Originally published online as doi:10.1189/jlb.0103037 on May 8, 2003
Published online before print May 8, 2003
(Journal of Leukocyte Biology. 2003;73:702-712.)
© 2003
by Society for Leukocyte Biology
Nondisposable materials, chronic inflammation, and adjuvant action
John A. Hamilton
Arthritis and Inflammation Research Centre and Cooperative Research Centre for Chronic Inflammatory Diseases, University of Melbourne, Department of Medicine, The Royal Melbourne Hospital, Parkville, Australia
Correspondence: Professor John Hamilton, Arthritis and Inflammation Research Centre, University of Melbourne, Department of Medicine, The Royal Melbourne Hospital, Clinical Sciences Building, Royal Parade, Parkville, Vic 3050, Australia. E-mail: jahami{at}unimelb.edu.au
 |
ABSTRACT
|
|---|
Why inflammatory responses become chronic and how adjuvants work remain unanswered. Macrophage-lineage cells are key components of chronic inflammatory reactions and in the actions of immunologic adjuvants. One explanation for the increased numbers of macrophages long term at sites of chronic inflammation could be enhanced cell survival or even local proliferation. The evidence supporting a unifying hypothesis for one way in which this macrophage survival and proliferation may be promoted is presented. Many materials, often particulate, of which macrophages have difficulty disposing, can promote monocyte/macrophage survival and even proliferation. Materials active in this regard and which can initiate chronic inflammatory reactions include oxidized low-density lipoprotein, inflammatory microcrystals (calcium phosphate, monosodium urate, talc, calcium pyrophosphate), amyloidogenic peptides (amyloid ß and prion protein), and joint implant biomaterials. Additional, similar materials, which have been shown to have adjuvant activity (alum, oil-in-water emulsions, heat-killed bacteria, CpG oligonucleotides, methylated bovine serum albumin, silica), induce similar responses. Cell proliferation can be striking, following uptake of some of the materials, when macrophage-colony stimulating factor is included at low concentrations, which normally promote mainly survival. It is proposed that if such responses were occurring in vivo, there would be a shift in the normal balance between cell survival and cell death, which maintains steady-state, macrophage-lineage numbers in tissues. Thus, there would be more cells in an inflammatory lesion or at a site of adjuvant action with the potential, following activation and/or differentiation, to perpetuate inflammatory or antigen-specific, immune responses, respectively.
Key Words: macrophage survival • oxidized LDL • alum • M-CSF • CSF-1
|
INTRODUCTION
|
|---|
The inflammatory response to tissue damage, among others, is normally a protective one but can become prolonged, leading to pathology, for example, in an atherosclerotic plaque, a granuloma, or a rheumatoid synovium. The reasons for the chronicity of such lesions are unknown. Macrophage-lineage cells are key components of inflammatory lesions, particularly those of a chronic nature. Presumably, there is a long-lasting stimulus to cells such as macrophages, which perpetuate the host reaction in some situations, but the nature of such signals remains to be elucidated. Macrophages are usually assumed to derive at a site of inflammation from the migration of circulating monocytes, although there are several reports indicating that local proliferation can occur, thereby offering an additional mechanism for the increase in macrophage numbers [1
2
3
4
5
]. Once there, macrophages can perpetuate inflammation by producing inflammatory mediators.
Adjuvants trigger the innate-immune system to elaborate the signals required for the initiation of an adaptive-immune response. Many immunologic adjuvants (for example, alum) are particulate in nature, but how they act to promote the immune reactions is uncertain. Many antigen-presenting (dendritic) cells (APCs/DCs) derive from macrophage-lineage cells so that internalization of such adjuvants is likely to be a key feature.
This article reviews the evidence for the concept that the induction of enhanced macrophage-lineage survival and even proliferation by a wide range of materials contribute significantly to the chronicity of inflammatory reactions and to the action of immunologic adjuvants. The materials and adjuvants in question are able to accumulate in macrophage-lineage cells, most likely because they are poorly degradable, thereby enabling them to provide a persistent stimulus. The consequences are more inflammatory macrophages and the development of a chronic lesion and/or more DCs and enhanced adjuvanticity. It is also proposed that if sufficient colony-stimulating factor (CSF)-1 is present to promote cell survival, then the same materials may induce macrophage-lineage proliferation, thus increasing cell number further.
|
MACROPHAGE NUMBERS AT INFLAMMATORY SITES
|
|---|
Like other hemopoietic cells in the adult, macrophages derive from bone marrow (BM) precursors and are maintained at steady-state levels by the balance among apoptosis, survival, and proliferation, under the influence of stimuli such as the cytokine macrophage-CSF (M-CSF or CSF-1) [6
]. At a site of injury or infection, their increased numbers are likely to derive mostly by migration of blood-borne monocytes into the damaged tissue site or site of infection. The eventual reduction in the number of inflammatory cells, such as macrophages, during normal tissue repair and turnover, for example, in a wound, is assumed to involve, in part, death of such cells by apoptosis, in addition to migration of such cells away from the resolving site [7
]. However, the lesion resolution, which normally occurs following repair of the injury or removal of the inciting stimulus, is sometimes delayed, leading to an excessive and uncontrolled inflammatory reaction, such as seen in granulomas, involving the maintenance of increased numbers of macrophages [7
, 8
]. Prolonged macrophage survival may contribute to the failure of such an inflammatory reaction to subside. There is also evidence, although not widely considered as a mechanism, that local proliferation of macrophages can contribute to their enhanced numbers in chronic lesions [1
2
3
4
5
].
|
OXIDIZED LOW-DENSITY LIPOPROTEIN (ox.LDL), FOAM CELLS, AND ATHEROSCLEROTIC PLAQUE
|
|---|
One important clinical example where increased numbers of macrophage-lineage cells persist is the atherosclerotic plaque. Atherosclerosis is now being more commonly viewed as a chronic inflammatory disease in which cholesterol ester-filled macrophages, or "foam" cells, are an early and prominent feature of the lesion [9
, 10
]. There is support for the notion that LDL becomes atherogenic after it has been modified in some way, for example, by oxidation [11
]. ox.LDL uptake through the so-called "scavenger receptors" can lead in vitro to the development of cholesterol ester-filled macrophages [9
]. There are a number of reports indicating that macrophage-derived foam cells are able to proliferate, particularly in the early stages of lesion development, in humans and rabbits [1
2
3
, 12
13
14
15
]. Therefore, it is possible that such proliferation may contribute to the development of atherosclerotic plaques by providing an additional mechanism to cell migration for the increased foam-cell numbers.
ox.LDL and macrophage survival/proliferation
Several studies have found that ox.LDL and acetylated LDL can induce macrophage-lineage cell survival and proliferation depending on the cell type under study [16
17
18
19
20
21
]. Murine peritoneal macrophages, resident in the peritoneal cavity or present in exudates, have been widely used in these studies. Our laboratory has advocated using murine BM-derived macrophages (BMM) because of their purity, the ease with which large numbers can be generated in vitro, the dependence of their survival on the physiologic regulator CSF-1, and the fact that they all have the capability of entering the cell cycle [18
]. It is important to note that human monocyte survival [19
] and proliferation have been reported to be fostered by ox.LDL. Some reports have found that the response of monocytes/macrophages to ox.LDL is, in fact, apoptosis [22
, 23
]; we have noted that toxic effects occur at high concentrations but that lower concentrations promote survival/proliferation [18
, 19
].
