Basic Science and Dialysis

Peritoneal Defenses Against Infection: Winning the Battle butLosing the War?

Randall J. Faull

Renal Unit, Royal Adelaide Hospital, Adelaide, South Australia

Peritoneal dialysis continues to be only a short- tomedium-term therapeutic option for most patients withend-stage renal failure (1, 2). The key to long-term suc-cess is maintaining the health and viability of the peri-toneal membrane, which is at odds with expecting it toperform a function for which it was not specifically de-signed. The most common reason for failure is peritoni-tis, which is also a major cause of morbidity and mor-tality in this population (3, 4). In many cases, peritonealdialysis has to be abandoned because of recurrent, over-whelming, and/or intractable peritoneal infection. Inmany other situations, though, the association betweenperitonitis and dialysis failure is more subtle and re-moved in time, where patients later develop slow onsetof failure of dialysis function and/or ultrafiltration, orare later found to have increasing peritoneal cavity scar-ring (5, 6). The latter occasionally progresses to life-threatening global sclerosis of the peritoneal cavity.

Many of the advances in the techniques used for peri-toneal dialysis have been driven by a desire to decreasethe rate and severity of peritonitis, and on the whole theyhave been remarkably successful given that the physicalisolation of the peritoneal cavity (one of its chief de-fenses against infection) is continuously breached. Un-fortunately this success in reducing the rate of peritonitis(typically now to an average of less than 1 episode/year/patient) has not translated into a striking increase in theduration of peritoneal dialysis viability, and it is in factnot uncommon for patients to experience dialysis failurewithout ever having suffered a clinical episode of peri-tonitis.

To a certain extent, lack of long-term survival seemsto be intrinsic to the technique itself, and chronic expo-sure of the peritoneal membrane to the unphysiologicaldialysis fluid (low pH, hyperosmolar, hyperglycemic,

lactate buffered) is most likely the major cause of thisdisappointing situation. The properties of the peritonealmembrane deteriorate with increasing duration of peri-toneal dialysis, leading to a gradual loss of ultrafiltrationfunction that is characterized by increased absorption ofsolute (glucose), poor fluid clearance, and increased lossof protein into the dialysate (7). Patients with these hightransport characteristics at the commencement of perito-neal dialysis, or those who develop them during thecourse of dialysis, have poorer technique and personalsurvival (7, 8).

With that background in mind, this review summarizesthe components of the immune defense of the peritonealcavity and will discusses their possible role, as well asthat of peritonitis, in the short-and long-term viability ofthe peritoneal membrane and of the technique of perito-neal dialysis.

The Peritoneal Defense System

Under normal circumstances the simple physical iso-lation of the peritoneal cavity acts as a powerful barrierto microbial invasion, but this is compromised in patientson peritoneal dialysis. Contamination of the cavity doesnot always lead to peritonitis, and part of this protectionis due to clearance of bacteria by peritoneal lymphatics(9). The peritoneal cavity itself, though, possesses a so-phisticated and complex intrinsic immune defense sys-tem against pathogenic microbes. The chief componentsof this system are cells (leukocytes, both resident andinfiltrating, peritoneal mesothelial cells, and peritonealfibroblasts) and the inflammatory factors they secrete(cytokines, prostaglandins, leukotrienes, chemoat-tractants). Other factors that will be discussed are op-sonins, and a recently described family of antimicrobialpeptides known as defensins. The review will end with adiscussion of the role of wound healing and repair of theperitoneal membrane, with emphasis on growth factorsin the peritoneal cavity and a potential link to peritonealfibrosis and ultrafiltration failure.

Address correspondence to: Randall J. Faull, PhD, RenalUnit, Royal Adelaide Hospital, North Terrace, Adelaide 5000,Australia. E-mail: rjfaull@medicine.adelaide.edu.au.Seminars in Dialysis—Vol 13, No 1 (January–February) 2000pp. 47–53

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Leukocytes in the Peritoneal Cavity

Macrophages are the predominant leukocytes residentwithin the peritoneal cavity. In normal individuals theperitoneal cavity contains 15–50 ml of fluid with ap-proximately 6 × 105 cells/ml, consisting of approxi-mately 90% macrophages, 5–10% lymphocytes, and<5% neutrophils (9, 10). The volume of peritoneal di-alysis fluid dramatically reduces the concentration offluid-phase leukocytes in the peritoneal cavity of dialysispatients. White cell differential counts in uninfected peri-toneal dialysate range from 20% to 95% for macro-phages, 2% to 84% for lymphocytes, and 0 to 27% forneutrophils (10). Macrophages are also present withinthe peritoneal membrane, in a submesothelial and peri-vascular location (11). Peritoneal macrophages are con-stantly depleted as the dialysate is discarded, and it isestimated that 3–4 × 107 are lost each day (10). They arereplaced by monocytes that migrate in from the intravas-cular space.

A number of studies indicate that peritoneal macro-phages in peritoneal dialysis patients are relatively im-mature, and that the immaturity becomes more pro-nounced with time on dialysis (12–15). They also havean activated phenotype, possibly as a result of exposureto the dialysate (10, 16, 17). In vitro peritoneal macro-phages from peritoneal dialysis patients seem to retaintheir antimicrobial phagocytic and bactericidal properties(10), but peritoneal dialysate has adverse effects on thesefunctions and the cells’ viability (18–20). Bicarbonate-buffered dialysate has less inhibitory effect on macro-phage function than the conventional lactate-buffered so-lutions (20). The macrophages secrete a broad range ofbiologically active factors that are relevant to the im-mune defense of the peritoneal cavity, including cyto-kines [interleukin (IL)-1a, IL-1b, tumor necrosis factor(TNF)-a, and IL-6 (10, 21)], chemoattractants [IL-8 andmonocyte chemotactic protein (MCP)-1 (21, 22)], pros-taglandins [TXB2 and PGE2 (21)] and leukotriene B4(23). Conventional peritoneal dialysate inhibits macro-phage production of TNF-a (24), but this is not observedin the presence of bicarbonate-buffered dialysate (25).The decreasing maturity of macrophages with time ondialysis is accompanied by a diminution in their ability tosecrete inflammatory cytokines (15).

