Activated Suppressor Cells Research Articles

Secretion of a suppressor cell inducing factor by an interleukin 3-dependent cell line with natural cytotoxic activity: III. Comparison with other interleukin 3-dependent cell lines

We previously described the interleukin 3 (IL-3)-dependent cell line, M1-A5, which has both natural cytotoxic (NC) and suppressor cell activities, the latter of which is mediated, in part, by the release of two cytokines which activate suppressor cells from unprimed lymphoid precursor cells. In this study we have compared the M1-A5 cell line with four other IL-3-dependent cell lines to determine whether these dual activities are universally associated with IL-3 dependence and to test the hypothesis that there is a direct relationship between the cytotoxic and the suppressive activities. The cell lines tested were a bone marrow derived Dexter culture derived line (FDC-P1), two Moloney leukemia virus induced leukemias (DA-1 and DA-3), and a mast cell line (PT18(A17)). All lines were dependent on IL-3 for survival but FDC-P1, DA-1, and DA-3 showed varying degrees of short-term proliferation in granulocyte-macrophage colony stimulating factor (GM-CSF). The cell lines all expressed asialo GM1 and Ly-5 surface markers but differed with respect to other markers. DA-1 expressed MAC-1, FDC-P1 and DA-3 expressed Thy-1, and PT18(A17) expressed receptors for the Fc portion of IgE. The cell lines varied greatly in their cytotoxic activity against WEHI-164. FDC-P1, DA-1, and PT18(A17) had low NC activity. DA-3 had consistently high activity, greater than that seen with M1-A5 cells. However, none of the cell lines secreted constitutively a suppressor cell inducing factor (SIF). In addition, it was demonstrated that recombinant murine TNF did not activate suppressor cells capable of inhibiting antibody synthesis and that anti-TNF did not block SIF activity, thus suggesting that TNF contamination of the M1-A5 derived SIF preparation is not responsible for the induction of suppressor cells. We conclude that suppressor cell inducing factors are not universally secreted by IL-3-dependent cell lines, that there is no correlation between NC and SIF activity, and that the dual activities of M1-A5 cells are not mediated by TNF.

Activated suppressor cell function in multiple sclerosis — clinical correlations

Activated suppressor cell function mediated by either freshly isolated peripheral blood mononuclear cells (MNCs), freshly isolated CD8 + lymphocytes or by CD8 + cell lines, has previously been found to be reduced compared to controls in multiple sclerosis (MS) patients with progressive disease (MS-P). In this study, we found that suppressor activity mediated by CD8 + cell lines, derived from MS patients with stable disease (MS-S) patients and maintained in culture for 14 days, was significantly greater (45 ± 6%) compared to that mediated by MS-P patients' CD8 + cells (11 ± 4%, P < 0.005). The MS-S suppressor values were, however, suggestively reduced compared to controls (60 ± 6%, P < 0.05). MNC-mediated suppressor values for the MS-S group (61 ± 5%) did not differ from the control group (67 ± 6%). Values for the MS-P group (7 ± 6%) were significantly reduced compared to MS-S and control groups. Cytotoxic activity mediated by CD8 + cell lines showing defective suppressor function did not differ from control values. The cell lines in MS and control did not differ with respect to their rate of proliferation in the presence of IL-2 and OKT3. Suppressor function in this assay was ablated if exogenous IL-2 was removed from the culture media. These data suggest that defective activated suppressor function is characteristic of the progressive form of MS, although a suppressor defect is also partially expressed in stable MS patients when CD8 + cell lines are studied.

Vaccination-induced activation of human blood T cells suppressing pneumococcal polysaccharide-specific B cells.