One feature of ox.LDL, which could contribute to its survival/proliferation capability, is that it is poorly metabolized within lysosomes of macrophages, thereby leading to its prolonged presence or at least that of some of its metabolites [24
25
26
27
28
].
Aggregation
Aggregation is a characteristic of extensively oxidized LDL [29
, 30
] and in addition to chemical modification, could contribute to the poor processing in macrophages [30
]. LDL aggregates are found in human atherosclerotic plaque [31
], and it has been proposed that aggregation in vivo of modified LDL is important in the eventual deposition of intracellular lipid [30
, 31
]. It has not been proven that LDL in lesions is oxidized sufficiently to be the dominant source of sterols in plaques or to be able to induce macrophage survival/growth. We found that aggregation of lightly oxidized LDL dramatically potentiated its ability to stimulate BMM–DNA synthesis [21
], indicating that extensive ox.LDL is not required for this response in vitro and perhaps in vivo. We also found in the same study that plaque-derived lipids could enhance BMM survival.
Effect of CSF-1 and granulocyte M-CSF (GM-CSF)
It is still unclear which factors attract monocytes into the intima in atheroma and control their subsequent differentiation into macrophages and foam cells. CSFs regulate the development of hemopoietic progenitor cells into mature cells by enhanced survival, proliferation, and differentiation [6
]. Two such CSFs are CSF-1 and GM-CSF. These CSFs can also act on the mature cells in the macrophage and/or granulocyte lineages, making it likely that they have a role in the inflammatory processes [32
, 33
]. CSF-1 circulates normally [34
], and its deficiency results in decreased macrophage-lineage numbers in many tissues [35
]. It and its receptor (c-Fms) have been detected in atherosclerotic plaques [36
, 37
]. Vascular endothelial and smooth muscle cells secrete CSF-1 and GM-CSF in vitro in response to a wide range of proinflammatory stimuli, including modified LDL [38
39
40
41
]; also, CSF-1 increases macrophage scavenger receptor expression and function in vitro, as well as cell adhesion [42
]. These findings have led to the suggestion that CSF-1 and GM-CSF production in the atheromatous plaque microenvironment could promote the recruitment and retention of mononuclear phagocytes and subsequent foam-cell formation [15
, 42
].
Given the evidence cited above that foam cells can be observed to be in cell cycle in atherosclerotic lesions and that CSF-1 and GM-CSF expression is associated with macrophage proliferation in such lesions [15
], we reasoned that plaque macrophages might themselves be proliferating under their influence. We found that treatment of BMM with ox.LDL gave rise to an enhanced proliferative response to CSF-1 and GM-CSF [18
]; a synergistic effect was noticeable at suboptimal CSF-1 doses. As CSF-1 circulates at low concentrations that maintain monocyte/macrophage survival [34
, 43
], we have proposed that when macrophages are "loaded" with ox.LDL, they are "primed" so that they are able to proliferate better in the presence of CSF-1 doses that are suboptimal, including "survival" doses, and that may be similar to those found in atheroma [18
] (see below). We have also proposed that more consideration be given in general to studying the effects of stimuli on macrophages in vitro in the presence of circulating CSF-1 concentrations [18
, 20
, 44
45
46
].
Molecular control of ox.LDL-induced macrophage survival/proliferation
A number of different receptors have been reported to be recognized by ox.LDL, including class A type I and II scavenger receptors [17
, 47
], CD36 [48
], Fc receptor [49
], lectin-like ox.LDL receptor-1 [50
], scavenger receptor-class B, type I [51
], and macrosialin [52
]. Further studies are needed to unravel this complexity. It has been claimed that ox.LDL induces differentiation of monocytes/macrophages into foam cells via a peroxisome proliferator-activated receptor-
-mediated mechanism [53
], although the significance of this finding has been questioned [54
, 55
].
Prior to commencing any analysis of the signal-transduction pathways governing ox.LDL-mediated induction of macrophage survival and DNA synthesis, it would seem important that the contribution of any endogenously produced CSF-1 or GM-CSF be determined. If such an indirect mechanism were significant, then signaling pathways governing CSF formation and/or action might be studied inadvertently instead of those relevant to any direct effect of ox.LDL. It has been reported that GM-CSF plays an essential role in ox.LDL-induced macrophage DNA synthesis [56
57
58
59
]. However, using BMM from CSF-deficient mice or blocking antibodies, we could find no evidence for an essential role for endogenous GM-CSF or CSF-1 in ox.LDL-induced macrophage survival and DNA synthesis [18
, 20
]. Using the formazan method for monitoring cell viability (a method we have advocated not to be used in this system [18
, 20
]), similar conclusions to ours have also been drawn by others [60
]. The reasons for these different findings regarding an essential role or not for endogenous GM-CSF are not clear, although they could reflect the different culture conditions used in the studies.
