Induction of T-suppressor cells by immunization with allogenic spleen cells in the H-2 system of mice]

The nature of suppressor cells induced by immunization with the allogenic spleen cells and inhibiting the DNA synthesis activation in the mixed lymphocyte culture was studied. Suppressor cells are resistant to mitomycin C and carrageenan. They are not inactivated by the treatment with rabbit anti-B- and anti-Ig- as well as with mouse antibodies (anti-Mls serum) against B lymphocytes in the presence of complement but eliminated by rabbit anti-lymphocyte and anti-T globulins and by mouse anti-theta serum. These T suppressor cells are concentrated in the large lymphocyte fraction in the ficoll gradient. Their blocking of the DNA synthesis activation is evidently non-specific.     

Modulation of the immunosuppressive effects of splenic macrophages in Fischer rats bearing adenocarcinoma 13762

Summary The nature of spleen cells in Fischer rats bearing a large size (>1 cm diameter) mammary adenocarcinoma 13762A (MAC) which block the immunostimulating capacities of MTP² (a synthetic immunomodulator) and suppress proliferation in vitro of splenic T and B lymphocytes by their respective mitogens was investigated. Splenic macrophages were recognized as the suppressor cells by (a) restoration of mitogenic responses by depletion of macrophages from spleen cell suspensions and (b) continued suppressor activity in spleen cell suspensions of tumor bearers devoid of viable T lymphocytes. Macrophage contact with T lymphocytes was required for the inhibition of T lymphocyte proliferation by concanavalin A as shown by (a) the absence of suppressor activity in supernatants derived from cultured suppressor macrophages, (b) lowering of the suppressor activity of intact macrophages after treatment with neuraminidase, (c) lowering of the suppressor activity of macrophages by addition of red cells to spleen cultures of tumor bearers indicating red cell interference with macrophage-T cell interaction and (d) lack of inhibiting action of suppressor macrophages on allogenic T lymphocyte proliferation showing macrophage T cell recognition for suppression.Animals bearing a large size tumor exhibited spleen hypertrophy and an increase in macrophage:lymphocyte ratio and a decrease in red cell:lymphocyte ratio. Splenic macrophages did not appear to be implicated in blocking antitumor immunity induction since (a) suppressor macrophages were absent in spleens during the inductive phase of the immune response and (b) MAC implanted in allogenic Wistar rats grew to about 2 cm diameter, induced splenic suppressor macrophages but the tumor was later rejected by the animals. Collectively the results suggest that suppressor macrophages are the result of increasing tumor volume rather than its cause.This study was supported by a grant from the National Cancer Institute of Canada Abbreviations used: Con A, Concanavalin A; LPS, lipopolysaccharide; PHA, phytohemagglutinin; MTP, maltose tetrapalmitate; MAC, mammary adenocarcinoma 13762; RPMI, Roswell Park Memorial Institute; TBR, tumor bearing rat; RBC, red blood cell     

Activation of human B lymphocytes. III. Concanavalin A-induced generation of suppressor cells of the plaque-forming cell response of normal human B lymphocytes.

Con A-induced suppression of the direct PFC response to polyclonal stimulation in human B cells has been described. Two types of experiments are presented. First, Con A was added directly to PWM-stimulated PB or tonsil cells resulting in a dose-dependent suppression of the PFC response, with maximal suppression occurring at a Con A concentration of 10 mug/ml. This suppression is completely removed by the simultaneous addition of alphaMM to the cultures. Secondly, Con A stimulation of tonsil or PB lymphocytes generated a population of cells which when added to autologous lymphocyte cultures induced a marked and reproducible suppression of the PFC response. The generation of suppressor cells is dependent on cell division and is blocked by alpha MM. Once generated the process of suppression is indpendent of the presence of Con A itself and is mediated by an activated lymphocyte population. These studies demonstrate a simple and reproducible model for the generation of a population of suppressor cells capable of inhibiting the direct PFC response to PWM-induced polyclonal activation of normal human B lymphocytes.     

