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A sensitive DNA-protein crosslinking approach has been used to characterize four inducible T-cell proteins (50 kDa, 55 kDa, 75 kDa, and 85 kDa) that specifically bind to kappa B enhancer elements. Partial proteolytic mapping revealed a distinct cleavage pattern for three of these proteins. These polypeptides are sequestered as inactive precursors in the cytosol of unstimulated T cells but can be converted into active forms in vivo by phorbol ester stimulation or in vitro by detergent treatment. The induction of these proteins by phorbol ester results in a strikingly biphasic pattern of nuclear expression with the 55-kDa and 75-kDa species appearing within minutes, whereas the 50-kDa and 85-kDa species appear only several hours after cellular stimulation. These data suggest that NF-kappa B-binding activity may not correspond to a single polypeptide but rather a family of at least four inducible and differentially regulated DNA-binding proteins that are expressed with distinct kinetics in human T lymphocytes.

Studies of NF-kappa B suggest that this enhancer binding activity corresponds to a family of at least four proteins (p50, p55, p75, and p85) differentially induced with biphasic kinetics during T cell activation. While p55 and p50 are closely related to the 50 kd DNA binding subunit of NF-kappa B, p75 and p85 exhibit DNA binding properties that distinguish them from this 50 kd polypeptide and its regulatory subunits I kappa B and p65. All four members of this kappa B-specific protein family are structurally related to the v-Rel oncoprotein and one, p85, appears identical to human c-Rel. v-Rel, but not nontransforming v-Rel mutants, binds to the kappa B enhancer and inhibits NF-kappa B-activated transcription from the IL-2 receptor alpha promoter and HIV-1 LTR. These findings suggest a Rel-related family of kappa B enhancer binding proteins and raise the possibility that the transforming activity of v-Rel is linked to its inhibitory action on cellular genes under NF-kappa B control.

The type I human T-cell leukemia virus (HTLV-I) encodes a 40-kD nuclear trans-regulatory protein termed Tax that transcriptionally activates the HTLV-I long terminal repeat (LTR), as well as select [corrected] cellular and heterologous viral promoters. Tax does not bind DNA specifically but, rather, acts in a more indirect manner. Tax activation of the HTLV-I LTR is mediated through constitutively expressed cellular factors that bind to cAMP response elements (CREs) present within the 21-bp enhancers of the LTR. In contrast, Tax transactivation of the interleukin-2 receptor-alpha gene (IL-2R alpha) and LTR of the type 1 human immunodeficiency virus (HIV-1) involves the induced nuclear expression of NF-kappa B. We now report the identification of missense mutations within the tax gene that functionally segregate these two pathways of trans-activation. Additionally, we demonstrate that the carboxyl terminus of the Tax protein, despite its acidic and predicted alpha-helical structure, is completely dispensable for trans-activation through either of these transcription factor pathways. Finally, we demonstrate that mutations within a putative zinc finger domain disrupt the nuclear localization of Tax and abolish trans-activation. These results demonstrate that Tax trans-activation of viral and cellular promoters involves at least two mechanisms of host transcription factor activation and suggest that this activation is likely mediated through distinct functional domains.

Using an enzyme-linked immunosorbent assay (ELISA) employing two monoclonal antibodies recognizing distinct epitopes on the interleukin 2 receptor (IL2R) alpha chain (Tac molecule), we previously demonstrated that activated lymphocytes release a soluble interleukin 2 receptor molecule (sIL2R) in vitro and in vivo. The sIL2R is biochemically and structurally related to Tac, but its precise origin and functional role remain to be defined. We report here that a single IL2R cDNA is sufficient to direct the synthesis of both cell-associated and soluble released IL2R molecules. Northern analysis of IL2R cDNA transfected L-cell lines revealed the presence of mRNA species unaccounted for by known transcription termination or internal splice sites. Nevertheless, S1 nuclease digestion studies failed to detect alternately spliced mRNA transcripts that specifically lack transmembrane or cytoplasmic domains and which may encode a secreted IL2R molecule. Therefore sIL2R does not appear to be the product of a unique post-transcriptional splicing event. In the absence of any post-translational modifications, sIL2R is most likely generated by enzymatic cleavage and release of cell surface Tac. This proteolytic release of Tac may be but one example of a common cellular mechanism for regulating the membrane expression of cell surface molecules.

Interleukin-2 (IL-2) was originally identified in 1976 as a growth factor for T lymphocytes. Since that time it has become an important mediator of immune function through its effects on the growth, development, and activity of T and B lymphocytes, natural killer cells, and lymphokine-activated killer cells. Only cells that bear a specific receptor for IL-2 respond to its immunoregulatory effects. Of all the lymphokine-receptor systems in immunology, perhaps most is known about the structure, function, and binding properties of IL-2 and its cognate receptor. There are two distinct, membrane-associated IL-2 binding components in the high-affinity IL-2 receptor: an alpha subunit and a beta subunit, which associate in a non-covalent manner. Each of these polypeptides can occur on the cell surface in the absence of the other and bind IL-2, although with only low or intermediate affinity relative to the high-affinity receptor complex. The primary structure of each chain has now been deduced from full-length cDNA. The rapid rate of association between IL-2 and the IL-2R alpha subunit is important in the formation of high-affinity binding sites, and the inducibility of the alpha gene contributes to the highly regulated and transient display of high-affinity IL-2R. The IL-2R beta chain controls the slow dissociation rate of IL-2 from the high-affinity receptor. Also, IL-2R beta appears centrally involved in internalization of IL-2 and signal transduction, functions mediated presumably through its long intracytoplasmic domain. However, the actual mechanism of signal transduction in the IL-2/IL-2R system remains undefined. IL-2R beta is a member of a novel family of cytokine-receptor proteins that includes receptors for IL-4, IL-6, and erythropoietin.

