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

The Monocytes That Repopulate in Mice After Cyclophosphamide Treatment Acquire a Neutrophil Precursor Gene Signature and Immunosuppressive Activity

Cyclophosphamide (CTX) is a major component of the chemotherapy conditioning regimens used in the clinic to prepare cancer patients for hematopoietic stem cell transplantation or adoptive T cell therapy. Previous studies have shown that CTX given at nonmyeloablative doses in mice and patients leads to expansion of myeloid cells within which the monocytic subset exhibits immunosuppressive activity. However, the ontogeny and gene expression signature of these CTX-induced monocytes are not well-defined. Here, we report that the expansion of myeloid cells is a default process intrinsic to hematopoietic recovery after chemotherapy. During this process, the monocytes repopulated in mice acquire immunosuppressive activity, which can persist long after cessation of chemotherapy. Moreover, monocytes acquire a gene signature characteristic of neutrophil precursors, marked by increased proliferative capability and elevated expressions of multiple primary and secondary granules. We provide evidence that CTX-induced myeloid cell expansion is regulated by DNA methyltransferase 1 (Dnmt1) and dependent on chemotherapy-induced microbial translocation. These findings help advance our understanding of the differentiation, heterogeneity, and function of myeloid cells repopulating after chemotherapy.

The renaissance of anti-neoplastic immunity from tumor cell demise

Cancer therapies can temporarily reduce tumor burdens by inducing malignant cell death. However, cancer cure is still far from realization because tumors often gain resistance to current treatment and eventually relapse. Accumulating evidence suggests that successful cancer interventions require anti-tumor immunity. Therapy-induced cell stress responses ultimately result in one or more cell death modalities, including apoptosis, autophagy, necroptosis, and pyroptosis. These irreversible dying processes are accompanied by active or passive release of cell death-associated molecular patterns (CDAMPs), which can be sensed by corresponding pattern recognition receptors (PRR) on tumor-infiltrating immune cells. This crosstalk with the immune system can reawaken immune surveillance in the tumor microenvironment (TME). This review focuses on immune-modulatory properties of anti-cancer regimens and CDAMP-mediated communications between cell stress responses and the immune contexture of TME. In addition, we describe how immunogenic cell death can elicit strong and durable anti-tumor immune responses.

Immunostimulatory Effects of Melphalan and Usefulness in Adoptive Cell Therapy with Antitumor CD4+ T Cells

The alkylating agent melphalan is used in the treatment of hematological malignancies, especially multiple myeloma. In the past, the usefulness of melphalan has been solely attributed to its cytotoxicity on fastgrowing cancerous cells. Although the immunomodulatory effects of melphalan were suggested many years ago, only recently has this aspect of melphalan's activity begun to be elucidated at the molecular level. Emerging evidence indicates that melphalan can foster an immunogenic microenvironment by inducing immunogenic cell death (ICD) as characterized by membrane translocation of endoplasmic reticulum protein calreticulin (CRT) and by release of chromatin-binding protein high-mobility group box 1 (HMGB1). In addition, the lympho-depletive effect of melphalan can induce the release of pro-inflammatory cytokines and growth factors, deplete regulatory T cells, and create space to facilitate the expansion of infused tumor-reactive T cells. These features suggest that melphalan can be used as a preparative chemotherapy for adoptive T-cell therapy. This notion is supported by our recent work demonstrating that the combination of melphalan and adoptive transfer of tumor-reactive CD4+ T cells can mediate potent antitumor effects in animal models. This review summarizes the recent advances in understanding and utilizing the immunomodulatory effects of melphalan.

Immunosuppressive myeloid cells induced by chemotherapy attenuate antitumor CD4+ T-cell responses through the PD-1-PD-L1 axis

