Contribution of a p75 interleukin 2 binding peptide to a high-affinity interleukin 2 receptor complex

A C-terminal tyrosine-based motif in the bile salt export pump directs clathrin-dependent endocytosis

The liver-specific bile salt export pump (BSEP) is crucial for bile acid-dependent bile flow at the apical membrane. BSEP, a member of the family of structurally related adenosine triphosphate (ATP)-binding cassette (ABC) proteins, is composed of 12 transmembrane segments (TMS) and two large cytoplasmic nucleotide-binding domains (NBDs). The regulation of trafficking of BSEP to and from the cell surface is not well understood, but is believed to play an important role in cholestatic liver diseases such as primary familial intrahepatic cholestasis type 2 (PFIC2). To address this issue, BSEP endocytosis was studied by immunofluorescence and a cell surface enzyme-linked immunosorbent assay (ELISA) endocytosis reporter system using a chimera of the interleukin-2 receptor α (previously referred to as Tac) and the C-terminal tail of BSEP (TacCterm). An autonomous endocytosis motif in the carboxyl cytoplasmic terminus of BSEP was identified. We define this endocytic motif by site-directed mutagenesis as a canonical tyrosine-based motif (1310) YYKLV(1314) (YxxØ). When expressed in HEK293T cells, TacCterm is constitutively internalized via a dynamin- and clathrin-dependent pathway. Mutation of the Y(1310) Y(1311) amino acids in TacCterm and in full-length human BSEP blocks the internalization. Subsequent sequence analysis reveals this motif to be highly conserved between the closely related ABCB subfamily members that mediate ATP-dependent transport of broad substrate specificity.

CONCLUSION

Our results indicate that constitutive internalization of BSEP is clathrin-mediated and dependent on the tyrosine-based endocytic motif at the C-terminal end of BSEP.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Modified DT-IL2 fusion toxin targeting uniquely IL2Ralpha expressing leukemia cell lines - Construction and characterization

Immunotoxins are fusion proteins of modified toxin conjugated to tumor cell selective ligand. Denileukin diftitox approved by FDA for treatment of CTCL is diphtheria toxin (DT)/IL2 fusion protein targeted to high affinity IL2R. Here, we have attempted to target the more uniquely expressed low affinity IL2R (IL2Ralpha). We designed four immunotoxins, SPRSV1 was designed to code for a single protein of DT (390) and IL2 (133) without any extra amino acids at the junction. SPRSV2 was designed to selectively target low affinity IL2R, it codes for DT (390) and IL2 (69). We also constructed SPRSV3 encoding for only DT (390) without any ligand, as negative control and SPRSV4 was designed similar to commercial equivalent denileukin diftitox, it codes for DT (387) and IL2 (133) with His at the junction. The cytotoxic activities of these immunotoxins were tested in various cell lines, cell lines lacking IL2R expression and healthy MNC were used as controls. The activities of SPRSV1 and SPRSV2 were comparable to that of SPRSV4. SPRSV2 exhibited potent cytotoxicity effectively targeted to alpha subunit of IL2R on various leukemia cell lines. Our studies also showed a negative correlation between CD25 expression and percentage cell viability after treatment with immunotoxins.

The structure of IL2 bound to the three chains of the IL2 receptor and how signaling occurs

The interleukin-2 molecule and receptor were the first of the interleukins to be discovered and characterized at the molecular level. Now after 20 years of effort, two groups have succeeded in determining the structure of IL2 bound to the external domains of the three receptor chains in a quaternary complex. What do we know now that we did not know before this structural information was available, and how do these new data help us to develop new therapies?

Use of an antibody against the soluble interleukin 2 receptor alpha subunit can modulate the stability and biodistribution of interleukin-2

The authors have previously reported that the soluble serum form of the alpha subunit of the IL-2 receptor (sIL-2Ralpha), whose natural half-life is approximately 40 min, survived much longer in the circulation when bound by a specific antibody. In the present study, the authors evaluated the extent to which sIL-2Ralpha protected IL-2 in freshly collected serum using biochemical analyses, and a functional CTLL-2 assay. In particular, sIL-2Ralpha protected IL-2 from forming complexes with alpha(2)-macroglobulin and from inactivation in vitro. In addition, the authors demonstrated that the anti-IL-2Ralpha monoclonal antibody 7G7/B6, which does not inhibit the binding of IL-2 to its binding site on sIL-2Ralpha, protected IL-2 from degradation and inactivation in vivo in the presence of sIL-2Ralpha. Both(125)I-labelled and unlabelled IL-2 were injected into mice preinjected with humanized anti-Tac (hTac) or 7G7/B6 and sIL-2Ralpha, or sIL-2Ralpha alone. Using size-exclusion HPLC, ELISA, and CTLL-2 cell proliferation assays, we observed that the presence of 7G7/B6 led to formation of complexes with sIL-2Ralpha and increased the serum levels of IL-2 more than 3- to 40-fold those of groups receiving IL-2 alone, sIL-2Ralpha, or hTac. Taken as a whole, these results suggest that the complex of 7G7/B6 and sIL-2Ralpha not only prolongs the survival of IL-2 in vivo, but also maintains the bioactivity of IL-2. The use of antibodies against endogenous soluble receptors could increase the in vivo survival of cytokines, protect their bioactivity and thereby facilitate their clinical use in the treatment of various malignancies and AIDS.

Protein kinase C in IL-2 signal transduction

Interleukin-2 (IL-2), secreted principally by activated helper T-cells, plays a pivotal role in the generation and regulation of the immune response. The various biologic functions of IL-2 have been the focus of intensive study over the years and have been well worked out. By contrast, an understanding of the intracellular signals coupled to the IL-2 receptor and responsible for mediating IL-2 effects in T-cells is far less developed, and the role that protein kinase C (PKC) may play in the various cellular responses to IL-2 receptor activation is unclear. In this article we will discuss IL-2, its receptors, and IL-2 signal transduction in relation to the physiological roles PKC activation may play in IL-2-mediated activation of T-cells and other hematopoietic cells.

