Synovial fluid lipoproteins: Review of current concepts and new directions

Seminars in

ARTHRITIS AND RHEUMATISM VOL 23, NO 2

OCTOBER 1993

S y n o v i a l F l u i d L i p o p r o t e i n s : Review of C u r r e n t C o n c e p t s a n d New Directions By P a m e l a E. Prete, Arzu G u r a k a r - O s b o r n e , and M o t i L. Kashyap Recent developments in plasma lipoprotein and apolipoprotein research have been striking, but few studies have focused on the analysis of lipoproteins in synovial fluid (SF). SF contains small amounts of lipoproteins and apolipoproteins. The lipid concentration of normal human SF is extremely low and is in sharp contrast to the concentrations found in plasma. Little is known about the lipids in pathological SF, but studies have noted increased cholesterol and lipoprotein content in rheumatoid arthritis (RA) SF ranging from 40% to 60% of the total plasma lipoproteins. Recently apolipoproteins AI, B and E have also been found to be in increased amounts in RA SF. Several theories have been

proposed to account for the increased presence of SF lipids in RA. Animal and human studies indicate the SF cholesterol, lipoproteins, and apolipoproteins may aggravate the inflammatory reaction within the synovial space. Research suggests an immunologic role for plasma lipoproteins on lymphocyte and monocytes in the blood and lymph. SF lipoproteins and apolipoproteins should be studied to define their actions within the synovial space. Copyright 9 1993by W.B. Saunders Company

YNOVIAL FLUID (SF), like plasma, re-

transporting nutrients from the synovial membrane to the joint cartilage with waste products traveling in the opposite direction. How it helps eliminate friction on the joint surface is under intensive study. SF, highly surface active, deposits phosphatidylcholine on the articular surface, rendering it hydrophobic. This results in excellent lubrication under a high load of kinetic friction. ~ The synovial cavity is in essence an extravascu-

S cently has been shown to contain limited amounts of certain lipids and apolipoproteins. Little is known about the composition of these lipid/lipoprotein complexes, their origin or metabolism in normal synovial fluid, their significance, or what happens to them in pathological states. This article reviews the current literature and theories about synovial fluid lipids with some comments about their role(s) in joint function. The best-described lipid in SF, cholesterol, is discussed along with the current theories of the pathophysiology of cholesterol crystal formation. Other lipids are described, and the significance of the differing amounts and types of lipoproteins found in different joint pathology. Last, the article explores the SF lipids/ lipoproteins and immune function along with the potential areas of future investigation. SYNOVIAL FLUID

We know that SF occupies the joint cavity, creating a low-friction articulating surface and

INDEX WORDS: Synovial fluid; lipoproteins; lipids; cholesterol; joint effusions.

From the Sections of Rheunmtology and Gerontology and the CholesterolCenter, Department of VeteransAffairs AIedical Center, Long Beach, CA; and the University of California, In'ine, CA. Pamela E. Prete, MD: Professor of Medichze, Universityof California, tn'ine, CA; Arzu Gurakar-Osborne, PhD: Research Associate, Unfi'ersityof California, In'hie, CA; Moti L. Kashyap, MD: Professorof Medicine, Unh'ersityof California, ln'ine, CA. Address reprintrequests to Pamela E. Prete,MD, 5901 E 7th St (111R), Long Beach, CA 9082Z Copyright 9 1993 by IV.B. Saunders Company 0049-0l 7219312302-000155.00/0

Seminars in Arthritis and Rheumatism, Vo123, No 2 (October), 1993: pp 79-89

79

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lar, extracellular tissue space where the fluids and electrolytes arrive as an ultrafiltrate of the plasma. Although many models have been suggested for the mechanism of the ultrafiltration, a recent model predicted a synovial circulation of extravascular fluid in which capillary filtrates entered a joint cavity as fluid and simultaneously exited via intercellular pathways distant from capillaries. 2 Some substances such as urea, amino acids, and uric acid diffuse freely into the synovial fluid and thus are present in concentrations similar to those of plasma, and other substances such as glucose are present in a much lower concentration. Proteins and globulins also are present in lower concentrations and appear to enter selectively into the fluid. Removal of small solutes takes place by diffusion back into the capillary network. Larger molecules such as proteins and lipids are believed to be cleared in part by the lymphatic system, although present studies suggest that lymphatic transport in the normal joint is low and almost nonexistent in pathological states? The solid matter can be removed by phagocytosis by macrophages? The relation between transsynovial flow and plasma osmotic pressure indicates the volume of SF is inversely correlated with plasma colloid osmotic pressure. 5 Interference in this dynamic synovial membrane system is reflected most apparently by a change in the volume of the SF. Thus, the viscoelastic parameters of the SF are believed to vary with SF volume. 6

