Non-immunogenic murine hepatocellular carcinoma Hepa1-6 cells expressing the membrane form of macrophage colony stimulating factor are rejected in vivo and lead to CD8+ T-cell immunity against the parental tumor.

Hepatocellular carcinoma is a lethal disease and methods that develop effective cellular-based immunotherapy are needed. We retrovirally transduced non-immunogenic mouse Hepa1-6 hepatoma cells with the gene encoding the membrane form of macrophage colony stimulating factor (mM-CSF). Excess recombinant M-CSF and phagocytosis-inhibiting chemicals blocked macrophage-mediated killing of cloned mM-CSF transfected Hepa1-6 hepatoma cells. Macrophages derived from Hck(-/-)Fgr(-/-) and Lyn(-/-) triple knockout mice, which are incapable of performing phagocytosis, failed to kill the mM-CSF transduced cells. The mM-CSF transfected tumor clones failed to grow when injected into C57BL/6 or C57L/J mice. Splenocytes from these vaccinated mice displayed cytotoxicity against parental Hepa1-6 cells, but not against B16 and CT-26 tumor cells in vitro. Mice that rejected the mM-CSF transfected Hepa1-6 tumor subsequently rejected parental Hepa1-6 cells but not the B16 melanoma cells when rechallenged. Elimination of the CD8+ effector cells by an anti-CD8 antibody and complement treatment prevented the adoptive transfer of anti-Hepa1-6-specific immunity into naive animals. Thus, mM-CSF provides a method of generating effective anti-tumor immune responses by macrophages and cytotoxic T cells against the parental Hepa1-6 cells. Our work suggests that mM-CSF transduced hepatoma cells could be used as a tumor vaccine to stimulate immune responses against hepatocellular carcinoma.

Dexamethasone increases the expression of membrane macrophage colony stimulating factor from retrovirally transduced tumor cells expressing macrophage colony stimulating factor.

Many different tumor cell types (breast, ovarian, glioma, liver and colon) were retrovirally transduced with the human macrophage colony stimulating factor (M-CSF) gene (either the membrane associated form [mM-CSF] or the secreted form [sM-CSF]). These cells were tested for their ability to display increased amounts of mM-CSF in response to dexamethasone. M-CSF-transfected tumor cells expressed additional mM-CSF in response to 18-72 h incubations with 3-15 microg/ml dexamethasone, while non-transfected parental cells were unaffected by this treatment. Increased mM-CSF protein expression on the M-CSF transduced cells was observed by flow cytometry and Western blotting using M-CSF specific antibodies. Northern blot analysis revealed an increase in the mM-CSF specific transcripts within the dexamethasone-treated mM-CSF transduced cells, but this was not seen within the non-transfected tumor cells that were treated with dexamethasone. ICAM-1 expression was unaffected by dexamethasone treatment, indicating that this response is mM-CSF specific. All trans-retinal and 1,25-dihydroxy vitamin D3 compounds that have been reported to induce M-CSF expression failed to increase mM-CSF. When dexamethasone-treated mM-CSF transfected clones were used as target cells for macrophage-mediated cytotoxicity assays, an increased killing with the dexamethasone-treated cells was seen. The macrophage-mediated cytotoxicity of these mM-CSF expressing tumor cells was blocked with excess recombinant M-CSF by saturating M-CSF receptors on the macrophage that is required for this form of tumor cell killing. This work suggests the possibility that dexamethasone may prove useful for vaccination purposes using mM-CSF retrovirally transfected tumor cells.

Membrane macrophage colony-stimulating factor on MADB106 breast cancer cells does not activate cytotoxic macrophages but immunizes rats against breast cancer.

Weakly immunogenic, but highly malignant, rat MADB106 breast cancer cells were retrovirally transduced with the membrane form of macrophage colony-stimulating factor (mM-CSF). The cloned mM-CSF-transfected MADB106 cells physically conjugated with macrophages, but were not killed by the macrophages in 48-h cytotoxicity assays. Macrophages killed the mM-CSF-expressing tumors in the presence of noncytotoxic doses of either taxol or taxol plus cisplatin. This indicated that macrophages bind to the mM-CSF expressed on the tumor cells, but for successful macrophage cytotoxicity to occur against mM-CSF-transduced tumor cells other factors must be present. The mM-CSF-transfected tumor cells were rejected when inoculated subcutaneously into normal rats. Cloned MADB106 tumor cells which expressed high amount of mM-CSF were rejected, while tumor cells that displayed lower levels of mM-CSF grew in 60% of the inoculated rats. The mM-CSF-transfected tumors that grew were smaller and had a greater amount of necrosis, compared to the viral vector tumors. Rats that spontaneously rejected the mM-CSF-transfected MADB106 cells showed rechallenge resistance to unmodified parental MADB106 and R3230Ac breast cancers, but not to the F98 glioma. These observations suggest that breast cancer-specific immunity was induced by the inoculation of mM-CSF-expressing MADB106 tumor cells.

