Lymphocytes induce resorption of cartilage by producing catabolin.

Mononuclear cells from pig blood, when cultured in the presence of lectins (concanavalin A or phytohaemagglutinin), release a factor that induces resorption of proteoglycan in explants of live bovine nasal cartilage. The factor had mol.wt. 20000 and pI 4.6-5.0, and was indistinguishable from the cartilage-catabolic protein catabolin isolated from culture medium of explants of pig synovial tissue. Since much of the catabolin from the mononuclear cells arose from the non-adherent population (98% lymphocytes) it was concluded that lymphocytes (and possibly monocytes) make catabolin.     

The cartilage-resorbing protein catabolin is made by synovial fibroblasts and its production is increased by phorbol myristate acetate.

Pig synovial fibroblasts were shown to produce a protein that caused live cartilage to resorb its proteoglycan matrix in vitro. Fibroblasts were obtained either from synovial tissue digest or by allowing them to grow out of explants. The population derived from the digests was homogeneous and free of macrophage-like cells after two passages, but was still producing the cartilage-resorbing protein after seven passages. The active protein was found to have Mr 20,000 on gell filtration, and pI 4.8 on isoelectric focussing in polyacrylamide gel. It was indistinguishable from a protein with the same activity from pig mononuclear leucocytes, which has been called catabolin. Production of the protein was increased if the synovial fibroblasts were cultured with the tumour promoter phorbol 12-myristate 13-acetate. Fibroblasts from other sources (joint capsule and peritoneum) also apparently made the protein. The possibility that catabolin is the same as interleukin-1 is discussed: if they are, then the results suggest that fibroblasts can make an interleukin-1-life protein.     

Characterization of catabolin, the major product of pig synovial tissue that induces resorption of cartilage proteoglycan in vitro

1. Pig synovium in organ culture produces material which induces living cartilage to resorb its proteoglycan in vitro. 2. The bioassay for this material was to measure glycosaminoglycan released from explants of bovine nasal-septal cartilage cultured for 8 days. The performance of the assay was greatly improved by adding cortisol succinate (0.1μg/ml). This decreased the release of glycosaminoglycan from unstimulated cartilage without inhibiting its response to catabolic factors from the synovium. 3. By using this improved assay it was shown that 90% of the active materials in synovial culture medium were retained by dialysis membrane. 4. An active protein was partially purified from synovial culture medium by (NH₄)₂SO₄ precipitation, ion-exchange chromatography, gel filtration and preparative isoelectric focusing. 5. This protein, called catabolin, had mol.wt. 17000 and pI4.6. 6. Synovial culture medium concentrated in dialysis tubing was subjected to gel chromatography and found to contain one major active component, which was eluted at the same position as the partially purified catabolin. 7. The synovial culture medium was not inactivated by heating (70°C for 10min), nor were diluted preparations of partially purified catabolin, but concentrated crude preparations were thermolabile. 8. These results suggest that catabolin is the major substance produced by the synovial tissue in culture which induces resorption of proteoglycan of living cartilage in vitro. 9. Other cultured soft connective tissues produced catabolin-like activity. The example of sclera is shown, and production was inhibited by cortisol succinate (0.1μg/ml). 10. It is suggested that catabolin may be a general product of soft connective tissues in culture, and its physiological function may be to induce resorption of connective-tissue matrix after injury.     

Identification of catabolin,a protein from synovium which induces degradation of cartilage in organ culture

Explants of porcine synovium produce a factor which causes degradation of the matrix of live cartilage in organ culture. Cartilage degradation was measured as release of glycosaminoglycan from explants of bovine nasal septum. Fractionation of synovial culture medium showed the factor to be a protein of 20,000 mol.wt. and iso-electric point pH = 4.6. The factor has been named catabolin.     

Effects of retinol and dexamethasone on cytokine-mediated control of metalloproteinases and their inhibitors by human articular chondrocytes and synovial cells in culture

Human articular chondrocytes in culture produced large amounts of specific mammalian collagenase, gelatinase and proteoglycanase when exposed to dialysed supernatant medium derived from cultured human blood mononuclear cells (mononuclear cell factor) or to conditioned medium, partially purified by fractionation with ammonium sulphate (60–90% fraction), from cultures of human synovial tissue (synovial factor). Human chondrocytes and synovial cells also released into culture medium an inhibitor of collagenase of apparent molecular weight about 30 000, which appeared to be similar to the tissue inhibitor of metalloproteinases synthesised by tissues in culture. The amounts of free collagenase inhibitor were reduced in culture media from chondrocytes or synovial cells exposed to mononuclear cell factor or synovial factor. While retinol inhibited the production of collagenase brought about by mononuclear cell factor or synovial factor, it restored the levels of inhibitor, which were reduced in the presence of mononuclear cell factor or synovial factor. Dexamethasone markedly reduced the production of collagenase by synovial cells, while only partially inhibiting factor-stimulated collagenase production by chondrocytes. Addition of puromycin as an inhibitor of protein synthesis reduced the amounts of both collagenase and inhibitor to control or undetectable levels.     

