The interleukin-1 family.

ARTICLE

The term interleukin-1 (IL-1) was suggested in 1979 during the "International Lymphokine Workshop" in Ermatingen, Switzerland, to designate a family of molecules produced by monocytic cells. Up until then each of these molecules was considered to be distinct and was referred to by a specific name using a nomenclature based on biological action. However, the comparison between different research at that point indicated such a similitude between the molecules concerned, that the proposal to group them under the same name was accepted [1].

The isolation of messenger RNA, the cloning of genes, their expression in bacteria and their sequencing have confirmed these analogies and have allowed to be shown that an IL-1 gene family exists both in man and in mice. This IL-1 family currently consists of 3 genes located in man on the long arm of chromosome 2 [2], the exon and intron organization of which indicates that they evolved from a common gene some 350 million years ago [3].

These genes encode for 3 distinct proteins, but presenting structural homologies. IL-1alpha and IL-1ß induce a wide variety of cell types to develop multiple functions and are therefore referred to as "agonist" molecules as opposed to the third member of the family, IL-1 receptor antagonist (IL-1Ra), which binds to the same receptors as IL-1alpha or ß but does not induce any intracellular signal and, therefore, acts as an inhibitor of IL-1 activity [3].

THE AGONISTS: IL-1alpha AND IL-1ß

Gene and protein structure

The first form of human IL-1 was cloned in 1984 by Auron et al. from peripheral blood monocytes. These authors isolated a cDNA coding for a precursor of 269 amino acids. A large N-terminal segment of this intra-cellular precursor was then cleaved to obtain a mature form of 153 amino acids, with a molecular weight of 17 kD and an iso-electric point of 7.0 [4]. A few months later, March et al. described the cloning and expression of two distinct forms of IL-1, derived from a library of human macrophage cDNA, calling them IL-1alpha and IL-1ß [5]. The IL-1ß sequence was identical to that previously isolated by Auron, whilst the IL-1alpha cDNA was homologous to the murine cDNA described at the same period by Lomedico [6]. As for the IL-1ß, the human IL-1alpha was firstly translated in the form of a precursor of 271 amino acids, then cleaved into a mature form of 159 amino acids with a molecular weight of 17 kD and an iso-electric point of 5.5 [5].

Human IL-1ß and IL-1alpha amino acid sequences are 26% homologous and the nucleotide sequences of their genes are 45% homologous [5, 7]. The genomic genes of the two IL-1 forms have also been cloned. They both contain 7 exons [8, 9].

A certain degree of polymorphism exists for these genes since two allelic forms are described for
IL-1ß [10] and several for IL-1alpha [11]. This polymorphism may be of clinical importance since a specific IL-1alpha allele is associated with juvenile polyarthritis [12] and a rare IL-1ß allele is related to the severity of rheumatoid arthritis erosions [13] or certain types of diabetes [14]. However, these findings remain to be confirmed.

IL-1alpha and ß are not only similar at gene level but also at the level of their three-dimensional structure since both are principally composed of 12 to 14 folds of ß-pleated sheets which assemble to form a trefoil-like structure [15]. In addition, both the arginine in position 4 on IL-1ß and that in position 12 on IL-1alpha, which are necessary for binding to receptors and for biological activity, occupy the same relative position in the crystallographic structure of these molecules [16].

Finally, neither the IL-1alpha nor the IL-1ß precursor contains a "signal sequence" characteristic of the proteins secreted. However, the mechanism leading to cleavage into mature forms and then to secretion is quite different for each of the two IL-1 forms.

Expression and synthesis

Despite their structural similarities, IL-1alpha and IL-1ß have in fact rather different mechanisms of expression, synthesis and secretion.

Firstly, the IL-1alpha gene does not contain sequences corresponding to the classical transcription initiation motif known as the "TATA box", whereas this motif is found in the IL-1ß gene [17]. The main elements regulating IL-1alpha expression are located upstream on the gene, with a positive regulatory element located quite near the transcription initiation site (­ 47 to ­ 103 bp) and a negative regulatory element much further away (­ 875 to ­ 3,600 bp) [18]. The transcription of the
IL-1ß gene seems to be controlled in a more complex way [19], but perhaps this appears to be the case because it has been more widely studied. There are two enhancing regions located far upstream on the gene, which, though independent, act in co-ordination. The first (­ 2,846 to ­ 2,896 bp) contains an element responding to cyclical AMP [17]. The second (­ 2,729 to ­ 2,782 bp) contains an element responding to lipopolyssacharide (LPS) called NF-IL-6 and a site analogous to the NFkB transcription factor [20]. Promoters nearer the transcription initiation site (­ 131 to + 14 bp) have also been identified. These are binding sites for the NFßA nuclear factor, which has very recently been shown to be analogous to the Spi-1/PU.1 factor described in monocyte cells [21, 22]. This observation is important for explaining the ability of peripheral blood monocytes to synthesize IL-1ß in response to a very weak concentration of LPS. In fact, monocytes express the Spi-1/PU.1 factor constitutively, which is not the case for the majority of other cells [23].

The transcription of IL-1alpha, as indeed that of IL-1ß, is stimulated during inflammatory and infectious processes by the immune complexes, certain coagulation and complement cascade proteins, substance P, and viral or bacterial products, in particular LPS [24]. It is also induced or enhanced by certain cytokines of lymphocyte origin, such as the granulocyte-macrophage colony stimulating factor (GM-CSF] [25] and interferon-gamma (IFN-gamma) [26, 27]. Monocytic cytokines such as tumour necrosis factor-alpha (TNF-alpha) also stimulate the production of IL-1 by endothelial cells [28], by monocytes [29] and by fibroblasts [30]. As for IL-1alpha and ß, each encourages its own production in endothelial cells [31] and in monocytes [32]. It has also been confirmed in vivo that the administration of IL-1 or TNF-alpha induces the appearance of detectable circulating levels of IL-1ß [33]. The IL-1ß gene does not, however, appear to be spontaneously expressed in normal monocytes. This is suggested by the negative results obtained using highly sensitive techniques such as the polymerase chain reaction (PCR) on mononuclear cells freshly isolated from the peripheral blood of healthy volunteers [3]. After stimulation in vitro by LPS, messenger RNA for both IL-1alpha and ß appears in the monocyte cells after 15 min, accumulates over 4 hours and then rapidly disappears, partly due to the appearance of a transcription repressor [34]. On the other hand, when the stimulation is carried out by IL-1 itself, the transcription of messenger RNA continues for more than 24 hours. However, this IL-1 self-enhancement is inhibited by IFN-gamma whose presence reduces the transcription of the IL-1ß gene stimulated by itself, whereas it increases the transcription induced by LPS [35].

Certain transcription stimuli, such as coagulation, adhesion, hypoxia and the complement C5a factor induce the accumulation of large quantities of IL-1ß mRNA in monocyte cells, without being followed by protein translation [36, 37]. This dissociation between transcription and translation is specific to IL-1ß and TNF-alpha, and probably constitutes one of the first regulatory mechanisms preventing a massive and inappropriate secretion of these cytokines. On the other hand, the addition of even a minimal quantity of LPS or IL-1 to these cells packed with untranslated messenger RNA results in a massive production of IL-1ß, the inhibition of translation having been lifted by a phenomenon which still remains unclear [37].

Finally, another stimulus type, composed of one or more molecules expressed at the surface of activated T lymphocytes, has recently been described and appears to constitute a very powerful activator of IL-1 synthesis [38, 39].

On the other hand, certain cytokines, such as interleukin-4 (IL-4), interleukin-10 (IL-10) and interleukin-13 (IL-13), have a suppressive effect on IL-1 synthesis. These cytokines are often grouped under the name of "T helper type 2" (Th2) and principally induce B lymphocyte expansion and immunoglobulin production, whilst inhibiting cell-mediated immunity. IL-4, IL-13 [40, 41] and IL-10 [42, 43] inhibit IL-1 production in cultured cells. The effect of IL-10 has also been confirmed in vivo in volunteers who had received an injection of this cytokine. It was noted that the production of IL-1, by cells isolated from the blood and then stimulated by LPS, fell substantially after an injection of IL-10 [44]. Other cytokines such as TGF-ß [45] and interleukin-6 (IL-6) [46] also tend to suppress IL-1 production. Finally, it has been shown that submitting subjects to a diet rich in unsaturated fatty acids (omega-3) reduces their capacity to produce
IL-1 [47-49].

There also exists an endocrine feedback mechanism for IL-1 production. This cytokine stimulates, via the production of hypothalamic and hypophysial hormones, the production of corticosteroids which have, in turn, an inhibiting effect on the production of IL-1 and TNF-alpha [50, 51].