Increases in intracellular Ca2+ [61
, 62
] and protein kinase C (PKC) [61
] have been implicated as early events in ox.LDL-induced macrophage proliferation. We have reported that ox.LDL stimulates extracellular-regulated kinase (Erk)-1, Erk-2, and phosphatidylinositol 3-kinase (PI-3K) activities in BMM but to a weaker extent than optimal CSF-1 concentrations [20
]. Inhibitor studies suggested at least a partial role for these kinases as well as p70S6-kinase in ox-LDL-induced macrophage survival and DNA synthesis. Hundal et al. [60
] have also found induction of Erk1/2 and PKB activities, and using the formazan method to monitor viable cell number and the use of inhibitors, they concluded that ox.LDL inhibits macrophage apoptosis through activation of the PI-3K/PKB pathway but not the Erk pathway. Further molecular analysis needs to be undertaken to define the molecular events governing the enhanced survival/proliferation; we maintain that any such analysis should give consideration to the influence of circulating CSF-1 concentrations. Recent studies have begun to incorporate microarray technology to monitor global gene expression in ox.LDL-treated macrophages [63
].
|
AMYLOIDOGENIC PEPTIDES, MACROPHAGES, AND NEURODEGENERATIVE DISEASE
|
|---|
There are increased numbers of microglial or macrophage-lineage cells in the brain in amyloid-containing plaques in Alzheimer’s disease (AD) and in the lesions of prion diseases [64
, 65
]. AD, like atherosclerosis, has been postulated to result from a chronic, inflammatory state [66
]. The functions of microglia in AD are not known, although they have been considered, for example, as plaque-attacking scavenger cells or as sources of cytokines and other inflammatory mediators. Conversely, they may mediate the clearance of amyloidogenic peptides from the extracellular space. Associated with the transmissible spongiform encephalopathies (prion diseases) is the conversion of the prion protein into an isoform that accumulates in affected individuals, often in the form of extracellular amyloid deposits [65
]. The amyloid ß (Aß) peptide, the major component of AD plaques, and the prion protein isoform decribed above show relative protease resistance and insolubility and form amyloid-like fibrils in vitro [67
, 68
]; also, they are only slowly degraded by macrophages/microglial cells [65
, 69
]. Engorgement of microglia with undigested Aß has been likened to the conversion of macrophages into foam cells [70
], and to extend the analogy further, Aß accumulation has recently been identified in atherosclerotic plaques [71
]. In addition, uptake of an Aß fragment microaggregate is mediated by the type A scavenger receptor on macrophages [70
, 72
]; a recent report, however, has claimed a CD36-mediated, Src-kinase-dependent production of inflammatory mediators in this system. [73
].
Amyloidogenic peptides and microglial and macrophage survival/proliferation
Aß and prion protein have been reported to be mitogenic for microglial cells [74
, 75
]. Because of the above-mentioned protease resistance and their fibrillar nature, as well as the analogy drawn in the literature for foam-cell development referred to above [70
], we determined whether the amyloidogenic peptides might behave like ox.LDL and promote macrophage survival/proliferation. We recently found that synthetic Aß 1–42 and prion protein 106–126 peptides promote BMM survival [46
]. If occurring in vivo, we suggested that enhanced survival of macrophage-lineage cells (macrophages and microglia) would be sufficient to lengthen their tenure in a lesion, leading to more cells being present in the brain lesions (see below).
Effect of CSF-1
It is likely, as with other macrophage-lineage cells in vivo, that brain microglia and macrophages will be exposed normally to low CSF-1 concentrations. In an AD brain, increased CSF-1 receptor and ligand have been observed [76
, 77
]. Thus, it is not unreasonable that the effects of amyloidogenic peptides on macrophages/microglial cells in vitro be studied in the presence of CSF-1. We reported recently that when BMM are exposed to Aß or prion protein peptides, they are able to proliferate more strongly in the presence of suboptimal levels of CSF-1, i.e., those that normally provide a survival signal and/or a weak, proliferative response in vitro [46
]. If this potentiation were occurring in vivo, then it could be contributing to the increased numbers of macrophages and microglia observed in AD lesions [64
] or to the glioses observed in prion disease [65
, 78
]. The enhanced numbers of macrophage-lineage cells in the brain as a result of the above-proposed mechanisms (i.e., enhanced survival or proliferation) would again mean that there are more cells available to produce inflammatory mediators [46
]. Macrophages and microglial cells have been shown to produce such mediators in response to these stimuli [79
, 80
].
|
CRYSTAL-INDUCED INFLAMMATION AND MACROPHAGES
|
|---|
Intra-articular, basic calcium phosphate (BCP; hydroxyapatite) crystal-deposition disease is associated with severe degenerative arthritis [81
], and the interaction of the crystals with inflammatory cells is considered to be a key factor [82
]. Deposition of calcium pyrophosphate dihydrate (CPPD) crystals has been associated with the acute inflammatory arthritis of "pseudogout" involving infiltration by mononuclear inflammatory cells and synovial hyperplasia [83
]; the initiation of acute gout by monosodium urate crystals in the synovium is associated with systemic, inflammatory manifestations [84
]. The degree of inflammation provoked experimentally by crystals in vivo is quite variable [85
]. The capacity of crystal-treated monocytes or macrophages to produce inflammatory cytokines in vitro is likewise variable [86
]. For example, human monocyte-derived macrophages can ingest monosodium urate crystals in the absence of concomitant proinflammatory cytokine synthesis [87
]. Exposure to talc crystals, present in aerosols of respirable talc or on surgical gloves, can lead, respectively, to an inflammatory reaction in the lung or to a macrophage-driven, granulomatous reaction with peritoneal adhesions [88
, 89
].
Crystals, macrophage survival, and DNA synthesis
As discussed, ox.LDL and the amyloidogenic peptides are protease-resistant and can form insoluble, aggregated structures. Therefore, it did not seem unreasonable to test whether the arthritogenic crystals listed above and talc might behave like them in favoring macrophage-lineage survival/proliferation as part of their proinflammatory action. We indeed found recently, as for ox.LDL and amyloidogenic peptides, that BCP, monosodium urate, talc, and to a lesser extent, CPPD crystals promote BMM survival and DNA synthesis [45
]; the latter response was particularly noticeable in the presence of low concentrations of CSF-1. We postulated that such enhanced macrophage survival or proliferation may contribute to the synovial hyperplasia noted in crystal-associated arthropathies, as well as to talc-induced inflammation and granuloma development. Thus, these crystals can be added to the list of poorly degradable materials having these effects on macrophages.
|
PARTICULATE ADJUVANTS
|
|---|
Adjuvants are used to promote an effective, immune response to an antigen. Many adjuvants are particulate in nature, for example, aluminium salts and mineral oil emulsions. These particular materials are also poorly degradable and are likely to accumulate in macrophage-lineage cells. The mode of action of adjuvants is not completely understood, one favored possibility being the depot effect involving entrapment of antigen at the site of injection and the attraction of various kinds of cells, including APCs and macrophages [90
]. Often, they have little intrinsic immunogenicity. APCs or DCs appear to have short half-lives in tissues [91
, 92
]. GM-CSF is implicated in the survival, development, activation, and recruitment of professional APCs from macrophage-lineage precursors and has adjuvant properties [93
].
