Biological expressions of lymphocyte activation : I. Effects of phytomitogens on antibody synthesis in vitro

FOXP3+ regulatory T cells and age-related diseases

Regulatory T (Treg) cells are critical for the maintenance of immune homeostasis. Dysregulation of Treg cells has been implicated in the pathogenesis of autoimmunity and chronic inflammation, while aging is characterized by an accumulation of inflammatory markers in the peripheral blood, a phenomenon known as 'inflammaging'. The relationship between Treg cells and age-related diseases remains to be further studied. Increasing evidence revealed that Treg cells' dysfunction occurs in aged patients, suggesting that immune therapies targeting Treg cells may be a promising approach to treat diseases such as cancers and autoimmune diseases. Furthermore, drugs targeting Treg cells show encouraging results and contribute to CD8⁺ T-cell-mediated cytotoxic killing of tumor and infected cells. In general, a better understanding of Treg cell function may help us to develop new immune therapies against aging. In this review, we discuss potential therapeutic strategies to modify immune responses of relevance for aging to prevent and treat age-related diseases, as well as the challenges posed by the translation of novel immune therapies into clinical practice.

Chronic Social Stress, Affiliation, and Cellular Immune Response in Nonhuman Primates

We report the first experimental study of the effect of long-term (over 2 years) exposure to a stressor on cellular immune response Forty-three male cynomolgus monkeys were randomly assigned to stable or unstable social conditions for 26 months The proportion of time spent in affiliative behaviors was assessed by observations made twice weekly T-cell immune response (mitogen-stimulated cell proliferation) was assessed weekly for 3 weeks immediately following the 26-month manipulation The possibility that affiliative behavior represents an attempt to cope with social stress was supported by greater affiliation among animals in the unstable condition than in the stable condition Animals in the unstable condition also demonstrated relatively suppressed immune response More affiliative animals showed enhanced immune response, with the beneficial effects of affiliation occurring primarily among unstable animals The data are interpreted as consistent with the stress-buffering hypothesis, that is, affiliation protects animals from the potentially pathogenic influences of chronic social stress

Immunological effects of acute and chronic nicotine administration in rats

We previously demonstrated that acute nicotine administration decreased the response of rat blood leukocytes (PBL) to concanavalin A (ConA). We now extend those findings to a comparison between the effects of acute and prolonged nicotine exposure (ten daily injections), on PBL and splenocytes (SL). A single injection suppressed the PBL response to ConA and phytohemagglutinin (PHA); tolerance developed by ten injections. In contrast, acute nicotine did not affect SL response to ConA and reduced the PHA response only at the highest concentration. Ten nicotine injections enhanced SL responsiveness to PHA. The only change in PBL subsets was an increase in CD8+ cells following ten injections.

Effects of heptaminol AMP amidate on suppressor and helper function of murine T cells

Heptaminol AMP amidate (HAA), a newly developed nucleotide derivative, was found to restore the immunosuppression in mice due to the induction of suppressor T (Ts) cells by concanavalin A (Con A) (50 micrograms/body). HAA also inhibited Con A-mediated in vitro induction of Ts cells. On the contrary, the administration of HAA in mice primed with keyhole lympet hemocyanin (KLH) (30 micrograms/body) caused an enhanced induction of antigen specific helper T (Th) cells. Effects of HAA on Ts and Th cells were found to be dependent on their level of induction. The administration of HAA also increased the spleen cell number and augmented the plaque forming cell response to some extent in cyclophosphamide treated mice. The present results suggested that HAA-mediated immunopotentiation was possible by a combined suppressive effect on Ts cells and enhancing effect on Th cells.