Articular cartilage cultured with catabolin (pig interleukin 1) synthesizes a decreased number of normal proteoglycan molecules.

A homogeneous preparation of catabolin from pig leucocytes caused a reversible dose-dependent (0.01-1 nM) decrease in the synthesis of proteoglycan in slices of pig articular cartilage cultured in serum-free medium. The monomers that were synthesized and secreted in the presence of catabolin had the same average hydrodynamic size and ability to aggregate as the controls, and the core protein was substituted with the same number of glycosaminoglycan chains. The chains were the same average length and charge as normal and were sulphated to the same extent as the controls. Newly synthesized extracellular proteoglycan was not preferentially degraded. A 2-3-fold increase in glycosaminoglycan synthesis occurred in control and catabolin-treated cartilage in the presence of beta-D-xyloside (1 mM), more than 80% being secreted into the medium as free chains. Decreased incorporation of sulphate was not reversed in the presence of lysosomal-enzyme inhibitors, and there was no evidence in pulse-chase experiments of increased intracellular degradation of glycosaminoglycan chains before secretion. It is concluded that catabolin-treated cartilage synthesizes a smaller number of normal proteoglycan molecules.     

The inhibition of human leucocyte elastase and chymotrypsin-like protease by elastatinal and chymostatin.

The ability of elastatinal and chymostatin, protease inhibitors of microbial origin, to inhibit human leucocyte proteases (EC 3.4.-) was studied. Elastatinal and chymostatin are capable of inhibiting the pancreatic enzymes elastase and chymotrypsin, respectively. It was found in these studies, with synthetic substrates, that elastatinal is a much weaker inhibitor of human leucocyte elastase than it is of porcine pancreatic elastase. Elastatinal caused no inhibition of the activity of human leucocyte chymotrypsin-like protease. Chymostatin was found to be a powerful inhibitor of human leucocyte chymotrypsin-like protease. Its affinity to the leucocyte protease was higher than its affinity to bovine pancreatic alpha-chymotrypsin. Chymsotatin had a weak inhibitory effect on the activity of human leucocyte elastase. Studies were also carried out on the ability of chymostatin to inhibit the release of 35SO2-4 from rabbit articular cartilage by human leucocyte chymotrypsin-like protease. Preincubation of the chymostatin with the protease before the latter was added to the 35SO2-4 -labeled cartilage caused inhibition of proteolysis as measured by 35SO2-4 release. Preincubation of chymostatin with 35SO2-4 -labeled cartilage prior to addition of the human chymotrypsin-like protease to the tissue also inhibited 35SO2-4 release. However, in the case of preincubation of cartilage with alpha1 -antitrypsin there was no such inhibition. It therefore appeared that chymostatin, unlike alpha1 -antitrypsin, was capable of penetrating the cartilage matrix and exerting its inhibitory effect upon the human leucocyte chymotrypsin-like protease that was subsequently added to the tissue.     

Retinoids and synovial factor(s) stimulate the production of plasminogen activator by cultured human chondrocytes. A possible role for plasminogen activator in the resorption of cartilage in vitro

Agents such as retinol, interleukin 1 and catabolin stimulate resorption of cultured cartilage. This process seems to be mediated by chondrocytes, but the mechanism by which breakdown occurs remains unknown. We have found that (10(-6)-10(-8) M) retinoic acid and (1 X 10(-6) M) retinol, in the presence or absence of a factor derived from cultured synovium (synovial factor), stimulate the degradation of fibrin by human chondrocytes in culture. Plasminogen was required for the enhancement of fibrinolysis, suggesting that the breakdown depended upon the production of plasminogen activators and subsequent liberation of plasmin. However, the chondrocytes did not release significant amounts of plasminogen activator, and the effects of the synovial factor and retinoids resulted from augmentation of the production or activity of enzymes which remained bound to the cell layer. The role of plasminogen in the resorption of cultured cartilage was also investigated. In the presence of plasminogen, (1 X 10(-8) M) retinoic acid or synovial factor stimulated the breakdown of cultured bovine nasal cartilage, but in the absence of plasminogen, the effect of synovial factor was abolished and that of retinoic acid reduced. However, in cultures containing both retinoic acid and synovial factor the resorption process was not affected by removal of plasminogen. Thus, the resorption of cartilage matrix in vitro may be partially mediated by plasminogen activators and plasmin.     