The ability of the Rex protein of the type I human T-cell leukemia virus (HTLV-I) to regulate expression of the retroviral gag and env structural genes post-transcriptionally is critically dependent on the presence of a Rex response element (RexRE). This cis-regulatory sequence is located within the retroviral 3' long terminal repeat and coincides with a predicted, highly stable RNA stem-loop structure. Rex action requires both the overall secondary structure intrinsic to the RexRE and specific sequences from one small subregion of this large structure. This small subregion likely forms a protein-binding site for Rex or a cellular RNA-binding factor. Whereas Rex can functionally replace the Rev protein of the type 1 human immunodeficiency virus (HIV-1) through its interaction with the analogous Rev response element (RevRE), distinct subregions of this HIV-1 RNA element mediate the responses to Rex and Rev. Strikingly, Rex acts as a dominant repressor of Rev action, following the deletion of the Rex responsive subregion of the RevRE. Similarly, Rev inhibits Rex function in a dominant manner when the Rev responsive subregion of the RevRE is deleted. Together, these findings suggest that Rex and Rev not only interact with their respective RNA response elements but also may either form mixed inactive multimers or interact with a common cellular factor(s). If binding of a common host protein is involved, this factor likely plays a central role either in spliceosome assembly or nuclear RNA transport.

Lymphokine-dependent T cell proliferation is regulated in part by the cell surface expression of high affinity interleukin-2 receptors (IL-2R). The functional, high affinity form of the IL-2 receptor is comprised of two ligand binding components, IL-2R alpha (Tac, p55) and IL-2R beta (p70/75). In the absence of the other subunit, IL-2R alpha and IL-2R beta bind ligand with only low or intermediate affinity, respectively. The inducible and transient expression of IL-2R alpha regulates the display of high affinity receptors, while IL-2R beta appears to contribute importantly to growth signal transduction. Although the primary structure of both receptor chains has now been elucidated, the mechanism of growth signal transduction through the high affinity IL-2R remains undefined. Of note, IL-2R beta belongs to a novel family of cytokine receptors including the binding proteins for IL-3, IL-4, IL-6, erythropoietin, and granulocyte-macrophage colony-stimulating factor. These various receptors may well utilize a common intracellular signalling pathway.

Interaction of interleukin 2 (IL2) with its high affinity membrane receptor complex (IL2R) is sufficient to induce proliferation of T lymphocytes. However, the biochemical mechanisms by which IL2 induces this process remain unresolved. The IL2R complex consists of at least two distinct polypeptides that bind IL2, a 75-kDa intermediate affinity subunit (IL2R beta) and a 55-kDa low affinity subunit (IL2R alpha). As indicated by Western blotting with anti-phosphotyrosine-specific antibodies and confirmed by phosphoamino acid analysis, we now demonstrate that interaction of the T cell growth factor interleukin 2 (IL2) with its high affinity receptor on IL2-sensitive human peripheral blood lymphoblasts induces tyrosine phosphorylation of proteins of 92, 80, 78, 70-75, and 57 kDa. IL2 induced tyrosine phosphorylation in YT 2C2 cells which express only the 75-kDa intermediate affinity IL2 binding molecule (IL2R beta) but not in cells which either express only the 55-kDa low affinity IL2 receptor molecule (IL2R alpha) or no IL2-binding sites. Therefore, IL2R beta, in the absence of IL2R alpha, appears sufficient to transduce the transmembrane signal leading to tyrosine phosphorylation. Two different antibodies reactive with phosphotyrosine specifically immunoprecipitated IL2R beta cross-linked to radiolabeled IL2. These findings suggest that IL2R beta is a substrate for the tyrosine kinase which is activated by IL2 binding to its receptor. Thus, like several other growth factor receptors, activation of the IL2R results in an increase in tyrosine phosphorylation with the receptor itself serving as one substrate.

CTC-SISS-B is an antigen-nonspecific suppressive lymphokine elaborated by an interleukin 2-dependent suppressor T cell line that produces noncytotoxic inhibition of human B cell but not T cell function. Like SISS-B, a soluble suppressive lymphokine present in the supernatants of Con A-activated peripheral blood T cell cultures, CTC-SISS-B is of 60,000 to 90,000 m.w., and its action is blocked by the simple sugar L-rhamnose. CTC-SISS-B inhibits human B cell Ig production and proliferation through a direct interaction with human B cells rather than through indirect effects on immunoregulatory T cells or monocytes. CTC-SISS-B suppression occurs through inhibition of an early event(s) in B cell activation since proliferation and Ig production by established human B cell lines are not inhibited by this lymphokine. Despite sharing many biochemical and biologic properties, CTC-SISS-B and gamma-interferon appear to be distinct mediators.

Resting human T lymphocytes do not express receptors for interleukin-2, but expression is rapidly induced by exposure to PHA. After maximal expression 2-3 days after stimulation, a progressive decline in receptor number is observed. Receptor expression can be augmented by reexposure to PHA. In this study we show that activators of protein kinase C including phorbol diester, phospholipase C, and the diacylglycerol congener diC8 also increase IL-2 receptor expression. Moreover, 5-azacytidine, which inhibits cytosine methyltransferase, and hydroxyurea, which inhibits ribonucleotide reductase, also increased receptor number. These studies demonstrate that IL-2 receptor expression can be altered in vitro, and that IL-2 receptor number, in combination with IL-2 secretion, may contribute to the regulation of IL-2-dependent immune responses.

