1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

Like that of many other chronic inflammatory and autoimmune diseases, the etiology of rheumatoid arthritis (RA) remains unknown, and many cell types (e.g., fibroblasts, macrophages, T and B lymphocytes, and neutrophils) and mediators (e.g., cytokines) have been implicated. A key role for macrophages has been suggested in part by successful treatment, particularly of RA, with blockade of tumor necrosis factor (TNF), which is widely considered to be produced by activated macrophages in inflamed tissue (1, 2). Although treatment of RA with disease-modifying antirheumatic drugs (DMARDs) has met with significant recent success, their mechanism of action is still debated, including if and how they modulate macrophage function (1, 3). The recent increase in the number of new targeted treatments for such diseases highlights the need for sensitive biomarkers that could be used in drug development and to help predict the response in individual patients, thereby guiding treatment decisions.

The main focus of this review is the regulation of macrophage populations, in particular their numbers in the RA synovium, as the paradigm both during disease progression and following biologic and chemical DMARD therapy. Data indicating that the number of synovial sublining macrophages is a good biomarker are incorporated into a discussion of the potential mechanisms controlling this parameter, both locally and systemically. There are many possible mechanisms that could regulate this cell number in tissue lesions, with altered cell trafficking being an obvious one. In addition to this possibility, evidence is presented for the concept that a contribution to the increased number of tissue macrophages could come from enhanced local macrophage survival and/or possibly their enhanced systemic generation from bone marrow precursors (i.e., monopoiesis) by the action of cytokines such as TNF, interleukin-1 (IL-1), and colony-stimulating factors (CSFs). We also discuss the possibility that certain DMARDs, such as methotrexate and gold compounds, reduce synovial macrophage numbers, at least in part, by suppressing this enhanced systemic macrophage lineage development or monopoiesis.

If these proposals have merit, then additional and more specific strategies to negatively regulate the survival of macrophages (and neutrophils), as well as their specific development from less mature precursors (i.e., myelopoiesis), may be fruitful approaches with applicability to RA and perhaps inflammatory and autoimmune diseases in general. Clinical trials targeting CSF activity in RA patients may be one such approach and may provide a test of the validity of the concepts mentioned above. While the emphasis in this article is on RA, some of the ideas may be relevant to the pathogenesis and control of other inflammatory and autoimmune conditions.

Synovial macrophage number as an RA biomarker

  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

Increased numbers of macrophages, which can be activated to produce mediators of inflammation, are a prominent feature of inflammatory lesions, in particular those of a chronic nature (4), and their depletion from inflamed tissue (5) and from the circulation (6) can have profound benefit. With regard to RA, it has been observed that the number of macrophages in the synovial tissue correlates with the degree of joint erosion (7, 8), and that increased numbers of macrophages are an early hallmark of active disease (9, 10).

Highlighting the importance of collecting data on the primary site of inflammation when possible, recent arthroscopic synovial biopsy findings support the notion that synovial sublining macrophage numbers can be used as a biomarker for disease severity, as well as a predictor of responsiveness to DMARD therapy (11). Following prior observations that reduced synovial inflammation and mononuclear cell infiltration were correlated with successful DMARD treatment (for review, see ref. 12), a randomized trial identified sublining macrophages as the best RA biomarker associated with clinical response to corticosteroids (13). These studies were extended across a wide range of interventions and kinetics. A strong correlation between the mean change in the Disease Activity Score (14) and the mean change in the number of sublining macrophages was noted (15). In addition to high sensitivity to change, CD68+ sublining macrophage numbers showed a clear distinction between effective and ineffective treatment (16) and were positively correlated with the clinical benefit of TNF blockade (17) and treatment with intraarticular corticosteroids (18). In summary, these results suggested that synovial sublining macrophages are a sensitive biomarker of response to treatment in RA patients.

Mechanisms controlling RA synovial macrophage numbers

  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

Local control.