Functional characterization of T lymphocytes grown in semisolid agar

T lymphocyte colony forming cells (TL-CFC) grown in agar in the presence of PHA were assayed for their capacity to induce or suppress polyclonal PWM dependent B lymphocyte differentiation into plasma cells. This was measured by identifying cells containing intracytoplasmatic immunoglobulins by direct immunofluorescence. To validate the helper and suppressor system used in this paper, the inductive capacity of unfractionated T lymphocytes and their subpopulations bearing Fc-receptors for IgM (TM) and for IgG (TG) was measured. The unfractionated T cells and the TM fraction showed helper activity, whereas the TG cells expressed suppressor activity. The TL-CFC grown in agar in the presence of PHA manifested helper activity at low cell concentration. However, increasing the TL-CFC concentration finally caused suppression of B cell differentiation. The suppressor effect could be abolished by prior irradiation of the TL-CFC before seeding them in agar. These results indicate that T cells grown in agar have the functional capacity of T helper and T suppressor cells to induce and suppress polyclonal PWM dependent B lymphocyte differentiation into plasma cells.     

Induction kinetics of human suppressor cells in the mixed lymphocyte reaction and influence of prednisolone on their genesis

The induction kinetics of human suppressor cells in mixed lymphocyte cultures (MLC) and the influence of prednisolone on the genesis of these suppressor cells is reported. We induced over 1 to 6 days suppressor cells in one-way MLC (MLC-1), the inhibitory activity of which was tested on a secondary MLC (MLC-2), and on responder cells alone, where lymphocytes were obtained from the same lymphocyte donors as for the MLC-1. In four experiments the degree of inhibition ($\text{x}\text{?}\text{±}\text{SE}$) when suppressor cells were induced for 2, 4, or 6 days was 38.5 ± 11.8, 79.5 ± 7, and 85 ± 6%, respectively, compared to 50.5 ± 9.4, 83.3 ± 7.8, and 85.3 ± 9.8% when 500 ng/ml prednisolone was added to the MLC-1. A similar inhibition pattern was observed when the generated suppressor cells were incubated with responder cells only. The inhibitory activity of these MLC-induced suppressor cells was abrogated by irradiation with 3000 R. Suppressor cells apparently are generated in MLCs between Days 1 and 4; furthermore, their genesis is not affected by usual therapeutic concentrations of prednisolone.     

Supernatant derived from a human hepatocellular carcinoma cell line (PLC/PRF/5) activates a population of T-suppressor cells

Harvest fluid derived from a primary hepatocellular carcinoma cell line (PLC/PRF/5) inhibited the incorporation of 3H-thymidine into PHA-activated human lymphocytes. A similar effect was observed when lymphocytes were pre-incubated with the tumour supernatant and washed prior to mitogen activation. Not only did the tumour supernatant inhibit 3H-thymidine incorporation by mitogen-activated lymphocytes, but it also inhibited production of the lymphokine leucocyte inhibitory factor (LIF). In experiments designed to establish whether a component of the tumour harvest fluid was activating a population of suppressor cells, normal mononuclear (MN) cells were treated with the PLC/PRF/5 or embryonic fibroblast supernatant for 48 h, after which they were washed and added to normal mitogen-activated lymphocyte cultures. Only cells pretreated with the PLC/PRF/5 supernatant suppressed mitogenesis. The cell responsible for the suppressor effect was a T cell, which after a further 24 h in culture liberated a suppressor factor responsible for inhibiting lymphocyte function. Although the nature of the factor/s in the PLC/PRF/5 supernatant responsible for activation of the T-suppressor cell population is unknown, it is suggested that this mechanism may be important in protecting the tumour from the immune response.     

Characteristics of T-Suppressors concentrated by elution from an allogeneic cell monolayer

Quantitative enrichment of the immune lymphocyte fraction with H-2 antigen-specified suppressor cells was obtained by their elution from the relevant allogeneic monolayer of target cells. This was accompanied by 2-3-fold gain in the content of T lymphocytes and DNA synthetizing cells, as well as in the total (3)H-thymidine incorporation resulting from an increase in the percentage of medium and large lymphocytes in the population and also in the proportion of DNA synthesizing small and medium lymphocytes. Complete abolition of the suppressor effect by treatment with anti-Thy-1 and anti-T antibodies rather than with anti-B serum, and the resistance of this effect to carrageenan and carbonyl iron indicate the T cell nature of the eluted suppressor cells.     

Factors contributing to impairment of the mixed lymphocyte reaction in leprosy.