Effect of oral vs. parenteral cyclophosphamide on in vitro IgA and IgG production by murine Peyer's patches and cultured jejunal fragments

The gut associated lymphoid tissue plays an important role in intestinal defenses, food allergy, oral tolerance, and certain intestinal diseases. This study describes the effect of either oral or parenteral cyclophosphamide on IgA and IgG production in the gut. Mice were treated with cyclophosphamide either IV or PO, and Peyer's patch cell cultures were established to evaluate mitogen induced production of IgA and IgG. To evaluate the effect of cyclophosphamide on the plasma cell rich lamina propria, segments of jejunum were cultured and overnight secretion of IgG and IgA were measured. We found, the secretion of IgA or IgG by jejunal fragments was not influenced by cyclophosphamide (IV or PO). Mitogen induced secretion of IgA and IgG by Peyer's patch cells was markedly decreased 24 hrs after drug administration, with significant recovery by day 7. Cell mixing experiments revealed that a single dose cyclophosphamide reduced the capacity of Peyer's patch B cells to secrete IgA or IgG when co cultured with normal T cells. This study demonstrates that a single dose cyclophosphamide can have profound effects on the gut immune system and that the drug has a similar effect when given either orally or parenterally.

Mice naturally tolerant to C5 have T cells that suppress the response to this antigen

We examined whether C5-sufficient mice which are naturally tolerant to this antigen have suppressor T cells to C5 humoral immune response. Two congenic strains of mice B10.D2 (NSN) and B10.D2 (OSN) differing only in the presence or absence of C5 were used. Irradiated (760 rds) sufficient hosts were reconstituted with a nonadherent spleen cell suspension from either sufficient or deficient mice or a mixture of both. Hemolytic C5 levels were assayed. Sufficient spleen cells appeared to prevent the drop of C5 level caused by anti-C5 antibody made by deficient spleen cells. Spleen cell suspensions from sufficient mice primed with deficient spleen cells exhibited better anti-C5 activity than normal sufficient spleen cell suspensions. This anti-C5 activity is abrogated by treatment of the NSN spleen cell suspensions obtained from NSN primed with OSN spleen cells with anti-Thy-1.2 antiserum and complement. Suppression of the humoral response to C5 failed to affect the anti-sheep red blood cell immune response. Suppressor T cells are resistant to low-dose irradiation, cortisone treatment and adult thymectomy. In contrast, they are sensitive to high doses of irradiation and both high and low doses of cyclophosphamide treatment. Thus, C5-sufficient mice, in contrast to C5-deficient mice, appear to have antigen-specific suppressor T cells which downregulate the humoral immune response to C5. In addition, we examined the relationship of these suppressor T cells to the state of tolerance in helper T cells of C5-sufficient mice. This was done in irradiated deficient mice which were repopulated with spleen cell suspensions selectively depleted of either Lyt-1+ or Lyt-2+ T cell subsets. These chimeras were challenged with murine C5 and both the primary and secondary immune response was measured by inhibition of the C5 hemolytic activity. It was found that only spleen cell suspensions of the deficient mice selectively depleted from the Lyt-2+ subset of T cells responded to the antigen both in the primary and secondary response. In contrast, either subset of T cells from the sufficient mice failed to respond. Thus, it appears that in sufficient mice helper T cells to C5 are intrinsically tolerant or physically and/or functionally deleted. In conclusion, the data suggest that both T cell compartments are unresponsive and play a role in the mechanism of tolerance to a physiologic antigen.

Induction of T-cell proliferation and enhancement of NK activity by supernatants from Con A-stimulated human peripheral blood mononuclear cells: a new lymphokine

Supernatants from human peripheral blood mononuclear cells activated by Con A contain a factor(s) that stimulates blastogenic activity of normal human peripheral blood mononuclear cells. This Con A supernatant (CAS) contains stimulatory activity for E-rosette positive lymphocytes (T cells) and requires adherent cells for stimulation of T-cell proliferation. CAS does not contain detectable amounts of IL-2 as determined by its inability to support CTLL cell growth. Nor does it contain IL-1 or interferon. Examination of functional activity of lymphocytes stimulated for 3 days by CAS revealed that NK activity is augmented. This supernate does not appear to have any direct effect on B-cell function, although it induces suppression of polyclonal PWM stimulation of immunoglobulins. Thus, CAS appears to contain a new cytokine with immunomodulating potential.