In order to study the regulation of the human B cell response to pneumococcal polysaccharides (PPS), anti-PPS-secreting cells (SC) and Ig-SC were quantitated in unstimulated and EBV-stimulated cultures of blood mononuclear cells (MNC) established before and one and two weeks after vaccination. In unstimulated cultures established one week after vaccination, significant numbers of anti-PPS-SC were detected; however, they were observed only in cultures depleted of T cells (mean: 62/10(6)), suggesting the presence of T cells suppressing in vivo-activated PPS-specific B cells. In EBV-stimulated cultures depleted of T cells, low numbers of anti-PPS-SC were observed before the vaccination, and the numbers of these cells increased significantly two weeks after the immunization (from 28 to 48/10(6)). In contrast, the numbers of anti-PPS-SC and total Ig-SC decreased in EBV-stimulated cultures of unseparated MNC established after the vaccination, suggesting T cell-mediated suppression. The findings were not explained by changes in the numbers of blood T helper (Leu 3+) or T suppressor (Leu 2+) cells following the vaccination. Possibly the vaccination induces the appearance in the blood of activated suppressor cells responsible for the suppression of B cells in unstimulated and EBV-stimulated cultures.

A role for histamine type II (H-2) receptor binding in production of the lymphokine, soluble immune response suppressor (SIRS).

Soluble immune response suppressor (SIRS) is an immunosuppressive protein produced by human and murine suppressor cells activated by a variety of agents. Because histamine has been reported to activate suppressor cells, the possibility that it also induced SIRS production was investigated. Human lymphocytes treated with 10(-4) M histamine for less than 1 hr released a suppressive substance into culture supernatants that was physically, functionally and antigenically similar to human SIRS. Cimetidine and ranitidine, structurally distinct histamine type II (H-2) receptor antagonists, prevented histamine-induced SIRS production. In further experiments, suppression of human polyclonal IgM PFC responses by Con A and interferons, substances that activate the SIRS pathway, was inhibited by H-2 receptor antagonists. Activation of lymphocytes to produce SIRS by Con A or interferons was blocked by cimetidine or ranitidine. These data demonstrate that production of SIRS is induced by histamine, and raise the possibility that H-2 receptor binding may play a role in the SIRS pathway.

Cytobiological studies in multiple myeloma.

Multiple myeloma (MM) is a heterogeneous disease in which the median survival time varies from a few months to more than 10 years. Clinical, kinetic, and immunological studies reflected such a heterogeneity. In clinical studies beta 2-microglobulin (beta 2-M), which is a well-known prognostic factor, was evaluated as an expression of tumor burden: a linear increase has been observed from stage I to stage III. No clear cutoff was detected between monoclonal gammopathy of undetermined significance (MGUS) and stage I MM. Kinetic studies have been performed by 3[H]thymidine or 5-bromo-2-deoxyuridine incorporation in bone marrow plasma cells. An analysis has been carried out between beta 2-M and the labeling index (LI). No correlation has been observed between these 2 parameters which identify different subgroups of poor prognosis patients. MGUS always showed an LI less than 1% allowing a clear distinction from stage I MM. A reduced survival time has been observed in patients with an LI greater than 2%. A subgroup of MM patients characterized by an LI greater than 2% and a decrease of M component greater than 50% within 3 months (early responders) has been identified. These patients showed a median survival time of about 10 months. Immunological studies identified a sharp decrease of 5' nucleotidase (5'NT) enzymatic activity in peripheral blood lymphocytes. This enzymatic defect is the biochemical marker of the expansion of a T subpopulation expressing low levels of 5'NT. This subpopulation was characterized by monoclonal antibodies and is represented by activated suppressor cells (OKM1+, DR+). Such an unbalancement in the T cell subpopulation was evident in poor prognosis patients. Clinical, kinetic and immunological studies identified new prognostic parameters that will be useful in the future therapeutic strategy of MM.

Secretion of a suppressor cell-inducing factor by an interleukin 3-dependent cell Line with natural cytotoxic activity. II. Range, potency, and kinetics of suppressive activity

We have shown previously that the interleukin 3 (IL 3)-dependent, M1-A5 cell line hasnatural cytotoxic (NC) activity and secretes a small molecular weight factor which activates suppressor cells capable of inhibiting the in vitro generation of antibody forming cells (PFC). In this paper, we show that the M1-A5 cells are capable of suppressing the generation of both PFC and cytotoxic T cells (CTL) as well as the proliferative response in a mixed lymphocyte reaction (MLR). Very little suppression of Concanavalin A (Con A)-stimulated proliferation is seen. Suppression is seen with as few as 10 2 M1-A5 cells, a 10 −4 dilution of a cell-free supernatant from the M1-A5 cells, or as few as 104 suppressor cells activated by the suppressor cell inducing factor (SIF) secreted by the M1-A5 cell line. Kinetic studies show that, in all cases, the cells or suppressive factors must be present in the initial 24 to 48 h of the culture for significant suppression. In contrast, a second IL 3-dependent cell line FDC-P1 is directly suppressive, but this suppression is clearly due to a different mechanism, since cell-free supernatants are not inhibitory and do not activate suppressor cells.