In an effort to clarify the mechanism by which certain plant lectins induce lymphocyte activation, we have studied amino acid (amino[14C]isobutyric acid) uptake in purified human lymphocytes exposed to mitogenic and nonmitogenic lectins. In confirmation of earlier work, mitogenic lectins (concanavalin A and two phytohemagglutinins) produced a dose-related, 2- to 10-fold increase in aminoisobutyric acid transport. Changes occurred as early as 2 hours and reached a maximum after 18 hours. The stimulation by concanavalin A was inhibited by alpha-methyl-D-mannoside but not by other selected monosaccharides, indicating that the effect is modulated through specific carbohydrate receptors. In contrast to the stimulation with concanavalin A and phytohemagglutinin, the nonmitogen wheat germ agglutinin inhibited aminoisobutyric acid transport, both in the presence and absence of the mitogenic lectins. The inhibition was seen over a broad wheat germ agglutinin dose range, was prevented by N-acetylglucosamine, a known inhibitor of wheat germ agglutinin binding, and did not appear to be associated with cytotoxicity. Comparative binding studies with radiolabeled concanavalin A and wheat germ agglutinin demonstrated that differences in transport occurred in cells containing comparable numbers of bound lectin molecules indicating that the failure of wheat germ agglutinin to stimulate a response was not a result of ineffective binding. The lack of stimulation by wheat germ agglutinin was not due to its inability to interact multivalently with membrane receptors since this lectin is divalent, produces capping and agglutination, and continues to inhibit aminoisobutyric acid transport even after the minimum valence was increased to 4 by cross-linking with glutaraldehyde. In contrast both divalent and tetravalent concanavalin A produced stimulation. Competitive binding studies with soluble wheat germ agglutinin or lectin attached to 300 A latex spheres revealed little or no competition for binding sites with radiolabeled concanavalin A, suggesting the two lectins are interacting with different receptors. This was further suggested by kinetic studies of aminoisobutyric acid transport which indicated that wheat germ agglutinin was probably affecting both the Vmax and Km of transport, whereas concanavalin A affected only the Vmax. While the mechanism by which concanavalin A and wheat germ agglutinin produce opposing effects on amino acid transport is not clear, since the two lectins appear to be interacting with different surface receptors we would suggest that they are perturbing microanatomically and functionally different domains on the lymphocyte plasma membrane.

Anti-Tac monoclonal antibody identifies the receptor for interleukin 2 (IL 2, or T cell growth factor) present on activated human T lymphocytes. By using tritiated anti-Tac, we now report a sensitive and specific binding assay to evaluate cell surface IL 2 receptor expression. IL 2 receptors on human peripheral blood lymphocytes can be detected within 6 hr after PHA stimulation. PHA-induced receptor expression is inhibited by actinomycin D and cycloheximide, but not by mitomycin C, suggesting a requirement for de novo RNA and protein synthesis, but not DNA synthesis. Scatchard analysis of [3H]-anti-Tac binding to lymphocytes stimulated with PHA for 3 days revealed from 20,000 to 60,000 molecules of antibody bound per cell, and a Kd of 1 to 3 x 10(-10) mol/l. Sequential binding studies of activated human lymphocytes maintained in long-term culture with IL 2 demonstrated a progressive decline in receptor number correlating with diminished growth rate. Restimulation with lectin or antigen increased the number of IL 2 receptors, suggesting that IL 2 dependent immune responses may be regulated, at least in part, by IL 2 receptor expression. Receptor number was also increased by PMA. Moreover, similar effects were produced by incubation with phospholipase C but not interleukin 1. Because both PMA and phospholipase C result in activation of protein kinase C, these data suggest the possibility that activation of protein kinase C may induce IL 2 receptor expression.

A human suppressor T cell maintained in long-term culture with conditioned medium containing interleukin 2 elaborates a suppressor factor(s) that specifically inhibits human polyclonal B cell immunoglobulin biosynthesis. This soluble immune suppressor supernate of immunoglobulin production (CTC-SISS-B) shares a number of features with the previously described suppressive mediator elaborated by concanavalin A-activated human peripheral T cells (SISS-B) including: (a) the inhibition by a noncytotoxic mechanism, (b) the suppression of immunoglobulin biosynthesis either through direct action on the B cell or indirect action via the monocyte, (c) the loss of inhibition in the presence of the monosaccharide L-rhamnose, (d) the elaboration by cells irradiated with 500 ro 2,000 rad, and (e) molecular weights of 60,000--90,000. Furthermore, the suppression by this mediator appears to be specific for B cell immunoglobulin production in that CTC-SISS B has no effect on T cell proliferation to mitogens, antigens, an allogeneic cells or on T cell-mediated cytotoxicity. These data indicate that one possible mechanism of suppressor T cell inhibition of human immunoglobulin production is via the generation of a lectinlike suppressor lymphokine that interacts with defined saccharide determinants on the cell surface of either the B cell or monocyte.