Stress and immune responses. II. Identification of stress-sensitive cells in murine spleen cells

The influences of restraint stress on the functions of T cells, B cells and adherent cells in antibody responses were investigated. Antibody response against sheep red blood cells (SRBC), a T cell-dependent antigen, in cultured splenocytes from restrained mice was reduced to about 40-50% of that from the control mice. Addition of normal T cells to these cultures, however, restored the suppressed response. Moreover, helper T cell activities were lowered in restrained mice. On the other hand, suppressor T cell activities induced by both concanavalin A (Con A) and SRBC were significantly decreased in restrained mice. However, the antibody responses to T cell-independent antigens in stressed mice were approximately 40% higher than the control response. These enhancement were also observed in T cell-depleted splenocytes. Polyclonal antibody response induced by lipopolysaccharide (LPS) was increased in stressed mice. Antigen presenting cell activities were little influenced by restraint stress. Proliferative response to Con A, but not that to LPS, was suppressed in splenocytes from restrained mice. These results suggest that both helper and suppressor activities of T cells are suppressed, but B cell activity is rather enhanced in splenocytes from restrained mice.

Effects of T mitogens on in vivo antibody production to a T-dependent antigen in lines of mice genetically selected for high or low in vitro responsiveness to PHA

The influence of genes which regulate the in vitro T-cell proliferative response to T mitogen upon in vivo antibody production to a T-dependent antigen was studied in two lines of mice genetically selected for a high or a low in vitro lymphocyte response to phytohaemagglutinin (PHA). Kinetics of agglutinin production to increasing doses of sheep erythrocytes was similar in the two lines, except for the titres of mercaptoethanol-resistant antibodies, which were slightly lower in the low-responder line. Treatment with mitogen prior to immunization modified the antibody response in the two lines differently. This finding would indicate that the genes which regulate in vitro stimulation of T cells by PHA also control in vivo activation of T-cell subsets involved in immunoresponsiveness.

A study of the mechanism of Con A-induced immunosuppression in vivo

The plaque-forming cell (PFC) response to sheep erythrocytes (SRBC) is suppressed in a dose-related manner when concanavalin A (Con A) is administered intravenously to mice prior to or after immunization with antigen. The magnitude of suppression as well as the duration of the Con A effect greatly depends on the concentration of antigen used for immunization. Although profound suppression of the anti-SRBC PFC response is observed in intact mice pretreated with Con A for 4-24 hr, spleen cells from these mice do not exhibit suppressive activity when transferred into normal recipients or when cotransferred with normal spleen cells into irradiated recipients. Moreover, the cells from Con A-treated mice respond as normal spleen cells to SRBC when transferred alone into irradiated hosts. Suppression of the anti-SRBC PFC is only observed when adoptive hosts of cells from Con A-treated mice are also injected with Con A within 48 hr (but not 72 hr) of cell transfer and immunization. This time course of responsiveness to the suppressive effects of Con A is similar to that observed in normal mice and in irradiated recipients of normal spleen cells. The immune response to SRBC is also suppressed in adoptive hosts of normal spleen cells that are pretreated with Con A 4-24 hr prior to irradiation and cell transfer. Although functionally inactive when transferred into adoptive hosts, spleen cells from mice pretreated with Con A for 4-24 hr can suppress a primary antibody response to SRBC in vitro. The suppressive activity, which cannot be detected in the spleens of mice when the interval between pretreatment and assay is longer than 24 hr, is present in a subpopulation that bears the Thy 1.2 and Lyt 2 phenotype. Taken together the results obtained in in vivo and in vitro functional assays suggest that a suppressor cell population is activated following in vivo treatment with Con A, but that the cells rapidly lose their state of activation when removed from a Con A environment. This phenomenon is in all probability responsible for the failure to demonstrate suppressive activity in the spleens of Con A-treated mice using in vivo functional assays.