Adjuvants and macrophage-lineage survival/proliferation
We mentioned above that ox.LDL, even more so when aggregated, promoted BMM survival and a proliferative response, particularly in the presence of low CSF-1 concentrations [21
]. We also found that another modified and poorly soluble protein, namely methylated BSA, could also do this (unpublished). Aggregated proteins, including methylated BSA, are adjuvants [94
]. Calcium phosphate also has adjuvant activity [44
]. We therefore wondered whether a range of particulate adjuvants, particularly if they are likely to be poorly degraded in macrophages, might also promote macrophage survival and DNA synthesis. We found that many poorly degradable, particulate adjuvants [for example, aluminium hydroxide (alum), oil-in-water emulsions, calcium-phosphate suspension (superfos), silica, and heat-killed bacteria] induced murine-macrophage survival and even DNA synthesis [44
]. These responses did not appear to be a result of endogenous GM-CSF or CSF-1. Synergy for the DNA synthesis response was noted in the presence of added GM-CSF or CSF-1.
After endocytosis and migration to lymph nodes in lymphatics, DCs are believed to die in the nodes, presumably by apoptosis; this is because they are not found in efferent lymphatics [95
] and, at least in some tissues such as lung [91
, 92
], normally have a short half-life. Regarding adjuvant action in this context, it is likely that macrophages and/or immature APCs will come into contact with the adjuvant at the site of injection. Based on our data summarized above on the particulate adjuvant-enhanced macrophage survival/proliferation, we have suggested that part of the action of certain particulate adjuvants may be to increase the number of immature DCs and macrophages at sites of immune reactions by providing survival signals [44
]; it could also be that there is a contribution from local cell proliferation, particularly as it is likely that GM-CSF and/or CSF-1 are present at the site of adjuvant action. Ultimate consequences would be more cells available to present antigen and with the potential to produce the relevant cytokines to enhance the immune response. In this connection, during an immune response, some of the large, nonlymphocytic cells in the afferent lymph have monocyte features, including phagocytic activity [95
]; in addition, lymph DCs have phagolysosomes containing debris. It would be of interest to know whether particulate adjuvants themselves can also induce maturation of the target cells to produce DC markers, for example, costimulatory molecules.
Seeing that GM-CSF has adjuvant activity and can also promote macrophage-lineage survival/proliferation, findings supporting our concept outlined above, it is possible that nonparticulate adjuvants may also have this property. As GM-CSF and other adjuvants can act synergistically in potentiating immune reactions in vivo [96
], the synergistic, proliferative, macrophage response between the particulate adjuvants and GM-CSF that we found [44
] may have some in vivo significance.
|
OTHER MATERIALS PROMOTING MACROPHAGE SURVIVAL/PROLIFERATION
|
|---|
Implant biomaterials
The interaction of macrophages and polyethylene-wear particles plays an important role in perpetuating chronic inflammation at the bone implant interface, leading to peri-implant osteolysis and mechanical failure of joint implants. Interaction of human monocyte-derived macrophages with these particles in vitro prolonged cell survival [97
].
Debris
A number of years ago, it was shown that "effete" cells and their debris could promote macrophage proliferation, and it was proposed that cell debris scavenged by macrophages may be important in inducing the growth of tissue macrophages in inflammation, in tumors, or in the normal steady state [98
]. More recently, it was reported that phagocytosis of apoptotic cells by macrophages led to their cytokine-independent survival but inhibition of their proliferation [99
]. The phagocytic uptake of apoptotic cells mediates survival through activation of Akt (PKB) and the effect on proliferation through inhibition of Erk-1 and Erk-2 [99
]. Inhibition of apoptosis and proliferation is an unusual pattern, as most survival factors, especially cytokines, simultaneously inhibit apoptosis and stimulate proliferation [100
, 101
]. Products released by necrotic cell death and stressed or damaged tissues can also act as powerful adjuvants [102
]. Perhaps there is a connection between these observations–such a connection is consistent with one of the major concepts proposed in this review (see below).
Microorganisms
As part of their virulence mechanisms to subvert host defense, many pathogenic microorganisms induce macrophage apoptosis [103
]. However, there are reports that monocyte/macrophage survival is enhanced. For example, infection of human monocytes with Mycobacterium bovisbacillus Calmette-Guerin (BCG) increased cell viability by preventing apoptosis [103
]. As we found in murine BMM [44
], heat-killed BCG also prevented apoptosis, indicating that replication of BCG is not required to prevent cell death. Leishmania donovani and Candida albicans infection enhance BMM and human monocyte viability, respectively [104
, 105
]. It has been hypothesized that microorganisms can modulate the apoptosis program to survive the host immune system [105
]. Inhibition of host cell apoptosis may protect an intracellular pathogen against immune attack outside the cell. Perhaps part of the adjuvant action of heat-killed organisms is a result of their ability to promote macrophage-lineage survival and perhaps can be viewed as mimicry of these pro-survival actions of live organisms.
CpG oligonucleotides
Nonmethylated CpG motifs are common in bacterial DNA but occur considerably less frequently in vertebrate DNA [106
]. Also, although CpG motifs in bacterial DNA are nonmethylated, the vast majority of C and G nucleotides are methylated in eukaryotes. The concept has therefore evolved that CpG dinucleotides and flanking nucleotides are recognized by cells of the immune system to discriminate pathogen-derived DNA from self-DNA [107
]. The powerful adjuvant effect of CpG oligonucleotides has been demonstrated [108
]; their proinflammatory and arthritogenic activities have also been reported [109
]. Consistent with one of the major concepts put forward in this review, namely that adjuvants and proinflammatory materials can promote macrophage-lineage survival, unmethylated CpG dinucleotides are capable of doing this [110
]. However, as for the uptake of apoptotic cells (see above), concomitant inhibition of macrophage proliferation was noted. Whether CpG dinucleotides and bacterial DNA are less-easily degraded than self-DNA is unknown. However, in this connection, CpG oligonucleotides have been noted to accumulate in BMM (David Hume, University of Queensland, Australia, personal communication). We propose in contrast that in macrophage-lineage cells, self-DNA, such as native LDL [24
25
26
27
28
], is likely to be fragmented quickly as part of the normal turnover.
|
"DAMAGED SELF" AND FOREIGNNESS
|
|---|
Many of the materials listed above are self-components that have been modified in some way, for example, ox.LDL, amyloid fibrils, necrotic cells, and methylated BSA, and others are microbial in origin. Two recent theories have addressed the question of what signals initiate an immune reaction and which need to be placed in the context of the ideas put forward in this review. Matzinger [111
, 112
] has introduced "a response to danger" hypothesis, whereby immunity might be guided by ancient signals sent by damaged and dying cells that can act as powerful adjuvants, thereby possibly accounting for antiself-immune responses in the absence of an associated microbial infection. Janeway and Medzhitov’s [113
] "self/non-self" theory predicts the interaction of innate and acquired immune responses through the recognition of specific pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs). It has been proposed recently, as a link between these theories, that in addition to PAMP-derived signals, some of the endogenously released "danger" signals could interact directly with the PRRs [112
, 114
]. As a consequence of these concepts, microbial adjuvants might mimic endogenous signs of damage [112
], and conversely, endogenous adjuvants might mimic microbial products [113
].
