Immune suppression induced by protoscoleces of Echinococcus multilocularis in mice. Evidence for the presence of CD8dull suppressor cells in spleens of mice intraperitoneally infected with E. multilocularis.

Immunoregulatory states induced by i.p. inoculation with the metazoan parasite Echinococcus multilocularis in the murine system were investigated. Proliferative responses and IL-2 production induced by Con A in spleen cells from BALB/c mice were significantly depressed at an early stage after infection with E. multilocularis protoscoleces (PSC). Addition of plastic-adherent cells from normal syngeneic mice to the nonadherent spleen cells from infected mice did not restore the depressed Con A responsiveness. On the other hand, exogenous IL-2 reconstituted completely the proliferative responses to Con A. Flow cytometry analysis revealed that CD4- CD8+ cells with a low density of CD8 Ag (CD8dull cells) increased in spleens from infected mice 2 weeks after inoculation. Addition of the spleen cell subpopulation containing the CD8dull cells, but not that depleted of the CD8dull cells, to normal spleen cells resulted in marked suppression of the Con A responses. These findings suggest that the CD8dull cells detected in spleens of mice inoculated with E. multilocularis PSC may play a key role in the suppressive regulation of immune responses. The relevance of the immune suppression seen in the early stages of experimental infection with E. multilocularis PSC to the eventual establishment of a host-parasite relationship is discussed.     

Murine leukemic cells inhibit IL 2 production by susceptible syngeneic splenocytes

BALB/c mice fail to mount significant cell-mediated immunity against the syngeneic virally induced leukemia MCDV-12, and die approximately 10 days after tumor inoculation. Our studies in vitro demonstrated that BALB/c splenocyte and irradiated MCDV-12 cell co-culture led to reduced alloreactivity, including depressed interleukin 2 (IL 2) production. Tumor-induced immune suppression was genetically restricted, antigen nonspecific, and alleviated in part by exogenous IL 2 administration in vitro. Furthermore, mitogen stimulation of IL 2 production, not requiring self or alloantigen recognition, was unaffected by MCDV-12 exposure. These results indicate that tumor cells may escape from immunosurveillance by reducing IL 2 production in the immediate tumor vicinity, and suggest a potential role for major histocompatibility complex antigens in the immunosuppression mechanism.     

Trypanosoma lewisi: stage-specific surface antigens and exoantigens recognized by monoclonal antibodies

The stage-specific expression of surface antigens by Trypanosoma lewisi was investigated using monoclonal antibodies directed against this parasite. Hybridomas secreting monoclonal antibodies were produced by the fusion of SP2/0-Ag 14 mouse plasmacytomas with spleen cells from rats infected previously with the Taliaferro strain of T. lewisi. Additivity enzyme-linked immunosorbent assays and indirect immunofluorescent antibody tests indicated the determinant recognized by monoclonal antibody TL40.3 (IgM) was different from those recognized by monoclonal antibodies TL40.1 (IgA), TL40.2 (IgM), and TL40.6 (IgG2 alpha). Monoclonal antibody TL40.3 agglutinated trypanosomes collected 3 days after parasite inoculation while monoclonal antibodies TL40.1, TL40.2, and TL40.6 agglutinated trypanosomes collected 6 days after inoculation. Since agglutinin titers against trypanosomes from irradiated (700 rad from a 60Co source) and nonirradiated rats were similar, expression of the antigens recognized by the monoclonal antibodies appeared to be independent of the immunological state of the host and the morphology of the parasite. The reproduction of T. lewisi in in vitro trypanostatic assays was inhibited only when the monoclonal antibodies were present in concentrations greater than or equal to those needed to agglutinate the trypanosomes. Monoclonal antibodies TL40.1 and TL40.3, but not TL40.2 and TL40.6, agglutinated erythrocytes collected later in the infection from irradiated, infected rats. None of the monoclonal antibodies agglutinated erythrocytes from nonirradiated, infected rats, from irradiated, noninfected rats or from nonirradiated, noninfected rats. This suggests that immunocompetent rats may make blocking antibodies against the exoantigens recognized by monoclonal antibodies TL40.1 and TL40.3.     