Mechanisms in interleukin-2 protection against glucocorticoid-induced apoptosis: regulation of AP-1 and glucocorticoid receptor transcriptional activities

We have used the gibbon ape leukemia cell line MLA-144 and its corticoid-sensitive subclone MLA-E7T to analyze the mechanisms whereby interleukin-2 (IL-2) can protect T cells against dexamethasone-induced apoptosis. MLA cells are characterized by the constitutive expression of intermediate affinity receptors for IL-2, together with IL-4 receptors. MLA-144 cells secrete IL-2 and are insensitive to dexamethasone, whereas MLA-E7T cells do not constitutively produce significant amounts of IL-2 and undergo apoptotic cell death in the presence of dexamethasone. Exogenous IL-2 was shown to protect MLA-E7T cells against the apoptotic effect of dexamethasone and to increase both the DNA binding and transactivating functions of activator protein-1 (AP-1). The functional relationship between AP-1 and glucocorticoid receptors transcriptional activities was further investigated using transient expression of reporter gene constructs whose transcriptions are regulated by promoters containing TPA-responsive elements or glucocorticoid-responsive elements. The data reported here demonstrate that in MLA-144 cells, IL-2 or PMA stimulation antagonizes the glucocorticoid receptor, whereas in MLA-E7T, synergistic effects are observed between dexamethasone and IL-2 or PMA for transactivation of MMTV-CAT. Taken together with the finding that IL-2 but not PMA protects MLA-E7T from dexamethasone-induced apoptosis, our results indicate that IL-2 does not induce such a protection by repressing the transcriptional activity of the glucocorticoid receptor.

Exercise and the immune system. Natural killer cells, interleukins and related responses

The main methods for the evaluation of natural killer (NK, CD16+ CD56+) cells, interleukins and related subsets of lymphocytes are briefly described. Moderate endurance exercise causes either no change or an increase in lymphocyte and NK cell counts, total T cell (CD3+) count, the ratio of T helper (CD3+ CD4+) to T suppressor (CD3+ CD8+) cells, mitogen-induced lymphocyte proliferation, serum immunoglobulin levels and in vitro immunoglobulin production. Plasma levels of interleukin-1 increase but interleukin-2 (IL-2) levels generally fall. Decreases in plasma IL-2 levels reflect increased expression of beta (CD122) receptors for IL-2, and thus increased binding of IL-2, changes in cell distribution or a lesser production of IL-2 by peripheral blood mononuclear cells. Exercise to exhaustion induces adverse changes in many of these indices of immune function, particularly if the physical activity is accompanied by psychological or environmental stress. Moderate, appropriately graded training reduces the adverse reactions initially associated with a given bout of exhausting exercise, and cross-sectional comparisons show an increased expression of beta IL-2 receptors on the peripheral blood mononuclear cells of trained individuals. However, excessive training, nutrient deficiency and/or muscle damage has adverse consequences for both the production of interleukins and the response of the immune system to these cytokines.

Giardiavirus-resistant Giardia lamblia lacks a virus receptor on the cell membrane surface

Giardia lamblia virus (GLV) is a small nonenveloped double-stranded RNA virus that infects specifically the parasitic protozoan G. lamblia. Among the many collected strains of G. lamblia, a few turn out to be highly resistant to the virus infection. Two of these strains, Ac and JH, were subjected to electroporation with the RNA from GLV-infected G. lamblia WB strain. Subsequent studies indicated the presence of GLV double-stranded RNA and GLV protein in the electroporated and propagated cells. Virus particles, released by the transfected cells into the culture medium, were capable of infecting the virus-sensitive G. lamblia WB strain. When the WB cells were incubated with GLV at 4 degrees C and treated with the bifunctional cross-linking reagent disuccinimidyl suberate, little GLV protein was detectable inside the cells by immunofluorescent staining. However, patches of fluorescent granules were found on the membrane surface of the cells, suggesting cross-linking of the viruses with a certain membrane component(s). Similar treatment of the resistant strains Ac and JH showed no fluorescence either inside or outside of the cells. Two other closely related parasitic protozoa, Tritrichomonas foetus and Trichomonas vaginalis, cannot be infected by GLV via either viral infection or RNA transfection. The [35S]cysteine-labeled protein profiles in Triton X-114 extracts of G. lamblia WB, Ac, and JH were compared. The profile of the WB strain differs clearly from that of Ac and JH. It remains to be seen, however, whether this difference is related at all to the different susceptibilities to GLV infection.

Expression of interleukin-2R alpha and interleukin-2R beta in Hodgkin's disease

Hodgkin and Reed-Sternberg cells, the putative malignant cells of Hodgkin's disease (HD), carry regularly the CD25 antigen that forms one chain of the interleukin-2 (IL-2) receptor (IL-2R alpha). To analyze the putative role of IL-2R expression in Hodgkin's disease, we have investigated the expression of both IL-2R alpha and IL-2R beta chains in HD-derived cell lines and in primary specimens from patients with HD. Expression of IL-2R alpha and IL-2R beta was detected in all HD-derived cell lines. In addition, soluble IL-2R alpha molecules were demonstrated in the supernatants of three of these cultured cell lines. In primary tissues, IL-2R alpha and IL-2R beta were seen in some but not all cases. Staining was detected in Hodgkin and Reed-Sternberg and in lymphoid cells. There was a remarkable difference in the pattern of expression, in that IL-2R alpha- but not IL-2R beta-positive cells from HD patients were clustered in frozen sections. We conclude from these data that IL-2R expression might be involved in the biology of HD.