What I am proposing in this review bears on why such signals in some cases can lead to an unwanted, persistent response, in the case of a chronic, inflammatory lesion, or can be used to the advantage, in the case of adjuvant action. My main hypothesis incorporates the endogenous (damaged self) and exogenous (microbial) stimuli discussed in the two theories [111
112
113
] but extends these theories by postulating the potential significance of these stimuli, particularly if poorly disposable, in prolonging macrophage-lineage survival/proliferation. It also helps to explain why particlates, such as talc, calcium-phosphate crystals, and alum, can lead to the establishment of chronic lesions or function as adjuvants–none of these materials is damaged self or microbial.
|
OTHER ACTIONS OF "NONDISPOSABLE" MATERIALS
|
|---|
As discussed above, if the concept of the pro-survival/proliferative actions of the non- or poorly disposable materials and adjuvants listed above were relevant, then it would mean that there would be more macrophage-lineage cells present at the site of interaction, capable of activation and/or differentiation in response to additional stimuli. However, it would seem likely that many of the agents discussed above would themselves also elicit inflammatory mediators when they interact with macrophage-lineage cells as a vital part of the progression to a chronic inflammatory lesion or as a key component of adjuvanticity. In other words, additional signals to those merely involving survival/proliferation may be imparted to the cell and would not necessarily have to result from another stimulus. There are reports that ox.LDL, amyloid fibrils, silica, and necrotic cells, among others, can induce such mediators in monocytes/macrophages [115
], including chemokines [116
]. A key pathway leading to these responses is likely to involve nuclear factor (NF)-
B activation [117
]. The contribution of endogenous cytokines or NF-B activation to the pro-survival properties of the agents in question should also therefore be considered. The issue of endogenously generated CSFs as pro-survival factors was addressed above. There is evidence for a role for NF-B in promoting cell survival in some systems [118
].
An important advance to our understanding of the early host-immune response to infection has been the identification of Toll-like receptors as important PRRs of the innate-immune system [113
]. As mentioned earlier, it has recently been suggested that in addition to pathogen-derived signals, some of the endogenously derived danger signals could interact directly with the PRRs [114
, 119
]. This implies that in chronic inflammatory diseases, there might be alternative stimuli to those provided by pathogens, which are maintaining or driving the responses through this receptor system. This particular hypothesis is dependent on cross-reactivity of endogenously generated ligands for PRRs. Some examples of this possibility are now beginning to appear in the literature [114
].
It would also not be unreasonable that following exposure to ox.LDL and adjuvants, among others, macrophage-lineage cells would differentiate or change their phenotype. Evidence for this concept has been presented, for example, for ox.LDL-induced differentiation of moncytes to foam cells [53
]; also, adjuvants themselves can mature DCs.
Antigen-presenting DCs initiate immune responses after they capture antigen from peripheral tissues and then migrate to lymph nodes where they efficiently interact with T cells. To study how monocytes become DCs in a tissue setting, a "reverse transmigration" model has been developed in which some monocytes differentiate into DCs in response to cues that are endogenous to an endothelial cell/collagen matrix system [95
]. It is interesting that in the context of the concepts outlined above, when phagocytic particulates such as zymosan are incorporated in the underlying collagenous matrix, reverse-transmigrated cells acquire phenotypic and functional features of mature, terminally differentiated DCs. Furthermore, a recent study has shown that the CD16+ subpopulation of monocytes has the greatest propensity among monocytes to develop into migratory DCs, and CD16 participates in the survival of these cells in response to zymosan activation [120
]. Perhaps the pro-survival functions of adjuvants are involved in the migration and maturational development of lymph-homing DCs, processes that now appear to be linked.
|
CONCLUSION
|
|---|
One prominent feature of chronic inflammatory/autoimmune lesions is the increased numbers of macrophage-lineage cells over a long time period. One commonly proposed mechanism for this persistent response is a "vicious cycle" created by the generation of self-antigens following tissue damage; another favored theory is that the chronic stimulus is propagated by an infectious insult. Many have also considered that the normal balance between cell survival and death by apoptosis is perturbed in some way in favor of the former. We have previously put forward one possible mechanism for this shift in such a balance via a so-called "CSF network" [32
, 33
]; in essence, the hypothesis considers that CSFs generated locally can promote survival/proliferation (and activation) of macrophage and granulocytic-lineage cells as a positive feedback loop, leading to the steady-state increase in inflammatory cells. In the current review, I have collected the evidence for another way in which macrophage-lineage cell numbers might be increased chronically. In this concept, non- or poorly disposable materials, often particulate or fibrillar in nature, deliver a pro-survival signal(s) to macrophage-lineage cells (Fig. 1
). Again, as discussed earlier in this review, the end result would be more cells at a site of inflammation being available to produce inflammatory mediators, directly in response to the pro-survival signal itself or induced by response to additional signals. It should be borne in mind that "toxic" effects on monocytes/macrophages of many of the materials discussed in this review have been reported in the literature. As we found for ox.LDL [19
, 21
], in my view, toxic effects most likely occur at high concentrations, and any pro-survival actions will occur at lower concentrations.
View larger version (16K):
[in this window]
[in a new window]
|
Figure 1. Effect of non- or poorly disposable materials on macrophage-lineage survival. For macrophage-lineage cells, uptake of non- or poorly disposable materials can shift the balance between apoptosis and survival in favor of the latter. The net, long-term result is more cells able to produce mediators for the perpetuation of a chronic, inflammatory lesion, for example, an atherosclerotic plaque, or as part of the action of adjuvants. The loaded cells may also differentiate, for example, into foam cells or to DCs to enhance adjuvant action. The pro-survival materials may or may not themselves provide the additional signals required for the other cellular responses.
|
|
As mentioned, many of the materials discussed could act in part through PRRs. If proinflammatory materials are continually being made available during a host response, then they will be able to signal to keep a macrophage-lineage cell alive, activate it, or induce differentiation; this is an obvious way in which a response to many stimuli might be extended. However, in addition, it is proposed in this review that a persistent signal could also be generated by certain agents of the type discussed as a result of the inherent resistance to breakdown with the potential to generate a chronic lesion [18
, 21
, 44
45
46
]. In other words, signaling occurs as long as the cell is attempting to remove the resistant material itself, presumably in lysosomes, or its decomposition product(s). It is also possible that as a result of the particulate or aggregate nature of some of the stimuli, cell entry will be by phagocytosis, thereby providing another potential signaling mechanism.