Lepromin-induced suppressor cells in patients with leprosy.

The possibility of an active mechanism of immunologic suppression in leprosy was explored by assessing the in vitro lymphocyte responses of 61 leprosy patients and 30 normal individuals to the mitogen Con A in the presence or absence of Dharmendra lepromin. Lepromin-induced suppression of Con A stimulation was found in 32 of 35 lepromatous patients and 15 of 15 borderline patients, but only 2 of 15 tuberculoid patients and 2 of 30 normal controls. Cell fractionation studies indicated at least two cell populations involved in the in vitro lepromin-induced suppressor activity, adherent cells and T gamma-cells.     

Cytokines involved in the immunosuppressor period in experimental fasciolosis in rats

The aim of this study was to evaluate the kinetics of the cytokines interferon-gamma, interleukin-2, interleukin-10 and interleukin-4 produced by spleen mononuclear cells stimulated by Con A during an experimental infection in rats with Fasciola hepatica. The proliferative response to Con A of Spm cells from rats infected with F. hepatica was significantly decreased on day 7 post-infection (P<0.006) and simultaneously an increase of interferon-gamma, interleukin-10 and interleukin-4 production along with a decrease of interleukin-2 by spleen mononuclear cells were observed. Interleukin-4 and interleukin-10 were involved in ablating cellular proliferation in vitro, as the addition of neutralising antibodies to either cytokine reversed the proliferative block. The addition of exogenous recombinant interleukin-2 also restored the proliferative response by spleen mononuclear cells obtained 7 days after infection from infected rats. At the same time, we found an increase in interleukin-10 production by peritoneal cells (in close contact with the flukes) and decreased nitric oxide levels. In addition, histological studies on the liver on day 7 after infection showed the presence of parasite inside migratory tunnels in the parenchyma, and polymorphonuclear leukocytes, predominantly eosinophils, around the parasite. The transient suppression in proliferative response mediated by cytokines interleukin-4 and interleukin-10 in the spleen, and diminution of nitric oxide production in the peritoneum could be mechanisms to evade the protective immune response during the first stages of liver penetration by the parasite.     

Role of interleukin 1 and interleukin 2 in rat and mouse arthritis models

The production of interleukin 1 (IL 1) and interleukin 2 (IL 2) by macrophages and lymphocytes from three animal models commonly used for rheumatoid arthritis, viz. adjuvant-induced and type II collagen-induced rat arthritis, and MRL/1 murine arthritis was studied. Although the peritoneal macrophages from adjuvant-arthritic rats in culture produced increased amounts of prostaglandin E2 (PGE2) and lower levels of IL 1 than the control group, cells from collagen-arthritic rats released normal levels of PGE2, but increased amounts of IL 1. After activation with lipopolysaccharides, the IL 1 production by macrophages from all groups was comparable. Addition of indomethacin did not significantly change the IL 1 production in any of these groups. In the absence of any exogenous mitogen, IL 2 production by the lymphocytes of adjuvant-arthritic rats was low, but could be restored to the normal levels when phytohemagglutinin A (PHA) or concanavalin A (Con A) was added. The lymphocytes from collagen-arthritic rats were capable of producing IL 2 without the need of any T cell mitogen. The lymphocytes from MRL/1 mice seemed to lack the functionality in terms of IL 2 production. The macrophagic IL 1 production in these animals was normal. Our data suggest that the type II collagen arthritis model may closely resemble human rheumatoid arthritis in which IL 1 and IL 2 production by the mononuclear cells is significantly enhanced.     

Cold stress-induced modulation of cell immunity during acute Toxoplasma gondii infection in mice.