The differential effect of 2-deoxyguanosine on concanavalin A-induced suppressor and cytotoxic activity

The effect of 2-deoxyguanosine (dGuo) on the generation in vitro of nonspecific suppressor cells in murine spleen cell cultures by concanavalin A (Con A) is examined. The experiments indicate that dGuo abrogates the generation of nonspecific suppressor activity by lectin stimulation of murine spleen cells. When comparisons were made between the effect of this nucleoside on the generation of suppressor and cytotoxic cells by Con A stimulation of murine spleen cells, it was found that dGuo only affected the generation of suppressor cells. The development of lectin-stimulated cytotoxicity was not affected by dGuo. In addition it was found that dGuo does not affect the NK activity of murine spleens.

Adjuvant effects of lithium chloride on human mononuclear cells in suppressor-enriched and suppressor-depleted systems

Lithium enhances several in vitro indices of immune function, including thymidine uptake by mitogen-stimulated human mononuclear cells. To further characterize the mechanism of action of lithium and to determine whether it acts by abrogating suppressor cell activity or by enhancing helper cell function, we have compared the effects of lithium on the mitogenic response of normal, suppressor-depleted and suppressor-enriched mononuclear cell preparations. In normal cultures, lithium enhanced thymidine uptake in response to concanavalin A (Con A) and phytohemagglutinin (PHA). In the suppressor-depleted cultures, thymidine uptake after Con A stimulation was significantly higher than in normal cultures, and was further enhanced by lithium. In the suppressor-enriched system, response to PHA was significantly lower than in normal cultures, and addition of lithium reversed the observed suppression. These results indicate that lithium may be enhancing thymidine uptake in response to mitogen at least in part by abrogating suppressor cell activity. The observed increase in thymidine incorporation in the suppressor-depleted cultures suggests that lithium may also have a direct stimulatory effect on helper cell activity.

OKT3 induces suppressor cells for mixed lymphocyte and PHA mitogenic responses in human peripheral lymphocytes

Human peripheral lymphocytes pretreated with the Orthoclone monoclonal anti-T cell antibody OKT3 for 48 h markedly suppressed the proliferative response of autologous lymphocytes in one-way MLC and the mitogenic response to PHA. The ability to induce suppression is specific to OKT3 since other monoclonal antibodies to human T cells (OKT1, OKT4 and OKT8) did not elicit similar responses, OKT3 is mitogenic but further proliferation of OKT3 pretreated lymphocytes was not required for the suppression of autologous lymphocytes since mitomycin-C treated cells were fully effective. Kinetic studies indicated that pretreatment of lymphocytes with OKT3 for 24 h was sufficient to induce marked inhibition of the mitogenic response of autologous lymphocytes to PHA whereas suppression in MLC was not observed until lymphocytes were pretreated for 48 h. These studies support the previous observations that OKT3 may be reacting with an important molecule on the T cell surface and that interaction of OKT3 with this molecule induces profound functional changes.

Comparative studies on superoxide anion production by polymorphonuclear leukocytes stimulated with various agents

Guinea-pig peritoneal polymorphonuclear leukocytes promoted superoxide anion (O2(-)) generation when stimulated with soluble antigen-antibody complex, concanavalin A or sodium dodecyl sulfate. The enhancement with antigen-antibody complex or concanavalin A was inhibited with diisopropyl fluorophosphate. On the other hand, the enhancement with sodium dodecyl sulfate was not affected by the inhibitor. L-1-pTosylamido-2-phenylethyl chloromethyl ketone (Tos-PheCH2Cl) and tetrahydrofuran also enhanced O2(-) generation even in the presence of diisopropyl fluorophosphate, while at low concentrations they inhibited O2(-) generation with antigen-antibody complex. These results indicate that a certain diisopropyl fluorophosphate-sensitive factor may be involved in the O2(-)-generating response of leukocytes to antigen-antibody complexes or concanavalin A, but not in that to sodium dodecyl sulfate, Tos-PheCH2Cl or tetrahydrofuran.