Pig catabolin is a form of interleukin 1. Cartilage and bone resorb, fibroblasts make prostaglandin and collagenase, and thymocyte proliferation is augmented in response to one protein.

Homogeneous catabolin from pig leucocytes induced proteoglycan breakdown, but not collagen breakdown, in explants of articular cartilage. It augmented lectin-induced proliferation of mouse thymocytes, stimulated production of prostaglandin E2 and collagenase by fibroblasts and chondrocytes, and increased Ca2+ release from mouse calvarial explants, all at concentrations down to 50 pM. In view of these effects it was concluded that pig catabolin is a form of interleukin 1.     

Human interleukin 1 mediates cartilage matrix degradation

Human monocyte factors mediate cartilage matrix degradation by activation of the resident chondrocytes. In the present work, the cartilage matrix-degrading activity of partially purified human monocyte-derived interleukin 1 has been investigated. Human monocyte or blood mononuclear cell culture supernatants were sequentially purified by phenyl-Sepharose and gel filtration chromatography, or by gel filtration chromatography, isoelectric focusing, or chromatofocusing. Column fractions were simultaneously tested by the standard method that defines interleukin 1 activity--costimulation of mouse thymocyte proliferation--and for matrix macromolecule release from living bovine cartilage explants in organ culture. The two activities showed identical profiles in purification steps that would discriminate according to molecular size, hydrophobicity, and net charge. Moreover, the thermal denaturation profiles of the material purified by chromatofocusing could not distinguish between thymocyte proliferating and matrix degrading activities. These results suggest that interleukin 1 which is present in inflammatory synovial fluids may play an important role in the mediation of cartilage damage in chronic inflammatory arthritides.     

Chondrocyte-mediated depletion of articular cartilage proteoglycans in vitro.

The degradation of proteoglycan was examined in cultured slices of pig articular cartilage. Pig leucocyte catabolin (10 ng/ml) was used to stimulate the chondrocytes and induce a 4-fold increase in the rate of proteoglycan loss from the matrix for 4 days. Material in the medium of both control and depleted cultures was mostly a degradation product of the aggregating proteoglycan. It was recovered as a very large molecule slightly smaller than the monomers extracted with 4M-guanidinium chloride and lacked a functional hyaluronate binding region. The size and charge were consistent with a very limited cleavage or conformational change of the core protein near the hyaluronate binding region releasing the C-terminal portion of the molecule intact from the aggregate. The 'clipped' monomer diffuses very rapidly through the matrix into the medium. The amount of proteoglycan extracted with 4M-guanidinium chloride decreased during culture from both the controls and depleted cartilage, and the average size of the molecules initially remained the same. However, the proportion of molecules with a smaller average size increased with time and was predominant in explants that had lost more than 70% of their proteoglycan. All of this material was able to form aggregates when mixed with hyaluronate, and glycosaminoglycans were the same size and charge as normal, indicating either that the core protein had been cleaved in many places or that larger molecules were preferentially released. A large proportion of the easily extracted and non-extractable proteoglycan remained in the partially depleted cartilage and the molecules were the same size and charge as those found in the controls. There was no evidence of detectable glycosidase activity and only very limited sulphatase activity. A similar rate of breakdown and final distribution pattern was found for newly synthesized proteoglycan. Increased amounts of latent neutral metalloproteinases and acid proteinase activities were present in the medium of depleted cartilage. These were not thought to be involved in the breakdown of proteoglycan. Increased release of proteoglycan ceased within 24h of removal of the catabolin, indicating that the effect was reversible and persisted only while the stimulus was present.     