In general, the increased numbers of macrophages at sites of inflammation can potentially occur in many ways, such as by heightened influx through chemotaxis (19, 20), reduced efflux (21), enhanced local survival/reduced apoptosis (22), and even by local proliferation in some cases, for which there is evidence from both clinical and animal studies (23–26). Chemokines are likely to be significant in this enhanced cellularity, as are adhesion molecules expressed on the surface of vascular endothelial cells (19, 20). Although the mechanisms governing likely cellular egress via lymphatics from the RA synovium and other sites of inflammation or autoimmunity are unknown, it could be that therapies such as TNF blockade increase lymphangiogenesis, thereby promoting efflux of cells out of the inflamed tissue, as part of their model of action (21). There are data supporting a crucial role of macrophages in lymphangiogenesis (27). It has also been proposed that the RA synovium can be viewed as exhibiting “inadequate apoptosis” (28), which might be relevant for the prolonged survival and constitutive activation of synovial cells, probably via the action of a number of possible cytokines.

In RA and other arthritides, modest proliferation has been described, at least in some reports, in a range of cell types, including macrophages (24, 29, 30). In early human atheroma, the predominant proliferating cell type is the macrophage lineage foam cell (26) and, during an inflammatory response in the brain, microglial proliferation is an early component of the activation of macrophage lineage foam cells by agents such as macrophage-CSF (M-CSF) (also known as CSF-1) and granulocyte–macrophage CSF (GM-CSF) (25).

Systemic control of the macrophage lineage.

There are a number of reports describing changes in the properties of both peripheral blood mononuclear cells (31–33) and purified monocytes (34) in RA, suggestive of some form of activation. Whether these changes are dependent upon signals (for instance, from cytokines emanating from arthritic lesions) or whether the circulating cells are inherently different due to prior exposure to another stimulus, such as a microorganism or an endogenous pathogen-associated molecular pattern, is unknown. The same issues are being considered with regard to other chronic inflammatory and autoimmune diseases.

Given that macrophages in inflamed tissue are derived mainly from circulating monocytes, which in turn are generated from less mature bone marrow precursors (promonocytes) (35, 36), it would not be an unreasonable possibility that increased tissue macrophage numbers at sites of chronic inflammation may be reflected by, and even be due in part to, up-regulated systemic myelopoiesis or, more specifically, monopoiesis. In this regard, it is intriguing that the numbers of sublining macrophages in one RA joint have been found to correlate with those in another joint from the same patient (37). It is also worth noting that there are data indicating that macrophages appearing at a site of inflammation (36), including in the synovium during inflammatory arthritis (38), are derived from recently divided precursors. Correlations between peripheral blood monocyte counts and, for example, peripheral arterial disease (39) and between these counts and rheumatoid disease have been demonstrated in some studies (40–42).

The reasons for the reported changes in monocyte numbers in these cases are unknown. They may be due, for example, to altered mobilization from the bone marrow and/or enhanced survival. However, the relevance of these changes to the increased numbers of macrophages in RA synovium discussed above remains to be established. Peripheral blood monocyte subsets have recently been identified (43–45) and, for reasons that are unclear, conflicting observations of their ratios have been reported in RA patients, with different studies showing an increased or reduced proportion of the CD14lowCD16+ subset (46, 47). The potential significance of these subsets is detailed below. Of possible relevance to the reported differences in monocyte numbers and subpopulations in the blood of patients with RA, previous reports have described bone marrow abnormalities in RA patients, such as abnormal myeloid cells (48–50), enhanced promonocyte DNA synthesis activity (51), increased numbers of HLA–DR and CD14-positive mononuclear cells (52), high myeloid precursor numbers and CSF activity in bone marrow next to affected joints (53, 54), and a defective proliferative capacity of hematopoietic progenitor cells (55).

In animal models of polyarthritis, enhanced myelopoiesis has been noted (56–59), and it has been reported that organ-specific and systemic autoimmune diseases originate from defects in hematopoietic stem cells (60, 61). For example, in the NZB mouse, there is a correlation between monocytosis and autoantibody production and subsequent development of fatal lupus nephritis (59). In addition, it has been proposed that the monocytosis observed in one murine model of lupus might be due to expanded precursor populations in the bone marrow (57), and in another such model there are elevated numbers of circulating and marrow-derived macrophage progenitor cells (62).

Based on such observations in clinical and animal studies, it has been proposed but not yet proven that bone marrow abnormalities may even initiate and drive RA pathogenesis (48, 49, 55, 63). In this context, the potential benefits of hematopoietic stem cell transplantation in RA have been debated (64, 65), and appropriate clinical trials are needed to clarify this issue.