Steroid-sensitive mechanism of soluble immune response suppressor production in steroid-responsive nephrotic syndrome.

Soluble immune response suppressor (SIRS), a lymphokine that suppresses antibody production and delayed type hypersensitivity in vivo, has been detected in urine and serum from certain patients with nephrotic syndrome. In the present paper, the relationship between SIRS production and nephrotic syndrome is further characterized. A striking correlation was found between detection of SIRS and the presence of steroid-responsive nephrotic syndrome (SRNS). A potential mechanism of SIRS production in SRNS patients was identified, in that lymphocytes from patients produced SIRS without requiring activation by exogenous agents, and incubation of normal lymphocytes with serum from patients activated the cells to secrete SIRS in culture. Although SIRS disappears rapidly from urine or serum after initiation of corticosteroid therapy, hydrocortisone (10(-6)-10(-7) M) did not block secretion of SIRS by activated suppressor cells. It did, however, inhibit in vitro activation of lymphocytes to produce SIRS by concanavalin A, interferon, or SRNS patient serum. The association of suppressor cell activation with SRNS and the sensitivity of both to steroids suggest that the pathogeneses of albuminuria and SIRS production are related.

Cyclic AMP as the second messenger for prostaglandin E in modulating suppressor cell-activation by natural suppressor/cytotoxic cells

The M1-A5 cell line, isolated from the spleen of a tumor bearing mouse, has been shown to secrete a suppressor cell-inducing factor (SIF) which activates suppressor cells from the spleen cells of unprimed mice. In an earlier publication we demonstrated that prostaglandins of the E series (PGE) regulate the release of SIF from M1-A5 cells. In the present study we provide evidence that cyclic adenosine 3′,5′-monophosphate (cyclic AMP) is the second messenger for PGE in regulating the release of SIF from M1-A5 cells. Our data show that PGE 1 and PGE 2 stimulate cyclic AMP production by M1-A5 cells. Furthermore, we show that drugs which: (1) elevate cyclic AMP levels (dibutyryl (db) cyclic AMP), (2) inhibit the breakdown of cyclic AMP (theophylline), or (3) stimulate adenylate cyclase (isoproternol and histamine) all restored suppressor cell activation by M1-A5 cells in which PG synthesis had been blocked by acetylsalicylic acid (ASA). Thus, these results are consistent with the hypothesis that cyclic AMP is the second messenger for PGE in modulating the release of SIF from M1-A5 cells.

Phenotypic analysis of lamina propria lymphocytes: Predominance of helper-inducer and cytolytic T-cell phenotypes and deficiency of suppressor-inducer phenotypes in Crohn's disease and control patients

To better define the nature of intestinal T cells, the phenotypes of isolated lamina propria lymphocytes (IPL) were determined in both Crohn's disease patients and control patients using combinations of monoclonal antibodies that have been found to correlate with particular immunoregulatory functions. Isolated LPL and autologous peripheral blood lymphocytes (PBL) were stained with multiple combinations of monoclonal antibodies and studied by dual immunofluorescence flow cytometry. In LPL, compared with PBL, there was a significant increase in the proportion of T cells having the Leu-3 +, Leu-8- and Leu-3 +, 2H4 − phenotypes (associated with helper-inducer function) and a corresponding decrease in the proportion of T cells having the Leu-3 +, Leu-8 + and Leu-3 +, 2H4 + phenotypes (associated with suppressor-inducer function). It was also found that in LPL, compared with PBL, the percentage of cells with the Leu-2 +, Leu-15 + phenotype (associated with suppressor-effector function) was significantly lower. However, the percentage of T cells with the Leu-2 +, 9.3 + phenotype (associated with cytolytic function) was similar in PBL and LPL in control patients. There were no major differences comparing Crohn's disease patients with control patients, except that the proportion of Leu-2 +, 9.3 + lymphocytes was higher in PBL in Crohn's disease patients. These results show that the lymphocyte subpopulations in the lamina propria differ from those in peripheral blood in having predominantly the phenotypes of helper-inducer and cytolytic T cells, whereas the phenotypes of suppressor-inducer cells and activated suppressor cells are less frequently observed.