Macrophages that kill glioma cells expressing the membrane form of macrophage colony stimulating factor are resistant to prostaglandin E2 and interleukin-10.

Malignant rat T9 glioma cells retrovirally transduced with the membrane form of macrophage colony stimulating factor (mM-CSF) were killed by bone marrow derived macrophages in 24 h cytotoxicity assays. Prostaglandin E2 (PGE) and interleukin-10 (IL10) were tested for their ability to block this tumoricidal reaction. Only at very high nonphysiological concentrations of PGE (10(-5) and 10(-6) M) was this cytotoxicity inhibited. Use of high doses of theophylline, a phosphodiesterase inhibitor, also prevented macrophages from killing the mM-CSF transduced target cells. IL10 did not alter the killing potential of the mM-CSF tumoricidal macrophages, even though IL10 reduced the production of nitric oxide by macrophages in response to tumor necrosis factor and lipopolysaccharide. IL10 enhanced the growth of bone marrow macrophages suggesting that IL10 has a complex role in the regulation of tumoricidal macrophages. Thus, the mM-CSF may be an ideal agent to treat tumors that utilize either of these two immunosuppressive defense mechanisms that may block other forms of treatment.

Development of systemic immunity to glioblastoma multiforme using tumor cells genetically engineered to express the membrane-associated isoform of macrophage colony-stimulating factor.

We investigated the ability of Fischer rat T9 glioblastoma cells transduced with cDNA genes for the secreted (s) or membrane-associated (m) isoform of M-CSF to elicit an antitumor response when implanted into syngeneic animals. Intracranial (i.c.) implantation of 1 x 10(5) T9 cells expressing mM-CSF (T9/mM-CSF) resulted in 80% tumor rejection. Electron microscopy of the T9/mM-CSF tumor site, 2-4 days postimplantation, showed marked infiltration by macrophages, many of which were in physical contact with the T9/mM-CSF cells. Animals that rejected T9/mM-CSF cells were resistant to i.c. rechallenge with T9 cells, but not syngeneic MadB106 breast adenocarcinoma cells, suggesting that T9-specific immunity can be generated within the brain via the endogenous APCs. Intracranial injection of parental T9, vector control (T9/LXSN), or T9 cells secreting M-CSF (T9/sM-CSF) was 100% fatal. Subcutaneous injection of 1 x 10(7) T9/sM-CSF, T9/LXSN, or parental T9 cells resulted in progressive tumors. In contrast, T9/mM-CSF cells injected s.c. were destroyed in 7-10 days and animals developed systemic immunity to parental T9 cells. Passive transfer of CD3+ T cells from the spleens of immune rats into naive recipients transferred T9 glioma-specific immunity. In vitro, splenocytes from T9/mM-CSF-immunized rats specifically proliferated in response to various syngeneic glioma stimulator cells. However, only marginal T cell-mediated cytotoxicity was observed by these splenocytes in a CTL assay against T9 target cells, regardless of restimulation with T9 cells. Subcutaneous immunization with viable T9/mM-CSF cells was effective in eradicating i.c. T9 tumors.

Macrophages kill T9 glioma tumor cells bearing the membrane isoform of macrophage colony stimulating factor through a phagocytosis-dependent pathway.

