Cytokines and Disease

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Cytokines and Disease Marc Feldmann* and Fionula M. Brennan Cytokine and Cellular Immunology Division, Kennedy Institute of Rheumatology, Imperial College School of Medicine, 1 Aspenlea Road, Hammersmith, London, W6 8LH, UK * corresponding author tel: +44(0)208 383 4406, fax: +44(0)208 563 0399, e-mail: [email protected] DOI: 10.1006/rwcy.2000.01003.

INTRODUCTION Whereas the health of an individual, organ or cell requires the precise maintenance of the function of the product of the entire  105 genes in the human genome, pathology is potentially simpler. It could be due to changes in a very small number of molecules, initially possibly one. Subsequently there is usually consequential damage to various cells and tissues, so that other genes become abnormally expressed as a consequence, and it can be difficult to assess what the critical early events were. With the involvement of cytokines in essentially all biological processes, such as in cell growth, differentiation, inflammation, immunity, repair, and fibrosis (see the chapter on Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity), it is apparent that cytokines will be involved in a multitude of, and probably all pathological processes. Their role in these processes will be diverse, in some instances cytokines being pathogenic, in others protective and probably most of the time, innocent bystanders. This means that unraveling the role of cytokines in diseases is not as simple as some might wish. The detection of a cytokine, upregulated at the site of disease or damage is merely the first step in that difficult path and in itself does not imply pathogenic relevance. It is very difficult to generalize about the role of cytokines in disease, not only because of the diversity (over 100 known cytokines), but also since diseases are enormously heterogeneous. Nevertheless, this chapter will attempt to provide an introduction to this very complex, and rapidly growing, medically and economically important field.

How can we establish whether a cytokine is relevant to a disease process? There are multiple strategies for evaluating the roles of cytokines in disease. A key distinction to make concerns the role of a particular cytokine in animal models or in human diseases. The former is considerably easier to establish, as an overexpression of that cytokine in transgenic mice (e.g. Keffer et al., 1991), total abrogation in knockouts (Marino et al., 1997), and infusion of large amounts of neutralizing antibodies (Thorbecke et al., 1992) are all possible, admittedly with various degrees of time and effort. This is not the case in humans, of course. As the closeness of animal models to the corresponding human disease is highly variable, with only some aspects of disease pathogenesis of complex human diseases reflected faithfully in any of the animal models, extrapolation from animal models is potentially perilous. So what is `important' in an animal model only provides a working hypothesis for what is happening in the related human disease. With authentic human diseases, other research strategies also yield clues concerning the role of cytokines. These include genetic susceptibility associated with cytokine polymorphisms (e.g. Wilson et al., 1997), differential expression at sites of disease compared to uninvolved tissue (e.g. Feldmann et al., 1996), increased levels in the blood of cytokines (e.g. Charles et al., 1999) or their soluble receptors (Cope et al., 1992). In vitro models of local human diseases, which involve culturing the tissue from the diseased site, can be very useful (Buchan et al.,

36 Marc Feldmann and Fionula M. Brennan 1988a, 1988b) if they continue to produce the same spectrum of `pathological' molecules considered or known to be produced in vivo. These in vitro models can be used for therapeutic tests with antibodies or other reagents to block cytokines that may influence measurable indices of disease activity (Brennan et al., 1989a). However, the only definitive proof of the importance of a cytokine in the pathogenesis of a human disease is in vivo, in a clinical trial in the affected population, using a cytokine inhibitor if the cytokine is considered detrimental or the cytokine itself if it is considered beneficial. Only a limited number of such studies have been performed to date, but it is inevitable that many more will be executed in the near future (Wendling et al., 1993; Paty et al., 1993; Cooper et al., 1993; Peest et al., 1995; Bresnihan et al., 1996; Keystone et al., 1998; Maini et al., 1998; van den Bosch et al., 1998).