Effects of concanavalin A-stimulated spleen cell supernate on the redevelopment of autoimmunity in NZB/NZW mice in induced remission

Because female NZB/NZW mice develop autoimmune abnormalities similar to those encountered in human systemic lupus erythematosus (SLE), a group of female NZB/NZW mice were used to study mechanisms of autoimmunity. These mice were treated daily with an immune suppressive material, 0.5 ml of a concanavalin A-stimulated spleen cell supernate (CONS), starting at 30 weeks of age after induced remission with prednisolone. This CONS treatment effectively reduced the proteinuria and the severity of the renal lesions, but failed to reduce the serum anti-DNA antibody level. Thus, the CONS effect on the autoimmunity in the NZB/NZW mice in induced remission appears to result from a more complicated mechanism than reduction in serum anti-DNA antibody level.

Antigen-induced helper and suppressor T cells in normal and agammaglobulinemic chickens

Spleen cells from chickens injected with sheep erythrocytes (SE) intravenously 2 to 14 days prior to culture were found to give faster and higher plaque-forming cell responses upon addition of antigen on day 2 rather than on day 0 of culture. Cell mixture experiments showed that this was due to the induction of suppressor T cells upon re-exposure to SE on day 0 of culture. Spleen cells taken on days 2 or 14, but not between days 4 and 7 after priming to SE were sensitive to suppression. The suppressor cells were resistant to gamma irradiation (1000 rd) and to mitomycin C, but were apparently lost after 2 days of culture in the absence of antigen. Pokeweed mitogen addition on day 0 of culture also induced suppressor cells, both in SE immune and in normal spleen. Similar suppressor cells were induced in cultures of primed spleen cells taken from agammaglobulinemic chickens. The response to Brucella abortus in vitro was not affected by induction of suppression for the anti-SE response. Suppression could also be shown after transfer of cell mixtures to irradiated recipients. Helper cell activity for the anti-SE response could readily be shown, both in vivo and in vitro, in primed spleen cells precultured for 2 days in the absence of antigen, and was also resistant to 1000 rd gamma irradiation and to mitomycin C.

Splenic immunoglobulin-secreting cells and their regulation in autoimmune mice

We have investigated in vitro the magnitude, nature, and regulation of spontaneous and mitogen-induced Ig secretion by splenic lymphocytes from several autoimmune murine strains (NZB, NZB X W, MRL/l BXSB) and appropriate, normal mice. All autoimmune strains had increased numbers of mature splenic B lymphocytes, which secreted and/or contained Ig, compared to age-matched normal strains. In NZB and NZB X W mice, the high frequency of mature B cells was apparent early in life, whereas in MRL/l and BXSB mice it was first noted shortly before the clinical onset of disease. Spleen cells from young autoimmune mice of all four strains secreted predominantly IgM, but with aging and the appearance of disease, the cells switched to IgG secretion predominantly. In contrast, spleen cells from normal mice were predominantly IgM, but with aging and the appearance of disease, the cells switched to IgG secretion predominantly. In contrast, spleen cells from normal mice were predominantly IgM secretors throughout the animals' lives. Approximately 15% of the total Ig-secreting cells in older NZB, NZB X W, and MRL mice were committed to secretion of anti-ssDNA antibodies. In both autoimmune and normal spleen cells, the B-cell population alone contained fewer secreting cells than the total lymphocyte population, indicating that T cells were required to achieve maximal levels of plaque-forming cells. Spleen cells of NZB and NZB X W mice had a greater response to lipopolysaccharide (LPS) than other autoimmune and normal strains. Responsiveness to LPS, as measured by the frequency of induced Ig-secreting cells, was considerably diminished with age and onset of disease in all autoimmune but not in normal strains. LPS-induced Ig secretion by B cells of autoimmune and normal mice was subject to regulation by splenic T cells. No significant differences were observed between concanavalin-A (Con A) stimulated spleen cells from young and older autoimmune mice and normal control strains in effectively suppressing spontaneous and LPS-induced Ig secretion. Moreover, B cells from autoimmune mice and from normal strains were equally receptive to Con A-induced suppressor signals. T cells from young and older NZB and BXSB mice added to a standard number of B cells from syngeneic young mice provided equal help in enhancing LPS-induced Ig secretion, and this help in turn was equivalent to that provided by T cells from normal mice of the same H-2 haplotype. The exception was the MRL/l strain; T cells from older animals provided considerably more help than T cells from young MRL/l or T cells from young and older H-2-compatible normal mice.

