Eicosanoids: lipid mediators of inflammation in transplantation

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Eicosanoids: lipid mediators of inflammation in transplantation

  1. 1. Springer Semin Immunopathol (2003) 25:215–227 DOI 10.1007/s00281-003-0132-4 © Springer-Verlag 2003 Eicosanoids: lipid mediators of inflammation in transplantation Paulo N. Rocha, Troy J. Plumb, Thomas M. Coffman Department of Medicine, Division of Nephrology, Duke University and Durham VA Medical Centers, Building 6/Nephrology, 508 Fulton Street, Durham, NC 27705, USA Abstract. Eicosanoids are a family of lipid mediators derived from the metabolism of arachidonic acid. Eicosanoids, such as prostanoids and leukotrienes, have a wide range of biological actions including potent effects on inflammation and immunity. It has been almost 20 years since the first reports emerged suggesting a role for eicosanoids in transplantation. Since then, a number of functions have been ascribed to these mediators, ranging from immunomodulation to regulation of allograft he- modynamics. In this review, we will highlight the effects of eicosanoids in trans- plantation, focusing particularly on evidence provided by gene targeting studies. In the future, pharmacological manipulation of eicosanoids and their receptors may provide a novel approach for controlling inflammation and promoting allograft ac- ceptance. Introduction Lipid mediators generated by the metabolism of arachidonic acid play key roles in a wide range of biological processes. As these compounds are synthesized from a common precursor, the 20-carbon fatty acid arachidonic acid, they are collectively termed eicosanoids. These mediators can be separated into distinct classes based on pathways of biochemical synthesis including: cyclooxygenase metabolites (prostanoids), 5-lipoxygenase metabolites (leukotrienes), P450 metabolites (HETES, EETs), and non-enzymatic metabolites (isoprostanes). Among their myri- ad of biological effects, eicosanoids can induce and regulate inflammatory re- sponses, actions that have been most clearly defined for two classes of eicosano- ids: prostanoids and leukotrienes. This review will focus on regulation of immuni- ty and inflammation by prostanoids and leukotrienes, highlighting their effects in organ transplantation. Rejection of an organ transplant is initiated by the specific interaction of recipient T cells with donor alloantigens [28]. The resulting activation and amplification of the alloimmune response triggers an intense inflammatory response within the graft. Correspondence to: Thomas M. Coffman
  2. 2. This local inflammatory response causes characteristic alterations in the physiologi- cal function of the graft that, if left unchecked, will eventually lead to graft destruc- tion. The inflammatory response in allograft rejection is complex, involving a num- ber of cellular effectors. The nature and intensity of the response are further shaped and orchestrated by a variety of soluble mediators. These mediators may act in sever- al distinct capacities including: recruitment and accumulation of inflammatory cells into the graft, regulation of immune cell function, and as direct effector pathways causing graft injury and dysfunction. Herein, we review evidence that suggests that lipid inflammatory mediators synthesized from arachidonic acid may act in each of these capacities during transplant rejection. Since these pathways can be manipulated pharmacologically, a comprehensive understanding of the diverse roles of eicosano- ids in transplantation may have direct clinical applications. Biosynthesis of eicosanoids The lipid bilayer that encircles eukaryotic cells contains the 20-carbon fatty acid ara- chidonic acid, the substrate for the production of eicosanoids. The first step in eico- sanoid biosynthesis is the release of arachidonic acid from membrane phospholipids by the actions of phospholipases. There are a number of mammalian phospholipases and their activity is regulated by hormones and cytokines [48, 54]. For example, pro- inflammatory cytokines such as IL-1 and TNF-α increase the activity of important phospholipases such as PLA2, contributing to increased production of eicosanoids during inflammatory states. By contrast, corticosteroids, which are critical compo- nents of immunosuppressive regimens used in transplantation, may inhibit PLA2 [72]. Once liberated from the cell membrane, arachidonic acid may become a sub- strate for various metabolic pathways that produce biological mediators. We will dis- cuss two of these: the cyclooxygenase (COX) and 5-lipoxygenase (5LO) pathways of arachidonic acid