Infection with Toxoplasma gondii in the acute phase results in nonspecific suppression of immunologic function in mice and humans. The present study examined the effects of a physical stressor, i.e., cold stress (CS), on macrophage function (nitrite production, parasite survival) and splenic blastogenesis in the acute phase of murine T. gondii infection. In our stress paradigm, female BALB/c mice were placed in cold water (1 +/- 0.5 C), 5 min each day for 8 days. Nitrite production and parasite survival were measured in cultured peritoneal macrophages obtained from mice subjected to CS after in vivo activation with interferon-gamma/lipopolysaccharide (CS + ACT), and in vitro infection with T. gondii tachyzoites. Peritoneal macrophages from CS + ACT mice showed decreased nitrite production compared to control but activated cells (ACT). Spleen cell proliferation to in vitro stimulation with the mitogens concanavalin A (Con A) and anti-CD3, and Toxoplasma lysate antigen (TLA) was measured in splenocytes obtained from BALB/c mice during the acute phase of infection with T. gondii. Mice subjected to CS and infection (CS + INF) had maximum splenocyte proliferation on days 8 and 15 followed by a subsequent decline on day 28 postinoculation (PI). In contrast, infected mice not subjected to stress (INF) showed decreased splenocyte proliferation on days 8 and 15 followed by an increase on day 28 PI. The rate of mortality was decreased in the CS + INF compared to the INF group during acute infection. These results suggest that CS may alter the pathogenesis of T. gondii infection by modulating acute-phase responses, provoking a state of transient disequilibrium between the host and parasite.     

Effect of concanavalin A treatment on the allogeneic response of mice to challenge with P 815 mastocytoma: interleukin 2 treatment reverses concanavalin A suppression in vivo

Mice injected repeatedly with concanavalin A (Con A) prior to and following challenge with P 815 mastocytoma are suppressed in their cell-mediated cytotoxicity responses. Earlier studies showed that pretreatment of the animals with silica to affect macrophage (M phi) functions reversed the Con A suppression. In the present paper we have shown that peritoneal exudate cells (PEC) induced/activated by ip injection of Con A were able to transfer suppression to normal mice. Separation of the PEC populations into adherent and nonadherent cells abrogated their capacity to transfer suppression. It was further shown that Con A is not functioning in this in vivo system to block effector activity of cytotoxic cells on target cells, and PEC induced with Con A were not directly cytotoxic to target P 815 cells. Finally, we were able to show that the cytotoxicity response of Con A-suppressed mice could be enhanced by treatment with concentrated culture supernatants of normal mouse spleen cells, rich in interleukin 2 (IL 2) activity. Attempts to detect a recently described mouse serum inhibitor of IL 2 in normal or Con A-treated mice were unsuccessful and spleen cells from Con A-treated mice lost their capacity to generate IL 2 in vitro when cultured under appropriate conditions. Taken together, these results suggest that suppression of cell-mediated immune responses in Con A-treated mice results from interruption of the normal generation of IL 2 helper effects necessary for the activation of cytotoxic effector T cells in vivo.     

Infection of white rat peritoneal macrophages with Toxoplasma gondii, (Coccidia: Sarcocystidae) after Trypanosoma lewisi (Kinetoplastida: Trypanosomatidae) infection

Peritoneal macrophages from Wistar rats, inoculated and non-inoculated with 10(6) T. lewisi trypomastigotes, were cultured and infected with 10(6) T. gondii tachyzoites. Multiplication rates of this parasite were studied after 1, 24 and 48 h of infection but there were not significant differences between the number of parasites found inside of macrophages coming, either from T. lewisi infected or non infected rats. On the other hand, in vivo studies of Toxoplasma multiplication inside peritoneal macrophages, showed that there is an increase of parasite number in cells from T. lewisi infected rats, as compared with those macrophages from non infected rats. This effect was statistically significant and was more evident after four days of infection. Therefore, it has been demonstrated that in vivo, but not in vitro T. lewisi infections, causes an important decrease of the natural resistance to T. gondii of the white rats, which is manifested by the major invasion and multiplication of the parasite inside of peritoneal macrophages.     

