CD28 and secretory immunoglobulin A-dependent activation of eosinophils: inhibition of mediator release by the anti-allergic drug, suplatast tosilate.

BACKGROUND: Eosinophils are major effector cells in allergic diseases. After their recruitment to sites of inflammation, they contribute to the pathophysiology of the disease by releasing granule proteins and cytokines. Suplatast tosilate (IPD-1151T), a new anti-allergic agent, has shown beneficial effect in the treatment of asthma, associated with reduced bronchoalveolar lavage eosinophil infiltration and eosinophilic cationic protein (ECP) release in serum and sputum. OBJECTIVE: We investigated whether suplatast tosilate could exert direct effects on human eosinophil activation. METHODS: Eosinophils from hypereosinophilic patients or normal donors were purified by Percoll gradient and the magnetic cell separation system. Chemotaxis was studied using the Boyden chamber technique using three chemoattractants, formyl-methionine-leucine-phenylalanine (fMLP), IL-5 and eotaxin. Oxidative metabolism was determined by a luminol-dependent chemiluminescence assay after activation with eotaxin or secretory IgA (sIgA). The release of ECP and eosinophil derived neurotoxin (EDN) was measured by radioimmunoassay and cytokine production was determined by ELISA following activation with sIgA or anti-CD28. RESULTS: The chemotactic response to fMLP, IL-5 and eotaxin was significantly inhibited by IPD-1151T. Suplatast tosilate was partially inhibiting the release of reactive oxygen species (ROS) induced by eotaxin and sIgA. Activation by sIgA and CD28 ligation resulted in the release of ECP and EDN, which was inhibited by IPD-1151T. Upon activation by anti-CD28, only IL-13 production was inhibited by IPD-1151T, whereas release of IL-2 and IFN-gamma was not affected. IL-10 release induced by sIgA was also inhibited by IPD-1151T. Additionally, the pro-inflammatory cytokine IL-6, which was secreted following anti-CD28 and sIgA stimulation, was strongly inhibited by IPD-1151T. CONCLUSION: Through inhibition of chemotaxis, IPD-1151T might limit the number of eosinophils at the inflammation site. Furthermore, it could reduce the pathological potential of eosinophils by inhibiting the release of ROS and cationic proteins, main inflammatory mediators produced by eosinophils. Moreover, the inhibition of immunoregulatory cytokines released by eosinophils could locally modify the immune response.

Inhibitory effects of ketotifen on eotaxin-dependent activation of eosinophils: consequences for allergic eye diseases.

BACKGROUND: The aim of this study was to investigate the effects of ketotifen on different parameters of human eosinophil functions, namely chemotaxis, oxidative metabolism and mediator release, induced after activation. METHODS: Eosinophils from hypereosinophilic patients or normal donors were purified by Percoll gradient and the magnetic cell separation system. Chemotaxis was studied using the Boyden chamber technique using three potent chemoattractants: formyl-methionine-leucine-phenylalanine (fMLP), interleukin (IL)-5 and eotaxin. Oxidative metabolism was determined by a luminol-dependent chemiluminescence assay after activation with eotaxin or secretory immunoglobulin A (sIgA). The release of eosinophil cationic protein (ECP) and eosinophil derived neurotoxin (EDN) was measured by radioimmunoassay after activation with sIgA. RESULTS: At pharmacologically active concentrations and in a dose-dependent manner, ketotifen significantly inhibited the chemotaxis of eosinophils to fMLP, IL-5 and eotaxin. The production of reactive oxygen species induced by eotaxin and sIgA was decreased by ketotifen, showing a more pronounced effect when cells were activated by eotaxin. Activation by sIgA resulted in ECP and EDN release, which was partially inhibited by ketotifen. CONCLUSIONS: Through inhibition of chemotaxis, ketotifen might limit the number of eosinophils at the inflammation site during allergic reaction. Furthermore, inhibition by ketotifen of main inflammatory mediators release suggests a potential role of the drug in limiting the pathological potential of eosinophils.

Human eosinophils and human high affinity IgE receptor transgenic mouse eosinophils express low levels of high affinity IgE receptor, but release IL-10 upon receptor activation.

