its effect on the stimulation of T lymphocyte proliferation, we used DNAStar 5.03 software to predict the structural diversity and biochemical character of IL2-B7.1(IgV+C). We then prepared fusion protein IL2-B7.1(IgV+C) by establishing its prokaryotic expression system, and tested its effect on the stimulation of T lymphocytes in vitro. The results indicated that IL2-B7.1(IgV+C) correctly formed a secondary structure in which both IL2 and B7.1(IgV+C) maintained their original hydrophilicity and epitopes. Western blot analysis revealed that IL2-B7.1(IgV+C) was efficiently expressed. Our analysis of CTLL-2 and T-cell proliferation showed that recombinant human (rh) IL2-B7.1(IgV+C) exerted the combined stimulating effects of both rhIL2 and rh B7.1(IgV+C) on cell proliferation, and that these effects could be blocked by adding either anti-IL2 or anti-B7.1 monoclonal antibodies. A >2-fold increase in [3H]TdR incorporation compared with that of cells treated with recombinant protein IL2, or B7.1(IgV+C) alone, revealed that rhIL2-B7.1(IgV+C) had dose-dependent synergetic effects on T-cell activation in the presence of anti-CD3 monoclonal antibody. We concluded that the augmented potency of rhIL2-B7.1(IgV+C) resulted in a stronger stimulation of T-cell proliferation than either rhB7.1(IgV+C) or rhIL2 alone.

Anti-CD3 scFv-B7.1 fusion protein expressed on the surface of HeLa cells provokes potent T-lymphocyte activation and cytotoxicityThis paper is one of a selection of papers in this Special Issue, entitled International Symposium on Recent Advances in Molecular, Clinical, and Social Medicine, and has undergone the Journal's usual peer-review process

The targeting of tumor cells by cytotoxic T lymphocytes is a promising strategy for biotherapy, but T cells require 2 signals via the T-cell receptor – CD3 complex and CD28 molecules for activation. To bridge the gap between cytotoxic T lymphocytes and tumor cells, our objective in this study was to describe the construction and the cell surface-anchored expression of a fusion protein, anti-CD3 scFv-B7.1, derived from inserting a fusion gene encoding anti-CD3 scFv and the extra-cellular domain of B7.1 fused by the splicing by overlap extension method into a mammalian expression vector, pDisplay. Transfection of the recombinant vector by electroporation into HeLa cells resulted in the production of protein migrating at approximately 57 kDa under reducing conditions. The expressed fusion protein could bind to T lymphocytes and induce strong T-cell activation. Meanwhile, a potent cytotoxicity was induced in the mixed culture of T-cell-modified tumor cells in a 96 h methyl-thiazolyl-diphenyl tetrazolium bromide assay. Our results indicate that this bifunctional protein, through activating T lymphocytes to lyse homologous human carcinomas, may be of potential value for T-cell-based immunotherapeutical treatment protocols in vivo.

Reduced monocyte CD86 expression in postinflammatory immunodeficiency

OBJECTIVE

Major surgery, polytrauma, stroke, and pancreatitis frequently lead to a compensatory anti-inflammatory response syndrome that often predisposes patients to lethal infections. This temporary postinflammatory immunodeficiency is characterized by altered function of blood monocytes. These cells show strongly reduced inflammatory and antigen-presentation capacity. Diminished monocyte expression of the major histocompatibility complex class II molecule human leukocyte antigen (HLA)-DR is a well-established diagnostic marker of this immunodeficiency. To further characterize the monocytic cells in this clinical state, we analyzed their expression of CD86, the most important co-stimulatory molecule.

DESIGN

Analysis of blood samples that entered the clinical immunologic diagnostics and of cells from an in vitro model of postinflammatory immunodeficiency.

SETTING

University laboratory.

SUBJECTS

Healthy donors and intensive care unit (ICU) patients at the university hospital.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The expression of HLA-DR on monocytes and of CD86 and CD80 on monocytes and B cells was analyzed by flow cytometry. Messenger RNA expression of CD86 was analyzed in isolated monocytes by real-time polymerase chain reaction on reverse transcribed. The normal range of monocyte CD86 expression in healthy subjects was established to be from 2128 to 5102 surface molecules per cell and was independent of age, gender, and leukocyte and monocyte count. The CD86 expression on monocytes in ICU patients correlated with HLA-DR expression. Approximately 40% of the ICU patients with long-term reduced monocyte HLA-DR expression had a long-term reduction of CD86 expression. Patients in whom the expression of both molecules was diminished had an unfavorable prognosis. The diminished number of CD86 surface molecules on monocytes was associated with reduced CD86 messenger RNA levels in these cells. The expression of CD86 in B cells was not diminished in immunodeficient patients. The expression of CD80 in both monocytes and B-cells was minimal in healthy donors and not clearly changed in patients.