Heterologous antibody responses in mice with chronic T. cruzi infection: depressed T helper function restored with supernatants containing interleukin 2

Antibody production to sheep erythrocytes (SRBC) or hapten-conjugated SRBC (TNP-SRBC) was studied in mice with chronic Trypanosoma cruzi infections. Studies in vivo demonstrated that both IgM and IgG anti-SRBC responses were suppressed during chronic infection. Secondary IgG responses were suppressed regardless of whether the primary immunization was given before or after infection. The ability of cells from infected mice to provide help for antibody production was examined in vitro. Anti-SRBC responses were restored to cultures of whole spleen cells from infected mice by the addition of interleukin 2 (IL 2)-rich supernatants, indicating that these cells were capable of antibody production when sufficient help was provided. T cells from SRBC-primed infected mice were unable to provide significant help to normal B cell/M phi cultures for in vitro anti-TNP or anti-SRBC responses. The percentages of Thy-1+, Lyt-1+, and Lyt-2+ spleen cells were not significantly different between normal and infected mice. Anti-TNP and anti-SRBC responses were restored to cultures that contained T cells from infected mice and normal B cell/M phi by the addition of IL 2-rich spleen cell supernatants. The suppression of in vitro antibody responses in mice with chronic T. cruzi infections was associated with a lack of T cell help, which was provided by exogenous spleen cell supernatant.     

Interleukin 2 deficiency in murine Leishmaniasis donovani and its relationship to depressed spleen cell responses to phytohemagglutinin

This study examined the kinetics and mechanisms of depressed spleen cell responses to phytohemagglutinin (PHA) that occur during Leishmania donovani infection of BALB/c mice. In co-culture experiments, neither spleen cells from infected animals nor parasite-infected macrophages suppressed PHA responses of normal spleen cells. In addition, parasite-mediated suppression of PHA-stimulated spleen cell proliferation could not be demonstrated. Mice with 2 wk of infection did manifest an impairment in spleen cell production of interleukin 2 (IL 2) and by 8 wk IL 2 activity in supernatants from these cells was reduced by approximately 95%. This finding was not explained by an alteration in the kinetics of IL 2 production. Furthermore, diminished IL 2 activity in supernatants of PHA-activated spleen cells from infected animals was not caused by suppressive factors in these fluids as shown by their inability to suppress IL 2 stimulation of IL 2-dependent T cells. When spleen cells from mice with 8 wk of infection were cultured with PHA and supplemented with exogenous IL 2, there was an approximately 48% increase in mitogenesis. These data indicate that abnormal PHA-induced spleen cell activation in BALB/c mice with L. donovani infection is associated with impaired production of IL 2. In addition, the observation that supplementation of spleen cells from infected mice with IL 2 resulted in partial reconstitution of the PHA response is consistent with a defect in IL 2 responsiveness.     

Trypanosoma cruzi-induced suppression of IL-2 production. I. Evidence for the presence of IL-2-producing cells

Mice infected with the protozoan parasite Trypanosoma cruzi, the causative agent of human Chagas' disease, are profoundly immunodepressed in their response to various Ag and mitogens. A key factor in this immunosuppression is the essential inability to produce the T cell growth factor IL-2. In this study we demonstrate that this failure to produce IL-2 in response to mitogen stimulation is not the result of the absence of production of soluble or membrane-bound IL-1 by macrophages. Limiting dilution analysis of the precursor frequency of IL-2 producers suggests that an adequate number of precursors for IL-2 production are present in the spleens of infected mice, but that their activity may be regulated by suppressor cells. The presence of precursor cells for IL-2 production is supported by experiments showing that the combination of calcium ionophores and PMA elicits IL-2 production by spleen cells from both normal and T. cruzi-infected mice. Although Con A can provide either of the signals necessary for IL-2 production, calcium flux or protein kinase C activation, to T cells from normal mice, Con A in combination with either calcium ionophore or phorbol ester failed to activate T cells from infected mice to produce IL-2. Preculture of spleen cells from infected mice for 48 to 72 h before addition of Con A results in near normal production of IL-2. This recovery of the capacity to produce IL-2 does not occur if parasite Ag is present during the preculture period. These results suggest that the inability of T cells from T. cruzi-infected mice to produce IL-2 in vitro in response to Con A is not due to the lack of IL-2-producing cells, but may be the result of the maturational state of the T cells or to the presence of a suppressor population.     

