The interleukin 1 receptor. Dynamics of interleukin 1 binding and internalization in T cells and fibroblasts

In this study, we have demonstrated that a murine T cell lymphoma, EL 4, and a murine fibroblast cell line, Swiss 3T3, possess a single class of high affinity interleukin 1 (IL 1) receptors that exist in a dynamic state of equilibrium that is influenced by IL 1. In the absence of IL 1, the IL 1 receptor appears to turnover with a t1/2 of approximately 11 hr. However, when cells are incubated in the presence of IL 1, the IL 1 receptor undergoes extensive ligand-induced down-regulation. IL 1 itself is internalized at 37 degrees C; 50% of the surface-bound IL 1 is internalized in 60 to 120 min. IL 1 does not undergo degradation for at least 6 hr after internalization. By using electron microscopy and autoradiography, we observed several important features of the internalization process. When cells having bound 125I-IL 1 at 4 degrees C were shifted to 37 degrees C, IL 1 moved from the cell membrane to the cytoplasm where it was found in proximity to nuclei or within lysosomes. IL 1 appeared to progressively accumulate in nuclei. Six hours after shifting cells to 37 degrees C, 30 to 35% of the internalized 125I-IL 1 is associated with the cell nucleus. The accumulation of relatively high levels of IL 1 in the nucleus raises the interesting possibility that IL 1 may not only interact in a highly specific manner with cell surface receptors, but also with potentially important nuclear receptors.     

Down-regulation of high affinity interleukin 2 receptors in a human tumor T cell line. Interleukin 2 increases the rate of surface receptor decay

We have described a human tumor T cell line, IARC 301, which constitutively expresses high affinity interleukin 2 (IL2) receptors, and showed that after binding to its receptors, IL2 is endocytosed and degraded. Here we present evidence that IL2 down-regulates its own high affinity receptors. Within 1 h, IL2 induces a 60% decrease in surface receptor expression. In order to maintain this down-regulation, IL2 concentration must be high enough for the receptors to be saturated throughout the incubation. The effect of IL2 on the kinetics of receptor internalization was investigated with two approaches. First, the initial rate of IL2 internalization was measured, and no difference could be detected whether the receptors were saturated with IL2 or only partially occupied. Second, the initial rate of surface receptor decay was followed and found to be significantly decreased in the presence of IL2. Although the half-life of IL2 receptors is very short in the absence of IL2, t 1/2 approximately 65 min, suggesting that these receptors are constantly endocytosed, it can still be reduced to t 1/2 approximately 25 min when the receptors are saturated with ligand. This suggests that occupied receptors are internalized faster than and independently from free receptors. The difference in internalization rates can explain the observed receptor down-regulation.     

Expression of interleukin 2 receptors on interleukin 3-dependent cell lines

Several mouse IL 3-dependent cell lines, IC2, LT4, FDC-P2, and PB-3C, derived from spleen or bone marrow cells were shown to express low affinity receptors for IL 2 (Kd; 0.5 to 8 X 10(-8) M). High affinity receptors for IL 2 were not detected on the IL 3-dependent cells within the experimental limitation of this study. The clones did not respond to IL 2 at all at the concentration as high as 25 micrograms/ml. The number of the receptors expressed on those clones was estimated to be 0.2 to 2 X 10(5)/cell, which is comparable with the number of those on IL 2-dependent T cell clones. Expression of IL 2 receptor was confirmed in mRNA levels for both IC2 and LT4 cells. A relatively low level expression of one (4.5 Kb) of four IL 2 receptor mRNA species was observed with those IL 3-dependent clones compared with IL 2-dependent T cells. It seems that these low affinity receptors may be expressed on IL 3-dependent cells that undergo differentiation or maturation in mast cell and some myeloid cell lineages.     

Modulation of interleukin 2 internalization and interleukin 2-dependent cell growth by antireceptor antibodies

The growth factor interleukin 2 (IL2) binds to and is internalized together with high-affinity surface receptors present on lymphoid cells. This endocytosis thus results in down-regulation of the receptors. However, it is not known if the internalization is relevant to the induction of cell growth. In the present study a rat monoclonal antibody to the P55 chain of the IL2 receptor was used to examine the role of receptor internalization in the IL2-dependent autocrine human tumor T cell line IARC 301. When given alone, this antibody did not inhibit IL2 binding, internalization, or IL2-dependent cell proliferation. However, crosslinking by anti-rat immunoglobulins, which did not affect binding of the growth factor, inhibited both IL2 internalization and cell proliferation. Besides offering a novel means for the specific inhibition of the uptake of IL2 bound to IL2 high-affinity receptors, the results are compatible with the association of this receptor-ligand uptake to the growth stimulation by IL2.     

