Inhibition of interleukin 1 activity by a factor in submandibular glands of rats

With the sequential use of ammonium sulfate precipitation, gel filtration and chromatofocusing, we have partially purified from extracts of the submandibular glands of rats a factor (referred to as submandibular gland's immunosuppressive factor or SMG-ISF) capable of inhibiting the in vitro proliferation of mitogen- and antigen-stimulated murine lymphocytes. The semi-purified suppressor fractions had an isoelectric point of 4.4 to 4.5 and consisted of at least three molecular species. These active fractions suppressed the mitogenic effects of Concanavalin A phytohemagglutinin, and lipopolysaccharide. In vitro immune reactions such as the mixed lymphocyte culture MLC reaction and the production of cytotoxic T lymphocytes (CTL) across major histocompatibility barriers in mice were also suppressed. These in vitro immunosuppressive effects required the addition of the suppressor fractions early after the initiation of the cultures and were reversed if the factor was removed from the cultures at least 48 to 72 hr before the completion of the assays. The active fractions did not affect the proliferation of CTLL 2 cells induced by interleukin 2 (IL 2), but inhibited the mitogenic and co-stimulatory effects of IL 1 on mouse thymocytes, and in this effect showed a dose-response relation suggestive of a competitive mechanism. These characteristics of SMG-ISF indicate a specific inhibition of the activity of IL 1.     

Interleukin inhibition by a parasite proteinase inhibitor, taeniaestatin

A proteinase inhibitor, taeniaestatin, isolated from the larval stage of the cestode Taenia taeniaeformis inhibits endogenous IL 2 generation in murine lymphocytes and IL 1 induced proliferation of murine thymocytes in a dose-dependent manner. However, taeniaestatin does not inhibit exogenous IL 2-induced proliferation of an IL 2-dependent cell line at any dose tested. These data indicate that the lack of IL 2 generation may be due in part to inhibition of a crucial cell-associated proteinase subsequent to cellular activation, or the lack of an effective IL 2 signal for differentiation. Our results are novel findings concerning molecular pathways for parasite inhibition of host immune responses, and suggest that selected proteinase inhibitors may be useful in clinical situations in which IL 1 or IL 2 are elevated.     

Interleukin 1 enhances synovial cell hyaluronate synthesis

Interleukin 1 enhances proliferation of murine thymocytes in the presence of lectins, and is also known to stimulate the release of prostaglandins and neutral proteases from a variety of cell types. We have previously shown that a factor isolated from the culture media of disaggregated lining cells of the human synovial membrane was indistinguishable from monocyte-derived interleukin 1. We report here that interleukin 1 from either source stimulates hyaluronate synthesis by synovial membrane cells. Upon gel filtration or isoelectric focusing of synovial cell supernatants, the hyaluronate-stimulatory activity co-fractionates with the interleukin 1 activity. Enhanced cell secretion of hyaluronate is a newly described metabolic effect of interleukin 1.     

Augmentation of IL 1-induced fibroblast PGE2 production by a urine-derived IL 1 inhibitor

IL 1, a monocyte-derived cytokine, has potent biologic effects in a variety of target tissues. The existence of naturally occurring inhibitors of IL 1 activity has been recently described; these inhibitors blocked one IL 1 effect: stimulation of thymocyte responses to mitogens. We examined the effect of one well-characterized inhibitor of IL 1, isolated from the urine of febrile patients, on a second IL 1 effect, stimulation of fibroblast PGE synthesis. In this system, purified preparations of the urinary inhibitor that completely blocked murine thymocyte proliferative responses to mitogen failed to block PGE synthetic responses to IL 1. Rather, inhibitor preparations markedly enhanced fibroblast PGE synthetic responses to IL 1. When partially purified inhibitor preparations were fractionated by ion exchange chromatography, inhibitory activity for the IL 1 effect on thymocytes and PGE stimulatory activity co-eluted. Augmentation of the IL 1-induced PGE response was seen with both low (1:1 unit) and high (400:1) ratios of inhibitor to IL 1. Inhibitor preparations alone did not stimulate fibroblast PGE synthesis. The augmentation of fibroblast PGE synthesis by inhibitor preparations was not due to contaminating endotoxin. Active inhibitor preparations contained less than 15 pg of endotoxin/U activity, and the PGE stimulatory effect was not blocked by the addition of polymyxin B, whereas polymyxin B reversed the effects of exogenous endotoxin. It appears that the inhibition of IL 1 effects by naturally occurring inhibitors may have target cell and/or functional specificity.     