CONCLUSIONS

The monocyte CD86 expression may be a helpful diagnostic variable in ICU patients.

Challenge with mammary tumor cells expressing MHC class II and CD80 prevents the development of spontaneously arising tumors in MMTV-neu transgenic mice

The HER-2/Neu oncogene has been implicated in human and mouse breast cancer. Indeed, transgenic MMTV-neu mice expressing this oncogene from the mammary tumor virus long terminal repeat develop spontaneous mammary tumors and die within 1 year of life. We have expressed the class II transactivator (CIITA) and/or the costimulatory molecule CD80 (B7.1) in a mammary carcinoma cell line (MCNeuA) derived from these mice. Class II transactivator directs the expression of MHC class II and the machinery for antigen processing and presentation by this pathway. When injected into MMTV-neu mice, tumor cells expressing CD80 or CD80 and CIITA, were rejected completely. In addition, following the rejection of dual expressing cells, 75% of the mice were protected against the development of subsequent spontaneous tumors. Cells expressing only CD80 or CIITA were not as effective as antitumor vaccines in preventing the development of spontaneous tumors. Thus, converting cancer cells into antigen presenting cells could represent an effective immunotherapy for breast cancer.

Primary tumor cells resected from cancer patients and decorated with a novel form of CD80 protein serve as effective antigen-presenting cells for the induction of autologous T cell immune responses ex vivo

Vaccination with autologous tumor cells genetically modified to express costimulatory molecules has shown utility for cancer immunotherapy in preclinical and limited clinical settings. Given the complicated nature of gene therapy, a practical alternative approach has been designed that relies on modification of the cell membrane with biotin and its "decoration" with a chimeric protein composed of the functional portion of human CD80 and core streptavidin (CD80-SA). We tested whether primary tumor cells resected from cancer patients can be decorated with CD80-SA and whether such cells serve as antigen-presenting cells (APCs) to generate autologous T cell responses ex vivo. Tumors and peripheral blood lymphocytes (PBLs) were collected from 14 lung, 9 colon, and 2 breast "treatment-naive" cancer patients presenting various clinical stages of the disease. Tumors were mechanically processed, irradiated, decorated with CD80-SA or control streptavidin (SA) protein, and used as APCs in ex vivo autologous T cell-proliferative and cytotoxicity assays. All tumor samples were modified with CD80-SA, albeit with various degrees of decoration ranging from 21.8 to 100%. CD80- SA-decorated cells generated significant proliferative responses in autologous T cells from 9 of 16 evaluable patients (p < 0.05). Proliferative responses were CD80-SA specific and heterogeneous, with stimulation indices ranging from 0.25 to 45. In 15 of 15 evaluable patients, CD80-SA-specific cytotoxic T cell responses against autologous tumors were generated, 11 of which were significant, with specific killing ranging from 5 to 70%. Taken together, these data demonstrate that primary tumor cells can be effectively decorated with CD80-SA and that such cells serve as APCs to induce autologous antitumor T cell responses.

Combination B7-Fc fusion protein treatment and Treg cell depletion therapy

PURPOSE

A B7.1 fusion protein consisting of the extracellular domains of human B7.1 and the Fc portion of human IgG1, called B7.1-Fc, was generated and evaluated for its antitumor potential when used alone or in combination with regulatory T (Treg) cell depletion.

METHODS

A human B7.1-Fc fusion protein was constructed, expressed, purified, and examined for its antitumor activity in experimental mouse tumor models.

RESULTS

Soluble B7.1-Fc showed costimulatory activity of T-cell proliferation in vitro, and when given in vivo, it induced complete regression of Colon 26 tumors after a 5-day treatment regimen. Parallel studies with human B7.2-Fc gave very similar results in the Colon 26 tumor model. Even in mice with established RENCA and Madison 109 tumors, which are poorly immunogenic, B7.1-Fc treatment slowed tumor growth dramatically. In these models, more potent antitumor activity was achieved when B7.1-Fc was used in combination with Treg depletion by i.p. administration of antibody PC61. Rechallenge experiments done with mice that had sustained complete tumor regressions showed that these mice had immunologic memory by their ability to reject subsequent implants. Histologically, B7.1-Fc treatment induced multiple areas of necrosis and infiltration of CD4+ and CD8+ T cells in tumors along with a concomitant dramatic increase in T-cell proliferation in tumor-draining lymph nodes.