Gangliosides inhibit T-lymphocyte proliferation by preventing the interaction of interleukin-2 with its cell surface receptors

Gangliosides are known to be actively shed from tumour cell membranes, and increased levels of circulating gangliosides may cause tumour-induced T-lymphocyte immunosuppression in vivo by interfering with the actions of interleukin-2 (IL-2). We have investigated the effect of gangliosides on the interaction of IL-2 with its cell surface receptors (IL-2R). Gangliosides inhibited IL-2-stimulated proliferation in synchronized populations of the IL-2-dependent cell lines CTLL-2 and HT-2. The immunosuppressive effect was most effective when gangliosides were added during the first 4 hr after IL-2-stimulation, indicating that they acted early in the IL-2 signalling pathway. Inhibition could be completely overcome by exogenous IL-2, suggesting that gangliosides inhibited growth solely by competing with IL-2R for available IL-2. In support of this proposal, gangliosides induced a concomitant dose-dependent decrease in binding of [125I]IL-2 to high-, medium- and low-affinity IL-2R. Ganglioside-treated cells recovered their high-affinity [125I]IL-2 binding after washing. The glycolipids also prevented chemical cross-linking of [125I]IL-2 to the p55/p75 complex, as well as to both IL-2R alpha (p55) and IL-2R beta (p75) independently. A thin-layer chromatography overlay technique was used to demonstrate that IL-2 binds directly to gangliosides, but not to simple neutral glycolipids or acidic lipids. Taken together, these findings indicate that gangliosides directly block the interaction of IL-2 with IL-2R, and may explain, in part, the immunosuppressive activities of gangliosides in vivo.

Mutation of Asp20 of human interleukin-2 reveals a dual role of the p55 alpha chain of the interleukin-2 receptor

Mutation of Asp20 in human interleukin-2 (IL-2) to Lys is known to result in an IL-2 molecule with unchanged binding to the p55 subunit of the IL-2 receptor, but with greatly decreased affinity for the p75 subunit (Collins, L., Tsien, W.-H., Seals, C. et al. Proc. Natl. Acad. Sci USA 1988. 85: 7709). Here we demonstrate that Lys20 IL-2 competed with a reduced (10-fold) affinity for high-affinity IL-2 receptors on two murine cell lines HT2 and CTLL. In parallel with this difference in receptor interaction, Lys20 IL-2 stimulated half-maximal HT2 cell proliferation at a 10-fold higher concentration than wild-type IL-2. However, half-maximal stimulation of CTLL cells required a 100-fold higher concentration of Lys20 IL-2. A similar 100-fold reduction in bioactivity of Lys20 IL-2 was observed for primary, activated, human or murine lymphocytes. Anti-p55 antibodies increased the concentration of Lys20 IL-2 required to stimulate HT2 cells to that required for CTLL cells. These data suggest that CTLL cells, while able to bind Lys20 IL-2 with high affinity, are lacking a p55-dependent function necessary for optimal stimulation. Therefore, p55 has a dual role, being important both for high-affinity IL-2 binding and for optimal cell triggering.

Pseudo-high affinity interleukin 2 (IL-2) receptor lacks the third component that is essential for functional IL-2 binding and signaling

Functional studies of the interleukin 2 receptor (IL-2R) of two (ED515-D and Kit225) IL-2-dependent and three (ED515-I, 3T3-alpha beta 11, and Hut102) IL-2-independent cell lines were done. All of these cell lines appeared to express high as well as low affinity IL-2R. However, ED515-I and 3T3-alpha beta 11, which expressed the IL-2R beta chain, did not bind IL-2 at all when IL-2 binding to their IL-2R alpha chain was blocked with anti-Tac monoclonal antibody, whereas the intermediate affinity binding in ED515-D, Kit225, and Hut102 cells remained. We tentatively called the high affinity IL-2R of the former cells pseudo-high affinity IL-2R. The dissociation constant of pseudo-high affinity IL-2R was higher than that of ordinary high affinity IL-2R. Internalization of cell-bound 125I-IL-2 into ED515-I and 3T3-alpha beta 11 cells was less efficient than that into ED515-D cells. The addition of IL-2 neither promoted cell growth nor upregulated IL-2R alpha chain expression in ED515-I and 3T3-alpha beta 11 cells. Furthermore, tyrosine phosphorylation of the cellular proteins (p120, p98, p96, p54, and p38) was induced or enhanced in response to the addition of IL-2 in ED515-D and Kit225 cells, but not in the cell lines expressing pseudo-high affinity IL-2R. Finally, 125I-IL-2 crosslinking followed by SDS-PAGE analysis showed an 80-kD band corresponding to p65 + IL-2, in addition to bands corresponding to IL-2R alpha and beta chain + IL-2 in cells bearing ordinary high affinity IL-2R but not in cells with pseudo-high affinity IL-2R. Taken together, we consider that another protein whose molecular mass is approximately 65 kD is functionally important in IL-2 binding and subsequent signal transduction and may be the third component of IL-2R.

CD4 and CD8 regulate interleukin 2 responses of T cells

To characterize the T-cell surface molecules involved in regulation of T-cell interleukin 2 (IL-2) responses, we established several monoclonal antibodies (mAbs) that inhibit IL-2 responses of freshly isolated CD8+ T cells and the IL-2-dependent cell line CTLL-2. Here we show that two inhibitory mAbs are directed against Lyt-2 (CD8 alpha). In fact, all anti-Lyt-2 mAbs tested were able to inhibit the IL-2 response of the Lyt-2- and L3T4-deficient cell line HT-2 after transfection with a Lyt-2 cDNA clone. Similarly, anti-L3T4 mAbs inhibited the IL-2 response of CD4-transfected HT-2 cells. These inhibitory effects of anti-CD4 and anti-CD8 mAbs occur on normal T lymphocytes, since they also were observed with CD4+ and CD8+ T-cell blasts, and are specific for IL-2 responses, since IL-4 responses of CD4- and CD8-transfected HT-2 cells were not affected by the anti-CD4 and anti-CD8 mAbs. The inhibitory effects of anti-CD4 or anti-CD8 mAbs could not be explained by interference with IL-2 binding and depended on CD4 and CD8 crosslinking, because F(ab')2 or Fab plus crosslinking second antibody, but not Fab alone, were effective. A mutant Lyt-2 molecule lacking the cytoplasmic region that mediates p56lck binding could not mediate the inhibitory effect upon crosslinking. These results suggest that CD4 and CD8 mediate negative regulation of T-cell IL-2 responses via cytoplasmically associated p56lck.