In recent years, immune-based therapies have become an increasingly attractive treatment option for patients with cancer. Cancer immunotherapy is often used in combination with conventional chemotherapy for synergistic effects. The alkylating agent cyclophosphamide (CTX) has been included in various chemoimmunotherapy regimens because of its well-known immunostimulatory effects. Paradoxically, cyclophosphamide can also induce suppressor cells that inhibit immune responses. However, the identity and biologic relevance of these suppressor cells are poorly defined. Here we report that cyclophosphamide treatment drives the expansion of inflammatory monocytic myeloid cells (CD11b(+)Ly6C(hi)CCR2(hi)) that possess immunosuppressive activities. In mice with advanced lymphoma, adoptive transfer (AT) of tumor-specific CD4(+) T cells following cyclophosphamide treatment (CTX+CD4 AT) provoked a robust initial antitumor immune response, but also resulted in enhanced expansion of monocytic myeloid cells. These therapy-induced monocytes inhibited long-term tumor control and allowed subsequent relapse by mediating functional tolerization of antitumor CD4(+) effector cells through the PD-1-PD-L1 axis. PD-1/PD-L1 blockade after CTX+CD4 AT therapy led to persistence of CD4(+) effector cells and durable antitumor effects. Depleting proliferative monocytes by administering low-dose gemcitabine effectively prevented tumor recurrence after CTX+CD4 AT therapy. Similarly, targeting inflammatory monocytes by disrupting the CCR2 signaling pathway markedly potentiated the efficacy of cyclophosphamide-based therapy. Besides cyclophosphamide, we found that melphalan and doxorubicin can also induce monocytic myeloid suppressor cells. These findings reveal a counter-regulation mechanism elicited by certain chemotherapeutic agents and highlight the importance of overcoming this barrier to prevent late tumor relapse after chemoimmunotherapy.

Immune-based mechanisms of cytotoxic chemotherapy: implications for the design of novel and rationale-based combined treatments against cancer

Conventional anticancer chemotherapy has been historically thought to act through direct killing of tumor cells. This concept stems from the fact that cytotoxic drugs interfere with DNA synthesis and replication. Accumulating evidence, however, indicates that the antitumor activities of chemotherapy also rely on several off-target effects, especially directed to the host immune system, that cooperate for successful tumor eradication. Chemotherapeutic agents stimulate both the innate and adaptive arms of the immune system through several modalities: (i) by promoting specific rearrangements on dying tumor cells, which render them visible to the immune system; (ii) by influencing the homeostasis of the hematopoietic compartment through transient lymphodepletion followed by rebound replenishment of immune cell pools; (iii) by subverting tumor-induced immunosuppressive mechanisms and (iv) by exerting direct or indirect stimulatory effects on immune effectors. Among the indirect ways of immune cell stimulation, some cytotoxic drugs have been shown to induce an immunogenic type of cell death in tumor cells, resulting in the emission of specific signals that trigger phagocytosis of cell debris and promote the maturation of dendritic cells, ultimately resulting in the induction of potent antitumor responses. Here, we provide an extensive overview of the multiple immune-based mechanisms exploited by the most commonly employed cytotoxic drugs, with the final aim of identifying prerequisites for optimal combination with immunotherapy strategies for the development of more effective treatments against cancer.

Integrin targeted oncolytic adenoviruses Ad5-D24-RGD and Ad5-RGD-D24-GMCSF for treatment of patients with advanced chemotherapy refractory solid tumors

The safety of oncolytic viruses for treatment of cancer has been shown in clinical trials while antitumor efficacy has often remained modest. As expression of the coxsackie-adenovirus receptor may be variable in advanced tumors, we developed Ad5-D24-RGD, a p16/Rb pathway selective oncolytic adenovirus featuring RGD-4C modification of the fiber. This allows viral entry through alpha-v-beta integrins frequently highly expressed in advanced tumors. Advanced tumors are often immunosuppressive which results in lack of tumor eradication despite abnormal epitopes being present. Granulocyte-macrophage colony stimulating factor (GMCSF) is a potent activator of immune system with established antitumor properties. To stimulate antitumor immunity and break tumor associated immunotolerance, we constructed Ad5-RGD-D24-GMCSF, featuring GMCSF controlled by the adenoviral E3 promoter. Preliminary safety of Ad5-D24-RGD and Ad5-RGD-D24-GMCSF for treatment of human cancer was established. Treatments with Ad5-D24-RGD (N = 9) and Ad5-RGD-D24-GMCSF (N = 7) were well tolerated. Typical side effects were grade 1-2 fatigue, fever and injection site pain. 77% (10/13) of evaluable patients showed virus in circulation for at least 2 weeks. In 3 out of 6 evaluable patients, disease previously progressing stabilized after a single treatment with Ad5-RGD-D24-GMCSF. In addition, 2/3 patients had stabilization or reduction in tumor marker levels. All patients treated with Ad5-D24-RGD showed disease progression in radiological analysis, although 3/6 had temporary reduction or stabilization of marker levels. Induction of tumor and adenovirus specific immunity was demonstrated with ELISPOT in Ad5-RGD-D24-GMCSF treated patients. RGD modified oncolytic adenoviruses with or without GMCSF seem safe for further clinical development.