Cytokines involved in a disease may be present at augmented, normal or low levels There is now definitive evidence that excess TNF is involved in the pathogenesis of two chronic inflammatory diseases, rheumatoid arthritis (RA) and Crohn's disease (Elliott et al., 1993; Van Dullemen et al., 1995; Present et al., 1999). The critical evidence comes from double-blind, randomized, placebo-controlled trials and has led to FDA approval of TNF inhibitors in these diseases (an anti-TNF antibody Remicade, in Crohn's and RA, a TNFR fusion protein Enbrel in RA). For real certainty of the role of a cytokine in disease more than one such trial and more than one anticytokine agent would be useful and this has been accomplished in RA. The role of TNF in these chronic inflammatory diseases is discussed in more detail in the chapter on TNF. This level of proof is difficult, expensive and time consuming to acquire, but there is no alternative. Negative clinical trial results, however, are more difficult to interpret and do not necessarily invalidate the hypothesis. Abundant animal experimentation was consistent with the importance of TNF in sepsis syndrome (Beutler et al., 1985; Van Zee et al., 1992). However, these studies clearly demonstrated benefit only if a TNF inhibitor was used at, before, or very soon after the injection of LPS or bacteria. However, human sepsis syndrome occurs relatively late after the onset of bacterial infection, and despite thousands of patients treated with TNF inhibitors, there have been no successes (Wherry et al., 1993; Fisher et al., 1996). What does this mean? There are at least two

possibilities. One is that TNF is not important in the later stages of sepsis as it presents clinically, or that the clinical trial design involving hundreds of trial centers makes it very difficult to discern the possible therapeutic effect. Whereas TNF is pathogenic in RA, there is evidence that the opposite may be true in systemic lupus erythematosus (SLE). The clues come from animal models of disease. (NZB  NZW) F1 mice spontaneously develop, in an age- and sex-related fashion, IgM and then IgG autoantibodies to doublestranded DNA (dsDNA), and subsequently other manifestations of SLE, especially nephritis. In these mice, treatment with anti-TNF antibody makes them worse (Jacob and McDevitt, 1988) as does treatment with IL-10 (Ishida et al., 1994), which would also reduce TNF synthesis. Analysis of these mice indicated that they were genetically low producers of TNF. What about human systemic lupus erythematosus? The results are not formally proven. There is evidence that a subset with lupus nephritis has a genetically low production of TNF (Jacob et al., 1990). As part of the anti-TNF therapy clinical trials in rheumatoid arthritis, two patients developed both clinical and serological manifestations of SLE, one including lupus nephritis (Sander and Rau, 1997). Thus it is very likely that low TNF production is one of the pathogenic steps in human SLE.

Cytokine equilibria As discussed in the chapter on the Introduction to the Role of Cytokines in Innate Host Defense and Adaptive Immunity, cells in vivo are never exposed to a single cytokine. They are exposed to whole battery, simultaneously and sequentially, and at various concentrations. So cells must integrate the multiple signals they receive, either individually or collectively, and translate them into biological effects. To help understand this, we have formulated over the years a number of helpful oversimplifications, which can be expressed in diagrams (Figure 1 and Figure 2). It may seem at first glance paradoxical that in inflammatory diseases both the pro- and anti-inflammatory cytokines may be upregulated. However, close investigation has revealed this to be the case, and one of the most important concepts is that of the cytokine equilibrium, illustrated in Figure 1. This applies, for example, to the ratios of proinflammatory to antiinflammatory cytokine mediators, with the net effect depending not on any one cytokine, but on the totality of expression, the equilibrium. Of course in any one tissue there is a complex mosaic of many such

Cytokines and Disease

Figure 1 Cytokine disequilibrium in rheumatoid arthritis. The cytokine profile at sites of inflammation such as rheumatoid synovial tissue is characterized by increased production of both pro- and anti-inflammatory molecules. Modified and reproduced with permission from Feldmann et al. (1996). TGF β MMP-3

IL-1 TNF

IL-10

sTNF-R

method increasing in popularity (because of its simplicity) is PCR for cytokine mRNA. This is a relatively poor tool for understanding the role of cytokines in disease, since cytokines are also abundantly expressed in health, and it is the protein, not the mRNA which signals to the cytokine receptors.

TIMP-1 TIMP-2

IL-1ra

MMP-1

Receptor expression Anti-inflammatory

Proinflammatory

Figure 2 Cytokine cascade in rheumatoid arthritis. Proinflammatory cytokines in rheumatoid arthritic synovium interact in a `network' or `cascade'. Modified and reproduced with permission from Feldmann et al. (1996). TIMP-1, TIMP-2 IL-10, IL-1ra, sTNF-R Anti-inflammatory Immune system

37

TNF

IL-1

Because of their action on very high-affinity receptors cytokines are potent signaling molecules. This means that the biological effect of cytokines also depends on the levels of the corresponding receptors expressed on their neighboring target cells. While the levels of cytokine receptors are far less variable than those of cytokines, and most cytokine receptors are constitutively expressed, levels of cytokine receptors are regulatable. The capacity to receive cytokine signals, especially for multichain receptor complexes, depends critically on receptor density, and so evaluating the role of cytokines in disease also involves an understanding of receptor expression. The variable expression of the IL-2 receptor  chain (CD25) is the best known example of variable cytokine receptor expression. Without CD25 the multifunction IL-2 receptor complex has a much lower affinity (Smith, 1988).