FcepsilonRI expressed by human eosinophils is involved in IgE-mediated cytotoxicity reactions toward the parasite Schistosoma mansoni in vitro. However, because receptor expression is low on these cells, its functional role is still controversial. In this study, we have measured surface and intracellular expression of FcepsilonRI by blood eosinophils from hypereosinophilic patients and normal donors. The number of unoccupied receptors corresponded to approximately 4,500 Ab binding sites per cell, whereas 50,000 Ab binding sites per cell were detected intracellularly. Eosinophils from patients displayed significantly more unoccupied receptors than cells from normal donors. This number correlated to both serum IgE concentrations and to membrane-bound IgE. The lack of FcepsilonRI expression by mouse eosinophils has hampered further studies. To overcome this fact and experimentally confirm our findings on human eosinophils, we engineered IL-5 x hFcepsilonRIalpha double-transgenic mice, whose bone marrow, blood, spleen, and peritoneal eosinophils expressed FcepsilonRI levels similar to levels of human eosinophils, after 4 days culture with IgE in the presence of IL-5. Both human and mouse eosinophils were able to secrete IL-10 upon FcepsilonRI engagement. Thus, comparative analysis of cells from patients and from a relevant animal model allowed us to clearly demonstrate that FcepsilonRI-mediated eosinophil activation leads to IL-10 secretion. Through FcepsilonRI expression, these cells are able to contribute to both the regulation of the immune response and to its effector mechanisms.

IL-4 production by human polymorphonuclear neutrophils.

Polymorphonuclear neutrophils (PMN) are phagocytic cells, able to secrete a large range of cytokines, including inflammatory cytokines, chemokines, as well as the Th1 cytokines interferon-gamma (IFN-gamma) and interleukin (IL)-12. Although PMN do not seem to express IL-10 and IL-13, no information exists on the ability of PMN to produce IL-4. Therefore intracellular flow cytometry was performed in the presence or absence of Brefeldin A. Similarly to eosinophils, freshly isolated neutrophils from normal donors contained low amounts of IL-4, which significantly increased upon culture with Brefeldin A (P < 0001). Immunostaining performed on cytospin preparations of normal granulocytes confirmed the presence of intracellular IL-4. Using a highly sensitive ELISA, the levels of IL-4 secreted by cultured PMN and peripheral blood mononuclear cells (PBMC) were compared. PBMC secrete up to 60 times more IL-4 as PMN but, in the presence of calcium ionophore, only PMN showed a slight but significant increase in IL-4 secretion (P < 0.05). In conclusion, we report here the presence within human PMN of intracellular IL-4, which can at least partly be released under calcium ionophore stimulation. The relevance of this production of IL-4 by human PMN is discussed.

Expression of CD28 and CD86 by human eosinophils and role in the secretion of type 1 cytokines (interleukin 2 and interferon gamma): inhibition by immunoglobulin a complexes.

Eosinophils are the source of various immunoregulatory cytokines, but the membrane molecules involved in their secretion have not been clearly identified. Here we show that peripheral blood eosinophils from hypereosinophilic patients could express membrane CD86 but not CD80. The T cell costimulatory molecule CD28 is also detected on the eosinophil surface. CD28 ligation but not CD86 ligation resulted in interleukin (IL)-2 and interferon (IFN)-gamma secretion by eosinophils, whereas IL-4, IL-5, and IL-10 were not detected. In contrast to T cells requiring two signals for effective stimulation, CD28 ligation alone was sufficient for optimal eosinophil activation. Eosinophil-derived IL-2 and IFN-gamma were biologically active, as supernatants from anti-CD28-treated cells were able to induce CTLL-2 proliferation and major histocompatibility complex class II expression on the colon carcinoma cell line Colo 205, respectively. Addition of secretory immunoglobulin (Ig)A-anti-IgA complexes, which could induce the release of IL-10, very significantly inhibited both CD28-mediated IL-2 and IFN-gamma release. These results suggest that the release of type 1 (IFN-gamma and IL-2) versus type 2 cytokines by eosinophils is not only differential but also dependent on cross-regulatory signals. They confirm that through activation of costimulatory molecules, eosinophils could function as an immunoregulatory cell involved in the release of both type 1 and type 2 cytokines.