Defective suppressor cell function mediated by T8+ cell lines from patients with progressive multiple sclerosis.

Activated suppressor cell function, induced with either concanavalin A or OKT3 and mediated by either unfractionated mononuclear cells or "panning" enriched T8+ cells, freshly isolated from peripheral blood, is reduced in patients with progressive multiple sclerosis (MS) as compared with control donors. In this study, we generated T8+ cell lines from the peripheral blood of these same patients and controls. Suppressor activity, mediated by T8+ cells exposed to OKT3 on days 1, 7, and 14 of culture and then treated with mitomycin C on day 16, was significantly reduced in the MS group (mean percent suppression 13% +/- 5) as compared with the control group (68% +/- 6, n = 8, p less than 0.001). No differences were noted in [3H]thymidine uptake by the OKT3-stimulated T8+ cell lines of MS and control groups. Mean percent suppression mediated by T4+ cell lines did not differ between MS and control groups (15% +/- 4, n = 3, vs 22% +/- 2, n = 4). These current data suggest that the previously observed defect in T8+ cell-mediated activated suppressor cell function in MS is a persistent one, favoring the postulate that the defect reflects intrinsic alterations in this cell population rather than a transient effect of serum factors on T8+ cell function.

Phenotypic and functional characteristics of active suppressor cells against IFN-gamma production in PHA-stimulated cord blood lymphocytes.

Cord blood mononuclear cells (MNC) were defective in their ability to produce interferon-gamma (IFN-gamma) on stimulation with phytohemagglutinin (PHA) or recombinant interleukin 2, whereas cord MNC could induce comparable amounts of IFN-gamma with adult controls on stimulation with a streptococcal preparation, OK-432. Moreover, irradiation of cord MNC with 1,500 rad before PHA stimulation could restore the IFN-gamma production. Kinetic studies indicated that such augmentation of IFN-gamma production by irradiation was evident when cord MNC were irradiated before or by 12 hr of PHA-stimulated culture. But irradiation after 18 hr or more of PHA stimulation did not exert any significant augmentation on IFN-gamma production by cord MNC. It seemed most likely that the ability of IFN-gamma production is already mature at birth, but radiosensitive suppressor effectors on IFN-gamma production are activated within cord MNC at an early stage of PHA stimulation, resulting in poor IFN-gamma production by cord MNC. PHA-induced IFN-gamma production by OKT3+, OKT4+, and OKT8- cord cells were markedly enhanced by irradiation with 1,500 rad before the culture. Coculture experiments disclosed that cord OKT4+ cells, but not OKT4- cells, when prestimulated with PHA for 24 hr, exerted active suppression on PHA-induced IFN-gamma production by adult MNC in a dose-dependent manner. These results suggested that radiosensitive suppressor effectors on IFN-gamma production were induced within the OKT4+ T cell subset of cord MNC on PHA stimulation.

Role of the spleen on immunosuppression in esophageal and gastric cancer.

To elucidate the role of the spleen on immunosuppression of gastric and esophageal cancer, suppressor cell activities of spleen cells (SCs), splenic vein lymphocytes (SVLs) and peripheral blood lymphocytes (PBLs) were investigated. Concanavalin-A induced suppressor cell (Con-AS) activity of SCs was significantly higher in patients with gastric cancer than in those with benign diseases. Higher Con-AS activity of SCs was observed in esophageal cancer patients with tumors located in the lower portion of the esophagus. In comparison with suppressor activities of SCs and SVLs, the decrease of the predominance of suppressor precursors in SCs and the increase of the spontaneously activated suppressor cells in SVLs were noted with the advance of the tumors. Culture supernatants from splenic adherent cells significantly induced suppressor cell activities as well as did sera from splenic venous blood. From these results, it is concluded that the generation of suppressor precursors in the spleen is dependent on the location of tumors and that the maturation of suppressor cells occurs in the spleen by factors released from splenic adherent cells, then migrates into the peripheral blood.

