9 Acute phase reactants in predicting disease outcome DOUGLAS THOMPSON JOHN TEMPLEMAN WHICHER R O S A M O N D E E L I Z A B E T H BANKS
The constellation of systemic responses which accompanies tissue damage is known as the acute phase response. Metabolic changes mobilize energy in the form of glucose, fatty acids and amino acids. Fever up-regulates enzymatic reactions in inflammatory and tissue cells, facilitating bacterial killing, with leucocytosis providing an increased supply of phagocytic cells. Hepatic production of acute phase proteins provides inflammatory, mediatory and inhibitory proteins within the tissue fluid. Complex interactions with the neuroendocrine system come into play, promoting cortisol and adrenocorticotrophic hormone release. Much evidence thus supports the concept that the acute phase response is an adaptive process conditioning the 'r~ilieu interieur' so that inflammation and the healing process may progress optimally. A number of components of the response have been used in various diseases to quantify inflammation, in particular fever and acute phase protein production. Many clinical studies have shown that plasma concentrations of acute phase proteins may be used to assess the mass and activity of inflammation and its response to therapeutic intervention. Such measurements can be very sensitive to small amounts of inflammation and may be used to monitor its presence, being particularly useful early on in inflammatory diseases such as arthritis. A C U T E PHASE PROTEINS
The acute phase proteins are a group of approximately 30 plasma proteins synthesized in increased amounts, principally by hepatocytes, following an inflammatory stimulus. Kushner (1982) has defined acute phase proteins as those whose concentration rises by 25 % or more following inflammation and has divided them into three groups (Kushner and Mackiewicz, 1987) on the basis of the magnitude of the increase (Table 1). When they are classified according to their known functions it is clear that they may all have roles to play in inflammation or the healing process which follows, particularly as Bailli~re's ClinicalRheumatology--
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Table 1. The major human acute phase proteins. Adapted from Kushner and Mackiewicz (1987).
Protein
Normal plasma concentration
Response time
Function
48-72 h 48-72 h
? Free radical scavenger Opsonization, chemotaxis, mast cell degranulation Promoter of fibroblast growth, inhibitor of T cells Proteinase inhibitor Proteinase inhibitor Scavenger of haemoglobin Clotting, fibrin matrix formation
Group l--about 50% increase
Caeruloplasmin Complement C3 Complement C4
0.15-0.6 g/litre 0.55-1.2g/litre 0.2-0.5 g/litre
Group I I - - 2 - 4 x increase
Orosomucoid
0.5-1.4 g/litre
24h
cq-Antitrypsin al-Antichymotrypsin Haptoglobin Fibrinogen
1.0-2.0 g/litre 0.3-0.6 g/litre 1.0-3.0 g/litre 2.0-4.5 g/litre
10 h 10 h 24 h 24 h
Group Ill--up to lO00x increase
C-reactive protein
0.07-8mg/litre
6-10 h
Serum amyloid A
1-30 mg/litre
6-10 h
Opsonization,?Complement activation ?Scavengerof cholesterol
mediators, inhibitors and scavengers of cell-derived products released from damaged tissue or macrophages (Table 1). In addition some m e m b e r s of the family m a y influence the immune response which often accompanies inflammation and the release of autologous antigens. The acute phase protein response is thus a physiological mechanism providing increased serum and tissue levels of proteins modulating inflammation, and it is probable that it affects both the nature of the inflammatory lesion and the healing process which follows it. The rate of increase in their plasma concentration and the incremental change which occurs following inflammation varies considerably among the acute phase proteins and reflects their induction by different cytokines and their molecular size, volume of distribution and rate of catabolism, both in the circulation and at the site of inflammation. In simple inflammation the pattern of acute phase response is characteristic for a particular species of animal. However, inflammation complicated by other processes such as intravascular coagulation or haemolysis may induce a different pattern of serum acute phase proteins due to the effect of differential catabolism of serum proteins.
C Y T O K I N E C O N T R O L OF ACUTE PHASE P R O T E I N PRODUCTION Following the realization that the liver is the m a j o r site of acute phase protein synthesis, a vast amount of experimental work has been directed towards elucidation of the chemical messengers involved in their synthesis and regulation. Although several cytokines have been implicated, the
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complexity of the acute phase response is only just becoming apparent and it is likely that synergistic interaction takes place not only between different cytokines, but also between cytokines, hormones and other molecules. In the 1980s interleukin (IL)-I was generally believed to be the major inducer of acute phase protein production. In vivo experiments using the newly available recombinant IL-1 (Sipe and Ramadori, 1986) appeared to substantiate the role of IL-1 implied by experiments using purified material (Kampschmidt and Mesecher, 1985), and a direct action on the liver was suggested (Sipe et al, 1988). However, a lack of correlation between in vivo and in vitro activity is now apparent. For example, in vivo administration of purified or recombinant human IL-1 causes the induction of fibrinogen in all species tested (Kampschmidt and Mesecher, 1985; Dinarello, 1988), although fibrinogen expression is not induced by IL-1 at the messenger RNA (mRNA) or protein level in hepatocytes or hepatoma cells from rat, mouse, rabbit or human (Darlington et al, 1986; Baumann et al, 1987a; Evans et al, 1987; Koj et al, 1987). It is now clear that IL-1 can induce a limited number of acute phase protein genes, and it has also been shown to suppress the induction of a number of acute phase proteins caused by other mediators such as IL-6 (Darlington et al, 1986; Koj et al, 1987; Andus et al, 1988). In some circumstances tumour necrosis factor (TNF) ~ and IL-lf3, cytokines with similar spectra of activity, have similar acute phase protein inducing capabilities. However, these cytokines elicit only a partial acute phase response in cultured liver cells compared with hepatocytes treated with crude supernatants from COLO 16 tumour cells or endotoxinstimulated monocyte supernatants (Darlington et al, 1986; Baumann et al, 1987a; Koj et al, 1987), pointing to the existence of other mediators of the acute phase protein response. The monocyte-derived hepatocyte-stimulating factor first described by Ritchie and Fuller (1981, 1983) and later by other groups (Baumann et al, 1984; Koj et al, 1984; Bauer et al, 1986; Darlington et al, 1986) proved to be identical to interferon ~32, hybridoma growth factor, 26 kDa protein and B cell stimulating factor, and was subsequently named IL-6. This molecule has since been recognized as the major inducer of acute phase protein synthesis, and the hepatocyte is thought to be its main physiological target. Gauldie and others (1988) have established that IL-6, in combination with IL-113 and glucocorticoid, accounts for all the hepatic-specific stimulation released by monocytes, and Baumann and co-workers (1987b) have described an additive effect of IL-6, IL-1 and glucocorticoids on some proteins. The current view is summarized in Table 2, which shows the acute phase proteins as distinct subsets based on differential induction by combinations of cytokines. A wide variety of cells, including fibroblasts, synovial cells and endothelial cells as well as hepatocytes themselves (Lotz et al, 1989), produce IL-6. Significant expression of IL-6 has been reported in normal human hepatocytes (Tovey et al, 1988), suggesting the possible involvement of an autocrine mechanism in liver stimulation. However, the failure to detect IL-6 mRNA expression in the liver of normal rats or those undergoing an acute phase response (Baumann and Gauldie, 1990) indicates that caution
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Table 2. Acute phase protein genes regulated by inflammatory cytokines. From Gauldie et al (1989), with permission. Induced by IL-1 and IL-6
al-Acid glycoprotein C3 Factor B Serum amyloidA Serum amyloidP Haptoglobin (rat) C-reactive protein (human) Hemopexin (rat) Synergy IL-1 and 1L-6
cq-Acid glycoprotein C3
Induced by IL-6 only
a2-Macroglobulin Fibrinogen cq-Antitrypsin Haptoglobin (human) cq-Antichymotrypsin Caeruloplasmin Cysteine proteinase inhibitor C1 esterase inhibitor Inhibition IL-1 on IL-6
Fibrinogen az-Macroglobulin Cysteine proteinase inhibitor
should be exercised when interpreting data using isolated hepatocytes cultured under non-physiological conditions such as in the absence of corticosteroids. QUANTITATION OF T H E ACUTE PHASE PROTEIN RESPONSE The acute phase protein response can either be measured directly using specific protein assays or indirectly by the use of integrated measures such as the erythrocyte sedimentation rate or plasma viscosity. More recently plasma cytokine concentrations have been measured during the acute phase response.