Cholesterol h7 SF The first lipid noted in SF was cholesterol. Early reports described a honey-golden, satinlike luster of the SF which was attributed to the crystalline cholesterol. Cholesterol crystals were first noted by Frerichs more than a century ago as traces of fat in bovine SF. 7 The relationship of SF cholesterol crystals to serum lipoprotein abnormalities has not been reported widely. In fact, despite numerous musculoskeletal manifestations known to occur in primary hypercholesterolemia (type IIA phenotype), cholesterol crystals are not seen in the SF of patients with this disorder. 8-~~ Early reports especially noted the presence of cholesterol crystals in the SF of rheumatoid arthritis (RA) patients and postulated that cholesterol accumulates in joints only where

PRETE, GURAKAR-OSBORNE, AND KASHYAP

excessive tissue destruction occurred. H-~5 Whether cholesterol or its crystals contribute to the inflammatory process or are a result of the tissue damage is still unknown.

Lipids and Apolipoprotehzs ht Normal SF Analysis of the cholesterol crystals' physical and chemical properties showed that they are actually composed of mixed lipids and small quantities of protein in addition to cholesterol. Investigators have confirmed that soluble lipoproteins and cholesterol are present in normal SF and proteolipids are integral parts of connective tissue. 16,17 Few early studies focused on measurement of lipoproteins in normal SF, preferring to concentrate on larger pathological effusions because normal SF is available in scant amounts and microtiter methods had not been developed for lipopr0tein analysis. In 1956, Schmid and MacNair, after assessing multiple postmortem samples, determined that lipoproteins were present in normal synovial fluid. 18 Normal SF contains trace amounts of phospholipids and cholesterol. H Phospholipids of the SF fluid were qualitatively similar to those in serum, ie, lecithin, sphingomyelin, and lysolecithin, ranging quantitatively from 1.3 to 1.5 mg/dL. Normal SF cholesterol was found in much lower amounts than in pathological specimens and ranged from 0.6 to 0.75 mg/dL. Neutral lipids were not detected in normal SF. Thus, it was determined that the presence of only trace amounts of phospholipid and even smaller quantities of sterol and the absence of neutral lipids in normal SF contrasted sharply with that found in normal human serum. A direct relationship between total protein concentration and the major lipid constituents was found to exist in normal SF. Early analyses of SF lipoproteins were based on ultracentrifugation studies that discussed types of lipoproteins found in terms of classes I through V. To describe these classes concisely according to the current density classification of lipoproteins: class I, chylomicrons; class II, very low-density lipoproteins (VLDL); class III, intermediate-density lipoproteins (IDL); class IV, low-density lipoproteins (LDL); class V, highdensity lipoproteins (HDL). Ultracentrifugation studies of SF lipoproteins by Small et a119 showed definite lipoprotein differences in nor-