Lymphocytes are a minor component of the residentleukocytes in terms of numbers, and the magnitude oftheir contribution to peritoneal defense is unclear and hasnot been studied in detail. Peritoneal lymphocytes havean activated phenotype, but also appear to functionallyimpaired (e.g., lower IL-2 production) (26). It is certainlypossible, based on other body systems, that the lympho-cytes have an important coordination and killing role inthe peritoneal cavity, but this is currently unknown.Likewise there has been little study of the role of residentneutrophils in “first-line” defense, but they have a majorrole in fighting peritonitis.

During episodes of peritonitis the resident cellular de-fense is greatly augmented by a dramatic influx of leu-kocytes from the blood. Many of these cells are macro-phages and lymphocytes, but the vast majority are neu-trophils that bring their impressive array of killing

functions. The peritoneal membrane is a highly vascularstructure (27, 28), and the blood flow is further aug-mented as a normal response to acute inflammation dur-ing episodes of peritonitis. This accounts for the in-creased membrane permeability and poor ultrafiltrationthat typically occurs during acute peritonitis.

Leukocyte extravasation during acute inflammationhas been intensively studied and characterized in recentyears (reviewed in 29, 30). Intravascular leukocytes mi-grate toward an inflammatory focus in response to agradient of chemoattractants released by cells at that site.In the case of the peritoneal cavity, the neutrophil che-moattractant IL-8 and the monocyte chemoattractantMCP-1 are produced by peritoneal macrophages and me-sothelial cells (see below) in response to inflammatorycytokines and bacterial products (e.g., lipopolysaccha-ride). These factors establish the necessary gradient toattract leukocytes passing through the peritoneal vascu-lature. The combined effects of inflammatory cytokinesand chemoattractants first induces leukocyte rollingalong the capillary endothelial cells, followed by firmeradhesion to the endothelium, leukocyte shape change,and migration out of the vessel by passing in between theendothelial cells. The rolling phase is mediated by leu-kocyte and endothelial adhesion molecules known as se-lectins (E-, L-, and P-selectin), while the latter steps in-volve adhesive interactions between integrins on the leu-kocytes (e.g.,aLb3, aMb3, a4b1) and members of theimmunoglobulin superfamily on the endothelial cells[e.g., intercellular adhesion molecule (ICAM)-1 and -2,vascular cell adhesion molecule (VCAM)-1] (29, 30).Both ICAM-1 and VCAM-1 are induced on endothelialcells by inflammatory cytokines, and the same factorsconvert leukocyte integrins to an activated, high-affinityconformation (31). Integrins act as tethering points onthe cell surface for the intracellular actin cytoskeleton,which reorganizes to change the shape of the cell as itslides between the endothelial cells. At this point an im-portant homophilic adhesive interaction occurs betweenplatelet endothelial cell adhesion molecule (PECAM)-1on the leukocyte and on the endothelial cell. The finalstep in extravasation takes the leukocytes to the extra-vascular connective tissue, after traversing the basementmembrane with the aid of secreted matrix-degrading me-talloproteinases.

This multistep model of leukocyte extravasation hasbeen extensively tested in vivo and in vitro (29, 30), andit is highly likely that the general description providedhere is consistent with the events in the peritoneal mem-brane. Much less is known about the final step in leuko-cyte entry into the peritoneum, when the cells traversethe single layer of mesothelial cells that line the cavity.Leukocyte transmigration would occur in the oppositedirection (i.e., basal to apical), but this event in epithelialcell layers (which have many similarities to the perito-neal mesothelial cells) has not been well studied. It isknown that peritoneal mesothelial cells basally expressboth ICAM-1 and VCAM-1 (11, 32) (in contrast to en-dothelial cells, which only express VCAM-1 after acti-vation), and that each is induced by inflammatory cyto-kines. Neutrophils traverse mesothelial cell layers invitro, preferentially in a basal to apical direction, in re-

48 Faull

sponse to an IL-8 gradient (most of the IL-8 secretion bythe mesothelial cells is apical) (33–35), and this is inhib-ited in a dose-dependent manner by blocking ICAM-1.Little is known about the steady influx of leukocytes(mainly macrophages) that is required to replace the resi-dent leukocytes removed during dialysis, although onefeature is that monocytes undergo significant changes intheir cell surface profile of adhesion molecules as theyenter the peritoneal cavity. In particular, peritonealmacrophages have a markedly decreased expression ofPECAM-1 compared to blood monocytes (36).

Peritoneal Mesothelial Cells

The peritoneal cavity is lined by a monolayer of me-sothelial cells, which are of mesenchymal origin but dohave epithelioid-like features. They represent the largestresident cell population in the peritoneal cavity. One ofthe important functions of the mesothelial cells is to pro-duce and secrete surfactant, which acts as a lubricantbetween the intra-abdominal organs (37). They normallyalso present a net fibrinolytic surface, but this is impaired(manifested by decreased tissue-type plasminogen acti-vator and increased plasminogen activator inhibitor type-1) and shifted toward a procoagulant phenotype (expres-sion of tissue factor) by inflammatory factors such asTNF-a, IL-1a, or lipopolysaccharide (LPS) (38, 39). Ex-cessive deposition of fibrin on the mesothelial cell sur-face can lead to the formation of fibrous connective tis-sue (6).