metabolism. Prostanoids and their role in transplantation The cyclooxygenase pathway The COX pathway of arachidonic acid metabolism is depicted in Fig. 1. Two distinct COX isoforms, COX-1 and COX-2, have been identified, cloned and sequenced [60]. Both COX isoforms carry out the identical biochemical function, catalyzing a two-step reaction that converts arachidonic acid to prostaglandin (PG)H2, a relatively unstable endoperoxide. Based on this function, these enzymes are also known as PGH synthases 1 and 2. Although their biochemical functions are similar, they share only about 60–65% homology at the amino acid level [59] and there are major differ- ences in the pattern and regulation of their expression. COX-1 is constitutively ex- pressed at low levels in virtually every nucleated cell. In contrast, COX-2 activity is not detectable in most adult tissues under normal conditions with the exception of the kidney and the brain. However, expression of COX-2 can be rapidly and highly up-regulated by a variety of stimuli that are associated with tissue injury and inflam- mation. Based on these expression patterns, a hypothesis was developed suggesting that COX-1 is responsible for the baseline production of prostanoids involved in 216 P.N. Rocha et al.
  3. 3. physiological functions such as gastric cytoprotection, while COX-2 is responsible for the increased output of prostanoids during inflammation and injury [11]. The mechanism of action of traditional non-steroidal anti-inflammatory drugs (NSAIDs) is COX inhibition and consequent reduction of prostanoid synthesis. Conventional NSAIDs such as ibuprofen, naproxen, and indomethacin inhibit both COX-1 and COX-2. The newer specific COX-2 inhibitors (coxibs) selectively inhibit COX-2. The efficacy of COX-2 inhibitors is similar to conventional NSAIDs, suggesting that the beneficial effects of NSAIDs for analgesia and anti-inflammatory effects are due to their inhibition of COX-2. Despite their similar efficacy, chronic use of COX-2 in- hibitors is associated with reduced gastrointestinal side effects [2] compared to con- ventional NSAIDs, suggesting that this adverse effect of non-selective NSAIDs is due to concomitant inhibition of COX-1. As shown in Fig. 1, PGH2 generated by COX is further metabolized by specific isomerases to form the various PGs and thromboxane (TX) A2. Once formed, the biological activity of prostanoids is short lived due to both inherent instability and specific enzymatic degradation. Thus, their primary actions are usually manifested in the local microenvironment in which they are generated. The cellular actions of prostanoids are mediated via specific G protein-coupled receptors, and the genes encoding many of these receptors have been cloned [38]. Mouse lines with targeted disruption of each prostanoid receptor have been subsequently generated, leading to a better understanding of the function of individual prostanoids in physiology and disease [39]. Eicosanoids and allograft rejection 217 Fig. 1. The Cyclooxygenase Pathway
  4. 4. Prostanoids in transplantation The general effects of prostanoids in immunity and inflammation have been re- viewed recently [69]. Depending on the specific metabolite that is produced and the cell type that is affected, prostanoids may stimulate or inhibit immune responses. Thus, they may have complex actions in responses, such as transplant rejection, that are characterized by activation of a diverse range of cellular and humoral elements. In this section, we will discuss the putative roles of components of the prostanoid system that have been implicated in transplantation. Prostaglandin E2 A broad range of physiological effects has been ascribed to PGE2. These protean ac- tions of PGE2 are mediated by four distinct receptors, EP (E prostanoid)1–4 , that are coupled to different G proteins and therefore utilize different intracellular signaling pathways. Differences in signaling and patterns of expression for the EP receptors provide a molecular basis for the diverse actions of PGE2. The generation and char- acterization of mice with targeted deletions of each EP receptor subtype has clarified our understanding of the role of these receptors in physiology and disease [39]. EP receptor isoforms can be detected on a broad range of immune cells including lymphocytes and macrophages. The major actions of PGE2 on immune responses tend to be inhibitory [69]. In macrophages, for example, PGE2 down-regulates the expression of MHC class II antigens, inhibits the production of pro-inflammatory mediators such as IL-12, TNF-α, and superoxide, and increases IL-10 production in response to LPS. This