Specific protein measurements As can be seen in Table 1, there is considerable variation amongst the acute phase proteins in terms of basal concentrations, incremental change and response time following the inflammatory stimulus. A number of studies have indicated that serum amyloid A (SAA) is the most sensitive of the acute phase proteins, sometimes rising above the reference range even when C-reactive protein (CRP) remains within it. In these studies CRP was measured by methods having a lower limit of sensivity at the upper limit of the reference range (approximately 10 mg/litre). The recent use of more sensitive methods with experimental minimal inflammation in man suggests that CRP may in fact increase by a similar increment, although within the reference range (Chambers et al, 1991). CRP
This is a sensitive acute phase protein with a short half-life, a rapid response time and large incremental change. Its catabolism is not affected by the type
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of inflammation and it is easily measured. For these reasons it has gained considerable popularity and has the largest database of disease-related change. Its major disadvantage is its wide reference range from 0.068 to 8.0 mg/litre, with a median value of 0.58 mg/litre (Claus et al, 1976). In mild inflammation and some viral infections typical values lie between 10 and 40mg/litre. Concentrations between 40 and 200mg/litre indicate acute inflammation and bacterial infection, while concentrations greater than 300 mg/litre are seen in extensive trauma and severe bacterial infections (Morley and Kushner, 1982). Many assays are available, such as enzyme immunoassays, radial immunodiffusion, electroimmunoassay, nephelometry or turbidimetry, with sensitivities of 5-10 mg/litre, but we would recommend the use of the more sensitive radioimmunoassay or the sensitive enzyme immunoassay, with sequential samples to monitor the response of inflammation to therapy.
SAA SAA, like CRP, is a very sensitive acute phase protein with a large incremental change. It may, however, be too sensitive, responding to such trivial infections as the common cold (Whicher et al, 1985). It also has major disadvantages due to difficulty in its measurement as there are no suitable standards and antibodies are difficult to produce and often of low titre. These problems are reflected in the widely varying reference ranges. Radial immunodiffusion assays suggest a reference range of 1-3 mg/litre, based on calibrants derived from tissue amyloid (Chambers and Whicher, 1983), while radioimmunoassays suggest a reference range of less than 200 ~g?titre, based on a purified SAA calibrant (Brandwein et al, 1984).
oLFAntichymotrypsin This protein does not show the same large incremental change of SAA and CRP and is slower to rise following inflammation (Table 1). It has a longer half-life than CRP or SAA and it has been suggested that this protein be measured in addition to CRP when it is desirable to have a longer-term window to assess changes in inflammation (Calvin et al, 1988). The measurement of this protein is simple, quick and cheap.
~l-Acid glycoprotein (orosomucoid) This is rather an insensitive acute phase protein and its measurement has little advantage over CRP or ~a-antichymotrypsin.
Haptoglobin, ~l-antitrypsin and fibrinogen These proteins are unsuitable as markers of the acute phase protein response since they can all be actively catabolized in certain disease states. Also, haptoglobin and r have common genetic variants resulting in deficiency and phenotype-specific reference ranges.