Regulation of interleukin 2 receptor expression on a human cytotoxic T lymphocyte clone, synergism between alloantigenic stimulation and interleukin 2

A human T cell clone (termed 40.2.6) established from a rejected human kidney allograft has been studied for its ability to express membrane IL 2 receptors in response to antigen (irradiated cells from the graft's donor) and recombinant IL 2 (rec-IL 2). On antigenic stimulation, the 40.2.6 clone produced low levels (0.15 U/ml) of IL 2 (peak at 24 hr) and incorporated (3H)thymidine (peak at 48 hr). This incorporation was strongly enhanced on addition of rec-IL 2 and was inhibited by the 33B31 antibody, an anti-human IL 2 receptor monoclonal antibody (Mab). The 125I-labeled 33B31 Mab has been used to quantify the density of IL 2 receptors on 40.2.6 cells. Cells not re-exposed to antigen or rec-IL 2 had a level of 33B31-binding sites which declined rapidly (10% of starting value after 2 days). This level remained much more stable when rec-IL 2 (1 U/ml) was present in the medium (80% at day 2). Antigen induced a three- to eightfold increase in the level of 33B31-binding sites which peaked at 24 hr and then declined. When a similar antigenic stimulation was performed in the presence of rec-IL 2 (1 U/ml), the level of 33B31-binding sites peaked at a higher value (eight- to 20-fold increase at day 2), and its subsequent decline was slower. These potentiating effects of rec-IL 2 were dose-dependent and occurred at low concentrations corresponding to the saturation by rec-IL 2 of high affinity IL 2 receptor sites. Finally, high affinity IL 2 receptors, as measured by the binding of 35S-labeled rec-IL 2, were found to be similarly up-regulated by antigen and rec-IL 2. Together, our results obtained on a monoclonal human T cell population with highly purified rec-IL 2 demonstrate that rec-IL 2 and antigen act in synergy to induce the expression of both high and low affinity membrane IL 2 receptors.     

Induction of high affinity IL 2 receptors on B cells responding to anti-Ig and T cell-derived helper factors

We have reported that IL 2 is one of the essential helper factors in culture supernatants from concanavalin A-activated spleen cells or T cell hybridomas that support proliferation and immunoglobulin secretion in B cell cultures responding to anti-immunoglobulin. Here we show that cells in such cultures consume IL 2 and bear high affinity IL 2 receptors detected by binding of purified, radiolabeled IL 2. Induction of high affinity IL 2 receptors depends on addition of both anti-immunoglobulin and helper factors, and does not occur in cultures given only anti-Ig, only helper factors, concanavalin A plus helper factors, or LPS. The majority of IL 2 receptors are on cells that also bear endogenous membrane immunoglobulin, because they are found in the membrane immunoglobulin-positive fraction when cultured cells are separated by fluorescence sorting after overnight culture in the absence of anti-immunoglobulin to allow reexpression of membrane immunoglobulin.     

High-affinity interleukin 2 receptor alpha and beta chains are internalized and remain associated inside the cells after interleukin 2 endocytosis.

High-affinity interleukin 2 (IL2) receptors on human T lymphocytes are multimeric complexes containing two IL2-binding polypeptides, alpha and beta chains of 50-55 and 70-75 kDa, respectively, associated by noncovalent bonds. IL2 binds to high-affinity IL2 receptors on the surface of T lymphocytes, mediates cell growth, and is internalized. In this paper, we used a biochemical method to directly identify the receptors components internalized together with the ligand. 125I-IL2-receptor complexes were solubilized with the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate, and IL2-binding polypeptides were identified by cross-linking with disuccinimidyl suberate. Under such conditions, the noncovalent association between alpha and beta is maintained. After IL2 internalization, two complexes of about 70 and 90 kDa, IL2 crosslinked to alpha and beta, respectively, were found inside the cells. Both components were immunoprecipitated with either anti-alpha or anti-beta monoclonal antibodies. This shows that the alpha and beta chains are found in an intracellular compartment after IL2 endocytosis, and remain associated as a ternary complex with IL2.     