Separation of mitogen-induced suppressor cells of human antibody-producing cells

Human antibody-forming cells were demonstrated by a plaque in agar technique following in vitro stimulation with either pokeweed mitogen or Cowan I strain of protein A-positive Staphylococcus aureus bacteria. We evaluated the effects on this antibody formation caused by the addition of cells which had been stimulated with PH A or Con A. Both Con A and PHA cells harvested after 3 days showed strong inhibition of pokeweed-induced plaque formation. The majority of the suppression could be accounted for by a blast fraction separated on 1g sedimentation gradients from the Con A or PHA cultures. Small cells from such cultures showed inhibition of PFC when added at high ratios (1:2), but this suppressive activity diluted out much more rapidly than that of the blast cells. No helper activity was noted with either small cells or blasts. Our studies indicate a T-cell blast as the suppressive fraction in Con A- or PHA-stimulated human lymphoid cells. While this T-cell suppression applies to T-dependent responses such as antibody stimulation with pokeweed mitogen, it does not have a substantial effect on Cowan I-induced plaque-forming responses. The finding that Cowan I-induced plaques could not be inhibited by Con A or PHA blasts indicates the T independence of this response.     

Concanavalin A triggers T lymphocytes by directly interacting with their receptors for activation

Anti-HLA-DR antibodies did not inhibit concanavalin A-(Con A) induced T cell proliferation or the generation of suppressor cells capable of inhibiting immunoglobulin synthesis in autologous mononuclear cells after pokeweed mitogen stimulation. Nylon-wool purified T cells (pretreated with anti-HLA-DR antibody and C) exposed to Con A acquired responsiveness to interleukin 2 (IL 2) and were able to absorb this growth factor, whereas nonlectin-treated cells did not respond to IL 2 and could not absorb it. In the presence of interleukin 1 (IL 1), Con A stimulated the synthesis of IL 2 in purified OKT4+ lymphocytes but not OKT8+ cells. However, in the absence of IL 1, neither resting OKT4+ nor Con A-treated OKT4+ cells produced IL 2. Con A by itself did not directly stimulate macrophages to synthesize IL 1, although it could do so in the presence of OKT4+ but not OKT8+ lymphocytes. In addition, Con A induced proliferation of purified T cells provided IL 1 was supplied to the cultures. Cyclosporin A rendered Con A-treated T cells unresponsive to IL 2, made lectin-stimulated OKT4+ lymphocytes unable to respond to IL 1, and inhibited the synthesis of IL 2. Furthermore, this drug abrogated the Con A-stimulated synthesis of IL 1 by acting on OKT4+ lymphocytes and not on macrophages. Finally, cyclosporin-A suppressed the proliferative response and the generation of suppressor T cells induced by Con A. The following are concluded: 1) HLA-DR antigens do not seem to play any role in the triggering of T cells by Con A, and macrophages participate in lectin-induced activation of T cells mainly by providing IL 1. 2) Cyclosporin-A inhibits activation of T cells by interfering with the mechanism by which Con A stimulates T lymphocytes. 3) Con A triggers T lymphocytes by directly interacting with their receptors for activation.     