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Integrated measures of the acute phase protein response
Erythrocyte sedimentation rate The erythrocyte sedimentation rate (ESR) is probably the most widely used measurement of the acute phase protein response. The ESR is a measure of the rate of sedimentation of erythrocytes in a vertical tube over a standard period of 1 h. The rate of sedimentation is governed by erythrocyte aggregation induced by large plasma proteins. Fibrinogen is particularly imPortant because of the combination of its asymmetric shape, large molecular size and high concentration. The relative contribution of plasma proteins to the ESR are: fibrinogen 55%, et2-macroglobulin 27%, immunoglobulins 11% and albumin 7% (Stuart and Whicher, 1988). Erythrocyte aggregation is also influenced by the number, shape, density and deformability of erythrocytes, so that the ESR increases with decreasing haematocrit. The effect of dietary lipids on the erythrocyte membrane is reflected in the circadian rhythm of the ESR. It cannot be performed on stored blood and is difficult to standardize, so that reference ranges should always be determined locally (International Committee for StandardiSation in Haematology, 1988). Because changes in ESR largely reflect changes in concentration of fibrinogen, The ESR is generally slow to change following an inflammatory stimulus (>24h) and is slow to resolve following the resolution of the inflammation (96-144 h). However, because the ESR also reflects changes in the concentration of several acute phase proteins as well as immunoglubulins and immune complexes, it does provide a wider screen for the detection of disease than the measurement of any one acute phase protein alone. The main advantages of the test, however, are its relative simplicity and cost. Plasma viscosity This measurement has become popular mainly because of the standarization and storage problems of the ESR. There are less problems with the plasma viscosity (PV) but anticoagulant and storage conditions are important. Other advantages over the ESR include low running costs, speed of assay and a more robust, sex-independent reference range. However, similarly to the ESR, the PV is disadvantaged by being largely dependent on the concentrations of slow-responding acute phase proteins such as fibrinogen and other non-acute phase proteins such as immunoglubulins (International Committee for Standardisation in Haematology, 1988). Cytokine measurements
There are now a number of commercially available immunoassay kits for the cytokines IL-1, IL-6 and TNF, which are thought to be the main mediators of the acute phase response. However, these assays are expensive and generally not very sensitive and therefore at present are used mainly for
ACUTE PHASE REACTANTS
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research purposes. It is not yet clear whether they are more sensitive than established measurements of the acute phase protein response or whether they can provide more useful diagnostic information. ACUTE PHASE REACTANTS AND DISEASE OUTCOME IN EARLY INFLAMMATORY ARTHRITIS Acute inflammatory arthritis may have several outcomes. For example, it may resolve relatively quickly, it may be viral with an occasionally protracted but good course, or it may lead to seropositive rheumatoid arthritis with varying degrees of functional disability. It is obviously important to be able to identify at an early stage those patients with inflammatory arthritis who may progress to a more severe and destructive disease, thus possibly meriting more aggressive therapy. However, relatively few studies have been carried out which have addressed this question satisfactorily. In a recent review of the literature (van der Heijde et al, 1988), it was found that no study of predictive factors in the outcome of newly diagnosed rheumatoid arthritis patients fulfilled the desired criteria of being prospective, including patients early after disease onset, and with regular standardized follow-up for a minimum of several years with clinical, laboratory and radiological investigation on each occasionl The heterogeneity and shortcomings of study design frustrate valid comparison and probably account for the lack of consensus concerning the best measurement to make for prediction of outcome. The issue is complicated by heterogeneity in three main areas: firstly, in the clinicaPand functional assessment of patients; secondly, in the statistical assessment of the data, for example whether the patient data is analysed in subgroups or as a whole in a study which contains patients with different diagnoses; and finally, the assessment is also complicated by a variety of therapies within the study group. The question of whether or not the group should be examined as a whole or in diagnostic subgroups depends very much on the questions being asked. When patients present early in disease, it is not always possible to assign them to a definite diagnostic category, a point in favour of examining the group as a whole. Analysing the data in diagnostic subgroups may be of particular value in providing information about a distinct aspect of the disease. Most of the studies examining predictive values in inflammatory arthritis centre round patients with established definite or classical rheumatoid arthritis, with a few examining seronegative spondylarthropathies and reactive arthritides. Few studies have examined the use of acute phase reactants in predicting the final outcome, whether measured as resultant disability or mortality. The measurement of the acute phase response used most often in these studies is the ESR, with PV and plasma CRP concentrations used less frequently. These studies are described below. One of the earliest prospective studies into prognostic markers in inflammatory arthritis which included a measure of the acute phase response as a parameter, namely the ESR, was that carried out on 307 patients with
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rheumatoid arthritis admitted to an Edinburgh hospital between 1948 and 1951 (Duthie et al, 1964). Two hundred of these patients were available for the final assessment approximately 9 years later, which was based both on functional capacity and disease activity, as assessed by the ESR, haemoglobin, and both joint and systemic involvement. It was concluded from this study that isolated ESR values were no guide to the subsequent disease course. However, when considering this conclusion it should be appreciated that less than 50% of the initial group had a disease duration of less then 1 year and that the study was limited to hospitalized patients, thereby excluding milder cases from the analysis. Similarly a study of 50 patients with rheumatoid arthritis, all presenting within 6 months of onset and assessed 6-monthly, failed to find any value in the initial ESR level for the prediction of disease outcome as assessed clinically and radiologically at an average of 3 (Masi et al, 1976) or 5 years (Feigenbaum et al, 1979). However, in a larger prospective study based on 102 patients with rheumatoid arthritis, a raised ESR at first assessment was associated with a less favourable outcome (Fleming et al, 1976). In this study, patients were recruited within a year of onset and divided into three prognostic groups depending on the outcome of their disease as assessed clinically, but not radiologically, at a mean of 4.5 years from onset. When examined over longer follow-up periods, the predictive power of acute phase response measurements is also not clear. A prospective study initially involving 100 patients with rheumatoid arthritis with a disease duration of less than 1 year examined either mortality, radiological status or functional outcome at follow-up periods of 8-11 years (Jacoby et al, 1973), 20 years (Rasker and Cosh, 1989) and 25 years (Reilly et al, 1990). The initial ESR (within 1 year of onset) did not correlate with clinical and radiological outcome when reassessed at between 8 and 14 years in the 83 survivors. However, of the survivors at 25 years (n = 37, 35 of whom were