Whereas the mixed lymphocyte reaction was essentially normal in inactive lepromatous leprosy and tuberculoid leprosy, it was severely impaired in active lepromatous leprosy. The impairment was found to be contributed by certain unknown factors in their plasma and subnormal reactivity of their T lymphocytes. The plasma derived from active lepromatous leprosy patients depressed the reaction of normal cells and normal plasma enhanced the reaction of active lepromatous lymphocytes. The cellular factor was studied by using a one-way reaction in which one of the two lymphocyte preparations was inactivated with mitomycin C. The impairment of blastogenesis of active lepromatous lymphocytes was partially reversed by substituting inactivated normal cells for similarly treated leprous cells, and conversely the response of normal allogeneic lymphocytes was depressed by substituting inactivated leprous lymphocytes as the stimulator cells.     

The immunoregulatory role of bone marrow. II. Characterization of a suppressor cell inhibiting the in vitro antibody response.

The addition of bone marrow cells (BMC) to spleen cell cultures suppressed the antibody response in a dose-dependent manner. This suppression required viable cells. Treatment of BMC with anti-thymocyte serum did not affect the suppressive activity and BMC, but not spleen cells, from nude mice inhibited the antibody response to the same degree as marrow from normal littermates. BMC which had been depleted of macrophages with antimacrophage serum or carbonyl iron showed increased suppressor activity. Furthermore, fractionation of BMC by velocity sedimentation and resetting revealed the suppressor cell to be a medium-to-large Fc receptor-positive lymphocyte. Absence of detectable B or T cell markers on the suppressor cell indicates this cell to be an Fc-positive null lymphocyte, possibly a precursor cell, which inhibits the response of mature lymphocytes     

Murine neonatal spleen contains natural T and non-T suppressor cells capable of inhibiting adult alloreactive and newborn autoreactive T-cell proliferation

The spleen of neonatal mice is known to be a rich source of cells capable of suppressing a variety of immune functions of adult lymphocytes in vitro. From such observations has emerged the concept that the gradual development in ability to express immune functions after birth is due in part to the parallel normal physiological decay of naturally occurring regulatory suppressor cells. There is, however, some confusion in the literature as to the exact nature of the newborn of the newborn inhibitory cell type(s). In contrast to most previous reports which detect only a single type of neonatal suppressor cell, usually a T cell, we show here that newborn spleen harbors both T and non-T inhibitory cells. Both types of suppressor cells could be shown to suppress the proliferative response of adult spleen to alloantigens as well as newborn T cells reacting against self-Ia antigen in the autologous mixed lymphocyte reaction (AMLR). Newborn suppressor T cells were characterized as being non-adherent to Ig-anti-Ig affinity columns, soybean agglutinin receptor negative (SBA-), and susceptible to lysis by anti-T-cell specific antiserum plus complement. Non-T suppressor cells were identified as non-phagocytic, SBA receptor positive (SBA+), and resistant to cytotoxic treatment with anti-T-cell antibodies and complement. The apparent controversy surrounding previous reports as to the T versus non-T nature of newborn suppressor cells can be reconciled by the present observation that both types of inhibitory cells coexist in the spleen. Furthermore, the demonstration that newborn suppressor cells can effectively regulate T-cell proliferative activity mediated by other newborn cells provides more direct support for the contention that such inhibitory cells play a physiological role in controlling immune responsiveness during early ontogeny.     

Suppression of lymphocyte alloreactivity by early gestational human decidua. I. Characterization of suppressor cells and suppressor molecules