Suppressor cells induced by purified protein derivative of tuberculin (PPD): the suppression is mediated by cells that proliferate in response to stimulation with PPD

Lymphocytes stimulated with purified protein derivative of tuberculin (PPD) were found to inhibit the PPD stimulation of fresh, autologous lymphocytes. This suppressor effect was exerted after preincubation with both high and low concentrations of PPD. Optimal suppression occurred after preincubation with PPD in concentrations of 5 micrograms/ml and higher, the same concentrations that gave optimal stimulation of DNA synthetsis in primary cultures. The suppressor effect was abolished completely by 'hot pulse' treatment and partly by treatment with colchicine during PPD preincubation, showing that the PPD-induced suppressr cells are generated by cell division. When fresh lymphocytes were incubated together with PPD-pretreated cells in cultures that were not stimulated with PPD, the PPD-stimulated lymphocytes exerted a stimulatory effect on the fresh lymphocytes. This effect was maximal for cells preincubated for 1 h with PPD, decreasing with increasing duration of preincubation with PPD. Possible explanations of this observation are discussed.

T-cell regulation of murine IgA synthesis

In studies reported here, the polyclonal activator lipopolysaccharide was used to stimulate the synthesis and secretion of IgM, IgA, and IgG in cultures of mouse lymphoid cells. The total immunoglobulin of each class which resulted was measured by specific double-antibody radioimmunoassays. The effect of Con A-activated T cells from various tissues on such immunoglobulin synthesis was then assessed. Variations in regulatory T-cell activity among the various lymphoid tissues for IgA but not for IgM or IgG was observed. In particular, Peyer's patches T cells were found to contain a high level of IgA T-cell helper activity compared to that of spleen or peripheral lymph node. The independent variation of T-cell regulatory activity for IgA as compared to that for IgM and IgG among the different tissues is most consistent with there being a separate subset of T cells specifically regulating IgA. The significance of these findings for the understanding of the secretory immune system is discussed.

Separation of mouse T cell subsets by a fluorescent activated cell sorter using fluorescence-labeled peanut agglutinin

Nylon non-adherent spleen T cells obtained from concanavalin A-injected mice were labeled with fluorescein-labeled peanut agglutinin and separated into peanut agglutinin-positive (PNA+) cells and peanut agglutinin-negative (PNA-) cells by a fluorescent activated cell sorter. PNA+ cells were found to exert marked suppressive effect on primary anti-sheep red blood cells antibody response, but PNA- cells did not affect the antibody response. From these results and from the sugar-binding specificity of PNA, suppressor T cells are supposed to possess abundant galactosyl residues exposed on the cell surface, which are not masked by sialyl residues.