Maturation and secretion

IL-1alpha and IL-1ß also differ substantially in relation to localisation, maturation and secretion since the former remains mainly intracellular, whilst the main part of the latter is secreted after cleaving by a specific protease. Due to the absence of a signal sequence, the immature forms, referred to as pro-IL-1alpha and pro-IL-1ß, remain in the cytosol after their translation and do not accumulate in any cell organelles. Unlike pro-IL-1ß, pro-IL-1alpha is just as active as the mature form, remains intracellular for the main part and acts at this level [52]. Moreover, IL-1alpha is only rarely found in the circulation or extra-cellular biological liquids and only in cases of serious illness when it may have its origin in lysed cells [53]. Pro-IL-1alpha, but not pro-IL-1ß, can however be cleaved by calcium-dependent membrane proteases called calpains, but this process remains an exception [54, 55]. IL-1alpha, or pro-IL-1alpha, functions rather like an autocrine intracellular messenger, in particular in endothelial and epithelial cells where it plays a role in the regulation of normal cellular differentiation [56]. It is possible that pro-IL-1alpha carries out its activities without leaving the cell which produces it. This hypothesis is supported by research showing that the introduction of an IL-1alpha antisense oligonucleotide modifies the growth of endothelial cells [57] and that complexes composed of pro-IL-1alpha and the IL-1 receptor bind to the DNA in the nucleus [58]. The origin of these complexes is still disputed. They may either be formed from intra-cellular receptors or originate from the internalization of membrane complexes [59, 60].

A small fraction of pro-IL-1alpha, synthesized by monocytes and B lymphocytes, is found at the surface of cells. In fact, myristyl groups bind to the lysines of 10 to 15% of the pro-IL-1alpha [61], which allows it to be transported to the surface of the cell where it can anchor by interactions with lectins [62, 63]. This membrane IL-1alpha is biologically active and its activity is neutralized by anti-IL-1alpha antibodies but not by anti-IL-1ß antibodies [24]. This activity can therefore be carried out paracrinely on neighbouring cells, in particular in inflammatory environments.

As far as IL-1ß is concerned, the greater part of pro-IL-1ß also remains in the cytosol until it is cleaved and transported to the extracellular environment. A small fraction of pro-IL-1ß is myristylated [61], as in the case of pro-IL-1alpha, but unlike the latter, pro-IL-1 does not anchor in the membrane and only has a very weak biological activity [64]. Monocyte cells secrete a small quantity of pro-IL-1ß by a mechanism which is still unknown, but which appears to be distinct from that for the secretion of the mature form [65]. In fact, pro-IL-1ß is not fully active biologically and is not secreted until it has undergone an intra-cellular cleaving into its mature form [66]. The specific enzyme responsible for this cleaving has recently been identified and is called the "IL-1ß converting enzyme" (ICE) [67, 68]. It is a cysteine protease which cleaves pro-IL-1ß between aspartic acid and alanine (aa 116-117). Another site for potential cleavage (aspartic acid in position 27) exists on the pro-IL-1ß molecule and probably explains the 22 kD IL-1ß form which is found in monocyte culture supernatants. ICE is highly specific to pro-IL-1ß and does not cleave pro-
IL-1alpha [69]. ICE is constitutively present in the majority of cells, but in the form of a 45 kD inactive precursor which requires two endo-cleavings before becoming an active heterodimer, composed of a 10 kD and a 20 kD chain containing the enzyme site [70, 71]. ICE activity is also regulated by the presence of a competitive inhibitor of the substrate consisting of a tetrapeptide whose presence reduces the maturation and secretion of IL-1ß and encourages the accumulation of pro-IL-1ß [68]. Moreover, it has very recently been shown that ICE has five isoforms, resulting from alternative splicings of messenger RNA, and that the ICE-alpha isoform is the most active, whereas ICE- is inactive and can have an inhibiting effect on enzyme activity by competition, [72].

In addition, a sequence homology has been shown to exist between the ICE gene and the ced-3 gene of the Caenorhabditis elegans nematode which plays a role in programmed cell death or apoptosis [73]. The over-expression of the ICE gene leads to increased apoptosis in cells transfected with this gene [74, 75]. However, ICE-deficient transgenic mice have a normally developed thymus and a normal apoptotic reaction to stress although the apoptosis induced by TNF or FAS protein activation is reduced [76]. These observations have led to numerous questions concerning the role of IL-1ß in apoptosis. However, currently it would appear that there exist other substrates for ICE and homologous enzymes, and that proteins, resulting from the activation of these enzymes and not belonging to the IL-1 family, play a major role in programmed cell death [23, 77]. It has, however, been recently shown that, in a model of apoptosis triggered by hypoxia on a cell line, IL-1ß could positively or negatively influence cell death depending on the moment when it was added to the culture [78]. It is extremely important to know more about the role of ICE and its substrates in apoptosis since therapies aiming to inhibit ICE and thus reduce IL-1ß secretion are under investigation. These could have a secondary effect of also inhibiting apoptosis and, therefore, of prolonging the survival of malignant cells or auto-reactive clones in auto-immune processes. It will be essential for any therapeutic inhibitors of ICE to be sufficiently specific to block only pro-IL-1ß cleaving and not the cleaving of the substrates of other enzymes related to ICE.

THE ANTAGONIST: IL-1Ra

Gene and protein structure

Initially called IL-1 inhibitor due to its biological activity, this protein with a molecular weight of 22 kD specifically blocks the effects of IL-1alpha and ß. It was first identified in the urine of patients suffering from fever or myelomonocytic leukaemia [79, 80], and subsequently in monocyte culture supernatant in the presence of immune adherence complexes [81]. After the corresponding gene was identified and cloned, it was established that its structure was homologous with that of IL-1alpha (18%) and ß (26%) [82, 83], and it was renamed IL-1Ra since it binds to the IL-1 receptors without transmitting any signal to the cell [84, 85].

Unlike the IL-1 genes, the IL-1Ra gene possesses a signal sequence which allows the protein to be secreted in the extra-cellular environment. However, an alternative splicing of the messenger RNA may occur which modifies the exon encoding for the signal sequence and leads to the translation of a protein which remains intra-cellular and which is, therefore, called icIL-1Ra [86, 87]. Both the secreted and the icIL-1Ra forms of IL-1Ra have exactly the same biological activity. The icIL-1Ra form is constitutively expressed in keratocytes and in digestive epithelial cells [86, 88], i.e. in the same cells as those where large quantities of intra-cellular pro-IL-1alpha are found. It has therefore been suggested that icIL-1Ra plays a regulating role in the intra-cellular activity of IL-1alpha [86, 89]. A second form of icIL-1Ra, also generated by alternative splicing of the messenger RNA, has recently been described in different types of cells and is called icIL-1Ra type 2 [90]. The inhibiting activity of this icIL-1Ra 2 is comparable to that of classical icIL-1Ra but the biological significance of this second type of icIL-1Ra is currently not understood.

In the majority of situations, IL-1Ra is secreted in the extracellular environment in the form of a 22 kD glycosylated protein whose biological activity is analogous to that of the 17 kD non-glycosylated protein [85]. IL-1Ra has a beta-trefoil crystallographic structure, composed of folds of ß-pleated sheets, very similar to that of IL-1alpha& and ß [91]. However, directed mutagenesis experiments have shown that, unlike IL-1ß which has two binding sites for its receptor, IL-1Ra has only one which would explain the absence of a signal transmission [92].

It is known that alternative splicing leads to icIL-1Ra, and the TATA site initiating transcription has been identified, but otherwise little is known about the IL-1Ra promoter and its transcription regulation [93]. Certain sites, resembling those usually binding NFIL6 and NFkB transcription factors, and LPS response elements, have been identified but are still being investigated in greater detail [93, 94]. On the other hand, it has been clearly shown that this gene is polymorphic with at least five allele forms [95, 96]. The occurrence of allele 2 increases significantly in patients suffering from psoriasis, ulcerous colitis [97, 98], systemic lupus erythematosus [99] and alopecia [100] whilst this association has not been noted in the case of rheumatoid polyarthritis or Crohn's disease. It is, therefore, tempting to suppose that these pathologies are linked to a drop in IL-1Ra production, but the relation between this particular allele and IL-1Ra synthesis has not yet been elucidated.

Production and regulation

The different mediators modulating the production of IL-1Ra have been studied in detail, particularly in monocyte cells. LPS is a powerful inducer of both IL-1ß and IL-1Ra in cultured cells but the expression kinetics of these proteins is different since maximum expression is achieved in 2 hours for IL-1ß and in 4 hours for IL-1Ra [40]. The same discrepancy has also been confirmed in vivo in human volunteers who had received an injection of LPS. A concentration peak for IL-1ß was noted in the serum after 1 hour whilst for IL-1Ra this peak was noted 2 hours after the injection [101]. Immune complexes and adherent IgG are also important inducers of IL-1Ra, whilst they only very slightly stimulate the production of IL-1ß [102, 103]. The paradox is that high concentrations of LPS reduce IL-1Ra expression and secretion induced by adherent IgG, without impairing the production of IL-1ß. The mechanism behind this inhibition is still not understood [102, 104]. However, it is clear from these observations that IL-1Ra and IL-1ß are produced by the same cells but are regulated in different ways.