Induction of IL 2 receptor expression and cytotoxicity of thymocytes by stimulation with TCF1

We investigated the role of T cell cytotoxicity inducing factor 1 (TCF1) in the induction of a cytotoxic T cell response. We found that help-deficient thymocyte cultures supplied with saturating amounts of purified IL 2 did not develop CTL in a 5-day culture. The expression of cytotoxicity was dependent on the addition of TCF1 derived from the T cell hybridoma K15. TCF1 also induced proliferation of thymocytes in the presence of IL 2. Only the PNA- thymocyte subpopulation responded to TCF1 with proliferation and cytotoxicity in the presence of IL 2. The monokine IL 1 also induced proliferation in this subpopulation but failed to induce cytotoxicity. IL 1 was further distinguished from TCF1 by inhibition of IL 1-induced but not TCF1-induced proliferation by anti-IL 1 antibodies. In addition, using anti-IL 2 receptor antibodies (AMT 13), we showed that TCF1 in the presence of IL 2 substantially increased IL 2 receptor expression in thymocytes. IL 1 had the same effect on induction of IL 2 receptor expression as TCF1. Because some effects of IL 1 and TCF1 are distinct and some overlap, we discuss whether IL 1 and TCF1 induce different subsets of PNA- thymocytes.     

Interleukin 2 responses of lpr and normal L3T4-/Lyt-2- T cells induced by TPA plus A23187

The major population of cells that accumulate abnormally in MRL/Mp-lpr/lpr lymphoid tissue is Thy-1+, L3T4-, and Lyt-2-. To clarify the functional potential of these cells, we examined their proliferation, interleukin 2 (IL 2) receptor expression, and IL 2 secretion by using as stimulants the combination of 12-O-tetradecanoylphorbol-2-acetate and A23187 (a calcium ionophore). Although the lpr T cells were capable of responding to these stimulants, the nature of the response and of the concentrations of ligand required differed sharply from the responses of normal adult T cells, and of adult L3T4-Lyt-2- thymocytes. There was a strong similarity but not identity when responses of 16 day fetal thymocytes were compared with those of lpr L3T4-Lyt-2- cells. The unusual functional properties of the lpr cells, such as high A23187 dose requirement for maximal proliferation, low percentage of IL 2 receptor-expressing cells, and low levels of IL 2 secretion, suggested that these cells are arrested at a stage of development similar to that of 16-day fetal thymocytes and before adult L3T4-/Lyt-2- thymocytes.     

Interleukin 1 and myelin basic protein synergistically augment adoptive transfer activity of lymphocytes mediating experimental autoimmune encephalomyelitis in Lewis rats

This investigation focused on the role of adherent accessory cells and their cellular product, interleukin 1 (IL 1), in cellular immune responses associated with experimental autoimmune encephalomyelitis (EAE) in Lewis rats. Guinea pig myelin basic protein (GPMBP)-sensitized lymph node cells (LNC) responded in culture with GPMBP by undergoing activation as measured by augmented transfer of EAE to syngeneic recipients, and proliferation as measured by [3H]thymidine incorporation. GPMBP-sensitized LNC, after depletion of adherent accessory cells, no longer responded to GPMBP in the EAE transfer activation assay. In contrast, aliquots of the same LNC preparation exhibited proliferative responses to GPMBP that were only partially reduced. Addition of irradiated thymocytes to adherent cell-depleted cultures fully reconstituted responsiveness to GPMBP in the activation assay and restored full reactivity to GPMBP in the proliferation assay. Furthermore, addition of either purified human IL 1 or recombinant human IL 1 to adherent cell-depleted cultures reconstituted reactivity to GPMBP in the EAE transfer activation assay and augmented GPMBP-specific proliferative responses. Anti-Ia monoclonal antibodies blocked GPMBP + IL 1-induced cellular activation of nonadherent LNC. These results demonstrate that both IL 1 and Ia molecules are important in the pathway leading to GPMBP-induced activation of EAE-inducing T lymphocytes. Furthermore, these results suggest that different accessory signals may be required for optimal induction of GPMBP-induced lymphocyte activation vs GPMBP-specific proliferative responses.     