Effect of selenium on the expression of high affinity interleukin 2 receptors.

Selenium (Se) is an essential nutritional factor that has been shown to affect the development and expression of cell-mediated immune responses. This study shows that dietary (2 ppm for 8 weeks) or in vitro (1 x 10(-7) M) supplementation with Se results in a significant increase in the number of high affinity interleukin (IL) 2-binding sites (Kd of 10(-11) M) on the surface of concanavalin A-stimulated lymphocytes from C57BL/6J mice, whereas Se deficiency (0.02 ppm for 8 weeks) has the opposite effect. Se supplementation or deficiency apparently alters the kinetics of IL-2 receptor expression. Supplementation with Se in vivo or in vitro resulted in an earlier expression of high affinity IL-2 receptors, whereas Se deficiency resulted in a delayed expression of lower numbers of receptors. To exert its effect on IL-2 receptor expression, Se must be present or absent in the cell environment 8-24 hr after stimulation, and it most likely affects processes in the cytoplasmic and/or nuclear compartments of activated lymphocytes. Thus, in the presence of continuous immunologic stimulation, the presence or absence of Se in the cell environment can result in an accelerated or delayed clonal expansion of immunocompetent lymphocytes, respectively.     

Interleukin 1-dependent induction of both interleukin 2 secretion and interleukin 2 receptor expression by thymoma cells

The macrophage-derived product, interleukin 1 (IL 1) is thought to play an important regulatory role in the proliferation of T lymphocytes; however, its mechanism of action is unknown. We describe in this report a variant subline of EL4 thymoma cells (EL4-6.1) that displays a high degree of responsiveness to IL 1. We show that recombinant IL 1 can induce both the secretion of interleukin 2 (IL 2) and the expression of IL 2 receptors (IL 2-R) by these cells. EL4-6.1 cells do not constitutively secrete IL 2, nor do they express IL 2-R; but when cultured in the presence of recombinant IL 1, they secrete detectable amounts of IL 2 (5 to 15 U/ml). In the presence of either suboptimal levels of phorbol ester (PMA) or Ionomycin, the addition of IL 1 resulted in up to an 80-fold enhancement in the amount of IL 2 secreted. Stimulation with IL 1 alone or in combination with Ionomycin was unable to induce detectable IL 2-R expression by EL4-6.1 cells. However, in the presence of suboptimal concentrations of PMA, IL 1 induced expression of about 3000 high affinity (dissociation constant, Kd of 31 pM) and 50,000 low affinity (Kd of 2800 pM) IL 2-R. These IL 2-R were functional, based on their ability to rapidly internalize IL 2. This model system will allow a detailed analysis of the mechanisms involved in the regulation of the immune response by IL 1 and IL 2.     

Proliferation of T lymphocytes in response to interleukin 2 varies with their state of activation

The interleukin 2 (IL 2) receptor on T lymphocytes can be upregulated by a variety of stimuli including antigen, lectin, and IL 2 itself. In this report, the direct binding of radiolabeled IL 2 and a quantitative bioassay of T cell responsiveness to IL 2 were used to determine the biological significance of upregulation of the murine IL 2 receptor. Antigen and lectin, and to a lesser extent IL 2, were found to cause an increase in the expression of the high affinity form of the IL 2 receptor on both a T cell clone and concanavalin A-induced T cell blasts. A 2-day stimulation with antigen resulted in an increase in the sensitivity of the T cell clone to IL 2, whereas activation with IL 2 caused a decrease in the sensitivity of these cells to subsequent stimulation with IL 2. Comparison of the direct binding and the functional data revealed that IL 2-preactivated T cells required a greater number of occupied high affinity IL 2 receptors to achieve a given fractional response than did unactivated T cells. These observations suggest that the sensitivity with which a T cell responds to IL 2 is not determined solely by the number of high affinity IL 2 receptors it bears.     