We examined the immunosuppressor role of the first trimester human decidua on lymphocyte alloreactivity in vitro in order to identify (1) the major cell classes in the decidua mediating the suppressor effect; (2) the stages in the lymphocyte alloreactive responses susceptible to the suppressor effects of the decidua; and (3) the precise nature of the suppressor molecules. Irradiated (2800 R), Ficoll-Paque-separated nucleated cells of the collagenase-dispersed early gestational (6.5-9.5 weeks menstrual age) decidua containing 70-94% typical decidual cells (identified on the basis of distinctive morphology and numerous cytoplasmic or surface markers) or their plastic-nonadherent fractions further enriched for decidual cells (approximately 96% pure) caused a strong dose-dependent suppression of the one way mixed lymphocyte reaction (MLR, i.e., proliferative response measured on Days 3, 4, or 5), when added at the onset of the mixed lymphocyte cultures (MLC). As few as 10(3) decidual cells caused a detectable inhibition of the MLR exhibited by 10(5)-1.5 X 10(5) responder lymphocytes. A smaller degree of suppression was noted with the plastic-adherent fractions of the early decidua (which retained all macrophages and granulocytes, but still included many decidual cells) or unfractionated cells of later gestational (10-13 weeks) decidua containing a higher incidence of leukocytes, granulocytes, and macrophages in particular, or the plastic-adherent fraction thereof, enriched for macrophages. Thus, decidual cells seem to represent an important suppressor cell class in the early gestational human decidua; however, suppression by decidual leukocytes, macrophages in particular, was also evident. The suppressor effect was unrelated to the major histocompatibility phenotype of the responder or the stimulator cells. It was not caused by cell crowding, since an equivalent number of irradiated K562 erythroleukemia cells had little effect on the MLR. The effect was exerted during both the initiation and the progression of the MLR. A delay in the addition of regulator cells progressively minimized the effect on the Day 4 MLR, but did not abolish it completely even when added as late as on Day 3. The major class of mediator molecules was identified as prostaglandins, primarily PGE2, on the basis of the following results: (1) the presence of indomethacin (10(-5) M) or varying dilutions of an anti-PGE2 antibody abrogated this suppression substantially or completely. (2) Addition of pure PGE2 (3 X 10(-7) to 1.1 X 10(-5) M), but not PGF2 alpha, reproduced a dose-dependent suppressor effect.(ABSTRACT TRUNCATED AT 400 WORDS)     

Suppressor factor from tumor-allosensitized spleen cells--its effect on in vitro proliferation of tumor cells and in vivo skin allograft survival

C57BL/6 mice are sensitized ip with allogeneic P-815 mastocytoma cells. Fifteen days later the spleen cells of the sensitized mice are used in the production of suppressor factor or treated with mitomycin and used as suppressor cells. Sensitized spleen cells incubated with the specific alloantigen (DBA/2 m-treated spleen cells) release suppressor factor (SF)² which inhibits cell proliferation in mixed lymphocyte culture (MLC) as well as the in vitro generation of cytotoxic cells (CML). SF is most effective when added eary during MLC. SF also inhibits mitogen responsiveness of normal spleen cells. In addition to inhibiting lymphocyte function in vitro, suppressor cells as well as SF inhibit the in vitro proliferation of tumor cells. This inhibition is specific for the tumor to which the suppressor cells are induced. The inhibition of tumor cell proliferation is not due to the presence of cytotoxic cells in the spleen of the tumor-allosensitized mice. Suppressor cells from neonatal mice do not inhibit the in vitro proliferation of tumor cells. SF injected iv into C57BL/6 mice decreases the mixed lymphocyte reactivity of the host spleen cells and decreases the ability of the host to reject skin allografts. We interpret these data to suggest that tumor-allosensitized spleen cells, and the SF they produce, not only affect lymphocyte function but also inhibit tumor cell proliferation. This dual effect of suppressor cells could be an important part of the immune surveillance against tumors.     

Comparative study of conditions for demonstrating memory T-cell receptors and other T-lymphocyte subpopulations immune to antigens of the H-2 complex

The immunological memory T cells assayed by the cytotoxic T lymphocyte (CTL) generation in the secondary mixed lymphocyte culture bear H-2 antigen-binding receptors as shown by the technique of the specific lymphocyte absorption on target cell (TC) monolayers of different H-2 origin. Memory T cells and specific suppressor T cells are demonstrated to be capable of adhering to native and fixed TC in a similar fashion, whereas CTL absorption appears to be two-fold reduced when the TC monolayer is fixed. The primary CTL precursors differ from memory T cells by a poor adherence to native TC which is not demonstrable at all when TC are fixed. The findings evidence the differences in receptor affinity (or structure) among the primary and secondary CTL precursors and the CTL themselves.     

Regulation of the activation of cytotoxic T lymphocytes by prostaglandins and antigens

The present study demonstrated the presence of two suppressor circuits in the regulation of the in vitro activation and differentiation of cytotoxic T lymphocytes (CTL); these suppressor circuits were mediated by prostaglandins (PG) and antigens, respectively. In intrinsic suppression, the activation of cytotoxic precursor cells was regulated by the host endogenous production of PG. When the regulation by PG was removed (e.g., using indomethacin), lymphokine-induced cytotoxic cells (LICC) were generated. This activation process can be induced in the absence of antigen or mitogen stimulation. In extrinsic suppression, the presence of antigen induced the generation of antigen-nonspecific suppressor T cells to restrict the expansion of antigen-unrelated cytotoxic lymphocyte clones, whereas the antigen-specific CTL clones were spared. The generation of antigen-specific helper cells further augmented the antigen-specific CTL response. These findings indicate that both antigen specific suppressor T cells and antigen nonspecific suppressor T cells are involved in the regulation of CTL responses. These suppressor circuits not only play an active role in monitoring the activation of CTL clones, they also help to determine the specificity and magnitude of the CTL response.     

