In our tissues there is a resident population of wandering white blood cells; the macrophages. Macrophages grab onto
and digest whatever "garbage" or foreign particles they come across. When they encounter and engulf invaders from the outside
world,such as bacteria from a wound.
The macrophages release chemicals that cause local blood vessels to leak fluid into the surrounding tissues,which give
rises rise to swelling. Vessels existing the area become narrowed,promoting a builup of blood resulting in localized redness.
Macrophages also release chemicals that attract other specilized white blood cells to the area. These new arrivals join
in the battle in various ways. The increased cellur chemical reactions (metabolism) in the irritated area generate warmth.
Some of the released,along with the mounting pressure in the swelling tissues, incite painful sensations. All of this
results in the classic signs of inflammation; pain,swelling,ewdness,and heat. When the invading agent has been eliminated,
the inflammation subsides and disappears.
There are several important differences between this scenario resulting from a wound and the inflammation that goes on
in RA. While much of the process is identical,the triggering event ;i.e.,what sets of the inflammation in RA ;is still not
While investigative study goes on for bacteria or viruses that might play a role. RA is regarded by many scientists as
not one disease but rather a common response to what could be a number of different stimuli.
In RA two of the types of white blood cells that answer the call of the macrophages, The T cells and B cells,somehow
escape the controls that normally monitor and regulate their activitis. They enter into a self-perpetuating cycle of inflammation
and damage ;the very heart of the problem in RA. They just keep on fighting,when,they should stop.
Macrophages appear to play a pivotal role in RA because they are numerous in the inflamed synovial membrane and at the
cartilage-pannus junction. They show clear signs of activation, such as overexpression of major histocompatibility complex
class II molecules, proinflammatory or regulatory cytokines and growth factors [eg IL-1, IL-6, IL-10, IL-13, IL-15, IL-18,
tumour necrosis factor (TNF)-a and granulocyte-macrophage colony-stimulating factor (GM-CSF)], chemokines and chemoattractants
[eg IL-8, macrophage inflammatory protein (MIP)-1 and monocyte chemoattractant protein (MCP)-1], metalloproteinases and neopterin.
It is unlikely that macrophages occupy a causal position in the pathogenesis of RA (except for their function as antigen-presenting
cells in the hypothesis of a primary autoimmune disorder). However, these cells of the innate immune system possess broad
proinflammatory, destructive and remodelling capacities, and considerably contribute to inflammation and joint destruction
both in the acute and chronic phases of RA. Also, activation of the monocytic lineage is not restricted to synovial macrophages,
but extends to circulating monocytes and other cells of the mononuclear phagocyte system, including precursors of the myelomonocytic
lineage in the bone marrow.
Thus, even before the cause of RA is identified [whether it is an infectious cause, or a clearly defined (auto)antigen
or a genetic alteration (or several)], several facts render monocytes/macrophages attractive therapeutic targets. The first
is the correlation between radiological progression of joint destruction and degree of synovial macrophage infiltration.
Second, the coincidence of therapeutic efficacy of conventional antirheumatic therapy with downregulation of functions
of the mononuclear phagocyte system is in accord with the increasing knowledge of macrophage-specific effects of such
Third, the efficacy of biological therapies directed at cytokines produced predominantly by macrophages has been demonstrated.
Fourth, conventional or experimental drugs can be targeted at macrophages, including their subcellular compartments.
The final factor in support of targeting monocytes/macrophages for treatment of RA is the differential activation of
intracellular signal transduction pathways that underlie different macrophage effector functions, in conjunction with the
availability of more specific inhibitors of key metabolic enzymes and/or particular signal transduction pathways.
Cells of the myelomonocytic lineage differentiate into several cell types that are involved in disease (ie monocytes/macrophages,
osteoclasts and dendritic cells). As a result of their marked plasticity, these differentiation pathways can be influenced
by particular pathophysiological stimuli (eg an excess/imbalance of cytokines or growth factors), resulting in altered differentiation
or maturation if regulatory mechanisms fail. In RA, such alterations have been already described at several levels (ie in
inflamed joints, peripheral blood and bone marrow).
In the RA synovial membrane, a first differentiation step is that from recently immigrated monocytes to mature macrophages.
These subsets differentially colonise the synovial sublining and lining layer, respectively, as well as the superficial and
deep layers of the lining. A possible functional diversity in these areas, which is emphasized by the expression of different
activation markers and adhesion molecules, may differentially contribute to disease progression.
Locally, synovial macrophages also differentiate into stimulatory or inhibitory subpopulations, which are known to influence
T-cell reactivity differentially. In experimental arthritis, for example, immunoregulatory macrophages greatly affect T-cell
responses to arthritogenic proteins and, upon nonselective targeting, influence clinical and histopathological improvement
In RA, macrophage subpopulations may be responsible for the separate synthesis of proinflammatory (eg IL-1 and TNF-a)
or regulatory cytokines (eg IL-10), the balance of which is critical to perpetuation of disease. A subset of synovial macrophages
may also exert a predominant role in angiogenetic processes.