Myeloid-derived suppressor cells: a novel therapeutic target

Immunotherapy and angiogenic inhibitors, used alone or in combination with chemotherapy, represent two promising cancer treatment programs. Each is limited by myeloid-derived suppressor cells (MDSCs), which accumulate in tumor-bearing hosts. MDSCs inhibit effector T-cell function and thus prevent the formation and execution of an effective antitumor immune response. Recently reported studies have shown that MDSCs also function to promote tumor-dependent angiogenesis as well as tumor metastasis, and to provide tumor resistance to antiangiogenic drugs. Insights into tumor-imposed dynamic changes in bone marrow function and myeloid cell development should fuel novel drug developments and novel applications of drugs currently in use. Such insights suggest that multitargeted receptor tyrosine kinase inhibitors, such as sunitinib, may be useful adjunctive agents for use in immunotherapy trials treating several tumor types.

Macrophage colony stimulating factor: not just for macrophages anymore! A gateway into complex biologies

Macrophage colony stimulating factor (M-CSF, also called colony stimulating factor-1) has traditionally been viewed as a growth/differentiation factor for monocytes, macrophages, and some female-specific tumors. As a result of alternative mRNA splicing and post-translational processing, several forms of M-CSF protein are produced: a secreted glycoprotein, a longer secreted form containing proteoglycan, and a short membrane-bound isoform. These different forms of M-CSF all initiate cell signaling in cells bearing the M-CSF receptor, called c-fms. Here we review the biology of M-CSF, which has important roles in bone physiology, the intestinal tract, cancer metastases to the bone, macrophage-mediated tumor cell killing and tumor immunity. Although this review concentrates mostly on the membrane form of human M-CSF (mM-CSF), the biology of the soluble forms and the M-CSF receptor will also be discussed for comparative purposes. The mechanisms of the biological effects of the membrane-bound M-CSF reveal that this cytokine is unexpectedly involved in many complex molecular events. Recent experiments suggest that a tumor vaccine based on membrane-bound M-CSF-transduced tumor cells, combined with anti-angiogenic therapy, should be evaluated further for use in clinical trials.

Cyclophosphamide Induces the Development of Early Myeloid Cells Suppressing Tumor Cell Growth by a Nitric Oxide-Dependent Mechanism

Adoptive immunotherapy with cyclophosphamide (Cy) increases the host resistance against tumor growth. The precise mechanism(s) by which this therapy enhances tumor suppression is unclear. Cy induces the development of early myeloid cells that may be strongly antiproliferative through NO production. These cells are similar to the natural suppressor cells found in normal bone marrow with a potential antitumor effect. Here we have addressed whether the development of NO-producing cells may be involved in this tumor resistance in Cy-treated mice. The results show a synergism between Cy treatment and tumor-specific lymphocytes transferred systemically (i.v.) or locally (Winn's assay) that results in a strong tumor suppression. Inhibition of NO production by N(G)-monomethyl-L-arginine at the site of tumor inoculation results in a loss of the protection achieved by the combined therapy. Cy-treated mice develop splenic early myeloid (CD11b, Gr-1, CD31 (ER-MP12), ER-MP20, ER-MP54) cells producing large amounts of NO upon T cell-derived signals (IFN-gamma plus CD40 ligation) able to inhibit tumor cell growth in vitro. Early myeloid cells (ER-MP54(+)) and cells expressing inducible NO synthase are increased at the site of tumor challenge in mice treated with the combined therapy, but not in those treated with Cy or immune cell transfer alone. Thus, Cy induces the expansion of early myeloid cells, inhibiting tumor cell growth by a mechanism involving NO. Both the recruitment and the activation of these myeloid cells at the site of tumor challenge appear to be dependent on the presence of tumor-specific lymphocytes.