Leukotriene B4 augments interleukin-2 receptor-beta (IL-2R beta) expression and IL-2R beta-mediated cytotoxic response in human peripheral blood lymphocytes

Interleukin-2 can induce cytolytic activity in human peripheral blood lymphocytes and this activation is mediated by the beta chain of the interleukin-2 receptor-beta (IL-2R beta). Leukotriene B4 (LTB4) is a potent lipid inflammatory mediator which induces IL-2 and interferon-gamma (IFN-gamma) production from T cells. We examined the ability of LTB4 to modulate IL-2-induced cytolytic activity. Peripheral blood lymphocytes which had been preincubated for 24 hr in the presence of LTB4 responded to 100-fold lower concentrations of IL-2 with an augmentation of natural killer (NK) cell cytotoxic activity. Furthermore, incubation of lymphocytes with graded concentrations of LTB4 augmented the proportion of IL-2R beta+ cells. Peak activity was seen at 10 nM LTB4 and was comparable to that of PHA. By two-colour cytofluorometry, the increased expression of IL-2R beta was found predominantly on CD56+ cells and to a lesser extent on CD8+ cells, while CD4+ cells were unaffected. These observations were correlated at the messenger RNA (mRNA) level with increased IL-2R beta mRNA accumulation following stimulation of purified CD56+ and CD8+ cells with LTB4. CD56-, CD8- cells did not respond to LTB4 by increased IL-2R beta mRNA accumulation. Our data indicate, for the first time, that LTB4 can markedly increase the sensitivity of non-major histocompatibility complex (MHC)-restricted cytotoxic lymphocytes to IL-2, in terms of IL-2-dependent cytotoxic responses, and that this sensitivity is associated with augmented IL-2R beta gene message and cell surface expression.

Selection of BoCD25 monoclonal antibodies by screening mouse L cells transfected with the bovine p55-interleukin-2 (IL-2) receptor gene

The bovine interleukin-2 receptor-alpha (IL-2R alpha) gene has been isolated and a rabbit antiserum against a fusion protein of the gene has been produced. However, the antiserum does not inhibit IL-2-dependent proliferation. Since a panel of monoclonal antibodies (mAb) to bovine activation antigens was available, we transfected the gene into mouse L fibroblasts, selected stable transfectants with the rabbit antiserum, and screened for antibodies that bound the transfected cells but not the untransfected cells. Three mAb were selected and all three precipitated a molecule of M(r) 55,000 (under reducing conditions) from activated cells, as expected from homology with mouse and human IL-2R alpha (CD25, Tac). One of the three mAb was a strong inhibitor of IL-2-dependent proliferation of bovine lymphocytes. Thus, the availability of transfected cells allowed us to establish quickly and unequivocally the antigenic specificity of a number of antibodies.

The multichain interleukin-2 receptor: a target for immunotherapy

Activation of resting T-lymphocytes induces synthesis of interleukin-2 (IL-2) and expression of cell surface receptors for this lymphokine. In contrast to resting normal T-cells that do not express high-affinity IL-2 receptors (IL-2R), abnormal T-cells of patients with leukemia-lymphoma, certain autoimmune disorders, and individuals rejecting allografts express this receptor. Exploiting this difference in receptor expression, antibodies to the IL-2 receptor have been used effectively to treat patients with leukemia and lymphoma. One approach is to use monoclonal antibodies produced in mice; the disadvantage is that they are highly immunogenic. In an effort to reduce the immunogenicity of the mouse monoclonal antibodies, monoclonal-antibody-mediated therapy has been revolutionized by generating humanized antibodies produced by genetic engineering in which the molecule is human except for the antigen-combining regions, which are retained from the mouse. Further, to increase its cytotoxic effectiveness, the monoclonal antibody has been armed with toxins or radionuclides. Alternatively, IL-2 itself has been linked to a toxin to kill IL-2 receptor-bearing cells. Thus, IL-2 receptor-directed therapy provides a new method for treating certain neoplastic diseases and autoimmune disorders and for preventing allograft rejection.

Regulation of IL-2 beta receptor expression and beta-chain mRNA by human thymocytes

The high affinity form of the human IL-2 receptor (IL-2R) has two known components, the IL-2R alpha (p55) and the IL-2R beta chain (p75). We have previously shown that recombinant IL-2 (rIL-2) could induce the expression of the alpha-chain (p55) on T cells and thymocytes, and increase this expression following suboptimal activation with concanavalin A (Con A) in combination with IL-2. An increase in the accumulation of IL-2R alpha-specific mRNA induced by rIL-2 in T cells and thymocytes had also been documented. We report here that the expression of IL-2R beta on the cell surface can be demonstrated on human thymocytes by the binding of Mik beta1, a MoAb directed against an epitope of the beta-chain. The IL-2R beta chain is constitutively expressed on freshly isolated thymocytes; this expression can be increased in thymocytes activated with Con A in combination with IL-2 or tetradecanoylphorbol 13-acetate (TPA). Blocking the formation of high affinity receptors with a MoAb directed against the alpha-chain of the receptor results in an increase in the display of IL-2R beta as evidenced by binding of MoAb Mik beta1. The accumulation of IL-2R-beta-specific mRNA is observed in freshly isolated thymocytes and it is increased in thymocytes cultured with rIL-2 alone, with Con A, and further enhanced by the addition of rIL-2 in combination with Con A or with TPA. Cyclosporine (CsA), which inhibits the accumulation of lymphokine-specific mRNA of thymocytes, does not inhibit the induction of the accumulation of IL-2R beta-specific mRNA. This is analogous to its effect on the expression of the alpha-chain (p55), and the accumulation of alpha-chain-specific mRNA.

