Cyclic AMP directly inhibits IL-2 receptor expression in human T cells: expression of both p55 and p75 subunits is affected.

Using purified human T lymphocytes stimulated in serum-free media with adhered anti-CD3 + exogenous IL-2, we have shown that elevated [cAMP]i (mimicked by CPT-cAMP or induced by the physiological agonist PGE2) directly inhibits mitogen-induced 1) [3H]thymidine incorporation by PBMC, purified T cells, and isolated CD4+ and CD8+ T cell subpopulations; 2) expression of both high- and low-affinity IL-2 receptors; 3) plasma membrane expression of both p55 and p75 subunits of the IL-2 receptor; and 4) expression of p55 mRNA, but not p75 mRNA. The decrease in p55 mRNA is not due to enhanced mRNA metabolism. We conclude that elevated [cAMP]i, acting directly on T cells, inhibits mitogenesis by decreasing IL-2 receptor expression. We discuss the possible physiological relevance for the multiple stages of T cell activation that are sensitive to elevated [cAMP]i.     

IL-4 inhibits IL-2 receptor expression and IL-2-dependent proliferation of human T cells

Recent studies have shown that IL-4 can affect lymphocyte responses to IL-2. To evaluate the effects of IL-4 on T cell responses to physiologically relevant stimuli, we studied normal human T cells cultured with a low concentration of anti-CD3 mAb and IL-2 in the presence and absence of added IL-4. The addition of IL-4 to cultures of T cells stimulated with anti-CD3 mAb and IL-2 reduced the proliferative response by 49 to 59%. The inhibitory effect was observed in 3-, 5-, and 7-day cultures. Inhibition was dose-dependent with maximal inhibition at concentrations greater than or equal to 5 to 10 U/ml IL-4. IL-4-mediated inhibition occurred early during the T cell response, inasmuch as addition of IL-4 after stimulation for 24 h did not result in significant inhibition. Phenotypic analyses of cells cultured in the presence of anti-CD3 mAb, IL-2, and IL-4 suggested that the mechanism of regulation by IL-4 involves the inhibition of IL-2R expression. The proportion of both CD4+ and CD8+ cells that expressed IL-2R in response to IL-2 was diminished in the presence of IL-4, although HLA-DR levels were unaffected. Soluble IL-2R was also reduced in supernatants of cultures stimulated with anti-CD3 mAb, IL-2, and IL-4 as compared to cultures stimulated with anti-CD3 mAb and IL-2. These findings indicate that when normal human T cells are stimulated in vitro in a manner that approximates a physiologic interaction with Ag in vivo, rIL-4 provides a potent inhibitory signal to IL-2 responsive cells that is likely mediated by IL-4-induced inhibition of IL-2R expression.     

p55 IL-2 receptor mRNA precursors in murine T lymphocyte nuclei

An unusual family of cDNA clones homologous to human p55 IL-2R sequences was isolated from the murine HT-2 Th cell line. These clones were mapped, partially sequenced, and compared with previously published human and mouse IL-2R sequences. They appear to consist of various combinations of exons and introns, suggesting that they are derived from p55 IL-2R mRNA precursors. The configuration of exons in the splicing intermediates indicates that the murine and human gene organizations are similar and that the 3' end of intron 3 is well conserved between the two species. RNA mapping experiments using nuclear, cytoplasmic, and total RNA and probes derived from various parts of the p55 IL-2R gene support and extend the sequence data. They indicate that detectable amounts of immature p55 IL-2R mRNA are found specifically in the cell nucleus of the HT-2 cell line. Similar data were obtained for the Th cell clone 52.3 and the cytotoxic T cell line CTLL. All these results indicate that the T cell nucleus contains significant amounts of immature p55 IL-2R mRNA.     

