Inhibition of an in vitro antibody response by a suppressor cell in normal bone marrow

Flow cytometric detection of CD11b+ Gr-1+ cells in nontumor-bearing mice: A propolis-elicited model

Myeloid-derived suppressor cells (MDSCs) are a heterogenous myeloid lineage population whose conventional surface phenotype is CD11b+ Gr-1+. Due to their rarity and fragility, analyses using primary isolated MDSCs are extremely difficult. However, counting CD11b+ Gr-1+ cells in associated tissues such as tumors and inflammatory lesions provides critical information regarding MDSC involvement in immune disorders in the tissues. Specific MDSC markers have not been identified, limiting our ability to apply histochemical approaches during MDSCs research. However, profiling surface antigens using multi-colorimetric flow cytometry enables us to easily monitor the abundance of MDSCs in vivo. Monitoring of mouse MDSCs and their subpopulations using flow cytometry is well established. In this article, I exemplify a conventional method of monitoring CD11b+ Gr-1+ cells in mouse adipose tissue after administration of Brazilian propolis ethanol extract, which is a strong inducer of MDSCs.

Redefining macrophage and neutrophil biology in the metastatic cascade

Tumor cells metastasize to distant organs through a complex series of events that are driven by tumor intrinsic and extrinsic factors. In particular, non-malignant stromal cells, including immune cells, modify tumor metastatic behavior. Of these cells, tumor-associated innate immune cells, particularly macrophages and neutrophils, suppress the cytotoxic activity of innate and adaptive killer cells and interact with tumor cells to promote their growth and malignancy. These findings in mouse cancer models suggest that targeting these sub-populations of immune cells holds therapeutic promise in treating metastatic disease. In this review, we describe the origin and role of the macrophages, neutrophils, and their progenitors in the metastatic cascade and suggest strategies that might enhance cancer therapy.

Characterization of natural suppressor cells in human bone marrow

Natural suppressor (NS) cells, which exert nonspecific suppressive activity in an unprimed manner, have been found in mouse, rabbit and monkey bone marrow (BM). In the present study, we characterize NS cells in human BM. NS activity was found in a fraction of low density (1.055-1.065 g/ml) BM cells that had been depleted of T cells, B cells, and monocytes. The NS activity was significantly decreased by the depletion of CD34+ or CD33+ cells but not CD56+ cells. The NS activity was indeed detected in isolated CD34+ cells and further enriched in CD34+CD33+ cells. Hematopoietic progenitor cells committed to the myeloid lineage were also enriched in the CD34+CD33+ cells, which significantly correlated to the NS activity. From these findings, it is strongly suggested that NS activity in human BM is exerted by the myeloid hematopoietic progenitors. Since cell-to-cell contact was not necessary for the action, NS cells seemed to secrete soluble mediator(s). Transforming growth factor-beta 1 and leukemia inhibitory factor were, however, not the candidates, based on experiments using neutralizing antibodies.

Pregnancy-associated suppressor cells in mice: functional characteristics of CD3+4-8-45R+ T cells with natural suppressor activity

Natural suppressor (NS) cells are MHC-unrestricted regulatory cells with non-specific inhibitory activity for immune responses. In adult mice, NS cells are characteristically found in bone marrow and in splenic tissue following total lymphoid irradiation and cyclophosphamide treatments. Recently, we have shown that the spleens of pregnant mice harbour a population of lymphocytes which resemble NS cells in terms of phenotype and inhibitory activity. In this study, we use positive and negative selection techniques to further characterize splenic pregnancy-associated NS (SPANS) cells as predominantly 'double negative' T cells (CD3+4-8-) bearing receptors for the lectins wheat germ agglutinin and soybean agglutinin, as well as expressing CD45R and the heat-stable J11d.2 antigen. Taken together, these findings lead us to conclude that SPANS cells belong to an immature T cell lineage. In keeping with their T cell phenotype, SPANS cells do not express the natural killer (NK) cell-specific markers NK2.1 and asialoGM1 and do not mediate lytic activity against NK-sensitive YAC-1 cells, although natural cytotoxic activity against WEHI-164 cells was found to co-purify with SPANS cells. Suppressive activity of SPANS cells in mixed lymphocyte reactions (MLR) is abolished by treatment with mitomycin C, suggesting that natural suppression in this system is a proliferation-dependent phenomenon. Preincubation of SPANS cells with conditioned medium from Con A-stimulated T cell cultures results in augmented NS activity, indicating that SPANS cells respond to T cell signals. Our data suggest that SPANS cells mediate suppression via the elaboration of a soluble suppressor factor since SPANS cells do not require cell-cell contact to mediate suppression and supernatants from short-term cultures of SPANS cell-enriched SBA+ pregnancy spleen cells inhibit MLR. We believe that SPANS cell may be important in regulating hematopoiesis and maternal anti-fetal immunity during murine pregnancy.