Expression of Th1 and Th2 immunoregulatory cytokines by human eosinophils.

Besides cytotoxic mediators, human eosinophils can produce proinflammatory and anti-inflammatory cytokines, as well as growth factors and chemokines. The demonstration that eosinophils from patients could produce IL-5, IL-4 and IL-2 suggested their participation in the regulation of immune response. In the present work, we have examined the presence of Th1 (IFN-gamma, IL-2) and Th2 (IL-4, IL-5, IL-10 and IL-13) cytokines in eosinophils from patients or donors by intracellular flow cytometry, and by immunocytochemistry. Whereas almost 100% eosinophils expressed intracellular IFN-gamma, IL-2 and IL-10, the expression of IL-4, IL-5 and IL-13 is more variable and increased in patients versus donors. The differential release by eosinophils of Th2 versus Th1 cytokines is suggested both by the decrease of the intracellular content after culture, restricted to IL-4, IL-5 and IL-13, associated with an accumulation of IL-4 and IL-5 in the presence of Brefeldin. These results indicate that, through the preferential release of Th2 cytokines, human eosinophils can participate in the polarization of the immune response.

Selectin and Lewis(x) are required as co-receptors in antibody-dependent cell-mediated cytotoxicity of human eosinophils to Schistosoma mansoni schistosomula.

Killing of Schistosoma mansoni larvae by human eosinophils via antibody-dependent cell-mediated cytotoxicity (ADCC) mechanisms requires adherence between effector cells and parasite targets. The role of adhesion molecules in this mechanism was investigated using blocking monoclonal antibodies (mAb) and soluble ligands. We show that, along with the Mac-1 alpha chain, interactions between selectins and LewisX-related structures, both expressed by eosinophils and parasite targets, play a critical part in the antibody-dependent cytotoxic function of eosinophils. To further elucidate the interactions between adhesion molecules and eosinophil Fc receptors, ADCC was performed with IgG1 or IgA mAb. We found that mAb directed against Mac-1 alpha chain or against LewisX could significantly inhibit the IgG1-, but not IgA cytotoxicity. This result might be explained, at least in part, by the inhibitory effect of these mAb on the release by eosinophils of eosinophil cationic protein, one of the major mediators involved in target killing. Taken together, these results suggest novel interactions between Fc receptors and selectins and LewisX-related structures which might act as co-receptors for eosinophil-mediated cytotoxicity.

Circulating growth-regulator oncogene alpha contributes to neutrophil priming and interleukin-8-directed mucosal recruitment into chronic lesions of patients with Crohn's disease.

Crohn's disease (CD) lesions are characterized by a marked neutrophilic infiltrate associated with enhanced mucosal IL-8, contrasting with low serum IL-8 levels. The aim of this study was to investigate the effects of circulating GROalpha and IL-8 on neutrophil priming and migration. The expression of surface molecules involved in rolling (CD62L, CD15) and firm adhesion (Mac-1 and LFA-1) to endothelial cells was assessed by flow cytometry, while the chemotactic response of PMN to IL-8 and to fMLP was investigated in a Boyden chamber assay. In addition, IL-8 and GROalpha levels were determined by ELISA in plasma samples and in culture supernatants of purified polymorphonuclear neutrophils (PMN) and peripheral blood mononuclear cells (PBMC) from patients with CD and healthy blood donors. This study revealed an upregulation of CD11b (Mac-1) membrane expression on circulating PMN from patients with CD, as assessed by the mean fluorescence intensity which reflects antigen density. Furthermore, an enhanced chemotactic response towards both fMLP and IL-8 of PMN from CD patients was observed. Despite often undetectable levels of circulating IL-8, all plasma samples were positive for GROalpha, with a significant increase in CD patients when compared to donors. In vitro, equivalent concentrations of GROalpha were able to increase the IL-8 driven chemotaxis of PMN. In conclusion, blood PMN from patients with CD showed an enhanced capacity to be recruited into inflammed intestinal mucosa, which could be due to an increased expression of CD11b (Mac-1) as well as an increased chemotactic response toward fMLP or IL-8. This priming effect of PMN in CD may partly occur through elevated circulating GROalpha levels.