Expression of glutamate decarboxylase isoform, GAD65, in human mononuclear leucocytes: a possible implication of C-terminal end deletion by Western blot and RT-PCR study

Human peripheral blood leucocyte was examined for the expression of glutamate decarboxylase (GAD). Peripheral blood from healthy individuals was fractionated into polynuclear leucocytes and mononuclear leucocytes. Cell lysate from the mononuclear leucocytes was analysed by SDS-PAGE and Western blot. With antibody raised against unique C-terminal sequence for GAD65, two protein bands at 30 and 80 kDa were detected. However, with anti-GAD65/67 antibody recognizing very end of C-terminal, about 40 residues toward C-terminal, no protein bands were observed. Expression of mRNA coding for the epitope of these two antibodies was examined by using PCR technique. Results showed an evidence that mononuclear leucocytes express GAD65 with its C-terminal segment truncated. Our results have suggested an expression of GAD with the novel molecular weight may be caused by possible mononuclear leucocyte specific splicing errors.     

Latent collagenase of rheumatoid synovial fluid is not of granulocytic origin

The physicochemical properties of three latent collagenases derived from rheumatoid synovial fluid, polymorphonuclear leucocytes and culture medium of rheumatoid synovium were compared. It has been shown that synovial fluid enzyme is similar to that of synovium collagenase from tissue culture and differs significantly in molecular size and protein charge from granulocyte collagenase. The results indicate that the latent, trypsin-activable collagenase present in rheumatoid synovial fluid is not of granulocytic origin and seems to derive from the synovial membrane.     

Modulation of the activity of sea bass (Dicentrarchus labrax) head-kidney macrophages by macrophage activating factor(s) and lipopolysaccharide

The aim of this study was to establish the requirements for macrophage activating factor (MAF) production by sea bass head-kidney leucocytes and the kinetics of macrophage activation when exposed to MAF-containing supernatants and/or lipopolysaccharide (LPS), a known macrophage stimulant. MAF activity was found in culture supernatants of total head-kidney leucocytes pulsed with 5 microg ml(-1)Con A, 5 or 10 ng ml(-1)PMA and 100 ng ml(-1)calcium ionophore, or 10 microg ml(-1)Con A alone, as assessed by the capacity to prime macrophages for enhanced production of reactive oxygen intermediates (ROI). Mixed leucocyte cultures from two or eight fish showed higher MAF activity after stimulation, indicating that a mixed leucocyte reaction was also important for MAF production. MAF-induced activation of macrophage cultures was highest at 18 h of exposure and was lost by 72 h except for MAF induced by Con A-stimulation alone. LPS primed macrophages for increased ROI production at early incubation times and down-regulated ROI production after 24 h. LPS had no effect in further stimulating the MAF-induced priming effect on production of ROI and down-regulated the MAF-priming by 48 h. Sea bass head-kidney macrophages did not show increased nitrite production when exposed to MAF and/or LPS, which may be related to their differentiation status.     

A 3D system for culturing human articular chondrocytes in synovial fluid

Cartilage destruction is a central pathological feature of osteoarthritis, a leading cause of disability in the US. Cartilage in the adult does not regenerate very efficiently in vivo; and as a result, osteoarthritis leads to irreversible cartilage loss and is accompanied by chronic pain and immobility (1,2). Cartilage tissue engineering offers promising potential to regenerate and restore tissue function. This technology typically involves seeding chondrocytes into natural or synthetic scaffolds and culturing the resulting 3D construct in a balanced medium over a period of time with a goal of engineering a biochemically and biomechanically mature tissue that can be transplanted into a defect site in vivo (3-6). Achieving an optimal condition for chondrocyte growth and matrix deposition is essential for the success of cartilage tissue engineering. In the native joint cavity, cartilage at the articular surface of the bone is bathed in synovial fluid. This clear and viscous fluid provides nutrients to the avascular articular cartilage and contains growth factors, cytokines and enzymes that are important for chondrocyte metabolism (7,8). Furthermore, synovial fluid facilitates low-friction movement between cartilaginous surfaces mainly through secreting two key components, hyaluronan and lubricin (9 10). In contrast, tissue engineered cartilage is most often cultured in artificial media. While these media are likely able to provide more defined conditions for studying chondrocyte metabolism, synovial fluid most accurately reflects the natural environment of which articular chondrocytes reside in. Indeed, synovial fluid has the advantage of being easy to obtain and store, and can often be regularly replenished by the body. Several groups have supplemented the culture medium with synovial fluid in growing human, bovine, rabbit and dog chondrocytes, but mostly used only low levels of synovial fluid (below 20%) (11-25). While chicken, horse and human chondrocytes have been cultured in the medium with higher percentage of synovial fluid, these culture systems were two-dimensional (26-28). Here we present our method of culturing human articular chondrocytes in a 3D system with a high percentage of synovial fluid (up to 100%) over a period of 21 days. In doing so, we overcame a major hurdle presented by the high viscosity of the synovial fluid. This system provides the possibility of studying human chondrocytes in synovial fluid in a 3D setting, which can be further combined with two other important factors (oxygen tension and mechanical loading) (29,30) that constitute the natural environment for cartilage to mimic the natural milieu for cartilage growth. Furthermore, This system may also be used for assaying synovial fluid activity on chondrocytes and provide a platform for developing cartilage regeneration technologies and therapeutic options for arthritis.     