In the RA synovial fluid, a subset of mononuclear cells presents with a double phenotype of activated T cells and macrophage-like
cells. Whether this promiscuity is related to plasticity of common precursors, or is the epiphenomenon of a central differentiation
defect remains to be clarified.
Alterations in the macrophage lineage are also evident in extrasynovial compartments. In the bone marrow, RA patients
with active or severe disease display faster generation of CD14+ myelomonocytic cells and faster differentiation into human
leucocyte antigen (HLA)-DR+ cells than do control individuals. Myeloid precursors are also elevated in the bone marrow adjacent
to rheumatoid joints, in correlation with the local levels of IL-1.
A common potential trigger for these bone marrow alterations may be the altered monokine (eg IL-1, IL-6, TNF-a) or growth
factor milieu (eg GM-CSF), which builds up in the circulation, and in the bone marrow of RA patients. On the other hand, the
in vitro differentiation of bone marrow precursor cells becomes insensitive to GM-CSF, suggesting intrinsic alterations in
the myelomonocytic lineage.
Possible bone marrow anomalies may also underlie the presence of highly proliferative potential colony-forming cells
in the peripheral blood of RA patients with severe disease and high incidence of interstitial pulmonary involvement, a factor
of poor prognosis in RA. Finally, bone marrow stromal cells also overexpress bone marrow stromal antigen (BST)-1, a pre-B-cell
growth factor that is significantly elevated in the sera of patients with severe RA, with growth inhibition effects on monocytes/macrophages.
These observations, as well as that of the existence of a macrophage activation syndrome in severe cases of systemic
juvenile RA, suggest that the spectrum of arthritis severity may be associated with the degree of systemic activation of monocytes/macrophages.
This is also supported by the extra-articular terminal differentiation of macrophages within rheumatoid nodules, the latter
a sign of clinical severity.
The involvement of the myelopoietic system in RA may also partly explain the mode of action of slow-acting antirheumatic
drugs, possibly targeting altered precursors, or that of stem-cell transplantation therapy.
Synovial membrane: In the RA synovial membrane, a surface layer of HLA-DR+, CD14+ and CD68+ macrophages is typically
followed by a layer of fibroblasts. Below the lining layer, macrophages are distributed in lymphoid aggregates or in diffuse
infiltrates, in the former case adjacent to activated CD4+ lymphoid cells and in the latter case near CD8+ T cells, suggesting
active participation in possible (auto)immune processes.
In addition, macrophages are located close to synovial fibroblast-like cells that display a typical morphology, which
are believed to be centrally involved in tissue destruction.
The degree of macrophage infiltration/activation correlates not only with the joint pain and general inflammatory status
of the patient, but also with the radiological progression of permanent joint damage, the disease feature that ultimately
determines quality of life.
In chronic RA, the prevalence of certain histological configurations may represent an important variable for the clinical
course. High TNF-a and IL-1ß production, for example, may be associated with granulomatous synovitis, a rare condition that
is more frequently associated with subcutaneous rheumatoid nodules.
Conversely, these cytokines appear to be modestly elevated in diffuse synovitis, which may be associated with seronegative
RA. These features may also explain some variability among studies on the abundance of TNF-a and/or TNF-a receptor expression
in the RA synovial membrane, and, possibly, the variable sensitivity to anti-TNF-a therapy.
Myeloid-related dendritic cells are also enriched in RA synovial compartments. Their efficacy as antigen-presenting cells
and their interdigitating location in perivascular lymphoid aggregates are optimal prerequisites for the presentation of putative
arthritogenic antigens to T cells and for the regulation of B cells.
Cartilage-pannus and bone-pannus junction: At the site of tissue destruction, macrophages express significant amounts
of the inflammatory cytokines IL-1, TNF-a and GM-CSF and contribute to the production of the proteases collagenase, stromelysin,
gelatinase B and leucocyte elastase.
Although gelatinase B levels positively correlate with disease progression and severity, the potential of macrophages
to degrade cartilage matrix components directly may be modest, assigning macrophages the position of amplifyers of the pathogenetic
cascade (especially via activation of fibroblasts) rather than primary effectors of tissue destruction. The situation may
be quite different at the bone-pannus junction, where osteoclasts derived from the myelomonocytic lineage strongly contribute
to bone erosion, possibly under the influence of local cell-cell contact and abundant cytokines.
Peripheral blood: The activation of circulating monocytes in RA, although unclear in its extent, is evidenced by the
following: spontaneous production of prostanoids and prostaglandin E2, cytokines, soluble CD14 and neopterin, the latter a
molecule exclusively produced by human mononuclear phagocytes in correlation with disease activity. Increased production of
the matrix-degrading enzyme gelatinase B and the metalloprotease inhibitor tissue inhibitor of metalloproteinase (TIMP)-1,expression
of manganese superoxide dismutase, a critical enzyme for the control of oxygen radicals,increased phagocytic activity; increased
integrin expression and monocyte adhesiveness; presence of activated suppressor monocytes; and, more generally, gene activation
with a pattern closely resembling the synovial activation pattern.