Cyclosporin A (CsA) is a potent immunosuppressive agent, now gaining wide application in human organ transplantation. The immunosuppressive activity of CsA is at least in part due to inhibition of lymphokine production by activated T lymphocytes. Specifically, inhibition of T-cell growth factor (TCGF; also designated interleukin 2) production appears to be an important pathway by which CsA impairs T-cell function. To define further both the specificity of CsA and the level at which it interferes with lymphokine gene expression, we have studied its effects on TCGF mRNA accumulation as well as TCGF gene transcription. These studies were performed with a cloned human leukemic T-cell line (Jurkat, subclone 32), which can be induced with phytohemagglutinin and phorbol 12-myristate 13-acetate to produce large amounts of TCGF. In these cells, high levels of TCGF mRNA were present in induced but not in uninduced Jurkat cells as judged by hybridization to a cloned human TCGF cDNA probe. CsA completely inhibited induced TCGF mRNA accumulation at concentrations of 0.3-1.0 microgram/ml, whereas low levels of appropriately sized TCGF mRNA were present at 0.01 microgram/ml. In nuclear transcription experiments, CsA inhibited the synthesis of TCGF transcripts in a dose-dependent manner with complete inhibition at a concentration of 1 microgram/ml. In contrast, CsA did not inhibit the expression of two other inducible genes, TCGF receptor and HT-3. Further, HLA gene expression was also less affected than TCGF in CsA-treated cells. These data suggest a relatively selective action of CsA on TCGF gene transcription.

The high-affinity IL-2-R complex is composed of at least two distinct IL-2-binding subunits, including p55 (Tac, IL-2-R alpha) and p70 (IL-2-R beta). Using a radiolabeled mAb specific for the p55 receptor subunit and cells expressing a homogeneous population of high-affinity binding sites, we demonstrate that p55 is co-internalized with p70 after IL-2 binding to the receptor complex. Endocytosis of p55 depends upon the presence of IL-2 in a form capable of effectively interacting with the p70 subunit. Whether IL-2 is required for high-affinity receptor assembly or triggering of the internalization of preassembled receptors remains unresolved. Together, these findings support the existence of a stable, high-affinity human IL-2-R membrane complex composed of at least the p55 and p70 receptor subunits and IL-2.

The possible modulatory role of microtubules or closely related colchicine-sensitive structures in the response of human lymphocytes to mitogenic lectins was investigated. Colchicine (0.1 to 10 muM) AND VINBLASTINE (0.1 TO 10 MUM) inhibited early [14C]-aminoisobutyric acid and late [3H]-thymidine uptake in phytohemagglutinin-and concanavalin A-stimulated human lymphocytes but failed to alter 45Ca uptake. Lumicolchicine, an inactive congener of colchicine, was ineffective in all three systems. Both microtubular agents accentuated and prolonged the early cyclic AMP response to lectin. Little or no alteration in cyclic AMP levels was seen with colchicine or vinblastine alone or in combination with PGE (10MUM) or epinephrine (1muM) suggesting that the effect on cyclic AMP metabolism is largely selective for lectin stimulation. Neither microtunular agent altered 125I-concanacalin A binding. Since the inhibition of DNA synthesis was throughout the culture period and early aminoisobutyric acid uptake is affected, it appears that these agents are acting on an early event, or events, in the activation sequence. Although the mechanism of the inhibition is not known, the effect of colchicine and vinblastine in prolonging the cyclic AMP response to lectin may be involved. Alternatively, alterations in microtubules assembly may exert effects on membrane architecture interfering with propagation of the stimulus from the membrane to the cell interior.

Complementary DNAs corresponding to the human receptor for interleukin-2 (IL-2) have been molecularly cloned, sequenced, and expressed in both COS-1 and L cells. The human genome appears to contain a single structural gene for this receptor located on the short arm of chromosome 10 (band 14-15). However, when transcribed, at least two families of mRNAs are produced, which vary in length due to the use of at least three different polyadenylation signals. Sequence analysis of the cloned cDNAs and S1 nuclease protection assays indicate an alternative pathway of mRNA processing for this receptor whereby a 216 base-pair segment contained within the protein coding region is spliced, resulting in an mRNA unable to encode a functional IL-2 receptor. In contrast, cDNAs corresponding to mRNA retaining this 216 base-pair region code membrane receptors that bind both IL-2 and anti-Tac (monoclonal anti-IL-2 receptor antibody). Analysis of the deduced amino acid sequence reveals that the receptor is composed of 272 amino acids including a signal peptide 21 amino acids in length. Hydrophobicity analysis suggests a single, 19 amino acid transmembrane domain. A short intracytoplasmic domain composed of 13 amino acids is present and contains two potential phosphate acceptor sites (serine and threonine but not tyrosine) as well as positively charged residues presumably involved in cytoplasmic anchoring. Two sites for N-linked glycosylation sites and numerous extracytoplasmic O-linked glycosylation sites are present.

Transcriptional induction of the gene encoding the alpha-subunit of IL-2R has been shown to be mediated by a sequence element (GGGGAATCTCCC) that is homologous to the NF-kappa B-binding site of the kappa Ig gene enhancer. In this report we demonstrate that the induced transcription of this gene by mitogen and by the tax gene product of the type-I human T cell leukemia virus is dependent upon an additional sequence motif (GGGCGTAGC) located approximately 10 bp downstream of the previously identified site. This newly identified motif binds a factor that is present in extracts derived from different cell types and does not appear to be required for basal promoter activity. We conclude that proteins binding at both sites act coordinately, leading to maximal induction of the receptor gene.