CONCLUSIONS

The B7.1-Fc fusion protein seems to be an effective antitumor agent especially in combination with Treg depletion. Its potency in stimulating immune responses and its human origin suggest that clinical studies may be warranted in the future.

CTLA-4 is constitutively expressed on tumor cells and can trigger apoptosis upon ligand interaction

CTLA-4 (CD152) is a cell surface receptor that behaves as a negative regulator of the proliferation and the effector function of T cells. We have previously shown that CTLA-4 is also expressed on neoplastic lymphoid and myeloid cells, and it can be targeted to induce apoptosis. In our study, we have extended our analysis and have discovered that surface expression of CTLA-4 is detectable by flow cytometry on 30 of 34 (88%) cell lines derived from a variety of human malignant solid tumors including carcinoma, melanoma, neuroblastoma, rhabdomyosarcoma and osteosarcoma (but not in primary osteoblast-like cultures). However, by reverse transcriptase-PCR, CTLA-4 expression was detected in all cell lines. We have also found, by immunohistochemistry, cytoplasmic and surface expression of CTLA-4 in the tumor cells of all 6 osteosarcoma specimens examined and in the tumour cells of all 5 cases (but only weakly or no positivity at all in neighbouring nontumor cells) of ductal breast carcinomas. Treatment of cells from CTLA-4-expressing tumor lines with recombinant forms of the CTLA-4-ligands CD80 and CD86 induced apoptosis associated with sequential activation of caspase-8 and caspase-3. The level of apoptosis was reduced by soluble CTLA-4 and by anti-CTLA-4 scFvs antibodies. The novel finding that CTLA-4 molecule is expressed and functional on human tumor cells opens up the possibility of antitumor therapeutic intervention based on targeting this molecule.

Dendritic cells derived from metastatic cancer patients vaccinated with allogeneic dendritic cell-autologous tumor cell hybrids express more CD86 and induce higher levels of interferon-gamma in mixed lymphocyte reactions

Dendritic cells (DCs) are highly effective antigen-presenting cells that, when derived from cancer patients, seem to be functionally deficient. Herein, we show that vaccination with allogeneic DC-autologous tumor cell hybrids affects the phenotype and improves the function of monocyte-derived DCs (Mo-DCs) from cancer patients. Mononuclear cells were isolated from patients' peripheral blood by density gradient centrifugation, and adherent cells were cultured in medium containing GM-CSF plus IL-4 and, after 5 days, TNF-alpha. After 2 more days, Mo-DCs were harvested and their CD80, CD86, and CD83 expression was assessed by flow cytometry. They were also used as stimulators in mixed lymphocyte reactions (MLR), where IFN-gamma production was measured by ELISA. Mo-DCs from unvaccinated patients expressed significantly lower levels of CD86, and tended to express lower levels of CD83 than Mo-DCs from healthy donors. However, Mo-DCs generated after hybrid cell vaccination presented increased expression of the same markers and induced significantly higher levels of IFN-gamma in MLR. These results indicate that the use of allogeneic DC-based cancer vaccines induces recovery of DC function in metastatic cancer patients and, therefore, could precede the use of autologous DCs for vaccine preparation. Such an approach could be relevant and should be investigated in clinical trials.

In situ expression and significance of B7 costimulatory molecules within tissues of human gastric carcinoma

AIM: To explore the role and significance of costimulatory molecules B7H1, B7H2 and ICOS within tissues of human gastric carcinoma and the possible mechanisms in tumor escape.

METHODS: mRNA expressions of costimulatory molecules including B7H1, B7H2, ICOS and B7-1 in tissues of human gastric carcinoma were investigated by in situ hybridization using digoxigenin-labeled oligonucleotide-probes. The tissue of chronic gastric ulcer was used as a control. All data were analyzed by SPSS statistic software.

RESULTS: At the site of gastric carcinoma, mRNA expression levels of B7H1, B7H2 and ICOS were much higher than that of B7-1. Their mRNA positive expression indexes were 0.512 ± 0.333, 0.812 ± 0.454, 0.702 ± 0.359 and 0.293 ± 0.253, respectively. The positively stained cells were mainly tumor infiltrating lymphocytes (TILs), and some tumor cells. The difference between them was greatly significant P < 0.005. The mRNA expression levels of four molecules were not correlated to the pathological grade and matastasis of gastric carcinoma.