Nitric oxide-producing CD11b+Ly-6G(Gr-1)+CD31(ER-MP12)+cells in the spleen of cyclophosphamide–treated mice: implications for T-cell responses in immunosuppressed mice

During recovery from intensive chemotherapy with cyclophosphamide (CTX), mice suffer a severe but transitory impairment in spleen cell proliferation to T-cell mitogens (Con A or anti-CD3 plus IL-2). Although CTX treatment reduced spleen T-cell cellularity, this cannot fully account for T-cell unresponsiveness. The results showed that CTX induces the colonization of spleen by an immature myeloid CD11b+Ly-6G+CD31+ population. Its presence closely correlated with the maximum inhibition of T-cell proliferation. Moreover, this suppressive activity was dependent on nitric oxide (NO) production in cultures since (1) higher amounts of nitric oxide and inducible nitric oxide synthase (iNOS) mRNA were produced in CTX spleen cells (CTX-SC) than in control splenocyte cultures and (2) NOS inhibitors greatly improved the proliferation of T lymphocytes. Nitric oxide production and suppressive activity were also dependent on endogenous interferon-γ (IFN-γ) production since anti–IFN-γ abrogated both effects. Finally, iNOS protein expression was restricted to a heterogeneous population of CD31+cells in which CD11b+Ly-6G+ cells were required to suppress T-cell proliferation. These results indicated that CTX might also cause immunosuppression by a mechanism involving the presence of immature myeloid cells with suppressor activity. This may have implications in clinical praxis since inappropriate immunotherapies in patients treated with intensive chemotherapy could lead to deleterious T-cell responses. (Blood. 2000;95:212-220)

Nitric oxide-producing CD11b+Ly-6G(Gr-1)+CD31(ER-MP12)+cells in the spleen of cyclophosphamide–treated mice: implications for T-cell responses in immunosuppressed mice

During recovery from intensive chemotherapy with cyclophosphamide (CTX), mice suffer a severe but transitory impairment in spleen cell proliferation to T-cell mitogens (Con A or anti-CD3 plus IL-2). Although CTX treatment reduced spleen T-cell cellularity, this cannot fully account for T-cell unresponsiveness. The results showed that CTX induces the colonization of spleen by an immature myeloid CD11b+Ly-6G+CD31+ population. Its presence closely correlated with the maximum inhibition of T-cell proliferation. Moreover, this suppressive activity was dependent on nitric oxide (NO) production in cultures since (1) higher amounts of nitric oxide and inducible nitric oxide synthase (iNOS) mRNA were produced in CTX spleen cells (CTX-SC) than in control splenocyte cultures and (2) NOS inhibitors greatly improved the proliferation of T lymphocytes. Nitric oxide production and suppressive activity were also dependent on endogenous interferon-γ (IFN-γ) production since anti–IFN-γ abrogated both effects. Finally, iNOS protein expression was restricted to a heterogeneous population of CD31+cells in which CD11b+Ly-6G+ cells were required to suppress T-cell proliferation. These results indicated that CTX might also cause immunosuppression by a mechanism involving the presence of immature myeloid cells with suppressor activity. This may have implications in clinical praxis since inappropriate immunotherapies in patients treated with intensive chemotherapy could lead to deleterious T-cell responses. (Blood. 2000;95:212-220)

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.

Effects of lysozyme dimer on humoral response to sheep erythrocytes in non-treated and cyclophosphamide-immunosuppressed mice

The effects of lysozyme dimer on humoral response to sheep erythrocytes (SRBC) and restoration of the response impaired by a single cyclophosphamide dose (200 mg/kg) were tested on mice. The effect of lysozyme dimer on the humoral response to SRBC in non-treated with cyclophosphamide mice was determined in relation to doses (0.2, 2, 20 or 200 micrograms/kg) and the time of the drug administration with respect to the antigen before or after SRBC immunization. Moreover, the effect of lysozyme dimer on the humoral response in cyclophosphamide-treated mice was studied depending on the dose applied and time of exposure to the drug in relation to SRBC. It has been found that lysozyme dimer potentiates the humoral response to SRBC in mice, resulting in an increased number of splenocytes producing haemolytic antibodies (PFC) and the total and 2-mercaptoethanol resistant level of anti-SRBC antibodies. A single exposure to lysozyme dimer gave the strongest stimulating action on SRBC when the doses of 2 or 20 micrograms/kg were administered 2 h prior to the antigen. The potentiating effect of the drug was reduced when it was administered 24 h before the antigen and also when single doses were as high as 200 micrograms/kg and as low as 2 micrograms/kg. Exposure to four doses of lysozyme dimer at 24 h intervals was more activating than a single injection. A strong potentiating effect on the specific response to SRBC was noted after four injections of lysozyme dimer at doses from 0.2 to 20 micrograms/kg. The effect of the drug did not depend on the time of exposure to the antigen. It has also been found that lysozyme dimer significantly reduces the suppressive effect of a high cyclophosphamide dose (200 mg/kg) on the humoral response of SRBC-immunized mice. The protective action of lysozyme dimer was dose- and time-dependent. The strongest protection was observed after three doses of 20 micrograms/kg administered prior to pharmacological immunosuppression. Reduction in the dose to 2 micrograms/kg and shorter treatment resulted in reduced protective effects. We have also found that the protective action of three doses of lysozyme dimer (2 or 20 micrograms/kg each) administered between cyclophosphamide injection and the antigen, or after antigen administration is weaker than such a treatment prior to cyclophosphamide immunosuppression.