Rat T9 glioma cells transfected with the gene for the membrane isoform of macrophage-CSF (mM-CSF) but not for the secreted isoform of M-CSF were directly killed by bone marrow-derived macrophages. Macrophage-mediated cytolysis of the mM-CSF-transfected clone was blocked by using chemical inhibitors of phagocytosis such as iodoacetate, 2-deoxyglucose, gadolinium chloride, and cytochalasin B. In contrast, macrophage-mediated killing of mM-CSF-expressing tumor cells was augmented by the microtubule inhibitor, colchicine. Use of nitric oxide and reactive oxygen intermediate inhibitors failed to alter the macrophage-mediated killing of the mM-CSF-transfected tumor cells. Photomicroscopy, using immunohistochemical staining with the anti-Hck Ab to distinguish macrophages from tumor cells, revealed that phagocytosis began within 2 h after addition of the mM-CSF-bearing tumor cells. Photocinematography confirmed that macrophages first phagocytosized and then lysed the internalized mM-CSF transfectant cells. Using annexin V and acridine orange staining techniques, macrophages phagocytosized living mM-CSF-transfected tumor cells.

Macrophages can recognize and kill tumor cells bearing the membrane isoform of macrophage colony-stimulating factor.

NBXFO hybridoma cells produced both the membrane and secreted isoforms of macrophage colony-stimulating factor (M-CSF). Murine bone marrow cells stimulated by the secreted form of M-CSF (sM-CSF) became Mac1+, Mac2+, Mac3+, and F4/80+ macrophages that inhibited the growth of NBXFO cells, but not L1210 or P815 tumor cells. In cytotoxicity studies, M-CSF activated macrophages and freshly isolated macrophages killed NBXFO cells in the presence of polymyxin B, eliminating the possibility that contaminating lipopolysaccharide (LPS) was responsible for the delivery of the cytotoxic signal. Retroviral-mediated transfection of T9 glioma cells with the gene for the membrane isoform of M-CSF (mM-CSF), but not for the secreted isoform of M-CSF, transferred the ability of macrophages to kill these transfected T9 cells in a mM-CSF dose-dependent manner. Macrophage-mediated killing of the mM-CSF transfected clone was blocked by using a 100-fold excess of recombinant M-CSF. Catalase, superoxide dismutase, and the nitric oxide inhibitor, N-omega-nitro-arginine methyl ester (NAME), did not effect macrophage cytotoxicity against the mM-CSF transfectant T9 clones. T9 parental cells when cultured in the presence of an equal number of the mM-CSF transfectant cells were not killed, indicating specific target cell cytotoxicity by the macrophages. Electron microscopy showed that macrophages were capable of phagocytosizing mM-CSF bearing T9 tumor cells and NBXFO hybridoma cells; this suggested a possible mechanism of this cytotoxicity. This study indicates that mM-CSF provides the necessary binding and triggering molecules through which macrophages can initiate direct tumor cell cytotoxicity.

Macrophage colony stimulatory factor-activated bone marrow macrophages suppress lymphocytic responses through phagocytosis: a tentative in vitro model of Rosai-Dorfman disease.

NBXFO hybridoma cells produced macrophage colony-stimulating factor (M-CSF), which stimulated the growth of murine bone marrow-derived macrophages with potent suppressor activity. These macrophages suppressed lymphocyte responses to mitogens and antigens in a dose-dependent manner. Using a transwell chamber, we demonstrated that macrophages needed physical contact with the lymphocytes to suppress lymphocyte proliferation on day 1 in the concanavalin A mitogen reaction. In addition, no soluble suppressor factor was detected at that time. The number of lymphocytes disappeared with time when they were cocultured with the macrophages. Electron microscopy revealed that the macrophage phagocytosized the lymphocytes after 7 1/2 h. Dextran sulfate, heparan, and fucoidan prevented the macrophages from suppressing the lymphocytes. This phenomenon resembles the human disease sinus histiocytosis, also called Rosai-Dorfman disease, in which macrophages (histiocytes) phagocytosize autologous lymphocytes; occasionally, this disease is associated with immunological abnormalities. Thus we believed that macrophage-activating cytokines, such as M-CSF, may stimulate macrophages to phagocytose lymphocytes in vivo.

The identification of the neonatal NBXFO hybridoma cell and its mediator.

The neonatal spleen:myeloma hybridoma cell, NBXFO, with immunosuppressive properties supported rodent hematopoietic colony formation. We identified this hybridoma to be an undifferentiated fibroblast that produced macrophage colony-stimulating factor (M-CSF). The bone marrow cells that grew in the presence of the NBXFO supernate were macrophages and were immunosuppressive towards lymphocytes. Neutralizing anti-M-CSF antibody partially inhibited the actions of the neonatal splenic suppressor cells. Neonatal splenocytes, but not the other parental cell line, FO, induced macrophage colony formation, possessed surface-associated M-CSF, and possessed M-CSF-specific transcripts. Therefore, we believe that the M-CSF-producing phenotype was contributed by a fibroblastic stromal cell and that these stromal cells could be responsible for the in situ generation of neonatal splenic suppressor cells.