Ig and T cell antigen receptor genes in their germ line form are separated DNA segments that are joined by recombinations during lymphocyte development. The analysis of Ig and T cell receptor gene arrangements has been of value in the study of lymphoid neoplasms. The identification of T cell receptor gene rearrangements taken in conjunction with studies of Ig gene rearrangements aids in the elucidation of the lineage (T cell or B cell) and the clonality of lymphoid populations of all series. The application of this molecular genetic approach has great potential for complementing conventional marker analysis, cytogenetics, and histopathology, thus broadening the scientific basis for the classification, diagnosis, and monitoring of the therapy of lymphoid neoplasia. IL 2 is a lymphokine synthesized by some T cells following activation. Resting T cells do not express IL 2 receptors, but receptors are rapidly expressed on T cells following the interaction of antigens, mitogens, or monoclonal antibodies with the antigen-specific T cell receptor complex. Normal resting T cells and most leukemic T cell populations do not express IL 2 receptors; however, the leukemic cells of all patients with HTLV-I-associated adult T cell leukemia examined expressed the Tac antigen. The constant display of large numbers of IL 2 receptors that may be aberrant may play a role in the uncontrolled growth of these leukemic T cells. Patients with the Tac antigen positive adult T cell leukemia are being treated with the anti-Tac monoclonal antibody directed toward this growth factor receptor.

Anti-Tac monoclonal antibody has been identified as a putative antibody against the receptor for T-cell growth factor (TCGF). We now show that: (i) TCGF blocks 85% of 3H-labeled anti-Tac binding to phytohemagglutinin-activated lymphoblasts and (ii) both anti-Tac and anti-TCGF immunoprecipitate a protein band that appears to represent TCGF crosslinked to its receptor on HUT-102B2 cells. In HUT-102B2 cells, the TCGF receptor is a Mr 50,000 glycoprotein with internal disulfide bond(s) and a pI of 5.5-6.0, and it represents approximately equal to 0.05% of total cellular de novo protein synthesis. It contains a peptide of Mr 33,000 that is processed to a mature form that includes N-linked and O-linked sugars and sialic acid.

The human T-lymphotropic viruses types I and II (HTLV-I and -II) have been etiologically linked with certain T-cell leukemias and lymphomas that characteristically display membrane receptors for interleukin-2. The relation of these viruses to this growth factor receptor has remained unexplained. It is demonstrated here that introduction of the trans-activator (tat) gene of HTLV-II into the Jurkat T-lymphoid cell line results in the induction of both interleukin-2 receptor and interleukin-2 gene expression. The coexpression of these cellular genes may play a role in the altering T-cell growth following retroviral infection.

We have investigated the biochemical basis for the activation of interleukin 2 receptor alpha-subunit (IL-2R alpha) gene expression in primary human T lymphocytes by a cytokine (tumor necrosis factor alpha), a T-cell mitogen (phorbol 12-myristate 13-acetate), and the transactivator protein (Tax) from the type I human T-cell leukemia virus. Using in vivo transfection techniques specificially designed for these primary T cells in conjunction with in vitro gel retardation and DNA footprinting assays, we found that activation of the IL-2R alpha promoter by each of these agents involves the induction of nuclear proteins that specifically interact with a kappa B-like enhancer element (i.e., an element resembling the immunoglobulin kappa-chain enhancer sequence recognized by transcription factor NF-kappa B). DNA-protein crosslinking studies revealed that primary T cells express at least three different inducible DNA-binding proteins (50-55, 70-75, and 80-90 kDa) that specifically interact with this IL-2R alpha kappa B element.

Interleukin 2 (IL-2) receptors on activated T cells exist in high- and low-affinity configurations, both of which share a ligand-binding component known as the Tac protein. Although almost all binding of IL-2 to such cells was inhibited by an antibody to Tac, the predominant component of binding on the natural killer (NK)-like cell line YT was resistant to this reagent. The ligand-binding component on YT cells also differed from Tac in its affinity constant (Kd approximately 8.2 X 10(-10) M vs. Kd approximately equal to 1.1 X 10(-8) M for low-affinity Tac sites) and in its susceptibility to inhibition by certain antibodies to IL-2. When the YT cells were stimulated in a manner that induced expression of the Tac protein, the IL-2 binding sites were converted to a high-affinity configuration (Kd approximately 1.8 X 10(-11) M). Thus, the original binding component on unstimulated YT cells appeared to combine with Tac and IL-2 to produce a high-affinity receptor complex. Use of bifunctional crosslinking agents following ligand binding to unstimulated YT cells yielded covalent IL-2-receptor complexes of 83 and 90 kDa. These complexes were similar in size to those derived from high-affinity receptors on activated T cells and shared a similar fragmentation pattern upon proteolysis. These results demonstrate the existence of a second IL-2 binding component in addition to the Tac protein and suggest that this component combines with Tac and IL-2 to form high-affinity receptor sites.