Effect of treatments on macrophage lineage numbers

  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

In this section, the concept that antirheumatic therapies and possibly antiinflammatory therapies in general may act partly by reducing macrophage lineage numbers, both locally in tissue and possibly systemically, will be explored further.

Cytokine blockade.

TNF, IL-1, and IL-6.

Blockade of the key proinflammatory cytokines TNF, IL-1, and IL-6 leads to reduced cellular infiltration, including reduced macrophage numbers, in RA synovial tissue (66, 67). Not unreasonably, this clinical benefit has usually been considered, particularly for the most studied TNF, to result from the inhibition of their pleiotropic activities involved in, for example, cell trafficking due to reduced chemotaxis and/or adhesion to the inflamed endothelium (68–70), control of the production of other mediators of inflammation (66), tissue damage (66), and enhanced efflux via increased lymphangiogenesis (21).

Although it has been reported that TNF has a prosurvival effect on monocyte/macrophages in vitro (71, 72), the issue of whether TNF neutralization can lead to apoptosis in tissue macrophages and/or blood monocytes has been debated. The divergent observations are probably due to the variety of conditions under which the studies were carried out (2, 73–75). It is possible that macrophage apoptosis could be induced over time following TNF blockade (73), possibly secondary to the decrease in inflammation rather than being the primary mechanism of action of anti-TNF therapy (2, 73). However, it should be noted that one of the difficulties associated with the measurement of apoptosis is that macrophages are extremely efficient at removing apoptotic cells from their immediate environment, making estimating the extent of apoptosis very difficult in their presence (76, 77).

With regard to possible systemic changes resulting from TNF blockade, such neutralization can also lead to a reduction in osteoclast precursors (i.e., a monocyte subpopulation) in the blood of patients with RA or psoriatic arthritis (78, 79) and to a reduction in peripheral blood monocyte numbers in RA (80) and Crohn's disease (81). These findings are consistent with the data indicating that TNF administration can promote myelopoiesis (82) and mobilization of cycling osteoclast precursors into peripheral blood from bone marrow (83). TNF has been linked to the abnormal bone marrow observed in RA patients (63). In addition, it should be noted that IL-1 and IL-6 can stimulate myelopoiesis (84), indicating the possibility that their blockade may have effects on the myelopoietic system that are similar to those of TNF neutralization.


In addition to their originally defined activities as hematopoietic growth and differentiation factors for precursor cells, the CSFs (for example, GM-CSF, M-CSF, and granulocyte CSF [G-CSF]) can act on mature hematopoietic cells, such as macrophages and/or neutrophils, and therefore potentially have proinflammatory effects (85–87). With regard to macrophages, M-CSF (also known as CSF-1) and GM-CSF can promote their survival and proliferation (i.e., increase cell numbers), as well as modify other functions, such as migration (87–89). Likewise, G-CSF can keep neutrophils alive and also activate them to produce mediators of inflammation (90). Since the levels of these CSFs have been found to be elevated at sites of inflammation, such as in RA synovial fluid and synovial tissue, they are candidate cytokines to maintain macrophage and/or neutrophil numbers in an inflammatory or autoimmune lesion (86, 87, 91, 92). Plasma M-CSF has recently been proposed to be a potential novel biomarker of disease activity in RA patients (93). Again, with regard to macrophage numbers, in the steady state the deficiency in synovial macrophages in mice with an inactivating mutation in the M-CSF gene (Csf1op/Csf1op mice) (94) indicates that M-CSF controls their numbers in this state.

The prosurvival effect of CSFs on myeloid populations is consistent with the “inadequate apoptosis” concept mentioned above for the RA synovium (28). Also, CSF action and production can be intimately linked, for example, with those of TNF and IL-1 (83, 86, 87, 95–101). All of these cytokines have been proposed to form part of a “CSF network” in which macrophages, neighboring T cells, and non-hematopoietic tissue cells (for example, fibroblasts, chondrocytes, endothelial cells, and osteoblasts) can communicate with each other to sustain chronic inflammatory reactions in a feedback loop (86–88, 102). As a corollary to this concept, GM-CSF and/or M-CSF blockade would reduce the numbers of activated macrophages in inflamed tissue, thereby significantly suppressing the degree of inflammation (86). The possibility that synovial fibroblasts may be a CSF source in the RA synovium in this “network” is consistent with this cell type being considered as a target for RA therapeutic strategies.