Proinflammatory IL-6, IL-8, GM-CSF etc. MMP-1, MMP-3

equilibria, in different spots; each cell has to summate the effects of signals on its cell surface, integrate them and then, if appropriate, produce secondary mediators, move, commit suicide, etc. Of much interest is the subdivision of CD4+ T lymphocytes into TH1 and TH2 cells. These cells are polarized into producing some but not all of the repertoire of cytokines that T cells can produce (Romagnani, 1991). TH1 cells, considered proinflammatory, produce IFN and IL-2; these are not produced by TH2 cells, which typically produce IL-4 and IL-5. TH2 cells are involved in allergic diseases. In the same way that it is the balance between proinflammatory/anti-inflammatory mediators that dictates the `outcome', rather than levels of either, so it is the balance between TH1 and TH2 and their products that dictates the effects. The concept of equilibrium emphasizes the need for accurate quantitation of cytokines in vitro, in animal models and in human pathology, if greater insight is to be gained. Poorly quantitative and indirect methods for measuring cytokine levels are of limited value. For example, one

Cytokine inhibitors may reflect cytokine expression Like all powerful entities, cytokines have their inhibitors. Cytokines function physiologically as local mediators and their inhibitors are involved in helping to localize their actions. As cytokine synthesis is sporadic, depending on local cell activation, but induced rapidly, it is consistent with the function of cytokine inhibitors that, unlike cytokines, they are present constitutively (e.g. Arend and Dayer, 1990), and are also inducible by the same type of stimuli as induce the corresponding cytokine, and perhaps also by the relevant cytokine itself. Hence the definition of a cytokine activity `profile' needs to also take into account the presence and quantities of the inhibitors. This can be done functionally by using bioassays that `automatically' take endogenous inhibitors into account (rather than the easy and hence popular ELISA assays). In some circumstances there are a variety of inhibitors, for example for IL-1, soluble type I and type II IL-1 receptor, and IL-1 receptor antagonist, and two soluble TNF receptors for TNF and LT.

38 Marc Feldmann and Fionula M. Brennan

The role of cytokines in a variety of biological, pathological processes and diseases While it is clearly not possible in an introductory chapter of reasonable size to discuss the role of cytokines in disease in any depth, we will attempt to illustrate the diversity of effects of cytokines in disease processes. In only a small number of these diseases is there sufficient information to definitely argue that cytokines play a key role in the disease process. This is only true so far for rheumatoid arthritis and Crohn's disease, where the blockade of TNF has reproducibly led to clear clinical benefit. However, these are unlikely to remain the only examples in this category. Because of the greater depth of knowledge in these diseases, we will start the more detailed discussion of cytokines in disease with these examples.

THE ROLE OF CYTOKINES IN INFLAMMATORY AND IMMUNE DISEASES The evident role of cytokines in immunity and inflammation has led to a widespread exploration of the role of cytokines in this group of diseases. Over the years the understanding of the cytokine network in these diseases has evolved dramatically. In the late 1980s and early 1990s it was widely believed that because a number of proinflammatory cytokines with closely overlapping properties (e.g. IL-1, TNF, GM-CSF) were expressed in inflammatory sites, blocking a single cytokine was unlikely to be clinically useful as the proinflammatory features would still be maintained by the remaining cytokines. Studies with antiTNF antibody in rheumatoid joint cell cultures first demonstrated that the expression of proinflammatory cytokines were coordinated (Brennan et al., 1989b), with TNF blockade leading to marked diminution of IL-1 bioactivity. These studies led to the concept of the cytokine cascade, as described in detail in the section on Rheumatoid arthritis. A wide spectrum of cytokines is involved in both the normal immune and inflammatory responses. Hence it is anticipated that in diseases of these systems, alterations in the levels of cytokines may occur, and could be important in the abnormal outcomes. There are clues from cytokine `knockouts' that lowered levels of certain mediators may be important in certain diseases. However, in the human diseases, it is much more likely that there is a reduced cytokine

level, rather than the abrogation of cytokine synthesis, which occurs with a targeted mutation. In this context, total lack of important immune mediators such as IL-2 and IL-10 lead to major immune and inflammatory disorders (Schorle et al., 1991; Kuhn et al., 1993). An excess of TNF in transgenic mice also produces a spectrum of pathology, with a destructive rheumatoid type of arthritis and a Crohn's like inflammatory bowel disease the most common manifestations (Keffer et al., 1991). In contrast, a lack of proinflammatory mediators, e.g. TNF, produces a much more subtle phenotype, discernible upon stressing the animals (Marino et al., 1997). There is a diverse spectrum of immune and inflammatory diseases, with differences in the phenotypes displayed. The most salient features are described below.