Other cytokines also play a modulating role in the production of IL-1Ra. Thus, interleukin-3 and
GM-CSF increase the production of IL-1Ra by monocytes [105], although this latter cytokine seems to be more a differentiating factor than a direct inducer of IL-1Ra, as we will subsequently see. IL-1 itself, whether alphaor ß, is also an inducer of IL-1Ra, but its direct activity in this field is quite weak and it seems, above all, to be an enhancing agent for other stimuli [89]. However, this influence of IL-1 activity on the production of IL-1Ra complicates the interpretation of research findings and could explain the effect of certain substances described as IL-Ra activators, but which, in actual fact, activate IL-1. It has thus been shown that TGF-ß induces the secretion of IL-1Ra in monocytes via IL-1 synthesis since this activity is completely blocked by the presence of anti-IL-1ß antibodies [106]. On the other hand, the Th2 cytokines, considered classically to be anti-inflammatory, such as IL-4 [107], IL-10 [108, 109] and IL-13 [110], are inducers and, more especially, enhancers of IL-1Ra synthesis with a direct effect, since it is not modified by the presence of an inhibitor of protein synthesis such as cyclohexymine.

Finally, IL-1Ra production, like that of IL-1, can be stimulated by direct contact with activated T lymphocytes. It is interesting to note that the surface molecules of these Th2 cells preferentially stimulate the production of IL-1Ra, whilst those of Th1 lymphocytes tend to induce IL-1 production [111].

The production capacity of IL-1Ra also depends on the degree of cell differentiation. We have, in fact, shown that peripheral blood monocytes, differentiated in vitro into macrophages by prolonged culture in the presence of GM-CSF, spontaneously produce IL-1Ra whilst their capacity to produce IL-1 diminishes [112].
This was initially observed when evaluating IL-1Ra by its biological activity, but it was subsequently confirmed by other researchers when ELISA techniques became available [113]. The macrophages differentiated in vivo, such as alveolar macrophages [114, 115] and synovial macrophages [116, 117], also produce a large quantity of IL-1Ra. Unlike monocytes, macrophages differentiated in vitro or in vivo do not increase the production of IL-1Ra in the presence of LPS [113, 115, 118], but in the presence of GM-CSF [115] or IL-4 [118, 119]. Another difference between monocytes and macrophages has also recently been shown at the level of the action of the C-reactive protein (CRP). This acute phase protein, stimulates the production of IL-1 and, to an even greater extent, IL-1Ra [120] in peripheral blood monocytes, but inhibits this in alveolar macrophages [121]. The increase in circulating levels of IL-1Ra following the injection of IL-6 [122] may be partially mediated by CRP, whose production by the hepatic cells is mainly induced by IL-6. However, it has been shown recently that IL-6 is also a direct inducer of IL-1Ra [123].

The secretion of IL-1Ra is not limited to monocyte cells. Polynuclear neutrophils (PMN) are also capable of producing large quantities of IL-1Ra in response to the same inducers as monocytes [124-126]. In addition, TNFalpha stimulates the production of IL-1Ra by PMN whilst it has very little effect on monocytes [89, 126]. The significance of this observation in vivo has been confirmed by showing that the injection of anti-TNF-alpha antibodies prevents the increase of plasma IL-1Ra induced by endotoxins in monkeys or man [127]. It is interesting to note that a pro-inflammatory cytokine, such as TNF-alpha, and certain anti-inflammatory cytokines, such as IL-4 and IL-10, but not IL-13 and TGF-ß, have a synergic effect on the production of IL-1Ra by PMN [128]. This highlights the fact that interactions in the cytokine network are too complex for an oversimplified classification to be applicable in all cases.

It has been shown, very recently, that IL-1Ra is also produced by hepatic cells, in response to stimulation by IL-1 or by a combination of IL-1 and IL-6, and that this cytokine is therefore related to the acute phase protein family [123].

Finally, fibroblasts [129] and keratocytes constitutively produce icIL-1Ra [86, 130].

IL-1 RECEPTORS

The three members of the IL-1 family share the same receptors. Substantial progress has been made recently in understanding the functioning of these receptors following the demonstration of the existence of an additional sub-unit called the IL-1 receptor accessory protein (IL-1R-AcP), even though this sub-unit has only, up to now, been positively identified in the mouse [131]. There are, however, indications of its expression in man, in particular in cerebral tissue [23]. The IL-1 receptors (IL-1R), therefore, also constitute a family of 3 molecules: two true receptors, called IL-1 receptor type I (IL-1RI) and IL-1 receptor type II
(IL-1RII), and IL-1R-AcP.

These receptors vary greatly in their capacity to transmit intra-cellular signals and can exist in either a transmembrane or a soluble form, which allows them to play a major role in the regulation of the biological activity of IL-1.

The three members of the IL-1R family belong to the immunoglobulin superfamily and present a certain degree of structural homology. In man, IL-1RI and IL-1RII genes are located on the long arm of chromosome 2, as are the members of the IL-1 family [132].

IL-1RI

IL-1RI is a glycoprotein expressed at the surface of numerous types of cells, but preferentially on endothelial cells, fibroblasts, chondrocytes, smooth muscle cells and T lymphocytes. Only a small number of IL-1RI molecules (50 to 200 receptors by cell) is present at the surface of cells but their number is sufficient to transmit the message in the presence of
IL-1 [133].

IL-1RI is a transmembrane monomeric molecule whose cytoplasmic domain with 213 amino acids does not seem to have any intrinsic kinase activity allowing a signal to be transmitted [134]. However, IL-1RI is functional and, indeed, is the only one of the two IL-1 receptors capable of transmitting a signal when it is occupied by IL-1 [135, 136].

The expression of IL-1RI is, therefore, a factor which could affect the biological activity of IL-1. The examination of the promoter of the IL-1RI gene shows that it resembles that of constitutively expressed proteins. This confirms the observation that numerous cells permanently express small quantities of IL-RI at their surface [137]. Substances such as phorbol esters, prostaglandin E2 (PGE2) dexamethasone, vitamin D3, IL-2 and IL-4 are, however, capable of increasing the expression of IL-1RI at the cell surface [3]. IL-1 plays rather a complex role in the modulation of its own receptor. It increases this in cells which synthesize PGE2 [138], but reduces it in T lymphocytes or when the synthesis of PGE2 is inhibited [139]. Finally, TGF-ß reduces the expression of IL-1RI [140].

IL-1R-ACP

IL-1R-AcP presents some homology with IL-1RI, both in respect to its extra-cellular domain and its cytoplasmic fragment. Little is known about this newly discovered molecule. Its presence explains, however, certain conflicting observations concerning the affinity of the receptor for its ligands which were not fully understood.

The model currently proposed suggests that IL-1 binds firstly with IL-1RI with a weak affinity. A conformational change then occurs in IL-1RI and allows the attachment of IL-1R-AcP and the formation of a high affinity complex [131]. The existence of different conformational modifications for IL-1RI has been confirmed by showing that sensitivity to proteolysis varies according to the ligand attached [141].

This model could also explain the inability of IL-1Ra to transmit a signal. In fact, the second receptor binding site, present on IL-1 molecules but not on IL-1Ra molecules, could be the site for binding to IL1R-AcP allowing the formation of the heterodimer [23]. However, this hypothesis implies that no signal can be transmitted before the formation of the heterodimer. This has not been proven as yet.

IL-1RII

The second IL-1 receptor (IL-1RII) is a 68 kD glycoprotein whose intra-cytoplasmic portion only has 29 amino acids [142], which is consistent with the fact that this receptor cannot transmit any signal [136, 143]. It is expressed in very small quantities on the same cells as IL-1RI, apart from endothelial cells, but it is distinctly predominant on B lymphocytes, monocytic cells, polynuclear neutrophils and haematopoietic cells [142]. Finally, although T lymphocytes have always been considered to preferentially express receptor type 1, Th2 clones also express a considerable amount of IL-1RII [144].

IL-1RII does not transmit any signal but is, on the other hand, capable of binding IL-1ß with great affinity and, therefore, can compete with IL-1RI for binding the latter, which amounts to reducing the biological activities of IL-1ß. For this reason it has been called a "decoy receptor" and is considered to be a negative regulatory factor for IL-1ß which remains trapped on this receptor which prevents it from inducing a biological signal [145].

IL-1Ra is also capable of binding to IL-1RII, but with an affinity one hundred-fold weaker than for
IL-1RI [146]. Thus there is little competition between the inhibiting activities of IL-1Ra and IL-1RII at target-cell surface level.