IL-4-dependent up-regulation of IL-4 receptor expression in murine T and B cells

This study examined mRNA levels and cell surface expression of IL-4 receptor (IL-4R) in murine T and B cells after incubation with IL-4. Northern blot analysis of mRNA levels of T cells isolated from mesenteric lymph nodes and spleen revealed that IL-4 induced a transient augmentation of IL-4R mRNA in a dose-dependent manner. Maximal levels of mRNA were detected as early as 5 h after initiation of culture. These data were complemented by studies examining the cell surface expression of IL-4R using an anti-IL-4R mAb. Resting T and B lymphocytes express IL-4R (T greater than B) and incubation of these cells with exogenous IL-4 increased IL-4R expression to a maximum after 24 h. This effect was abolished after addition of anti-IL-4 antibody. Continuous incubation of T cells in the presence of high concentrations of IL-4 resulted in a down-regulation of IL-4R expression. Addition of the protein synthesis inhibitor cycloheximide blocked the induced increases in IL-4R expression, indicating the requirement for de novo protein synthesis. Both the levels of mRNA and cell surface expression of IL-4R were not affected by addition of exogenous IL-2, and IL-4 regulation of IL-4R expression was not influenced by the immunosuppressive drug cyclosporin A. These data demonstrate that in T and B cells, IL-4 induces a transient up-regulation of IL-4 mRNA levels that is subsequently reflected in increased numbers of IL-4R displayed on the cell surface. This regulation of IL-4R expression by IL-4 provides an important mechanism for amplification of IL-4-dependent activation pathways.     

Different induction of p55 and p70/75 interleukin-2 receptor subunits in human cells

There are two interleukin-2 receptor (IL-2R) subunits (p55 and p70/75) on human lymphocytes. Induction of the expressions of these IL-2R subunits was examined by the protein kinase-C (PK-C) activator (phorbol myristate acetate, PMA) and the calcium ionophore, ionomycine (IM). IM induced predominantly p70/75 expression on human T and B cells as indicated by the results of chemical crosslinking studies and binding assays. In contrast, PMA induced p55 expression significantly. These results suggest that the calcium-calmodulin and PK-C pathways regulate p70/75 and p55 expressions differently, and indicate that these intracellular signal messengers could control the responsiveness to IL-2, changing the affinity and number of receptors in vivo.     

Differential expression of the IL-2 receptor subunits, p55 and p75 on various populations of primary peripheral blood mononuclear cells

Unstimulated PBL were examined for expression of IL-2R subunits, IL-2Rp55 and IL-2Rp75, by two-color flow cytometric analyses using mAb. NKH-1+ non-T non-B cells expressed IL-2Rp75 but not IL-2Rp55, and the IL-2Rp75 sites on purified NKH-1+ cells were determined to be 1630 sites/cell by binding of 125I-labeled TU27 mAb specific for IL-2Rp75. In the CD4+ T cell population, IL-2Rp55+ cells were significantly detected, but little or marginally of the IL-2Rp75+ cells. However, IL-2Rp75+ cells were significantly detected, but little of the IL-2Rp55+ cells in the CD8+ T cell population. The IL-2Rp75 sites on CD8+ T cells were estimated at approximate 180-410 sites/cell. In the CD4+ T cells, expression of IL-2Rp75 as well as IL-2Rp55 was induced by stimulation with PHA. IL-2Rp75+ cells, but not IL-2Rp55+ cells, were also detected in the CD14+ monocyte population. In the CD20+ B cell population, a small number of IL-2Rp55+ cells was detected, but little of the IL-2Rp75+ cells.     

Activation by IL-2, but not IL-4, up-regulates the expression of the p55 subunit of the IL-2 receptor on IL-2- and IL-4-dependent T cell lines

We have investigated the effects of IL-2 and IL-4 on different parameters of T cell activation using three T cell lines. The Th cell line L14 and the cytotoxic T cell line C30.1, both grown in IL-2-containing medium, and a line derived from C30.1 cells (line 1) cultured in IL-4 for a prolonged period were studied. All three cell lines could be activated with IL-2 or IL-4. T cell stimulation by either IL-2- or IL-4-induced identical patterns of cell size enlargement and transferrin receptor expression. However, only IL-2 up-regulated cell-surface expression of the p55 subunit of the IL-2R (p55 IL-2R) as measured by flow cytometry and RIA. This difference was also reflected by the accumulation of soluble p55 IL-2R in the culture medium. No significant increase in expression of membrane or soluble forms of p55 IL-2R was detected after IL-4 stimulation. mAb specific for p55 IL-2R which block IL-2-induced T cell growth did not affect IL-4-mediated T cell proliferation indicating that p55 IL-2R is not involved in IL-4-mediated T cell growth. Analysis of IL-4R expression performed on line 1 using biotinylated IL-4 revealed that IL-4, but not IL-2, is capable of increasing IL-4R expression. Together these results suggest that during IL-2- or IL-4-induced T cell proliferation, each lymphokine specifically up-regulates its own receptor.     