Interleukin-2 (IL-2) is a lymphokine synthesized by some T cells following activation. Resting T cells do not express IL-2 receptors but receptors are rapidly expressed on T cells following the interaction of antigens, mitogens, or monoclonal antibodies with the antigen specific T-cell receptor complex. Using anti-Tac a monoclonal antibody that recognizes the IL-2 receptor, the receptor has been purified. The receptor is a 33 kdalton peptide that is post-translationally glycosylated to a 55 kdalton mature form. Mature receptors contain both N-linked and O-linked sugars and are both sulfated and phosphorylated. Using an oligonucleotide probe, based on the N-terminal amino acid sequence, cDNAs encoding this receptor have been cloned, sequenced and expressed. The addition of anti-Tac to in vitro culture systems blocks the IL-2 induced DNA synthesis of IL-2 dependent T-cell lines and inhibits soluble auto- and alloantigen induced T-cell proliferation. Furthermore, it prevents the generation of cytotoxic and suppressor effector T cells. The anti-receptor antibody also inhibits lectin stimulated immunoglobulin synthesis and the sequential expression of late appearing activation antigens on T cells. Normal resting T cells and most leukemic T-cell populations do not express IL-2 receptors however the leukemic cells of all patients with human T-cell leukemia/lymphoma virus (HTLV-I) associated, adult T-cell leukemia (ATL) examined expressed the Tac antigen. In HTLV-I infected cells the 42 kdalton long open reading frame (LOR) protein encoded in part, by the pX region of HTLV-I may act as a transacting transcriptional activator that induces transcription of the IL-2 receptor gene thus providing an explanation for the constant association of HTLV-I infection of lymphoid cells and IL-2 receptor expression. The constant display of large numbers of IL-2 receptors which may be aberrant in the ATL cells may play a role in the uncontrolled growth of these leukemic T cells. Patients with the Tac positive ATL are being treated with an anti-Tac monoclonal antibody directed towards this growth factor receptor.

cDNAs for human interleukin-2 receptor were recently cloned and sequenced (Leonard et al., 1984, Nature 311, 626-631; Nikaido et al., 1984, Nature 311, 631-635; Cosman et al., Nature 312, 768-771). In the studies reported here, we describe the expression of a cDNA clone for the human interleukin-2 receptor in E. coli using an "open reading frame" expression vector pMR100. The inserted cDNA was expressed in E. coli transformants as a tripartite fusion polypeptide fused to the lambda cI protein at its amino terminus and to beta-galactosidase at its carboxy terminus. We demonstrate that the bacterially produced IL-2 receptor protein can bind to IL-2.

Transferrin receptor expression is essential for the proliferation of both normal and malignant T cells. While transferrin receptor expression in normal T cells is tightly coupled to interleukin-2 receptor expression, transferrin receptor expression in malignant cells is usually constitutive and is released from this constraint. Temporally, the appearance of these membrane receptors is preceded by changes in the expression of the proto-oncogenes c-myc and c-myb. In addition, although an increase in the level of intracellular free calcium occurs early in the sequence of T-cell activation, the activation events dependent on this calcium flux have not been resolved. In the present study we report that diltiazem, an ion channel-blocking agent that inhibits calcium influx, arrested the growth in vitro of both normal and malignant human T cells in the G1 phase of the cell cycle. However, diltiazem did not inhibit the expression of c-myc or interleukin-2 receptor mRNA and protein in normal mitogen-activated T cells or the constitutive expression of c-myc and c-myb mRNA in malignant T cells (T acute lymphoblastic leukemia cells). In contrast, diltiazem prevented the induction of transferrin receptor (mRNA and protein) in normal T cells and caused a progressive loss of transferrin receptor (mRNA and protein) in malignant T cells. These data demonstrate that diltiazem can dissociate several growth-related processes normally occurring in G1 and thereby disrupt the biochemical cascade leading to cell proliferation.

To investigate the mechanism by which immune activation augments replication of the human immunodeficiency virus type 1 (HIV-1) in infected T cells, four different classes of T cell mitogens were evaluated for their effects on the HIV-1 long terminal repeat (LTR). Phytohemagglutinin (PHA), a mitogenic lectin; phorbol 12-myristic 13-acetate, a tumor promoter; ionomycin, a calcium ionophore; and tat-1, the trans-activator protein from the human T cell leukemia/lymphoma virus type I (HTLV-I) each stimulated the HIV-1 LTR. Studies of deleted forms of the LTR supported a central role in these responses for the HIV-1 enhancer, which alone was sufficient for mitogen inducibility, but also suggested that other 5' positive and negative regulatory elements contribute to the overall magnitude of the response. Synergistic activation of the HIV-1 LTR (up to several thousandfold) was observed with combinations of these mitogens and the HIV-1--derived tat-III protein. Cyclosporin A, an immunosuppressive agent, inhibited PHA-mediated activation of the HIV-1 LTR but was without effect in the presence of other mitogens. Thus, HIV-1 gene expression and replication appear to be regulated, via the HIV-1 LTR, by the same mitogenic signals that induce T cell activation.

To study human T cell suppression of immunoglobulin (Ig) synthesis with homogeneous populations of immunoregulatory cells, human suppressor T cell hybridomas were prepared by somatic cell fusion of concanavalin A-activated peripheral blood T cells with hypoxanthine-guanine phosphoribosyltransferase-(HGPRT, EC 2.4.2.8) deficient human leukemic CEM T cells. After selection in hypoxanthine-aminopterin-thymidine (HAT) medium and cloning by limiting cell dilution, two human T cell hybridomas were identified that produced 60 to 80% suppression of in vitro polyclonal immunoglobulin production when cocultured with pokeweed mitogen- (PWM) stimulated peripheral blood lymphocytes. Further, one of the suppressor T cell hybridomas constitutively secreted a soluble suppressor factor(s) (TsF) of m.w. 70,000 to 85,000 daltons, which produced reversible noncytotoxic inhibition of lectin-activated B cell Ig production. In contrast, this TsF did not inhibit lectin- or antigen-induced T cell proliferation, nor did it interfere with the generation or effector function of cytotoxic T cells. Additional studies indicated that this Tsf acts directly on B cells or monocytes rather than indirectly modulating the activity of immunoregulatory T cells. In summary, these studies suggest that techniques of somatic cell fusion may provide a valuable approach to further study human immunoregulatory cell-cell interactions as well as provide a source of sufficient quantities of important lymphokines for further purification and characterization.