In so far as IL-1RII tends to be an inhibitor of IL-1, the result of its expression is the opposite to that of IL-1RI. In fact, when IL-1RI expression increases, the biological activity of IL-1 increases, whereas when IL-1RII expression increases, the biological activity of IL-1 is inhibited. The validity of this reasoning has been demonstrated by transfecting fibroblasts, which only expressed IL-1RI, with the IL-1RII gene and observing that the more IL-1RII is expressed on the surface, the less these cells are capable of responding to IL-1 [147]. The increased expression of IL-1RII, observed in the presence of dexamethasone, IL-4 and IL-13, also confirms the anti-inflammatory effect of these substances [148-150].

The soluble forms of the receptors

The regulation of IL-1 biological activity is even more complex due to the existence of soluble forms for the two types of receptors. They result from the proteolytic cleavage of the extra-cellular portion of membrane receptors and are found in the serum and the urine of healthy subjects [143, 151], as well as in various inflammatory biological fluids, particularly the synovial fluid [152]. In normal individuals, the serum levels of the soluble form of IL-1RI (IL-1sRI) are roughly 3 ng/ml, whilst those of IL-1sRII are twice as high [153]. Higher concentrations are found in pathological situations such as infections [154]. These soluble receptors, by binding to IL-1 in the extra-cellular environment, prevent its subsequent binding to membrane receptors and act, therefore, as inhibitors, but by a mechanism different to that of IL-Ra.

The affinity of these soluble receptors for the different members of the IL-1 family differs from that of membrane receptors, as summarized in Table 1.

To summarize, it has been noted that IL-1sRI binds preferentially to IL-1alpha and IL-1Ra, whereas IL-1sRII has a greater affinity for IL-1ß. These observations should be compared with those made when studying the biological effect of IL-1 in the presence of these different inhibitors. Thus, as their respective affinities indicate, IL-1sRI inhibits more the activity of IL-alpha than that of IL-1ß, whilst IL-1sRII has the reverse effect. More interestingly, the combination in vitro of IL-1Ra and IL-1sRI results in a reduction in the inhibition induced by each of these inhibitors separately, especially as regards the activity of IL-1ß. On the other hand, the combination of IL-1RA and IL-1sRII has the opposite effect i.e. it increases the inhibition of the activity of IL-1 [156, 157]. The reduction in the effect of IL-1Ra by IL-1sRI could be explained by the fact that IL-1Ra has a strong affinity for IL-1sRI and that it binds therefore to this soluble receptor before binding to membrane IL-1RI, which is, therefore, free to bind IL-1 and to transmit a biological signal [157]. However, this hypothesis has not been confirmed by research into affinity constants which show, to the contrary, that the affinity of IL-1Ra for the membrane receptor is two thousand-fold greater than its affinity for IL-1sRI [156].

These observations have important implications for any therapeutic use of these soluble forms of receptors. It appears, in fact, that the use of IL-1sRI could be dangerous since it could blunt the inhibiting effect of endogenous IL-1Ra and could therefore exacerbate the inflammatory process. On the other hand, the use of IL-1sRII could be beneficial (Figure 1). However, we must wait until these observations are confirmed in vivo in animal models before drawing any final conclusions.

In this context, it is interesting to note that certain viral proteins present a homology of some 30% with IL-1sRII. These proteins are capable of binding to IL-1 and, thus, of preventing the host from producing an adequate inflammatory response [158, 159].

CONCLUSION

REFERENCES

1. Aarden L A, Brunner T K, Cerrottini J C, Dayer J M, De Weck A L, Dinarello C A, Di Sabato G, Farrar J J, Gery I, Gillis S, Handschumacher R E, Henney C S, Hoffmann M K, Koopman W J, Krane S M, Lachman L B, Lefkowits I, Mishell R I, Mizel S B, Oppenheim J J, Paetkau V, Plate J, Rollinghoff M, Rosenstreich D, Rosenthal A S, Rosenwasser L J, Schimpl A, Shim H S, Simon P L, Smith K A, Wagner H, Watson J D, Wecker E, Wood D D. 1979. Revised nomenclature for antigen-non specific T cell proliferation helper factors (letter). J. Immunol. 123: 2928.

2. Webb A C, Collins K L, Auron P E, Eddy R L, Nakai H, Byers M G, Haley W M, Shows T B. 1986. Interleukin-1 gene assigned to long arm of human chromosom 2. Lymphokine Res. 5: 77.

3. Dinarello C A. 1994. The interleukin-1 family: 10 years of discovery. (Review) FASEB J. 8: 1314.

4. Auron P E, Webb A C, Rosenwasser L J, Mucci S F, Rich A, Wolff S M, Dinarello C A. 1984. Nucleotide sequence of human monocyte interleukin-1 precursor cDNA. Proc. Natl. Acad. Sci. USA 81: 7907.

5. March C J, Mosley B, Larsen A, Cerretti D P, Braedt G, Price V, Gillis S, Henney C S, Kronheim S R, Grabstein K, et al. 1985. Cloning, sequence and expression of two distinct human interleukin-1 complementary DNAs. Nature 315: 641.

6. Lomedico P T, Gubler U, Hellmann C P, Dukovich M, Giri J G, Pan Y C, Collier K, Semionow R, Chua A O, Mizel S B. 1984. Cloning and expression of murine interleukin-1 cDNA in Escherichia coli. Nature 312: 458.

7. Gubler U, Chua A O, Stern A S, Hellmann C P, Vitek M P, De Chiara T M, Benjamin W R, Collier K J, Dukovich M, Familletti P C, et al. 1986. Recombinant human interleukin-1alpha: purification and biological characterization. J. Immunol. 136: 2492.

8. Clark B D, Collins K L, Gandy M S, Webb A C, Auron P E. 1986. Genomic sequence for human prointerleukin-1beta: possible evolution from a reverse transcribed prointerleukin-1alpha gene [published erratum appears in Nucleic Acids Res. 1987 Jan 26; 15(2): 868]. Nucleic Acids Res. 14: 7897.

9. Furutani Y, Notake M, Fukui T, Ohue M, Nomura H, Yamada M, Nakamura S. 1986. Complete nucleotide sequence of the gene for human interleukin-1alpha [published erratum appears in Nucleic Acids Res. 1986 Jun 25; 14(12): 5124]. Nucleic Acids Res. 14: 3167.

10. Di Giovine F S, Takhsh E, Blakemore A I, Duff G W. 1992. Single base polymorphism at ­ 511 in the human interleukin-1beta gene (IL-1beta). Hum. Mol. Genet. 1: 450.

11. Bailly S, Di Giovine F S, Blakemore A I, Duff G W. 1993. Genetic polymorphism of human interleukin-1alpha. Eur. J. Immunol. 23: 1240.

12. McDowell T L, Symons J A, Ploski R, Forre O, Duff G W. 1995. A genetic association between juvenile rheumatoid arthritis and a novel interleukin-1alpha polymorphism. Arthritis Rheum. 38: 221.

13. Di Giovine F S, Crane A, Gough A, De Vries N, van de Putte L B A, Emery P, Duff G W. 1994. A genetic polymorphism in the IL-1ß gene is associated with joint erosions in rheumatoid arthritis. Cytokine 6: 545 (abstract).

14. Pociot F, Molvig J, Wogensen L, Worsaae H, Nerup J. 1992. A TaqI polymorphism in the human interleukin-1beta (IL-1beta) gene correlates with IL-1beta secretion in vitro. Eur. J. Clin. Invest. 22: 396.

15. Murzin A G, Lesk A M, Chothia C. 1992. Beta-Trefoil fold. Patterns of structure and sequence in the Kunitz inhibitors interleukins-1beta and 1alpha and fibroblast growth factors. J. Mol. Biol. 223: 531.

16. Nanduri V B, Hulmes J D, Pan Y C, Kilian P L, Stern A S. 1991. The role of arginine residues in interleukin-1 receptor binding. Biochim. Biophys. Acta 1118: 25.

17. Shirakawa F, Saito K, Bonagura C A, Galson D L, Fenton M J, Webb A C, Auron P E. 1993. The human prointerleukin-1beta gene requires DNA sequences both proximal and distal to the transcription start site for tissue-specific induction. Mol. Cell. Biol. 13: 1332.

18. Furutani Y. 1994. Molecular studies on interleukin-1alpha. (Review) Eur. Cytokine Netw. 5: 533.

19. Auron P E, Webb A C. 1994. Interleukin-1: a gene expression system regulated at multiple levels. (Review) Eur. Cytokine Netw. 5: 573.

20. Tsukada J, Saito K, Waterman W R, Webb A C, Auron P E. 1994. Transcription factors NF-IL-6 and CREB recognize a common essential site in the human prointerleukin-1beta gene. Mol. Cell. Biol. 14: 7285.

21. Buras J A, Monks B G, Fenton M J. 1994. The NF-ßA-binding element, not an overlapping NF-IL-6-binding element, is required for maximal IL-1ß gene expression. J. Immunol. 152: 4444.

22. Fenton M J, Lodie T, Buras J. 1994. NF-ßA is identical to the ETS family member PU.1 and is structurally altered following LPS activation. Cytokine 6: 558.