Elicitation of the reproduction-inhibiting antibody ablastin by serum exoantigens of Trypanosoma lewisi

Serum exoantigens of Trypanosoma lewisi were collected 5 days after infection from immunocompetent (untreated) rats and rats immunosuppressed by treatment with either hydrocortisone acetate or dexamethasone. Normal rats were then immunized with pooled, whole exoantigen-containing serum from 1 of these 3 sources plus alum as an adjuvant, and the immune sera produced were tested individually. All contained agglutinating (trypanocidal) antibodies to both antigenic variants of T. lewisi, but only about two-thirds showed precipitating activity with exoantigens in gels. More importantly, however, when these antisera were thoroughly adsorbed with living trypanosomes (from immunocompetent hosts) to remove agglutinating antibody only and then tested for ablastic activity in vitro, all showed significant (P less than 0.01) reproduction-inhibiting activity, comparable to that shown by ablastic serum collected from rats that experienced a natural infection. Antisera from control rats similarly immunized with normal rat serum were negative in all antibody tests. The exoantigens of T. lewisi are, therefore, a complex mixture of immunogens that are related to the known immune responses to the parasite and can elicit the formation of ablastic antibody with the same biological properties as that produced during a natural infection.     

In situ and in vitro characterization of the cellular immune response in erythema nodosum leprosum

We sought to evaluate cell-mediated immune responses in erythema nodosum leprosum (ENL), a reactional state occurring in lepromatous leprosy. Skin biopsies from patients with leprosy were studied with monoclonal antibodies against T lymphocyte antigenic determinants, interleukin 2 (IL 2), and IL 2 receptors (Tac) by using immunoperoxidase staining of frozen sections. Peripheral blood lymphocytes from 18 ENL patients were tested in vitro for lepromin-induced suppression of Con A stimulation. Serial studies of seven lepromatous patients who developed ENL during the course of the study showed increases in both the Leu-3a:Leu-2a ratio and the number of IL 2-positive cells. IL 2-positive cells comprised 0.3% of the cells in all of the ENL lesions studied as compared with the 0.03% found in nonreactional lepromatous lesions (P less than 0.001). Lepromin-induced suppression of the Con A response, present in nonreactional lepromatous patients, significantly decreased in patients developing the ENL reaction, but returned after recovery from ENL. These changes in tissues and peripheral blood suggest that the pathogenesis of ENL is related to cell-mediated immune processes. Despite these immunologic changes, however, ENL patients do not recover antigen-specific skin tests or eliminate Mycobacterium leprae.     

Suppression of Con A mitogen-induced proliferation of normal spleen cells by macrophages from chickens with hereditary muscular dystrophy

Spleen cells from chickens with hereditary muscular dystrophy (MD) give low blastogenic responses to the T cell mitogen concanavalin A (Con A) while exhibiting normal mitogen stimulated blastogenic responses to the T cell mitogen phytohemagglutinin (PHA). The addition of MD spleen cells to normal spleen cells caused a marked suppression of the Con A response of the normal cells while not affecting the PHA response of the normal cells. The suppressive activity by the MD spleen cells requires viable cells and is contact mediated. The suppressive activity is attributed to the presence in MD spleens of a population of suppressor cells with characteristics typical of macrophages. The suppressor cell activity was not removable by complement-mediated lysis using anti-T or anti-B sera, but it was reversible by treatment with carrageenan or carbonyl iron magnet, by passage through a Sephadex G-10 column, and by adherence to plastic petri dishes or glass beads. MD spleen cells depleted of the suppressor cell population remained unable to respond to Con A.     