Immunotoxins and cytokine toxin fusion proteins

Paul Ehrlich first suggested the simple and elegant concept of creating specific cell toxins or "magic bullets" through the fusion of cell-specific antibodies and toxins. In practice it has proven difficult to create safe and effective "magic bullets." In the past several years, several immunotoxins have been applied to clinical testing. These immunotoxins have been created by the biochemical coupling of cell- or lineage-specific monoclonal antibodies to plant toxins or fragments thereof. These immunotoxins have been used to treat bone marrow transplant recipients and patients with autoimmune disorders. In recent years, another strategy has also been pursued to create hybrid toxins. Rather than use antibodies as the targeting moiety, cytokines have been used to target a select population of cells bearing a high copy number of receptors for the specific cytokine. Rather than biochemically couple a cytokine to the toxin, the cytokine and toxin are fused by a peptide bond established via genetic engineering. A prototype IL-2 diphtheria toxin-related fusion protein is now being tested in the clinic for treatment of hematopoietic malignancies and autoimmune disorders.

Lymphokine receptor-directed therapy: a model for immune intervention in leukemia, autoimmunity, and immunodeficiency

Activation of resting T cells induces the synthesis of interleukin-2 (IL-2) and expression of its high-affinity receptor that involves both a 55-kDa IL-2 binding peptide identified by the anti-Tac monoclonal antibody and a 75-kDa IL-2 binding peptide associated in a receptor complex. The IL-2 receptor is proving to be an extraordinarily important therapeutic target since it is expressed by the abnormal T cells in patients with certain lymphoid malignancies or autoimmune disorders and in individuals rejecting allografts whereas it is not expressed by normal resting cells. IL-2 receptor directed monoclonal antibodies, genetically engineered humanized antibodies, and antibodies armed with toxins or radionuclides represent novel therapeutic agents for these clinical conditions.

Interleukin-2 lengthens extrajunctional acetylcholine receptor channel open time in mammalian muscle cells

The effect of interleukin-2 (rIL-2) on nicotinic acetylcholine receptors (nAChR) was examined on cultured muscle fibres isolated from the flexor digitorum brevis muscle (FDB) of the rat and on aneural mouse cultured C2 myotubes. Intracellular measurement of the sensitivity to iontophoretically applied ACh demonstrated that the sensitivity of the extrajunctional nAChRs in cultured fibres showed a transient increase after application of rIL-2 (2,000-3,000 units/ml). Cell-attached patch-clamp experiments on the same fibres proved that rIL-2 (2,000 units/ml) induces a significant increase in the mean open time of the extrajunctional nAChR channel. The other channel parameters were not significantly modified. The same applied also to aneural mouse patch-clamped C2 myotubes exposed to rIL-2 (2,000 units/ml). In freshly dissociated fibres no effects on nAChR channels were observed following rIL-2 application. 125I-rIL-2 binding experiments on either 7-day cultured or freshly dissociated adult muscle fibres showed that a specific binding with a Kd of 2.07 +/- 0.4 nM develops in cultured fibres but fails to occur immediately after dissociation. It is concluded that rIL-2 modulates the duration of extrajunctional nAChR channels in both myotubes and adult muscle cells, and that this effect is probably due to the activation of a second messenger system.

Suppressor cells and T cell synergy in the primary MLR: interleukin-2 is required for the maintenance of rat CD8+ suppressor cells

The requirements for maintenance of allospecific CD8+ Ts cells generated in the rat primary MLR were investigated. Allospecific CD8+ Ts cells rapidly lose their activity over 72 hr in secondary culture with media alone, whereas low concentrations of rIL-2 (less than 1 U/ml) are able to maintain potent CD8+ Ts cell activity. This Ts cell activity is maintained at rIL-2 concentrations which do not result in significant cell proliferation. Therefore, cell proliferation per se is not a requirement to maintain Ts cell activity, although the CD8+ Ts cells can proliferate to rIL2 in a concentration-dependent manner. An anti-IL-2 receptor monoclonal antibody significantly inhibited the maintenance of Ts cell activity. Two-color flow cytometric analysis demonstrated that Ts cells cultured in rIL-2 maintain upregulation of their high-affinity IL-2 receptor. Although allospecific Ts cells maintained in secondary culture with rIL-2 for 48 hr maintained antigen specificity, there is also the induction of an antigen-nonspecific population. After 168 hr in secondary culture the Ts cells have lost allospecificity, although Ts activity can be maintained with rIL2 in continuous culture for up to 4 weeks.

Functional p75 interleukin-2 receptor expression on the fresh blast cells in childhood acute lymphoblastic leukemia with natural killer cell properties

Neoplastic cells of childhood acute lymphoblastic leukemia (ALL) with natural killer (NK) cell properties were studied for the expression of p75 interleukin-2 receptors (IL-2R) and the receptor functions. Freshly prepared blast cells from a patient with ALL had NK cell properties: (1) the phenotype such as CD56+, CD2+, E-rosette+, CD3-, and CD19-; and (2) the presence of spontaneous cytotoxicity against NK-sensitive K562 target cells. Although p55 Tac antigen was not detectable, there was the expression of p75 IL-2R on the freshly prepared blast cells: 70% of the cells reacted with Mik-beta 1 monoclonal antibody against p75 IL-2R as determined by flow cytometry. Two-color flow cytometry revealed that the blast cells expressed both p75 IL-2R and NKH-1. NK activity of the blast cells was augmented by their treatment with 1,000 U/ml recombinant IL-2 (rIL-2): the cytotoxicity level as percentage lysis increased to 38.7% from 22.0% when the normal lymphocyte value increased to 62.1% from 46.2%. Although the blast cells possessed no apparent level of proliferative capacity, the addition of 1,000 U/ml rIL-2 yielded a 2.7-fold increase in their thymidine uptake. These results demonstrate the expression of functional p75 IL-2R on the patient's blast cells with NK cell properties.