Immune presensitization and local intrauterine defences as determinants of success or failure of murine interspecies pregnancies.

Putatively immuno-incompetent Mus musculus females exhibited failure to support pregnancy of Mus caroli embryos. These results for M. musculus females (i.e. treated by cyclosporine A, of the nu/nu genotype, and as an interspecies chimaera) can be explained in immunological terms. Mus musculus females possessed pre-sensitized cytotoxic T cells against Mus caroli antigen. Nu/nu mice possessed activated NK cells and macrophages, and selectively discriminated against Mus caroli embryos early in pregnancy unlike normal +/+ females; the requirement for T cells to activate non-specific cytotoxic effector mechanisms was bypassed in nu/nu mice. Mus caroli are not inbred, and interspecies chimaeras which are tolerant of the antigens on the Mus musculus donor strain were not tolerant of cells from unrelated Mus caroli. Interspecies chimaeras also behaved as if they were pre-sensitized to Mus caroli. Our results show that Mus caroli embryos recruit fewer active suppressor cells even when gestating in Mus caroli decidua as compared to Mus musculus embryos in Mus musculus decidua and that the ability of Mus caroli placental cells to directly inhibit cytotoxic effector cell killing was inherently less than the inhibitory activity of placental cells from Mus musculus. Mus caroli embryos therefore appear to be less well defended against maternal immune attack even when gestating in a uterus possessing compatible Mus caroli decidual tissue.

Secretion of a suppressor cell inducing factor by an interieukin-3 dependent cell line with natural cytotoxic activity

This report describes the morphology, surface markers, growth requirements, and functional activity of the M1-A5 cell line, which was established by the limiting dilution of spleen cells from a mouse bearing a large methylcholanthrene-induced fibrosarcoma, The M1-A5 cells share many of the morphological features of large granular lymphocytes and, in addition, express asialo GM1 and Ly-5 surface markers which are commonly found on natural killer cells (NK) cells. There is no expression of T-cell differentiation antigens, surface immunoglobulin, or the granulocyte/ macrophage marker, MAC-1. M1-A5 cells are dependent on exogenous growth factor(s) for survival and will proliferate if cultured in interleukin 3 (IL-3), but not in interleukin 1 (IL-1), interleukin 2 (IL-2), or granulocyte/macrophage colony stimulating factor (GM-CSF). In addition, the M1-A5 cells do not absorb IL-2. Despite their morphology and surface characteristics, the M1-A5 cells do not lyse NK targets such as YAC-1 and RLM1 in 4- or 18-hr cytotoxic assays but do lyse the natural cytotoxic (NC) susceptible target, WEHI-164, and to a very small extent, the M-1 fibrosarcoma cells, in an 18-hr assay. Thus they exhibit NC-like cytotoxic activity. In addition, the M1-A5 cells secrete a small molecular weight factor which activates suppressor cells capable of inhibiting antibody synthesis by cocultured syngeneic spleen cells.

Induction of suppression by a murine nonspecific suppressor-inducer cell line (M1-A5). II. The role of prostaglandins.

A murine nonspecific suppressor-inducer cell line (M1-A5) was generated from the spleen cells of a mouse bearing an advanced methylcholanthrene-induced fibrosarcoma. We previously demonstrated the capability of M1-A5 cells to activate suppressor cells from the spleen cells of unprimed mice. We demonstrate here that induction of suppression by M1-A5 cells was blocked by acetylsalicylic acid (ASA) and ibuprofen at concentrations which block prostaglandin (PG) synthesis. Maximal blockade of the induction of suppression by M1-A5 cells was seen when ASA was added at the initiation of culture, and it required inhibition of PG synthesis at the level of the inducer (M1-A5 cells) population. However, ASA blockade of suppressor cell activation by M1-A5 cells was not due to ASA acetylating suppressor-inducing factor. Exogenously added PGE1, PGE2, and PGI2, but not PGF2 alpha or PGD2, were able to restore the inducing capability of M1-A5 cells, which had been blocked by ASA. However, PGE1, PGE2, or PGI2 did not reconstitute an inactive suppressor-inducing factor. These results suggest that PG act to modulate the release of suppressor-inducing factor from M1-A5 cells.