Many of the agents listed as being capable of enhancing cell survival also have been shown to have adjuvant activity. Therefore, an additional concept outlined above is that the pro-survival action of poorly disposable (e.g., alum, oil-in-water emulsions) and other (e.g., GM-CSF) adjuvants is a key component of adjuvant activity (Fig. 1)
. Activation and/or subsequent differentiation of the target cells into more mature DCs are also likely to occur.
The macrophage-lineage cells eventually dispose of some of the stimuli considered above, and there is therefore likely to be a continuum in the time frame of the respective pro-survival effects. This issue is considered in Figure 2
, where the inverse correlation between material "disposability" on the one hand and the resultant lesion chronicity and/or adjuvanticity on the other are depicted.
View larger version (20K):
[in this window]
[in a new window]
|
Figure 2. Inverse relationship between material "disposability" and the chronicity of an inflammatory reaction or adjuvanticity. For reasons outlined in Figure 1
, materials that are the least "disposable" by macrophage-lineage cells (for example, talc and alum) are more likely per se to be the most potent at perpetuating an inflammatory reaction and/or to have the potential to make the best adjuvants. Materials that are more readily "disposable" (for example, dead cells) would provide pro-survival signals only while being degraded.
|
|
In Figures 1
and 2
, as in most studies with monocytes/macrophages in vitro, the cells are shown as being treated in the absence of exogenously added CSF-1. However, macrophage-lineage cells in vivo are likely to be exposed to pro-survival concentrations of this cytokine unless macrophage-lineage cell numbers rise, leading to its consumption [121
]. Therefore, it is also suggested [18
, 20
, 21
, 44
, 46
] that monocyte/macrophage biology in vitro should be considered in the presence of these steady-state concentrations of CSF-1. As presented above, when BMM are treated with the poorly disposable materials under these conditions, a proliferative response in fact ensues (Fig. 3A
). This shift in the nature of the response to different CSF-1 concentrations is also depicted in Figure 3B
. In my view, the macrophage proliferation observed at sites of inflammation in many reports [1
2
3
4
5
] (see earlier) is not widely enough considered as a mechanism for increasing their numbers at such sites, particularly over the lengthy time frames involved in chronic lesions. In this connection, it should be noted and again, not widely recognized, that there is now strong evidence for local control of macrophage generation in the steady state, for example, in the peritoneum [122
]. However, whether a particular macrophage-lineage population proliferates following dual exposure to the stimuli, depicted in Figure 3A
and 3B
, will be determined by its proliferative capability.
View larger version (20K):
[in this window]
[in a new window]
|
Figure 3. Effect of non- or poorly disposable materials in the presence of CSF-1 (M-CSF). Macrophage-lineage cells in vivo are most likely to be exposed to at least pro-survival concentrations of CSF-1, thus shifting the balance between apoptosis and survival in favor of the latter. If a certain macrophage population, such as BMM, also internalizes non- or poorly disposable materials under these conditions, they can proliferate (A). In other words, pro-survival or circulating levels of CSF-1 can become mitogenic if the macrophages are also loaded with non- or poorly disposable materials (B); i.e., there is a shift in the CSF-1 dose response for proliferation. Solid line, Normal CSF-1 dose response for macrophage survival and proliferation; the extent of the latter response increases with ligand concentration. Dashed line, CSF-1 dose response of macrophages "loaded" with non- or poorly disposable material.
|
|
|
ACKNOWLEDGEMENTS
|
|---|
The author is a Senior Principal Research Fellow of the National Health and Medical Research Council of Australia. R. Sallay and E. Tully are thanked for typing the manuscript.
Received January 22, 2003;
revised March 3, 2003;
accepted March 4, 2003.
|
REFERENCES
|
|---|
- Rosenfeld, M. E., Ross, R. (1990) Macrophage and smooth muscle cell proliferation in atherosclerotic lesions of WHHL and comparably hypercholesterolemic fat-fed rabbits Arteriosclerosis 10,680-687[Abstract/Free Full Text]
- Gordon, D., Reidy, M. A., Benditt, E. P., Schwartz, S. M. (1990) Cell proliferation in human coronary arteries Proc. Natl. Acad. Sci. USA 87,4600-4604[Abstract/Free Full Text]
- Spagnoli, L. G., Orlandi, A., Santeusanio, G. (1991) Foam cells of the rabbit atherosclerotic plaque arrested in metaphase by colchicine show a macrophage phenotype Atherosclerosis 88,87-92[CrossRef][Medline]
- Jutila, M. A., Banks, K. L. (1988) Increased macrophage division in the synovial fluid of goats infected with caprine arthritis-encephalitis virus J. Infect. Dis. 157,1193-1202[Medline]
- Yamada, M., Naito, M., Takahashi, K. (1990) Kupffer cell proliferation and glucan-induced granuloma formation in mice depleted of blood monocytes by strontium-89 J. Leukoc. Biol. 47,195-205[Abstract]
- Metcalf, D. (1989) The molecular control of cell division, differentiation commitment and maturation in haemopoietic cells Nature 339,27-30[CrossRef][Medline]
- Greenhalgh, D. G. (1998) The role of apoptosis in wound healing Int. J. Biochem. Cell Biol. 30,1019-1030[CrossRef][Medline]
- Thompson, C. B. (1995) Apoptosis in the pathogenesis and treatment of disease Science 267,1456-1462[Abstract/Free Full Text]