Growth of certain myeloid leukemic cells can be stimulated by interleukin-2

The effect of recombinant interleukin-2 (IL-2) on the proliferation of T-cell depleted leukemic blasts was evaluated in 23 patients with acute myelogenous leukemia (AML). For this purpose, the effect of IL-2 on cell growth, [3H]-thymidine incorporation into the blasts and the expression of IL-2 receptors on cell surface using T-cell depleted blasts were studied. The results showed that IL-2 stimulated [3H]-thymidine incorporation significantly in blasts of 8 out of 23 cases of AML. An IL-2 induced increase in cell number was directly demonstrated in seven out of eight IL-2 responsive patients studied. IL-2 stimulated the proliferation of blasts in monocytic lineage (M4 and M5), but not all M4/M5 leukemics responded to rIL-2. Stimulation of the growth of leukemic cells was not correlated with the expression of Tac antigen on the cell surface, but it was significantly correlated with the expression of IL-2 receptor (IL-2R) beta chain on the cell surface. These results indicate that IL-2 is an active growth factor in certain myeloid leukemia cells, especially of monocytic type.

Regulation of expression of interleukin 2 receptors upon triggering of the TCR-CD3 complex on human T lymphocytes

Monoclonal antibodies reactive with CD3 molecular complex can induce antigen-associated early biochemical changes in purified, monocyte-depleted resting T cell populations and synergize with interleukin 2 (IL2) in the induction of T-cell proliferation. Interleukin 2 mediates its effects via two receptor molecules of apparent 70-75 kD (p70/p75) and 50-55 kD (p50/55) molecular weights respectively. Using radioactive IL2 and bi-functional cross-linking chemistry, we are able to determine that incubation of purified, monocyte-depleted, resting T cells with anti-CD3 (OKT3) antibody induces a significant and selective increase in the expression of p70/75 IL2 receptors from their low constitutively expressed levels. This event occurs in the complete absence of cellular proliferation. Although IL2 also causes the upregulation of p70/75 molecules, it is the synergistic action of both antibody and lymphokine which is needed for the induction of significant amounts of the p50/55 IL2 receptors and the concomitant cellular proliferation. The effect of anti-CD3 on p70/75 receptor expression is specific, as determined by the inability of a non-related (anti-CD2) monoclonal antibody of the same subclass (IgG2a) to induce a similar effect. The Ca++ ionophore ionomycin, under conditions that cause significant intracellular Ca++ influx cannot by itself mediate upregulation of IL2 receptor expression in T cells. Since anti-CD3 itself can induce intracellular Ca++ increase in purified T cells, the finding with the ionophore suggests that the intracellular Ca++ accumulation alone cannot account for the IL2 receptor molecular events described here. Addition of PMA induces both p70/75 and p50/55 IL2 receptor upregulation, as well as IL2-dependent proliferation. Although resting T cells constitutively express p70/75 receptors, under our experimental conditions and with the concentration of IL2 used, these molecules cannot transduce the lymphokine signal efficiently. Thus, in a physiologic context, a simple interpretation of our data could be that upon interaction of the TCR/CD3 with antigen a selective upregulation of p70/75 IL2 receptors renders them competent of not only binding the lymphokine, but also transducing its signal. The latter event leads to the expression of p50/55 receptors and subsequent proliferation. Whether an increase in the numbers of these receptors is all that is needed or additional events are necessary merits further investigation.

Isolation of high-affinity murine interleukin 2 receptors as detergent-resistant membrane complexes

Murine T cells and T cell lines bearing high- and low-affinity receptors for interleukin (IL) 2 were chemically cross-linked to radiolabeled IL 2 and subjected to differential detergent extractions to evaluate the extent of IL 2 receptor association with the nonionic detergent-resistant framework of the plasma membrane. Low-affinity receptors were readily solubilized by nonionic detergent extraction of whole cross-linked cells, while solubilization of high-affinity receptors required a stronger ionic detergent suggesting their association with a membrane structure that is resistant to nonionic detergents. To achieve physical separation of low- and high-affinity receptors, cells cross-linked to 125I-labeled IL 2 were centrifuged through a sucrose barrier containing Triton X-100. Alternatively, Triton X-114 extracts of plasma membrane fractions were partitioned into aqueous and detergent phases. By either approach, high-affinity receptors differed from low-affinity ones by their increased density and consisted of detergent-resistant complexes containing p55-p75 heterodimers. The low-affinity receptors, on the contrary, were of low density and consisted exclusively of detergent-soluble p55 subunits. High density and resistance to nonionic detergent extraction of high-affinity IL 2 receptors suggest their integration into lateral microdomains of the detergent-resistant framework of the plasma membrane.

Analysis of a soluble mutant des-methionine interleukin-2 receptor alpha chain (Tac protein) produced by transfected mammalian cells

By using recombinant DNA technology the cytoplasmic and trans-membrane domain of the human interleukin-2 receptor alpha chain (IL-2R alpha, Tac) and of a mutant protein lacking methionine-residues 18, 25, 44, 88, 92, 126, 149, 167, 205, and 209 (des-Met IL-2R alpha) encoded by a chemically and enzymatically synthesized gene, were deleted. This leads to secretory expression of soluble wild-type and des-Met mutant Tac protein of 42-45 kDa after transfection of BHK-21 cells. Transfectants secreted up to 1.6 micrograms soluble wild-type IL-2R alpha protein/10(6) cells in 24 h into the culture medium. LTK- cell lines, expressing a large number of wild-type and des-Met mutant low-affinity IL-2R alpha of 50-55 kDa on their surface, shed a truncated form of the Tac protein of about 40 kDa into the culture medium. In contrast to wild-type IL-2R alpha, shedding of mutant Tac protein is strongly reduced. This phenomenon might be the result of higher protein stability of the mutant receptor which may also explain the about 10 times higher surface expression of des-Met IL-2R alpha in LTK- cells. There are no significant differences in the biosynthesis and post-translational modification of mutant or wild-type Tac proteins either in transfected LTK- or BHK-21 cells as analysed by pulse/chase labeling experiments.