reviewed), the group with the better functional outcome had a significantly lower mean ESR at 1 year post-onset than the poorer outcome groups, a poorer outcome being associated with a persistently raised ESR (Reilly et al, 1990). When reviewed after 20 or 25 years, the ESR levels at 1 year post-onset were not found to correlate with mortality, although patients with persistently high ESR and seropositivity tended to do badly (Rasker and Cosh, 1989). Other studies which have examined the usefulness of the initial ESR levels in predicting disease outcome have also failed to find a significant correlation (Sherrer et al, 1986; Scott et al, 1987), but these were studies in which patients with early disease were in the minority. Although not significant, the latter study did find that a high ESR at the onset of the study tended to be associated with a poorer functional outcome. Larger studies have been carried out as the result of international collaboration, but the picture still remains far from clear as to whether measurements of the acute phase response at onset are predictive. In a Soviet-Finnish co-operative prognostic study which consisted of 136 and 139 patients respectively, fulfilling (at least) the American Rheumatism Association diagnostic criteria for probable rheumatoid arthritis and with a
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disease duration of less than 6 months, patients were assessed clinically and radiologically at presentation and after 3 years (Isomaki et al, 1984). The prognostic importance of the signs and symptoms at the onset of arthritis was evaluated separately in the two centres due to differences in assessment and patient groups. In the Finnish patients, the initial ESR was significantly higher for the patients eventually assigned to the poorer outcome groups (assessed clinically and radiologically) but this was not so for the Soviet group, whereas serum CRP at onset was significantly higher in the poorer outcome groups for both the Finnish and Soviet patients. Fibrinogen and orosomucoid were not significant indicators. A smaller study (n = 72) over a shorter follow-up period which also examined the usefulness of both ESR and CRP measurements failed to find any association between either early CRP concentrations or ESR with functional outcome, as assessed by limitation of wrist extension (Reeback and Silman, 1984). In a more recent study (Woolf et al, 1991) recruited 100 patients attending an early synovitis clinic within 3 months of onset of inflammatory joint disease. The follow-up data on 88 patients was obtained at a minimum of 5 years later, with 56 patients examined in person, 25 by questionnaire and seven from medical records; assessments were made clinically and on the basis of radiographs of hands where available. The group of patients, 41% of whom had rheumatoid arthritis, was examined as a whole. A raised CRP and PV at presentation had a poor specificity (38 and 35 %, respectively) and sensitivity (29 and 65 %, respectively) for the group with functional disability (n = 26). This is probably due at least in part to the mixed group, as the figures improved when the 36 patients with rheumatoid arthriti~ were analysed as a subgroup. Several studies have examined longitudinal variations in acute phase proteins. Most of these studies are concerned with the association of such markers with disease activity during the period of assessment (reviewed by McKenna, 1988) rather than with the future actual predictive value of any apparent trends early in the disease. Many of the patients used in such studies do not have early disease and have been subjected to various therapies which may, it is argued in some cases, in themselves influence the acute phase response as an indicator of the underlying disease activity. The studies which have examined the actual predictive ability of such trends are discussed below. It should be noted, however, that in most of these studies the patients are mixed in terms of disease duration, with a variable proportion having early disease. Although in the earlier mentioned study by Duthie et al (1964) isolated ESR measurements were of no predictive value in terms of outcome, sequential measurements were related to functional outcome. Patients were grouped according to their highest ESR reading obtained on admission, on discharge and at the first assessment (24 months) and this was related to their functional capacity at the end of the study (mean = 9 years). Generally, the higher the ESR, the worse the functional outcome, with the exception of the group with the highest ESR results, who did relatively well. Scott et al (1985) examined the effects of second-line antirheumatic drugs
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on the course and progression of rheumatoid arthritis. Fifty-six patients were studied, of whom 14 had a disease duration of less than 1 year. A second group of 15 patients on non-steroidal anti-inflammatory therapy were also studied. The 53 of the total patients who had an initial ESR of 30 mm/h or more were divided into two groups--those whose E S R fell by 25 mm/h or who had an E S R of 20 mm/h or less at 6 months, and those showing a lesser or no fall at all. Only those patients whose E S R fell showed no significant progression in the subsequent 6 months. Both these groups had an initial mean E S R of 50 mm/h. This suggests that a falling ESR is a n important predictor of subsequent radiological progression. In 60 patients with early juvenile chronic arthritis--20 in each of the pauciarticular, polyarticular and systemic onset groups--plasma CRP levels were significantly higher in the systemic group than the polyarticular group (mean 120 mg/litre vs 59.7 mg/litre; Gwyther et al, 1982). A good correlation was seen between plasma CRP and the E S R in early disease. CRP and E S R tended to fall in systemic illness when the disease state improved. However, in the five patients in whom amyloidosis developed within 5 years of onset, the CRP remained consistently high and the correlation with E S R was not as good. Additionally, in patients who later developed amyloidosis, similar high plasma CRP concentrations were found over the months or years before amyloidosis was confirmed. Tunn and Bacon (1992) measured at initial presentation a wide range of clinical and laboratory variables in 112 patients referred to an early arthritis clinic with < 6 months joint symptoms. Univariate analysis was unable to predict persistence. Multivariate analysis also showed overlap between the self-limiting and persistent disease groups but identified a subset of useful variables. The combination of a positive R A latex with an E S R > 30 mm/h carried a relative risk for persistence of 4.33 with 94% specificity but only 69% sensitivity. SUMMARY From the studies which are reviewed above, it is generally apparent that in terms of the acute phase response, the initial findings in early inflammatory arthritis (particularly rheumatoid arthritis, with which the majority of such studies are concerned) have little predictive value for either the functional outcome or mortality. The wide interindividual variability in these measurements is also likely to limit their clinical usefulness as predictors of disease outcome. The trend in certain acute phase reactants may be more useful in indicating disease activity, although the number of satisfactory studies in this area is very limited. REFERENCES Andus T, Geiger T, Hirano T et al (1988) Regulation of synthesis and secretion of major rat acute phase proteins by recombinant human intefleukin-6 (BSF-2/IL-6) in hepatocyte primary cultures. European Journal of Biochemistry 173: 287-293.