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.

Further characterization of murine bone marrow-derived natural suppressor cells. Potential relationships between NS and NK/LAK activities

Murine bone marrow (BM) cells regulate a variety of immune responses via an endogenous natural suppressor (NS) activity. We demonstrate that BM-derived NS activity resides in an enriched fraction of large, low-density cells which have a high proliferative rate. Complement-dependent lysis of BM cells by antibody directed against markers of Veto and NK/LAK cells had no effect on NS activity. The BM of SCID mice and their littermate C.B-17 possessed normal NS activity. Conversely, the BM of NK-deficient C57 beige mice displayed reduced NS activity as compared to normal C57 black mice. Long-term BM cultures (LTBMC) generated in medium containing supernatants of Con A-stimulated (CAS) rat spleen cells resulted in the emergence of a population of cells which possessed NS activity greater than that of fresh BM cells. The LTBMC were also potent effectors of NK activity, as compared to fresh BM, which had little NK activity. Thus, while NS, NK/LAK, and Veto cells are all nonspecific effectors of immune suppression, the exact relationship between them is not clear.

IL-3, IL-4, and IL-6 enhance IFN-gamma-dependent bone marrow natural suppressor activity

The ability of murine bone marrow (BM) natural suppressor (NS) cells to suppress a Con A proliferation assay was greatly enhanced by supernatant obtained from the T cell hybridoma D9C1.12.17. Of the lymphokines produced by this hybridoma, three were found to enhance suppression: interleukin-3 (IL-3), IL-4, and IL-6. These molecules enhanced suppression of both unirradiated and irradiated (2000 R) BM cells indicating that augmented suppression was not just due to proliferation of NS cells. The ability of all three of the lymphokines to enhance BM suppression could be blocked by anti-interferon-gamma (IFN-gamma) antibody. These results indicate that (1) NS cell activity is not radiosensitive and (2) that two signals may be required for maximal NS cell suppression, one being a lymphokine-mediated signal and the other IFN-gamma.

Natural suppressor-like cells in local graft-vs-host disease

We examined the generation of suppressor cells in the popliteal lymph nodes (PLN) of F1 recipients of parental spleen cells in rats, i.e., a local form of graft-vs-host disease (GVHD). PLN cells of F1 recipients, or subpopulations of these cells, were tested for their ability to suppress the mixed lymphocyte culture and cell-mediated lympholysis. Suppressor cells were consistently generated in this local GVHD reaction. In contrast to others, we found no evidence that suppression was mediated by T cells. Instead, the suppressor cells had the CD5-, CD8+, asialo-GM1+ phenotype characteristic of rat NK cells. The suppression exerted by these cells was nonspecific and was not mediated by a veto effect. The suppressor cells did not kill alloreactive T cell blasts, but had strong NK activity. These cells appear to be of donor origin. These local-GVHD-associated NK-like suppressor cells are similar, but not identical, to the natural suppressor cells described by other investigators in various experimental systems.

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.