Neuroregulation of natural suppressor cell activity

The neuronal control of murine natural suppressor (NS) cell activity was studied in mice following lesion of the arcuate nucleus of the hypothalamus performed by the postnatal treatment with monosodium glutamate (MSG). The suppressor activity was evaluated as the ability of spleen or bone marrow cells (suppressors) to suppress the mitogen-induced lymphocytes proliferation of allogeneic Balb/c spleen cells (indicators). The study was carried out on three different inbred strains of mice (C57BL/6, DBA/2 and CBA). The results demonstrated that, in all strains used, the treatment of mice (12 day-old) with MSG led to a marked reduction of natural suppressor cell activity. This reduction was observed in both spleen and bone marrow, the lymphoid organs in which the NS cell activity is usually present. A reduced NS activity was also present in the bone marrow of MSG-treated adult mice, suggesting a permanent alteration of NS cell activity. These results provide additional support on neuromodulation of immune system.

Cyclophosphamide treatment of an SJL murine B-cell lymphoma increases the proportion of suppressive CD8+ over tumor-stimulatory CD4+ T-lymphocytes

Lymphomas (formerly reticulum cell sarcomas (RCS)) which develop spontaneously in SJL mice are a murine counterpart of human low grade B-cell non-Hodgkin's lymphoma. Tumor cells stimulate proliferation of syngeneic CD4+ T-lymphocytes which secrete cytokines required for tumor growth. Cyclophosphamide treatment of tumor-bearing mice (RCS/Cy) decreases in vitro tumor-stimulated CD4+ T-cell proliferation and, in turn, tumor growth, in part, through the suppressive action of CD8+ T-lymphocytes. In RCS/Cy compared to untreated tumor-bearing (RCS5) mice we report marked in vivo decreases in: (1) the activation (CD44HI/CD45RBLO phenotype) and proliferation (S + G2M phases of the cell cycle) of CD4+ T-lymphocytes; (2) the ratio of activated and/or proliferating CD4+ to CD8+ T-lymphocytes, and; (3) the proliferation of tumor cells. Also, depletion of CD8+ T-lymphocytes from RCS/Cy mice abrogated much of the efficacy of the RCS/Cy treatment and led to changes in lymphoid populations more reminiscent of those in RCS5 than RCS/Cy mice. The data support our hypothesis that the RCS/Cy treatment achieves its efficacy by preventing the predominance of CD4+ over CD8+ T-lymphocytes which is essential to maximum tumor growth in RCS5 mice. The results imply that analogous B-cell lymphomas in humans also may be treatable by shifting the T-cell balance toward inhibitory CD8+ rather than the tumor-stimulatory CD4+ T-lymphocytes.

Heterogeneity of splenic natural suppressor cells induced in mice by treatment with cyclophosphamide

Administration of high dose cyclophosphamide (CY, 200 mg/kg body weight) to adult mice induces transient, nonspecific suppressor activity in the spleen of treated animals. Characterization of the CY-induced natural suppressor (NS) cells which inhibit mixed lymphocyte reactions revealed a heterogeneous population of lymphocytes expressing the CD8 T cell marker and the B220 B cell marker, as well as cells bearing the granulocyte-monocyte marker CD11b. On a cell per cell basis the most potent of these suppressors were found to be positive for CD11b. Inhibitory activity was also detected in the CD8-, CD11b-, B220- compartment of CY-spleen, suggesting the presence of null NS cells. The fact that several phenotypically distinct cell populations contribute to the overall inhibitory effect of CY-spleen cells indicates that natural suppression defines an activity rather than a specific cell type. Interestingly, NS activity was observed to reside solely within the fraction of CY-spleen that is agglutinable with soybean agglutinin or wheat germ agglutinin, suggesting that expression of receptors for these plant lectins is a universal characteristic of CY-induced NS cells, regardless of their lineage. CY-spleen cell-mediated suppression of lymphoproliferative responses was found to be partially dependent on DNA synthesis and totally dependent on protein synthesis, but did not require cell-cell contact, indicating the production of soluble suppressor factor(s).