SYNOVIAL FLUID LIPOPROTEINS

mal SF from normal serum. All SFs studied contained greatly increased percentages of H D L even though normal serum contained far more total lipoprotein per gram of total globulin quantitatively. The total lipoprotein detected in SF was always less than 40% of that in serum. Only a minute amount of class I (chylomicrons) was detected in SF. Class II lipoprotein was present but considerably reduced compared with serum. Classes III and IV were present in SFs in comparable relative percentages of the total serum lipoproteins. These studies suggest that the most significant finding in SF appeared in the class V HDI_s. All SFs studied contained greatly increased quantities in class V~ (HDL1), approximately 18% of the total lipoprotein, whereas only 4% of the total serum lipoprotein fell in this area. The components of class Vb or HDL3 were present in SF in comparable serum percentages. In 1962, Schur and Sandson studied the lipid content of different classes of SF and noted a decrease in 13 lipoprotein in normal SF compared with pathological joint effusions. 2~ Recent studies have increased understanding of lipoproteins found in noninflammatory SF by analyzing carrier proteins of lipoprotein, apolipoproteins AI, B, and E. 21,z2Apolipoprotein AI (apo AI) is the major carrier protein of H D L particles. Apolipoprotein B (apo B) is an apolipoprotein common to VLDL and LDL. Apolipoprotein E (apo E) functions in lipid transport both as a ligand for the LDL apo B and E receptor and as a postulated receptor for chylomicron remnants. SF apolipoproteins are found in a relatively small ratio compared with plasma levels. Apo Al and B make up less than 1% of their respective plasma concentrations, whereas apo E levels are increased and present in SF at approximately 5% of the plasma concentration. 21 Furthermore, apo has been shown to associate with lipoproteins of c~-electrophoretic mobility (HDL), supporting the early ultracentrifugation studies by Small et a119 before the discovery of apo E that showed substantial amounts in normal SF of class V~ (HDLI), the major H D L subclass that carries apo E. There is a clear need for more detailed assessment of synovial apolipoproteins and lipoproteins. Differences between specimen collection and laboratory methods can account for

81

apparent discrepancies in reported data. Although recent developments in lipoprotein and apolipoproteins have been striking, problems of standardization and other aspects of apoprotein quantification persist.

Lipids and Apolipoproteins bt SF of RA and Other Arthropathies The lipid constituents of SF have not been studied extensively in pathological states. Because of the many early reports of increased cholesterol levels in the effusions of RA, a few studies focused on the lipoprotein analysis of the synovial fluid of this disease. Some SF data also exist in noninflammatory arthritic conditions such as osteoarthritis (OA). Ropes and Bauer described increased cholesterol levels in RA SF. 15 Schmid and MacNair studied one patient with RA and another with traumatic arthritis and reported increased a~ and [31 lipoprotein levels in the SF. ~8 Chung et al t2 found that SF in both RA and OA contained substantial amounts of cholesterol, phospholipids, and triglycerides, with the phospholipid content and ratio of total cholesterol to phospholipid differing from those of serum. Bole found increased amounts of cholesterol, phospholipid, and neutral lipids in RA SF ranging from 40% to 60% of the lipid concentration in serum, n Triglyceride content was increased only slightly in RA SF. Whereas normal SF had little or no neutral lipids, the major neutral lipid in RA SF was found to be esterifled cholesterol. The percentage composition of the increased phospholipid in RA SF was similar to that of the serum but differed from normal SF in that the mean percentage of lecithin was decreased and the percentages of sphingomyelin and lysolecithin were increased. In the ultracentrifugation studies of pathological SF, Small et al noted an increase in total lipoprotein level compared with normal SF (but still lower than in the patient's serum), especially in class II (VLDL), IV (LDL), V~ (HDL~), and Vb (HDL3) lipoprotein in active RA. 19 These amounts were nearly twice those found in the noninflammatory SF of OA. Another inflammatory arthritis, gout, also appeared to have an intermediate level of these same SF lipoproteins compared with both RA and OA. More recently, Prete et al confirmed the

82

PRETE, GURAKAR-OSBORNE, AND KASHYAP

increased lipid and cholesterol levels in RA SF compared with normal and noninflammatory joint fluid and significant increases in SF apo A! and B levels. In these studies, SF apo AI and B were found in lower concentrations than in plasma. However, a significant correlation between plasma and SF apo A! and B was found, suggesting that SF apoproteins may be plasma derived. 22Apo E, already enriched in noninflammatory SF, also is increased in joint effusions of RA. 2~ Lipoprotein values in plasma and SF in normal and pathological states are summarized in Table 1.

rum lipids into the synovial space, reduced rate of removal of SF lipids, local lymphatic obstruction due to chronic inflammation, and degradative release of lipids from abnormal synovial tissue.

