Altered expression of V1a receptors mRNA in the brain and kidney after myocardial infarction and chronic stress

Vasopressin released during myocardial infarction and in response to stress regulates blood pressure through multiple actions exerted in the brain, cardiovascular system and kidney. The aim of the present study was to determine whether myocardial infarction influences expression of vasopressin V1a receptor (V1aR) mRNA and protein in the brain and kidney and whether stress has an impact on expression of these parameters during the post-infarct state. Male, adult Sprague Dawley rats were subjected to myocardial infarction or sham surgery. Seven days later some rats were exposed to mild stress for 4weeks whereas other stayed at rest. Tissue fragments were harvested from four groups of rats (control, infarct, stress, infarct+stress). Expression of V1aR mRNA (Real time PCR) was determined in the preoptic, diencephalic, mesencephalopontine and medullary regions of the brain and in the renal cortex and medulla. Protein V1aR expression (Western blotting) was determined in the brain mesencephalopontine region and in the kidney medulla. In the preoptic, diencephalic, and mesencephalopontine regions, V1aR mRNA expression was significantly lower in the infarcted rats than in the sham-operated unstressed controls. The infarcted rats manifested also lower expression of V1aR protein in the mesencephalopontine region than the other groups. The stressed group demonstrated significantly higher V1aR mRNA expression in the brain medulla and in the renal cortex and renal medulla than the control group. In all brain regions and in the kidney, V1aR mRNA expression was significantly higher in the stressed rats than in the infarcted rats. The stressed rats showed also higher expression of V1aR protein in the renal medulla than the other groups. It is concluded that myocardial infarction and chronic stress cause significant but differential changes in the regulation of V1a receptors expression in the brain and the kidney.

B cell activating factor, its role in autoimmunity, and targeting in autoimmune diseases

B cell activation factor (BAFF), a recently identified member of the tumour necrosis factor (TNF) family, is a key survival factor during B cell maturation and is essential for the development of B cell tolerance. Breakdown of the regulation of BAFF expression results in excessive BAFF production that impairs B cell tolerance and leads to autoimmune phenomena. Consistent with this, BAFF levels are elevated in plasma of patients with various autoimmune diseases. BAFF is considered to be one of the principal factors that regulate the size and composition of B cell compartment. BAFF acts as an important driving factor for B cell hyperplasia and autoantibody production in autoimmune processes. Thus BAFF has become a very attractive target for the treatment of autoimmune diseases with an altered B cell function. Results of clinical trials have confirmed a crucial role of BAFF in the pathogenesis of rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). BAFF inhibitors in the treatment of RA, SLE and other autoimmune diseases are under intensive investigation. However, BAFF biology remains poorly understood. Nonetheless, results of the ongoing studies may enable the development of a new generation of BAFF inhibitors with more selective efficacy and increased safety (Fig. 2, Ref. 92). Full Text (Free, PDF) www.bmj.sk.

Decreased expression of integrins by hematopoietic cells in patients with rheumatoid arthritis and anemia: relationship with bone marrow cytokine levels

BACKGROUND AND OBJECTIVES

In order to gain a better insight into the pathogenesis of the anemia of chronic disease (ACD) accompanying rheumatoid arthritis, we analyzed the density of the integrins very late antigen (VLA) 4 and VLA-5 on the surface of erythroblasts from bone marrow in patients with rheumatoid arthritis. We also measured the concentration of interleukin (IL) 3 and tumor necrosis factor (TNF) alpha in bone marrow. Finally, we analyzed the relationship between integrin expression on hematopoietic cells and the degree of anemia and concentration of cytokines in bone marrow in patients with rheumatoid arthritis.

RESULTS

Patients with rheumatoid arthritis who also had ACD were found to have lower hemoglobin levels and higher C-reactive protein and erythrocyte sedimentation rate compared to patients who had rheumatoid arthritis without ACD or osteoarthritis of the hip. The mean bone marrow concentration of IL-3 was elevated in patients with rheumatoid arthritis and ACD compared to those without ACD or patients with osteoarthritis. IL-3 concentration in bone marrow showed a significant negative correlation with VLA-4 and VLA-5 expression on erythroblasts, but only in patients with rheumatoid arthritis and ACD.

CONCLUSION

Patients with rheumatoid arthritis and ACD have abnormal erythroblasts (decreased VLA density), possibly through an effect on early stages of erythroblast development. Increased levels of IL-3 and the negative correlation between IL-3 concentration in bone marrow and expression of the integrins VLA-4 and VLA-5 may suggest positive feedback between erythroblasts and IL-3, probably associated with decreased sensitivity of bone marrow erythroblasts to IL-3.