Temperature sensitivity of interleukin-dependent murine T cell proliferation: Q2 mapping of the responses of peanut agglutinin-negative thymocytes

Recent studies in our laboratory have shown that IL 1-dependent mitogenic activity is present in apparently homogeneous preparations of rabbit endogenous pyrogen. This finding suggested that the mitogenic and pyrogenic activities of this molecule might serve a common goal. Initial studies on the temperature dependence of interleukin-dependent thymocyte mitogenesis suggested that high temperature sensitivity was associated with the action of IL 1 but not that of IL 2. The present study has used an expanded range of temperatures and refined tissue culture conditions to further examine the relative temperature dependence of thymocyte mitogenesis due to IL 1 or IL 2. Both the pI 5 and pI 7 species of rabbit IL 1 evoke highly temperature-sensitive responses from mouse thymocytes in the presence of a suboptimal dose of PHA and from peanut agglutinin-negative (PNA-) thymocytes in the absence of PHA. IL 2 also evokes a highly temperature-sensitive response from unseparated thymocytes in the presence of PHA. However, in the absence of PHA, vigorous responses by either unseparated or PNA- thymocytes to IL 2 alone lack strong temperature sensitivity. The temperature-dependent responses of both unseparated and PNA- thymocytes to either IL 1 or IL 2 have been analyzed by Q2 mapping, a determination of the temperature intervals most sensitive to temperature changes. By using this mode of analysis, we have found that IL 1 and IL 2 generate distinct Q2 maps, and that PHA transforms the shape of the IL 2-derived Q2 map but not that of IL 1. The possible significance of the temperature sensitivity of IL 1- and IL 2-driven reactions is discussed with respect to the biological functions of inflammation and fever.     

Human thymic epithelial cells produce interleukin 1

Although the thymus plays a critical role in generation of immunocompetent T lymphocytes, the precise role of the epithelial component of the thymus in the induction of T cell proliferation and maturation remains unknown. Since interleukin 1 (IL 1) is required for mature T cell activation, we have determined whether human thymic epithelial (TE) cells produce IL 1. By using a system for longterm culture of human TE cells, we found that human TE cells produced an IL 1-like factor (TE-IL 1) that augmented the proliferation of C3H/HeJ mouse thymocytes to phytohemagglutinin. IL 1 activity (20 to 200 U/ml) was detected in supernatants of TE cultures from all individuals (2 to 13 yr old) tested. IL 1 activity was also detected in supernatants of TE cultures from a 17-wk fetus but not from a 10-wk fetus. Production of TE-IL 1 was dependent on TE cell density and time in culture with optimal TE-IL 1 activity observed at 10(6) TE cells/ml after 48 to 72 hr of culture. With the use of high performance liquid chromatography, TE-IL 1 chromatographed as a molecule of 18,000 to 20,000 relative molecular mass, and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, TE-IL 1 migrated at 15,000 to 17,000 Mr. With the use of isoelectrofocusing gels, charge heterogeneity of TE-IL 1 was demonstrated with two major isoelectric points of 5.7 to 5.8 and 6.9 to 7.0. Polyclonal antibody to human monocyte IL 1 markedly inhibited the TE-IL 1 activity. In indirect immunofluorescence assay of frozen human thymic sections, rabbit anti-IL 1 antibody reacted with epithelial cells in human thymic cortex and medulla. Furthermore, high performance liquid chromatography-purified TE-IL 1 augmented human thymocyte proliferation to suboptimal concentrations of phytohemagglutinin. Thus, thymic epithelial cells are capable of providing an intrathymic source of IL 1-like cytokine (TE-IL 1), which affects thymocyte proliferation. We propose that TE-IL 1 may play an important role in intrathymic proliferation and differentiation of human thymocytes.     

Study of the biological activities of toxic shock syndrome toxin-1. I. Proliferative response and interleukin 2 production by T cells stimulated with the toxin