Differential analysis of gene patterns in RA monocytes collected upon initial and final therapeutic leukapheresis (a
procedure that induces clinical remission in severe RA, presumably by reducing the degree of monocyte activation) has recently
shed light on gene expression at different stages of monocyte activation. In addition to the expected cytokines (IL-1a, IL-1ß,
IL-6, TNF-a), gene activation in the florid stage of disease has also been documented for growth-related oncogene protein
(GRO)-a/melanoma growth-stimulatory activity, MIP-2/GRO-ß, ferritin, a1-antitrypsin, lysozyme, transaldolase, Epstein-Barr
virus encoded small nuclear RNA (EBER)-1/EBER-2-associated protein, thrombospondin-1, an angiotensin receptor-II carboxyl-terminal
homologue, the RNA polymerase-II elongation factor, and for five unknown/functionally undefined genes.
Because a number of these molecules (IL-1a, IL-1ß, IL-6, TNF-a, GRO-a, ferritin, lysozyme and thrombospondin-1) are also
(over)expressed in RA joints, monocytes appear to be preshaped in a 'rheumatoid' phenotype before their entry into the inflamed
synovial tissue. This is further supported by overexpression of the human cartilage glycoprotein gp39, a late macrophage differentiation
marker, in circulating monocytes and tissue macrophages. The findings regarding novel or functionally undefined genes also
indicate that the extent of systemic monocyte/macrophage activation remains to be fully explored.
Cell-cell interaction: A significant part of macrophage effector responses occurs in the absence of soluble stimuli,
through cell contact-dependent signalling with several inflammatory cells.
T cell-macrophage interaction: Accessory, inflammatory, effector and inhibitory macrophage functions can be stimulated
by paraformaldehyde-fixed T cells or plasma membranes of T cells, provided that these are preactivated and express the surface
molecules that are involved in such stimulation (eg CD69). In response to such interaction, monocytes produce metallproteinases,
IL-1a and IL-1ß. Also, T cells that are prestimulated in an antigen-mimicking manner stimulate TNF-a and IL-10 production,
once they are in contact with monocytes.
Conversely, fixed T cells stimulated in an antigen-independent manner (ie with IL-15, IL-2, or a combination of IL-6
and TNF-a) induce monocyte production of TNF-a, but not IL-10. These findings have resulted in the hypothesis that early RA
reflects antigen-specific T cell-macrophage interactions. Conversely, chronic RA may be associated with antigen-independent
interactions, which are dominated by an exuberant cytokine milieu. This model would also explain the relative paucity of IL-10
in the RA synovial membrane.
Fibroblast-macrophage interaction: Because macrophages and fibroblasts appear to be the most active cells in the RA synovium,
their interaction is particularly interesting in view of the resulting inflammation and tissue damage. Indeed, the mere contact
of these cells elicits the production of IL-6, GM-CSF and IL-8. The cytokine output can be enhanced or downregulated not only
by addition of proinflammatory or regulatory cytokines (eg IL-4, IL-10, IL-13 or IL-1 receptor antagonist), but also by neutralization
of the CD14 molecule.
Also, in vitro model systems show significant cartilage degradation by cocultures of mouse fibroblasts and macrophages,
a response markedly exceeding that observed with each culture alone.
More recent studies have shown that purified human synovial fibroblasts cocultured with myelomonocytic cell lines induce
cartilage degradation in vitro. The mechanism of this response, however, seems to be paracrine- and not cell contact-mediated,
because cartilage degradation is most effectively blocked by a mixture of anti-IL-1 and anti-TNF-a monoclonal antibodies.
Soluble stimuli ;Proinflammatory cytokines: IL-15, an IL-2-like cytokine with chemoattractant properties for memory T
cells, is highly augmented in RA synovial fluid, being produced by lining layer cells, including macrophages. Notably, IL-15-stimulated
peripheral blood or synovial T cells induce macrophages to produce IL-1ß, TNF-a, IL-8 and MCP-1, but not IL-10. Because IL-15
is also produced by macrophages themselves, this cytokine may (re)stimulate T cells, possibly contributing to a self-perpetuating
The lymphokine IL-17, which is produced in approximately 90% of RA synovial explant cultures, but only in 16% of osteoarthritis
cultures, strongly stimulates macrophages to produce IL-1 and TNF-a. The induction of TNF-a production can be completely reversed
by addition of IL-10. IL-17, which is present in T cell-rich areas of RA synovial samples, is exclusively produced by T-helper
(Th)0 or Th1 clones that are derived from the synovial membrane or synovial fluid of RA patients. In addition, IL-17 indirectly
induces the formation of osteoclasts from progenitor cells and enhances the production of nitric oxide (NO) in articular chondrocytes,
thus potentially contributing to cartilage and bone destruction.
In the RA synovial membrane IL-18, a cytokine of the IL-1 family, is expressed most prominently in CD68+ macrophages
that are contained in lymphoid aggregates. CD14+ macrophages of the RA synovial fluid also express the IL-18 receptor. IL-18,
either alone or in concert with IL-12 and IL-15, strongly enhances the production of IFN-?, TNF-a, GM-CSF and NO by cultured
synovial cells. Treatment with recombinant murine IL-18 markedly aggravates experimental arthritis, indicating that IL-18
has proinflammatory effects in this disorder.