Increased IL-4 production and attenuated proliferative and pro-inflammatory responses of splenocytes from wild-caught rats (Rattus norvegicus)

Wild animals, unlike their laboratory counterparts, live amidst an abundance of pathogens and parasites. The presence of such immune stimulation from the time of birth likely has a profound effect on the development and stasis of the immune system. To probe potential differences between the immune systems of wild and laboratory animals, the response to mitogen (Con A) of splenocytes from wild rats was evaluated in vitro and compared with results from lab-rat-derived splenocytes. Although the response to mitogen is ubiquitous in splenocytes from laboratory animals regardless of strain or even species, splenocytes derived from wild rats were unresponsive to mitogen as judged by upregulation of activation markers and proliferation. Further, splenocytes from wild rats produced almost 10-fold less IL-2 and TNF-alpha in response to mitogen than did splenocytes from laboratory rats. In addition, mitogen stimulation resulted in an almost 100-fold greater production of IL-4 in wild-rat-derived splenocytes than in lab-rat-derived splenocytes. Perhaps surprisingly, these differences were observed in the absence of differences between wild and laboratory animals in the ratio of CD4+/CD8+ T cells or in the relative numbers of T cells, B cells and monocytes in the splenocyte population. These observations may have substantial implications for the hygiene hypothesis and provide considerable insight into the roles played by the environment during immune system development and modulation.     

Suppression of lymphocyte proliferative responses: characterization of the suppressor and kinetics of suppression

Culturing spleen cells for 2 or more days in the absence of mitogenic stimulation results in the generation of suppressor cells that can effectively inhibit the proliferative responses of freshly prepared spleen cells to mitogen or alloantigen stimulation. Suppression does not appear to be mediated by prostaglandins nor other soluble factors produced during the preculture period. The suppressor cell is described as a plastic-adherent Thy-1.2-, IgM-, FcR+ macrophage-like cell. Significant suppression of Con A responses can be detected at suppressor to target ratios as low as 1:100. The plastic-adherent suppressor is capable of terminating Con A-induced proliferation of spleen cells whether added at the onset of the Con A response or added as late as 48 hr after mitogenic stimulation. The suppressed spleen cell population displays an absence of large blast cells and a decrease in surface density of Thy-1.2 determinants.     

The role of IL 1 in the antigen-specific activation of murine class II-restricted T lymphocyte clones

The role of IL 1 in the antigen-specific activation of class II-restricted T lymphocytes was examined by using a model system consisting of cloned WEHI 5 B lymphoma accessory cells and class II-restricted, soluble antigen- or alloantigen-reactive T cell clones. The addition of exogenous recombinant IL 1 to the T cell cultures resulted in a significant enhancement of the antigen-specific T cell proliferation response, but at best, only small increases in IL 2 release. Goat IgG anti-IL 1 antibodies were added to the T cell cultures to assess their effect on T cell activation. The IL 1 enhancement of the T cell proliferation response was inhibited by the anti-IL 1 antibodies in a dose-dependent manner. In contrast, only modest levels (10 to 25%) of proliferation inhibition were observed in T cell cultures containing either WEHI 5 or splenocyte accessory cells but no exogenous IL 1. When the anti-IL 1 antibodies were added to primary mixed lymphocyte cultures stimulated by WEHI 5 cells in the absence of exogenous IL 1, no significant inhibition of proliferation was observed. A small but statistically significant proliferation inhibition was observed when anti-IL 1 antibodies were added to mixed lymphocyte reaction cultures stimulated by splenocytes. Two-color cytofluorometric analysis of the effects of IL 1 on antigen-activated T cell clones demonstrated that under suboptimal stimulation conditions, IL 1 stimulated a small but significant increase in the number of T cells bearing IL 2 receptors. In the presence of optimal numbers of WEHI 5 accessory cells, IL 1 enhanced T cell proliferation in the absence of a detectable increase in the number of T cells bearing IL 2 receptors, the number of IL 2 receptors per T cell, or the levels of IL 2 released. Finally, exogenous IL 1 can be added as late as 18 to 24 hr after culture initiation without significantly reducing its ability to enhance the T cell proliferation response. These data indicate that IL 1 has pleiotropic effects on murine T lymphocytes and can function to enhance T cell activation at multiple points during the activation sequence.