Release of neutral proteinases from mononuclear phagocytes and synovial cells in response to cartilaginous wear particles in vitro

Cartilaginous wear particles were retrieved from synovial fluid aspirates of human diarthrodial joints and added to cultures of human or murine mononuclear phagocytes or human synovial cells. In each case, addition of the wear particles elevated the production of proteinases active at neutral pH against collagen, gelatin, azocasein and the synthetic pentapeptide phenylazobenzyloxycarbonyl-L-Pro-L-Leu-Gly-L-Pro-D-Arg. Synovial cells secreted more than five times as much collagenase as the same number of the other cells. All types of cell secreted significant quantities of enzymes active against the noncollagenous substrates. Mild treatment of the spent media with trypsin stimulated all of these enzymic activities. The spent culture media of synovial cells which had been exposed to cartilaginous wear particles released hydroxyproline and glycosaminoglycan from powdered cartilage, indicating the production of enzymes which degrade both the collagen and proteoglycan of the cartilaginous matrix. Cultures of mononuclear phagocytes, in contrast, while solubilizing chondroitin sulphate from cartilage, released very little hydroxyproline. The ability of wear particles to elicit these effects suggests a role for them in the pathogenesis of oesteoarthritis and other types of joint deterioration.     

Protein kinase and phosphatases from human polymorphonuclear leucoytes.

Purified preparations of human polymorphonuclear leucocytes contain a protein kinase in the cytosol which is stimulated by cyclic AMP and cyclic IMP but not by other cyclic nucleotides. The holoenzyme had a molecular weight of 66000 estimated by gel filtration; when it was incubated with histone or cyclic AMP, it dissociated into two smaller subunits of molecular weight 45000 and 30000; the former remained cyclic AMP-sensitive, whereas the latter had become independent of added cyclic AMP. By means of substrate-affinity chromatography on histone-Sepharose 4B, cyclic [3H5AMP-binding activity (regulatory or R subunit) could be resolved into two peaks of enzyme activity, one again independent of added cyclic AMP, with a molecular weight of 30000 (catalytic or C subunit). Also by means of substrate-affinity chromatography it was possible to resolve 'specific' polymorphonuclear leukocyte histone phosphatases from 'non-specific' phosphomonesterases capable of dephosphorylating histone previously phosphorylated by the protein kinase. Specific histone phosphatase displayed greatest affinity for histone-Sepharose 4B, followed by acid p-nitrophenyl phosphatase, and the unretained acid beta-glucerophosphatase. Polymorphonuclear leucocyte histone phosphatase, purified approx. 40-fold, was further resolved from the other phosphatases by gel filtration on Sephadex G-150 from which it was eluted with apparent molecular weights of 45000 and 18700. The apparent Km values for dephosphorylation of histone are 4.3 X 10-6M and 3.6 X 10-6M. Most (69%) of cytoplasmic histone phosphatase was found in the cell sap, whereas 20% remained tightly associated with polymorphonuclear leucocyte lysosomes from which it could not be solubilized by treatments (Triton X-100, freeze-thawing) that released approx. 70% of lysosomal beta-glucuronidase or acid phosphatases. Although both soluble and particulate enzymes required 5-10 mM-Mn2 for maximal activation, and showed a pH maximum of 6.5-7.0, only the particulate enzyme was partly inhibited by ammonium molybdate. Polymorphonuclear leucocyte histone phosphatases were neither inhibited nor stimulated by those cyclic nucleotides that greatly stimulate the protein kinase of the same subcellular fraction     