- Ross, R. (1993) The pathogenesis of atheroslerosis: a perspective for the 1990s Nature 362,801-809[CrossRef][Medline]
- Pasceri, V., Yeh, E. T. (1999) A tale of two diseases: atherosclerosis and rheumatoid arthritis Circulation 100,2124-2126[Free Full Text]
- Steinberg, D., Parthasarathy, S., Carew, T. E., Khoo, J. C., Witztum, J. L. (1989) Modifications of low-density lipoprotein that increase its atherogenicity N. Engl. J. Med. 320,915-924[Medline]
- Villaschi, S., Spagnoli, L. G. (1983) Autoradiographic and ultrastructural studies on the human fibro- atheromatous plaque Atherosclerosis 48,95-100[CrossRef][Medline]
- Katsuda, S., Coltrea, M. D., Ross, R., Gown, A. M. (1993) Human atherosclerosis IV. Immunocytochemical analysis of cell activation and proliferation in lesions of young adults Am. J. Pathol. 142,1787-1792[Abstract]
- Rekhter, M. D., Gordon, D. (1995) Active proliferation of different cell types, including lymphocytes, in human atherosclerotic plaques Am. J. Pathol. 147,668-677[Abstract]
- Wang, J., Wang, S., Lu, Y., Weng, Y., Gown, A. M. (1994) GM-CSF and M-CSF expression is associated with macrophage proliferation in progressing and regressing rabbit atheromatous lesions Exp. Mol. Pathol. 61,109-118[CrossRef][Medline]
- Yui, S., Sasaki, T., Miyazaki, A., Horiuchi, S., Yamazaki, M. (1993) Induction of murine macrophage growth by modified LDLs Arterioscler. Thromb. 13,331-337[Abstract/Free Full Text]
- Martens, J. S., Reiner, N. E., Herrera-Velit, P., Steinbrecher, U. P. (1998) Phosphatidylinositol 3-kinase is involved in the induction of macrophage growth by oxidized low density lipoprotein J. Biol. Chem. 273,4915-4920[Abstract/Free Full Text]
- Hamilton, J. A., Myers, D., Jessup, W., Cochrane, F., Byrne, R., Whitty, G., Moss, S. (1999) Oxidized LDL can induce macrophage survival, DNA synthesis, and enhanced proliferative response to CSF-1 and GM-CSF Arterioscler. Thromb. Vasc. Biol. 19,98-105[Abstract/Free Full Text]
- Hamilton, J. A., Whitty, G., Jessup, W. (2000) Oxidized LDL can promote human monocyte survival Arterioscler. Thromb. Vasc. Biol. 20,2329-2331[Free Full Text]
- Hamilton, J. A., Byrne, R., Jessup, W., Kanagasundaram, V., Whitty, G. (2001) Comparison of macrophage responses to oxidized low-density lipoprotein and macrophage colony-stimulating factor (M-CSF or CSF-1) Biochem. J. 354,179-187[CrossRef][Medline]
- Hamilton, J. A., Jessup, W., Brown, A. J., Whitty, G. (2001) Enhancement of macrophage survival and DNA synthesis by oxidized-low-density-lipoprotein (LDL)-derived lipids and by aggregates of lightly oxidized LDL Biochem. J. 355,207-214[CrossRef][Medline]
- Reid, V. C., Mitchinson, M. J., Skepper, J. N. (1993) Cytotoxicity of oxidized low-density lipoprotein to mouse peritoneal macrophages: an ultrastructural study J. Pathol. 171,321-328[CrossRef][Medline]
- Hardwick, S. J., Hegyi, L., Clare, K., Law, N. S., Carpenter, K. L., Mitchinson, M. J., Skepper, J. N. (1996) Apoptosis in human monocyte-macrophages exposed to oxidized low density lipoprotein J. Pathol. 179,294-302[CrossRef][Medline]
- Hoppe, G., O’Neil, J., Hoff, H. F. (1994) Inactivation of lysosomal proteases by oxidized low density lipoprotein is partially responsible for its poor degradation by mouse peritoneal macrophages J. Clin. Invest. 94,1506-1512
- Lougheed, M., Zhang, H. F., Steinbrecher, U. P. (1991) Oxidized low density lipoprotein is resistant to cathepsins and accumulates within macrophages J. Biol. Chem. 266,14519-14525[Abstract/Free Full Text]
- Roma, P., Bernini, F., Fogliatto, R., Bertulli, S. M., Negri, S., Fumagalli, R., Catapano, A. L. (1992) Defective catabolism of oxidized LDL by J774 murine macrophages J. Lipid Res. 33,819-829[Abstract]
- Dhaliwal, B. S., Steinbrecher, U. P. (2000) Cholesterol delivered to macrophages by oxidized low density lipoprotein is sequestered in lysosomes and fails to efflux normally J. Lipid Res. 41,1658-1665[Abstract/Free Full Text]
- Jessup, W., Kritharides, L. (2000) Metabolism of oxidized LDL by macrophages Curr. Opin. Lipidol. 11,473-481[CrossRef][Medline]
- Tertov, V. V., Orekhov, A. N., Sobenin, I. A., Gabbasov, Z. A., Popov, E. G., Yaroslavov, A. A., Smirnov, V. N. (1992) Three types of naturally occurring modified lipoproteins induce intracellular lipid accumulation due to lipoprotein aggregation Circ. Res. 71,218-228[Abstract/Free Full Text]
- Hoff, H. F., Zyromski, N., Armstrong, D., O’Neil, J. (1993) Aggregation as well as chemical modification of LDL during oxidation is responsible for poor processing in macrophages J. Lipid Res. 34,1919-1929[Abstract]
- Steinbrecher, U. P., Lougheed, M. (1992) Scavenger receptor-independent stimulation of cholesterol esterification in macrophages by low density lipoprotein extracted from human aortic intima Arterioscler. Thromb. 12,608-625[Abstract/Free Full Text]
- Hamilton, J. A. (1993) Rheumatoid arthritis: opposing actions of haemopoietic growth factors and slow-acting anti-rheumatic drugs Lancet 342,536-539[CrossRef][Medline]
- Hamilton, J. A. (2002) GM-CSF in inflammation and autoimmunity Trends Immunol 23,403-408[CrossRef][Medline]
- Bartocci, A., Pollard, J. W., Stanley, E. R. (1986) Regulation of colony-stimulating factor 1 during pregnancy J. Exp. Med. 164,956-961[Abstract/Free Full Text]
- Cecchini, M. G., Dominguez, M. G., Mocci, S., Wetterwald, A., Felix, R., Fleisch, H., Chisholm, O., Hofstetter, W., Pollard, J. W., Stanley, E. R. (1994) Role of colony stimulating factor-1 in the establishment and regulation of tissue macrophages during postnatal development of the mouse Development 120,1357-1372[Abstract]