Rheumatoid arthritis The role of cytokines in the pathogenesis of rheumatoid arthritis (RA) has been the most extensively investigated of the human diseases since the mid-1980s (reviewed by Feldmann et al., 1996). RA is a chronic inflammatory disease that affects the peripheral synovial joints, resulting in inflammation and swelling, and eventually leading to destruction of the underlying connective tissue. The synovial membrane lining the joint capsule in RA becomes infiltrated with cells from the blood, mainly activated monocytes, T and B cells, while the tissue-resident cells, such as fibroblasts and endothelium, become activated and increase in number. The eventual destruction of the underlying cartilage and bone is now thought to result from the activities of these cells in the synovium and the cytokine and enzyme products that they release. In particular the `pannus' tissue, which migrates from the synovium into the cartilage, is thought to represent the `moving edge' of inflammation and, as such, is an important source of proinflammatory cytokines and other mediators. Cytokine Expression in RA Synovial Tissue Using a combination of immunohistological, mRNA studies, and protein assays, studies from our own group and others have documented that many cytokines are produced in RA synovium. These investigations have indicated the abundance of proinflammatory cytokines (Fontana et al., 1982; Buchan et al., 1988a, 1988b; Di Giovine et al., 1988; Eastgate et al., 1988; Hirano et al., 1988; Hopkins et al., 1988; Hopkins and Meager, 1988; Houssiau et al., 1988; Saxne et al., 1988; Brennan et al., 1989b;

Cytokines and Disease Chu et al., 1991; Deleuran et al., 1992; Wood et al., 1992), hematopoietic growth factors (Firestein et al., 1988; Xu et al., 1989; Alvaro-Garcia et al., 1989; Haworth et al., 1991), chemokines (Brennan et al., 1990a; Koch et al., 1991, 1992, 1993, 1994a, 1994b; Akahoshi et al., 1993; Hosaka et al., 1994), and fibrotic growth factors. These are summarized in Table 1 and reviewed by Feldmann et al. (1996). In the main, activated macrophages, fibroblasts, and endothelium produce the most abundant cytokines. In contrast there is a paucity of T cell-derived cytokines (Firestein et al., 1988; Saxne et al., 1988; Buchan et al., 1988a; Brennan et al., 1989a; Xu et al., 1989), leading to an ongoing debate regarding the importance of T cells in RA, especially in its late stages (Firestein and Zvaifler, 1990; Panayi et al., 1992). Recently, however, using more sensitive techniques, the presence of T cell-derived cytokines, IFN , IL-2, and lymphotoxin, which are characteristic of TH1 cells, has been described in RA tissue (Ulfgren et al., 1995a, 1995b; Steiner et al., 1999).