On the basis of many studies in recent years, the me-sothelial cells are now recognized as key regulators ofintraperitoneal homeostasis, and particularly as activeparticipants in peritoneal immune defense (40, 41). Asdescribed above, they form the final barrier to leukocyteentry into the peritoneal cavity and are also an importantsource of the gradient of the leukocyte chemoattractantsIL-8, MCP-1, and RANTES (32–35) following stimula-tion by secreted products of the peritoneal macrophages(IL-1 and TNF-a). Peritoneal mesothelial cells produce along and ever-increasing list of biologically active sub-stances, including a number that are directly relevant tothe generation of an acute inflammatory response in theperitoneal cavity [e.g., IL-1a and IL-1b (42), TNF re-ceptors (43), IL-6 (44), and the vasodilatory prostaglan-dins PGE2 and PGI2 (45)]. PGE2 production by culturedmesothelial cells is increased by high glucose, at least inpart due to the hyperosmolarity (46).

The most efficient initial interactions between perito-neal macrophages and pathogenic microbes probably oc-cur directly at the mesothelial surface, particularly inperitoneal dialysis patients, where there is a major dilu-tion effect from the large volume of dialysate (40). Ac-tivation of the mesothelial cells induces a proadhesivephenotype, and the consequent attachment of leukocytesis a key early event in microbial recognition and destruc-tion (47). This is supported by in vitro studies showingthat macrophages adhere to cultured mesothelial cells,particularly after cytokine stimulation (32), as do patho-genic staphylococci (48, 49). It has been proposed thatthe initial response of macrophages to pathogenic bacte-

ria, including production of inflammatory cytokines, ac-tivates the mesothelial cells, leading to a synergistic am-plification of the immune response (40).

Peritoneal Fibroblasts

Fibroblasts reside within the interstitium that separatesthe peritoneal membrane capillaries from the mesothelialcells, and techniques have been developed for specifi-cally growing these cells in vitro (50). Studies with cul-tured peritoneal fibroblasts suggest that they may also betargets for proinflammatory cytokines, and in turn thatthey may themselves contribute to the acute inflamma-tory response. For example, IL-1 and TNF-a induce cul-tured peritoneal fibroblasts to secrete IL-6, IL-8, PGE2,and PGI2 (50). Macrophage-mediated stimulation of theperitoneal fibroblasts may be an important additionalpathway within the peritoneal defense system.

Opsonins

Macrophage recognition and phagocytosis of mi-crobes within the peritoneal cavity is facilitated by op-sonins that coat the microorganisms. Normal peritonealfluid contains opsonins such as IgG, C3, and fibronectin,but these are significantly diluted by dialysate in patientshaving peritoneal dialysis (9). It is controversial whetherthis dilution effect predisposes individuals to episodes ofperitonitis.

Defensins

Potentially important contributors to the peritoneal im-mune defense are a family of small antimicrobial pep-tides known as “defensins.” Multicellular organisms(plants and animals) have developed small peptides withbroad antimicrobial properties (“natural antibiotics”) thatare important contributors to their innate immunityagainst the myriad of microbes they encounter duringtheir life span (51). They have the advantage of func-tioning without the need for high specificity or memory,in contrast to the adaptive immunity represented by an-tibodies and lymphocytes. An important class of thesemolecules in humans and other animals are defensins,which are 3.5–4.5 kDa cyclic polypeptides (52, 53).They have activity at concentrations of 1–100mg/mlagainst gram-positive and gram-negative bacteria, myco-bacteria, fungi, and enveloped viruses (including HIV).Their targeting of microbes is partly due to their highlycationic structure, which complements the anioniccharge of the microbial cell surface. Defensin subunitsenter the lipid membranes and assemble into biologicallyactive, pore-forming multimeric complexes within thelipid bilayer. The resultant increase in membrane perme-ability, and subsequent depletion of ATP, inhibits cellu-lar respiration, and death ensues (52, 54).

Human defensins are widely distributed on mucosalepithelial surfaces, in body fluids, and in the microbici-dal organelles of phagocytic cells (52, 53, 55, 56). Theyhave been classified into two related groups, known asa-andb-defensins (51, 52). Four of thea-defensins [HNP

49PERITONEAL DEFENSES AGAINST INFECTION

(human neutrophil peptide)-1 to -4] are stored in highconcentrations in granules of phagocytes (granulocytesand some macrophages). The concentrations of HNP-1,-2, and -3 in plasma in normal volunteers are approxi-mately 40 ng/ml, but increase to greater than 1mg/mlduring severe infections. The other two known humana-defensins [HD (human defensin)-5 and -6] are re-stricted to Paneth cells at the bases of crypts betweenintestinal villi. HNP-1 to -4 make up 5% of the totalprotein of human granulocytes and are the principal pro-teins in their azurophilic granules. They are deliveredinto the phagocytic vacuoles by granule fusion and arereleased from granules in response to neutrophil activat-ing factors such as IL-8, phorbol myristate acetate/ionomycin, and fMLP (57). HNP-1 and -2 are also potentchemoattractants for T lymphocytes (57) and monocytes(58), and HNP-1 to -3 treatment of airway epithelial cellsincreases production of the mRNA and protein for theneutrophil chemoattractant IL-8 (59). Subcutaneous ad-ministration of HNP-1 and -2 into Balb/c mice induces aneutrophil and mononuclear cell infiltrate (57).a-de-fensins are also mitogenic for epithelial cells and fibro-blasts (60).