Local Synthesis Newcombe and Cohen administered sodium acetate-1 14C in the chylous joint fluid space of a patient with classical RA in an attempt to assess the contribution of local lipid synthesis to the phenomenon of cholesterol accumulation. ~4 Comparison of the data from both SF and serum indicated that local biosynthesis of cholesterol had taken place. Isotopic studies performed for qualitative fatty acid and cholesterol analyses confirmed that cholesterol synthesis in the joint space is in part a local phenomenon. ~m2,26 Many investigators who have studied and reported increased SF lipid concentrations as well as cholesterol in RA also agree with this view. RA SF contains a variety of inflammatory cells that have been shown in other circumstances to synthesize both neutral lipids and phospholipids. Bole noted that the inflammatory cells contained in a polyvinyl-induced granuloma have the capability of incorporating 14C into phospholipids and neutral lipids, z7 This also is true of the cells in carrageenan-induced granulomas. 28 Studies by Buchanan 29 and Marks et al 3~also indicate that leukocytes possess this capacity of synthesizing lipids. The hallmark increased white

Theories for hzcreased Lipids, Cholesterol, attd Cholesterol Crystal Formation ht RA SF Griffin et al, 23 Bland et al, 24 and Meyers and Watermeyer, zs in their early investigations, attempted to explain the increased lipid and cholesterol presence in RA SF in terms of their metabolic, physiological, and structural function, suggesting a relationship between lipid metabolism and disorders of connective tissue, eg, RA. Because several studies confirmed that levels of lipid and cholesterol and its crystals were curiously increased in RA SF, interest turned to defining the source of the lipids and cholesterol and the mechanism of cholesterol crystal formation and its role in inflammation and tissue damage in the synovial joint cavity. The current theories accounting for the presence of cholesterol and the cholesterol crystal in SF include local synthesis of cholesterol by the synovium, selective lipoprotein transport of se-

Table 1: Ranges for Lipid and A p o l i p o p r o t e i n V a l u e s in P l a s m a and SF of Normal, OA, and RA P a t i e n t s

Normal Plasma SF OA Plasma SF RA Plasma SF

TC

PL

TG

apo AI

apo B

apo E

148-269 7-8

172-278 13-15

< 165 0

1,400-1,900 NA

775-1,560 NA

35-73 NA

127-252 4-169

166-288 26-98

90-410 12-59

1,024-1,550 53-521

597-1,190 10-458

11-34 5-9

137-210 76-130

162-192 40-140

64-136 17-100

839-1,959 339-1,055

721-947 180-386

9-29 5-14

Abbreviations:SF, synovial fluid; OA, osteoarthritis; RA, rheumatoid arthritis; TC, total cholesterol; PL, phospholipid; TG, triglycerides; apo, apolipoprotein; NA, data not available. NOTE. Rangesof values for TC, PL, and TG in milligrams per deciliter; ranges for apo AI, B, and E in micrograms per milliliter. Data summarized and compiled from several published studies. Tl,1z,18.zl.22.8~

SYNOVIAL FLUID LIPOPROTEINS

blood cell count found in all rheumatoid fluids could allow for this mechanism of cholesterol production. Whether the large number of leukocytes persisting in RA SF have this capability has not yet been investigated. Cholesterol exists as crystals within the human body under a variety of circumstances. Effusions containing such crystals have been obtained from pericardial and pleural spaces of patients with myxedema, tuberculosis, neoplastic and idiopathic arthritis, and R A 31-38 a s well as all the effusions found in patients with RA, especially the joint space. H Analysis of lipid composition of these crystal-containing effusions has indicated that effusions freed of crystalline lipid qualitatively resemble that of plasma while ihe crystals isolated are found not to be pure cholesterol but to contain small to moderate amounts of cholesterol ester, triglyceride, phospholipids, and protein and exist in two geometric f o r m s . 39

Hemorrhage--Red Blood CellDegradation The presence of intact red blood cells in pericardial fluid and SF indicates that plasma itself enters the space and directly contributes cholesterol crystals to these fluids. Ayres and Haymaker 4~ suggest that hemorrhage plays an important role in the formation of cholesterol crystals in body fluids. It may be speculated that the plasma first enters into effusion as red blood cells that then lyse to release heme, globulin, and cell stroma, which is made up of proteolipids (protein, phospholipid, and cholesterol). Having thus entered the effusion, the proteolipids that arise from cell components and are therefore distinct from plasma lipoproteins are then degraded partially, releasing the component lipids that recombine with soluble lipoprotein of the SF and/or aggregate into crystals of mixed lipid. As tempting as this theory is to explain cholesterol crystallization, Ayres and Haymaker could not use this theory to explain the formation of cholesterol in effusions in certain disease states such as diabetes mellitus, hypertension, and arteriosclerosis.