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Kampschmidt RF & Mesecher M (1985) Interleukin 1 from P388D1: effects upon neutrophils, plasma iron and fibrinogen in rats, mice and rabbits. Proceedings of the Society for Experimental Biology and Medicine 179: 197-200. Koj A, Gauldie J, Regoeczi E, Sauder DN & Sweeney GD (1984) The acute phase response of cultured rat hepatocytes. System characterisation and the effect of human cytokines. Biochemical Journal 224: 505-514. Koj A, Kurdowska A, Magielska-Zero D et al (1987) Limited effects of recombinant human and murine interleukin-1 and tumour necrosis factor on production of acute phase proteins by cultured rat hepatocytes. Biochemistry International 14: 553-560. Kushner I (1982) The phenomenon of the acute phase response. Annals of the New York Academy of Sciences 389: 39-48. Kushner I & Mackiewicz A (1987) Acute phase proteins as disease markers. Disease Markers 5: 1-11. Lotz M, Zuraw BL, Carson D A & Jirik FR (1989) Hepatocytes produce interleukin-6. Annals of the New York Academy of Sciences 557: 509-511. Masi AT, Maldonad0-Cocco JA, Kaplan SB et al (1976) Prospective study of the early course of rheumatoid arthritis in young adults: Comparison of patients with and without rheumatoid factor positivity at entry and identification of variables correlating with outcome. Seminars in Arthritis and Rheumatism 5" 299-326. McKenna F (1988) Clinical and laboratory assessment of outcome in rheumatoid arthritis. British Journal of Rheumatology 27(supplement 1): 12-20. Morley JJ & Kushner I (1982) Serum C-reactive protein levels in disease. Annals of the New York Academy of Sciences 389: 406-418. Rasker JJ & Cosh Jm (1989) Course and prognosis of early rheumatoid arthritis. Scandinavian Journal of Rheumatology Supplement 79: 45-56. Reeback J & Silman A (1984) Predictors of outcome at two years in patients with rheumatoid arthritis. Journal of the Royal Society of Medicine 77: 1002-1005. Reilly PA, Cosh JA, Maddison P J, Rasker JJ & Silman AJ (1990) Mortality and survival in rheumatoid arthritis: a 25 year prospective study of 100 patients. Annals of the Rheumatic Diseases 49: 363-369. Ritchie DG & Fuller GM (1981) An in vitro bioassay for leukocytic endogenous mediator(s) using cultured rat hepatocytes. Inflammation 5: 287-299. Ritchie DG & Fuller GM (1983) Hepatocyte stimulating factor: a monocyte-derived acute phase regulatory protein. Annals of the New York Academy of Sciences 408: 490-502. Scott DL, Dawes PT, Fowler PD et al (1985) Anti-rheumatic drugs and joint damage in rheumatoid arthritis. Quarterly Journal of Medicine 54- 49-59. Scott DL, Coulton BL, Symmons DPM & Popert AJ (1987) Long-term outcome of treating rheumatoid arthritis: results after 20 years. Lancet i: 1108-1111. Sherrer YS, Bloch DA, Mitchell DM, Young DY & Fries JF (1986) The development of disability in rheumatoid arthritis. Arthritis and Rheumatism 29: 494--500. Sipe JD & Ramadori G (1986) Site of SAA/AA synthesis. In Marrink J & Van Rijswijk MH (eds) Amyloidosis, pp 319-328. Dordrecht: Martinus Nijhoff. Sipe JD, Johns MA, Ghezzi P & Knapschaefer G (1988) Modulation of serum amyloid A gene expression by cytokines and bacterial cell wall components. Advances in Experimental Medicine and Biology 243: 193-201. Stuart J & Whicher JT (1988) Tests for detecting and monitoring the acute phase response. Archives of Disease in Childhood 63: 115-117. Tovey MG, Content J, Gresser I e t al (1988) Genes for IFN-beta-2 (IL-6) tumour necrosis factor and IL-1 are expressed at high levels in the organs of normal individuals. Journal of Immunology 141: 3106-3110. Tunn EJ & Bacon PA (1992) Differentiating persistent from self-limiting synovitis in an early arthritis clinic. British Journal of Rheumatology (in press). van Der Heijde DMFM, Van Riel PLCM, Van Rijswijk MH & Van De Putte LBA (1988) Influence of prognostic features on the final outcome in rheumatoid arthritis: a review of the literature. Seminars in Arthritis and Rheumatism 17: 284-292. Whicher JT, Chambers RE, Higginson J, Nashef L & Higgins PG (1985) Acute phase response of serum amyloid A protein and C reactive protein to the common cold and influenza. Journal of Clinical Pathology 38: 312-316. Woolf AD, Hall AND, Goulding NJ et al (1991) Predictors of the long-term outcome of early synovitis: a 5-year follow-up study. British Journal of Rheumatology 30: 251-254.
The constellation of systemic responses which accompanies tissue damage is known as the acute phase response. Metabolic changes mobilize energy in the form of glucose, fatty acids and amino acids. Fever up-regulates enzymatic reactions in inflammatory and tissue cells, facilitating bacterial killing, with leucocytosis providing an increased supply of phagocytic cells. Hepatic production of acute phase proteins provides inflammatory, mediatory and inhibitory proteins within the tissue fluid. Complex interactions with the neuroendocrine system come into play, promoting cortisol and adrenocorticotrophic hormone release. Much evidence thus supports the concept that the acute phase response is an adaptive process conditioning the 'r~ilieu interieur' so that inflammation and the healing process may progress optimally. A number of components of the response have been used in various diseases to quantify inflammation, in particular fever and acute phase protein production. Many clinical studies have shown that plasma concentrations of acute phase proteins may be used to assess the mass and activity of inflammation and its response to therapeutic intervention. Such measurements can be very sensitive to small amounts of inflammation and may be used to monitor its presence, being particularly useful early on in inflammatory diseases such as arthritis. A C U T E PHASE PROTEINS
The acute phase proteins are a group of approximately 30 plasma proteins synthesized in increased amounts, principally by hepatocytes, following an inflammatory stimulus. Kushner (1982) has defined acute phase proteins as those whose concentration rises by 25 % or more following inflammation and has divided them into three groups (Kushner and Mackiewicz, 1987) on the basis of the magnitude of the increase (Table 1). When they are classified according to their known functions it is clear that they may all have roles to play in inflammation or the healing process which follows, particularly as Bailli~re's ClinicalRheumatology--
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Table 1. The major human acute phase proteins. Adapted from Kushner and Mackiewicz (1987).
Protein
Normal plasma concentration
Response time
Function
48-72 h 48-72 h
? Free radical scavenger Opsonization, chemotaxis, mast cell degranulation Promoter of fibroblast growth, inhibitor of T cells Proteinase inhibitor Proteinase inhibitor Scavenger of haemoglobin Clotting, fibrin matrix formation
Group l--about 50% increase
Caeruloplasmin Complement C3 Complement C4
0.15-0.6 g/litre 0.55-1.2g/litre 0.2-0.5 g/litre
Group I I - - 2 - 4 x increase
Orosomucoid
0.5-1.4 g/litre
24h
cq-Antitrypsin al-Antichymotrypsin Haptoglobin Fibrinogen
1.0-2.0 g/litre 0.3-0.6 g/litre 1.0-3.0 g/litre 2.0-4.5 g/litre
10 h 10 h 24 h 24 h
Group Ill--up to lO00x increase
C-reactive protein
0.07-8mg/litre
6-10 h
Serum amyloid A
1-30 mg/litre
6-10 h
Opsonization,?Complement activation ?Scavengerof cholesterol
mediators, inhibitors and scavengers of cell-derived products released from damaged tissue or macrophages (Table 1). In addition some m e m b e r s of the family m a y influence the immune response which often accompanies inflammation and the release of autologous antigens. The acute phase protein response is thus a physiological mechanism providing increased serum and tissue levels of proteins modulating inflammation, and it is probable that it affects both the nature of the inflammatory lesion and the healing process which follows it. The rate of increase in their plasma concentration and the incremental change which occurs following inflammation varies considerably among the acute phase proteins and reflects their induction by different cytokines and their molecular size, volume of distribution and rate of catabolism, both in the circulation and at the site of inflammation. In simple inflammation the pattern of acute phase response is characteristic for a particular species of animal. However, inflammation complicated by other processes such as intravascular coagulation or haemolysis may induce a different pattern of serum acute phase proteins due to the effect of differential catabolism of serum proteins.