Two humanb-defensins (hBD-1 and -2) have beendescribed. They are largely confined to epithelium, andunlike thea-defensins are not contained within intracel-lular granules.b-defensin expression is increased at sitesthat are constantly exposed to and colonized by micro-organisms (61, 62). HBD-1 was first isolated from he-modialysate of humans with end-stage renal failure (63)and is expressed in the kidney tubules, urogenital tissues,and the oral mucosa (64–66). It is a major antimicrobialfactor in human airway surface fluid, where it is pro-duced by the respiratory epithelial cells (67). The highNaCl content of airway fluid from patients with cysticfibrosis inactivates the hBD-1, which may contribute sig-nificantly to their compromised immunity. HBD-2 wasfirst isolated from human skin (68), and expression of itsmRNA in cultured keratinocytes is up-regulated by TNF-a, gram-positive and gram-negative bacteria, andCan-dida albicans. It is also up-regulated in inflamed epider-mis (69). Expression of the equivalent bovineb-defensin[tracheal antibiotic peptide (TAP)] is markedly up-regulated by lipopolysaccharide fromPseudomonas ae-ruginosa(70).

The expression ofa-defensins (HNP-1 to -4) byphagocytes andb-defensins by epithelium suggests thatthey may be important factors in the defense of the peri-toneal cavity in normal patients and in patients onCAPD. We (unpublished observations) and others (71)have found that peritoneal leukocytes from CAPD pa-tients expressa-defensins. We have also made the novelobservation thatb2 defensin (hBD2) is expressed in theperitoneal membrane and by cultured peritoneal meso-thelial cells (manuscript in preparation). There are manyunanswered questions concerning defensins in the peri-toneal cavity, including the extent of their contribution toperitoneal defense and the effects of peritoneal dialysison their function. This may be a fruitful area of researchand have potential therapeutic implications. Their com-bination of broad antimicrobial properties without appar-ent acquired resistance, chemical resistance, and lack of

antigenicity suggests that they may be commercially use-ful topical or systemic agents for prophylaxis and treat-ment of infections.

Growth Factors and Repairing thePeritoneal Membrane

Maintenance of a healthy and viable peritoneal mem-brane, enabling both effective peritoneal defense andlong-term successful peritoneal dialysis, also depends onefficient and orderly repair of the membrane after it isdamaged by peritonitis. Shedding of damaged mesothe-lial cells occurs during the acute inflammatory response,which will leave denuded areas requiring repair. There isalso evidence that peritoneal dialysis causes continuousdamage to the peritoneal membrane (20, 72), and we (73)and others (74) have observed that a marked shedding ofperitoneal mesothelial cells occurs soon after peritonealdialysis commences (upon first exposure to the peritone-al dialysate). This suggests that repair of a wounded peri-toneal membrane occurs continuously during peritonealdialysis, with accelerated activity following acute peri-tonitis. One of the most common sequelae of acute peri-tonitis or abdominal cavity surgery is the development offibrous adhesions in the peritoneal cavity. As describedabove, a contributing factor to this appears to be the lossof the mesothelial cell fibrinolytic phenotype duringacute inflammation. Histologic examination of peritonealmembranes from long-term dialysis patients shows pro-gressive membrane fibrosis, vascular damage, and lossof the mesothelial cell layer (5, 75), implying that thewound repair process becomes progressively disorderedand eventually unable to effectively cope with the recur-rent insults.

Wound repair at other sites in the body (e.g., skin) hasbeen extensively studied (76, 77), but there has beenlittle examination of that process in the peritoneal cavityof patients on peritoneal dialysis. A number of growthfactors have important roles in normal wound repairthrough their various effects on cell proliferation, celladhesion and migration, angiogenesis, and production ofextracellular matrix molecules. They include members ofthe epidermal growth factor (EGF) family [EGF, TGF-a,and heparin-binding (HB)-EGF], fibroblast growth factor(FGF)-1, -2, -4, and -7, PDGF, and TGF-b1, -b2, and-b3 (76). Activated macrophages are a prominent sourceof a number of these growth factors, and the residentperitoneal macrophages may serve this role in the peri-toneal cavity. An additional important aspect of woundrepair that is likely to occur in the peritoneal cavity isremodelling of extracellular matrix by matrix metallo-proteinases (MMPs). Pleural and peritoneal mesothelialcells produce and secrete 72 kDa and 92 kDa gelatinases(type IV collagenases, or MMP2 and MMP9) as well asthe counterregulatory tissue inhibitor of metalloprotein-ases (TIMP) (78).

Mitogenic activity is present in the dialysis effluent ofperitoneal dialysis patients (79), and peritoneal mesothe-lial cells synthesize TGF-b1 and -b2 (but not TGF-b3)(80). IL-1 treatment of cultured mesothelial cells stimu-lates further production of TGF-b1 and -b2 (80), and

50 Faull

high glucose increases TGF-b1 (81). Peritoneal macro-phages from patients on peritoneal dialysis also produceTGF-b1 (82). This is interesting with respect to perito-neal membrane fibrosis, as TGF-b1 and -b2 stimulateproduction of a collagen-rich matrix, and have been im-plicated in pathogenic fibrosis in many sites in the body,whereas TGF-b3 appears to have “antiscarring” effects(76). TGF-b1 is mitogenic for peritoneal mesothelialcells (6) and promotes their synthesis and secretion ofplasminogen activator inhibitor-1, which may impairdegradation of fibrin and of unorganized matrix compo-nents (83).

