Polarized secretion of IL-6 and IL-8 by human retinal pigment epithelial cells

A number of cell types situated along interfaces of various tissues and organs such as the peritoneum and the intestine have been shown to secrete inflammatory cytokines in a polarized fashion. Retinal pigment epithelial (RPE) cells are positioned at the interface between the vascularized choroid and the avascular retina, forming part of the blood–retina barrier. These cells are potent producers of inflammatory cytokines and are therefore considered to play an important role in the pathogenesis of ocular inflammation. Whether cytokine secretion by these cells also follows a vectorial pattern is not yet known, and was therefore the subject of this study. Monolayers of human RPE cells (primary cultures and the ARPE-19 cell line) cultured on transwell filters were stimulated to produce IL-6 and IL-8 by adding IL-1β (100 U/ml) to either the upper or the lower compartment. After stimulation, the human RPE cell lines showed polarized secretion of IL-6 and IL-8 towards the basal side, irrespective of the side of stimulation. The ARPE-19 cell line also secreted IL-6 and IL-8 in a polarized fashion towards the basal side after basal stimulation; polarized secretion was, however, not apparent after apical stimulation. The observation that human RPE cells secrete IL-6 and IL-8 in a polarized fashion towards the choroid may represent a mechanism to prevent damage to the adjacent fragile retinal tissue.     

Soluble IL-2 receptor and tumour necrosis factor-alpha in plasma of haemophilia patients infected with HIV.

We measured plasma concentrations of soluble receptors for IL-2 (sIL-2R) and tumour necrosis factor-alpha (TNF-alpha) in 149 haemophilia patients. Soluble IL-2R levels were elevated in 37% of 62 HIV-seronegative patients (mean 570 +/- 27 U/ml versus 361 +/- 17 U/ml in the control group, P less than 0.0001), in 78% of 68 HIV-seropositive patients (928 +/- 49 U/ml, P less than 0.0001), and in 95% of 19 AIDS/ARC patients (1578 +/- 199 U/ml, P less than 0.0001 compared with controls and with HIV-seronegative patients; P less than 0.005 compared with HIV-seropositive asymptomatic patients). A negative correlation was observed between sIL-2R, relative and absolute numbers of CD4+ cells (P less than 0.0001), and CD4/CD8 ratios (P less than 0.0001). There was also a negative correlation between sIL-2R in plasma and the cellular expression of IL-2R (P less than 0.001). We found a significant association of sIL-2R and plasma neopterin (P less than 0.0001). With progression of the disease from HIV-seronegative to seropositive without symptoms and to full manifestation of AIDS/ARC, sIL-2R plasma levels increased. The highest levels were found at the time of diagnosis of AIDS/ARC, but the levels decreased again during the following 18 months. Eight per cent of HIV-seronegative patients, 32% of HIV-seropositive patients, and 24% of patients with AIDS/ARC had increased plasma TNF-alpha. We conclude that sIL-2R and TNF-alpha plasma levels are elevated in HIV-infected haemophilia patients and that sIL-2R is a marker for disease progression from asymptomatic HIV-seropositive to AIDS/ARC.     

Increased spontaneous secretion of IL-6 from B cells of patients with B chronic lymphatic leukaemia (B-CLL) and autoimmunity.

We studied B cells from 18 patients with B-CLL, six of them with autoimmune haemolytic anaemia, for spontaneous secretion of IL-6. Our aim was to determine whether the increased incidence of autoimmune disease found in B-CLL patients is associated with enhanced spontaneous IL-6 secretion. IL-6 was measured by the effect of B cell supernatants on the proliferation of an IL-6 dependent plasmacytoma cell line T1165. The highest IL-6 values (7.4 +/- 1.8 U/ml) were measured in supernatants derived on day 3 of culture from lymphocytes of the six patients with B-CLL and concomitant autoimmune disease. The maximal IL-6 values for 10 patients with B-CLL only were 2.8 +/- 0.3 U/ml and for 10 age-matched controls, 0.8 +/- 0.3 U/ml (P less than 0.01, each group compared with the other). We conclude that there is an association between B-CLL, autoimmune disease and the spontaneous in vitro secretion of IL-6. Further studies are needed to determine whether the IL-6 secretion plays a role in the pathogenesis of autoimmune disease in patients with B-CLL.     

Interferon-gamma stimulates the secretion of IL-1, but not of IL-6, by glomerular mesangial cells.