23. Dinarello C A. 1996. Biologic basis for interleukin-1 in disease. (Review) Blood 87: 2095.

24. Dinarello C A. 1991. Interleukin-1 and interleukin-1 antagonism. (Review) Blood 77: 1627.

25. Sisson S D, Dinarello C A. 1988. Production of interleukin-1 alpha, interleukin-1beta and tumor necrosis factor by human mononuclear cells stimulated with granulocyte-macrophage colony stimulating factor. Blood 72: 1368.

26. Ucla C, Roux-Lombard P, Fey S, Dayer J M, Mach B. 1990. Interferon gamma drastically modifies the regulation of interleukin-1 genes by endotoxin in U937 cells. J. Clin. Invest. 85: 185.

27. Collart M A, Belin D, Vassalli J D, De Kossodo S, Vassalli P. 1986. Gamma interferon enhances macrophage transcription of the tumor necrosis factor/cachectin, interleukin-1, and urokinase genes, which are controlled by short-lived repressors. J. Exp. Med. 164: 2113.

28. Nawroth P P, Bank I, Handley D, Cassimeris J, Chess L, Stern D. 1986. Tumor necrosis factor/cachectin interacts with endothelial cell receptors to induce release of interleukin-1. J. Exp. Med. 163: 1363.

29. Dinarello C A, Cannon J G, Wolff S M, Bernheim H A, Beutler B, Cerami A, Figari I S, Palladino M A Jr, O'Connor J V 1986. Tumor necrosis factor (cachectin) is an endogenous pyrogen and induces production of interleukin-1. J. Exp. Med. 163: 1433.

30. Le J M, Weinstein D, Gubler U, Vilcek J. 1987. Induction of membrane-associated interleukin-1 by tumor necrosis factor in human fibroblasts. J. Immunol. 138: 2137.

31. Warner S J, Auger K R, Libby P. 1987. Interleukin 1 induces interleukin-1. II. Recombinant human interleukin-1 induces interleukin-1 production by adult human vascular endothelial cells. J. Immunol. 139: 1911.

32. Dinarello C A, Ikejima T, Warner S J, Orencole S F, Lonnemann G, Cannon J G, Libby P. 1987. Interleukin-1 induces interleukin-1. I. Induction of circulating interleukin-1 in rabbits in vivo and in human mononuclear cells in vitro. J. Immunol. 139: 1902.

33. Dinarello C A. 1989. Interleukin-1 and its biologically related cytokines. (Review) Adv. Immunol. 44: 153.

34. Fenton M J, Vermeulen M W, Clark B D, Webb A C, Auron P E. 1988. Human pro-IL-1beta gene expression in monocytic cells is regulated by two distinct pathways. J. Immunol. 140: 2267.

35. Schindler R, Ghezzi P, Dinarello C A. 1990. IL-1 induces IL-1. IV. IFN-gamma supresses IL-1 but not lipopolysaccharide-induced transcription of IL-1. J. Immunol. 144: 2216.

36. Kaspar R L, Gehrke L. 1994. Peripheral blood mononuclear cells stimulated with C5a or lipopolysaccharide to synthesize equivalent levels of IL-1beta mRNA show unequal IL-1beta protein accumulation but similar polyribosome profiles. J. Immunol. 153: 277.

37. Schindler R, Clark B D, Dinarello C A. 1990. Dissociation between interleukin-1ß mRNa and protein synthesis in human peripheral blood mononuclear cells. J. Biol. Chem. 265: 10232.

38. Vey E, Zhang J H, Dayer J M. 1992. IFN-gamma and 1, 25(OH)2D3 induce on THP-1 cells distinct patterns of cell surface antigen expression, cytokine production, and responsiveness to contact with activated T cells. J. Immunol. 149: 2040.

39. Isler P, Vey E, Zhang J H, Dayer J M. 1993. Cell surface glycoproteins expressed on activated human T cells induce production of interleukin-1beta by monocytic cells: a possible role of CD69. Eur. Cytokine Netw. 4: 15.

40. Vannier E, Miller L C, Dinarello C A. 1992. Coordinated anti-inflammatory effects of interleukin-4: interleukin-4 suppresses interleukin-1 production but up-regulates gene expression and synthesis of interleukin-1 receptor antagonist. Proc. Natl. Acad. Sci. USA 89: 4076.

41. Hart P H, Vitti V F, Burgess D R, Whitty G A, Piccoli D S, Hamilton J A. 1989. Potential anti-inflammatory effects of interleukin-4: suppression of human monocyte tumor necrosis factor, interleukin-1 and prostaglandin E2. Proc. Natl. Acad. Sci. USA 86: 3803.

42. De Waal Malefyt R, Abrams J, Bennett B, Figdor C G, de Vries J E. 1991. Interleukin-10 (IL-10) inhibits cytokine synthesis by human monocytes: an autoregulatory role of IL-10 produced by monocytes. J. Exp. Med. 174: 1209.

43. Wang P, Wu P, Siegel M I, Egan R W, Billah M M. 1994. IL-10 inhibits transcription of cytokine genes in human peripheral blood mononuclear cells. J. Immunol. 153: 811.

44. Chernoff A E, Granowitz E V, Shapiro L, Vannier E, Lonnemann G, Angel J B, Kennedy J S, Rabson A R, Wolff S M, Dinarello C A. 1995. A randomized, controlled trial of IL-10 in humans. Inhibition of inflammatory cytokine production and immune responses. J. Immunol. 154: 5492.

45. Chantry D, Turner M, Abney E, Feldmann M. 1989. Modulation of cytokine production by transforming growth factor beta. J. Immunol. 142: 4295.

46. Schindler R, Mancilla J, Endres S, Ghorbani R, Clark S C, Dinarello C A. 1990. Correlations and interactions in the production of interleukin-6 (IL-6), IL-1, and tumor necrosis factor (TNF) in human blood mononuclear cells: IL-6 suppresses IL-1 and TNF. Blood 75: 40.

47. Endres S, Ghorbani R, Kelley V E, Georgilis K, Lonnemann G, van der Meer J W, Cannon J G, Rogers T S, Klempner M S, Weber P C, et al. 1989. The effect of dietary supplementation with n-3 polyunsaturated fatty acids on the synthesis of interleukin-1 and tumor necrosis factor by mononuclear cells. N. Engl. J. Med. 320: 265.

48. Molvig J, Pociot F, Worsaae H, Wogensen L D, Baek L, Christensen P, Mandrup-Poulsen T, Andersen K, Madsen P, Dyerberg J, et al. 1991. Dietary supplementation with omega-3-polyunsaturated fatty acids decreases mononuclear cell proliferation and interleukin-1beta content but not monokine secretion in healthy and insulin-dependent diabetic individuals. Scand. J. Immunol. 34: 399.

49. Meydani S N, Lichtenstein A H, Cornwall S, Meydani M, Goldin B R, Rasmussen H, Dinarello C A, Schaefer E J. 1993. Immunologic effects of national cholesterol education panel step-2 diets with and without fish-derived N-3 fatty acid enrichment. J. Clin. Invest. 92: 105.

50. Besedovsky H, del Rey A, Sorkin E, Dinarello C A. 1986. Immunoregulatory feedback between interleukin-1 and glucocorticoid hormones. Science 233: 652.

51. Berkenbosch F, van Oers J, del Rey A, Tilders F, Besedovsky H. 1987. Corticotropin-releasing factor-producing neurons in the rat activated by interleukin-1. Science 238: 524.

52. Mosley B, Urdal D L, Prickett K S, Larsen A, Cosman D, Conlon P J, Gillis S, Dower S K. 1987. The interleukin-1 receptor binds the human interleukin-1alpha precursor but not the interleukin-1beta precursor. J. Biol. Chem. 262: 2941.

53. Wakabayashi G, Gelfand J A, Jung W K, Connolly R J, Burke J F, Dinarello C A. 1991. Staphylococcus epidermidis induces complement activation, tumor necrosis factor and interleukin-1, a shock-like state and tissue injury in rabbits without endotoxemia. Comparison to Escherichia coli. J. Clin. Invest. 87: 1925.

54. Kobayashi Y, Yamamoto K, Saido T, Kawasaki H, Oppenheim J J, Matsushima K. 1990. Identification of calcium-activated neutral protease as a processing enzyme of human interleukin-1alpha. Proc. Natl. Acad. Sci. USA 87: 5548.

55. Kavita U, Mizel S B. 1995. Differential sensitivity of interleukin-1alpha and -ß precursor proteins to cleavage by calpain, a calcium dependent protease. J. Biol. Chem. 270: 27758.

56. Hauser C, Saurat J H, Schmitt A, Jaunin F, Dayer J M. 1986. Interleukin-1 is present in normal human epidermis. J. Immunol. 136: 3317.