Cyclophosphamide treatment antagonizes the in vitro development of Mycobacterium lepraemurium-induced suppressor cell precursors

The in vitro-inducible maturation of splenic suppressor cell precursors detected during the early phase of Mycobacterium lepraemurium infection can be abrogated when a high dose of cyclophosphamide (Cy) is inoculated to infected mice 2 days before assay. The drug does not act directly on adherent suppressor cell precursors, but rather inhibits their activation by a non-adherent cell subset whose phenotype has not yet been elucidated. It was established by flow cytometry analyses, that despite a marked increase in the total number of splenic non-adherent cells following M. lepraemurium infection, the effect of Cy on Ia+, Thy-1+, CD4+ and CD8+ cells in infected mice was comparable to that observed in normal controls. It was not possible to determine the duration of the inhibiting effect of Cy on non-adherent regulatory cells, because the drug was itself inducing suppressor cells from 7 days after inoculation. By the time spleen cell suspensions were totally free of Cy-induced suppressor cells, infection-dependent suppressor cell precursors were once again detected, indicating that Cy treatment did not prevent their in vivo accumulation. Therefore, even though M. lepraemurium-induced adherent suppressor cell precursors are themselves fully resistant to Cy, their development is transiently abrogated by the drug, most probably through the impairment of a non-adherent cell subset regulating their maturation.

Induction of non-specific suppressor cells and myeloregulatory effects of an immunomodulatory azaspirane, SK&F 105685

Administration of the immunosuppressive agent, SK&F 105685, has demonstrated immunosuppressive activity in several animal models of autoimmunity such as adjuvant arthritis and experimental autoimmune encephalomyelitis. The mechanism of action of SK&F 105685 in these autoimmune disease models appears to be the induction of non-specific suppressor cells (SC) detected in the spleen and bone marrow of treated animals. In this study we have examined the kinetics of SC appearance in the spleen and bone marrow following treatment with 30 mg/kg/day, p.o., for 1-6 days. SC activity was apparent following a single dose and increased with successive treatments. Treatment with SK&F 105685 also resulted in significantly enhanced myelopoiesis as measured by a 128% increase in the frequency of bone marrow myeloid progenitors (CFU-GM). Mechanistic studies indicated that in vitro treatment of bone marrow stromal cell cultures with SK&F 105685 upregulated the production of colony stimulating activity (CSA) detectable in a rat CFU-GM assay. Further, in vitro studies revealed that the SC in the bone marrow or spleens of SK&F 105685-treated rats admixed with normal marrow cells inhibited CFU-GM formation at a six-fold less cell concentration than cells obtained from control rats. These in vitro results suggest that the SK&F 105685-induced myelopoiesis is regulated by the subsequent generation of SC.

Immunomodulatory activity and non-specific suppressor cell generation by spirogermanium in murine and rat models of cell-mediated immunity

Spirogermanium (SG) is a metal-containing compound reported to have antitumor, antiarthritic, antimalarial and immunoregulatory activity. In this study we have demonstrated that treatment of mice and rats with spirogermanium results in an inhibition of autoimmune disease and cell-mediated immune (CMI) responses. Prophylactic administration of SG inhibited the development of adjuvant-induced arthritis and the DTH response to purified protein derivative (PPD) in Lewis strain rats. SG treatment was also able to alleviate the symptoms of experimental autoimmune encephalomyelitis (EAE) induced in Lewis rats. In two strains of mice, BDF1 and C57B1/6, the DTH response to sheep red blood cells could be suppressed by intraperitoneal (i.p.) administration of SG. The spleens of both mice and rats that have been treated with this drug contain suppressor cells which inhibit the response of normal cells to concanavalin A (Con A) and the mixed lymphocyte reaction. In addition, the generation of cytotoxic T cells (CTL) in the murine MLR is abrogated in the presence of these suppressor cells. The suppressor cells were radiation-resistant (2000 rad), indomethacin-insensitive and were not depleted by treatment with anti-Thy-1.2 antiserum plus complement. These results suggest that SG modulates cell-mediated immune responses in vivo by the induction of non-specific suppressor cells.