Immunological unresponsiveness of self-recognizing B lymphocytes to the PBA property of a soluble T cell factor.

Spleen cells cultured in the presence of Con A became activated to polyclonal anti-body synthesis. This effect was found to be mediated by a polyclonal factor released by the activated T cells. Such a factor, contrary to any other polyclonal B cell activator, so far tested, failed to induce resting B cells to synthetize antibodies capable of lysing autologous albumin coupled SRBC. The Unresponsiveness was found to be specific since Con A activated spleen cells were capable of lysing heterologous albumin and HGG coupled SRBC. Moreover, neither active suppressor cells, nor soluble suppressor factors seem to be involved in the lack of response to autologous antigens. It is concluded that the clones of B cells carrying the polyclonal receptor for the T factor and Ig receptors for self are eliminated or functionally inactivated. The implication of these findings for the mechanism of self-non-self discrimination are discussed in relation to the mechanism of immunocyte triggering.     

Characteristics of the splenic suppressor cell--target cell interaction in experimental African trypanosomiasis

We have examined further the relationship between immunosuppression and suppressor cell activity in experimental African trypanosomiasis. In the present study we describe the nature of the interaction between splenic suppressor macrophages from Trypanosoma rhodesiense-infected C57BL/6 mice and target effector cells in the primary in vitro PFC response to SRBC. Suppressor cell potential was expressed only when cell-cell contact of a noncytolytic nature was established between infected spleen cells and normal splenic responder cells. Isolation of suppressor cells from responder cells by a cell-impermeable membrane completely abrogated suppression. Similarly, supernatant fluids from infected spleen cell cultures could not passively transfer suppression. Suppressor cells did not act via prostaglandin synthesis in that indomethacin failed to restore responsiveness to infected spleen cells or to passively suppressed normal cultures. Inhibition of DNA synthesis by irradiation of mitomycin C treatment did not block suppressor cell function, but suppressor cell effects were inhibited by exposure of infected spleen cells to silica particles or to heat treatment. We conclude that suppressor cell effects in experimental African trypanosomiasis are consistent with a suppressor macrophage acting via a noncytolytic cell-cell interaction with responder target cells.     

Altered immune response patterns in murine syngeneic pregnancy: presence of natural null suppressor cells in maternal spleen identifiable by monoclonal antibodies

Expression of certain autologous lymphocyte-activating antigenic determinants on the developing embryo is known to provide a stimulus for maternal anti-fetal autoproliferative responses. If left unregulated these responses could exert negative influences on the reproductive process by converting to autoaggressive forms of immune reactivity. In normal circumstances, immunological reactions of this nature are therefore likely to be under the control of pregnancy-associated immunoregulatory elements found within the maternal/fetal environment. In the present investigation we describe a naturally occurring splenic inhibitory cell type devoid of conventional T, B, and macrophage surface markers associated with syngeneic murine pregnancy that is capable of exerting potent immunosuppressive effects on an in vitro expression of fetal/newborn T cell autoreactivity, namely the autologous mixed lymphocyte reaction (AMLR). Maternal spleen cells inhibitory for AMLR were found to be highly resistant to cytotoxic pretreatment with a panel of conventional antisera directed against T cell-specific antigenic determinants. The non-T nature of the natural splenic suppressor cell was further indicated by experiments showing that purified spleen T cells had no inhibitory activity. Pregnancy spleen cell populations that were effectively depleted of macrophages retained full ability to inhibit AMLR. Maternal suppressor activity could be localized to the spleen cell population bearing receptors for the B cell-specific lectin, soybean agglutinin (SBA). A panel of monoclonal antibodies prepared against enriched populations of suppressor cells was screened and selected for specific reactivity using an ELISA against glutaraldehyde-fixed SBA+ spleen cell subpopulations from pregnant versus virgin animals. Several of the monoclonals developed against suppressor-enriched spleen cell populations from isopregnant as well as allopregnant animals were effective in reducing or eliminating suppressor cell activity following cytotoxic pretreatment in the presence of complement. The novel set of anti-suppressor monoclonal antibodies described here should prove useful in furthering the isolation and characterization of pregnancy-associated suppressor cells and in determining their relationship to natural suppressor cell populations described in other systems.     