The mitogenic and interleukin 2 (IL 2) production-inducing effects of toxic shock syndrome toxin-1 (TSST-1) on murine lymphocytes were investigated. TSST-1, an exotoxin produced by Staphylococcus aureus recovered from patients with toxic shock syndrome (TSS), is thought to be a causative agent of the syndrome. TSST-1 was mitogenic for splenic T cells and peanut agglutinin (PNA)-negative thymocytes, but not for T cell-depleted spleen cells, PNA-positive thymocytes or IL 2-dependent CTLL 2-cells. A factor mitogenic for CTCC-2 cells with a molecular weight of 30-35 kdaltons was obtained by stimulating spleen cells with TSST-1 and it was absorbed by CTLL-2 cells, indicating that the factor is IL 2. For substantial amounts of IL 2 to be produced, 10 ng or more of TSST-1 per ml and 48 hr or more of incubation were required. Removal of T cells abrogated the IL 2 production by spleen cells. T cells obtained by the nylon wool column method alone produced IL 2 on TSST-1 stimulation in the presence of either macrophages or a macrophage lysate containing interleukin 1. However, T cells obtained by a combination of the nylon wool column method and anti-Ia antibody treatment produced IL 2 in the presence of macrophages but not of the macrophage lysate, indicating that IL 2 production by TSST-1-stimulated T cells is absolutely dependent on the presence of accessory cells.     

Characterization of fibroblast proliferation factors elaborated by antigen- and mitogen-stimulated guinea pig lymph node cells: Differentiation from lymphocyte-derived chemotactic factor for fibroblasts,lymphocyte mitogenic factor,and interleukin 1

Guinea pig lymph node cells stimulated in culture by T-cell mitogens or sensitizing antigens release ~60,000- and ~16,000-mol wt proteins that induce normal guinea pig fibroblasts to proliferate in vitro. These fibroblast proliferation factors can be separated from lymphocytederived chemotactic factor for fibroblasts and from lymphocyte mitogenic factor by gel filtration employing Sephadex G-100. The 16,000-mol wt fibroblast proliferation factor was found to coelute with interleukin 1 (IL 1) from gel filtration columns. When the 16,000 molecular weight factor was further analyzed by anion exchange-high-performance liquid chromatography five major peaks containing IL 1 activity were obtained, only one contained fibroblast proliferation activity, suggesting forms of IL 1 exist that are not mitogenic for fibroblast. Occasionally, a large-molecular-weight inhibitor of fibroblast proliferation was detectable in void volume fractions from gel filtration of supernatant from antigen-stimulated lymph node cell cultures. This inhibition was accompanied by gross aggregation of fibroblasts. These studies suggest that fibroblast accumulation at sites of certain cell-mediated immune reactions in vivo may in part be attributable to the release of mediators by lymphocytes and, or macrophages that induce fibroblast growth.     

Interleukin 2 does not induce phosphatidylinositol hydrolysis in activated T cells

Hydrolysis of phosphatidylinositol-4,5-bisphosphate to diacylglycerol and myoinositol-1,4,5-trisphosphate is thought to be a primary event in the activation of cells by some growth factors, mitogenic lectins, and oncogenes. The mechanism whereby interleukin 2 (IL 2) binding to its receptor on activated T lymphocytes leads to cell proliferation has not been determined. Because the mitogenic has not been determined. Because the mitogenic action of IL 2 resembles that of some growth factors, the possible role of phosphatidylinositol breakdown in the activation of T cells by IL 2 was examined. In human or murine IL 2-sensitive cells, incubation with IL 2 did not alter the rate of turnover of phosphatidylinositol, phosphatidylinositol-5-phosphate, phosphatidylinositol-4,5-bisphosphate, or phosphatidylcholine in 32PO4-loaded cells. IL 2 also did not alter either the isotopic labeling of diacylglycerol or [3H]arachidonic acid release from cells. In addition, IL 2 did not alter the rate of formation of the phosphatidylinositol breakdown products myoinositol-1,4,5-trisphosphate, myoinositol-1,4-bisphosphate, or myoinositol-1-phosphate. In contrast, under similar conditions, IL 2 induced significant increases in [3H]thymidine incorporation and cell proliferation. Mitogenic lectins such as concanavalin A and phytohemagglutinin gave significant changes in isotopic labeling of phosphoinositols, diacylglycerols, and phosphatidylinositols, indicating that phosphatidylinositol hydrolysis induced by mitogenic lectins was detectable in the assay systems. IL 2, in contrast to other growth factors, does not appear to signal cells by increasing phosphatidylinositol breakdown.     