EGF is mitogenic for peritoneal mesothelial cells andinduces a morphologic change from their usual epitheli-oid appearance toward an extended, more fibroblastoidshape (73, 84). This alteration is reminiscent of epithe-lial-mesenchymal transformation, where epithelial cellsacquire a more contractile, motile, and proliferative phe-notype during wound repair and embryonic development(85). Mesothelial cells undergo significant morphologicchanges during the repair process in vivo, adapting a“reactive” cuboidal morphology shortly after injury (86).We have shown that EGF increases peritoneal mesothe-lial cell expression of matrix adhesion receptors and in-creases their migration over and toward collagen. Both ofthese alterations, as well as the increased proliferationinduced by EGF, should facilitate mesothelial cell repairof the wounded peritoneal membrane in vivo (73). How-ever, we have been unable to demonstrate expression ofEGF in the peritoneal cavity of CAPD patients. Insteadwe have found expression of HB-EGF mRNA and pro-tein by peritoneal macrophages, cultured peritoneal me-sothelial cells, and in biopsies of the peritoneal mem-brane (manuscript in preparation). Each of the receptorsfor members of the EGF family [human EGF receptor(HER)-1, -2, -3, and -4] (87) are also expressed by peri-toneal macrophages and mesothelial cells. HB-EGF, likeEGF, is mitogenic for mesothelial cells, induces theirfibroblastoid transformation, and increases mesothelialcell adhesion and migration over collagen (manuscript inpreparation). Its presence in the peritoneal cavity may bea crucial element in regulating peritoneal membrane re-covery and reepithelialization following peritonitis.

TGF-b1 and HB-EGF (as well as other growth factors,including FGF and PDGF) are also mitogenic for fibro-blasts and stimulate their production of matrix molecules(76). PDGF and basic FGF, as well as peritoneal dialy-sate, are mitogenic for peritoneal fibroblasts (50, 88). Asdescribed above, peritoneal dialysis causes continuousinjury in the peritoneal cavity, and thus persistent acti-vation of the mesothelial cells and macrophages. Con-tinuous production of growth factors by these cells in thissetting, while useful for repair of the peritoneal mem-brane, might also be the genesis of peritoneal membranefibrosis. When severe mesothelial injury occurs, such asduring frank infection, the likelihood of fibrosis in-creases further (86).

Growth factors may also be involved in the progres-sive development of ultrafiltration failure in peritonealdialysis patients due to rapid absorption of glucose,which is associated with a marked increase in the numberof peritoneal microvessels (27). FGF and vascular endo-

thelial growth factor (VEGF) are both potent angiogenicfactors (76), and VEGF is produced by peritoneal mac-rophages in other settings (89, 90). Recent evidence alsosuggests that IL-8, produced by peritoneal macrophagesand mesothelial cells, is a powerful angiogenic factor aswell as a neutrophil chemoattractant (91). It is possiblethat an important adverse effect of peritoneal dialysateand/or peritonitis is induction of angiogenic factors bymesothelial cells and peritoneal macrophages, and sostimulation of peritoneal membrane angiogenesis.

Conclusions

The peritoneal cavity possesses a complex and sophis-ticated defense system that is subjected to unusual andrepeated insults during the course of peritoneal dialysis.It is primarily designed for dealing with acute insults,such as peritonitis, and generally is extremely effectivein that role. However, the nature of peritoneal dialysisdictates that the defense system is continuously acti-vated, and some of the long-term complications of peri-toneal dialysis, such as peritoneal fibrosis and ultrafiltra-tion failure, may be a consequence of this artificial situ-ation. The dialysate routinely used for peritoneal dialysiscan be blamed for much of this misuse of peritonealdefenses. Development of more physiologic and less in-trusive peritoneal dialysis solutions is probably the keyto peritoneal dialysis becoming a truly long-term treat-ment option.

Acknowledgments

Some of the work referred to in this article was sup-ported by grants from the Australian Kidney Foundation,the Royal Adelaide Hospital, the University of Adelaide,and Baxter Healthcare Corporation. I would like to grate-fully acknowledge the assistance of Dr. David Leavesleyand Jodie Stanley with the laboratory work, and SisterMelinda Yelland and the nursing staff of the Renal Unitat the Royal Adelaide Hospital for assistance in supply-ing patient material.

References1. Gokal R, Oreopoulos DG: Is long-term technique survival on continuous

ambulatory peritoneal dialysis possible?Perit Dial Int 16:553–555, 19962. Disney APS, for the Australia and New Zealand Dialysis and Transplant

Registry: Demography and survival of patients receiving treatment forchronic renal failure in Australia and New Zealand.Am J Kid Dis25:165–175, 1995

3. Gokal R, Mallick NP: Peritoneal dialysis.Lancet353:823–828, 19994. Piraino B: Peritonitis as a complication of peritoneal dialysis.J Am Soc

Nephrol9:1956–1964, 19985. Dobbie JW, Lloyd JK, Gall CA: Categorization of ultrastructural changes in

peritoneal mesothelium, stroma and blood vessels in uremia and CAPD pa-tients, inAdvances in Continuous Ambulatory Peritoneal Dialysis, edited byKhanna R, Nolph KD, Prowant B, et al., Toronto, Peritoneal Dialysis BulletinInc., 6:3–12, 1990

6. Dobbie JW: Pathogenesis of peritoneal fibrosing syndromes (sclerosing peri-tonitis) in peritoneal dialysis.Perit Dial Int 12:14–27, 1992

7. Davies SJ, Phillips L, Griffiths AM, et al: What really happens to people onlong-term peritoneal dialysis?Kidney Int54:2207–2217, 1998

8. Churchill DN, Thorpe KE, Nolph KD, et al: Increased peritoneal membranetransport is associated with decreased patient and technique survival forcontinuous peritoneal dialysis patients.J Am Soc Nephrol9:1285–1292, 1998

9. Brulez HFH, Verbrugh HA: First-line defense mechanisms in the peritonealcavity during peritoneal dialysis.Perit Dial Int 15:S24–S34, 1995