pattern of reduced IL-12 and enhanced IL-10 production by antigen-presenting cells may promote differentiation of naïve T cells towards a Th2 phenotype and B lympho- cyte isotype switching to IgE. Using a panel of EP-deficient mouse lines, our laboratory showed that the inhibitory effects of PGE2 on T cell function are mediated by EP2 re- ceptors, whereas both EP2 and EP4 act to suppress macrophage functions [40]. As both of these receptors are linked to G proteins, this finding is consistent with previous work showing that many of the inhibitory actions of PGE2 upon immune cells can be repro- duced by cAMP or maneuvers that increase intracellular cAMP concentration. Consistent with its immunosuppressive actions, PGE2 analogues have been shown to prolong the survival of renal [65], cardiac [10, 24], and small-intestinal [27] allo- grafts in rats as well as skin allografts in mice [1]. Human transplantation studies also indicate a favorable effect of PGE2. In a double-blind, placebo-controlled study of 77 renal allograft recipients receiving a cyclosporine- and prednisone-based regi- men, treatment with the PGE2-analogue misoprostol was associated with a 50% re- duction in acute rejection episodes [36]. Moreover, at the end of follow up, renal function, assessed by serum creatinine and creatinine clearance, was significantly better in the patients receiving misoprostol compared to controls. However, this ben- eficial effect of misoprostol could not be confirmed in a subsequent analysis. None- theless, additional studies researchers have demonstrated beneficial effects of a PGE analogue in patients after liver transplantation [20, 67]. Along with its immunosup- pressive effects, PGE2 is also a vasodilator. Thus, beneficial hemodynamic effects may contribute to the efficacy of PGE2 in these settings. The chemical and pharmacological characteristics of complex lipid molecules such as PGE2 have limited their therapeutic applications in transplantation and other 218 P.N. Rocha et al.
  5. 5. clinical settings. Furthermore, these compounds stimulate all EP receptors, and at higher concentrations may activate other non-EP prostanoid receptors. In the future, identification of small molecules that could be used to target specific EP receptors might allow more widespread clinical applications. Prostaglandin I2 PGI2 (also known as prostacyclin) is the major prostanoid produced by vascular en- dothelium. It acts as a vasodilator and potently inhibits platelet aggregation. In con- trast to PGE2, the biological actions of PGI2 are mediated by a single IP receptor. Along with its vascular effects, generation and characterization of IP-deficient knockout mice has revealed an unexpected role for IP receptors in mediating pain re- sponses [37]. A potential role for PGI2 to modulate immune responses was first suggested by the studies of Leung et al [29]. In these studies, the addition of relatively high con- centrations of PGI2 to lymphocyte cultures from immunized mice reduced cytolytic activity by approximately 20%. Conversely, a PGI2 synthase inhibitor augmented cell-mediated immunity as measured by splenocyte proliferation and cytotoxicity as- says. Although this work suggested that PGI2 might regulate cellular immune re- sponses, it is possible that other prostanoid receptors might have been affected by the high concentrations of PGI2 that were used. In addition, during PGI2 synthase inhibi- tion, there may have been shunting of PGH2 metabolism towards the production of other pro-inflammatory prostanoids. Future studies using IP receptor knockout mice should help clarify the role of PGI2 and its receptor in immunity. There is limited experience with PGI2 in the setting of transplantation. However, a preliminary study reported prolongation of renal allograft survival in rabbits by a prostacyclin analogue given in combination with cyclosporine [44]. A role for pros- tacyclin deficiency in the thrombotic microangiopathy associated with cyclosporine has also been suggested [45]. Thromboxane A2 The actions of TXA2 tend to oppose those of PGE2 and PGI2. TXA2 is a potent vaso- constrictor and platelet aggregant. It is the major prostanoid produced by platelets. Activated macrophages are also a major source of TX. Similar to PGI2, there appears to be only a single T prostanoid (TP) receptor. However, different TP receptor iso- forms may be generated by alternative splicing of the single TP receptor gene. Un- like PGE2 and PGI2, TXA2 appears to enhance cellular immune responses. For ex- ample, in mixed lymphocyte cultures, the TXA2 synthase inhibitor carboxyheptyl imidazole reduced antigen-specific proliferation and cytotoxicity [29]. However, as discussed above, interpretation of this experiment is complicated by the possibility of endoperoxide shunting toward production of suppressor prostanoids such as PGE2 or PGI2 [29], as well as the poor specificity and potency of imidazole [4]. Nonethe- less, later studies showed that specific TP receptor antagonists could also inhibit the proliferation of unprimed mouse spleen cells in the mixed lymphocyte reaction [53]. Recently, we have confirmed the existence of potent actions of TP receptors in pro- moting cellular immune responses using TP-deficient mice [Thomas et al, submitted]. Eicosanoids and allograft rejection 219