Tissue Degradation Hamerman and Schubert 4~ summarize the pathological changes in RA as inflammation, proliferation, and necrosis, characterized by

83

edema, intense infiltration of the membrane by lymphocytes and plasma cells, formation of "nodes" of lymphocytes, and perivasculitis and multiplication of capillaries. Proliferative changes in the synovial membrane are marked by hyperplasia of the lining cells and exuberant growth of villi. The synovial membrane extends as a pannus over the surface of articular cartilage, penetrates the cartilage, and erodes it. Necrosis occurs in the form of eosinophilic degeneration called fibrinoid and is observed on the edge of synovial membrane. Ropes and Bauer 15 suggest that this tissue destruction is responsible for the cholesterol crystals found in the SF of patients with RA. In this situation, it is again likely that the origin of synovial cholesterol is degraded proteolipids in addition to any influx or de novo synthesis of cholesterol. Thus, although tissue destruction within the joint cavity is quite marked, this cannot be the only factor in the genesis of cholesterol found there. The deposition of free crystalline cholesterol in the pericardial sac has been postulated to be caused by very slow absorptive properties of the pericardium allowing slow degradation of lipoprotein macromolecule complexes. 36Similar notions have been offered to account for the deposition of cholesterol in the intimal layers of arterial walls 37 and possibly the anterior chamber of the eye.3s The corollary of this applied to the synovial membrane seems plausible but will require study.

Phospholipases and Turnoverof Phospholipids Differences in SF lipoprotein composition may be related to phospholipases active in RA SF. Evidence exists that levels of phospholipases, especially A2, are increased in the SF of RA patients? z,43Increased turnover of phospholipids by RA macrophages has been demonstrated by Bomalaski et a l . 44

Semipermeable Membrane As discussed previously, lipid concentrations are extremely low in normal human SF fluid and are in sharp contrast with the concentrations found in the serum and the increased content found in RA SF. In the normal state, lipids (lipoproteins) appear to be prevented from entering the synovial cavity freely. There is ample evidence that increased permeability of

84

the synovial membrane to plasma proteins is responsible for greater lipid concentrations of SF in RA. According to Kushner and Somervile, passage of plasma proteins from blood into synovial space through synovial vascular walls and interstitial hyaluronate is related to molecular size and degree of inflammation. 45 The studies of Bole ix and Chung et a112also suggest that most lipid in the SF is derived from the blood. Human synovium is readily permeable to homologous 1311 serum albumin and globulin, and there is little difference in the rate of absorption of these two proteins after intraarticular injection. Radioactive labeled albumin and ",/-globulin appear in the peripheral blood within 15 minutes. 46 Similar experiments with radiolabeled lipoproteins into the synovial space have not been reported.

Select&e Lipid Transport There is evidence of selective lipid transport into SF by the synovial membrane. Viikari et ai 47 showed that the ratio of triglycerides to cholesterol was always lower than in serum, suggesting that the synovial membrane remains rather impermeable to VLDL, even in active RA. In a recent study we showed a direct correlation between plasma and SF total cholesterol and apo A! and B concentrations. Lower concentrations of the lipids in SF suggested a gradient between the plasma and SF cavity.22 Furthermore, preliminary data from gel electrophoresis of SF lipoproteins by our group 4s show little or no pre-13 lipoprotein bands in any SFs studied. Table 2 illustrates our preliminary results showing three bands in RA and OA plasma, e~l, pre-13, and 13. RA plasma showed minimal differences from normal lipoprotein distributions, a slight decrease in pre-13, and an increase in 13 band compared with OA plasma values. Significantly, the RA SF eL band was markedly decreased (P < .05) and the 13 band was increased (P < .05) compared with OA SF, and both RA and OA SF showed no pre-[3 bands. This finding may also suggest that part of the cholesterol and phospholipids in these fluids may not be transposed as charged lipoproteins. These results show significant increases in percent distribution in the a band of OA SF and in the 13 band of RA SF. These data taken in toto suggest that the synovial membrane is