Prostaglandin E2 induces up-regulation of murine macrophage beta-endorphin receptors.

Cultured murine bone marrow macrophages specifically bound 125I-labeled beta-endorphin. Binding was displaceable by 100 times molar excess of full-length beta-endorphin but was insensitive to the opioid receptor antagonist, naloxone. Binding was inhibited by beta-endorphin's C-terminal tetrapeptide, lys-lys-gly-glu, but not by the truncated N-terminal 27 amino acid fragment, indicating that binding of beta-endorphin to this receptor is dependent on its C-terminus. Macrophages incubated for 24 h with 10(-8)-10(-5) M prostaglandin E2 showed a dose-dependent increase in beta-endorphin binding, implying receptor up-regulation. This was also observed in response to the phosphodiesterase inhibitor, isobutylmethylxanthine, indicating that regulation of these receptors may be mediated through a cAMP-dependent process. This is the first demonstration that beta-endorphin receptor expression can be positively regulated.

Hematopoietic colony stimulatory factor formation by murine mesangial cells: gene expression and biological activity.

This study examined the ability of mesangial cells to synthesize colony-stimulating factors (CSF), cytoregulatory peptides associated with the differentiation and proliferation of hematopoietic cells. Conditioned media obtained from SV-40 transformed murine mesangial cells stimulated the growth of murine bone marrow progenitor cells of the myeloid series. Differential analysis of these cells showed the presence of both macrophages and granulocytes. Cellular identification of bone marrow colonies stimulated in response to mesangial cell conditioned media was examined by flow cytometric analysis and revealed the presence of F4/80 antigen positive macrophages (67%) and Gran-1 antigen positive granulocytes (21%). Neutralizing antibodies to macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) but not antibody to interleukin-3 (IL-3), or stem cell factor (SCF) significantly inhibited the growth of the progenitor cells induced by mesangial cell conditioned media. Utilizing Northern blot analysis, murine mesangial cells expressed mRNA transcripts for M-CSF, GM-CSF, and granulocyte colony-stimulating factor (G-CSF). Further studies were performed to determine optimal incubation conditions for mesangial cell CSF gene expression. These studies revealed that both GM-CSF and G-CSF mRNA were maximally expressed at early time points (4 and 8 h of incubation), while M-CSF mRNA expression remained unchanged during the incubation of mesangial cells from 4-48 h. Incubation of mesangial cells with various concentrations of fetal bovine serum (FBS, 0.5-15%) markedly increased the mRNA expression of M-CSF, GM-CSF and G-CSF in a dose-dependent manner. These studies indicated that transformed murine mesangial cells are able to synthesize and secrete biologically active CSF that are associated with the migration and proliferation of circulating mononuclear cells in the glomerulus. Furthermore, observations regarding the role of duration of incubation and the media concentration of FBS on mesangial cell CSF mRNA expression may provide useful data to understand the optimal conditions for studies that examine the gene expression of basal or inducible CSF in mesangial cells.

Detection of interleukin 2 receptors on murine lymphocytes using fluorescent interleukin 2.

Murine interleukin 2 receptors found on freshly isolated and on in vitro activated lymphocytes were identified using a fluorescent interleukin 2 (IL2F). Three percent of freshly isolated small thymocytes bound the IL2F; these cells appeared to be dual CD4 and CD8 positive cells. Ten percent of the larger thymocytes also bound the IL2F; phenotypically, these cells were more heterogenous in their CD4/CD8 composition than the small IL2F+ thymocytes. Freshly isolated splenocytes bound more IL2F than did the thymocytes. Twenty-four percent of the small splenocytes were IL2F+ and they were mostly B220+ cells. Half of the larger splenocytes were IL2 receptor positive and these cells consisted of B and T cells. Using mitogen stimulated splenocytes, three times as many LPS stimulated B220+ blasts bound the fluorescent IL2 than freshly isolated large B220+ cells; this level of IL2F binding was maintained for four days. Of the Con A blasts, more CD8+ cells (30%) bound IL2F than did CD4+ blasts (19%); these cells maintained this level of IL2F binding for only three days. The IL2F binding could be completely inhibited by excess unlabeled IL2 and could be inhibited by 92% using a monoclonal antibody directed against the IL2 binding region of the IL2 alpha receptor, indicating that IL2F can bind to both IL2 alpha and IL2 beta receptors.