IL-1 activity in culture supernatant and cell lysate from rat mesangial cells stimulated with interferon-gamma (IFN-gamma) was measured by a thymocyte proliferation assay. While IFN-gamma alone had no effect on the secretion or the intracellular pool of IL-1, the enhancement by IFN-gamma of IL-1 secretion in response to lipopolysaccharide (LPS) was observed. The stimulatory effect of culture supernatant on thymocyte proliferation was abrogated by preincubation with the anti-IL-1 antibody. At least 4-h incubation with IFN-gamma and LPS was required to detect enhancing effect of IFN-gamma. The addition of as little as 1 U/ml IFN-gamma significantly increased IL-1 secretion in the presence of 10 micrograms/ml LPS. The IL-6 activity in culture supernatants was determined by measurement of thymidine uptake in mouse IL-6-dependent cell line (MH60.BSF2). Mesangial cells secreted IL-6 in culture supernatant without additional stimuli and LPS distinctly increased it as described previously. However, in contrast to IL-1 production, no effect of IFN-gamma on IL-6 secretion was observed in the presence or absence of LPS. Moreover, we determined whether enhanced IL-1 release is associated with Ia expression on mesangial cells. IFN-gamma alone and the combination with LPS induced marked expression of Ia antigen, whereas LPS alone did not. We conclude that IFN-gamma stimulates the production of IL-1, but not IL-6, by mesangial cells and suggest an important role of IFN-gamma in the pathogenesis of glomerulonephritis by regulating the mesangial production of IL-1 and the accessory cell function of mesangial cells.     

B cells capable of spontaneous IgG secretion in cerebrospinal fluid from patients with multiple sclerosis: dependency on local IL-6 production.

Cerebrospinal fluid (CSF) from multiple sclerosis (MS) patients contains B cells capable of spontaneous IgG secretion in vitro. This study analyses the function and regulation of these cells. CSF cells obtained from nine MS patients actively produced IgG during 2-3 days in culture, and the activity decreased when CSF cells were cultured in serum-free medium. CSF cells from four controls did not secrete detectable IgG in vitro. Further experiments revealed that IL-6 played a role on MS CSF IgG-secreting cells, as can be deduced from the following findings: (i) the addition of exogenous IL-6, but not of other cytokines, to serum-free cultures restored missing CSF cell IgG secretion (ii) the inclusion of anti-IL-6, but not of control, blocking MoAb reduced IgG secretion by CSF cells in fetal calf serum (FCS)-containing cultures; and (iii) CSF cells were capable of active IL-6 production in the presence of FCS. These results suggest that endogenous IL-6 production by MS CSF cells seems to be responsible for inducing CSF IgG-secreting B cells to reach terminal differentiation.     

IL-4 enhances IEC-6 intestinal epithelial cell proliferation yet has no effect on IL-6 secretion

Intestinal epithelial cells (IEC) form an important line of defence at the intestinal mucosa by providing a barrier to lumenal contents and also by their ability to secrete various inflammatory cytokines. Recently, several T cell-derived cytokines have been shown to regulate specific IEC functions. In this study, the effect of IL-4 on IEC proliferation and secretion of the inflammatory cytokine IL-6 was investigated using the non-transformed rat IEC-6 intestinal epithelial cell line. Recombinant rat (rr)IL-4 was found to enhance IEC-6 cell proliferation over 4 days of culture, and this enhancement was dose-dependent. Further studies using specific antibodies confirmed that IL-4 induced the effect and that the effect was not mediated by autocrine-produced transforming growth factor-alpha. However, IL-4 did not induce IL-6 secretion by the IEC-6 cells, nor did it alter IL-1β-induced IL-6 secretion. These results indicate that T cells may be capable of regulating IEC proliferation via the secretion of IL-4 without altering the capacity of the IEC to function in the inflammatory response by secreting IL-6.     

Autostimulatory effects of IL-6 on excessive B cell differentiation in patients with systemic lupus erythematosus: analysis of IL-6 production and IL-6R expression.