PRETE, GURAKAR-OSBORNE, AND KASHYAP

Table 2: Average Percent Distribution of Lipoproteins in SF and Plasma in RA and OA Patients by Agarose Gel Electrophoresis

RA Plasma SF OA Plasma SF

Q

Pre-13

p

30 ___6 32 • 8

15 • 3 0

55 --- 11 6 6 * --- 13

30 • 7 48* • 6

22 • 8 0

48 • 8 4 8 * • 13

Abbreviations: SF, synovial fluid; RA, rheumatoid arthritis; O,~

osteoarthritis. NOTE. Percent distribution was measured semiquantitatively by laser densitometry, which calculates the percentage of the total area composed by each band (a, pre-13, and 13) peak, Because the percent distributions of individual RA and OA patients are averaged, totals may not always equal 100%. n = 19 RA and 11 OA patients. *P < .05, RAv OA.

semipermeable to lipids and lipoproteins. Changes in vascular permeability in the diseased rheumatoid synovium may be presumed to occur because of proliferation and hyperemia of the synovium. Increased diffusion of lipids from the blood into the joint cavity is a likely consequence and possibly an important cause for the increased lipid content in RA SF aside from local synthesis. Because a qualitative difference exists in RA lipid composition, an alternative explanation for the lipid differences could result from "absorption rate differences for different classes of lipoproteins. ''12 Lipids would be selectively removed faster or slower from the synovial space. For example, the reason phospholipid levels are lower than cholesterol levels in the SF may be that phospholipids are reabsorbed at a faster rate than cholesterol, resulting in the higher cholesterol-to-phospholipid ratio in RA SF.

L)vnphatic Obstntction Chylous collections resulting from lymphatic obstruction have been well recognized and described. 49 The changes demonstrated by lymphography mentioned in this work strongly suggest that a phenomenon of lymphatic fistulation as well as other mechanisms result in increased SF lipids. Although no direct evidence of lymphatic obstruction is found in RA to account for much of the SF cholesterol accumulation, it may be worth speculating that

SYNOVIAL FLUID LIPOPROTEINS

this is a possibility as exemplified in the extreme by filarious chylous arthritisP ~

Significance of Cholesterol Oystals, Cholesterol, and Lipoprotehzs ht SF Despite many studies on the origin of cholesterol and lipids in SF, the role of lipoproteins in normal SF is not understood. Although it can be postulated that synoviai lipids should function to allow SF to "lubricate the joint" together with hyaluronate, lipoproteins may be the normal metabolic byproduct of the SF ultrafiltrate. What is intriguing is the presence of increased amounts of cholesterol and cholesterol crystals in pathological joint effusions. How or if cholesterol or its crystals so abundantly present in all rheumatoid and other chronic joint fluids contribute to the inflammatory process is an important question. In the 1960s, investigators could not agree that SF cholesterol crystals contributed to the articular inflammatory process. Zuckner et al, 13 in studying several synovial effusions from RA patients, found some evidence the cholesterol crystals from these patients could stimulate an inflammatory response. In contrast, Griffin and Bole studied an RA patient with hypercholesterolemia and found no convincing evidence that cholesterol crystals heightened the inflammatory response in SF or tissue. 23 A pioneering work by Spain and Aristizaba151 indicated that injection of cholesterol crystals into the subcutaneous tissue and knee joint of the rabbit promoted local inflammatory reactions. They showed that a local tissue reaction could arise in rabbits to gel foam implants containing various triglycerides, cholesterol, and cholesterol esters. Bland et al z4 also injected rabbit knee joints with a suspension of cholesterol crystals. They then killed the animals and examined the knee joints histologically. Knees injected with crystals showed acute synovitis with necrosis and proliferative fibrosis. It was concluded that cholesterol crystals may be inflammatory or add an inflammatory component to the basic pathology in RA. Ettlinger and Hunder reviewed patients with cholesterol crystals in SF and concluded that the presence of these crystals usually is seen in and represents chronic persistent synovitis. 52