C Y T O K I N E C O N T R O L OF ACUTE PHASE P R O T E I N PRODUCTION Following the realization that the liver is the m a j o r site of acute phase protein synthesis, a vast amount of experimental work has been directed towards elucidation of the chemical messengers involved in their synthesis and regulation. Although several cytokines have been implicated, the
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complexity of the acute phase response is only just becoming apparent and it is likely that synergistic interaction takes place not only between different cytokines, but also between cytokines, hormones and other molecules. In the 1980s interleukin (IL)-I was generally believed to be the major inducer of acute phase protein production. In vivo experiments using the newly available recombinant IL-1 (Sipe and Ramadori, 1986) appeared to substantiate the role of IL-1 implied by experiments using purified material (Kampschmidt and Mesecher, 1985), and a direct action on the liver was suggested (Sipe et al, 1988). However, a lack of correlation between in vivo and in vitro activity is now apparent. For example, in vivo administration of purified or recombinant human IL-1 causes the induction of fibrinogen in all species tested (Kampschmidt and Mesecher, 1985; Dinarello, 1988), although fibrinogen expression is not induced by IL-1 at the messenger RNA (mRNA) or protein level in hepatocytes or hepatoma cells from rat, mouse, rabbit or human (Darlington et al, 1986; Baumann et al, 1987a; Evans et al, 1987; Koj et al, 1987). It is now clear that IL-1 can induce a limited number of acute phase protein genes, and it has also been shown to suppress the induction of a number of acute phase proteins caused by other mediators such as IL-6 (Darlington et al, 1986; Koj et al, 1987; Andus et al, 1988). In some circumstances tumour necrosis factor (TNF) ~ and IL-lf3, cytokines with similar spectra of activity, have similar acute phase protein inducing capabilities. However, these cytokines elicit only a partial acute phase response in cultured liver cells compared with hepatocytes treated with crude supernatants from COLO 16 tumour cells or endotoxinstimulated monocyte supernatants (Darlington et al, 1986; Baumann et al, 1987a; Koj et al, 1987), pointing to the existence of other mediators of the acute phase protein response. The monocyte-derived hepatocyte-stimulating factor first described by Ritchie and Fuller (1981, 1983) and later by other groups (Baumann et al, 1984; Koj et al, 1984; Bauer et al, 1986; Darlington et al, 1986) proved to be identical to interferon ~32, hybridoma growth factor, 26 kDa protein and B cell stimulating factor, and was subsequently named IL-6. This molecule has since been recognized as the major inducer of acute phase protein synthesis, and the hepatocyte is thought to be its main physiological target. Gauldie and others (1988) have established that IL-6, in combination with IL-113 and glucocorticoid, accounts for all the hepatic-specific stimulation released by monocytes, and Baumann and co-workers (1987b) have described an additive effect of IL-6, IL-1 and glucocorticoids on some proteins. The current view is summarized in Table 2, which shows the acute phase proteins as distinct subsets based on differential induction by combinations of cytokines. A wide variety of cells, including fibroblasts, synovial cells and endothelial cells as well as hepatocytes themselves (Lotz et al, 1989), produce IL-6. Significant expression of IL-6 has been reported in normal human hepatocytes (Tovey et al, 1988), suggesting the possible involvement of an autocrine mechanism in liver stimulation. However, the failure to detect IL-6 mRNA expression in the liver of normal rats or those undergoing an acute phase response (Baumann and Gauldie, 1990) indicates that caution
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Table 2. Acute phase protein genes regulated by inflammatory cytokines. From Gauldie et al (1989), with permission. Induced by IL-1 and IL-6
al-Acid glycoprotein C3 Factor B Serum amyloidA Serum amyloidP Haptoglobin (rat) C-reactive protein (human) Hemopexin (rat) Synergy IL-1 and 1L-6
cq-Acid glycoprotein C3
Induced by IL-6 only
a2-Macroglobulin Fibrinogen cq-Antitrypsin Haptoglobin (human) cq-Antichymotrypsin Caeruloplasmin Cysteine proteinase inhibitor C1 esterase inhibitor Inhibition IL-1 on IL-6
Fibrinogen az-Macroglobulin Cysteine proteinase inhibitor
should be exercised when interpreting data using isolated hepatocytes cultured under non-physiological conditions such as in the absence of corticosteroids. QUANTITATION OF T H E ACUTE PHASE PROTEIN RESPONSE The acute phase protein response can either be measured directly using specific protein assays or indirectly by the use of integrated measures such as the erythrocyte sedimentation rate or plasma viscosity. More recently plasma cytokine concentrations have been measured during the acute phase response.
Specific protein measurements As can be seen in Table 1, there is considerable variation amongst the acute phase proteins in terms of basal concentrations, incremental change and response time following the inflammatory stimulus. A number of studies have indicated that serum amyloid A (SAA) is the most sensitive of the acute phase proteins, sometimes rising above the reference range even when C-reactive protein (CRP) remains within it. In these studies CRP was measured by methods having a lower limit of sensivity at the upper limit of the reference range (approximately 10 mg/litre). The recent use of more sensitive methods with experimental minimal inflammation in man suggests that CRP may in fact increase by a similar increment, although within the reference range (Chambers et al, 1991). CRP
This is a sensitive acute phase protein with a short half-life, a rapid response time and large incremental change. Its catabolism is not affected by the type
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of inflammation and it is easily measured. For these reasons it has gained considerable popularity and has the largest database of disease-related change. Its major disadvantage is its wide reference range from 0.068 to 8.0 mg/litre, with a median value of 0.58 mg/litre (Claus et al, 1976). In mild inflammation and some viral infections typical values lie between 10 and 40mg/litre. Concentrations between 40 and 200mg/litre indicate acute inflammation and bacterial infection, while concentrations greater than 300 mg/litre are seen in extensive trauma and severe bacterial infections (Morley and Kushner, 1982). Many assays are available, such as enzyme immunoassays, radial immunodiffusion, electroimmunoassay, nephelometry or turbidimetry, with sensitivities of 5-10 mg/litre, but we would recommend the use of the more sensitive radioimmunoassay or the sensitive enzyme immunoassay, with sequential samples to monitor the response of inflammation to therapy.
SAA SAA, like CRP, is a very sensitive acute phase protein with a large incremental change. It may, however, be too sensitive, responding to such trivial infections as the common cold (Whicher et al, 1985). It also has major disadvantages due to difficulty in its measurement as there are no suitable standards and antibodies are difficult to produce and often of low titre. These problems are reflected in the widely varying reference ranges. Radial immunodiffusion assays suggest a reference range of 1-3 mg/litre, based on calibrants derived from tissue amyloid (Chambers and Whicher, 1983), while radioimmunoassays suggest a reference range of less than 200 ~g?titre, based on a purified SAA calibrant (Brandwein et al, 1984).
oLFAntichymotrypsin This protein does not show the same large incremental change of SAA and CRP and is slower to rise following inflammation (Table 1). It has a longer half-life than CRP or SAA and it has been suggested that this protein be measured in addition to CRP when it is desirable to have a longer-term window to assess changes in inflammation (Calvin et al, 1988). The measurement of this protein is simple, quick and cheap.