39

Table 1 Cytokine expression in rheumatoid arthritis Cytokine

Expression mRNA

Protein

IL-1, IL-1

+

+

TNF

+

+

Proinflammatory

LT

+



IL-6

+

+

GM-CSF

+

+

M-CSF

+

+

LIF

+

+

Oncostatin M

+

+

IL-2

+

+

IL-3

ÿ

ÿ

IL-7





IL-9





IL-12

+

+

Cytokine Networks in RA Synovial Tissue

IL-15

+

+

While the studies described above document the spectrum of cytokines produced in an inflammatory tissue such as that in RA, these observations do not indicate which are of major importance with respect to disease pathology, nor do they address the complex interactions between the cytokines. These issues have been addressed using an ex vivo system in which dissociated rheumatoid synovial tissue cells are cultured for short periods without exogenous stimulation, and the spontaneous production of cytokines detected in the presence or absence of cytokine antagonists (reviewed by Feldmann et al., 1996). Prolonged cytokine expression was detected, at both the mRNA level which was not due to increased half-life (Buchan et al., 1988b) and the protein level, mimicking what was presumed to occur in vivo. As it had been reported that TNF was a strong inducer of IL-1 production (Nawroth et al., 1986) we investigated the effect of neutralizing TNF on IL-1 synthesis. It was found that blockade of TNF but not TNF (LT) in these RA synovial cultures significantly inhibited IL-1 bioactivity (Brennan, 1989). As both IL-1 and TNF induce the resorption of cartilage and bone (Gowen et al., 1983; Saklatvala et al., 1985; Dayer et al., 1985; Thomas et al., 1987) this suggested that TNF blockade could have potent effects upon the disease process (Figure 2). Since there are many other possible inducers of IL-1, such as GM-CSF, IFN , and immune complexes, these results demonstrated the predominance of TNF, and led us to investigate whether other

IFN,

+

+

IFN

+

IL-17

+

+

IL-18

+

+

IL-4



ÿ

IL-10

+

+

IL-11

+

+

Immunoregulatory

IL-13

+

+

TGF

+

+

IL-8

+

+

GRO

+

+

Chemokines

MIP-1

+

+

MCP-1

+

+

ENA-78

+

+

RANTES

+

+

Mitogens FGF

+

+

PDGF

+

+

VEGF

+

+

40 Marc Feldmann and Fionula M. Brennan

Regulation of TNF in Rheumatoid Arthritis TNF production is a highly regulated process which includes involvement of both the 50 promoter and the 30 untranslated region of the gene (Beutler and Cerami, 1989). The 30 UTR contains an AU-rich motif, which regulates translation of the mRNA and tissue expression. Indeed replacement of this AU region in the human TNF gene expressed in transgenic mice results in the spontaneous development of arthritis (Keffer et al., 1991). In rheumatoid joints, the TNF is produced predominantly from CD68+ macrophage-like cells. The factor(s) that induce TNF synthesis in RA synovial joints have obviously been of much interest. Blockade of one likely candidate, IL-1, with the IL-1 receptor antagonist had no effect on TNF production (Butler et al., 1995). Others have suggested that the IL-2-like cytokine IL-15, which is found in RA synovium, may be of importance (McInnes et al., 1997). Thus, IL-15 was observed to activate T cells, which then induced TNF synthesis in monocytes in a cognate-dependent manner, and in vivo, arthritis disease in rodents was prevented with a soluble IL-15 receptor protein (McInnes et al., 1996; Ruchatz et al., 1998). However, it remains

unclear whether a gene such as IL-15, which contains multiple start codons, is translated efficiently into protein (Tagaya et al., 1996) in rheumatoid tissue. We have focused upon the hypothesis that TNF synthesis in RA tissue is T cell dependent, based upon the observation that removal of T cells from the monocytic/macrophage population markedly reduces the capacity of the latter to make TNF. Surprisingly, the few monocytic cells left in the T cellenriched fraction, together with the T cells, release more TNF than T cell-depleted synovial macrophages (Brennan, unpublished; Figure 3). Direct cell contact between monocytes and T cells provides for a potent stimulus for TNF and IL-1 production (Isler et al., 1993; Sebbag et al., 1997). The nature of the signals from T cells to macrophages is not well understood but has been studied. Using normal T cells/cell lines, several groups have investigated and proposed some possibilities, e.g. CD69 and LFA-1 (Isler et al., 1993; Manie et al., 1993), VLA4 (Laffon et al., 1991), (Alderson et al., 1993; Wagner et al., 1994; Kiener et al., 1995; Shu et al., 1995) as well as unidentified proteins in the range 25±35 kDa (Isler et al., 1993). We have observed additionally, that the manner in which the T cell is stimulated influences which cytokine are produced by the monocyte. Thus T cells activated with anti-CD3 antibodies mimicking an antigen signal induce abundant TNF and IL-10 synthesis in monocytes when co-cultured (Parry et al., 1997). In contrast, T cells activated for 8 days with a cocktail of cytokines (IL-2, TNF, and IL-6) when fixed and co-cultured with monocytes induce TNF but not IL-10 synthesis (Sebbag et al., 1997) (Figure 4). This raises the interesting possibility that chronic inflammation can be maintained as a consequence of proinflammatory cytokine production Figure 3 TNF production in RA synovial joint cell cultures is T cell dependent. Rheumatoid synovial cells were cultured either as a total population or without CD3+ T cells (removed by magnetic bead enrichment). TNF production was measured by ELISA. 750