Since GM-CSF, M-CSF, and G-CSF administration can promote myelopoiesis and mobilize myeloid precursors from the bone marrow into blood (84), it is possible that CSFs formed in an inflamed lesion may also appear in the circulation and have similar direct systemic effects (86, 87). It is also conceivable that CSF expression may contribute to the myelopoietic activity of TNF and IL-1 mentioned earlier, since their expression can be up-regulated by these classic proinflammatory cytokines. Some support for this “CSF network” concept comes from data from arthritis models and other models of inflammation showing that CSF (GM-CSF, M-CSF, and G-CSF) administration and depletion can have dramatic effects on disease progression (87, 103–109). As a result, antibody-based clinical trials targeting GM-CSF and M-CSF action in RA have recently commenced. Both the background rationale for these trials and some potential issues have been reviewed previously (87).

Methotrexate and leflunomide.

Several mechanisms of action have been proposed for methotrexate and leflunomide as effective antiinflammatory and immunomodulating agents (3, 110–112). As noted above, administration of each of them can lead to decreased numbers of sublining macrophages in the RA synovium (113, 114). Again, a proposed mechanism to explain their effectiveness is reduced cell traffic into inflamed joints (113, 114) and, for methotrexate, adenosine-induced immune suppression is widely considered to be important (3, 110–112). Based on in vitro findings of methotrexate treatment, it has been suggested that the disruption of the cell cycle may be the initial step in the apoptotic sequence of dying cells and may explain the antiproliferative effects of the drug (111). It inhibits in vitro the replication of human macrophage leukemic cell lines but not that of synovial macrophages, leading to the suggestion that it might inhibit the recruitment of immature and inflammatory monocytes to sites of inflammation and could reduce the survival of those cells in the inflamed synovial tissue (114, 115).

Methotrexate and leflunomide inhibit cell replication, and leukopenia can be a side effect of their administration (36, 116, 117). One of us has previously suggested that myelosuppression forms a part of the action of methotrexate as a DMARD (86). Interestingly, at low and effective doses, methotrexate, either alone (118) or in combination with other chemical DMARDs (119), can lead to reduced leukocyte counts, including those for monocytes.

Other DMARDs.

Various mechanisms have been put forward to explain the efficacy of gold salts, sulfasalazine, D-penicillamine, and antimalarials (chloroquine and hydroxychloroquine) in RA, including direct effects on synovial macrophages (3). In the context of this review, it is interesting that gold salts are taken up predominantly by macrophage lineage cells (120).

Particularly for the first 3 drugs, myelosuppression can be a problem (86, 121–123). Similar to the findings of the studies of methotrexate referred to above (118, 119), in RA patients receiving sulfasalazine, moderate reductions in leukocyte counts were correlated with reduced disease severity (123). Based on the in vivo (124) and in vitro sensitivity of myeloid cell development from precursor cells, with human myeloid precursor cells being particularly sensitive (122, 125), it has been proposed that the myelosuppressive activity of gold compounds and sulfasalazine, in addition to that of D-penicillamine, could form part of their clinical efficacy (86, 122, 125–127), possibly by opposing the action of the CSFs (86) (Figure 1).

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Figure 1. Proposed model of the mechanism of control of macrophage lineage numbers in rheumatoid arthritis (RA) and other inflammatory and autoimmune diseases. Clinical data have demonstrated that the number of synovial sublining macrophages is a good biomarker of disease severity in RA and of the response to treatment with disease-modifying antirheumatic drugs (DMARDs). In this model, the sublining “inflammatory” macrophages are continually derived from relatively immature “inflammatory” blood monocytes, and their increased numbers are maintained locally by stimuli (such as tumor necrosis factor [TNF], granulocyte–macrophage colony-stimulating factor [GM-CSF], or M-CSF), by their effects on cell trafficking in and out of the synovium (heightened migration and/or reduced efflux), and/or by prosurvival activities. One or all of these cytokines may also act systemically, either directly or indirectly via other mediators (not shown), to stimulate myelopoiesis, to mobilize promonocytes (which may be cycling promonocytes) from bone marrow to blood, and/or to promote blood monocyte survival. The relationship between the synovial sublining and intimal macrophage populations is unknown. A similar scenario may apply in other inflammatory and autoimmune diseases. It is also speculated that, like specific cytokine-neutralizing therapies, chemical DMARDs, such as methotrexate and gold compounds, in addition to possibly opposing the local trafficking and prosurvival effects of these cytokines in RA (not shown), may suppress at least some of the systemic changes in macrophage lineage dynamics driven by such cytokines. Other likely actions of TNF, CSFs, and DMARDs are not depicted, nor are the effects of other mediators.