10. Lewis S, Holmes C: Host defense mechanisms in the peritoneal cavity of

51PERITONEAL DEFENSES AGAINST INFECTION

continuous ambulatory peritoneal dialysis patients.Perit Dial Int 11:14–21,1991

11. Suassuna JHR, Das Neves FC, Hartley RB, et al: Immunohistochemicalstudies of the peritoneal membrane and infiltrating cells in normal subjectsand in patients on CAPD.Kidney Int46:443–454, 1994

12. Goldstein CS, Bomalaski JS, Zurier RB, et al: Analysis of peritoneal mac-rophages in continuous ambulatory peritoneal dialysis patients.Kidney Int26:733–740, 1984

13. McGregor SJ, Brock JH, Briggs JD, Junor BJR: Release of hydrogen per-oxide and expression of HLA DR and transferrin receptors by monocytes andperitoneal macrophages from patients undergoing continuous ambulatoryperitoneal dialysis and normal controls.Clin Immunol Immunopathol56:151–158, 1990

14. Moughal NA, McGregor SJ, Brock JH, et al: Expression of transferrin re-ceptors by monocytes and peritoneal macrophages from renal failure patientstreated by continuous ambulatory peritoneal dialysis (CAPD).Eur J ClinInvest21:592–596, 1991

15. McGregor SJ, Topley N, Jorres A, et al: Longitudinal evaluation of perito-neal macrophage function and activation during CAPD. maturity, cytokinesynthesis and arachidonic acid metabolism.Kidney Int49:525–533, 1996

16. Bos HJ, van Bronswijk H, Helmerhorst TJM, et al: Distinct subpopulationsof elicited human macrophages in peritoneal dialysis patients and womenundergoing laparoscopy: a study on peroxidatic activity.J Leukoc Biol43:172–178, 1988

17. Davies SJ, Suassuna J, Ogg CS, Cameron JS: Activation of immunocompe-tent cells in the peritoneum of patients treated with CAPD.Kidney Int 36:661–668, 1989

18. Alobaidi HM, Coles GA, Davies M, Lloyd D: Host defence in continuousambulatory peritoneal dialysis: the effect of the dialysate on phagocyticfunction.Nephrol Dial Transplant1:16–21, 1986

19. Verbrugh HA, Keane WF, Hoidal JR, et al: Peritoneal macrophages andopsonins: antibacterial defense in patients undergoing chronic peritoneal di-alysis.J Infect Dis147:1018–1029, 1983

20. Jorres A, Williams JD, Topley N: Peritoneal dialysis solution biocompatibil-ity: inhibitory mechanisms and recent studies with bicarbonate-buffered so-lutions.Perit Dial Int 17:S42–S46, 1997

21. Topley N, Mackenzie R, Jorres A, Coles GA, Davies M, Williams JD:Cytokine networks in continuous ambulatory peritoneal dialysis: interactionsof resident cells during inflammation in the peritoneal cavity.Perit Dial Int13:S282–S285, 1992

22. Bauermeister K, Burger M, Almanasreh N, et al: Distinct regulation of IL-8and MCP-1 by LPS and interferon-g-treated human peritoneal macrophages.Nephrol Dial Transplant13:1412–1419, 1998

23. Mackenzie RK, Coles GA, Williams JD: The response of human peritonealmacrophages to stimulation with bacteria isolated from episodes of continu-ous ambulatory peritoneal dialysis-related peritonitis.J Infect Dis163:837–842, 1991

24. Hart PH, Jones CA, Finlay-Jones JJ: Inflammatory fluids regulate TNF-a,but not IL-1b, production by human peritoneal macrophages. A study ofpatients on continuous ambulatory peritoneal dialysis with peritonitis.J Leu-koc Biol 53:309–319, 1993

25. Mackenzie RK, Holmes CJ, Moseley A, et al: Bicarbonate/lactate- and bi-carbonate-buffered peritoneal dialysis fluids improveex vivoperitoneal mac-rophage TNFa secretion.J Am Soc Nephrol9:1499–1506, 1998

26. Lewis SL, Kutvirt SG, Cooper CL, et al: Characteristics of peripheral andperitoneal lymphocytes from patients.Perit Dial Int 13:S273–S277, 1993

27. Krediet RT: The peritoneal membrane in chronic peritoneal dialysis.KidneyInt 55:341–356, 1999

28. Nagy JA: Peritoneal membrane morphology and function.Kidney Int50:S2–S11, 1996

29. Springer TA: Traffic signals for lymphocyte recirculation and leukocyteemigration: the multistep paradigm.Cell 76:301–314, 1994

30. Carlos TM, Harlan JM: Leukocyte-endothelial adhesion molecules.Blood84:2068–2101, 1994

31. Faull RJ, Ginsberg MH: Inside-out signalling through integrins.J Am SocNephrol7:1091–1097, 1996

32. Jonjic N, Peri G, Bernasconi S, Sciacca FL, Colotta F, Pelicci PG, Lanfran-cone L, Mantovani A: Expression of adhesion molecules and chemotacticcytokines in cultured human mesothelial cells.J Exp Med176:1165–1174,1992

33. Topley N, Brown Z, Jorres A, et al: Human peritoneal mesothelial cellssynthesize interleukin-8. Synergistic induction by interleukin-1b and tumornecrosis factor-a. Am J Pathol142:1876–1886, 1993

34. Zeillemaker AM, Mul FPJ, van Papendrecht AAGMH, et al: Polarized se-cretion of interleukin-8 by human mesothelial cells: a role in neutrophilmigration.Immunology84:227–232, 1995

35. Li FK, Davenport A, Robson RL, Loetscher P, Rothlein R, Williams JD,Topley N: Leukocyte migration across human peritoneal mesothelial cells isdependent on directed chemokine secretion and ICAM-1 expression.KidneyInt 54:2170–2183, 1998