Bacterial/viral components: The ability of bacterial toxins or superantigens to initiate proinflammatory responses that
are characterized by secretion of macrophage-derived cytokines is relevant in view of the possible micro-organism aetiology
of RA. Lipopolysaccharide, for example, binds to macrophages through the CD14 receptor and, in vitro, stimulates the production
of IL-1ß, TNF-a and MIP-1a.
Staphylococcal enterotoxin B, also a potent macrophage-activating factor in vitro and in vivo, enhances arthritis in
MRL-lpr/lpr mice. In this case, anti-TNF-a therapy reverses both the severe wasting effects of staphylococcal enterotoxin
B and the incidence of arthritis, indicating that TNF-a is central in this system.
Lipoarabinomannan, a mycobacterial lipoglycan that is involved in attenuating host immune responses and entry of mycobacteria
into macrophages, induces monocyte chemotaxis, as well as selective production of IL-12 and subsequent differentiation of
T cells toward a Th1-like phenotype. It also stimulates macrophages to produce TNF-a, GM-CSF, IL-1, IL-6, IL-8 and IL-10.
More generally, the prolonged persistence of obligate or facultative intracellular pathogens in macrophages may directly
lead to the development of arthritis.
This is the case with the Ross River virus, which causes human epidemic polyarthritis, or with the caprine-arthritis
encephalitis lentivirus, which causes a disorder in which viral replication within macrophages correlates with the severity
of tissue pathology. Further interesting cases are arthritis associated with human immunodeficiency virus-1 infection, which,
as with all other manifestations of this disease, is due to virus tropism for macrophages, and that associated with human
Hormones: RA undergoes clinical fluctuations during the menstrual cycle and during pregnancy. These observations have
a pathophysiological basis in the expression of sex hormone receptors on RA synovial macrophages. Indeed, physiological concentrations
of oestrogens stimulate the production of the proinflammatory cytokine IL-1 by RA macrophages. Conversely, higher oestrogen
concentrations inhibit IL-1 production, perhaps mimicking the clinical improvement that occurs during pregnancy. Thyroid or
other neuroendocrine hormones can also influence RA, at least partly through actions on macrophages.
Regulation of monocyte/macrophage activation in rheumatoid arthritis
Regulatory cytokines: The anti-inflammatory cytokine IL-4 is believed to play a protective role in arthritis, although
its virtual absence from synovial samples points to the lack of protective mechanisms, rather than to active regulation. This
Th2-like cytokine downregulates monocyte/macrophage cytotoxicity and cytokine production , including that of TNF-a and TNF-a
receptors, as well as IL-15-induced chemokine production. Notably, IL-4 decreases IL-1ß production while increasing IL-1 receptor
antgonist production, thus suggesting a 'coordinated' anti-inflammatory approach.
IL-4 also decreases the mRNA production of cyclo-oxygenase 2 and cytosolic phospholipase A2, thereby reducing the levels
of prostaglandin E2. In RA, IL-4 decreases monokine production in ex vivo synovial specimens, or TNF-a receptor expression
by synovial fluid macrophages. Importantly, IL-4 reduces bone resorption as well as synovial proliferation in vitro.
Consistently, experimental therapy with IL-4 clearly suppresses streptococcal cell wall-induced arthritis, a strongly
macrophage-dependent model. In considering a combined antimacrophage and immuno-therapeutic approach in autoimmunity, IL-4
should be the elective molecule because of its ability to regulate macrophages and to shift the cytokine pattern from a Th1-like
to a Th2-like predominance, however, induction of fibrosis may limit its therapeutic applicability.
IL-10 is a macrophage-derived cytokine with clear autocrine functions. Accordingly, IL-10 reduces HLA-DR expression and
antigen presentation in monocytes and inhibits the production of proinflammatory cytokines, GM-CSF and Fc? receptors by synovial
macrophages. Consistently with cytokine and chemokine downregulation, IL-10 clearly suppresses experimental arthritis.
In spite of IL-10 elevation in serum and synovial compartments of RA patients, some studies have suggested a relative
deficiency in IL-10. A combined IL-4/IL-10 deficiency may therefore shift the cytokine balance to a proinflammatory predominance.
Because of the autocrine effects of IL-10, this may further enhance the 'rheumatoid' macrophage imprint. Although recombinant
IL-10 appears to be an optimal candidate for treatment of RA, the induction of soluble and membrane TNF-a receptors on monocytes
and synovial fluid macrophages, as well as the lack of efficacy in some cohorts of RA patients, currently question the broad
applicability of this treatment.
IL-11 is present in synovial membrane, synovial fluid and sera of RA patients, and blockade of endogenous IL-11 increases
endogenous TNF-a production in RA synovium in vitro, whereas recombinant human IL-11 reduces the production of TNF-a and IL-12
from activated macrophages. IL-11 may represent another example of a paracrine regulatory loop, because IL-1a and TNF-a synergistically
stimulate the production of IL-11 in rheumatoid synovial fibroblasts. Treatment with IL-11 decreases clinical severity and
prevents joint damage in murine collagen-induced arthritis, which is in accord with high levels of IL-11 mRNA in clinically
uninvolved paws. Clinical studies with recombinant human IL-11 have been initiated in several inflammatory conditions, including
Similarly to IL-4 and IL-10, IL-13 exerts suppressive effects in experimental arthritis, probably through a selective
effect on monocytes/macrophages. In RA, IL-13 is produced by synovial fluid mononuclear cells, which, when exposed to exogenous
IL-13, diminish their own production of IL-1 and TNF-a.