Cyclophosphamide-induced suppressor cells in mice: suppression of the antibody response in vitro and characterization of the effector cells

It was found earlier that nonspecific suppressor cells obtained from the spleen of mice injected with cyclophosphamide (Cy) at a dose of 200 mg/kg body wt are nonadherent, surface Ig negative, and Thy-1 negative. It is now reported that Cy-induced suppressor (Cy-S) cells suppress the in vitro primary and secondary humoral responses in a dose-dependent manner. Suppressor activity is a property of a low density cell that is found in the spleen from 5 to 11 days after Cy administration. Cy-S cells are negative for a number of T-cell markers and are nonadherent to plastic. They are not agglutinated by peanut agglutinin and do not acquire the Thy-1 antigen after treatment with thymosin, suggesting that they are not T-cell precursors. While NK activity was detected in the Cy-S cell preparations, this activity was removed by treatment with antibodies specific for the Qa-5 antigen and complement without affecting the suppressor activity. Cy-S cells are, therefore, distinct from B cells, T cells, NK cells, and macrophages.

Suppression of the polyclonal B-cell responses by concanavalin A-treated bone marrow B cells in vitro

A suppressor cell that inhibits the development of a polyclonal antibody response of splenic B cells to lipopolysaccharide is generated in the bone marrow cell culture in response to a mitotic dose (10 micrograms/ml) of concanavalin A (Con A). The Con A-responding suppressor cell is radioresistant and found in a bone marrow B (BM-B) cell population of normal as well as athymic mice. The suppressor activity of Con A-treated BM-B cells was consistently higher (P less than 0.01-0.0001) than those of untreated BM-B and fresh BM cells. The BM-B cell population recovered from short-term (3-day) cultures with Con A contained about 65% surface immunoglobulin (Ig)-positive cells, about 6% T cells, and less than 0.5% plastic-adherent cells, the latter two of which did not contribute to the suppressive activity. Thus, cytolytic treatment with various anti-T-cell antibodies could not eliminate the suppressive activity of the Con A-treated BM-B cells, and the Con A-treated macrophage population provided no significant suppression. The Con A-treated BM-B cells adherent to anti-Ig or anti-Con A dishes exhibited highly enriched suppressive activity. It was therefore concluded that an immature B-cell population of bone marrow could develop in response to stimulation with Con A into surface Ig-positive suppressor cells, contributing to the regulation of nonspecific B-cell responses.

T cell-derived B cell growth and differentiation factors. Dichotomy between the responsiveness of B cells from adult and neonatal mice

In these studies we have determined the molecular weights of B cell growth factor (BCGF) (less than 20,000), and B cell differentiation factors (BCDF) that induce immunoglobulin M (IgM) secretion (BCDF mu) (30-60,000) and IgG secretion (BCDF gamma) (less than 20,000). Thus, the molecular weight of BCDF mu is distinct from that of BCGF and BCDF gamma; BCGF and BCDF gamma cannot be distinguished. In addition, BCGF, BCDF mu, and BCDF gamma are distinguishable by their presence or absence in different supernatants from a panel of mitogen-induced T cell clones. These results suggest that the three lymphokines are different. This conclusion is supported by their differential biological effect on B cells from adult and neonatal mice. Thus, treatment with anti-Ig induces B cells from adult mice to proliferate and this proliferation is sustained by BCGF. In contrast, even in the presence of BCGF, anti-Ig does not induce B cells from neonatal mice to proliferate. However, BCDF mu and BCDF gamma induce IgM and IgG secretion in B cells, respectively, from both adult and neonatal mice. Thus, mature B cells can both clonally expand and differentiate in response to anti-Ig, BCGF, and BCDF, whereas immature B cells can only differentiate. The poor response of neonatal B cells to anti-Ig and BCGF may partially explain the relative immunoincompetence of immature B cells.