The role of cytokines in graft-versus-host reactions and disease.

CD4+ and CD8+ T lymphocytes are responsible for the initiation of graft-versus-host disease (GVHD) which limits the efficacy of human allogeneic bone marrow transplantation. Activated T cells are a rich source of cytokines in vitro, and many of these same molecules are also found in graft-versus-host reactions (GVHR) and GVHD. However, T cells are not the only source of these soluble mediators. Cytokines control the actions of many cell types both in vitro and in vivo, so these soluble factors can easily influence the actions of both the engrafted hematopoietic cells and the host's cells. The complex manner in which cells and cytokines interact in GVHR and GVHD is reviewed here. We speculate that cytokine cascades play major roles in many aspects of GVHR and GVHD.

The effects of 16,16-dimethyl PGE2 and phosphodiesterase inhibitors on Con A blastogenic responses and NK cytotoxic activity of mouse spleen cells.

Prostaglandins function as down regulators of immune responses probably by increasing the concentration of intracellular cAMP. Phosphodiesterase inhibitors, which prevent the breakdown of cAMP, also increase the intracellular levels of cAMP. Prostaglandins and phosphodiesterase inhibitors have both been shown to suppress immune responses in vitro. In this study 16,16-dimethyl PGE2 (dm-PGE2), added in vitro, suppressed the mouse spleen cell concanavalin A (Con A) response by 38% and natural killer (NK) activity by 53%. Addition of the phosphodiesterase inhibitors, theophylline, RO20-1724, or dipyridamole, decreased both the Con A response and NK activity by at least an additional 30%. We also demonstrate that treatment with dm-PGE2 and theophylline in vivo is more effective than either compound alone in inhibiting NK activity of both untreated mice and mice treated with polyinosinic-polycytidylic acid. These studies support the hypothesis that the immunosuppressive effect of dm-PGE2 is mediated by cAMP and suggest that treatment with a combination of dm-PGE2 and phosphodiesterase inhibitors can augment this immunosuppressive effect.

The effects of protein kinase inhibitors on lymphocyte blastogenesis and GVHD-induced splenomegaly.

The activation of the protein kinase C (PKC) pathway plays an integral part in the proliferation of many cell types including lymphocytes. We report that the PKC inhibitor H-7 caused inhibition of three commonly studied blastogenic responses (Con A, LPS, and MLR) with the strongest suppression being detected in the MLR. In contrast, HA1004, a potent inhibitor of cyclic nucleotide-dependent protein kinases, did not alter the blastogenic response but occasionally caused augmentation. The phenothiazine compounds studied inhibited the Con A and, to a lesser extent, the LPS responses. One of the compounds, promethazine-HCl, was effective in vivo in inhibiting splenomegaly resulting from the induction of graft vs. host disease. Our studies support the involvement of PKC in lymphoid blastogenesis. They also suggest that agents that can inhibit PKC activity may be useful in inducing immunosuppression in vivo.

The effect of recombinant murine interferon-gamma on the hematopoietic and immunological parameters of mice bearing metastatic Lewis lung carcinoma tumors.

The in vitro and in vivo effects of murine recombinant interferon-gamma (rIFN-gamma) on hematopoietic and immune parameters of normal mice and of mice bearing metastatic variant Lewis lung carcinoma (LLC-C3) tumors were assessed. The in vitro addition of rIFN-gamma to bone marrow or spleen cells from normal and LLC-C3-bearing mice reduced their capacity to grow into colonies in soft agar (CFU) and minimized their immune suppressive activities. In vivo studies showed that when LLC-C3 tumor-bearing mice were injected with rIFN-gamma for 2 days prior to sacrifice, there was a reduction in femoral bone marrow cellularity, CFU, and suppressor cell activity. In contrast, spleen cells of tumor-bearing mice that were injected with rIFN-gamma showed reduced blastogenesis, and increased spleen cellularity, CFU, and suppressor cell activity. Thus, short-term rIFN-gamma treatment of LLC-C3-bearing mice may be beneficial with regard to the bone marrow because it caused a decrease in hematopoiesis and suppressor cell activity, whereas it may be detrimental in the spleen because it appeared to stimulate hematopoiesis and increase splenic suppressor cell activity. The dichotomy between the in vitro versus in vivo effects of rIFN-gamma on splenic hematopoiesis and suppressor activity may be due to the stimulation of production of colony-stimulating factor (CSF) activities by spleen cells of rIFN-gamma-treated mice. Our results suggest that the tumor stimulation of hematopoiesis and its associated appearance of immune suppressor cells can be both positively and negatively altered by rIFN-gamma.