Differentiation of eosinophils from cord blood cell precursors: kinetics of Fc epsilon RI and Fc epsilon RII expression.

Expression of Fc epsilonRI and Fc epsilonRII/CD23 was examined by immunocytochemistry and flow cytometry on eosinophils differentiated from human cord blood cells in the presence of human interleukin-3 (rhIL-3), granulocyte/macrophage colony stimulating factor (rhGM-CSF) and interleukin-5 (rhIL-5) and on blood eosinophils purified from normal donors or patients with idiopathic hypereosinophilic syndrome (HES). On cord blood derived eosinophils, Fc epsilonRI expression started at 1 week of culture and increased to reach a plateau at 3 weeks of culture. Fc epsilonRII/CD23 appeared slightly later, after 2 weeks of culture, and the percentage of Fc epsilonRII/CD23-positive eosinophilic cells increased and stayed in plateau. Fc epsilonRI expression on cord blood derived eosinophils was downregulated after culture with interleukin-2 (rhIL-2), interleukin-4 (rhIL-4), rhIL-5, interferon-alpha (rhIFN-alpha), interferon-gamma (rhIFN-gamma). In contrast, the expression of Fc epsilonRII/CD23 on cord blood derived eosinophilic cells was upregulated after culture with rhIL-4, rhIL-5 and rhIFN-gamma, and downregulated with rhIL-2 and rhIFN-alpha. Fc epsilonRI was expressed on about 30% normal donor eosinophils as well as on normodense eosinophils from HES patients but significantly decreased on hypodense eosinophils. In contrast, Fc epsilonRII/CD23, expressed on a very small proportion of normal donor eosinophils, increased from normodense to hypodense eosinophils. These results suggest that Fc epsilonRI on eosinophils might represent one differentiation antigen expressed relatively early, with decreased expression through maturation or activation, whereas Fc epsilonRII/CD23 might rather be considered as a marker of eosinophil activation.

Interferon-gamma enhances tumor necrosis factor-alpha production by inhibiting early phase interleukin-10 transcription.

The ability of cytokine synthesis inhibitory factor or interleukin-10 (IL-10) and interferon-gamma (IFN-gamma) to modulate the production of tumor necrosis factor (TNF-alpha) induced by lipopolysaccharide (LPS) was examined in mouse bone marrow-derived macrophages (BMDM). IFN-gamma profoundly enhances LPS-stimulated TNF-alpha production, whereas IL-10 is markedly inhibitory, demonstrating the opposing effects of IFN-gamma and IL-10 on BMDM. Early neutralization of endogenously produced, LPS-stimulated IL-10 markedly enhanced short term TNF-alpha production, an effect further amplified by the absence of IFN-gamma priming. The regulatory effects of IFN-gamma and IL-10 apparently occurred at the translational (or post-translational) level, with TNF-alpha mRNA steady-state levels remaining unchanged. Furthermore, IFN-gamma exerts its enhancing effect on TNF synthesis by the transcriptional inhibition of IL-10. This in vitro finding was also confirmed in vivo. In the absence of LPS, IFN-gamma was not capable of inducing TNF-alpha production in BMDM, indicating that LPS or other signals are necessary for transcriptional activation. Reduced but significant TNF-alpha production in LPS-injected IFN-gamma receptor -/- mice suggests that IFN-gamma is not an absolute requirement and that other cytokines or cell types contribute in a secondary fashion to the priming of LPS-induced TNF-alpha production in vivo.

Growth inhibition signalled through the interleukin-4/interleukin-13 receptor complex is associated with tyrosine phosphorylation of insulin receptor substrate-1.