IL-4 potentiates the IL-2-dependent proliferation of mouse cytotoxic T cells

In addition to the regulation of B lymphocyte growth and differentiation, the cytokine IL-4 (BSF-1) exerts effects on T lymphocytes and other bone marrow-derived lineages. We show here that recombinant mouse IL-4 synergizes with low levels of IL-2 to increase the yield of cytotoxic activity in a primary MLR, and the proliferation of both cloned IL-2-dependent CTL lines and cells obtained from a primary MLR. IL-4 did not induce the proliferation of any of several cloned CTL cell lines on its own. It also did not replace IL-2 in stimulating the growth or reactivation of quiescent, antigen-dependent CTL clones. However, IL-4 was synergistic with IL-2 after reactivation of the quiescent cells with antigen plus IL-2. Enhancement by IL-4 of the IL-2-driven proliferation of an antigen-independent line was blocked by the addition of anti-IL-4 monoclonal antibody. Although incubation of the CTL clones with IL-4 or with IL-2 plus IL-4 induced a transient increase in the expression of the mRNA encoding the 55 kDa IL-2 receptor, no change in the number or affinity of IL-2 receptors because of IL-4 was detected. This suggests that IL-4 does not potentiate the IL-2 response by altering IL-2 receptor levels. Instead, we propose that the synergistic effect of IL-4 is mediated by a different signalling mechanism from that used by IL-2.     

Direct identification of the murine IL-2 receptor p55-p75 heterodimer in the absence of IL-2

The proteins cross-linked to the IL-2R p55 subunit were biochemically compared in distinct cell populations that varied in their capacity to express high affinity IL-2R. We directly cross-linked p75 to p55 in the absence of IL-2 for the cell populations that bear only high affinity IL-2R. Furthermore, although no endogenous IL-2 production was detected, p75 was readily cross-linked to p55 for EL4J-3.4, a p55 transfectant of EL4 that bears high affinity IL-2R. These results strongly suggest that high affinity IL-2R exist as a preformed heterodimer of p55 and p75 which do not require IL-2 for their association. Furthermore, cross-linking of three other proteins of apparent Mr of 100,000, 135,000, and 180,000 to p55 was also seen, raising the possibility of a more complex subunit composition for the IL-2R.     

Molecular area involved in the in-vitro dimerization of the murine p55 IL-2 receptor.

In order to study structure-function relationships of the M(r) 55,000 subunit of the murine IL-2R (p55 IL-2R) by epitope mapping, we have expressed the p55 IL-2R molecule in a cell-free translation system. Under these in vitro conditions, we detected the expected p55 IL-2R polypeptide initiated at Met 1 and, in addition, two internally initiated molecules at Met 64 and Met 105. In this report we describe that from such a mixture, containing three molecular species of p55 IL-2R, mAb 135D5 immunoprecipitated the polypeptide initiated at Met 105 although this N-terminally truncated form of p55 IL-2R does not contain its epitope located between amino acids 72-88. This observation can be taken as a further evidence for the formation of p55 IL-2R dimers. Finally, we identified the region implicated in the formation of p55 IL-2R dimers close to the transmembrane region of the molecule.     

Structure-function study of the p55 subunit of murine IL-2 receptor by epitope mapping.

Using a cell-free translation system we have expressed the Mr 55,000 subunit of the murine IL-2R (p55 IL-2R), which binds IL-2 with low affinity (Kd = 10 nM). Mutants and truncated forms of p55 IL-2R have been used to map the epitopes recognized by three anti-p55 IL-2R mAb: 135D5, 7D4, and 2E4. The mAb 135D5 inhibits IL-2 binding to p55 IL-2R and recognizes an epitope located between amino acids 64 to 125. This epitope can be mimicked by a synthetic peptide corresponding to the region defined by residues 72 to 88. However, the mAb 7D4 and 2E4 do not affect the IL-2 binding to p55 IL-2R. These mAb recognize an epitope of p55 IL-2R lying between residues 125 to 212 that can be mimicked with a peptide corresponding to amino acids 188 to 208. A strong correlation emerged between the experimental results on epitope mapping and predictions of potential antigenicity of murine p55 IL-2R. In addition, we described two internal initiation sites of p55 IL-2R mRNA under the in vitro conditions used leading to the production of significant amounts of N-terminal truncated p55 IL-2R proteins.     