Animal models of human systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a human autoimmune disease of unknown etiology. Clinical, serologic, immunologic, and pathologic findings are highly variable in different patients and at different times in the same patient. Murine and canine animal models of SLE have been found with clinicopathologic abnormalities resembling those observed in humans. Each animal model has unique characteristics; taken together they reflect the spectrum of disease in human SLE.Investigations in the animals have suggested that genetic, hormonal, immunologic, viral, and other environmental factors contribute to and modify the expression of disease. Where analogous studies are available for humans, the same factors have been found to modify disease expression in a similar fashion. Together, these studies have helped to clarify the multifactorial basis for SLE.The best characterized abnormalities are immunologic. These include excessive B cell function with the formation of large amounts of autoantibodies, and T cell abnormalities which include defects in T cell regulatory function as well as certain T cell effector functions.The animal models of SLE also serve as convenient test subjects for newer therapeutic modalities. It is hoped that further study of the animal models will provide a more rational approach to therapeutic modulation of disease in humans with SLE.

Experimental allergic encephalitis: study of cellular immunity during disease suppression

Administration in complete Freund's adjuvant of encephalitogenic protein (EP), derived from central nervous tissue to guinea pigs, regularly results in the development of experimental allergic encephalitis (EAE) which leads to the death of the animals. Administration of EP in incomplete Freund's adjuvant at an appropriate time will completely suppress the clinical development of disease. Results reported herein show that animals receiving suppressive injections of EP for 7 days show depression of lymphocyte DNA synthesis and macrophage migration inhibition, but not of skin reactivity, in response to EP immediately following the injections, and subsequently show recovery of lymphocyte reactivity but do not develop clinical manifestations of EAE. Humoral or other factors may prevent the development of disease in these animals. Guinea pigs receiving injections of EP for 14 days show profound and prolonged depression of lymphocyte reactivity to EP and macrophage migration inhibition. Possible mechanisms for these results include a diminished number or function of reactive cells or activity of a population of cells with the capacity to suppress cellular immune responses. Nonspecific suppression of reactivity to an unrelated antigen during the suppressive injections was not observed.

T cell-dependent suppression of antibody production. I. Characteristics of suppressor T cells following tolerance induction

Specific immunological tolerance was induced in adult CBA mice by a single injection of deaggregated human IgG (dHGG). Spleen cells taken 7 to 42 days later, produced consistent suppression of a DNP-HGG collaborative antibody response on adoptive transfer into heavily irradiated recipients. Noncentrifuged F(ab')2 fragments of HGG were as effective as dHGG in the production of suppressor cells. Suppression was antigen-specific since HGG-tolerant cells failed to abrogate either a DNP-keyhole limpet hemocyanin collaborative response or antibody production to the noncross-reactive antigen, horse erythrocytes. Pretreatment of the tolerant cell population with anti-Thy-1 serum and complement reversed the suppressive effect. However, purified tolerant T cells obtained by passage through nylon wool or anti-Ig columns were less effective than the original spleen cells in mediating suppression. Analysis of the cell types appearing in the column effluents indicated that the reduction in suppressive activity is best explained by retention of T cells rather than macrophages. Different T cell populations, however, were retained on the two types of columns. In the case of anti-Ig columns, these consisted of Ly-2,3+, Ia+ effector cells, whereas nylon wool columns caused depletion of Ly-1,2,3+ cells which are known to act as amplifiers of suppression. Suppression could not be explained in terms of delay in differentiation of antibody-forming cell precursors since the effect persisted for up to 15 days after transfer of tolerant cells. The demonstration of a reduction in serum anti-DNP and anti-HGG antibodies excluded the possibility of antibody production in sites other than the spleen. A role for anti-carrier antibody-antigen complexes in mediating the effector phase of suppression was rendered unlikely by the finding that the suppressive effect of tolerant cells persisted in the absence of detectable anti-HGG antibody production. Effector T cells mediating suppression in this system were shown to bear the phenotype Ia+, Ly-2,3+ as judged by the effect of pretreatment with appropriate antisera and complement. They were spleen-seeking, but were not detected in the thymus or recirculating lymphocyte pool. Adult thymectomy failed to cause a significant reduction in suppressive activity by tolerant spleen cells indicating that at least a major component of the immediate precursors is not of recent thymic origin.