Introducing avidin-biotin complex ELISA for anti-DNA antibody, the mechanism of in vitro production of anti-ssDNA antibody as well as of polyclonal immunoglobulin mediated by an IL-6-IL-6R loop was studied in patients with systemic lupus erythematosus (SLE). Regardless of the presence or absence of T cells, B cells from SLE patients could produce IgG anti-ssDNA antibody as well as total IgG without any stimulation. Low density B cells obtained by Percoll gradient density centrifugation responded to rIL-6 to produce IgG and IgG anti-ssDNA antibody. rIL-2 and rIL-4 had lesser effects on the differentiation of low density B cells. In fact, IL-6R was preferentially expressed on low density B cells from active SLE patients, as detected by anti-IL-6R MoAb, MT18, which did not inhibit IL-6 binding. SLE B cells, especially high density B cells, produced greater amounts of IL-6 in culture supernatants than did T cells, regardless of whether disease was active or inactive. Normal T cells and B cells did not produce significant amounts of IL-6. Thus, endogenous IL-6 produced by high density B cells bound to the IL-6R preferentially expressed on the low density B cells, and drove them into terminal differentiation, especially in active SLE patients. Further, addition of polyclonal anti-IL-6 or anti-IL-6R MoAb (PM1), which inhibited IL-6 binding, both inhibited IgG anti-ssDNA antibody as well as total IgG production by SLE B cells in a dose-dependent manner. These results suggest that interruption of the autocrine IL-6 loop would be of therapeutic value in SLE.     

Dysregulation in IL-12 secretion by neutrophils from HIV-infected patients

It is generally believed that neutrophils from HIV-infected patients are functionally competent, but several studies have shown impairment in neutrophil fungal killing and cytokine production. In this study we evaluated the ability of neutrophils from healthy donors and HIV-infected patients to produce IL-12 in response to stimulation with Candida albicans, lipopolysaccharide (LPS) and Cryptococcus neoformans (acapsular and encapsulated), with and without MoAb opsonization. Neutrophils from healthy donors secreted IL-12 in response to LPS or C. albicans but not in response to encapsulated or acapsular C. neoformans, regardless of MoAb opsonization. Surprisingly, neutrophils from HIV-infected patients demonstrated constitutive IL-12 production, although these cells were not responsive to LPS stimulation. The inability of MoAb to C. neoformans capsular polysaccharide to promote IL-12 production by neutrophils excludes phagocytosis and/or CD16 cross-linking in this process, and distinguishes neutrophils from monocytes. Our results provide additional evidence for cytokine dysregulation in neutrophils from HIV-infected patients. Furthermore, the IL-12 response of neutrophils and monocytes to CD16 stimulation appears to be different, suggesting differences in the role of these phagocytic cells during the inflammatory response.     

Cytokine-specific ELISPOT assay single cell analysis of IL-2, IL-4 and IL-6 producing cells

In order to assess cytokine-producing cells at the single cell level, the cytokine-specific ELISPOT assay has proven to be an important and sensitive method. The purpose of this study was to adapt this method to elucidate individual cells producing murine IL-2, IL-4 or IL-6. In order to establish these cytokine-specific ELISPOT assays, IL-2-, IL-4- and IL-6-specific cDNA transfected myeloma cell lines, e.g., X63-Ag8-653 X2, X63-Ag8-653 X4 and X63-Ag8-653 X6, respectively, were used as specific cytokine-producing cells. In the IL-2 ELISPOT assay, the coating reagent, monoclonal antibody (mAb) rat IgG2a anti-mouse IL-2 (CR #40014) was used while rabbit IgG polyclonal anti-mouse IL-2 was employed for detection of IL-2 spot forming cells (SFC). The mAbs anti-mouse IL-4, BVD4-1D11 and BVD6-24G2 were selected as capture and detection antibodies for enumeration of IL-4 SFC. For the IL-6 ELISPOT assay, anti-mouse IL-6 (MP5-20F3) mAb was used for coating and MP5-32C11 mAb was used for detection of IL-6 SFC. When IL-2 producing X63-Ag8-653 X2 cells were subjected to these three different ELISPOT assays, IL-2-specific SFC were only noted with the IL-2 ELISPOT system. In the case of IL-4 SFC, only X63-Ag8-653 X4 cells formed specific spots using the tandem of BVD4-1D11 and BVD6-24G2 mAbs. IL-6-specific spots developed in MP5-20F3 mAb pre-coated wells containing X63-Ag8-653 X6 cells, when developed with mAb anti-IL-6 (MP5-32C11). Addition of cycloheximide (50 μg/ml) inhibited formation of IL-2, IL-4 and IL-6 SFC by approximately 90%. When an unrelated mAb was used as detection antibody in these three different cytokine-specific ELISPOT assays, IL-2-, IL-4-and IL-6-specific SFC were not detected. Further, when concanavalin A stimulated T cells from Peyer's patch of normal mice were subjected to the respective cytokine-specific ELISPOT assay, IL-2, IL-4 and IL-6 SFC were enumerated. These results have shown that cytokine-specific IL-2, IL-4 and IL-6 ELISPOT assays have now been established and will allow analysis of the frequency of cytokine-secreting cells at the single cell level.     