Induction of growth inhibition in human colorectal carcinoma cell lines by interleukin (IL)-4 and IL-13 was associated with the neophosphorylation of a 170 kDa cellular protein, identified as insulin receptor substrate-1 (IRS-1) by immunoprecipitation. Tyrosine phosphorylation of IRS-I was also induced by insulin and insulin-like growth factor I. Sublines of colorectal carcinoma cells unresponsive to growth modulation by IL-4, IL-13 or insulin-like growth factor I-induced growth did not phosphorylate IRS-1. A functional, multimeric IL-4 receptor complex was present on all carcinoma cell lines with a subunit composition of 65 kDa, 75 kDa and the previously characterized 130 kDa band as demonstrated by affinity cross-link with 126I labelled IL-4. The 65 kDa subunit is novel whereas the 75 kDa band represents the common IL-2 receptor gama-chain the novel 65 kDa receptor was present as a double band and bound primarily 125I-labelled IL-13. The present study demonstrates the involvement of a novel chain other than the gama-chain in the receptor complexes of IL-4 and IL-13 and and post-receptor tyrosine phosphorylation of IRS-1. The association of IRS-1 with growth inhibitory signals in carcinoma cells suggests a novel mechanism of tumour growth control.

Effect of rapamycin on the expression of the IL-2 receptor (CD25).

The immunosuppressive macrolide rapamycin inhibits cytokine-driven proliferation of lymphocytes, acting at a later stage of T lymphocyte activation than the related compound FK506 or cyclosporin, which block IL-2 transcription. However, the effect of rapamycin on the expression of the IL-2 receptor alpha-chain (CD25) is less well documented. This study has investigated the effect of rapamycin on mRNA levels of CD25 and membrane expression of IL-2 receptor in human primary T lymphocytes activated by various stimuli. Rapamycin surprisingly inhibits CD25 upregulation subsequent to anti-CD3 or ionomycin stimulation. These effects are not secondary to an IL-2-mediated CD25 up-regulation, as rapamycin inhibits neither IL-2 synthesis nor IL-2-induced CD25 mRNA. Interestingly, sensitivity to rapamycin correlates with the requirement of de novo protein synthesis, as demonstrated by anisomycin inhibition of both ionomycin- and CD3-induced CD25 transcription. Thus, rapamycin inhibition of T cell activation may involve not only IL-2-driven proliferation, but also suppression of CD25 up-regulation.

Multiple immune abnormalities in tumor necrosis factor and lymphotoxin-alpha double-deficient mice.

To investigate the roles of tumor necrosis factor (TNF) and lymphotoxin (LT)-alpha in the development and function of the immune system, the Tnf and Ltalpha genes were simultaneously inactivated in mice by homologous recombination. These mutant mice are highly susceptible to Listeria monocytogenes infection and resistant to endotoxic shock induced by the combined administration of D-galactosamine (D-GaIN) and lipopolysaccharide (LPS). Their splenic microarchitecture is disorganized, characterized by the loss of the clearly defined marginal zone, ill defined T and B cell areas, and absence of MAdCAM-1 and reduced ICAM-1, VCAM-1 and Mac-1 expression. They are devoid of peripheral lymph nodes and Peyer's patches, and show a strong reduction of IgA+ plasma cells in the intestinal lamina propria. The alymphoplasia is accompanied by a marked B lymphocytosis and reduced basal lg levels. Ig depositions in the renal glomerulus and a strong up-regulation of MHC class I antigen expression on endothelial cells of different tissues are observed. The primary humoral immune response towards sheep red blood cells reveals a defective IgG isotype switch, while that against vesicular stomatitis virus is normal. The cytotoxic T cell responses are attenuated, although still effective, against vaccinia, lymphocytic choriomeningitis virus (LCMV-ARM) and LCMV-WE. In conclusion, the combined inactivation of Tnf and Ltalpha confirms their essential role in the normal development and function of the immune system.

Role of interferon-gamma in interleukin 12-induced pathology in mice.