57. Maier J A, Voulalas P, Roeder D, Maciag T. 1990. Extension of the life-span of human endothelial cells by an interleukin-1alpha antisense oligomer. Science 249: 1570.

58. Weitzmann M N, Savage N. 1992. Nuclear internalisation and DNA binding activities of interleukin-1, interleukin-1 receptor and interleukin-1/receptor complexes. Biochem. Biophys. Res. Commun. 187: 1166.

59. Mizel S B, Kilian P L, Lewis J C, Paganelli K A, Chizzonite R A. 1987. The interleukin-1 receptor. Dynamics of interleukin-1 binding and internalization in T cells and fibroblasts. J. Immunol. 138: 2906.

60. Curtis B M, Widmer M B, De Roos P, Qwarnstrom E E. 1990. IL-1 and its receptor are translocated to the nucleus. J. Immunol. 144: 1295.

61. Stevenson F T, Bursten S L, Fanton C, Locksley R M, Lovett D H. 1993. The 31 kDa precursor of interleukin-1alpha is myristoylated on specific lysines within the 16 kDa N-terminal propiece. Proc. Natl. Acad. Sci. USA 90: 7245.

62. Brody D T, Durum S K. 1989. Membrane IL-1: IL-1alpha precursor binds to the plasma membrane via a lectin-like interaction. J. Immunol. 143: 1183.

63. Kaplanski G, Farnarier C, Kaplanski S, Porat R, Shapiro L, Bongrand P, Dinarello C A. 1994. Interleukin-1 induces interleukin-8 secretion from endothelial cells by a juxtacrine mechanism. Blood 84: 4242.

64. Jobling S A, Auron P E, Gurka G, Webb A C, McDonald B, Rosenwasser L J, Gehrke L. 1988. Biological activity and receptor binding of human prointerleukin-1beta and subpeptides. J. Biol. Chem. 263: 16372.

65. Beuscher H U, Gunther C, Rollinghoff M. 1990. IL-1beta is secreted by activated murine macrophages as biologically inactive precursor. J. Immunol. 144: 2179.

66. Black R A, Kronheim S R, Cantrell M, Deeley M C, March C J, Prickett K S, Wignall J, Conlon P J, Cosman D, Hopp T P, et al. 1988. Generation of biologically active interleukin-1beta by proteolytic cleavage of the inactive precursor. J. Biol. Chem. 263: 9437.

67. Cerretti D P, Kozlosky C J, Mosley B, Nelson N, van Ness K, Greenstreet T A, March C J, Kronheim S R, Druck T, Cannizzaro L A, et al. 1992. Molecular cloning of the interleukin-1beta converting enzyme. Science 256: 97.

68. Thornberry N A, Bull H G, Calaycay J R, Chapman K T, Howard A D, Kostura M J, Miller D K, Molineaux S M, Weidner J R, Aunins J, et al. 1992. A novel heterodimeric cysteine protease is required for interleukin-1beta processing in monocytes. Nature 356: 768.

69. Howard A D, Kostura M J, Thornberry N, Ding G J, Limjuco G, Weidner J, Salley J P, Hogquist K A, Chaplin D D, Mumford R A, Schmidt J A, Tocci M J. 1991. IL-1-converting enzyme requires aspartic acid residues for processing of the
IL-1ß precursor at two distinct sites and does not cleave 31 kDa IL-1alpha. J. Immunol. 147: 2964.

70. Wilson K P, Black J A, Thomson J A, Kim E E, Griffith J P, Navia M A, Murcko M A, Chambers S P, Aldape R A, Raybuck S A, Livingston D J. 1994. Structure and mechanism of interleukin-1ß converting enzyme. Nature 370: 270.

71. Gu Y, Wu J, Faucheu C, Lalanne J L, Diu A, Livingston D J, Su M S. 1995. Interleukin-1beta converting enzyme requires oligomerization for activity of processed forms in vivo. EMBO J. 14: 1923.

72. Wu M X, Daley J F, Rasmussen R A, Schlossman S F. 1995. Monocytes are required to prime peripheral blood T cells to undergo apoptosis. Proc. Natl. Acad. Sci. USA 92: 1525.

73. Yuan J, Shaham S, Ledoux S, Ellis H M, Horvitz H R. 1993. The C. elegans cell death gene ced-3 encodes a protein similar to mammalian interleukin-1beta-converting enzyme. Cell 75: 641.

74. Miura M, Zhu H, Rotello R, Hartwieg E A, Yuan J. 1993. Induction of apoptosis in fibroblasts by IL-1beta-converting enzyme, a mammalian homolog of the C. elegans cell death gene ced-3. Cell 75: 653.

75. Faucheu C, Diu A, Chan A W, Blanchet A M, Miossec C, Herve F, Collard-Dutilleul V, Gu Y, Aldape R A, Lippke J A, et al. 1995. A novel human protease similar to the interleukin-1beta converting enzyme induces apoptosis in transfected cells. EMBO J. 14: 1914.

76. Kuida K, Lippke J A, Ku G, Harding M W, Livingston D J, Su M S, Flavell R A. 1995. Altered cytokine export and apoptosis in mice deficient in interleukin-1ß converting enzyme. Science 267: 2000.

77. Patel T, Gores G J, Kaufmann S H. 1996. The role of proteases during apoptosis. (Review) FASEB J. 10: 587.

78. Friedlander R M, Gagliardini V, Rotello R J, Yuan J. 1996. Functional role of interleukin-1ß (IL-1ß) in IL-1ß-converting enzyme-mediated apoptosis. J. Exp. Med. 184: 717.

79. Balavoine J F, De Rochemonteix B, Williamson K, Seckinger P, Cruchaud A, Dayer J M. 1986. Prostaglandin E2 and collagenase production by fibroblasts and synovial cells is regulated by urine-derived human interleukin-1 and inhibitor(s). J. Clin. Invest. 78: 1120.

80. Seckinger P, Williamson K, Balavoine J F, Mach B, Mazzei G, Shaw A, Dayer J M. 1987. A urine inhibitor of interleukin-1 activity affects both interleukin-1alpha and 1beta but not tumor necrosis factor alpha. J. Immunol. 139: 1541.

81. Arend W P, Joslin F G, Massoni R J. 1985. Effects of immune complexes on production by human monocytes of interleukin-1 or an interleukin-1 inhibitor. J. Immunol. 134: 3868.

82. Eisenberg S P, Evans R J, Arend W P, Verderber E, Brewer M T, Hannum C H, Thompson R C. 1990. Primary structure and functional expression from complementary DNA of a human interleukin-1 receptor antagonist. Nature 343: 341.

83. Carter D B, Deibel M R, Jr., Dunn C J, Tomich C S, Laborde A L, Slightom J L, Berger A E, Bienkowski M J, Sun F F, McEwan R N, et al. 1990. Purification, cloning, expression and biological characterization of an interleukin-1 receptor antagonist protein. Nature 344: 633.

84. Seckinger P, Isaaz S, Dayer J M. 1988. A human inhibitor of tumor necrosis factor alpha. J. Exp. Med. 167: 1511.

85. Hannum C H, Wilcox C J, Arend W P, Joslin F G, Dripps D J, Heimdal P L, Armes L G, Sommer A, Eisenberg S P, Thompson R C. 1990. Interleukin-1 receptor antagonist activity of a human interleukin-1 inhibitor. Nature 343: 336.

86. Haskill S, Martin G, van Le L, Morris J, Peace A, Bigler C F, Jaffe G J, Hammerberg C, Sporn S A, Fong S, et al. 1991. cDNA cloning of an intracellular form of the human interleukin-1 receptor antagonist associated with epithelium. Proc. Natl. Acad. Sci. USA 88: 3681.

87. Butcher C, Steinkasserer A, Tejura S, Lennard A C. 1994. Comparison of two promoters controlling expression of secreted or intracellular IL-1 receptor antagonist. J. Immunol. 153: 701.

88. Hammerberg C, Arend W P, Fisher G J, Chan L S, Berger A E, Haskill J S, Voorhees J J, Cooper K D. 1992. Interleukin-1 receptor antagonist in normal and psoriatic epidermis. J. Clin. Invest. 90: 571.

89. Arend W P. 1993. Interleukin-1 receptor antagonist. (Review) Adv. Immunol. 54: 167.

90. Muzio M, Polentarutti N, Sironi M, Poli G, De Gioia L, Introna M, Mantovani A, Colotta F. 1995. Cloning and characterization of a new isoform of the interleukin-1 receptor antagonist. J. Exp. Med. 182: 623.

91. Vigers G P, Caffes P, Evans R J, Thompson R C, Eisenberg S P, Brandhuber B J. 1994. X-ray structure of interleukin-1 receptor antagonist at 2.0-Å resolution. J. Biol. Chem. 269: 12874.