IL-2-dependent proliferation of thymic accessory cells

Phagocytic cells of the thymic reticulum are Ia-positive accessory cells continuously produced in long term thymic stroma cultures. We show in the present paper that phagocytic cells of the thymic reticulum have R for IL-2. They share this property with splenic accessory cells produced in vitro by the same technique but have the unique property of proliferating in the presence of rIL-2. This proliferation is not enhanced by Con-A, is not linked to a lymphoid contaminant, and is specifically inhibited by an anti-IL-2R mAb.     

Trypanosoma cruzi reduces the number of high-affinity IL-2 receptors on activated human lymphocytes by suppressing the expression of the p55 and p70 receptor components

We previously established that Trypanosoma cruzi, the causative agent of Chagas' disease, has the ability to suppress expression of the p55 component of the IL-2R by activated human PBMC. We explored in this work whether the parasite alters the expression of high affinity IL-2R, responsible for the internalization of IL-2 and signal transduction. Radiobinding measurements revealed that the trypanosome indeed inhibited the expression of high affinity IL-2R. Thus, a considerably smaller number of 125I-IL-2 molecules was necessary to saturate the IL-2R on PHA-stimulated PBMC cocultured with T. cruzi than those of control PBMC that had not been exposed to the organisms. Scatchard analysis of equilibrium binding data showed that, in the presence of T. cruzi, the number of high affinity IL-2R per cell was reduced by approximately 80%. The Kd for IL-2 binding to the fewer IL-2R expressed on PBMC exposed to T. cruzi was not significantly different from that of IL-2R on nonsuppressed PBMC. Independent measurements made after cross-linking 125I-IL-2 to its specific receptors with disuccinimidylsuberate showed that both the p55 and p70 components of the IL-2R were markedly suppressed and to comparable extents. These results demonstrate for the first time that T. cruzi suppresses the expression of high affinity IL-2R by human cells, including the p70 chain of the heterodimeric IL-2R. It is noteworthy that the in vitro model system we used in this work to study the mechanisms whereby T. cruzi may induce the immunosuppression that accompanies acute Chagas' disease also lends itself to the exploration of the regulatory mechanisms governing the expression of IL-2R by human PBMC.     

Mycophenolic acid inhibits IL-2-dependent T cell proliferation,but not IL-2-dependent survival and sensitization to apoptosis

Mycophenolic acid (MPA), the active metabolite of the immunosuppressive drug mycophenolate mofetil, is a selective inhibitor of inosine 5'-monophosphate dehydrogenase type II, a de novo purine nucleotide synthesis enzyme expressed in T and B lymphocytes and up-regulated upon cell activation. In this study, we report that the blockade of guanosine nucleotide synthesis by MPA inhibits mitogen-induced proliferation of PBL, an effect fully reversed by addition of guanosine and shared with mizoribine, another inhibitor of inosine 5'-monophosphate dehydrogenase. Because MPA does not inhibit early TCR-mediated activation events, such as CD25 expression and IL-2 synthesis, we investigated how it interferes with cytokine-dependent proliferation and survival. In activated lymphoblasts that are dependent on IL-2 or IL-15 for their proliferation, MPA does not impair signaling events such as of the extracellular signal-regulated kinase 2 and Stat5 phosphorylation, but inhibits down-regulation of the cyclin-dependent kinase inhibitor p27(Kip1). Therefore, in activated lymphoblasts, MPA specifically interferes with cytokine-dependent signals that control cell cycle and blocks activated T cells in the mid-G(1) phase of the cell cycle. Although it blocks IL-2-mediated proliferation, MPA does not inhibit cell survival and Bcl-x(L) up-regulation by IL-2 or other cytokines whose receptors share the common gamma-chain (CD132). Finally, MPA does not interfere with IL-2-dependent acquisition of susceptibility to CD95-mediated apoptosis and degradation of cellular FLIP. Therefore, MPA has unique functional properties not shared by other immunosuppressive drugs interfering with IL-2R signaling events such as rapamycin and CD25 mAbs.     