Interleukin 12 (IL-12) activates natural killer (NK) and T cells with the secondary synthesis and release of interferon-gamma (IFN-gamma) and other cytokines. IL-12-induced organ alterations are reported for mice and the pathogenetic role of IFN-gamma is investigated by the use of mice deficient in the IFN-gamma receptor (IFN-gamma R-/-). IL-12 caused a rapid infiltration of liver and splenic red pulp with activated macrophages; this and increased NK cells resulted in a fivefold increase of splenic weight in wild-type mice. Splenomegaly was associated with myelosuppression and decreasing peripheral leukocyte counts. IL-12-induced changes in wild-type mice were associated with markedly increased IFN-gamma serum levels and up-regulation of major histocompatibility complex (MHC) class I and II expression in various epithelia. IL-12 induced a qualitatively similar macrophage infiltration in IFN-gamma R-/- mice, less marked splenomegaly (to 2 x normal), and no MHC upregulation. Strikingly increased vascular endothelial intercellular adhesion molecule-1 expression was apparent in both IFN-gamma R-/- and IFN-gamma R+/+ mice. Restricted to mutant mice was a severe, invariably lethal, interstitial, and perivascular pulmonary macrophage infiltration with diffuse pulmonary edema. Extensive quantitative reverse transcriptase polymerase chain reaction analysis revealed an increase of only IL-6 and IL-10 pulmonary gene transcripts in IFN-gamma R-/- mice compared with wild-type mice. IL-12-induced myelosuppression is due to IFN-gamma-release from NK cells and T cells, and is associated with macrophage activation and distinct MHC class I and II antigen upregulation. The pulmonary pathology in IFN-gamma R-/- mice, however, reveals a toxic potential for IL-12 and suggests that endogenous IFN-gamma plays a protective role in preventing fatal pulmonary disease in these mice.

In situ detection of cyclosporin A: evidence for nuclear localization of cyclosporine and cyclophilins.

BACKGROUND: The immunosuppressant cyclosporin A (CsA) forms a trimolecular complex with cyclophilin (CPH) and calcineurins (CN) and inhibits CN phosphatase activity. Inhibition of CN phosphatase by CsA prevents the dephosphorylation of a nuclear factor in the cytosol and its nuclear translocation to the nucleus. EXPERIMENTAL DESIGN: The intracellular distribution of CPH and CN was investigated in permeabilized Jurkat T lymphocytes and MRC fibroblasts using biochemical techniques and confocal microscopy. The site of CsA binding was identified in situ using a photoaffinity label derivative of CsA followed by immunodetection. RESULTS: Cyclophilin A (CPH-A) and CN display essentially a cytosolic localization by immunofluorescence, and additional nuclear CPH-A and CN are evidenced by Western blot analysis of purified nuclei and immunofluorescence. By contrast, cyclophilin B (CPH-B) has a punctuate and reticular distribution pattern in cytoplasm, indicating an association with the endoplasmatic reticulum, but its main location is in the nuclear matrix, sparing the nucleolar region. For the in situ detection of CsA binding sites, a photolabile cyclosporine derivative (PL-CS) was used that allowed the detection of covalently bound CsA by Ab. Using the biologically active PL-CS, a punctate cytoplasmatic and nuclear immunoreactivity was obtained, which was specific and competed only with active cyclosporine derivatives. Nuclear CPH-A and -B were labeled by PL-CS, and trimolecular complexes of labeled CPH and CN were obtained by chemically cross-linking nuclear extracts. CONCLUSIONS: We describe herein the accessibility of CsA to the nucleus, the presence and labeling in situ of nuclear CPH and CN. The current models of CsA action predict that CsA-CPH complexes inhibit CN activity in the cytosol. However, our present findings invite the hypothesis that CPH may capture the drug into the nucleus and target regulatory proteins or transcriptional control elements.

Interleukin-2 receptor (CD25) upregulation on human T-lymphocytes: sensitivity to immunosuppressants is defined by the mode of T-lymphocyte activation.

We investigated the transcription and expression of the interleukin-2 receptor (IL-2R, CD25) in human T-lymphocytes after different modes of T-lymphocyte stimulation in the presence of the immunosuppressants cyclosporin (CsA) and tacrolimus (FK506) as well as the structurally related macrolide rapamycin. We demonstrate that CsA and FK506 inhibited IL-2R (CD25) gene transcription and protein expression after stimulation by anti-CD3 or ionomycin, but not by phorbol ester or IL-2. Rapamycin, which does not affect IL-2 transcription, surprisingly inhibited IL-2R upregulation after anti-CD3- or ionomycin-induced stimulation, but not by phorbol ester or IL-2. Interestingly, the protein synthesis inhibitor anisomycin inhibited anti-CD3- or ionomycin-, but not phorbol ester- and IL-2-induced CD25 gene transcription indicating the dependence on de novo protein synthesis of the former pathways. These data suggest that the pharmacological effect of the imunosuppressants tested on IL-2R gene expression may be predicted by the requirement of protein synthesis of a given activation pathway.