Context-specific roles for paracrine IL-6 in lymphomagenesis

A basic requirement for the development of complex organ systems is that the cellular response to identical environmental cues can vary significantly between distinct cell types and developmental stages. While it is well established that paracrine signaling can similarly elicit diverse responses in distinct tumor types, the relevance of developmental stage-specific signaling responses to tumor development remains unclear. Here, we show that the same microenvironmental factor, IL-6, can both promote and prevent lymphoma development by acting on cells at distinct stages of hematopoietic development. Specifically, paracrine IL-6 signaling promotes the survival of transplanted hematopoietic stem cells following lethal irradiation, allowing for the persistence and expansion of progenitor cells bearing a cancer-promoting alteration. Conversely, IL-6 signaling also initiates a paracrine secretory program in the bone marrow that promotes B-cell differentiation and inhibits the development of B-cell malignancies. Thus, stage-specific responses to cytokines may promote progenitor cell expansion while also inhibiting neoplastic development within a single developmental lineage. Once transformed, the resulting B-cell lymphomas again use paracrine IL-6 signaling as a survival signal, highlighting the ability of tumor cells to co-opt pathways used for stem cell protection. These data not only suggest a complex regulation of tumor development by the preneoplastic microenvironment, but also that this regulation can decisively impact the outcome of well-established tumor modeling approaches.     

缺血性鼠脑中的IL-6基因表达

IL-6是一多效应的细胞因子,在宿主的防卫和急性炎症反应中起重要的作用[1]。它的表达异常和失调与多种疾病发病有关,如肿瘤、Alzheimer’s病、不同类型的脑损害。为探讨IL-6与缺血性脑损害的关系,我们用PT-PCR方法对缺血性鼠脑不同部位IL-6mRNA表达进行了研究,报道如下:1材料和方法1.1动物模型研制ICR小鼠18只,每组6只,雌雄各半,重45～50g。用线栓法研制成右侧大脑中动脉栓塞模型[2],即25%戊拉坦1000mg/kg,IP麻醉。手术显微镜下,颈部正中切开,分离右颈总动脉,用5～0号进口尼龙线约10～12mm长,头端涂用cilicon,插入颈内动脉抵大脑中…     

Inverse regulation of interleukin-6 (IL-6) and IL-6 receptor in histamine deficient histidine decarboxylase-knock-out mice

Interleukin-6, a multifunctional cytokine upon binding to its receptor on hepatocytes regulates production of acute phase proteins involved in local and systemic inflammation. Gene expression and biosynthesis of IL-6 and its receptor (IL-6 R/gp130) is under complex regulation. Histamine, in addition to its principal role in immediate type hypersensitivity has been described to modulate IL-6 production and expression of IL-6 receptor. In this study, the IL-6 and IL-6 receptor expression was examined in histamine deficient histidine decarboxylase (HDC) knock-out mouse model. Our data suggest that in histamine deficient mice the inducibility of IL-6 is significantly reduced, whilst more IL-6 receptor/gp130 mRNA expresses in the liver than in wild type (HDC^+/+) mice. These in vivo findings confirm earlier in vitro results and emphasize the efficacy of antihistamines in local IL-6 related processes.     

HIV-induced IL-6/IL-10 dysregulation of CD4 cells is associated with defective B cell help and autoantibody formation against CD4 cells

To analyse CD4 cell cytokine secretion and helper/suppressor function at a clonal level we established 446 CD4⁺ T cell clones (TCC) in four healthy controls, three HIV^? haemophilia patients, four CDC II,III and four CDC IV patients. Spontaneous TCC secretion of Th1 cytokines (IL-2, interferon-gamma (IFN-γ)) and Th2 cytokines (IL-4, IL-6, IL-10) was determined by ELISA. TCC helper and suppressor functions were tested in a pokeweed mitogen (PWM)-stimulated allogeneic co-culture system using a reverse haemolytic plaque assay for assessment of B cell responses. There was a significant association of TCC surface marker expression (Leu-8, CD45RA) with TCC IL-6 secretion in healthy controls (P < 0.01), HIV^? patients (P 0.001) and CDC II,III patients (P 0.01) but not in CDC IV patients. Likewise, TCC expression of Leu-8 and CD45RA was significantly associated with TCC suppressor function in healthy controls (P 0.0005) but not in HIV-infected patients. A reduced TCC helper frequency (10% of TCC) and an enhanced TCC suppressor frequency (> 80% of TCC) were detected only in those HIV-infected patients who showed an excessively increased TCC IL-6 secretion (> 70% of TCC) together with a significantly diminished TCC IL-10 secretion (10% of TCC). CD4 cell autoantibodies also were found only in patients with this type of cytokine dysregulation. These data indicate that CD4 cell surface markers lose their functional relevance in HIV-infected patients. HIV-induced IL-6/IL-10 dysregulation of CD4⁺ T cells, i.e. the up-regulation of spontaneous IL-6 and down-regulation of spontaneous IL-10 secretion, appears to be involved in inducing CD4 helper defects and may promote autoantibody formation against CD4 cells.     