Requirements for T cell activation by OKT3 monoclonal antibody: role of modulation of T3 molecules and interleukin 1

The requirements for activation of human peripheral blood T cells by the mitogenic monoclonal antibody OKT3 were examined. OKT3 binds to a T cell molecule, T3, associated with the T cell antigen receptor and involved in T cell activation. Activation of T cells by OKT3 requires signals provided by accessory cells and is IL 2 dependent. In the presence of accessory cells, OKT3 induces loss of T3 molecules from the cell surface, production of IL 2, expression of IL 2 receptors, and proliferation. Modulation of T3 molecules by OKT3 can be induced in the absence of accessory cells with anti-mouse IgG. These T cells, however, are not induced to express IL 2 receptors or secrete IL 2. The addition of IL 1 induces expression of IL 2 receptors, but does not induce IL 2 secretion or proliferation. Thus, peripheral blood T cells appear to have different requirements for activation compared with antigen-specific T cell clones that can be induced to produce IL 2 when stimulated with OKT3 and IL 1. Expression of IL 2 receptors does not require modulation of T3 molecules, because the binding of OKT3 to T cells in the presence of IL 1 alone is sufficient to induce IL 2 receptor expression. The results suggest that IL 2 secretion depends on cross-linking and modulation of T3 molecules, and additional, as yet undefined, accessory cell signals. The expression of IL 2 receptors and proliferation of T cells can be induced in the absence of these signals when exogenous IL 2 is provided.     

Direct activation of human resting T cells by IL 2: the role of an IL 2 receptor distinct from the Tac protein

High concentrations of interleukin 2 (IL 2) were shown to produce a delayed but pronounced proliferation of purified resting T cells in the apparent absence of other activation signals. Because these stimulatory effects of IL 2 occurred in the absence of detectable Tac+ cells, the possibility that IL 2 might be initially interacting with an IL 2 binding protein distinct from the Tac protein was studied. Chemical cross-linking studies with 125I-IL 2 revealed the presence of an IL 2 binding protein distinct from the Tac protein on the surface of these unstimulated T cells. This second IL 2 receptor has an estimated molecular size of 70,000 daltons, lacks reactivity with the anti-Tac antibody, and appears to be identical to the p70 protein recently proposed as a component of the high affinity IL 2 receptor. Scatchard analysis of IL 2 binding assays performed with the unactivated T cells revealed approximately 600 to 700 p70 sites per cell and an apparent Kd of 340 pM. These data indicate that the p70 protein present on resting T cells binds IL 2 with an intermediate affinity compared with the previously recognized high and low affinity forms of the receptor and may account for the high concentration of IL 2 needed to induce resting T cell proliferation. To investigate the early biologic consequences of IL 2 binding to the p70 protein, potential changes in the expression of genes involved in T cell activation were examined. Northern blotting revealed the rapid induction of c-myc, c-myb, and Tac mRNA after stimulation of resting T cells with a high concentration of IL 2. The anti-Tac antibody did not inhibit IL 2 induced expression of these genes, suggesting that the p70 protein rather than the Tac antigen or the high affinity IL 2 receptor complex mediated this signal. However, in contrast to these early activation events, the anti-Tac antibody significantly inhibited IL 2 induced T cell proliferation. This finding implicates the high affinity form of the IL 2 receptor in the proliferative response of the IL 2 activated T cells. Thus these data support a two step model for the induction of resting T cell proliferation by high doses of IL 2 involving the initial generation of an activation or "competence" signal through the p70 protein and a subsequent proliferation or "progression" signal through the high affinity form of the receptor.     

The murine interleukin 2 receptor. Irreversible cross-linking of radiolabeled interleukin 2 to high affinity interleukin 2 receptors reveals a noncovalently associated subunit

The murine interleukin 2 (IL-2) receptor is a 55- to 60-kDa glycoprotein (p58) that binds IL-2 at a high and low affinity. In this investigation, we have identified sublines of EL4 that vary in their capacity to express high affinity IL-2 receptors after transfection of the IL-2 receptor cDNA. These and other cell populations were used to determine whether unique membrane molecules were specifically associated with the high affinity IL-2 receptor. Irreversible chemical cross-linking of [125I]IL-2 to only high affinity IL-2 receptors resulted in detection of IL-2 cross-linked to p58 as a 70- to 75-kDa band and other complexes of 90 to 95 kDa, 115 kDa, 150 kDa, 170 to 190 kDa, and 245 kDa. Antibodies specific for p58 resulted in precipitation of each of these complexes. However, disruption of noncovalent interactions prior to immunoprecipitation resulted in an inability to detect the material at 90 to 95 kDa. Therefore, we conclude that this complex most likely represented IL-2 cross-linked to a 75- to 80-kDa subunit that was noncovalently associated with p58. The other complexes greater than 150 kDa may represent these subunits cross-linked to each other. The detection of all the cross-linked complexes larger than 75 kDa appeared to be directly related to formation of high affinity IL-2 receptors because IL-2 was cross-linked only to p58 for three cell lines that exclusively expressed low affinity IL-2 receptors. Thus, high affinity murine IL-2 receptors are comprised of at least one alpha (p58)- and beta (p75)-subunit. Our data also raise the possibility of a more complex subunit structure.     