  6. 6. Our studies suggest that TP receptors potentiate calcium-dependent signals triggered by T cell receptor engagement. Along with its effects on T cells, TXA2 may also promote immune responses by enhancing macrophage functions. For example, stimulation of TP receptors on mac- rophages induces expression of MHC class II molecules [61]. In addition, TX facili- tates synthesis of pro-inflammatory cytokines such as TNF-α and IL-1 [3]. A role for TXA2 in rejection was first suggested by the studies of Foegh et al. [13] showing that excretion of TX metabolites was enhanced during episodes of acute rejection in human renal allograft recipients. Subsequent studies from several laboratories demonstrated increased production of TX in animal models of rejection [8, 26, 68] and showed that TXA2 causes reversible hemodynamic changes in grafts during rejection [8, 34]. A more profound effect on graft function was observed when TX inhibitors were given continuously from the time of transplantation [7]. In these studies, a functional benefit was observed only when substantial inhibition of renal TX production was accomplished and this could only be achieved by direct in- fusion of the TX synthase inhibitor into the renal artery supplying the graft. Lack of effect of TX synthase inhibition at later stages of rejection was associated with in- complete suppression of TXA2 production. The lack of efficacy may also relate to generation of PG endoperoxides such as PGH2 that may accumulate when TX syn- thase is inhibited. These compounds may act as agonists at the TP receptor and thus reduce the effectiveness of therapy. Using TP knockout mice in an aggressive model of cardiac allograft rejection, we found that the absence of TP receptors on the recipient alone was not sufficient to prolong graft survival [Thomas et al, submitted]. However, if recipients are treated with sub-therapeutic doses of cyclosporine, graft survival is prolonged in TP-defi- cient recipients compared to wild-type controls. The prolonged graft survival ob- served in TP-deficient recipients is associated with significant amelioration in the se- verity of histopathological manifestations of rejection. This suggests that a contribu- tion of TP-mediated actions to promote cellular immunity can be uncovered when the rejection response is attenuated. Moreover, these findings also imply potential additive or cooperative actions of TP blockade with calcineurin inhibition. The beneficial effects of TXA2 inhibition during acute rejection probably extend further than alterations in immune responses. For example, in acute renal allograft rejection, the initial fall in glomerular filtration rate is often much greater than would be expected based on the severity of pathological changes in the graft [14, 21, 52]. This likely represents potent vasoconstriction mediated by molecules produced as a part of the inflammatory response within the rejecting allograft. Since TXA2 is a powerful vasoconstrictor whose production is increased during acute rejection, it would be a likely candidate for such a role. A similar rationale was used by Pierucci et al. [43] to explain the improvement in renal function observed after the acute infu- sion of a TXA2 antagonist in humans with lupus nephritis. Nonetheless, it was later shown that chronic TXA2 blockade profoundly altered the course of renal disease in lupus prone mice, effects that go beyond the vasodilation [62]. In transplantation, TXA2 has also been implicated as a potential mediator of cy- closporine nephrotoxicity. In animal studies, production of TXA2 is increased during experimental chronic cyclosporine toxicity [5, 42]. Furthermore, administration of TXA2 synthase inhibitors and TP receptor blockers are beneficial in this setting [64]. Smith et al. [57] showed similar beneficial effects of TX synthase inhibition in hu- man kidney transplant recipients with cyclosporine nephrotoxicity. In a subsequent 220 P.N. Rocha et al.