Rat thymic epithelial cells in culture constitutively secrete IL-1 and IL-6.

To study the in vitro interactions between rat thymic non-lymphoid cells and thymocytes, we established a system for long-term cultivation of thymic epithelial cells (TEC). TEC were cultivated and successfully propagated for over 8 months in RPMI 1640 medium containing 15% FCS, dexamethasone, insulin, epidermal growth factor, and poly-L-lysin as an adhesive matrix. Their epithelial nature has been confirmed using monoclonal anti-cytokeratin (CK) antibodies. More than 95% of these cells were reactive with K 8.13 and CK 8 mAbs, which are pan-epithelial markers for rat TEC in situ. An epithelial cell clone (TE-R 2.5) established from a long-term TEC culture was 100% reactive with these anti-CK antibodies. Phenotypic analysis of TEC cultures was performed by a large panel of mAbs reactive with a subset of rat TEC or CK polypeptides as well as UIex europaeus agglutinin I using a streptavidin-biotin immunofluorescence assay. Although the results obtained demonstrated phenotypic heterogeneity among these cells, most cultures, including the TE-R 2.5 clone, were of subcapsular/medullary phenotype. Medium conditioned by TEC cultures exhibited IL-1 and IL-6 activities when tested on D10S and B9 sensitive cell lines, respectively. Cytokine activities were neutralized (IL-1) or significantly inhibited (IL-6) by specific polyclonal antibodies. In addition, both anti-IL-1 and anti-IL-6 antibodies reacted with TEC in culture and epithelial (CK-positive) cells on thymic cryostat sections, indicating that thymic epithelium provides an important intrathymic source for molecules contributing to T cell activation.     

卵巢癌细胞IL-6、IL-8及其受体表达的研究

目的分析比较五种常见的上皮性卵巢癌细胞系IL-6、IL-8及其受体表达的差异。方法IL-6、IL-8的表达分别采用RT-PCR和ELISA法进行检测,IL-6受体（IL-6Rα和gp130）及IL-8受体（IL-8RA和IL-8RB）的表达采用免疫印迹技术进行测定。结果①五种上皮性卵巢癌细胞均组成性表达IL-6和IL-8。IL-6和IL-8在CAOV-3细胞中的表达水平均最高,而在HO-8910PM细胞中的表达水平均最低,IL-6在SKOV-3、HO-8910、OVCAR-3细胞中的表达水平依次降低,IL-8在OVCAR-3、SKOV-3、HO-8910细胞中的表达水平依次降低。②五种上皮性卵巢癌细胞均表达IL-6Rα、gp130及IL-8RA;除CAOV-3细胞外,其它细胞均表达IL-8RB。结论本研究旨在筛选表达IL-6和IL-8及其相应受体的细胞株,为研究IL-6、IL-8与卵巢癌发生、发展关系奠定基础,同时也为今后卵巢癌的免疫治疗提供一个新的思路。     

Recombinant human interleukin 6 (B cell stimulatory factor 2) enhances immunoglobulin secretion by single murine hapten-specific B cells in the absence of cell division