The question regarding whether IL-16 is proinflammatory or anti-inflammatory is still under debate. In the RA synovial
membrane, IL-16 is present in CD68- synovial fibroblasts  and CD8+ T cells. In a human synovium/severe combined immunodeficiency
mouse chimera, IL-16 behaves as an anti-inflammatory cytokine by strongly reducing levels of mRNA for IFN-?, IL-1 ß and TNF-a
[but not for transforming growth factor (TGF)-ß], and by decreasing the numbers of cells that express IFN-? and TNF-a in the
human implant. IL-16 therefore appears to be yet another potential candidate for RA treatment, but one that requires care
in view of its strong T-cell chemo attractant features.
Tumour necrosis factor-a: TNF-a is a pleiotropic cytokine that increases the expression of cytokines, adhesion molecules,
prostaglandin E2, collagenase and collagen by synovial cells. In RA, TNF-a is mostly produced by macrophages in the synovial
membrane and at the cartilage-pannus junction and is believed to be a proximal cytokine in the inflammatory cascade.
While, on average, approximately 5% of synovial cells express TNF-a mRNA in situ, the degree of TNF-a expression in the synovial
tissue appears to depend on the prevailing histological configuration.
The importance of TNF-a is evidenced by several experimental and clinical observations: lymph node TNF-a production precedes
clinical synovitis in experimental arthritides, neutralization of TNF-a suppresses collagen-induced arthritis and reduces
inflammation in human/murine severe combined immunodeficiency arthritis; TNF-a, in combination with IL-1, is a potent inducer
of synovitis; transgenic, deregulated expression of TNF-a causes development of chronic arthritis; the TNF-a levels in the
synovial fluid correlate with the number of lining macrophages and with the degree of radiologically assessed bone erosion;
and serum TNF-a and soluble TNF receptors 1 and 2 are increased in active, systemic juvenile rheumatoid arthritis, including
some cases of macrophage activation syndrome.
TNF-a exists as a membrane-bound and a soluble form. Transmembrane TNF-a appears to be involved in local, cell contact-mediated
processes, and appears the prime stimulator of the R75 receptor. Interestingly, the transgenic expression of this form alone
is sufficient to induce chronic arthritis; likewise, a mutant membrane TNF-a, which utilizes both R55 and R75 receptors, can
also cause arthritis . Conversely, the soluble form of TNF-a, shed via metalloprotease cleavage from the membrane-bound form,
primarily stimulates the R55 receptor, acting transiently and at a distance.
Tumour necrosis factor-a receptors: TNF-a receptors are found in synovial tissue and fluid of RA patients, especially
in severe disease. There are two known TNF-a receptors: R55 (TNF-a receptor 1; high-affinity receptor) and R75 (TNF-a receptor
2; low-affinity receptor). The resulting stable R55 or transient R75 character of the ligand-receptor complex mediates different
cellular responses to soluble and transmembrane TNF-a.
In general, TNF-a receptors can operate independently of one another, cooperatively, or by 'passing' TNF-a to one another.
This complexity may explain the tremendous sensitivity of target cells to minute concentrations of TNF-a, as well as the considerable
variety of its effects. TNF-a receptors can also be shed, binding to soluble TNF-a and hence acting as natural inhibitors
Consistent with the pivotal role of TNF-a in arthritis, clinical trials with intravenous administration of chimaeric
anti-TNF-a monoclonal antibodies or TNF-a receptor constructs have shown remarkable efficacy in acute disease, with long-term
safety and slowing-down of radiological disease progression.
Also, in accordance with the activating role of TNF-a in leucocyte extravasation, anti-TNF-a treatment reduces endothelial
E-selectin and vascular cell adhesion molecule-1 in synovial samples, as well as the levels of E-selectin, intercellular adhesion
molecule-1, IL-8 and vascular endothelial growth factor in the circulation. This downregulation may lead to deactivation of
the vascular endothelium and suppression of angiogenesis, as substantiated in animal studies.
Interleukin 1: In RA, IL-1 gene expression is found predominantly in CD14+ macrophages , and IL-1 levels in the synovial
fluid significantly correlate with joint inflammatory activity. This cytokine, which is believed to act in sequence after
TNF-a, appears to mediate most of the articular damage in arthritis, because it induces proteoglycan degradation and inhibition
of proteoglycan synthesis. Also, IL-1 induces the production of the metalloproteases stromelysin and collagenase, and enhances
bone resorption. In RA, the balance between IL-1 and its physiological inhibitor IL-1 receptor antagonist is shifted in favour
of IL-1, indicating a dysregulation that may be crucial in promoting chronicity.