Murine interleukin 2 receptor beta chain: dysregulated gene expression in lymphoma line EL-4 caused by a promoter insertion

The functional, high-affinity interleukin 2 receptor (IL-2R) consists of at least two receptor components, IL-2R alpha (p55) and IL-2R beta (p70-75). The cDNA encoding the murine IL-2R beta has been isolated by using the previously cloned cDNA for human IL-2R beta as a probe. Analysis of the cDNA revealed that the murine IL-2R beta shows a marked homology with the human IL-2R beta and that it is also structurally related to other cytokine receptors such as erythropoietin receptor. The cDNA-directed murine IL-2R beta formed high-affinity IL-2R in conjunction with the endogenous IL-2R alpha in a murine pro-B-cell line and could transduce IL-2-induced growth signal. In mouse lymphoma line EL-4, the IL-2R beta gene was found to be rearranged by the insertion of the long terminal repeat sequence of an intracisternal A particle, giving rise to constitutive expression of the IL-2R beta mRNA.

Demonstration of high-affinity interleukin-2 receptors on B-chronic lymphocytic leukemia cells: functional and structural characterization

Functional and structural characteristics of interleukin 2 (IL-2) receptors on B-cell chronic lymphocytic leukemia (B-CLL) cells were analyzed by a proliferation assay, IL-2 binding assay and cross-linking study. In the 3H-thymidine incorporation assay, purified B-CLL cells from four out of sixteen cases, in which the percentage of Tac antigen (Tac Ag) positive cells in peripheral blood lymphocytes ranged from 0 to 48.8%, responded to IL-2 (100 U/ml) after both 3- and 6-day incubation. No relationship was found between the responsiveness to IL-2 and the percentage of Tac Ag positive cells. In the radiolabeled IL-2 binding assay, however, B-CLL cells from all seven cases examined, including three cases with mitogenic response to IL-2 and four cases without mitogenic response, were shown to have both high- and low-affinity receptors. The number of high- and low-affinity receptors per cell ranged from 29-186 and from 420 to 1,800, respectively. Furthermore, with the affinity cross-linking method p55 (Tac Ag) and p70/75 were found even in cases without mitogenic response in their B-CLL cells. In conclusion, the B-CLL cells so far examined possessed high-affinity IL-2 receptors consisting of p55 and p70/75; nevertheless, this was not sufficient to respond to the mitogenic signal of IL-2.

Evidence that the level of the p55 component of the interleukin (IL) 2 receptor can control IL 2 responsiveness in a murine IL 3-dependent cell

Although the role of interleukin 2 (IL 2) in mature T lymphocyte function is well documented, its effect on hematopoietic progenitor cells is less well characterized. Here we have used recombinant retroviruses to transduce and express a cDNA clone encoding the p55 component of the human IL 2 receptor (h-p55), in a murine IL 3-dependent cell line, BAF3. While the parental cells do not respond to IL 2, the h-p55-expressing cells proliferate upon treatment with recombinant IL 2 after an initial lag period. The responsiveness of individual cell clones is correlated with their level of h-p55 expression, and can be inhibited by Tac monoclonal antibody. Furthermore, growth at limiting IL 2 concentrations selects a subset of cells expressing higher h-p55 levels from a bulk population. Detailed 125I-labeled IL 2 binding analysis on the highest h-p55-expressing clone detects the presence of 200 high-affinity (KD = 25 pM) IL 2 receptors. We therefore propose that the level of h-p55 expression governs the formation of high-affinity receptors, and hence IL 2 responsiveness in BAF3 cells.

Suppression of interleukin 2-dependent human T cell growth in vitro by prostaglandin E (PGE) and their precursor fatty acids. Evidence for a PGE-independent mechanism of inhibition by the fatty acids

PGE represent oxygenation products of polyunsaturated essential fatty acids and are important regulators of cell-mediated immune responses. Because oils enriched in such fatty acids reduce inflammation and tissue injury in vivo, we examined the effects of these PGE precursors on IL-2-driven growth of human T lymphocytes. Dihomogamma linoleic acid (DGLA), AA, and their metabolites (PGE1 and PGE2, respectively) strongly inhibited short- and long-term growth of IL-2-dependent T cell cultures; EPA was much less inhibitory and its product, PGE3, failed to suppress IL-2 responses. Short-term pretreatment of the cells with DGLA or AA and removal of the fatty acids before the proliferation assay resulted in a smaller reduction in [3H]TdR incorporation. PGE and fatty acids did not alter the number of high affinity IL-2 binding sites on the T cell cultures but reduced the percentage of cells expressing CD25 and HLA class II molecules. No PGE was detected in supernatants from the fatty acid-treated cultures. Moreover, indomethacin, a cyclooxygenase inhibitor, did not reverse the antiproliferative effects of the fatty acids. Together, these findings indicate that fatty acids can inhibit IL-2-driven T cell growth via a PGE-independent mechanism and might be relevant to inflammatory diseases associated with persistent T cell activation.

Transient analysis for antiproliferative gene activity

A subset of tumor suppressor genes presumably functions by the inhibition of cellular proliferation; however, antiproliferative activity after transfection with putative suppressor genes has been difficult to demonstrate and often requires lengthy selection either in nude mice or in vitro. A rapid alternative is presented here that utilizes a gene encoding a surface marker protein to identify transfectants in a transient expression assay. In this assay the labeling index, rate of DNA synthesis, cell-cycle distribution, and surface receptor display are measured by flow cytometry. Human beta-interferon, a gene with proven antiproliferative activity, was studied using the transient analysis system. The beta-interferon gene was introduced into human tumor cells along with the marker gene encoding the 55-kDa subunit of the human interleukin 2 receptor. Within a few days of transfection, analysis of transfectants by flow cytometry revealed a decrease in the fraction of cells in G2/M and an increase in the fraction of cells in G1/G0 and S phases. The distortion of the cell cycle was accompanied by as much as a 69% reduction in the rate of DNA synthesis and, in some experiments, an unanticipated increase in the labeling index. Therefore, cells accumulating in S phase were not blocked but continued to synthesize DNA although at a reduced rate. These studies on DNA synthetic rates revealed the caveat that screening for antiproliferative candidate genes with a labeling index alone could, in certain circumstances, exclude potentially interesting sequences from further consideration. Although this transient analysis system was developed for studies on cellular proliferation, it may prove suitable for phenotypic assays on other genes as well.