High levels of IL-17 in rheumatoid arthritis patients: IL-15 triggers in vitro IL-17 production via cyclosporin A-sensitive mechanism

Recent data suggest that IL-15 plays an important role in the pathogenesis of rheumatoid arthritis. In the present study, we hypothesized that elevated in the joints of rheumatoid arthritis, but not osteoarthritis, patients, IL-15 may exert its proinflammatory properties via the induction of IL-17, a cytokine known to stimulate synoviocytes to release several mediators of inflammation including IL-6, IL-8, GM-CSF and PGE2. To test this hypothesis, we first measured the levels of IL-17 and IL-15 using specific ELISA and found that synovial fluids of patients with rheumatoid arthritis, but not with osteoarthritis, contain high levels of these cytokines. A strong correlation between IL-15 and IL-17 levels in synovial fluids was observed. Among tested factors, LPS and TNF-alpha failed, IL-15 and IL-2 were equipotent, and PMA + ionomycin was far more efficient in the induction of IL-17 secretion by PBMCs isolated from healthy blood donors. Interestingly, synovial fluid cells, in contrast to PBMCs isolated from patients with rheumatoid arthritis, but not osteoarthritis, respond to PMA + ionomycin with much lower, comparable to IL-15-triggered IL-17 secretion. Moreover, PMA + ionomycin-triggered IL-17 secretion is completely or partially blocked in the presence of low doses of cyclosporin A or high doses of methylprednisolone, respectively. IL-15-triggered IL-17 secretion by PBMCs was completely inhibited by these drugs. Thus, our results suggest for the first time that IL-15 may represent a physiological trigger that via cyclosporin A and steroid sensitive pathways leads to the overproduction of IL-17 in the joints of rheumatoid arthritis patients.

Immunoglobulin-cytokine fusion molecules: the new generation of immunomodulating agents

Advances in the understanding of genetic engineering, protein expression, and Ig function have come together to allow for the rapid synthesis and production of a novel generation of potent immunomodulating reagents. Selective approaches that allow the isolation of desired specificities will be elucidated through meticulous engineering techniques. This, in turn, may eventually result in the fulfillment of the tremendous potential of engineered proteins for therapeutic and diagnostic applications.

Interleukin-15 signals T84 colonic epithelial cells in the absence of the interleukin-2 receptor beta-chain

Interleukin-15 (IL-15) shares many biological functions with interleukin-2 (IL-2) due to common receptor components. IL-15 binds to the IL-2 receptor (IL-2R) beta-chain and the common gamma-chain receptor in addition to one other IL-15 binding receptor protein (IL-15R alpha). Both IL-2R beta- and gamma-chains are required to promote cell growth in hematopoietic cells. The colonic cryptlike epithelial cell line T84 contains the common gamma-chain but lacks the IL-2R beta-chain. We report IL-15R alpha-chain mRNA in T84 cells with the use of reverse transcriptase-polymerase chain reaction. T84 and normal colonic epithelial cells bind a FLAG-IL-15 fusion protein in immunoperoxidase and flow cytometric experiments. In addition, IL-15, but not IL-2, accelerates and enhances the development of transepithelial resistance across T84 monolayers in a dose-dependent fashion. We conclude that normal and T84 colonic epithelial cells express IL-15R alpha and are able to bind IL-15. IL-15 can deliver a nonproliferative functional signal in the absence of IL-2R beta-chain in T84 cells.

Distribution of IL-15 receptor alpha-chains on human peripheral blood mononuclear cells and effect of immunosuppressive drugs on receptor expression

IL-15, a newly described cytokine exerting IL-2-like in vitro activities, binds to and induces proliferation of cells co-expressing IL-15R alpha, IL-2R beta, and IL-2R gamma chains. To study the expression of human IL-15R alpha chains, we have utilized tagged human IL-15 protein and FACS analysis. In contrast to resting cells, mitogen-activated macrophages, NK cells, and CD4+ and CD8+ T cells express IL-15R alpha chains. Neither IL-2R alpha nor IL-2R beta chains are required for IL-15 binding. Dexamethasone, but not cyclosporine or rapamycin, blocks mitogen-induced IL-15R alpha expression. Dexamethasone-pretreated cells respond to IL-15 poorly, while the response to IL-2 is not affected. Thus, despite structural and functional similarities between IL-2R alpha and IL-15R alpha chains, the activation-triggered mechanisms of induction are different. Since IL-15R alpha chain is necessary and sufficient for IL-15 binding, regulation of IL-15R alpha expression may represent a new target for T cell-directed pharmacologic intervention.