Generation of cytotoxic T lymphocytes from thymocyte precursors to trinitrophenyl-modified self antigens. II. Establishment of a T cell hybrid clone constitutively producing killer-helper factor(s) (KHF) and functional analysis of released KHF

T cell hybridoma lines were constructed by fusion of Mycobacterium tuberculosis-primed and boosted BALB/c T cells with the AKR-derived T lymphoma cell line BW5147. Certain of the hybridomas prepared in this manner secreted constitutively into their culture supernatants biologically active molecules that displayed precursors of cytotoxic T cell activating properties characteristic of killer-helper factor (KHF). Cell surface analysis revealed that the hybridomas were indeed somatic cell hybrids between the two respective partner cells used for fusion. KHF properties of these hybridoma supernatants were verified by their capacity to stimulate peanut agglutinin-binding (PNA+) C3H/He thymocytes to respond in vitro to 2,4,6-trinitrophenyl(TNP)-modified syngeneic stimulator cells in conjunction with suboptimal doses (10 U/ml) of interleukin 2 (IL 2) for the generation of H-2-restricted, TNP-reactive cytotoxic T cells. The biologically active molecules secreted by a T cell hybrid clone (2Y4) were, like conventional KHF, distinct from IL 1, IL 2, or immune interferon (IFN-gamma). The partially purified KHF derived from 2Y4 cells shows activity at apparent m.w. range of 34,000 to 60,000 on gel permeation, and is relatively homogeneous with respect to isoelectric point, which was approximately 4.5 to 4.7. The partially purified 2Y4-KHF is able to augment proliferation of as well as the expression of IL 2 receptors on PNA+ thymocytes in conjunction with IL 2. Finally, addition of 2Y4-KHF on day 0, followed by the addition of IL 2 on day 2 for 7 days of culture was effective in generating potent CTL responses, whereas addition of IL 2 on day 0, followed by the addition of 2Y4-KHF on day 2 to the culture was ineffective.     

Lymphokine-like activity of 8-mercaptoguanosine: induction of T and B cell differentiation

Lymphocyte proliferation and differentiation result from ordered cellular interactions governed by soluble products (lymphokines). Dissecting the individual steps in these processes has been difficult, due to a paucity of pure lymphokines. Recently, it was reported that the derivatized ribonucleoside 8-mercaptoguanosine (8MGuo) has both mitogenic and differentiative effects on murine B cells. In the present studies, we tested 8MGuo for its ability to stimulate both B and T cell responses. In contrast to the murine studies, 8MGuo does not stimulate rat B cells to proliferate and, when tested for B cell growth factor-like activity, no stimulation was observed. The addition of 8MGuo (0.5 to 1 mM final concentration) to mitogen-stimulated B cells led to a marked increase in IgM and a modest increase in IgG secretion. When mixed with conditioned medium, 8MGuo acted synergistically in stimulating secretion of both isotypes, arguing that 8MGuo has both B cell-differentiating factor-mu (BCDF-mu) and BCDF-gamma activity. 8MGuo had no IL 2-like activity when tested on a mouse IL 2-dependent cell line, and no IL 1-like activity on addition to mouse thymocytes with or without submitogenic doses of lectin. However, when added to cultures of murine allogeneic cells in which the stimulating cell populations had been heat-inactivated, 8MGuo induced the generation of specific allogeneic cytotoxic T lymphocytes. Together, these results suggest that a simple derivatized nucleoside can induce both T and B cell differentiation without concomitant proliferation, and thus represent a unique probe for studying events in lymphocyte differentiation.     

Lymphokine regulation of activated (G1) lymphocytes. I. Prostaglandin E2-induced inhibition of interleukin 2 production

PGE2-induced inhibition of the proliferatory response of PHA-stimulated human PBL and Con A-stimulated murine thymocytes was analyzed by flow cytometry. It was found that the activation process (G0-G1a transition) was not influenced by PGE2 over a wide range of concentrations (10(-10) to 10(-6) M), nor was the formation of IL 2 receptors inhibited. Similarly, the viability of human lymphocytes was practically unaltered. In contrast, the IL 2-dependent cell cycle event (G1a-G1b transition), which is required for proliferation, was inhibited in a dose-dependent fashion. The addition of IL 2-containing supernatants to such cultures prevented the PGE2-mediated block in the G1a phase and reconstituted a normal lymphocyte proliferation. Furthermore, lower IL 2 titers were measured in supernatants from PHA-stimulated human PBL treated with PGE2. These findings strongly suggest that PGE2 primarily exerts its inhibitory effect on lymphocyte proliferation through an inhibition of IL 2 production.     