36. Faull RJ, Wang J, Stavros W: Changes in the expression of adhesion mol-ecules as peripheral blood monocytes differentiate into peritoneal macro-phages.Nephrol Dial Transplant11:2037–2044, 1996

37. Dobbie JW: Surfactant protein A and lamellar bodies: a homologous secre-

tory function of peritoneum, synovium and lung.Perit Dial Int 16:574–581,1996

38. Van Hinsbergh VWM, Kooistra T, Scheffer MA, van Bockel JH, van MuijenGNP: Characterization and fibrinolytic properties of human omental tissuemesothelial cells. Comparison with endothelial cells.Blood 75:1490–1497,1990

39. Sitter T, Spannagl M, Schiffl H, et al: Imbalance between intraperitonealcoagulation and fibrinolysis during peritonitis of CAPD patients: the role ofmesothelial cells.Nephrol Dial Transplant10:677–683, 1995

40. Topley N, Williams JD: Role of the peritoneal membrane in the control ofinflammation in the peritoneal cavity.Kidney Int46:S71–S78, 1994

41. Hjelle JT, Miller-Hjelle MA, Dobbie JW: The biology of the mesotheliumduring peritoneal dialysis.Perit Dial Int 15:S13–S23, 1995

42. Douvdevani A, Rapoport J, Konforty A, Argov S, Ovnat A, Chaimovitz C:Human peritoneal mesothelial cells synthesize IL-1a andb. Kidney Int46:993–1001, 1994

43. Douvdevani A, Einbinder T, Yulzari R, et al: TNF-receptors on humanperitoneal mesothelial cells: regulation or receptor levels and shedding byIL-1a and TNFa. Kidney Int50:219–228, 1996

44. Topley N, Jorres A, Luttmann W, et al: Human peritoneal mesothelial cellssynthesize interleukin-6: induction by IL-1b and TNFa. Kidney Int43:226–233, 1993

45. Topley N, Petersen MM, Mackenzie R, et al: Human peritoneal mesothelialcell prostaglandin synthesis: induction of cyclooxygenase mRNA by perito-neal macrophage-derived cytokines.Kidney Int46:900–909, 1994

46. Sitter T, Haslinger B, Mandl S, et al: High glucose increases prostaglandinE2 synthesis in human peritoneal mesothelial cells: role of hyperosmolarity.J Am Soc Nephrol9:2005–2012, 1998

47. Muller J, Yoshida T: Interaction of murine peritoneal leukocytes and meso-thelial cells: in vitro model system to survey cellular events on serosal mem-branes during inflammation.Clin Immunol Immunopathol75:231–238, 1995

48. Glancey G, Cameron CJ, Ogg C, Poston S: Adherence ofStaphylococcusaureusto cultures of human peritoneal mesothelial cells.Nephrol Dial Trans-plant 8:157–162, 1993

49. Haagen IA, Heezius HC, Verkooyen RP, et al: Adherence of peritonitis-causing Staphylococci to human peritoneal mesothelial cell monolayers.JInfect Dis161:266–273, 1990

50. Jorres A, Ludat K, Sander K, Dunkel K, Lorenz F, Keck H, Frei U, Gahl GM:The peritoneal fibroblast and the control of peritoneal inflammation.KidneyInt 50:S22–S27, 1996

51. Boman HG: Peptide antibiotics and their role in innate immunity.Annu RevImmunol13:61–92, 1995

52. Ganz T, Lehrer RI: Defensins.Pharmacol Ther66:191–205, 199553. Ganz T, Lehrer RI: Antimicrobial peptides of vertebrates.Curr Opin Immu-

nol 10:41–44, 199854. White SH, Wimley WC, Selsted ME: Structure, function, and membrane

integration of defensins.Curr Opin Struct Biol5:521–527, 199555. Zhao C, Wang I, Lehrer RI: Widespread expression of beta-defensin hBD-1

in human secretory glands and epithelial cells.FEBS Lett396:319–322, 199656. Rice WG, Ganz T, Kincade JM, Selsted ME, Lehrer RI, Parmley RT: De-

fensin-rich dense granules of human neutrophils.Blood 70:757–765, 198757. Chertov O, Michiel DF, Xu L, et al: Identification of defensin-1, defensin-2,

and CAP37/azurocidin as T-cell chemoattractant proteins released from in-terleukin-8-stimulated neutrophils.J Biol Chem271:2935–2940, 1996

58. Territo MC, Ganz T, Selsted ME, Lehrer R: Monocyte-chemotactic activityof defensins from human neutrophils.J Clin Invest84:2017–2020, 1989

59. van Wetering S, Mannesse-Lazeroms SPG, van Sterkenburg MAJA, et al:Effect of defensins on interleukin-8 synthesis in airway epithelial cells.Am JPhysiol272:L888–L896, 1997

60. Murphy CJ, Foster BA, Mannis MJ, Selsted ME, Reid TW: Defensins aremitogenic for epithelial cells and fibroblasts.J Cell Physiol155:408–413,1993

61. Stolzenberg ED, Anderson GM, Ackermann MR, et al: Epithelial antibioticinduced in states of disease.Proc Natl Acad Sci USA94:8686–8690, 1997

62. Shonwetter BS, Stolzenberg ED, Zasloff MA: Epithelial antibiotics inducedat sites of inflammation.Science267:1645–1648, 1995

63. Bensch KW, Raida M, Magert H-J, et al: hBD-1: a novelb-defensin fromhuman plasma.FEBS Lett368:331–335, 1995

64. Valore EV, Park CH, Quayle AJ, Wiles KR, McCray PB, Ganz T: Humanb-defensin-1: an antimicrobial peptide of urogenital tissues.J Clin Invest101:1633–1642, 1998