We have assessed the role of recombinant human IL-6 (r-hu-IL-6)In promotion of early actlvation, proliferation, and immunoglobulin(lg)secretion amongst single hapten-specific murine splenic B cellsin vitro it was found that r-hu-IL-6 acting alone was able toinduce early B cell activation in a proportion of B cells, asmeasured by a significant increase in cell diameter within 24h. An enhanced effect was seen in the concomitant presence ofa ‘T-Independent’ antigen. None of the B cells activatedby r-hu-IL-6 appeared to divide, as the frequencies of proliferatingclones Induced by either medium alone or antigen alone werevirtually identical whether r-hu-IL-6 was present or absent.However, assay of the culture supernatants for the presenceof Ig by ELlSA revealed that r-hu-IL-6 effected a significant2-fold increase in the frequency of B cells secreting Ig. Thus,the prlme effect of r-hu-IL-6 appears to be to recruit moreprecursor B cells into Ig secretion, rather than to promoteproliferation or to enhance the amount of Ig secreted by pre-committedbut non-cycling B cells. Delayed addition experiments showedthat r-hu-IL-6 enhanced Ig secretion late in the activationpathway. Kinetics studies demonstrated detectable Ig secretionas early as day 2, and when taken with its apparent abilityto Induce early activation, these flndings suggest that IL-6is not exclusively a late-acting Interleukin. Studies with sizefractionated hapten-spectflc B cells showed the larger B cellsto be preferentially responsive to r-hu-IL-6. The data presentedstrongly suggest that IL-6 may play a role In B cell maturationas a polyclonal stimulator of low rate Ig secretion. The bioactivityof IL-6 In this slngle cell system thus differed from that ofIL-4 and/or IL-5. These findings are discussed with relevanceto B cell activation in vivo.     

血清IL-6水平在神经胶质瘤预后中的意义

目的探讨血清白细胞介素-6在神经胶质瘤患者预后中的潜在意义。方法收集手术前182位神经胶质瘤病人和49位正常供血者的血清样本。使用酶联免疫吸附试验测定白细胞介素-6水平。与血清癌胚抗原(CEA)、甲胎球蛋白(AFP)和C-反应蛋白(CRP)水平比较,评估白介素-6的临床意义。结果神经胶质瘤患者血清白介素-6水平明显高于健康对照组。血清白介素-6水平与癌胚抗原密切相关,但是与AFP和CRP关系不大。患者血清IL-6水平与患者TNM分期显著相关。此外,高血清的IL-6水平与神经胶质瘤的不良预后相关联。结论血清IL-6是神经胶质瘤的预后因子。     

IL-6 in malignant pleural effusions and its augmentation by intrapleural instillation of IL-2.

The levels and activities of endogenous IL-6 in malignant pleural effusions due to lung cancer before and during daily intrapleural instillations of recombinant IL-2 were examined by enzyme immunoassay and bioassay using an IL-6-dependent murine hybridoma cell line MH60.BSF2. Before therapy, malignant pleural effusions contained various levels of IL-6. Daily intrapleural instillation of recombinant IL-2 for treatment of malignant pleurisy resulted in significant augmentation of the levels and activities of IL-6 in the pleural effusions. On fractionation of the pleural effusion by chromatography, one major peak of material with a mol. wt of 24 kD showed IL-6 activity. In contrast, no significant level of tumour necrosis factor-alpha or IL-1 beta was detectable in pleural effusions before or during local IL-2 therapy. These data suggest that IL-2 is an important regulatory factor of secondary IL-6 production.     

IL-6 and soluble IL-6 receptors (sIL-6R and sgp130) in human pleural effusions: massive IL-6 production independently of underlying diseases

IL-6, soluble IL-6 receptor (sIL-6R) and soluble gp130 (sgp130) levels were measured in sera and pleural effusions from 42 patients with metastatic carcinoma, non-Hodgkin's lymphoma, tuberculosis, cardiac failure and miscellaneous diseases. Pleural IL-6 levels measured by ELISA were very high in all patient groups (mean 34.8 ± 15.3 ng/ml) without significant difference according to diseases. IL-6 was shown to be biologically active in a proliferative assay. Serum IL-6 levels were low (0.049 ± 0.014 ng/ml) and did not correlate with pleural fluid levels. Pleural IL-6 levels correlated with the number of polymorphonuclear cells in pleural fluid (P< 0.03). Pleural sIL-6R levels (76 ± 8 ng/ml) were always lower than serum levels (196 ± 12 ng/ml; P< 0.0001) but correlated with them (P< 0.01). Pleural sIL-6R and albumin levels correlated (P< 0.01), suggesting a transudation of sIL-6R from the serum. Pleural sgp130 levels (10.9 ± 1.0 ng/ml) were lower than serum levels (24.6 ± 2.8 ng/ml; P< 0.002). After gel filtration of pleural fluid, the bulk of IL-6 (>90%) was recovered in a 15 000–30 000 fraction, corresponding to the expected mol. wt of free IL-6. These results suggest a production and a sequestration of IL-6 in the pleural cavity in all studied conditions.