  7. 7. study, however, the oral administration of the TX synthase inhibitor CGS 12970 for 4 weeks did not improve renal function or plasma flow in patients with cyclosporine nephrotoxicity [58]. Along with its direct effects on T cell functions, TXA2 may also influence the maturation and development of the T cell repertoire. For example, TP receptors are expressed at high levels in the thymus, most prominently in immature thymocyte populations [70]. Stimulation of TP receptors on these cells induces programmed cell death, suggesting that TP receptors on thymocytes might play a role in selection of maturing T lymphocytes. Studies by Remuzzi et al. [46] suggest that the actions of TP receptors in the thymus may be critical for the development of tolerance to kid- ney allografts. Using a rat model in which tolerance to an allograft can be induced by intra-thymic injection of donor-derived MHC peptides, these authors found that ad- ministration of TP receptor antagonists prevented allograft tolerance induction. While these studies support a role for TP receptors in the thymus in promoting toler- ance to an allograft, the mechanism and sites of action of this effect have not been defined. Global inhibition of prostanoid synthesis Because NSAIDs are potent anti-inflammatory agents, it has been inferred that pros- tanoids have dominant actions in promoting inflammation. However, as we have dis- cussed above, the role of prostanoids in inflammation and immunity is complex, and it is clear that some PGs, such as PGE2 and PGI2, have potent immunosuppressive actions. In vitro studies indicate that COX blockade with indomethacin actually enhances cellular immune responses, as evidenced by proliferation and cytotoxicity assays [29]. Although these findings suggest that global COX blockade might have deleterious effects in transplantation, there currently are few animal or human studies evaluating this issue. A recent publication by Ma et al. [32] addressed the role of se- lective COX-2 inhibition in allograft rejection using a rat model of cardiac transplan- tation [32]. They found a significant prolongation in allograft survival in rats treated with the COX-2 inhibitor. This prolongation in allograft survival was associated with significant attenuation in histological changes at days 3 and 5, as well as a reduction in the number of apoptotic cardiomyocytes. Taken together, these findings suggest that COX-2-derived prostanoids exert a dominant pro-inflammatory role during allo- graft rejection. Leukotrienes and their role in transplantation The 5-lipoxygenase pathway The 5-LO pathway for arachidonic acid is depicted in Fig. 2. The initial step in this pathway is the conversion of membrane-derived arachidonic acid into the unstable intermediate leukotriene (LT) A4 by the enzyme 5-LO [17]. LTA4, in turn, can be hydrolyzed to form LTB4, or conjugated with glutathione to form LTC4. These reac- tions are catalyzed by LTA4 hydrolase and LTC4 synthetase, respectively. LTC4 is converted into LTD4 and LTE4 by extracellular metabolism and these three mole- cules are collectively termed cysteinyl-leukotrienes (CysLTs) or peptidoleukotrienes. Eicosanoids and allograft rejection 221
  8. 8. Similar to prostanoids, the biological effects of LT are mediated by a family of G protein-coupled leukotriene receptors. LTB4 exerts most of its known actions by binding BLT1 receptors on leukocytes. A second B leukotriene receptor, BLT2, has recently been identified and cloned but its functions are not clear. Two receptors, CysLT1 and CysLT2, mediate the actions of (CysLTs). Similar to prostanoids, the production of LTs is increased during acute allograft rejection [34, 63]. This section will provide a brief discussion of the biological properties of these molecules, with emphasis on their role in the alloimmune response. Leukotriene B4 Neutrophils are the main cellular source of LTB4 but this molecule is also synthe- sized by macrophages [30]. LTB4 is a potent