CONCLUSION: ICOS-B7H costimulatory pathway may be predominant at the site of gastric carcinoma. B7-1mRNA might be the basis of ICOS-B7H interaction. ICOS-B7H interaction induces the production of IL-10 which inhibits the antitumor immune responses. Therefore, it is supposed that ICOS-B7H costimulatory pathway may be involved in the negative regulation of cell-mediated immune responses.

What's the difference between CD80 and CD86?

CD28 and CD152 have crucial yet opposing functions in T-cell stimulation, in which CD28 promotes but CD152 inhibits T-cell responses. Intriguingly, they share two ligands, CD80 and CD86, but at present there is no clear model for understanding whether a ligand will promote or inhibit responses. Current perceptions are based around the concept that CD86 is the initial co-stimulatory ligand based on its more abundant and earlier expression pattern; CD80 has a role following antigen-presenting-cell activation. We describe an alternative view in which CD80 is the initial ligand, responsible for maintaining aspects of immune tolerance through interactions with CD152. These inhibitory functions can then be over-ridden by the upregulation of CD86 on dendritic cells as a result of inflammatory stimuli, leading to immune activation.

Dual expression of CD80 and CD86 produces a tumor vaccine superior to single expression of either molecule

A murine model for neuroblastoma, Neuro-2a (N2a), was used to establish a model tumor vaccine. An aggressive subclone of N2a and the less aggressive parental line were transfected with CD80, CD86, or both molecules and stable lines were established. The less aggressive N2a expressing either CD80 or CD86 induced anti-tumor immunity. In contrast, dual expression of CD80 and CD86 was required to initiate a protective anti-tumor immune response against the aggressive subclone. Control of tumor growth was dependent on CD8+ lymphocytes that infiltrated dual-expressing (CD80 and CD86) lesions. These tumor-infiltrating lymphocytes (TIL) exhibited a non-classical mechanism of tumor cell lysis that may require both the up-regulation of cell surface molecules on the tumor and the subsequent lytic activity normally associated with CD8+ TIL. Although Fas was up-regulated by the tumor in the presence of IFN-gamma, N2a and transfected N2a cell lines were not sensitive to Fas-mediated lysis.

Blockade of CD86 in BALB/c mice infected with Leishmania major does not prevent the expansion of low avidity T cells

The interactions between CD28 and its ligand CD86 are critical for the regulation of T cell responses. However, it is not clear whether CD4+ T cells expressing low and high avidity TCR are equally dependent on CD28 costimulation for their activation and expansion. To address this issue, we have used multimers of I-Ad molecules linked to a peptide derived from the Leishmania major homolog for the receptor of activated C kinase (LACK) antigen to compare the fate of LACK-specific CD4+ T cells in Leishmania-infected BALB/c mice which have been treated or not with anti-CD86 mAb. Although the administration of anti-CD86 mAb did not completely prevent the expansion of LACK-specific T cells, their frequency and number were markedly reduced. In mice treated with anti-CD86 mAb as well as in control animals, L. major induced the clonal expansion of LACK-specific T cells which expressed a canonical low avidity Valpha8/Vbeta4 TCR. Taken together, our results suggest that the molecular interactions between CD28 on T cells and CD86 on APC serve to amplify and modulate T cell responses without promoting breadth in the TCR repertoire.

Negative effect of CTLA-4 on induction of T-cell immunity in vivo to B7-1+, but not B7-2+, murine myelogenous leukemia