The Rex proteins of types I and II human T-cell leukemia viruses (HTLV-I, HTLV-II) are required for expression of the viral structural gene products, gag and env and, thus, are essential for the replication of these pathogenic retroviruses. The action of Rex is sequence specific, requiring the presence of a cis-acting Rex response element located in the 3' long terminal repeat. This element corresponds to a predicted RNA secondary structure and functions in an orientation-dependent but position-independent manner. Rex acts through this response element to stimulate the nuclear export of the unspliced or singly spliced viral mRNA species encoding the virion structural proteins that are normally excluded from the cytoplasm. Although the Rex proteins of HTLV-I and HTLV-II can also function via the related Rev response element present in the env gene of the type I human immunodeficiency virus (HIV-1), the analogous HIV-1 Rev protein is unable to act on the HTLV-I Rex response element. This nonreciprocal pattern of genetic complementation by Rex and Rev suggests that these viral trans-regulators may interact directly with their RNA response elements.

Both the interleukin-2 receptor-alpha (Tac, p55, IL-2R alpha) gene and the long terminal repeat (LTR) of type 1 HIV are activated by various T cell mitogens. We now demonstrate that an inducible 86 kd nuclear protein termed HIVEN86A specifically binds to both the enhancer element of the HIV-1 LTR and a closely related 12 bp sequence present in the 5' regulatory region (-267 to -256) of the IL-2R alpha gene. The interaction of these binding sites with HIVEN86A and perhaps other cellular proteins such as NF-kappa B appears importantly involved in mitogen activation since the isolated IL-2R alpha promoter binding site or single elements of the normally duplicated HIV-1 enhancer alone are sufficient to confer mitogen inducibility on an unresponsive heterologous promoter. Together these findings suggest that the normal action of an inducible nuclear DNA binding protein(s) involved in the regulation of IL-2R alpha gene expression can be subverted by the HIV-1 provirus to promote activation of retroviral gene transcription.

In this review, Warner Greene and colleagues discuss recent studies that have revealed an intriguing molecular interplay between two pathogenic human retroviruses, HIV-1 and HTLV-1, and certain cellular genes that normally control T-cell growth. Activation of T cells during an immune response results in the induction of select transcription factors that bind specifically to kappa B enhancer elements present in both the IL-2R alpha and IL-2 genes. Normal T-cell growth is in part regulated by the transient expression of these genes. The Tax protein of HTLV-1 induces these same kappa B-specific proteins, but in contrast to immune stimulation, HTLV-1 infection of T cells leads to constitutive IL-2R alpha gene expression and immortalization. A second human retrovirus, HIV-1, can subvert the normal action of the kappa B-binding factors induced by these immune stimuli. Rather than promoting T-cell growth, these factors may augment viral replication and promote T-cell death.

Adult T-cell leukemia is a progressive disease produced by infection of mature T-cells with the human T-lymphotropic virus-I (HTLV-I). These retrovirus infected T-cells express large numbers of receptors for interleukin 2 (or T-cell growth factor). Due to the presence of these receptors, these leukemic T-cells can be selectively killed in vitro by monoclonal anti-interleukin 2 receptor antibody covalently linked to the A chain of the plant toxin, ricin (anti-TAC-A), suggesting that such immunotoxins may be useful in the therapy of this disease. In this report we demonstrate that the lysosomotropic agent ammonium chloride and the carboxylic ionophore monensin substantially potentiate the cytotoxicity of anti-TAC-A on HUT 102 cells, a long-term cultured HTLV-I infected T-cell line. Anti-TAC-A alone produces half-maximal inhibition of protein synthesis in HUT 102 cells at a concentration of 2.2 X 10(-10) M (the 50% inhibitor, concentration). Addition of ammonium chloride or monensin augments the potency of anti-TAC-A killing 100-fold (50% inhibitory concentration = 2.5 X 10(-12) M) and 400-fold (50% inhibitory concentration = 8 X 10(-13) M), respectively; furthermore, these agents accelerate rate of anti-TAC-A intoxication and increase the specific killing of interleukin 2 receptor-bearing leukemic cells. At concentrations which cause only minor harm to colony forming hematopoietic progenitor cells (granulocyte-erythroid, monocytic, megakaryocytic colony forming unit, granulocyte-macrophage colony forming unit, macrophage colony forming unit, and granulocyte colony forming unit), anti-TAC-A alone is able to eliminate greater than 99.99% of an HTLV-I infected T-cell population. In the presence of ammonium chloride or monensin, respectively, a 3.5- and 20-fold greater cytoreduction of HTLV-I infected T-cells is achieved. Combined treatment with anti-TAC-A and monensin may offer an efficient and highly selective means of purging bone marrow of patients with adult T-cell leukemia, which may then be used for autologous marrow transplantation.

We demonstrate that purified interleukin 2 (IL-2) can directly upregulate IL-2 receptor expression on phytohemagglutinin-activated T lymphocytes maintained in culture until IL-2 receptor expression had markedly declined. The IL-2-induced increase in IL-2 receptor number is maximal within 12 hr, requires new RNA and protein synthesis, and is mediated by an interaction of ligand with the high-affinity receptors for IL-2. IL-2 stimulation results in increased accumulation of IL-2 receptor mRNA within 4 hr, while an increase in IL-2 receptor gene transcription is detected within 30 min in isolated nuclei. In addition, IL-2 incubation results in increased amounts of c-myc and transferrin receptor mRNA, but it does not augment levels of mRNA encoding the beta chain of the T-cell receptor for antigen. These results demonstrate that IL-2 can directly upregulate transcription and expression of its own receptor and, therefore, indicate that IL-2 may regulate IL-2-dependent immune responses, in part, by influencing the expression of IL-2 receptors.