Requirement for prostaglandin E 1 (PGE 1) for the secretion of suppressor cell inducer factors by spleen cells of tumor-bearing mice

We have demonstrated previously that spleen cells from mice bearing M-1 fibrosarcomas release low molecular weight factors capable of activating suppressor cells from unprimed normal spleen cells. Using Markbrook vessels to separate inducer and precursor populations, we found in the previous paper that cyclooxygenase inhibitors blocked the activation of suppressor cells and that this activation could be restored by exogenous PGE 1. In this paper we have examined the site of action of the prostaglandins in the activation of suppressor cells. To do so, we tested cell-free supernatants from cultured spleen cells of tumor-bearing mice (inducer cells) for the ability to activate suppressor cells from unprimed normal spleen cells (precursor cells). Supernatants from acetyl salicyclic acid (ASA) treated inducer cells did not activate suppressor cells and exogenous PGE 1 could not restore the activity of this supernatant. In contrast, if inducer cells were treated with ASA and then incubated with PGE 1, the supernatant was capable of activating suppressor cells. No role for prostaglandins at the level of the precursor cells or the effector suppressor cells was seen. These data suggest that the inducer cells in tumor-bearer spleens require prostaglandins for the release of an inducer factor but that prostaglandins are not required for the action of this factor on the precursor cells or for the effector function of the activated suppressor cells.

Parameters Involved in the Induction and Abrogation of the Lethal Graft‐versus‐Host Reaction Directed against non‐H‐2 Antigens

The grafting of cells from donors incompatible for non-H-2 antigens alone can lead to GvHR mortality in up to 100% of lethally irradiated adult recipients. GvHR severity correlates with the number of mature immunocompetent cells present in the bone marrow inoculum. Histologic and clinical manifestations of GvHR observed in these mice differ from those seen when GvHR is induced across an H-2 barrier. The number of non-H-2 genes capable of influencing GvHR mortality is probably great, and their effects may vary as a function of sex. The non-H-2 genes influence GvHR mortality mainly via their interactions, the consequences of which are complex and can result in either cumulative or suppressive effects. GvHR mortality is considerably reduced by donor immunization, shortly before grafting, against host-specific non-H-2 antigens; and it is virtually abrogated by an additional immunization of the donors against nonspecific (foreign) H-2 antigens. Three weeks after grafting, these "protected" mice are easily distinguishable from those undergoing lethal GvHR, as assessed by both clinical appearance and histologic examination; in contrast, they are nearly indistinguishable from control mice grafted with syngeneic cells. However, depending upon the conditions used for the immunization, an additional immunization against nonspecific H-2 antigens can lead to acceleration rather than suppression of GvHR mortality; this phenomenon is not seen, under the same experimental conditions, after immunization against specific non-H-2 antigens alone. It is therefore suggested that a "second signal" provided by an additional nonspecific stimulus can potentiate either the establishment of specific suppression or the activation of a secondary ("positive") response. Suppressive effects of the specific and nonspecific immunizations are cumulative, and both treatments activate suppressor cells. The intensity of suppression induced by both specific and nonspecific immunizations is antigen dose-dependent. At equivalent antigen doses the specific immunization is considerably more effective than the nonspecific immunization, and is detectable after injection of as few as 2.5 X 10(5) cells. In both cases, irradiation of the immunizing cells abolishes the suppression induced by the lower cell doses tested, while it merely decreases the intensity of the suppression induced by the higher cell doses tested. The impairment of suppression after irradiation of the immunizing cells is not attributable to a modification of their homing pattern, but to the fact that proliferation of the immunizing cells, which leads to an augmentation of the antigen dose, is abolished by irradiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of suppression by a murine nonspecific suppressor-inducer cell line (M1-A5)