Agonist-independent internalization and activity of a C-terminally truncated somatostatin receptor subtype 2 (delta349)

The rat somatostatin receptor subtype 2 (SSTR2) is rapidly internalized and phosphorylated in the presence of somatostatin 14 (SST14). Several C-terminal deletion constructs of SSTR2 have been investigated for their ability to undergo agonist-dependent internalization by using biochemical ligand binding assays and confocal microscopic analysis. Whereas mutant receptors lacking either 10 (delta359), 30 (delta339), or 44 (delta325) amino acid residues at the C terminus required SST14 for internalization, a construct lacking the last 20 amino acids (delta349) was detected mostly intracellularly and independently of the presence of the agonist. When internalization was blocked by sucrose, the delta349 receptor remained at the cell surface, strongly indicating that this mutant is internalized in an agonist-independent fashion. An increased affinity for agonists as measured in membrane binding assays and a reduced level of forskolin-stimulated cyclic AMP accumulation in human embryonic kidney cells expressing delta349 are properties that are characteristic of agonist-independent receptor activity. Delta349 is not phosphorylated detectably in the absence of agonist, demonstrating that phosphorylation per se is not a prerequisite for internalization of SSTR2. This observation is in line with data obtained for the delta325 mutant, which was internalized in an agonist-dependent manner, but not phosphorylated in either the presence or absence of SST14. We conclude that truncation of the SSTR2 C terminus at position 349 leads to agonist-independent, constitutive activity and internalization.     

Dissection of the role of macrophages in triggering T lymphocytes for interleukin 2 production by monoclonal antibody OKT3

Unfractionated human peripheral blood mononuclear cells produce a small amount of interleukin 2 (IL 2) by stimulation with a monoclonal anti-T3 antibody (OKT3) in vitro. The IL 2 production could be greatly augmented by the addition of a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). In the presence of TPA, the T cell enriched fraction deprived of macrophages did not produce IL 2, but the T cells pulse-incubated with OKT3 and reconstituted with macrophages efficiently produced IL 2 in subsequent culture in the presence of TPA as did T cells reconstituted with OKT3-pulse-incubated macrophages. The stimulating effect of OKT3 in the presence of macrophages was inhibited dose-dependently by the addition of immunoglobulins, particularly by mouse IgG2a which is the same isotype as that of the OKT3 antibody, showing that it inhibits by blocking the binding of OKT3 to Fc receptors on macrophages. The same extent of IL 2 production was induced in T cells when paraformaldehyde-fixed macrophages were substituted for intact macrophages. Remarkable IL 2 production was also induced by OKT3 when latex beads coated with rabbit anti-mouse IgG2a antibody and TPA were added to the culture. It was confirmed that the production induced by these stimulations was due to an increase of IL 2 mRNA. These results show that effective signals for IL 2 production are generated by efficient crosslinking of T3 molecules which results from multi-interaction of T3 molecules on the T cell membrane and anti-T3 antibody molecules on macrophage membrane or on the surface of the latex particle.     

Immunologic effects of interleukin 2 in primary immunodeficiency diseases

Five children with primary deficiencies of T cell function were studied to assess the effects of highly purified exogenous Interleukin 2 (IL 2) on their in vitro T cell responses. The lymphocytes from one child with Nezelof's T cell deficiency demonstrated absence of endogenous IL 2 production and improved proliferative responses to mitogen or alloantigen in the presence of exogenous IL 2. Moreover, during in vitro mixed lymphocyte culture in the presence of exogenous IL 2, his lymphocytes were able to develop into cytotoxic effector cells. A second child with Nezelof's syndrome demonstrated a different type of defect. The lymphocytes from this child had less impairment of endogenous IL 2 production. Although IL 2 increased the proliferation of his cells in response to PHA, similar augmentation was not seen after stimulation with OKT3 or alloantigen. In cell-mediated cytotoxicity assays, after mixed lymphocyte culture, natural killer-like activity was strongly boosted in the cultures that contained IL 2, but T cell-mediated cytotoxicity was not. The lymphocytes from three patients with severe combined immunodeficiency did not show improved proliferative responses in the presence of IL 2. Thus, only one of the five patients demonstrated the combination of defective endogenous IL 2 production, but preservation of the ability to respond appropriately to exogenous IL 2. This child may therefore have suffered from a T cell defect pathophysiologically similar to that seen in nude or aged mice.     