Distribution of IL-15 receptor alpha-chains on human peripheral blood mononuclear cells and effect of immunosuppressive drugs on receptor expression

IL-15, a newly described cytokine exerting IL-2-like in vitro activities, binds to and induces proliferation of cells co-expressing IL-15R alpha, IL-2R beta, and IL-2R gamma chains. To study the expression of human IL-15R alpha chains, we have utilized tagged human IL-15 protein and FACS analysis. In contrast to resting cells, mitogen-activated macrophages, NK cells, and CD4+ and CD8+ T cells express IL-15R alpha chains. Neither IL-2R alpha nor IL-2R beta chains are required for IL-15 binding. Dexamethasone, but not cyclosporine or rapamycin, blocks mitogen-induced IL-15R alpha expression. Dexamethasone-pretreated cells respond to IL-15 poorly, while the response to IL-2 is not affected. Thus, despite structural and functional similarities between IL-2R alpha and IL-15R alpha chains, the activation-triggered mechanisms of induction are different. Since IL-15R alpha chain is necessary and sufficient for IL-15 binding, regulation of IL-15R alpha expression may represent a new target for T cell-directed pharmacologic intervention.

Analysis of IL-2, IL-4 and their receptors in clonally-related cell lines derived from a patient with a progressive cutaneous T-cell lymphoproliferative disorder

Three clonally related T-cell lymphoma lines (PB-1, 2A, and 2B) were examined for expression of IL-2, IL-4, and their receptors. All three lines were derived from a single patient who had an atypical, progressive T-cell lymphoproliferative disorder involving primarily skin (Davis, T.H. et al. 1992, N. Engl. J. Med. 326:1115). The PB-1 cell line was obtained from a relatively early, clinically indolent stage of the cutaneous lymphoma, whereas the 2A and 2B lines were established from a late, aggressive stage of the lymphoma. Reverse-transcriptase PCR performed with primer pairs specific for IL-2 and IL-4 showed that no mRNA coding for these cytokines was present in any of the lines with the exception of IL-4 mRNA in the 2A line. No IL-4 protein, however, was found in any of the cell lines including 2A by immunocytochemical staining with anti-IL-4 mAb. Accordingly, no bioactive IL-4 was present in the supernatants of these lines. In contrast, all three T-cell lymphoma lines contained mRNA for IL-2R alpha, IL-2R beta, IL-4R and common gamma chain. Immunocytochemical analysis revealed that only the PB-1 line stained strongly with mAbs specific for IL-2R alpha, IL-2R beta, and IL-4R whereas the 2A and 2B lines showed only limited staining with these mAbs. In contrast to expression of IL-2R alpha and IL-4R primarily on the cell surface, IL-2R beta was localized mainly in the cell cytoplasm. Testing supernatants of the cell lines by ELISA for the presence of soluble alpha chain of the IL-2R (sIL-2R) has shown that only PB-1 secreted a large amount of sIL-2R, whereas the 2A and 2B lines secreted lesser amounts. Furthermore, the PB-1 cells expressed a relatively large number of IL-4R as determined by IL-4 binding studies using an IL-4-alkaline phosphatase fusion protein. The remaining two lines displayed only limited binding of IL-4. Addition of IL-2 and/or IL-4 to the culture medium did not modulate growth of PB-1 and the other two lines. These findings may indicate that at least some types of T-cell lymphoma evolve from cells which lose the capacity to synthesize T-cell autocrine growth factors such as IL-2 and IL-4, and show progressive loss of receptors for these cytokines.

Intragraft IL-15 transcripts are increased in human renal allograft rejection

IL-15, a novel growth factor made by a variety of cells, stimulates T cell proliferation in a fashion similar to IL-2. IL-2 transcripts are not routinely found in rejecting human renal allografts at the time of clinically evident rejection. However, T cell proliferation continues as the rejection progresses. We postulated that IL-15 may be actively transcribed during clinical rejection and account, at least in part, for the ongoing T cell expansion. RNA was extracted from renal biopsies and reverse transcribed to cDNA which was used as template for competitive PCR. IL-2 mRNA was detected in just 3 of the 45 biopsy samples. IL-15 transcripts were detected in all renal biopsy specimens and was significantly increased in specimens obtained from rejecting as compared with nonrejecting renal allografts. IL-15 transcription correlates with rejection and may play an important role in T cell mediated rejection.