A murine nonspecific suppressor-inducer cell line (M1-A5) was established by the limiting dilution method from the spleen cells of a mouse bearing an advanced methylcholanthrene-induced fibrosarcoma. Indirect immunofluorescence studies demonstrated that the M1-A5 cells were Thy-1 −, sIg −, Ly-5 +, MAC −, and 45% asialo GM1 +. The M1-A5 cells were able to activate suppressor cells from unprimed, syngeneic normal spleen cells. These activated cells inhibited antibody production by cocultured syngeneic lymphoid cells. Induction of suppression by the M1-A5 cells was via the release of a suppressor-inducing factor, which was found to be protein in nature. Kinetic studies showed that when M1-A5 cells were separated from NSC by a dialysis tubing in Marbrook vessels, the M1-A5 cells required a minimum of 8 hr incubation period before suppressor cell activity could be demonstrated in precursor cells. On the other hand, induction of suppression by the suppressor-inducing factor required a minimum of 3 hr exposure of the precursor cells to the factor.

Activation of suppressor T cells by low-molecular-weight factors secreted by spleen cells from tumor-bearing mice

The spleens of mice bearing large M-1 fibrosarcomas have been shown to contain several populations of cells which nonspecifically suppress antibody synthesis by cocultured normal spleen cells. It has now been shown that the spleens of tumor-bearing mice also contain inducer cells which secrete soluble factors capable of activating suppressor T cells from unprimed precursor cells. The activated suppressor cells are Thy 1 +, Lyt 1 +2 + and secrete a soluble suppressive factor. They inhibit the in vitro generation of antibody-forming cells by cocultured normal spleen cells stimulated by T-cell-dependent antigens. They do not, however, suppress the antibody response to T-cell-independent antigens and do not inhibit antibody synthesis by cocultured nude mouse spleen cells cultured with T-cell-dependent antigens and exogenous helper factors. In addition, suppression is blocked if conditioned medium containing T-cell growth factors is added to the suppressor cell assays. These data suggest that cells in the spleens of tumor-bearing mice secrete inducing factors which activate suppressor cells. These activated suppressor cells in turn secrete soluble suppressor factors which inhibit antibody synthesis, possibly by interfering with the synthesis or release of T-cell growth factors.

Induction of suppressor cells by a tumor-derived suppressor factor

Murine fibrosarcomas produce a factor that activates suppressor cells to inhibit expression of delayed-type hypersensitivity (DTH) responses to dinitrochlorobenzene (DNCB). This tumor-derived suppressor factor (TDSF) was partially purified by preparative isoelectric focusing of spent medium and 3 M KCl extracts of cultured methylcholanthrene-induced and spontaneous fibrosarcomas of C 3H/He mice. Incubation of 1 μg/ml of a fraction, isoelectric pH < 2.9, with normal syngeneic spleen cells for 1–6 hr at 37 °C induced suppressor cells that inhibited the primary DTH response to DNCB upon intraperitoneal transfer to normal C 3H/He mice. TDSF was not present in extracts of either syngeneic embryonic fibroblasts or normal spleen cells or in medium conditioned by normal peritoneal exudate cells but was present in 3 M KCl extracts of and the spent medium from four different cultured murine fibrosarcomas. TDSF activity was not restricted at the major histocompatibility complex. The suppressor cells inhibited the efferent limb of the DTH response because (1) hyporesponsive recipients of TDSF-treated spleen cells had splenic effector T cells capable of transferring DTH to DNCB into naive secondary recipients and (2) the ability of Lyt 1 +,2 − effector T dth cells to transfer a secondary DTH response to DNCB was inhibited by co-incubation with macrophages or Lyt 1 −,2 + T cells treated with TDSF. Preliminary biochemical analysis suggested that TDSF was an RNA-protein complex. Thus, several murine fibrosarcomas produced a soluble factor that activated splenic suppressor cells to depress the immune response to nonneoplastic antigens. These suppressor factors represent a novel group of regulatory molecules which may be ribonucleoprotein complexes.