A chimera of interleukin 2 and a binding variant of aerolysin is selectively toxic to cells displaying the interleukin 2 receptor

Aerolysin is a bacterial toxin that binds to glycosylphosphatidylinositol-anchored proteins (GPI-AP) on mammalian cells and oligomerizes, inserting into the target membranes and forming channels that cause cell death. We have made a variant of aerolysin, R336A, that has greatly reduced the ability to bind to GPI-AP, and as a result it is only very weakly active. Fusion of interleukin 2 (IL2) to the N terminus of R336A-aerolysin results in a hybrid that has little or no activity against cells that do not have an IL2 receptor because it cannot bind to the GPI-AP on the cells. Strikingly, the presence of the IL2 moiety allows this hybrid to bind to cells displaying high affinity IL2 receptors. Once bound, the hybrid molecules form insertion-competent oligomers. Cell death occurs at picomolar concentrations of the hybrid, whereas the same cells are insensitive to much higher concentrations of R336A-aerolysin lacking the IL2 domain. The targeted channel-forming hybrid protein may have important advantages as a therapeutic agent.     

High- and low-affinity interleukin 2 receptors: distinctive effects of monoclonal antibodies

We previously established several mouse hybridoma cell lines producing monoclonal antibodies against the human interleukin 2 (IL 2) receptor molecule. As they bind to both high- and low-affinity IL 2 receptors, their effects on binding of 125I-labeled IL 2 to high- and low-affinity receptors were examined by Scatchard plot analysis. Two of these monoclonal antibodies, HIEI and H-47, reduced the IL 2 binding affinity of high-affinity receptors from a Kd of 14 to 20 pM to a Kd of 110 to 140 pM, but slightly raised that of low-affinity receptors. These two antibodies scarcely affected the numbers of high- and low-affinity receptors. On the other hand, H-31 completely blocked IL 2 binding to both high- and low-affinity receptors, and H-A26 slightly reduced the affinities of both high- and low-affinity receptors, from 17 pM to 28 pM and from 28 nM to 54 nM, respectively. H-48 had little affect on IL 2 binding to high- or low-affinity receptors. By use of these monoclonal antibodies, the inhibitory effect of IL 2 on growth of an HTLV-I-immortalized T cell line was demonstrated to be transmitted from high-affinity, but not low-affinity, receptors.     

Bacterial lipopolysaccharide-induced interferon-gamma production: roles of interleukin 1 and interleukin 2

Bacterial lipopolysaccharide (LPS) induced human peripheral blood mononuclear cells (PBMC) to produce interferon-gamma (IFN-gamma). Monocytes play a mandatory accessory role in this process, because purified T lymphocytes failed to produce IFN-gamma in response to LPS and the addition of 2% monocytes to T cell cultures resulted in an optimal LPS-induced IFN-gamma production. IFN-gamma production was abolished in the presence of monoclonal antibodies specific for HLA-DR antigen. Addition of exogenous interleukin 2 (IL 2) markedly enhanced IFN-gamma secretion by PBMC induced with LPS. The addition of anti-Tac antibody specific for IL 2 receptors abrogated IFN-gamma production, suggesting that an interaction of IL 2 with IL 2 receptors was involved. By using a specific antibody binding assay, LPS was shown to amplify IL 2 receptor expression on PBMC, whereas exogenous IL 2 showed only a negligible enhancing effect on the expression of its own receptors. Interleukin 1 (IL 1), a product of LPS-stimulated monocytes, potentiated IL 2-induced IFN-gamma production in the absence of LPS. Neither IL 1 nor IL 2 alone induced IFN-gamma production in purified T lymphocyte cultures. When added together, however, substantial levels of IFN-gamma were induced. An enhanced IL 2 receptor expression on T cells was also demonstrated as a result of the combined action of IL 1 and IL 2. These results suggest that induction of IFN-gamma by LPS is due mainly to the generation of IL 1 and an enhanced expression of IL 2 receptors.