Cellular immunity in Q fever: modulation of responsiveness by a suppressor T cell-monocyte circuit

Human infection with the rickettsia Coxiella burnetii presents as an acute flulike primary Q fever, as a subacute granulomatous hepatitis, or, rarely, as chronic endocarditis. We have previously described lymphocyte unresponsiveness to Coxiella antigen in patients with Q fever endocarditis. This unresponsiveness was antigen specific and was mediated in part by adherent suppressor cells. In this report we show that the adherent suppressor cells work via prostaglandin E2 (PGE2)4 production. Addition of the cyclooxygenase inhibitor indomethacin to cultures of PBMC from patients with endocarditis or chronic laboratory exposure resulted in consistent increases in Coxiella-specific lymphocyte proliferation. The degree of increase in proliferation induced by indomethacin correlated strongly with the amount of PGE2 produced in a 4-hr culture stimulated by Coxiella antigen, but it also correlated with the sensitivity to inhibition of mitogenesis by PGE2. The suppressor mechanism was antigen nonspecific, because induction of suppression in vitro by Coxiella antigen also suppressed Candida-induced proliferation when both antigens were present in the same culture. Addition of indomethacin to these antigen cocultures totally reversed the Coxiella-induced suppression, confirming the evidence above that the nonspecific effector mechanism of suppression was prostaglandin (PG)-mediated. Elicitation of suppression, however, was antigen specific and involved a T cell-monocyte suppressor circuit. Supernatants from Coxiella-stimulated immune T cells and from the suppressor subset (OKT8+-enriched) of those T cells, but not unstimulated immune cells, induced augmented PGE2 production by unrelated nonimmune PBMC. We conclude that the lymphocyte unresponsiveness characterizing patients with Q fever endocarditis is modulated in part by an antigen-specific T suppressor cell which secretes a lymphokine to stimulate PGE2 production by adherent cells.     

Mechanism of histamine-induced inhibition of lymphocyte response to mitogens in mice

Histamine induced, in mice, an inhibition of lymphocyte response to PHA and LPS, at molar concentrations ranging from 10^?3 to 10^?9M. This inhibition occurs as a specific interaction between histamine and T lymphocytes bearing H2-type receptors for this hormone (H + cells) and Ly 2 membrane antigens. Two features of the suppressive activity of this T-cell subpopulation were observed: (i) when histamine is added at the beginning of the culture period with PHA or LPS, it activates the suppressor activity of H + cells which act on the lymphocyte population responding to PHA and LPS; (ii) preincubation of spleen lymphocytes with histamine for 24 hr induces suppressor cells which inhibit the response to PHA, but not to LPS, of syngeneic lymphocytes in a coculture system, and which are radiosensitive. The role of PHA as a second stimulus of histamine-induced suppressor cells, and the relation between these cells and PHA or Con A-induced suppressor cells, are discussed.     

The selective growth of murine newborn-derived suppressor cells and their probable mode of action

Naturally occurring suppressor cells residing in the spleens of newborn mice of less than 5 days old are known to suppress various lymphocyte activities. A population of these suppressor cells can be maintained and expanded in the supernatants derived from Wehi-3 cells. These suppressor cells, designated as Wehi-3-expanded neonatal splenocytes (WENS), can suppress mixed lymphocyte reactions (MLR) and T and B cell mitogen responses without any genetic restrictions. The WENS bear the Ly-5, J11d, and class I molecules. WENS suppression is not mediated through an interleukin 1 or interleukin 2 absorptive mechanism. To achieve maximum suppression of MLR, WENS must be present for at least 24 hr. WENS inhibited the proliferation of Wehi-164 cells but not other tumor cells. The inhibition of Wehi-164 growth was due to the action of natural cytotoxic cells, because WENS lysed Wehi-164 cells but not the natural killer target cell YAC-1. Maximum lysis of Wehi-164 by WENS required 18 to 24 hr. Five WENS cell lines were cultured for more than 6 mo; three of the cell lines lost their capacity to lyse Wehi-164 targets (natural cytotoxicity) and simultaneously lost their natural suppressor activity. The two WENS lines that retained natural cytotoxicity also retained natural suppressor activity. Thus, natural suppressor cells may manifest their suppression through a natural cytotoxicity mechanism.