85

From these studies it would seem that cholesterol crystals, once considered a curious phenomenon of joint effusion, have the potential to aggravate the inflammatory reaction locally and possibly contribute to the chronicity of the pathological state and destruction common to many arthropathies. Despite several decades of study, SF cholesterol or its crystals has not been implicated directly as a spontaneous cause of an inflammatory joint effusion as other biological crystals have. In 1961, Faires and McCarty first proposed that the monosodium urate (MSU) crystal seen in the SF in gout induced the joint inflammation. 53 Much work has been done in elucidating the mechanism for this crystal-induced arthritis. Now there is strong evidence that the cholesterol contained in the membrane is necessary for pathological lysis of the plasma membrane by MSU crystals that occur in gout. After phagocytosis of protein-coated crystals, the proteins on the crystal would be digested by lysosomal enzymes in the phagolysosome, allowing the crystal membrane interaction to take place in two consecutive steps: (1) crystal-membrane binding by the crystals, and (2) lysis of the phagolysosomal membrane with release of the lysosomal enzymes into the cytoplasm leading to inflammation. In the case of MSU-membrane interaction, the crystals are negatively charged because of the oxygen atoms on the crystal surface. The MSU can associate with the plasma membrane by hydrogen binding with the crystals acting as the hydrogen acceptor in an interaction with the positively charged membrane phospholipid polar head groups (hydrogen donor). Weissmann and Rita showed that the membranes and liposomes not bearing cholesterol could not be lysed by MSU crystals, therefore reinforcing the importance of membrane cholesterol to the progression of inflammation in g o u t . 54 Some investigators have focused on the possibility that cholesterol molecules and their carrier proteins, apolipoproteins, adsorbed to the crystal surfaces could be a critical determinant of inflammatory potential. Terkeltaub et a155 have identified apo B-bearing lipoproteins as major inhibitory species in plasma for MSU crystal-induced stimulation of human neutrophils and platelets. Terkeltaub et al also found

86

that apo E inhibits the capacity of MSU crystals to stimulate the neutrophil. 21 LDLs inhibit the cellular response to MSU by binding to the crystal surface, thereby physically inhibiting particle-cell interaction and phagocytosis. 56 Thus it may be an important modulator of crystalinduced inflammation. Indeed, the effects of lipids, lipoproteins, cholesterol crystals, and apolipoproteins on cell functions and interactions within the SF are complex and require further investigation.

LipMs, Apolipoprotehls, and the Immune Response The role of various types of lipids in reactivity, regulation, and development of the immune System is not well understood. Immune cells are yery membrane active so that lipids as membrane components must be involved with the activity of the cell. It is only recently appreciated that lipids and lipoproteins may play a dynamic role in lymphocyte/macrophage function above that of a structural component. The understanding of the interaction of lipids and the immune system has developed along three investigative lines: the effects of dietary lipids and fats on plasma lipoprotein metabolism and general immune responsivity; the effects of cholesterol and steroid metabolism on lymphocyte function; and the immunoregulatory role of lipid molecules on immune cell reactivity. It is beyond the scope of this review to discuss the myriad reports of the role of dietary fats on plasma lipoprotein metabolism and the resultant effects on cancer and disease. It also is not possible to discuss all the ramifications of steroid, prostaglandins, leukotrienes, and eicosanoids and their inhibitors on cell membrane interactions in the blood or lymph. The published research on plasma lipoproteins and their effects on lymphocytes and monocytes in the blood and lymph may give some insight into the effects of SF lipoproteins on the immunoreactivity of cells in the SF space. Plasma lipoproteins regulate immune cell function and metabolism. 57-63Lipoproteins from normal and hyperlipidemic patients inhibit both mitogen- and antigen-stimulated lymphocyte responses in vitro. 64,65In these studies chylomicrons and lipoproteins of lighter density such as