The intragraft gene activation of markers reflecting T-cell-activation and -cytotoxicity analyzed by quantitative RT-PCR in renal transplantation

T-cell activation is the key event in the development of acute allograft rejection and precedes clinically apparent organ damage. We have performed competitive RT-PCR to quantify the intragraft gene expression for T-cell associated cytokines (IL-2, IL-4, IL-7, IL-15), CTLA4 and cytotoxic lymphocyte specific molecules to test their potential as rejection markers and to further elucidate mechanisms involved in graft rejection. RNA was isolated from snap-frozen portions of core biopsies obtained for the evaluation of graft dysfunction in 34 adults and 8 children. Reverse transcription derived cDNA was coamplified with a known amount of a competitor (a mutated target gene fragment) and normalized for the house keeping gene GAPDH. IL-2, the principal T-cell growth factor and IL-4 were not detectable in any biopsy at the time of histologically apparent rejection. Transcripts of the novel cytokine IL-15 were found in all dysfunctioning grafts and in two donor kidneys prior to reperfusion. CTLA-4, expressed in activated T-cells after costimulation by CD28 was uniformly present post transplantation, but not in the two donor kidneys. Transcripts for IL-7 (p < 0.001), IL-15 (p < 0.0005), CTLA4 (p = 0.04), granzyme B (p < 0.00015) and perforin (p < 0.0003) showed a significant correlation to acute rejection episodes. Heightened gene expression declined rapidly after initiation of rejection treatment. Fas-ligand mRNA gene expression was upregulated in both acute and chronic rejections. While this study shows that competitive RT-PCR is a reliable diagnostic tool to detect acute rejection in renal core biopsies, a future challenge will be to identify molecular markers of evolving rejections utilizing RT-PCR in sequential samples of fine needle aspirations, urine and blood.

Muscarinic cholinergic receptors of rat lymphocytes: effect of antigen stimulation and local brain lesion

Expression of muscarinic cholinergic receptors (m-AchR) on thymocytes and lymphocytes supports a hypothesis of direct interaction between autonomic innervation and the immune system. Using a muscarinic antagonist, [3H]quinuclidinyl benzilate ([3H]QNB), we revealed different characteristics of m-AchR expression in lymphocytes isolated from the spleen, peripheral blood or thymus of Wistar rats. To further explore the mechanisms controlling m-AchR expression in lymphocytes we have addressed two questions: (i) whether local brain lesions will affect [3H]QNB binding activity of lymphocytes and (ii) whether different antigens will alter m-AchR expression in lymphocytes. We report here that electrolytic lesions of the area hypothalamica anterior resulted in a significant increase in [3H]QNB-specific binding in thymocytes 7 days after neurosurgery. In contrast, local lesions of other hypothalamic structures (area preoptica medialis, area hypothalamica posterior) or sensomotor cortex did not affect m-AchR expression in thymocytes. No apparent changes of [3H]QNB-binding activity were found in lymphocytes isolated from rat spleen or peripheral blood after operation. Both T-cell-dependent (sheep red blood cells) and T-cell-independent (Vi antigen of Salmonella typhi) antigens induced a significant increase in [3H]QNB-specific binding in spleen lymphocytes while [3H]QNB-binding activity of peripheral blood lymphocytes did not change. A different pattern of lymphocyte m-AchR expression found in various lymphoid tissues after immunization or brain lesions suggests a local involvement of cholinergic mechanisms in neuroimmune interaction.

Interleukin-2 (IL-2) induces tyrosine kinase-dependent translocation of active raf-1 from the IL-2 receptor into the cytosol

Stimulation of the interleukin-2 (IL-2) receptor results in phosphorylation and activation of cytosolic Raf-1 serine/threonine kinase. Herein, we report that enzymatically active Raf-1 is physically associated with the IL-2 receptor beta chain (p75) in T-cell blasts. Following stimulation with IL-2, Raf-1 dissociates from the IL-2 receptor complex and translocates to the cytosol. Genistein, a protein tyrosine kinase inhibitor, prevents the dissociation of enzymatically active Raf-1 from the ligand-stimulated IL-2 receptor complex. These data favor a model of IL-2 receptor activation in which an IL-2-activated protein tyrosine kinase phosphorylates the IL-2 receptor and/or receptor-bound Raf-1. Following tyrosine phosphorylation, enzymatically active Raf-1 dissociates from the IL-2 receptor and translocates into the cytosol.