The effects of E series prostaglandins on blastogenic responses in vitro and graft vs. host responses in vivo.

In this study the ability of prostaglandin E1 (PGE1), Misoprostol (a stable analog of PGE1), and 16,16-dimethyl PGE2 (a stable analog of PGE2) to suppress immune responses in vitro and in vivo was determined. All of the compounds caused a titratable (10(-6) to 10(-9) M) suppression of Con A blastogenesis and the mixed lymphocyte response whereas there was only slight inhibition of the LPS response. When either 16,16-dimethyl PGE2 (30 ug/mouse) or Misoprostol (60 ug/mouse) was administered daily in vivo, there was a significant suppression of splenomegaly in F1 mice (C57Bl/6 x CBA) which had been injected with parental (C57Bl/6) spleen cells. We conclude that prostaglandins of the E series can function as immunosuppressive reagents both in vitro and in vivo. In the future they may serve to augment existing forms of immunosuppressive therapy.

Suppressor alveolar macrophages in mice bearing metastatic Lewis lung carcinoma tumors.

The capacity of alveolar macrophages from mice injected with a metastatic Lewis lung carcinoma variant, LLC-C3, to regulate T-cell Con A blastogenesis and NK cytotoxicity was studied. During the first 5 days after subcutaneous tumor injection, alveolar macrophages were stimulatory to Con A blastogenesis of normal spleen cells. After 5 days, the alveolar macrophages shifted to become suppressive. The suppressive activity was extensive by day 11, when the primary and metastatic tumor foci were first detectable. The tumor-bearer alveolar macrophages also suppressed NK cytotoxicity. Alveolar macrophage suppressive activity was sensitive to indomethacin, suggesting a prostaglandin-dependent suppressor mechanism. Suppression was not mediated by the production of hydrogen peroxide or superoxide, as it was insensitive to catalase or superoxide dismutase. When normal alveolar macrophages were cultured with LLC-C3 supernatants for over 12 hours, suppressive activity was induced. The results of these studies show that alveolar macrophages of tumor bearers become suppressive with progressive tumor growth and might, thus, facilitate the development of pulmonary metastases.

Stimulation of suppressor cells in the bone marrow and spleens of high dose cyclophosphamide-treated C57Bl/6 mice.

Systemic administration of a single dose (300 mg/kg) of cyclophosphamide (Cy) induced the appearance of a population of suppressor cells in the bone marrow and spleens of mice. Suppressor cells were assayed by their capacity to inhibit the concanavalin A (Con A) blastogenesis or the mixed-lymphocyte response of normal C57Bl/6 spleen cells. Cy-induced bone marrow (Cy-BM) suppressor cells were present as early as 4 days following Cy therapy and their activity gradually decreased over the next 2 weeks. Cy-induced splenic (Cy-Sp) suppressor cells were maximally present on Days 6 through 10 following Cy therapy. Studies were performed to characterize the suppressor cells of bone marrow obtained 4 days after Cy treatment and of normal bone marrow (N-BM). Some suppressor activity was present in normal bone marrow. N-BM suppressor cells resembled cells of the monocyte/macrophage lineage in that they were slightly adherent to Sephadex G-10, sensitive to L-leucine methyl ester (LME), and insensitive to treatment either with anti-T-cell antibody and complement or with anti-immunoglobulin antibody and complement. Their suppressive activity was abrogated by incubation with either indomethacin or catalase. Cy-BM suppressor cells were also resistant to treatment with anti-T-cell and anti-immunoglobulin antibody and complement but were not adherent to Sephadex G-10 and not sensitive to LME. Their suppressive activity was partially eliminated by indomethacin alone or in combination with catalase. We conclude that Cy chemotherapy induces the appearance of a population of immune suppressive cells and that these cells appear first in the bone marrow and subsequently in the spleen.