TNFα (g/ml)

cytokines produced in the RA synovial cultures may also be TNF? dependent. It was found that GM-CSF, IL-6, and IL-8 (Haworth et al., 1991; Butler et al., 1995) were also downregulated by anti-TNF in these ex vivo cultures. In order to evaluate the role of IL-1 in the synovial cytokine network, IL-1 receptor antagonist was used, and it was found that IL-6, IL-8 were downregulated, but not TNF (Butler et al., 1995). These results clearly pointed to the possibility that blocking TNF may be useful therapeutically and it also highlighted the central or `pivotal' role of TNF in this inflammatory disease (Brennan et al., 1992). Most of the studies described above have been performed upon tissue from late-stage disease. Thus it is unclear whether or what cytokines are involved in promoting the earlier events, such as the TH1/proinflammatory immune response, which may involve IL-12 or IL-18, the most potent inducers of IFN . The role of IL-12 has been investigated in murine collagen-induced arthritis in DBA/1 mice. Thus, IL-12 coadministered with collagen type II boosts arthritis in these animals, and disease is ameliorated if IL-12 action is blocked with anti-IL-12 antibody (Malfait et al., 1998), which is particularly effective in conjunction with anti-TNF (Butler et al., 1999). Little is yet known about IL-18 expression in rheumatoid arthritis, but it is expected to be present at the early stages (Yamamura et al., 1997).

500

250

0

Total

T cell depleted

Day 2

Total

T cell depleted

Day 5

Cytokines and Disease Figure 4 T cell activation modulates the cytokine profile produced by monocytes following T cell monocyte cognate interaction. Macrophage

TNFα + IL-10

TNFα

Cytokine stimulus ‘bystander activation’

Antigen stimulus ‘specific activation’

Figure 5 Blockade of IL-11 activity enhances TNF production: synergy with IL-10. Rheumatoid arthritis synovial membrane cells (pooled data from five experiments) were cultured in triplicate, treated for 48 hours with neutralizing anti-IL-11 mAb (10 g/mL), neutralizing anti-IL10 mAb (10 g/mL) or both antibodies together, and supernatants tested for TNF by ELISA and illustrated as % level of TNF control. TNF production was significantly increased in cell cultures treated with anti-IL-11 mAb (2-fold), anti-IL-10 mAb (3-fold) or both antibodies (22-fold) compared with basal production, anti-IL-11 mAb-, and anti-IL-10 mAb-treated synovial membrane cell cultures, respectively. Modified and reproduced with permission from Hermann et al. (1998).

from macrophages, induced by `bystander' T cells, which themselves are activated by the cytokine products of the macrophage. In the absence of insufficient immunoregulation by cytokines such as IL-10, chronic inflammation ensues.

αIL-10 (10 µg/ml)

Anti-inflammatory Cytokines

αIL-11 (10 µg/ml)

The evaluation of cytokine expression in rheumatoid synovium revealed that some cytokines with chiefly anti-inflammatory properties were also resent in abundant quantities in rheumatoid synovium. These include TGF and IL-10 (Brennan et al., 1990a; Fava et al., 1991; Katsikis et al., 1994). In contrast, IL-4 production has not usually been identified, and its lack may be one of the factors accounting for the relative TH1 T cell preponderance in rheumatoid joints. There are conflicting data concerning IL-13 (Ulfgren et al., 1995b; Isomaki et al., 1996) and the expression and role of IFN/ is not well defined (Hopkins and Meager, 1988). The type I interferons have both pro- and antiinflammatory activities and it is not clear which, if either, predominates in RA. However, the recent evidence that IFN induces STAT4 phosphorylation in TH1 CD4+T cells (Rogge et al., 1997) in common with IL-12, may suggest that increased IFN in RA may predispose to TH1 rather than TH2 predominance. Our own interest has focused on the inhibitory cytokines, which are present in abundance in RA synovial tissue, and include TGF , IL-10, and IL-11. However, while TGF displays several anti-inflammatory effects, the addition to RA joint cell cultures had little effect on the spontaneous production of proinflammatory cytokines (Brennan et al., 1990a), suggesting that the quantities already present were probably at maximal effective concentration. In contrast, the addition of IL-10 diminished the production of IL-1 and TNF furthermore, blockade of endogenous IL-10 enhanced TNF levels by 3±4-fold (Katsikis et al., 1994). This suggests that IL-10 is an important immunoregulator of inflammation, and

41

αIL-11 + αIL-10

* ** * p