Intoxication of high affinity IL-2 receptor positive thymocytes blocks early stages of T cell maturation

Early murine fetal thymocytes express functional, high affinity IL-2 receptors as determined by: (i) the presence of IL-2R beta chain (p75) mRNA; (ii) IL-2 (10 U/ml) induced cell proliferation/cellular maturation in lobe submersion cultures (LSC). Under the influence of IL-2, early thymocytes differentiate in vitro into more mature, early single positive CD4-CD8+ followed in vivo by double positive CD4+CD8+ and single positive CD4+CD8-T and CD4-CD8+ thymocytes. Specific intoxication of high affinity IL-2R positive thymocytes by recombinant interleukin-2-diphtheria toxin-related fusion protein (DAB486-IL-2) results in transient, dose dependent blockade of in vivo and in vitro thymocyte maturation. DAB486-IL-2 induced effects upon in vivo maturation are reversible within 2 weeks after cessation of drug administration. Taken together, these results demonstrate the expression of functional, high affinity IL-2 receptors on early thymocytes. Elimination of high affinity IL-2 receptor positive thymocytes with DAB486-IL-2 results in transient blockade of T cell maturation. Since DAB486-IL-2 is now in clinical trial, it is reassuring to note that it does not permanently disrupt thymic maturation.

Nicotinic receptors of rat lymphocytes during adjuvant polyarthritis

Resting rat lymphocytes and thymocytes express specific [3H]nicotine binding sites with Kd values 7.5 and 10 nM, respectively. Resting T lymphocytes express 5-7 times as many binding sites (1,500/cell) as do unfractionated thymocytes (250/cell), suggesting that only a subset of thymocytes express nicotinic binding sites. Neither macrophages nor B cells bear nicotinic receptors. [3H]nicotine binding to lymphocytes is competitively inhibited in the presence of the cholinergic agonist carbamylcholine with IC50 = 100 microM. The number of receptors expressed on draining lymph node lymphocytes is upregulated during the course of adjuvant induced polyarthritis: maximum of [3H]nicotine binding is observed at day 21 of the disease with a subsequent decline reaching basal level at day 36. These results show the presence of [3H]nicotine binding sites on T lymphocytes and suggest their involvement in the later immune phase of adjuvant induced polyarthritis.

Interleukin-2 receptor regulates activation of phosphatidylinositol 3-kinase

Interleukin-2 (IL-2) stimulates proliferation of T lymphocytes and is involved in the activation of both natural killer and lymphokine-activated killer precursor cells. The intracellular messengers which mediate IL-2-dependent events have not yet been identified. IL-2 receptor is not a protein-tyrosine kinase. Activation of a cellular protein-tyrosine kinase and direct association of a protein-tyrosine kinase activity with the IL-2 receptor occurs within minutes of IL-2 stimulation. We investigated the activation of phosphatidylinositol 3-kinase (PI 3-kinase) in IL-2-mediated signal transduction using the IL-2-dependent murine T-cell line, CTLL-2, and human phytohemagglutinin-stimulated peripheral blood lymphocytes (phytohemagglutinin blasts). Within a minute following stimulation of these cells with IL-2, PI 3-kinase activity could be detected in antiphosphotyrosine (anti-P-Tyr) antibody immunoprecipitates. IL-2 triggered a direct association of PI 3-kinase with the IL-2 receptor as detected in immunoprecipitates using anti-IL-2 receptor beta chain antibody. In vivo labeled CTLL-2 cells have a time-dependent increase in D-3-phosphorylated polyphosphoinositides following stimulation with IL-2. This is the first group of second messengers identified in IL-2-mediated signal transduction.