Chemokines: Several studies have documented the existence of a positive feedback between the macrophage-derived TNF-a
and IL-1 and chemotactic factors for monocytes, IL-8 and MCP-1. These factors are produced by synovial macrophages in an autocrine
manner. The highest levels of secreted IL-8 are observed in patients with seropositive RA, indicating a correlation with a
particularly vigorous macrophage activation. Significantly, IL-8 derived from synovial macrophages is a powerful promoter
of angiogenesis, thus providing a link between macrophage activation and the prominent neovascularization of the RA synovium.
Anti-inflammatory/regulatory cytokines: Macrophages also generate anti-inflammatory cytokines, most notably IL-1 receptor
antagonist and IL-10 (the latter described under Regulatory cytokines, above), both cytokines being engaged in autocrine regulatory
Differentiated macrophages constitutively express IL-1 receptor antagonist, which binds to IL-1 receptors without evoking
physiological responses. Significantly, this protein is upregulated by proinflammatory cytokines, including IL-1 itself or
GM-CSF, inducing strong anti-inflammatory effects. By means of this feedback mechanism, macrophages can therefore contribute
to the termination of inflammatory reactions. The critical relevance of IL-1 in chronic RA, as well as the imbalance between
IL-1 and IL-1 receptor antagonist, constitute the rationale for apparently successful therapy with IL-1 receptor antagonist.
Cytokines with a dual role in arthritis: Interleukin-6 ;IL-6 is the most strikingly elevated cytokine in RA, especially
in the synovial fluid during acute disease. The acute rise is consistent with the role of IL-6 in acute-phase responses. However,
although IL-6 levels in the synovial fluid correlate with the degree of radiological joint damage, and IL-6 and soluble IL-6
receptors promote the generation of osteoclasts, this cytokine may also protect cartilage in acute disease but promote excessive
bone formation in chronic disease.
Although IL-6 is mostly produced by synovial fibroblasts and only partly by macrophages , two findings suggest that the
striking IL-6 rise is a prominent outcome of macrophage activation: the morphological vicinity of IL-6-expressing fibroblasts
with CD14+ macrophages in the RA synovial tissue and the in vitro and coculture studies that showed that IL-1 stimulates IL-6
Transforming growth factor-ß: In RA, different forms of TGF-ß and TGF-ß receptors are expressed by macrophages in the
lining and sublining, as well as at the cartilage-pannus junction and in the synovial fluid. TGF-ß is a main regulator of
connective tissue remodelling, controlling both matrix production and degradation. In experimental animals, TGF-ß can induce
synovial inflammation, but can also suppress acute and chronic arthritis.
In the latter case, TGF-ß can act as a regulatory molecule, especially by counteracting some of the effects of IL-1,
including metalloprotease production and phagocytosis of collagen. On the other hand, synovial fluid TGF-ß induces the expression
of the Fc?RIII macrophage receptor, the stimulation of which induces release of tissue-damaging reactive oxygen species.
The potential relevance of Fc?Rs for the pathogenesis of arthritis has recently been re-emphasized. Likewise, TGF-ß is
a potent chemotactic factor for leucocytes, promoting monocyte adhesion and possibly resulting in excessive inflammatory infiltration
in chronic disease. Another limit to the therapeutic potential of TGF-ß is the enhancement of synovial fibroblast proliferation
The inhibiting effects of TGF-ß on metalloproteases are also controversial. While inhibition of gelatinase B and collagenase
suggest a protective role in tissue destruction, the increase collagenase-3 in chondrocytes may aggravate cartilage damage.
The effects of TGF-ß on TIMP are also ambiguous, because the regulation of matrix metalloproteinase and TIMP may depend on
different tissue domains (superficial versus deep cartilage layers) and may also vary for intracellular and extracellular
digestion of collagen. Metalloproteases themselves can also affect TGF-ß, by regulating the shedding of latent TGF-ß that
is attached to decorin, and thereby creating a loop that may enhance disease.
Nitric oxide and reactive oxygen species: In RA, variable numbers of synovial lining macrophages may represent a source
of NO. Synovial cells exposed to NO increase their TNF-a production, possibly adding to the mechanisms that promote synovitis.
NO may be relevant to arthritis also because of its effects on bone remodelling . However, NO can also exert protective effects
in experimental autoimmunity, limiting perhaps the antiarthritic impact of selective antagonists for human inducible NO synthase.
RA macrophages also produce reactive oxygen species, which are involved in distal inflammatory processes. Enhanced mitochondrial
production of reactive oxygen species in RA blood monocytes correlates with plasma levels of TNF-a, confirming the stimulatory
effect of this cytokine on the production of reactive oxygen species .
In general, the link between macrophage activation and molecules with dual activity (be they macrophage products or external
stimuli, the latter being soluble, membrane-bound, stimulatory, or suppressive factors) and the number of different
receptors that are involved in such stimulation may result not only in potent proinflammatory functions of macrophages, but
also in powerful anti-inflammatory and tissue repair functions. Finally, macrophage stimuli and macrophage responses rely
heavily on autocrine regulatory mechanisms, which makes it difficult to discern the cause from the effect.