Long-term interleukin-6 administration stimulates sustained thrombopoiesis and acute-phase protein synthesis in a small primate--the marmoset.

Interleukin-6 (IL-6) has been ascribed significant roles in both hematopoiesis and the immune response, although its contribution to host defence as a whole is poorly understood. Because short-term IL-6 treatment was previously shown to stimulate megakaryocytopoiesis, we investigated the effect of long-term administration of IL-6 on megakaryocytopoiesis and other systemic parameters in nonhuman primates. We chose a small primate, the marmoset (Callithrix jacchus), which enabled long-term administration at high doses. Recombinant human IL-6 (rhIL-6) administered at doses of up to 1,000 micrograms/kg/d over 4 and 9 weeks caused a sustained twofold to threefold increase of thrombocyte counts, peaking at 4 weeks. Thrombocyte counts declined thereafter, despite continuing IL-6 administration. The number of bone marrow megakaryocytes at 4 and 9 weeks was not increased compared with controls, but the ploidy grade was augmented, suggesting that IL-6 effects are restricted to mature megakaryocytes in vivo. An acute-phase protein response was observed within 24 hours after the first IL-6 administration and reached a maximum after 1 week of IL-6 administration at 25 micrograms/kg. Serum C-reactive protein, haptoglobin, and ceruloplasmin were increased, whereas albumin and transferrin levels declined. The acute-phase protein response was not associated with any morphologic evidence of hepatocellular damage. The increased levels of Ig and soluble IL-2 receptor in the serum levels reflected systemic immunostimulation. There was no evidence of renal mesangioproliferative pathology. Antibodies against rhIL-6 developed within 2 weeks, continuously increasing during the course of the study. High titers of neutralizing antibodies appeared concomitantly with the decrease in platelet counts and decline in acute-phase proteins. Therefore, despite the pleiotropic effects of IL-6 observed in vitro, long-term administration of IL-6 caused a selective and sustained stimulation of thrombopoiesis in marmosets that was only ablated by the appearance of neutralizing antibodies, and high doses were well tolerated in marmosets. A long-term targeting of IL-6 to cells of the megakaryocytic lineage, without evoking general toxicity, confirms the potential therapeutic usefulness of rhIL-6 for the chronic treatment of thrombocytopenic patients.

Demonstration of ternary immunophilin-calcineurin complexes with the immunosuppressants cyclosporin and macrolide FK506.

The specificity of cyclosporin A (CsA) binding to the major intracellular receptor proteins, cyclophilin A and B, as well as the interaction of CsA with the phosphatase calcineurin were investigated. Binding of photoaffinity-labeled CsA (PL-CS), a photoaffinity probe of CsA, to recombinant human cyclophilin A and B is saturable and specific. Non-specific PL-CS binding to calcineurin is observed in the absence of cyclophilin and calmodulin. In the presence of cyclophilin, cyclosporin-calcineurin binding becomes specific. Ternary complexes containing an equimolar ratio of cyclophilin A or B, PL-CS and calcineurin are resolved using the chemical-crosslinking technique. The formation of these complexes is specific, calcium- but not calmodulin-dependent, and is only inhibitable by cyclosporins, which bind cyclophilin. The drug-immunophilin complex binds to the calcineurin A subunit. The proteolytic 43 kDa product of calcineurin A retains binding properties, suggesting that the C-terminal domains are not necessary for complex formation. A trimeric complex of FKBP-calcineurin is also formed with FK506, but not with rapamycin. As expected, these complexes are only competed with by homologous derivatives. Chemical crosslinking of photolabeled Jurkat T-cells strongly suggests that drug-calcineurin complexes are of biological relevance.