Stimulated kidney tubular epithelial cells express membrane associated and secreted TNF alpha

Tumor necrosis factor-alpha (TNF alpha) is a pleiotropic, pro-inflammatory peptide cytokine which promotes immune renal injury, and participates in T cell activation. It is produced by macrophages, T cells, and some non-hematopoietic cells, and is cytotoxic in picogram quantities. As renal tubular epithelial cells (TEC) bearing MHC class II (Ia) antigens and adhesion molecules (ICAM-1) can act as immune accessory cells, the ability of TEC to produce costimulatory cytokines could augment TEC accessory capacity in vivo. We report that transformed TEC express low levels of TNF alpha in response to LPS or IL-1 alpha as a secreted product and as a cytotoxic membrane associated molecule displayed on the cell surface. Surface labelling and immunoprecipitation studies of TEC detect a number of bands including a prominent 26 kD protein, which is the predicted size of TNF alpha precursor. TNF alpha mRNA transcripts were also detected by in situ hybridization in cortical tubules of C3H/FeJ mice injected with LPS, demonstrating the capacity of normal tubular epithelial cells to express TNF alpha in vivo. This report demonstrates for the first time the ability of kidney tubular cells to express TNF alpha protein and that membrane associated TNF alpha is not limited to hematopoietic cells. The function of small amounts of TNF displayed on the surface of tubular cells may be amplified by the abundance of these cells within the renal cortex, and may allow TEC to modulate immune responses within the kidney during inflammation.

Muscarinic antagonist binding to intact rat thymocytes

The presence and properties of muscarinic receptors on intact rat thymocytes have been studied by the use of the muscarinic antagonists [3H]-3-quinuclidinyl benzilate (3H-QNB), [3H]-4-N-methylpiperidinyl benzilate ([3H]-NMBP) and [3H]-N-methylscopolamine (3H-NMS). The course of binding of 3H-QNB reveals a maximum at 5 min, and shows a subsequent decrease of bound radioactivity, suggesting internalization of the receptor 3H-3-QNB complex. This phenomenon has also been studied by the use of another muscarinic antagonist, 3H-NMPB, which has a faster on and off rate than 3H-QNB, and which may be rapidly displaced by an excess of unlabeled muscarinic antagonist, atropine. Bound 3H-NMPB is highly susceptible to atropine displacement only within the first two minutes of incubation with thymocytes at 37 degrees C. The kinetics of binding of the less lipophilic muscarinic antagonist 3H-NMS to thymocytes, show no maximum as a function of incubation time and 3H-NMS is susceptible to displacement by atropine up to 30 min incubation time with the thymocytes. These data suggest that binding of lipophilic benzilate type antagonists (3H-QNB and 3H-NMPB) may be followed by internalization and/or isomerization of the receptor-benzilate antagnist complex while the 3H-NMS-receptor complex is not subject to these processes.

Rat thymocytes release a factor which inhibits muscarinic ligand binding

Rat thymocytes incubated in RPMI medium for 120-300 min release a soluble factor of molecular weight below 10,000 Da which inhibits the binding of 3H-labeled muscarinic antagonists in an uncompetitive manner, i.e. it reduces maximal specific binding (Bmax) without changing the affinity of the ligand (KD). This factor inhibited muscarinic antagonist binding on thymocytes and rat cerebral cortex cellular membranes. Thymocytes from hydrocortisone-treated rats produced more factor per mg cell protein than thymocytes from untreated rats. The activity of the factor was unaffected by incubation with Cd2+ (1 mM) or Zn2+ (1 mM) or EDTA (1 mM), and its protein nature is supported by the following findings: it was trypsin sensitive, heat denaturated at 56 degrees C and precipitated by (NH4)2SO4 (40%, w/v). Inhibition by the factor was apparently irreversible after 1 h of incubation.

Muscarinic receptors and receptor-mediated actions on rat thymocytes

Rat thymocytes possess a single class of saturable, high affinity binding sites for muscarinic antagonists of the benzilate type such as [3H]3-quinuclidinyl benzilate ([3H]3-QNB). The average number of receptors per cell is 3000 and the equilibrium dissociation constant of [3H]3-QNB on intact cells is 7.5 nM. In the work reported here we found that perturbation of the thymocyte membrane by addition of phytohemagglutinin (4 micrograms/ml) caused a transient increase in muscarinic antagonist binding, and hydrocortisone (100 mg/kg s.c.) treatment of rats for 2 days prior to sacrifice increased the average number of muscarinic receptor sites on thymocytes by 100%. Atropine treatment, which in other tissues causes increased muscarinic receptor concentration, did not alter the receptor number on thymocytes. Binding of carbachol to the receptor on intact cells resulted in inhibition of cAMP synthesis and stimulation of cGMP synthesis. These muscarinic agonist effects were each inhibited by the simultaneous addition of the muscarinic antagonist atropine (5 X 10(-5) M). No stimulation of phosphatidylinositol turnover by muscarinic agonists was observed.