92. Evans R J, Bray J, Childs J D, Vigers G P, Brandhuber B J, Skalicky J J, Thompson R C, Eisenberg S P. 1995. Mapping receptor binding sites in interleukin-(IL)-1 receptor antagonist and IL-1ß by site-directed mutagenesis. Identification of a single site in IL-1Ra and two sites in IL-1ß. J. Biol. Chem. 270: 11477.

93. Lennard A C. 1995. Interleukin-1 receptor antagonist. (Review) Crit. Rev. Immunol. 15: 77.

94. Smith M F Jr, Eidlen D, Arend W P, Gutierrez-Hartmann A. 1994. LPS-induced expression of the human IL-1 receptor antagonist gene is controlled by multiple interacting promoter elements. J. Immunol. 153: 3584.

95. Lennard A, Gorman P, Carrier M, Griffiths S, Scotney H, Sheer D, Solari R. 1992. Cloning and chromosome mapping of the human interleukin-1 receptor antagonist gene. Cytokine 4: 83.

96. Tarlow J K, Blakemore A I, Lennard A, Solari R, Hughes H N, Steinkasserer A, Duff G W. 1993. Polymorphism in human
IL-1 receptor antagonist gene intron 2 is caused by variable
numbers of an 86-bp tandem repeat. Hum. Genet. 91: 403.

97. Mansfield J C, Holden H, Tarlow J K, Di Giovine F S, McDowell T L, Wilson A G, Holdsworth C D, Duff G W. 1994. Novel genetic association between ulcerative colitis and the anti-inflammatory cytokine interleukin-1 receptor antagonist. Gastroenterology 106: 637.

98. Bioque G, Crusius J B, Koutroubakis I, Bouma G, Kostense P J, Meuwissen S G, Peña A S. 1995. Allelic polymorphism in IL-1ß and IL-1 receptor antagonist (IL-1Ra) genes in inflammatory bowel disease. Clin. Exp. Immunol. 102: 379.

99. Blakemore A I, Tarlow J K, Cork M J, Gordon C, Emery P, Duff G W. 1994. Interleukin-1 receptor antagonist gene polymorphism as a disease severity factor in systemic lupus erythematosus. Arthritis Rheum. 37: 1380.

100. Tarlow J K, Clay F E, Cork M J, Blakemore A I, McDonagh A J, Messenger A G, Duff G W. 1994. Severity of alopecia areata is associated with a polymorphism in the interleukin-1 receptor antagonist gene. J. Invest. Dermatol. 103: 387.

101. Granowitz E V, Santos A A, Poutsiaka D D, Cannon J G, Wilmore D W, Wolff S M, Dinarello C A. 1991. Production of interleukin-1-receptor antagonist during experimental endotoxaemia. Lancet 338: 1423.

102. Arend W P, Smith M F Jr, Janson R W, Joslin F G. 1991. IL-1 receptor antagonist and IL-1ß production in human monocytes are regulated differently. J. Immunol. 147: 1530.

103. Poutsiaka D D, Clark B D, Vannier E, Dinarello C A. 1991. Production of interleukin-1 receptor antagonist and interleukin-1ß by peripheral blood mononuclear cells is differentially regulated. Blood 78: 1275.

104. Marsh C B, Pope H A, Wewers M D. 1994. Fcgamma receptor cross-linking down-regulates IL-1 receptor antagonist and induces IL-1ß in mononuclear phagocytes stimulated with endotoxin or Staphylococcus aureus. J. Immunol. 152: 4604.

105. Jenkins J K, Arend W P. 1993. Interleukin-1 receptor antagonist production in human monocytes is induced by IL-1alpha, IL-3, IL-4 and GM-CSF. Cytokine 5: 407.

106. Wahl S M, Costa G L, Corcoran M, Wahl L M, Berger A E. 1993. Transforming growth factor-ß mediates IL-1-dependent induction of IL-1 receptor antagonist. J. Immunol. 150: 3553.

107. Wong H L, Costa G L, Lotze M T, Wahl S M. 1993. Interleukin (IL)-4 differentially regulates monocyte IL-1 family gene expression and synthesis in vitro and in vivo. J. Exp. Med. 117: 775.

108. Jenkins J K, Malyak M, Arend W P. 1994. The effects of interleukin-10 on interleukin-1 receptor antagonist and interleukin-1ß production in human monocytes and neutrophils. Lymphokine Cytokine Res. 13: 47.

109. Seitz M, Loetscher P, Dewald B, Towbin H, Gallati H, Baggiolini M. 1995. Interleukin-10 differentially regulates cytokine inhibitor and chemokine release from blood mononuclear cells and fibroblasts. Eur. J. Immunol. 25: 1129.

110. Muzio M, Re F, Sironi M, Polentarutti N, Minty A, Caput D, Ferrara P, Mantovani A, Colotta F. 1994. Interleukin-13 induces the production of interleukin-1 receptor antagonist (IL-1Ra) and the expression of the mRNA for the intracellular (keratinocyte) form of IL-1Ra in human myelomonocytic cells. Blood 83: 1738.

111. Chizzolini C, Chicheportiche R, Burger D, Dayer J M. 1997. Human Th1 cells preferentially induce interleukin-IL-1ß while Th2 cells induce IL-1 receptor antagonist production upon cell/cell contact with monocytes. Eur. J. Immunol. 27: 171.

112. Roux-Lombard P, Modoux C, Dayer J M. 1989. Production of interleukin-1 (IL-1) and a specific IL-1 inhibitor during human monocyte-macrophage differentiation: influence of GM-CSF. Cytokine 1: 45.

113. Janson R W, Hance K R, Arend W P. 1991. Production of IL-1 receptor antagonist by human in vitro-derived macrophages. Effects of lipopolysaccharide and granulocyte-macrophage colony-stimulating factor. J. Immunol. 147: 4218.

114. Galve-de Rochemonteix B, Nicod L P, Junod A F, Dayer J M. 1990. Characterization of a specific 20 to 25 kD interleukin-1 inhibitor from cultured human lung macrophages. Am. J. Respir. Cell Mol. Biol. 3: 355.

115. Janson R W, King T E Jr, Hance K R, Arend W P. 1993. Enhanced production of IL-1 receptor antagonist by alveolar macrophages from patients with interstitial lung disease. Am. Rev. Respir. Dis. 148: 495.

116. Roux-Lombard P, Modoux C, Vischer T, Grassi J, Dayer J M. 1992. Inhibitors of interleukin-1 activity in synovial fluids and in cultured synovial fluid mononuclear cells. J. Rheumatol. 19: 517.

117. Koch A E, Kunkel S L, Chensue S W, Haines G K, Strieter R M. 1992. Expression of interleukin-1 and interleukin-1 receptor antagonist by human rheumatoid synovial tissue macrophages. Clin. Immunol. Immunopathol. 65: 23.

118. Moore S A, Strieter R M, Rolfe M W, Standiford T J, Burdick M D, Kunkel S L. 1992. Expression and regulation of human alveolar macrophage-derived interleukin-1 receptor antagonist. Am. J. Respir. Cell Mol. Biol. 6: 569.

119. Ohashi P S, Oehen S, Aichele P, Pircher H, Odermatt B, Herrera P, Higuchi Y, Buerki K, Hengartner H, Zinkernagel R M. 1993. Induction of diabetes is influenced by the infectious virus and local expression of MHC class I and tumor necrosis factor-alpha. J. Immunol. 150: 5185.

120. Tilg H, Vannier E, Vachino G, Dinarello C A, Mier J W. 1993. Anti-inflammatory properties of hepatic acute phase proteins: preferential induction of interleukin-1 (IL-1) receptor antagonist over IL-1beta synthesis by human peripheral blood mononuclear cells. J. Exp. Med. 178: 1629.

121. Pue C A, Mortensen R F, Marsh C B, Pope H A, Wewers M D. 1996. Acute phase levels of C-reactive protein enhance IL-1ß and IL-1Ra production by human blood monocytes but inhibit IL-1ß and IL-1Ra production by alveolar macrophages. J. Immunol. 156: 1594.

122. Tilg H, Trehu E, Atkins M B, Dinarello C A, Mier J W. 1994. Interleukin-6 (IL-6) as an anti-inflammatory cytokine: induction of circulating IL-1 receptor antagonist and soluble tumor necrosis factor receptor p55. Blood 83: 113.

123. Gabay C, Smith M F, Eidlen D, Arend W P. 1996. Interleukin-1 receptor antagonist (IL-1Ra) is an acute-phase protein. J. Clin. Invest. 99: 2930.

124. McColl S R, Paquin R, Ménard C, Beaulieu A D. 1992. Human neutrophils produce high levels of the interleukin-1 receptor antagonist in response to granulocyte/macrophage colony-stimulating factor and tumor necrosis factor-alpha. J. Exp. Med. 176: 593.

125. Re F, Mengozzi M, Muzio M, Dinarello C A, Mantovani A, Colotta F. 1993. Expression of interleukin-1 receptor antagonist (IL-1Ra) by human circulating polymorphonuclear cells. Eur. J. Immunol. 23: 570.