OKT8 antibody inhibits OKT3-induced IL 2 production and proliferation in OKT8+ cells

We studied IL 2 production and proliferation induced by OKT3 mitogenic monoclonal antibody in the OKT8+ T cell subset. OKT3 antibody induced IL 2 production and proliferation in OKT8+ cells in a typical time-dependent manner: maximal IL 2 levels were found in 24 hr culture supernatants; maximal proliferation was found on day 3. OKT3 antibody was mitogenic over a wide range of concentrations (0.125 to 500 ng/ml). The presence of OKT8 antibody (greater than or equal to 100 ng/ml) in these cultures resulted in almost complete inhibition of IL 2 production and proliferation. Kinetic studies demonstrate that OKT8 antibody suppresses both IL 2 production and response to exogenous IL 2 in OKT8+ cells when added within the first 2 hr of culture. After 14 to 20 hr of culture, addition of OKT8 only blocks IL 2 production but not the IL 2 response of activated OKT8+ cells. The specificity of inhibition by OKT8 antibody of OKT3 mitogenicity on OKT8+ cells was confirmed by the failure of Leu-I and OKT4 antibody to produce the same effect and by the lack of inhibition by OKT8 antibody of OKT3-induced IL 2 production and proliferation in OKT4+ cells.     

Spontaneous production of a suppressor factor by the human macrophage-like cell line U937. I. Suppression of interleukin 1, interleukin 2, and mitogen-induced blastogenesis in mouse thymocytes

The human macrophage-like cell line U937 spontaneously produced a nondialyzable factor that inhibited interleukin 1 (IL 1), interleukin 2 (IL 2), and phytohemagglutinin (PHA)-induced blastogenesis in mouse thymocytes. The suppression by U937 supernatant factor occurred independently of the concentration of IL 1 or PHA, indicating that it was noncompetitive. The U937 suppressor factor was not cytotoxic for thymocytes, nor did it affect the spontaneous proliferation of T lymphoblastoid cell lines and U937. Physicochemical characterization showed that the U937 suppressor factor was nondialyzable, partially inactivated by heat treatment (56 degrees C), ammonium sulfate (67% saturation) precipitable, sensitive to pH 2.5, and resistant to freeze-thawing. Molecular weight of the factor inhibiting co-mitogenic IL 1 activity was approximately 85,000, as estimated by gel filtration. The U937 cell line may provide a model for the study of mechanisms and mediators of immunosuppression by mononuclear phagocytes.     

Interleukin 1-driven secretion of interleukin 2 is highly temperature-dependent

To investigate the temperature dependence of T lymphocyte activation, we have examined the IL 1-induced secretion of IL 2 by the murine T lymphoma cell line LBRM-33-1A5. During 24-hr incubations, there are modest increases in IL 2 secretion as culture temperatures are increased from 33 degrees to 37 degrees C, but IL 2 secretion declines at higher temperatures. However, the kinetics of IL 2 release during the first 8 hr of culture are highly temperature-dependent. The rate of IL 2 release increases linearly with temperature over the range from 33 degrees to 41 degrees C, demonstrating temperature coefficients (Q10) greater than 70. In contrast, IL 2-promoted proliferation of a continuous T cell line is much less temperature-dependent with Q10 values of less than 4.0.     

Interleukin 1 production by human polymorphonuclear neutrophils

The purpose of this study was to determine whether human polymorphonuclear neutrophils (PMN), which share a common cell lineage with macrophages, could produce factors such as IL 1. Other properties which these two cell types share are their phagocytic nature and the common receptor and antigens on their cell surfaces. IL 1, in many of its physical, biochemical, and functional characteristics, is found to resemble endogenous pyrogen (EP). PMN have been cited as a possible cell source of EP, but there have also been reports in which the capacity of PMN to produce EP has been questioned. This study shows that normal human PMN can be stimulated by particulate agents such as zymosan and soluble agents such as phorbol myristic acetate to produce a factor(s) which induces proliferation of mouse thymocytes, i.e., PMN IL 1. This PMN IL 1 was released from PMN in a dose- and time-dependent fashion. PMN IL 1 was nondialyzable, was heat-labile, and was inactivated at pH below 5 and above 8. PMN IL 1 stimulated the proliferation of normal human synovial fibroblasts and caused release of a neutral protease (plasminogen activator) from synovial cells. The synovial and thymocyte-proliferating capacity of PMN IL 1 was not affected by the protease inhibitor aprotinin or by soybean trypsin inhibitor. Gel filtration studies estimate the m.w. of PMN IL 1 to be approximately 13,000 to 17,000.