Physical, biologic, and phenotypic properties of natural regulatory cells in murine bone marrow

A lymphocyte-enriched fraction of murine bone marrow (BML) contains natural regulatory cells (NRC) that can inhibit, on a dose-dependent basis, proliferative and cytotoxic responses to alloantigens in a mixed lymphocyte culture. The objective of this study was to investigate the characteristics of the cells responsible for this phenomenon in CBA mice. Maximal suppression was obtained with BML cells themselves rather than cell products. Light-scatter analysis of NRC on the fluorescence-activated cell sorter demonstrated them to be larger than small lymphocytes, and their sedimentation in discontinuous Percoll gradients showed the cells to be of heterogeneous density. This heterogeneity is further reflected by the fact that both plastic adherent and nonadherent BML are suppressive. NRC must be viable in order to mediate suppression; they are cortisone-resistant and are not affected by doses of gamma irradiation up to 1,000 R. NRC are not T or B lymphocytes or Ia-bearing macrophages. The involvement of mature granulocytes and macrophages in natural suppression is unlikely in that NRC do not bear Fc receptors. Elimination of cells from BML with the natural killer (NK) surface marker Asialo GM1 does not abrogate suppression. NRC are capable of mediating suppression across major and minor histocompatibility complex barriers. While lymphoid cells are prominent in BML, the contamination of this marrow fraction with immature granulocytes and monocytes makes a morphologic identification of NRC difficult. These characteristics are most consistent with NRC begin immature marrow cells of undetermined lineage. The relationship of NRC to naturally occurring marrow suppressor cells described in other systems is not yet clear and awaits experimental clarification.

Immunosuppressor cells from newborn mouse spleen are macrophages differentiating in vitro from monoblastic precursors

Newborn mouse spleen, whose cells strongly suppress the in vitro humoral response of adult spleen cells, is essentially a hematopoietic organ. It contains a large percentage of proliferating cells, among which about 50% are erythroblasts (identified by their spectrin content) and about 15% are cells of the myelocytic and monocytic lineage. Lymphoid cells are a minority, with about 20% B and only 1-2% T lymphocytes. After a 4 days, a culture of newborn spleen cells contains 5-10 times more macrophages than that of an adult spleen. Most of these macrophage precursors from the newborn spleen are proliferating cells, partially glass- or plastic-adherent, which differentiate in culture into activated macrophages producing large amounts of plasminogen activator. It is this macrophage excess which is responsible for the immunosuppressive effect of newborn spleen cells in culture, as indicated by (a) the effect of silica particles added to the cultures, which both relieve the suppression and prevent the accumulation of macrophages and (b) the suppression of the humoral response of adult spleen cells when they are cultured on the adherent cells from a newborn but not from an adult spleen. The suppressive effect of macrophages seems to result, at least in part, from the production of prostaglandin, since it can be relieved by indomethacin or aspirin. Suppression is not related to arginine depletion of the medium or to production of an excess of plasminogen activator. T lymphocytes from newborn spleen or lymph nodes have no suppressive capability.

The ability of horse serum to support an in vitro antibody response

It was found that 10% horse serum (HrS) could be used to support an in vitro antibody response to SRBC. It could replace the fetal calf serum (FCS) in both the culture medium and the nutritional feed. The response could be improved by the addition of 2-mercaptoethanol or by including both FCS (5%) and HrS (5%) in the culture medium.

Splenic plaque-forming cells (PFC) and stem cells (CFU-s) during acute phenylhydrazine-induced enhanced erythropoiesis

The erythroid status and levels of splenic plaque-forming cells (PFC) to sheep red blood cells (SRBC) were monitored in mice subsequent to acute phenylhydrazine (PHZ)-induced hemolytic anemia. From ferrokinetic measurements, we noted a shift in erythropoiesis from bone marrow to spleen. The levels of splenic PFC were significantly depressed following PHZ-induced erythroid differentiation. Although this immune depression may reflect competition at the stem cell levels, whereby pluripotent stem cells (CFU-s) are preferentially differentiated into the erythroid line at the expense of lymphopoietic pathways, other possibilities cannot be excluded. In this regard, we have shown that loading of the mononuclear phagocyte system (MPS) by PHZ-damaged erythrocytes effected profound depressions in splenic PFC numbers. Lastly, in addition to the well-documented increases in CFU-s migration from marrow to spleen during enhanced erythropoiesis, we noted increased migration of B lymphocytes (as assessed by PFC) in marrow-shielded lethally-irradiated mice given PHZ. We also provide data which show that PHZ-damaged RBC evoke increased migration of CFU-s in normal mice, indicating a possible involvement of the MPS in stem cell migration.