Anti-CD3 antibody-induced expression of both p55 and p75 chains of the high affinity interleukin-2 receptor on human T lymphocytes is inhibited by cyclosporin A

The inhibitory effect of cyclosporin (CsA) was investigated on human lymphocytes stimulated by anti-T-cell antibodies (anti-CD3 and -CD2) or mitogenic lectins. Whereas inhibition of cell proliferation (50%) occurred at 10 ng/ml CsA after cell activation via CD3 or CD2, higher CsA concentrations (300 ng/ml) were necessary to inhibit lectin-mediated cell activation (PHA, Con A). Exogenous recombinant interleukin-2 (rIL-2) partially reversed the inhibitory effect on antibody-stimulated cells only; however, at higher CsA concentrations (300 ng/ml) proliferation was again inhibited. Thus, CsA affected IL-2R expression and/or function at higher concentrations (300 ng/ml). CsA had no effect on receptor function as measured on IL-2-dependent cell growth of CTLL cells or preactivated lymphocytes. However, CsA inhibited both high and low affinity receptor expression as shown by [125I]IL-2 equilibrium binding studies on anti-CD3-stimulated cells. Cross-linking studies revealed that both p55 (TAC) and p75 chains of the IL-2R were not induced at low CsA concentrations (10 ng/ml). However, addition of rIL-2 reversed CsA inhibition of IL-2R expression. It is concluded that CsA, at least in anti-CD3-stimulated cells, inhibits IL-2R expression and cell proliferation with similar potency. Exogenous rIL-2 reverses CsA inhibition of IL-2R expression. This might be due to binding of rIL-2 to receptors which escape CsA inhibition, thereby up-regulating receptor expression which is drug resistant.

Characterization of the bovine receptor(s) for interleukin-2

The binding properties of the bovine receptors (IL-2R) for the lymphokine interleukin-2 (IL-2) have been examined using activated bovine lymphoid cells and human recombinant [125I] IL-2. The results of these binding studies indicate that the bovine IL-2R in many ways resembles the receptors for human and mouse IL-2, but with some differences. Equilibrium binding experiments revealed the presence of two classes of bovine IL-2R, one with a KD approximately 20 pM, representing approximately 400-1300 sites per cell, the other with a KD approximately 6 nM, representing approximately 20,000-50,000 sites per cell. A study of the time course of IL-2R appearance on the cell surface of activated bovine lymphocytes showed that both high- and low-affinity receptors appear rapidly following stimulation, with maximum levels of expression being reached within about 48-96 hr. Lymphoid cell proliferation, as monitored by [3H]thymidine [( 3H]TdR) incorporation, increased in parallel with the expression of high-affinity IL-2R. Measurements of the association/dissociation kinetics showed that IL-2 binds to (t1/2 approximately 10 seconds), and dissociates from (t1/2 approximately 20 seconds) the low-affinity bovine IL-2R very rapidly. In contrast, IL-2 binds rapidly to (t1/2 approximately 40 seconds), but dissociates slowly from (t1/2 approximately 8.5 hr) the high-affinity bovine IL-2R. In previous work, our laboratory has molecularly cloned the cDNA coding for the bovine IL-2 and IL-2R (p55, Tac) proteins. Comparisons of the deduced amino acid sequences of these bovine proteins with those of the homologous human and mouse proteins revealed a high degree of evolutionary conservation among these mammalian proteins. Our present IL-2/IL-2R binding studies are also consistent with such a close evolutionary relationship, but leave unresolved the molecular basis for the previously observed species specificity of the bovine IL-2/IL-2R receptor-ligand system.

Modulation of interleukin 2 high-affinity binding by lymphocyte-derived tetrahydrobiopterin: pterins as potential participants in the control of interleukin 2 receptor assembly

In this report, we have examined whether (6R)-tetrahydrobiopterin (H4biopterin) modulates the binding of interleukin 2 to high-affinity sites of the cloned mouse cytotoxic T-lymphocyte clone CTLL-2. Scatchard plot analysis of the equilibrium binding data reveals increased affinity when the cells are exposed simultaneously to interleukin 2 and to the pterin. The Kd values are statistically significantly reduced from 1.4 x 10(-11) M to 0.78 x 10(-11) M interleukin 2. The dissociation kinetics of the ligand were followed at 4 degrees C after equilibrium binding under high-affinity conditions (1.2 x 10(-10) M interleukin 2). In the presence of H4 biopterin, the dissociation rate constant (k-1) decreases from 6.2 x 10(-3) min-1 to 3.0 x 10(-3) min-1 and the half-time for dissociation increases from 106.8 min to 218.0 min. As a third approach interleukin 2 was bound to the surface of cells under high-affinity conditions by incubation in the cold and the internalization kinetics upon warming were determined. Sigmoidal-shaped kinetics of endocytosis in control cells indicate that the internalization rates increase only gradually. The presence of H4 biopterin causes an apparent immediate transition from higher-order kinetics to a linear response so that maximum internalization rates are reached immediately upon warming. The data show that lymphocyte-derived H4 biopterin in vitro at concentrations ranging from 2-8 x 10(-7) M modulates interleukin 2 high-affinity binding and that H4 biopterin potentially participates in the control of interleukin 2 receptor assembly.