Experimental arthritides with histological similarities to human RA have confirmed the role(s) of macrophages and/or
their products in synovitis, including the following: macrophage infiltration and activation patterns; migration mechanisms
of bone marrow-derived monocytes; maturation into effector and immunoregulatory subpopulations; activation of circulating
monocytes and extra-articular macrophages; leading role of TNF-a; sensitivity to cytokine-based therapy, including gene therapy
approaches; and sensitivity to reduction of monocyte recruitment or depletion of activated macrophages/ osteoclasts.
In addition to the TNF-a transgenic models of arthritis, two recent transgenic manipulations are worth mentioning. The
first, a back-cross of TNF-a transgenic mice with arthritis-susceptible DBA/1 mice, results in arthritis with enhanced production
of TNF-a, IL-1 and IL-6 in the synovial membrane, but with remarkable paucity of macrophage and lymphocyte infiltration.
The second, a back-cross of a T-cell receptor transgene with the non-obese diabetes (NOD) strain (K/B × N), is characterized
by erosive arthritis with predominant macrophage infiltration and other striking similarities to human RA: synovitis of the
distal joints, lymphocyte clustering close to macrophage infiltrates and predominant polymorphonuclear leucocyte exudation
in the synovial fluid.
Strikingly, the autoantigen recognized by both T cells and B cells in this model has recently been identified as the
ubiquitous glycolytic enzyme glucose-6-phosphate isomerase, lending strong support to the concept that general breakage of
tolerance can lead to aggressive arthritis.
The definition of the role of macrophage-derived cytokines in the
perpetuation of RA, of the pathophysiological and therapeutic dichotomy between inflammation and cartilage destruction, and
of the crucial significance of activated macrophages in the synovial membrane in relationship to permanent joint damage have
prompted a radical re-evaluation of the current regimens of anti-inflammatory and disease-modifying treatments. In addition,
research concerning conventional agents with antimacrophage effects is now aimed at potentiating such effects.
Disease-modifying antirheumatic therapy: Empirically introduced disease-modifying antirheumatic drugs (DMARDs) possess
a whole array of anti-macrophage effects.
Gold compounds: Administration of gold compounds to RA patients results in gold accumulation in the lysosomes of synovial
macrophages, especially in lysosome-rich sublining macrophages. In monocytes, gold compounds inhibit Fc and C3 receptor expression,
oxygen radical generation and IL-1 production. Through their effects on macrophages as accessory cells, gold compounds also
inhibit T-cell proliferation in response to antigen or mitogen. Gold compounds inhibit the production of IL-1, IL-8 and MCP-1
and decrease monocyte chemotaxis in vitro.
In the synovial lining, this is paralleled by a significant decrease in macrophage numbers and IL-1, IL-6 and TNF-a production.
In vitro, gold salts also inhibit angiogenic properties of macrophages, probably through their thiol moiety, which colocalizes
in macrophage lysosomes together with gold. In experimental arthritis, gold salts seem much more effective as long-term disease-modifying
drugs than as anti-inflammatory agents.
Methotrexate: One of the most effective DMARDs, methotrexate also impairs chemotaxis of blood monocytes and monokine
production, while increasing the production of cytokine inhibitors, including soluble TNF-a receptor R75. Because methotrexate
shifts the IL-1/IL-1 receptor antagonist balance in favour of IL-1 receptor antagonist, this drug may pharmacologically correct
the imbalance between these two mediators . A change in the monokine balance, including that of TNF-a , may indirectly
cause a selective decrease in collagenase production in the synovial tissue. A surprising caveat of methotrexate therapy,
however, is the accelerated formation of rheumatoid nodules in some patients. MTX is usually introduced at low dose e.g.,7.5
mg,oral and raised-gradually. If response is subpar,subcutaneous injection of MTX should be introduced-equivalent;25 mg.
Antimalarials: Antimalarials are endowed with significant antirheumatic effects in early RA , but are apparently less
effective than gold. They are are attractive because of their limited toxicity. Their tropism for lysosomes is probably
the cause for their slow accumulation in macrophages, in which they inhibit the release of arachidonate and the production
of prostaglandin E2 via phospholipase A2 inhibition . At high concentrations, antimalarials also inhibit the production of
IL-1 and TNF-a in lipopolysaccharide-stimulated macrophages. With higher concentrations, some undesired side effects tends
to arise,therefore that is another consideration.
Corticosteroids: The potent anti-inflammatory effects of corticosteroids in RA can be at least partly explained by transcriptional
downregulation of the inflammatory cytokines IL-1 and IL-6, or, as recently reported, transcriptional and post-transcriptional
downregulation of TNF-a in monocytes.