~l-Acid glycoprotein (orosomucoid) This is rather an insensitive acute phase protein and its measurement has little advantage over CRP or ~a-antichymotrypsin.
Haptoglobin, ~l-antitrypsin and fibrinogen These proteins are unsuitable as markers of the acute phase protein response since they can all be actively catabolized in certain disease states. Also, haptoglobin and r have common genetic variants resulting in deficiency and phenotype-specific reference ranges.
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D. T H O M P S O N ET A L
Integrated measures of the acute phase protein response
Erythrocyte sedimentation rate The erythrocyte sedimentation rate (ESR) is probably the most widely used measurement of the acute phase protein response. The ESR is a measure of the rate of sedimentation of erythrocytes in a vertical tube over a standard period of 1 h. The rate of sedimentation is governed by erythrocyte aggregation induced by large plasma proteins. Fibrinogen is particularly imPortant because of the combination of its asymmetric shape, large molecular size and high concentration. The relative contribution of plasma proteins to the ESR are: fibrinogen 55%, et2-macroglobulin 27%, immunoglobulins 11% and albumin 7% (Stuart and Whicher, 1988). Erythrocyte aggregation is also influenced by the number, shape, density and deformability of erythrocytes, so that the ESR increases with decreasing haematocrit. The effect of dietary lipids on the erythrocyte membrane is reflected in the circadian rhythm of the ESR. It cannot be performed on stored blood and is difficult to standardize, so that reference ranges should always be determined locally (International Committee for StandardiSation in Haematology, 1988). Because changes in ESR largely reflect changes in concentration of fibrinogen, The ESR is generally slow to change following an inflammatory stimulus (>24h) and is slow to resolve following the resolution of the inflammation (96-144 h). However, because the ESR also reflects changes in the concentration of several acute phase proteins as well as immunoglubulins and immune complexes, it does provide a wider screen for the detection of disease than the measurement of any one acute phase protein alone. The main advantages of the test, however, are its relative simplicity and cost. Plasma viscosity This measurement has become popular mainly because of the standarization and storage problems of the ESR. There are less problems with the plasma viscosity (PV) but anticoagulant and storage conditions are important. Other advantages over the ESR include low running costs, speed of assay and a more robust, sex-independent reference range. However, similarly to the ESR, the PV is disadvantaged by being largely dependent on the concentrations of slow-responding acute phase proteins such as fibrinogen and other non-acute phase proteins such as immunoglubulins (International Committee for Standardisation in Haematology, 1988). Cytokine measurements
There are now a number of commercially available immunoassay kits for the cytokines IL-1, IL-6 and TNF, which are thought to be the main mediators of the acute phase response. However, these assays are expensive and generally not very sensitive and therefore at present are used mainly for
ACUTE PHASE REACTANTS
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research purposes. It is not yet clear whether they are more sensitive than established measurements of the acute phase protein response or whether they can provide more useful diagnostic information. ACUTE PHASE REACTANTS AND DISEASE OUTCOME IN EARLY INFLAMMATORY ARTHRITIS Acute inflammatory arthritis may have several outcomes. For example, it may resolve relatively quickly, it may be viral with an occasionally protracted but good course, or it may lead to seropositive rheumatoid arthritis with varying degrees of functional disability. It is obviously important to be able to identify at an early stage those patients with inflammatory arthritis who may progress to a more severe and destructive disease, thus possibly meriting more aggressive therapy. However, relatively few studies have been carried out which have addressed this question satisfactorily. In a recent review of the literature (van der Heijde et al, 1988), it was found that no study of predictive factors in the outcome of newly diagnosed rheumatoid arthritis patients fulfilled the desired criteria of being prospective, including patients early after disease onset, and with regular standardized follow-up for a minimum of several years with clinical, laboratory and radiological investigation on each occasionl The heterogeneity and shortcomings of study design frustrate valid comparison and probably account for the lack of consensus concerning the best measurement to make for prediction of outcome. The issue is complicated by heterogeneity in three main areas: firstly, in the clinicaPand functional assessment of patients; secondly, in the statistical assessment of the data, for example whether the patient data is analysed in subgroups or as a whole in a study which contains patients with different diagnoses; and finally, the assessment is also complicated by a variety of therapies within the study group. The question of whether or not the group should be examined as a whole or in diagnostic subgroups depends very much on the questions being asked. When patients present early in disease, it is not always possible to assign them to a definite diagnostic category, a point in favour of examining the group as a whole. Analysing the data in diagnostic subgroups may be of particular value in providing information about a distinct aspect of the disease. Most of the studies examining predictive values in inflammatory arthritis centre round patients with established definite or classical rheumatoid arthritis, with a few examining seronegative spondylarthropathies and reactive arthritides. Few studies have examined the use of acute phase reactants in predicting the final outcome, whether measured as resultant disability or mortality. The measurement of the acute phase response used most often in these studies is the ESR, with PV and plasma CRP concentrations used less frequently. These studies are described below. One of the earliest prospective studies into prognostic markers in inflammatory arthritis which included a measure of the acute phase response as a parameter, namely the ESR, was that carried out on 307 patients with
400
D. T H O M P S O N ET A L
rheumatoid arthritis admitted to an Edinburgh hospital between 1948 and 1951 (Duthie et al, 1964). Two hundred of these patients were available for the final assessment approximately 9 years later, which was based both on functional capacity and disease activity, as assessed by the ESR, haemoglobin, and both joint and systemic involvement. It was concluded from this study that isolated ESR values were no guide to the subsequent disease course. However, when considering this conclusion it should be appreciated that less than 50% of the initial group had a disease duration of less then 1 year and that the study was limited to hospitalized patients, thereby excluding milder cases from the analysis. Similarly a study of 50 patients with rheumatoid arthritis, all presenting within 6 months of onset and assessed 6-monthly, failed to find any value in the initial ESR level for the prediction of disease outcome as assessed clinically and radiologically at an average of 3 (Masi et al, 1976) or 5 years (Feigenbaum et al, 1979). However, in a larger prospective study based on 102 patients with rheumatoid arthritis, a raised ESR at first assessment was associated with a less favourable outcome (Fleming et al, 1976). In this study, patients were recruited within a year of onset and divided into three prognostic groups depending on the outcome of their disease as assessed clinically, but not radiologically, at a mean of 4.5 years from onset. When examined over longer follow-up periods, the predictive power of acute phase response measurements is also not clear. A prospective study initially involving 100 patients with rheumatoid arthritis with a disease duration of less than 1 year examined