chemotactic and chemokinetic factor for neutrophils. LTB4 increases leukocyte adhesion to endothelial cells and extravasation into tissues. LTB4 also promotes production of pro-inflammatory cytokines, such as IL-1, IL-2 and INF-γ by T cells and monocytes [49–51]. In addition, LTB4 up-regu- lates the expression of integrins such as CD11b [56]. LTB4 may also act as a ligand for peroxisome proliferator-activated receptor α (PPARα), which has been shown to have anti-inflammatory effects [9]. LTB4 acts via two G protein-coupled receptors, BLT1 and BLT2. BLT1 has a high affinity for LTB4, and is expressed in leukocytes. BLT2 is expressed in many tissues, and compared to BLT1 has a lower binding affinity for LTB4 [73, 75–77]. Yokomizo et al. [74] recently showed that BLT1 is expressed in highest concentration in human 222 P.N. Rocha et al. Fig. 2. The 5-Lipoxygenase Pathway
  9. 9. monocytes, whereas BLT2 is most highly expressed in CD4+ and CD8+ T lympho- cytes and CD19+ B lymphocytes. Although the precise function of BLT2 is not known, these differing expression patterns suggest distinct biological roles for the two receptors. Mangino et al. [34] documented increased levels of LTB4 in the cortex of reject- ing renal allografts. A functional role for increased LTB4 in rejection was clearly demonstrated by Weringer et al. [71], who showed that administration of the LTB4 antagonist CP-10596 prolonged survival of mouse heart allografts. Although the treatment did not substantially affect cellular infiltrates or levels of MHC class II an- tigen expression, it caused a marked reduction in the proportion of CD11b-express- ing cells in the graft. Administration of the LTB4 receptor antagonist was also associ- ated with a delayed peak in graft-reactive serum IgG levels. Cysteinyl-leukotrienes The CysLTs are synthesized by eosinophils, mast cells and macrophages [30]. They stimulate smooth muscle contraction and are potent inducers of bronchoconstriction and arteriolar constriction. In addition, CysLTs increase permeability of the post-cap- illary venule promoting plasma extravasation. LTC4 may also be important in mobi- lizing dendritic cells from peripheral tissue to lymph nodes as mice deficient in the LTC4 transporter, multidrug resistance-associated protein 1 (MRP1), have impaired migration of dendritic cells from the skin to lymph nodes; normal mobilization is re- stored by LTC4 [47]. While pharmacological studies had indicated the existence of two distinct CysLT re- ceptors, they have only recently been cloned and characterized [19, 23, 31, 41, 66]. These receptors, CysLT1 and CysLT2 are G protein-coupled receptors that can be found in numerous tissues. In humans, CysLT1 mRNA is present in the spleen, lung tissue, smooth muscle cells and macrophages; CysLT2 mRNA has likewise been isolated from a wide variety of tissues, including peripheral blood leukocytes, eosinophils, monocytes, lung macrophages, airway smooth muscle, cardiac purkinje cells, adrenal medulla and brain. Ogasawara et al. [41] cloned the mouse CysLT1 and CysLT2 receptors and charac- terized their function and tissue distribution in two different strains, C57BL/6 and 129. CysLT1 mRNA levels were found to be highest in skin, lung, small intestine and macro- phages, whereas CysLT2 was found in highest levels in spleen, lung, and small intestine. Interestingly, there was a much higher overall expression of both receptors in the C57BL/6 strain. Furthermore, mouse CysLT2 receptors were inhibited by a compound that had previously been shown to be a specific antagonist of the human CysLT1. The CysLTs are potent inducers of inflammation, and receptor antagonists of the CysLTs are used clinically in the treatment of asthma [18]. Specific contributions of CysLTs in inflammation have also been evaluated using mice with targeted disrup- tion of the LTC4 synthase (LTC4S) and CysLT1 genes. Using LTC4S knockout mice, Kanaoka et al. [25] showed that the absence of LTC4S significantly decreased vascu- lar permeability in zymosan-induced peritonitis. A similar response was observed in CysLT1 gene knockout mice [33]. These alterations in vascular permeability oc- curred without any effect on neutrophil influx. In a rat model of kidney transplantation, our laboratory showed that generation of LTC4 was enhanced in allografts compared to isografts and there was a correlation be- tween LTC4 levels and cellular infiltrates [6]. In a similar model, treatment with an Eicosanoids and allograft rejection 223