126. Malyak M, Smith M F Jr, Abel A A, Arend W P. 1994. Peripheral blood neutrophil production of interleukin-1 receptor antagonist and interleukin-1ß. J. Clin. Immunol. 14: 20.

127. Jelkmann W E, Fandrey J, Frede S, Pagel H. 1994. Inhibition of erythropoietin production by cytokines. Implications for the anemia involved in inflammatory states. Ann. NY Acad. Sci. 718: 300.

128. Marie C, Pitton C, Fitting C, Cavaillon J M. 1996. IL-10 and IL-4 synergize with TNF-alpha to induce IL-1Ra production by human neutrophils. Cytokine 8: 147.

129. Chan L S, Hammerberg C, Kang K, Sabb P, Tavakkol A, Cooper K D. 1992. Human dermal fibroblast interleukin-1 receptor antagonist (IL-1Ra) and interleukin-1ß (IL-1ß) mRNA and protein are co-stimulated by phorbol ester: implication for a homeostatic mechanism. J. Invest. Dermatol. 99: 315.

130. Gruaz-Chatellard D, Baumberger C, Saurat J H, Dayer J M. 1991. Interleukin-1 receptor antagonist in human epidermis and cultured keratinocytes. FEBS Lett. 294: 137.

131. Greenfeder S A, Nunes P, Kwee L, Labow M, Chizzonite R A, Ju G. 1995. Molecular cloning and characterization of a second subunit of the interleukin-1 receptor complex. J. Biol. Chem. 270: 13757.

132. Sims J E, Painter S L, Gow I R. 1995. Genomic organization of the type I and type II IL-1 receptors. Cytokine 7: 483.

133. Shirakawa F, Tanaka Y, Ota T, Suzuki H, Eto S, Yamashita U. 1987. Expression of interleukin-1 receptors on human peripheral T cells. J. Immunol. 138: 4243.

134. Sims J E, March C J, Cosman D, Widmer M B, MacDonald H R, McMahan C J, Grubin C E, Wignall J M, Jackson J L, Call S M, et al. 1988. cDNA expression cloning of the IL-1 receptor, a member of the immunoglobulin superfamily. Science 241: 585.

135. Stylianou E, O'Neill L A, Rawlinson L, Edbrooke M R, Woo P, Saklatvala J. 1992. Interleukin-1 induces NF-kappa B through its type I but not its type II receptor in lymphocytes. J. Biol. Chem. 267: 15836.

136. Sims J E, Gayle M A, Slack J L, Alderson M R, Bird T A, Giri J G, Colotta F, Re F, Mantovani A, Shanebeck K, Grabstein K H, Dower S K. 1993. Interleukin-1 signaling occurs exclusively via the type I receptor. Proc. Natl. Acad. Sci. USA 90: 6155.

137. Ye K, Dinarello C A, Clark B D. 1993. Identification of the promoter region of human interleukin-1 type I receptor gene: multiple initiation sites, high G+C content, and constitutive expression. Proc. Natl. Acad. Sci. USA 90: 2295.

138. Takii T, Akahoshi T, Kato K, Hayashi H, Marunouchi T, Onozaki K. 1992. Interleukin-1 up-regulates transcription of its own receptor in a human fibroblast cell line TIG-1: role of endogenous PGE2 and cAMP. Eur. J. Immunol. 22: 1221.

139. Takii T, Hayashi H, Marunouchi T, Onozaki K. 1994. Interleukin-1 down-regulates type I interleukin-1 receptor mRNA expression in a human fibroblast cell line TIG-1 in the absence of prostaglandin E2 synthesis. Lympho. Cytokines Res. 13: 213.

140. Dubois C M, Ruscetti F W, Palaszynski E W, Falk L A, Oppenheim J J, Keller J R. 1990. Transforming growth factor beta is a potent inhibitor of interleukin-1 (IL-1) receptor expression: proposed mechanism of inhibition of IL-1 action. J. Exp. Med. 172: 737.

141. Einstein R, Jackson J R, d'Allessio K, Lillquist J S, Sathe G, Porter T, Young P R. 1996. Type I IL-1 receptor : ligand specific conformation differences and the role of glycosylation in ligand binding. Cytokine 8: 206.

142. McMahan C J, Slack J L, Mosley B, Cosman D, Lupton S D, Brunton L L, Grubin C E, Wignall J M, Jenkins N A, Brannan C I, et al. 1991. A novel IL-1 receptor, cloned from B cells by mammalian expression, is expressed in many cell types. EMBO J. 10: 2821.

143. Sims J E, Giri J G, Dower S K. 1994. The two interleukin-1 receptors play different roles in IL-1 actions. (Review) Clin. Immunol. Immunopathol. 72: 9.

144. McKean D J, Podzorski R P, Bell M P, Nilson A E, Huntoon C J, Slack J, Dower S K, Sims J. 1993. Murine T helper cell-2 lymphocytes express type I and type II IL-1 receptors, but only the type I receptor mediates costimulatory activity. J. Immunol. 151: 3500.

145. Colotta F, Dower S K, Sims J E, Mantovani A. 1994. The type II "decoy" receptor: a novel regulatory pathway for interleukin-1. (Review) Immunol. Today 15: 562.

146. Granowitz E V, Clark B D, Mancilla J, Dinarello C A. 1991. Interleukin-1 receptor antagonist competitively inhibits the binding of interleukin-1 to the type II interleukin-1 receptor. J. Biol. Chem. 266: 14147.

147. Re F, Sironi M, Muzio M, Matteucci C, Introna M, Orlando S, Penton-Rol G, Dower S K, Sims J E, Colotta F, Mantovani A. 1996. Inhibition of interleukin-1 responsiveness by type II receptor gene transfer: a surface "receptor" with anti-interleukin-1 function. J. Exp. Med. 183: 1841.

148. Colotta F, Re F, Muzio M, Bertini R, Polentarutti N, Sironi M, Giri J G, Dower S K, Sims J E, Mantovani A. 1993. Interleukin-1 type II receptor: a decoy target for IL-1 that is regulated by IL-4. Science 261: 472.

149. Re F, Muzio M, De Rossi M, Polentarutti N, Giri J G, Mantovani A, Colotta F. 1994. The type II "receptor" as a decoy target for interleukin-1 in polymorphonuclear leukocytes: characterization of induction by dexamethasone and ligand binding properties of the released decoy receptor. J. Exp. Med. 179: 739.

150. Colotta F, Saccani S, Giri J G, Dower S K, Sims J E, Introna M, Mantovani A. 1996. Regulated expression and release of the IL-1 decoy receptor in human mononuclear phagocytes. J. Immunol. 156: 2534.

151. Symons J A, Eastgate J, Duff G W. 1991. Purification and characterization of a novel soluble receptor for interleukin-1. J. Exp. Med. 174: 1251.

152. Arend W P, Malyak M, Smith M F Jr, Whisenand T D, Slack J L, Sims J E, Giri J G, Dower S K. 1994. Binding of
IL-1alpha, IL-1ß, and IL-1 receptor antagonist by soluble IL-1 receptors and levels of soluble IL-1 receptors in synovial fluids. J. Immunol. 153: 4766.

153. Orencole S F, Fantuzzi G, Vannier E, Dinarello C A. 1995. Circulating levels of IL-1 soluble receptors in health and after endotoxin or IL-2 administration. Cytokine 7: 642 (abstract).

154. Giri J G, Wells J, Dower S K, McCall C E, Guzman R N, Slack J, Bird T A, Shanebeck K, Grabstein K H, Sims J E, Alderson M R. 1994. Elevated levels of shed type II IL-1 receptor in sepsis. Potential role for type II receptor in regulation of IL-1 responses. J. Immunol. 153: 5802.

155. Sims J E, Dower S K. 1994. Interleukin-1 receptors. (Review) Eur. Cytokine Netw. 5: 539.

156. Symons J A, Young P R, Duff G W. 1995. Soluble type II interleukin-1 (IL-1) receptor binds and blocks processing of
IL-1ß precursor and loses affinity for IL-1 receptor antagonist. Proc. Natl. Acad. Sci. USA 92: 1714.

157. Burger D, Chicheportiche R, Giri J G, Dayer J M. 1995. The inhibitory activity of human interleukin-1 receptor antagonist is enhanced by type II interleukin-1 soluble receptor and hindered by type I interleukin-1 soluble receptor. J. Clin. Invest. 96: 38.

158. Spriggs M K, Hruby D E, Maliszewski C R, Pickup D J, Sims J E, Buller R M, van Slyke J. 1992. Vaccinia and cowpox viruses encode a novel secreted interleukin-1-binding protein. Cell 71: 145.

159. Alcamí A, Smith G L. 1992. A soluble receptor for interleukin-1ß encoded by vaccinia virus: a novel mechanism of virus modulation of the host response to infection. Cell 71: 153.