A recombinant amphotropic retrovirus was used to introduce the protein-coding region of the IL-2 receptor cDNA derived from HUT-102 cells into human CEM leukemic T-cells that lack these receptors. CEM T-cells that contained the virus expressed functional IL-2 receptors that transiently mediated five- to tenfold increases in [3H]thymidine incorporation following the addition of picomolar quantities of IL-2. Although IL-2 responsiveness was subsequently lost, it could be reinduced by cellular activation with the OKT11 monoclonal antibody. This phenotype also proved unstable with progressive time in culture. Despite the loss of IL-2 responsiveness, the infected CEM T-cells continued to express Tac antigen and displayed 50 to 200 high-affinity IL-2 receptors per cell that bound IL-2 with a dissociation constant of 4.3 pM. This affinity is fully equivalent to that detected on activated normal T-cells (2 to 50 pM). The apparent molecular size of the Tac antigen on these cells (55,000 to 60,000 daltons) was comparable to that on normal activated T-cells but 5000 daltons larger than the aberrant IL-2 receptors on HUT-102 cells. These data demonstrate that expression of a human IL-2 receptor cDNA in human T-cells results in high-affinity IL-2 receptor display that transiently imparts an IL-2 responsive state of growth. These results also raise the possibility that the T11 surface receptor may play an important regulatory role in high-affinity IL-2 receptor expression.

We studied purified subpopulations of lymphocytes from patients with the acquired immunodeficiency syndrome (AIDS) in order to determine whether intrinsic defects in lymphocyte function, aside from those due to alterations in lymphocyte numbers, were present. Mitogen-stimulated DNA synthesis, production of gamma interferon, production of interleukin-2, and expression of interleukin-2 receptors, although variably decreased in unseparated cell populations, were normal in populations of purified T-cell subsets. In contrast, DNA synthesis in response to the soluble protein antigen tetanus toxoid was decreased in both unseparated and purified T-cell subpopulations. Cell-mixing experiments demonstrated that the hyporesponsiveness of the unfractionated lymphocytes from patients with AIDS was not due to active suppression. We conclude that the lymphocytes of patients with AIDS, although capable of undergoing a normal degree of blast transformation and lymphokine production after mitogenic stimulation, have an intrinsic defect in their ability to recognize and respond to soluble antigen.

Stable expression of the 40-kDa transactivator protein (Tax) from the type I human T-cell leukemia virus (HTLV-I) in Jurkat T cells leads to the activation and sustained expression of certain cellular genes that are transiently induced during normal T-cell growth. Cellular genes induced by Tax include those encoding the alpha subunit of the high-affinity interleukin 2 receptor (Tac), interleukin 2, and granulocyte/macrophage colony-stimulating factor. Tax induction of the interleukin 2 gene is synergistically amplified by mitogens that augment cytoplasmic levels of calcium. These changes in the pattern of cellular gene expression reflect a specific action of Tax, as they are undetectable in isogenically matched control cell lines expressing antisense tax cDNA. The spectrum of cellular genes regulated by Tax appears to be restricted: several other T-cell genes, either inducibly or constitutively expressed, are unaffected by this viral protein. These cell lines constitutively expressing Tax provide valuable reagents to explore the molecular basis for Tax action and to delineate the full spectrum of cellular genes regulated by this retroviral gene product.

The rex gene of the type I human T-cell leukaemia virus (HTLV-I) encodes a phosphorylated nuclear protein of relative molecular mass 27,000 which is required for viral replication. The Rex protein acts by promoting the cytoplasmic expression of the incompletely spliced viral messenger RNAs that encode the virion structural proteins. To identify the biologically important peptide domains within Rex, we introduced a series of mutations throughout its sequence. Two distinct classes of mutations lacking Rex biological activity were identified. One class corresponds to trans-dominant repressors as they inhibit the function of the wild-type Rex protein. The second class of mutants, in contrast, are recessive negative, rather than dominant negative, as they are not appropriately targeted to the cell nucleus. These results indicate the presence of at least two functionally distinct domains within the Rex protein, one involved in protein localization and a second involved in effector function. The trans-dominant Rex mutants may represent a promising new class of anti-viral agents.

Human peripheral blood mononuclear cells (PBMC) activated with the mitogenic lectin concanavalin A (Con A) elaborate a soluble immune suppressor supernatant (SISS) that contains at least 2 distinct suppressor factors. One of these, SISS-B, inhibits polyclonal B cell immunoglobulin production whereas the other, SISS-T, suppresses T cell proliferation to both mitogens and antigens. The latter mediator is discussed in the companion paper. Characteristics of the human soluble suppressor of B cell immunoglobulin production (SISS-B) include: 1) inhibition by a noncytotoxic mechanism, 2) loss of activity in the presence of the monosaccharide L-rhamnose, 3) appearance within 8 to 16 hr after the addition of Con A, 4) elaboration by cells irradiated with 500 or 2000 rads, 5) production by highly purified T cells, 6) stability at pH 2.5 but instability at 56 degrees C, and 7) m.w. of 60 to 80,000. These data indicate that after Con A activation, selected T cells not only become potent suppressor cells, but also generate a soluble saccharide-specific factor(s) that inhibits polyclonal immunoglobulin production by human B cells.