PRETE, GURAKAR-OSBORNE, AND KASHYAP

VLDL, IDL, and LDL were potent inhibitors, whereas H D L had little inhibitory effect. Suppression of mitogen-induced T-lymphocyte proliferation in vitro by plasma lipoproteins has been investigated further at the level of inductive and terminal e~'ents. 66 Again, the potent inhibitory lipoproteins are those with the hydrated densities less than 1.063 g/mL: chylomicrons, VLDL, IDL, and LDL. These contain apo B and E in addition to a complex mixture of neutral and polar lipids. These suppression events are contingent on the binding of lipoprotein to the cell surface but do not require internalization of the membrane-bound lipoprotein. Another species of lipoproteins called the LDL inhibitor also suppresses lymphocyteproliferative responses to antigen. 59,6~ Other investigators have noted that VLDL but not H D L derived from the ascitic fluid of MM46 tumor-bearing mice can inhibit antibodydependent monocyte/macrophage tumor lysis. 67 Nakayasu et ai 6s found that VLDL could selectively suppress one type of accessory cell function without being cytotoxic to that cell. The accessory cells that function to promote an interleukin 2-responsive state were unaffected by lipoproteins. The lipoprotein carrier proteins, apo B and E, also effect lymphocyte function. 69,7~ For example, apo E, besides its function in lipid redistribution and tissue repair, can be apo E-bearing lipoproteins and apo E polypeptide suppress lymphocyte activation. 7~ Some evidence exists that apo E secretion by the monocyte/macrophage can be regulated and increased by exposure to cellular debris, platelets, 7z and oxidized LDL 73 and suppressed by endotoxin, 74thus lending support to a role of intraarticular apo E production to reflect and modulate the inflammatory responses to antigens in synovial tissue. 75 Major changes in host lipid metabolism occur during infections 76 and in some chronic disease states. In nephrotic patients, these plasma lipoprotein fluctuations (ie, increase in LDLs) are believed to contribute to generalized immunosuppression in these patients by suppressing the macrophage and accessory cell function. 77 Thus, it appears that plasma lipoproteins are macromolecular complexes that circulate in the

SYNOVlAL FLUID LIPOPRO~'EINS

87

blood and lymph and are natural modulators of the immune system. Studies on the effects of lipoproteins on lymphocyte and macrophage functions within the synovial space are highly desirable but as yet unavailable. The changes in the constituents of plasma lipoproteins that occur in autoimmune diseases and their effects on autoimmune responses have not been studied. Lahita et a178 recently detected antibodies to apo A1 and B in the serum of systemic lupus erythematosus (SLE) patients. They suggested that apo AI antibodies impair the transport function, altering levels of lipoproteins and contributing to the advanced atherosclerosis known to be part of the natural progression of SLE disease. Changing lipoprotein constituents may play a pivotal role in many manifestations of autoimmune diseases. In RA, Svenson et al correlated clinical activity of RA with leyels of certain serum lipoproteins. 79 Patients with active RA had plasma LDL concentrations reduced to 20% to 30% of those in healthy controls. Decreased plasma VLDL percentages correlated with the degree of inflammation present, and RA activity governed the degree of altered lipoprotein metabolism seen in these patients. It is obvious that the interactions of lipids and

cholesterol and lipid metabolism with immunoreactivity in normal and diseased states are both diverse and complex. Devising the studies and experiments to gain understanding of this complexity remains an important challenge. CONCLUSION

Lipids are small but important constituents of normal SF. More studies are needed to understand their origin and function within the normal synovial joint. Alterations in synovial lipids occur in pathological SF states. Clearly, SF levels of cholesterol and apo AI, B, and E are markedly increased in RA. Definition of the mechanisms accounting for these increased lipid and apolipoprotein levels would give important insights into the pathogenesis and perpetuation of chronic arthritis, potentially yielding new therapeutic options. The relationship of SF lipids and apolipoproteins, if any, to the inflammatory aspects of articular disease remains to be explored. Only studies in crystalline arthritis show SF lipoproteins to have a potentiating role in the inflammatory response. The clinical significance of these findings and the effect of synovial lipids and apolipoproteins in other arthropathies and autoimmune diseases need further investigation.

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