65. Schnapp D, Reid CJ, Harris A: Localization of expression of human betadefensin-1 in the pancreas and kidney.J Pathol186:99–103, 1998

66. Krisanaprakornkit S, Weinberg A, Perez CN, Dale BA: Expression of thepeptide antibiotic humanb-defensin 1 in cultured gingival epithelial cells andgingival tissue.Infect Immunol66:4222–4228, 1998

67. Goldman MJ, Anderson GM, Stolzenberg ED, et al: Humanb-defensin-1 isa salt-sensitive antibiotic in lung that is inactivated in cystic fibrosis.Cell88:553–560, 1997

68. Harder J, Bartels J, Christophers E, Schroder J-M: A peptide antibiotic fromhuman skin.Nature387:861, 1997

69. Liu L, Wang L, Jia HP, Zhao C, Heng HHQ, Schutte BC, McCray PB, GanzT: Structure and mapping of the humanb-defensin HBD-2 gene and itsexpression at sites of inflammation.Gene222:237–244, 1998

70. Diamond G, Russell JP, Bevins CL: Inducible expression of an antibiotic

52 Faull

peptide gene in lipopolysaccharide-challenged tracheal epithelial cells.ProcNatl Acad Sci USA93:5156–5160, 1996

71. Jones S, Donovan KL, Williams JD, et al: Human neutrophil defensins havechemoattractant activity for mononuclear cells (MNC) but not neutrophils(PMN): possible role in cell recruitment into the peritoneal cavity.J Am SocNephrol9:284A, 1998

72. Van Bronswijk H, Verbrugh HA, Bos HJ, et al: Cytotoxic effects of com-mercial continuous ambulatory peritoneal dialysis (CAPD) fluids and ofbacterial exoproducts on human mesothelial cells in vitro.Perit Dial Int9:197–202, 1989

73. Leavesley DI, Stanley JM, Faull RJ: Epidermal growth factor modifies theexpression and function of extracellular matrix adhesion receptors expressedby peritoneal mesothelial cells from patients on CAPD.Nephrol Dial Trans-plant, 14:1205–1217, 1999

74. Betjes MG, Bos HJ, Krediet RT, Arisz L: The mesothelial cells in CAPDeffluent and their relation to peritonitis incidence.Perit Dial Int 11:22–26,1991

75. Pollock CA, Ibels LS, Eckstein RP, et al: Peritoneal morphology on main-tenance dialysis.Am J Nephrol9:198–204, 1989

76. Martin P: Wound healing—aiming for perfect skin regeneration.Science276:75–81, 1997

77. Martin P, Hopkinson-Woolley J, McCluskey J: Growth factors and cutaneouswound repair.Prog Growth Factor Res4:25–44, 1992

78. Marshall BC, Santana A, Xu QP, et al: Metalloproteinases and tissue inhibi-tor of metalloproteinases in mesothelial cells. Cellular differentiation influ-ences expression.J Clin Invest91:1792–1799, 1993

79. Molina S, Selgas R, de Castro MF, Vara F: Presence of different mitogenicactivities in the nocturnal peritoneal effluent of patients treated with CAPD.Adv Perit Dial 9:190–194, 1993

80. Offner FA, Feichtinger H, Stadlmann S, et al: Transforming growth factor-bsynthesis by human peritoneal mesothelial cells. Induction by interleukin-1.Am J Pathol148:1679–1688, 1996

81. Wang T, Ghen YG, Ye RG, et al: Effect of glucose on TGF-b1 expressionin peritoneal mesothelial cells.Adv Perit Dial 11:7–10, 1995

82. Wang T, Ghen YG, Ye RG, et al: Enhanced expression of TGF-b by peri-toneal macrophages in CAPD patients.Adv Perit Dial 11:11–14, 1995

83. Rougier JP, Guia S, Hagege J, et al: PAI-1 secretion and matrix depositionin human peritoneal mesothelial cell cultures: transcriptional regulation byTGF-b1. Kidney Int54:87–98, 1998

84. Stylianou E, Jenner LA, Davies M, et al: Isolation, culture and characteriza-tion of human peritoneal mesothelial cells.Kidney Int37:1563–1570, 1990

85. Hay ED: An overview of epithelio-mesenchymal transformation.Acta Anat154:8–20, 1995

86. Strange CJR, Tomlinson C, Wilson R, et al: The histology of experimentalpleural injury with tetracycline, empyema, and carrageenan.Exp Mol Pathol51:205–219, 1989

87. Raab G, Klagsbrun M: Heparin-binding EGF-like growth factor.BiochimBiophys Acta1333:F179–F198, 1997

88. Beavis MJ, Williams JD, Hoppe J, Topley N: Human peritoneal fibroblastproliferation in 3-dimensional culture: modulation by cytokines, growth fac-tors and peritoneal dialysis effluent.Kidney Int51:205–215, 1997

89. McLaren J, Prentice A, Charnock-Jones DS, et al: Vascular endothelialgrowth factor is produced by peritoneal fluid macrophages in endometriosisand is regulated by ovarian steroids.J Clin Invest98:482–489, 1996

90. Xiong M, Elson G, Legarda D, Leibovich SJ: Production of vascular endo-thelial growth factor by murine macrophages. Regulation by hypoxia, lactate,and the inducible nitric oxide synthase pathway.Am J Pathol153:587–598,1998

91. Hogaboam CM, Steinhauser ML, Chensue SW, Kunkel SL: Novel roles forchemokines and fibroblasts in interstitial fibrosis.Kidney Int54:2152–2159,1998

53PERITONEAL DEFENSES AGAINST INFECTION