- Rosenfeld, M. E., Yla-Herttuala, S., Lipton, B. A., Ord, V. A., Witztum, J. L., Steinberg, D. (1992) Macrophage colony-stimulating factor mRNA and protein in atherosclerotic lesions of rabbits and humans Am. J. Pathol. 140,291-300[Abstract]
- Salomon, R. N., Underwood, R., Doyle, M. V., Wang, A., Libby, P. (1992) Increased apolipoprotein E and c-fms gene expression without elevated interleukin 1 or 6 mRNA levels indicates selective activation of macrophage functions in advanced human atheroma Proc. Natl. Acad. Sci. USA 89,2814-2818[Abstract/Free Full Text]
- Rajavashisth, T. B., Andalibi, A., Territo, M. C., Berliner, J. A., Navab, M., Fogelman, A. M., Lusis, A. J. (1990) Induction of endothelial cell expression of granulocyte and macrophage colony-stimulating factors by modified low-density lipoproteins Nature 344,254-257[CrossRef][Medline]
- Sieff, C. A., Niemeyer, C. M., Mentzer, S. J., Faller, D. V. (1988) Interleukin-1, tumor necrosis factor, and the production of colony-stimulating factors by cultured mesenchymal cells Blood 72,1316-1323[Abstract/Free Full Text]
- Schrader, J. W., Moyer, C., Ziltener, H. J., Reinisch, C. L. (1991) Release of the cytokines colony-stimulating factor-1, granulocyte-macrophage colony-stimulating factor, and IL-6 by cloned murine vascular smooth muscle cells J. Immunol. 146,3799-3808[Abstract]
- Filonzi, E. L., Zoellner, H., Stanton, H., Hamilton, J. A. (1993) Cytokine regulation of granulocyte-macrophage colony stimulating factor and macrophage colony-stimulating factor production in human arterial smooth muscle cells Atherosclerosis 99,241-252[CrossRef][Medline]
- de Villiers, W. J., Fraser, I. P., Hughes, D. A., Doyle, A. G., Gordon, S. (1994) Macrophage-colony-stimulating factor selectively enhances macrophage scavenger receptor expression and function J. Exp. Med. 180,705-709[Abstract/Free Full Text]
- Tushinski, R. J., Oliver, I. T., Guilbert, L. J., Tynan, P. W., Warner, J. R., Stanley, E. R. (1982) Survival of mononuclear phagocytes depends on a lineage-specific growth factor that the differentiated cells selectively destroy Cell 28,71-81[CrossRef][Medline]
- Hamilton, J. A., Byrne, R., Whitty, G. (2000) Particulate adjuvants can induce macrophage survival, DNA synthesis, and a synergistic proliferative response to GM-CSF and CSF-1 J. Leukoc. Biol. 67,226-232[Abstract]
- Hamilton, J. A., McCarthy, G., Whitty, G. (2001) Inflammatory microcrystals induce murine macrophage survival and DNA synthesis Arthritis Res 3,242-246[CrossRef][Medline]
- Hamilton, J. A., Whitty, G., White, A. R., Jobling, M. F., Thompson, A., Barrow, C. J., Cappai, R., Beyreuther, K., Masters, C. L. (2002) Alzheimer’s disease amyloid beta and prion protein amyloidogenic peptides promote macrophage survival, DNA synthesis and enhanced proliferative response to CSF-1 (M-CSF) Brain Res 940,49-54[CrossRef][Medline]
- Sakai, M., Miyazaki, A., Hakamata, H., Sasaki, T., Yui, S., Yamazaki, M., Shichiri, M., Horiuchi, S. (1994) Lysophosphatidylcholine plays an essential role in the mitogenic effect of oxidized low density lipoprotein on murine macrophages J. Biol. Chem. 269,31430-31435[Abstract/Free Full Text]
- Endemann, G., Stanton, L. W., Madden, K. S., Bryant, C. M., White, R. T., Protter, A. A. (1993) CD36 is a receptor for oxidized low density lipoprotein J. Biol. Chem. 268,11811-11816[Abstract/Free Full Text]
- Stanton, L. W., White, R. T., Bryant, C. M., Protter, A. A., Endemann, G. (1992) A macrophage Fc receptor for IgG is also a receptor for oxidized low density lipoprotein J. Biol. Chem. 267,22446-22451[Abstract/Free Full Text]
- Sawamura, T., Kume, N., Aoyama, T., Moriwaki, H., Hoshikawa, H., Aiba, Y., Tanaka, T., Miwa, S., Katsura, Y., Kita, T., Masaki, T. (1997) An endothelial receptor for oxidized low-density lipoprotein Nature 386,73-77[CrossRef][Medline]
- Acton, S. L., Scherer, P. E., Lodish, H. F., Krieger, M. (1994) Expression cloning of SR-BI, a CD36-related class B scavenger receptor J. Biol. Chem. 269,21003-21009[Abstract/Free Full Text]
- Ramprasad, M. P., Fischer, W., Witztum, J. L., Sambrano, G. R., Quehenberger, O., Steinberg, D. (1995) The 94- to 97-kDa mouse macrophage membrane protein that recognizes oxidized low density lipoprotein and phosphatidylserine-rich liposomes is identical to macrosialin, the mouse homologue of human CD68 Proc. Natl. Acad. Sci. USA 92,9580-9584[Abstract/Free Full Text]
- Tontonoz, P., Nagy, L., Alvarez, J. G., Thomazy, V. A., Evans, R. M. (1998) PPARgamma promotes monocyte/macrophage differentiation and uptake of oxidized LDL Cell 93,241-252[CrossRef][Medline]
- Li, A. C., Brown, K. K., Silvestre, M. J., Willson, T. M., Palinski, W., Glass, C. K. (2000) Peroxisome proliferator-activated receptor gamma ligands inhibit development of atherosclerosis in LDL receptor-deficient mice J. Clin. Invest. 106,523-531[Medline]
- Rosen, E. D., Spiegelman, B. M. (2000) Peroxisome proliferator-activated receptor gamma ligands and atherosclerosis: ending the heartache J. Clin. Invest. 106,629-631[Medline]
- Biwa, T., Hakamata, H., Sakai, M., Miyazaki, A., Suzuki, H., Kodama, T., Shichiri, M., Horiuchi, S. (1998) Induction of murine macrophage growth by oxidized low density lipoprotein is mediated by granulocyte macrophage colony-stimulating factor J. Biol. Chem. 273,28305-28313[Abstract/Free Full Text]
- Biwa, T., Sakai, M., Matsumura, T., Kobori, S., Kaneko, K., Miyazaki, A., Hakamata, H., Horiuchi, S., Shichiri, M. (2000) Sites of action of protein kinase C and phosphatidylinositol 3-kinase are distinct in oxidized low density lipoprotein-induced macrophage proliferation J. Biol. Chem. 275,5810-5816
TOP
ABSTRACT
INTRODUCTION