  10. 10. LTC4 receptor antagonist decreased vascular rejection but had no effect on graft sur- vival [63]. Global inhibition of leukotriene synthesis Much of the experimental work evaluating the role of LTs in allograft rejection has relied on the use of 5-LO inhibitors to globally suppress LT production. In published studies, pharmacological inhibitors of 5-LO have consistently been shown to have beneficial effects on allograft function and survival. For example, in a canine model of kidney transplantation, treatment with a combined 5-LO and COX inhibitor pre- served glomerular filtration rate, reduced cellular infiltrates, and inhibited the gener- ation of LTs by the grafts [34, 35]. Similarly, using a rat kidney transplant model, our laboratory showed that 5-LO inhibition improved allograft survival, inhibited MHC class II expression in the allograft, and preserved allograft morphology [63]. Similar beneficial actions of 5-LO inhibitors have been demonstrated in cardiac allografts in rats [12] and in pancreas transplants in dogs [22, 55]. While pharmacological inhibition of 5-LO generally improves allograft function and survival, similar beneficial effects were not observed with targeted deletion of the 5lo gene. In a mouse model of kidney transplantation, survival of kidney allo- grafts in 5-LO-deficient recipients was significantly reduced compared to wild-type controls [15]. Poor graft survival was not associated with differences in renal hemo- dynamics, allograft histopathology, cytokine mRNA expression, or generation of other eicosanoids such as prostanoids or lipoxins. Although these results are surpris- ing in view of the multiple studies documenting beneficial effects of 5-LO inhibitors, they are consistent with previous studies showing that 5-LO deficiency also acceler- ated mortality from autoimmune disease in male MRL-lpr/lpr mice [16]. There has otherwise been substantial concordance between studies using knock- outs and pharmacological inhibitors of 5-LO in various models of inflammation. Thus, the apparent discrepancy between transplant studies using 5-LO inhibitors and the 5-LO knockout mice is perplexing. Nonetheless, the adverse effects of the 5lo mutation in renal allograft rejection suggest an unexpected protective effect of a 5-LO product in this model. While the bulk of current evidence indicates that LTs act primarily as pro-inflammatory lipid mediators, this study along with our previous ex- periments in the MRL-lpr/lpr model [16] indicate a potentially beneficial action of 5-LO metabolites in renal inflammation. Conclusions Lipid mediators that are generated as a consequence of the inflammatory response in transplant rejection have wide-ranging effects on immunity and allograft physiology. Cloning and sequencing of genes encoding enzymes and receptors in the COX and 5-LO pathways along with generation of lines of mice with targeted disruption of these genes have provided opportunities to precisely define their functions in trans- plantation. Such studies should lead to a better understanding of the relative contri- butions of eicosanoids to alloimmune responses, graft injury, and transplantation tol- erance. Our expectation is that such studies may lead to new approaches for control- ling inflammation that would be more specific and less toxic than current broadly acting anti-inflammatory drugs such as corticosteroids. 224 P.N. Rocha et al.
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