Corticosteroids may also affect the balance of the functionally distinct membrane-bound and soluble TNF-a . Interestingly,
in vitro studies suggest that addition of low doses of IL-4 and IL-10 decreases the dose of corticosteroids that is necessary
to downregulate TNF-a, possibly via a coordinated attack on activated macrophages. Corticosteroid also decrease the production
of IL-8 and MCP-1 ; this, once optimally exploited, may limit the self-perpetuating ingress of monocytes into the inflamed
Nonsteroidal anti-inflammatory drugs: Aspirin reduces the production of prostaglandin E2 through acetylation of the isoforms
1 and 2 of the cyclo-oxygenase. Although its use is limited by the gastric side effects (mostly dependent on cyclo-oxygenase-1
inhibition), there is a renewed interest in aspirin derivatives that potently and selectively inactivate the inducible cyclo-oxygenase-2
isoform in isolated macrophages and in local inflammation
Cyclo-oxygenase-2-dependent mechanisms selectively induce the production of IL-6 , the cytokine that is most highly
increased in the RA synovial fluid. Aspirin may affect macrophages also by decreasing the TNF-a production via nuclear factor-?B
Leukapheresis: In RA, repeated leukapheresis has proven effective in depleting the bloodstream of activated monocytes
, leading to reduced expression of differentiation markers, DR antigens, cytokines, neopterin and prostaglandin E2.
Apoptosis-inducing agents: A theoretical way of counteracting activated macrophages or osteoclasts is to eliminate them
physically from the inflammatory foci, or from systemic loci that are relevant to disease. Incorporation of liposome-encapsulated
bisphosphonates by activated macrophages, for example, induces apoptotic cell death in these cells, a process that circumvents
secondary tissue damage by restraining cellular organelles in apoptotic vesicles. Systemic application to arthritic rats counteracts
not only joint swelling, but also joint destruction and subchondral bone damage .
The DMARD bucillamine also appears to display apoptotic effects in monocyte lines . Notably, liposome encapsulation can
also be exploited for selective delivery of macrophage-modulating drugs and for gene therapy constructs .
Control of gene transcription: The transcription of most cytokine genes in monocytes/macrophages depends on the activation
of NF-?B and nuclear factor-?M transcription factors or that of the activator protein (AP)-1 complex. In RA synovial
macrophages, the expression of NF-?B is much more pronounced than that of AP-1 , a selectivity that may bear important therapeutic
Accordingly, the antiarthrtic effects of IL-4 may be based on the selective suppression of NF-?B in macrophages. IL-10
also downregulates the production of proinflammatory monokines, inhibiting the nuclear factors NF-?B, AP-1, or nuclear factor-IL-6
. Unlike IL-4, IL-10 can also enhance degradation of the mRNA for IL-1 and TNF-a.
Vitamin D3: Vitamin D3 may exert major immunomodulatory effects in RA that are possibly based on the effects on activated
synovial macrophages, resulting in positive regulation of the anti-inflammatory cytokines IL-4 and TGF-ß. Accordingly, dietary
supplementation of vitamin D3 in collagen-induced arthritis suppresses or prevents arthritis. These immunomodulatory effects
may directly or indirectly underlie the observation that, in RA, the serum levels of vitamin D3 are inversely correlated with
parameters of disease activity . Vitamin D3 supplementation may therefore represent a simple but effective therapy for RA,
although it may increase the risk of developing renal concrements.
Gene therapy in arthritis: Gene therapy approaches have been applied to counteract IL-1 and TNF-a, cytokines that are
predominantly produced by macrophages. Gene therapy with IL-1 receptor antagonist has proven efficacious in a number of experimental
arthritis models. The same applies to therapeutic overexpression of the soluble IL-1 type I receptor-IgG and the type I soluble
TNF-a receptor-IgG fusion proteins, although the latter appears less effective.
Recently, gene therapy approaches have also been extended to anti-inflammatory cytokines (ie IL-4, IL-10, IL-13 and TGF-ß
). An interesting approach of gene therapy is to achieve 'molecular synovectomy', either by expression of herpes simplex virus-thymidine
kinase, with subsequent administration of ganciclovir , or by overexpression of Fas ligand, resulting in synovial cell apoptosis
Gene therapy aimed at neutralizing proinflammatory macrophage products, overexpressing macrophage-regulating mediators,
or simply eliminating overly activated macrophages therefore carries some promise for the treatment of arthritis. Gene Therapy
research has a way to go-yet.
Although it is unlikely that macrophages represent the 'initiators' of the pathogenetic cascade in RA, it is believed
that these cells act as amplifiers of local and systemic inflammation, with a direct contribution to matrix degradation. Locally,
macrophages are involved in recruitment and activation of inflammatory cells, cell contact, or cytokine-mediated activation/
differentiation of neighbouring cells, secretion of matrix-degrading enzymes, and neovascularization. At a systemic level,
macrophages amplify disease via the acute phase response network, production of TNF-a, development of bone marrow differentiation
anomalies, and chronic activation of circulating monocytes.
Although identifying the diease aetiology remains the only means to fully silence RA, efforts to therapeutically address
activated monocytes/macrophages have the advantage of striking the very cell population that mediates/amplifies most of the
irreversible cartilage destruction, thus minimizing adverse effects on other cells that may have no (or marginal) effects
on joint damage.
The development of more sophisticated means to reduce the numbers of activated macrophages (eg by selective apoptosis-inducing
agents), to inhibit activation signals and/or their specific macrophage receptors, or to selectively counteract the macrophage
products that act as disease amplifiers, raises the hope of optimizing the therapeutic success against joint inflammation
and irreversible joint damage.