either mortality, radiological status or functional outcome at follow-up periods of 8-11 years (Jacoby et al, 1973), 20 years (Rasker and Cosh, 1989) and 25 years (Reilly et al, 1990). The initial ESR (within 1 year of onset) did not correlate with clinical and radiological outcome when reassessed at between 8 and 14 years in the 83 survivors. However, of the survivors at 25 years (n = 37, 35 of whom were reviewed), the group with the better functional outcome had a significantly lower mean ESR at 1 year post-onset than the poorer outcome groups, a poorer outcome being associated with a persistently raised ESR (Reilly et al, 1990). When reviewed after 20 or 25 years, the ESR levels at 1 year post-onset were not found to correlate with mortality, although patients with persistently high ESR and seropositivity tended to do badly (Rasker and Cosh, 1989). Other studies which have examined the usefulness of the initial ESR levels in predicting disease outcome have also failed to find a significant correlation (Sherrer et al, 1986; Scott et al, 1987), but these were studies in which patients with early disease were in the minority. Although not significant, the latter study did find that a high ESR at the onset of the study tended to be associated with a poorer functional outcome. Larger studies have been carried out as the result of international collaboration, but the picture still remains far from clear as to whether measurements of the acute phase response at onset are predictive. In a Soviet-Finnish co-operative prognostic study which consisted of 136 and 139 patients respectively, fulfilling (at least) the American Rheumatism Association diagnostic criteria for probable rheumatoid arthritis and with a
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disease duration of less than 6 months, patients were assessed clinically and radiologically at presentation and after 3 years (Isomaki et al, 1984). The prognostic importance of the signs and symptoms at the onset of arthritis was evaluated separately in the two centres due to differences in assessment and patient groups. In the Finnish patients, the initial ESR was significantly higher for the patients eventually assigned to the poorer outcome groups (assessed clinically and radiologically) but this was not so for the Soviet group, whereas serum CRP at onset was significantly higher in the poorer outcome groups for both the Finnish and Soviet patients. Fibrinogen and orosomucoid were not significant indicators. A smaller study (n = 72) over a shorter follow-up period which also examined the usefulness of both ESR and CRP measurements failed to find any association between either early CRP concentrations or ESR with functional outcome, as assessed by limitation of wrist extension (Reeback and Silman, 1984). In a more recent study (Woolf et al, 1991) recruited 100 patients attending an early synovitis clinic within 3 months of onset of inflammatory joint disease. The follow-up data on 88 patients was obtained at a minimum of 5 years later, with 56 patients examined in person, 25 by questionnaire and seven from medical records; assessments were made clinically and on the basis of radiographs of hands where available. The group of patients, 41% of whom had rheumatoid arthritis, was examined as a whole. A raised CRP and PV at presentation had a poor specificity (38 and 35 %, respectively) and sensitivity (29 and 65 %, respectively) for the group with functional disability (n = 26). This is probably due at least in part to the mixed group, as the figures improved when the 36 patients with rheumatoid arthriti~ were analysed as a subgroup. Several studies have examined longitudinal variations in acute phase proteins. Most of these studies are concerned with the association of such markers with disease activity during the period of assessment (reviewed by McKenna, 1988) rather than with the future actual predictive value of any apparent trends early in the disease. Many of the patients used in such studies do not have early disease and have been subjected to various therapies which may, it is argued in some cases, in themselves influence the acute phase response as an indicator of the underlying disease activity. The studies which have examined the actual predictive ability of such trends are discussed below. It should be noted, however, that in most of these studies the patients are mixed in terms of disease duration, with a variable proportion having early disease. Although in the earlier mentioned study by Duthie et al (1964) isolated ESR measurements were of no predictive value in terms of outcome, sequential measurements were related to functional outcome. Patients were grouped according to their highest ESR reading obtained on admission, on discharge and at the first assessment (24 months) and this was related to their functional capacity at the end of the study (mean = 9 years). Generally, the higher the ESR, the worse the functional outcome, with the exception of the group with the highest ESR results, who did relatively well. Scott et al (1985) examined the effects of second-line antirheumatic drugs
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D. THOMPSONET AL
on the course and progression of rheumatoid arthritis. Fifty-six patients were studied, of whom 14 had a disease duration of less than 1 year. A second group of 15 patients on non-steroidal anti-inflammatory therapy were also studied. The 53 of the total patients who had an initial ESR of 30 mm/h or more were divided into two groups--those whose E S R fell by 25 mm/h or who had an E S R of 20 mm/h or less at 6 months, and those showing a lesser or no fall at all. Only those patients whose E S R fell showed no significant progression in the subsequent 6 months. Both these groups had an initial mean E S R of 50 mm/h. This suggests that a falling ESR is a n important predictor of subsequent radiological progression. In 60 patients with early juvenile chronic arthritis--20 in each of the pauciarticular, polyarticular and systemic onset groups--plasma CRP levels were significantly higher in the systemic group than the polyarticular group (mean 120 mg/litre vs 59.7 mg/litre; Gwyther et al, 1982). A good correlation was seen between plasma CRP and the E S R in early disease. CRP and E S R tended to fall in systemic illness when the disease state improved. However, in the five patients in whom amyloidosis developed within 5 years of onset, the CRP remained consistently high and the correlation with E S R was not as good. Additionally, in patients who later developed amyloidosis, similar high plasma CRP concentrations were found over the months or years before amyloidosis was confirmed. Tunn and Bacon (1992) measured at initial presentation a wide range of clinical and laboratory variables in 112 patients referred to an early arthritis clinic with < 6 months joint symptoms. Univariate analysis was unable to predict persistence. Multivariate analysis also showed overlap between the self-limiting and persistent disease groups but identified a subset of useful variables. The combination of a positive R A latex with an E S R > 30 mm/h carried a relative risk for persistence of 4.33 with 94% specificity but only 69% sensitivity. SUMMARY From the studies which are reviewed above, it is generally apparent that in terms of the acute phase response, the initial findings in early inflammatory arthritis (particularly rheumatoid arthritis, with which the majority of such studies are concerned) have little predictive value for either the functional outcome or mortality. The wide interindividual variability in these measurements is also likely to limit their clinical usefulness as predictors of disease outcome. The trend in certain acute phase reactants may be more useful in indicating disease activity, although the number of satisfactory studies in this area is very limited. REFERENCES Andus T, Geiger T, Hirano T et al (1988) Regulation of synthesis and secretion of major rat acute phase proteins by recombinant human intefleukin-6 (BSF-2/IL-6) in hepatocyte primary cultures. European Journal of Biochemistry 173: 287-293.
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