Characteristics of tumor necrosis factor production in rheumatoid arthritis

The biological effects of tumor necrosis factor (TNF) include the enhancement of fibroblast proliferation, the secretion of collagenase and prostaglandin E2 (PGE2) by fibroblasts, and the resorption of bone and cartilage, suggesting a role for this cytokine in arthritic conditions. To investigate this, we measured the levels of TNF in synovial fluids and evaluated its secretion by synovial fluid mononuclear cells and tissues from patients with rheumatoid arthritis, osteoarthritis, and seronegative arthritis and normals. TNF was found to be secreted in all arthritic conditions but not in normals. The levels of TNF were highest in synovial fluid and correlated with interferon-gamma (IFN-gamma) levels but not PGE2. The production of TNF was stable in a single joint for 3 to 6 months. Using immunohistochemical staining, TNF was localized to mononuclear cells in the lining layer, sublining, and perivascular areas of synovial tissue. The secretion of TNF by rheumatoid synovial fluid mononuclear cells was inhibited by PGE2, while IFN-gamma enhanced its production in those cells which were spontaneously secreting TNF. Our data suggest that TNF may play a role in various arthritic diseases.     

Cleavage of cartilage oligomeric matrix protein (thrombospondin-5) by matrix metalloproteinases and a disintegrin and metalloproteinase with thrombospondin motifs.

Cartilage oligomeric matrix protein (COMP) is a pentameric glycoprotein present in cartilage, tendon and ligament. Fragments of the molecule are present in the diseased cartilage, synovial fluid and serum of patients with knee injuries, osteoarthritis and rheumatoid arthritis. Although COMP is a substrate for several matrix metalloproteinases (MMPs), the enzymes responsible for COMP degradation in vivo have yet to be identified. In this study we utilised well-established bovine cartilage culture models to examine IL-1alpha-stimulated COMP proteolysis in the presence and absence of MMP inhibitors. COMP was released from bovine nasal cartilage, in response to IL-1alpha, at an intermediate time between proteoglycans and type II collagen, when soluble MMP levels in the culture medium were undetectable. The major fragment of COMP released following IL-1alpha-stimulation migrated with an apparent molecular mass of approximately 110 kDa (Fragment-110) and co-migrated with both the major fragment present in human arthritic synovial fluid samples and the product of COMP cleavage by purified MMP-9. However, the broad-spectrum MMP and ADAM inhibitor BB94 only partially inhibited the formation of Fragment-110 and failed to inhibit COMP release significantly. Therefore the results of these studies indicate a role for proteinases other than MMPs in the degradation of COMP in bovine cartilage. It was further demonstrated that purified COMP was cleaved by ADAMTS-4, but not ADAMTS-1 or -5, to yield a fragment which co-migrated with Fragment-110. Therefore this is the first demonstration of COMP as a substrate for ADAMTS-4, although it remains to be determined whether this enzyme plays a role in COMP degradation in vivo.     

Regulation of hepatic acute phase protein synthesis by products of interleukin 2 (IL 2)-stimulated human peripheral blood mononuclear cells

Cancer patients injected with recombinant human IL 2 develop marked changes in serum concentrations of hepatic acute-phase proteins. To determine if this acute-phase response involves a change in the rate of hepatic protein synthesis and if it is due to a direct effect of IL 2 on hepatocytes, human hepatoma-derived hepatocytes (Hep-3B cells) were incubated in medium containing IL 2 or in culture supernatants from IL 2-activated human peripheral blood mononuclear cells (PBMNC). The rate of synthesis of two acute-phase proteins, complement protein factor B and albumin, was determined by the incorporation of a radiolabeled amino acid precursor into newly synthesized protein as measured by analytical gel electrophoresis of immunoprecipitates. IL 2 in concentrations from 1 to 1000 U/ml had no effect on the synthesis of factor B or albumin; conversely, there was a dose-dependent increase in the rate of synthesis of factor B and decrease in albumin synthesis mediated by culture supernatants of IL 2-activated PBMNC. The magnitude of the effect of acute-phase protein synthesis was dependent on the IL 2 concentration used for the activation of PBMNC. The rate of factor B synthesis increased approximately 4.0-fold in the presence of culture supernatants of PBMNC activated with either opsonized heat-killed Staphylococcus albus or with 1000 U/ml IL 2. Preincubation of the IL 2-activated PBMNC culture supernatants with an antiserum specific for recombinant IL 1-beta completely neutralized the capacity of the supernatants to stimulate factor B synthesis, whereas antisera specific for human IL 1-alpha or for tumor necrosis factor had no effect. These results indicate that the indirect effect of IL 2 on hepatic acute phase protein synthesis is mediated by IL 1-beta.