IL-12p70-dependent Th1 induction by human B cells requires combined activation with CD40 ligand and CpG DNA

The detection of microbial molecules via Toll-like receptors (TLR) in B cells is not well characterized. In this study, we found that both naive and memory B cells lack TLR4 (receptor for LPS) but express TLR9 (receptor for CpG motifs) and produce IL-6, TNF-alpha, and IL-10 upon stimulation with CpG oligonucleotides (ODN), synthetic mimics of microbial DNA. Consistent with the lack of TLR4, purified B cells failed to respond to LPS. Similar to CpG ODN, CD40 ligand (CD40L) alone induced IL-6, TNF-alpha, and IL-10. Production of these cytokines as well as IgM synthesis was synergistically increased when both CpG ODN and CD40L were combined. Unlike IL-6, TNF-alpha, and IL-10, the Th1 cytokine IL-12p70 was detected only when both CpG ODN and CD40L were present, and its induction was independent of B cell receptor cross-linking. CpG ODN did not increase the capacity of CD40L-activated B cells to induce proliferation of naive T cells. However, B cells activated with CpG ODN and CD40L strongly enhanced IFN-gamma production in developing CD4 T cells via IL-12. Together, these results demonstrate that IL-12p70 production in human B cells is under the dual control of microbial stimulation and T cell help. Our findings provide a molecular basis for the potent adjuvant activity of CpG ODN to support humoral immune responses observed in vivo, and for the limited value of LPS.     

Cell surface T3 expression requires the presence of both alpha- and beta-chains of the T cell receptor

We have identified a surface T3- Jurkat variant which has a defective alpha-chain but which possesses an intact beta-chain. The transfection of a functional mouse alpha-chain into this human T cell induces the expression of surface T3 molecules associated with mouse alpha-human beta-heterodimers detected by anticlonotypic antibodies. Treatment of the transfectant with anti-T3, anti-mouse Ti-alpha, or anti-human Ti-beta antibodies clears all Ti-T3 complexes from the surface. These results demonstrate that functional alpha- and beta-chains are both required for expression of T3 on the cell membrane, and that the Ti heterodimers present and associated with T3 on Jurkat cells involve only alpha- and beta-chains.     

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

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

IL-1 and IL-4 modulate IL-1 receptor expression in a murine T cell line

The combination of IL-1 and IL-4 stimulates the proliferation of certain murine T cell populations. Although this effect has been best characterized for a number of murine type 2 Th cell (Th2) clones, the mechanism(s) by which these cytokines effect this response is unclear. We have examined the effects of IL-1 and IL-4 on IL-1R expression by MD10 cells, and IL-1-responsive murine T cell line. These cells bear specific IL-1R, which bind human and murine IL-1 alpha and -beta. The measured apparent IL-1R dissociation constant ranged from 41 to 255 pM using 125I-HrIL-1 alpha. Cross-linking studies demonstrated two different 125I-HrIL-1 alpha binding complexes having Mr of 70,000 and 130,000 to 156,000. When removed from passage conditions and placed in non-growth factor-supplemented media, MD10 IL-1R expression spontaneously increased two- to fourfold over the first 11 to 12 h of culture followed by a decline. This phenomenon is partially inhibitable by cycloheximide suggesting that protein synthesis is involved. In agreement with other reports, HrIL-1 alpha down-regulated the expression of its own receptor with an ED50 of between 1 and 10 pM HrIL-1 alpha for this effect. In most experiments, low amounts of HrIL-1 alpha (1.0, 0.1 pM) significantly augmented IL-1R expression. Scatchard analysis of data obtained with all HrIL-1 alpha treatment conditions showed that the effects were due to a change in receptor number, not affinity. Significantly, purified murine IL-4 (MpIL-4) augmented MD10 IL-1R expression in both a time- and dose-dependent fashion. In the presence of 50 U/ml MpIL-4, MD10 IL-1R expression increased two- to threefold after 24 h without a change in receptor affinity. When MpIL-4 (50 U/ml) and various amounts of HrIL-1 alpha (.01-1000 pM) were co-added, the down-regulatory effect of high levels of HrIL-1 alpha was significantly antagonized. When added to cultures after 24 h of HrIL-1 alpha (100 pM) treatment, MpIL-4 reversed the IL-1R down-regulatory effect induced by high levels of HrIL-1 alpha. Finally, when combined in MD10 proliferation assays, MpIL-4 synergistically enhanced the proliferation of MD10 cells treated with suboptimal levels of HrIL-1 alpha.(ABSTRACT TRUNCATED AT 400 WORDS)