B7 molecules provide important costimulatory signals to T cells, and B7 genes have been introduced into B7-negative tumor cells to enhance their immunogenicity. However, the role of B7 molecules in inducing tumor immunity is controversial because of conflicting results and reports of differential signaling through the B7 molecules and their ligands CD28 and CTLA-4. In this study, we compared the effect of B7-1 (CD80) and B7-2 (CD86) on the induction of T-cell immunity to C1498, a murine myelogenous leukemia. When cultured with exogenous cytokines in vitro, C1498/B7-1 and C1498/B7-2 induced syngeneic CD8+ T cells to kill parental C1498. In vivo, C1498/B7-1 grew progressively after subcutaneous injection, whereas C1498/B7-2 completely regressed after transient growth in naive mice. Spontaneous rejection of C1498/B7-2 resulted in immunity to challenge doses of C1498 and C1498/B7-1. Antibody-depletion studies in vivo showed that CD8+ T cells rejected C1498/B7-2, whereas only natural killer cells affected the growth of C1498/B7-1. Two approaches were used to determine whether preferential interaction of B7-1 with CTLA-4 contributed to the failure of C1498/B7-1 to activate CD8+ T cells in vivo. First, CTLA-4 specific monoclonal antibody was used to block B7-1-CTLA-4 interaction. Second, CTLA-4 deletional mutant (-/-) bone marrow chimeras were used as tumor hosts. In both systems, there was a significant increase in the rate of rejection of C1498/B7-1 tumors. Resistance to C1498/B7-1 in CTLA-4(minus sign/minus sign) hosts was mediated by CD8+ T cells. Blocking or deletion of CTLA-4 did not affect the growth of parental C1498, indicating that B7-1 was important for the induction of CD8+ T-cell immunity in the absence of CTLA-4.

Dendritic cells from patients with myeloma are numerically normal but functionally defective as they fail to up-regulate CD80 (B7-1) expression after huCD40LT stimulation because of inhibition by transforming growth factor-beta1 and interleukin-10

Limited response to idiotype vaccination in patients with myeloma suggests that there is a need to develop better immunotherapy strategies. It has been determined that the number of high-potency CMRF44+CD14-CD19- dendritic cells (DCs) in the blood of patients with myeloma (range, 0.03%-0.8% of mononuclear cells [MNCs]; n = 26) was not significantly different from that in controls (range, 0.05%-0.8% of MNCs; n = 13). Expression of the costimulatory molecules CD80 and CD86 on DCs from these patients (mean, 29%+/-17% of MNCs and 85%+/-10% of MNCs, respectively) was also normal (mean, 29%+/-17% and 86%+/-16% of MNCs, respectively). Up-regulation of CD80 expression in response to stimulation by human huCD40LT + interleukin (IL)-2 was significantly reduced on the DCs of patients with myeloma during stable disease (n = 9) and was absent during progressive stages (n = 7) of disease. Similar effects were seen on B cells but not on monocytes of the same group of patients. CD86 expression on DCs was high before (86%) and after (89%) stimulation. Inhibition of CD80 up-regulation was neutralized by either anti-transforming growth factor (TGF)-beta1 or anti-IL-10. Up-regulation of CD80 on DCs of controls was inhibited by rTGF-beta1 in a dose-dependent manner. Serum TGF-beta1 and IL-10 levels were normal in most patients studied. Cytoplasmic TGF-beta1 was increased in plasma cells during progressive disease. Thus patients with myeloma have normal numbers of DCs, but CD80 expression may fail to be up-regulated in the presence of huCD40LT because of tumor-derived TGF-beta1 or IL-10. Autologous high-potency DCs may have to be tested for CD80 up-regulation and biologically modified ex vivo before idiotype priming for immunotherapy.

Evidence of a role for CD200 in regulation of immune rejection of leukaemic tumour cells in C57BL/6 mice

Increased expression of the molecule CD200 in mice receiving renal allografts is associated with immunosuppression leading to increased graft survival, and altered cytokine production in lymphocytes harvested from the transplanted animals. Preferential production of IL-4, IL-10 and TGFbeta occurs on donor-specific restimulation in vitro, with decreased production of IL-2, IFNgamma and TNFalpha. These effects are enhanced by simultaneous infusion of CD200 immunoadhesin (CD200Fc) and donor CD200 receptor (CD200r) bearing macrophages to transplanted mice. C57BL/6 mice do not normally resist growth of EL4 or C1498 leukaemia tumour cells. Following transplantation of cyclophosphamide-treated C57BL/6 with T-depleted C3H bone marrow cells, or for the EL4 tumour, immunization of C57BL/6 mice with tumour cells transfected with a vector encoding the co-stimulatory molecule CD80 (EL4-CD80), mice resist growth of tumour challenge. Immunization of C57BL/6 mice with EL4 cells overexpressing CD86 (EL4-CD86) is ineffective. Protection from tumour growth in either model is suppressed by infusion of CD200Fc, an effect enhanced by co-infusion of CD200r+ macrophages. CD200Fc acts on both CD4+ and CD8+ cells to produce this suppression. These data are consistent with the hypothesis that immunosuppression following CD200-CD200r interaction can regulate a functionally important tumour growth inhibition response in mice.