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These potent and widely used antiinflammatory drugs have many effects on numerous cell types, including inflammatory macrophages (3). Therefore, local effects, such as suppression of proinflammatory cytokine production, leading to reduced trafficking and retention of inflammatory cells, are likely to be quite important. Although they can cause granulocytosis, monocytopenia, and lymphopenia, possibly due to margination (128), drugs in this class can suppress myelopoiesis (129), which again might contribute to reduced macrophage and neutrophil infiltration into the RA synovium (13, 130) (Figure 1).

All classes of DMARDs can reduce macrophage numbers in the inflamed RA synovium. The relative effects of different antirheumatic and antiinflammatory drugs in general on tissue macrophage survival (and possibly proliferation), as well as on local trafficking and retention, need to be established. Likewise, the significance of any systemic inhibition of myelopoiesis or reduction in macrophage lineage numbers (3, 119, 123) remains to be determined.

Macrophage lineage heterogeneity

  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

Macrophage polarization.

Many macrophage subpopulations have been defined based on surface markers (131), although it has recently been concluded that the number of these subpopulations is an exponential function of the number of markers, and that no marker distinguishes dendritic cells from macrophages (132). Recent research has demonstrated the extent of the heterogeneity and even the plasticity of macrophage populations, and the term “polarization” is now widely used (19, 131). As a generalization, macrophages with these different activation states or phenotypes, which have generated a great deal of recent interest, have been termed M1 macrophages and M2 macrophages, with the former considered to be more involved in inflammation and host defense and the latter having an antiinflammatory or tissue repair function. These states have also been referred to as “classical activation” or “alternative activation,” respectively. Typical stimuli for the M1 macrophages are interferon-γ and lipopolysaccharide (LPS), while typical stimuli for the M2 phenotype are IL-4 and IL-13 (19, 131). As a simplistic generalization, the former have a TNFhighIL-12highIL-10lowiNOS2high phenotype, and the latter have a TNFlowIL-12lowIL-10higharginasehigh phenotype.

Most classification of tissue macrophages at sites of inflammation has been done for solid tumors (19). However, studies with tissue macrophages from other diseases are now appearing with the goal of “switching” from one state to the other in order to control the particular pathologic condition to the advantage of the host. RA synovial tissue or synovial fluid macrophages have not been categorized as M1 or M2 macrophages, and the nature of any such polarization may depend on whether a “flare” is occurring. One other stimulus that favors the M2 state is glucocorticoids (19), and this “switching” may form part of the action of this class of drugs at the macrophage level. It would be interesting to know whether the effects of DMARDs on macrophage populations (1) could be incorporated into this concept.

Peripheral blood monocyte subpopulations.

Another burgeoning area of research in macrophage lineage biology is the recent identification of peripheral blood monocyte subsets (43–45, 133). In humans, 2 main monocyte populations have been delineated, namely, the major CD14+CD16− and the minor CD14lowCD16+ subsets (43). As mentioned above, there have been conflicting findings regarding the ratio of these subpopulations in the blood of RA patients (46, 47). The CD14+CD16− subpopulation is considered to be the less mature, while the more mature CD14lowCD16+ monocytes have been called “inflammatory” monocytes (44, 134), in part because their level may be elevated in blood in inflammatory conditions, such as RA (135), and in part because they produce TNF in response to LPS (44, 134). However, it is unknown whether the latter subpopulation simply expands in response to the inflammatory milieu in the body and therefore may be an indicator rather than a driver of inflammation (44). Therefore, the designation “inflammatory” monocytes may be premature in the absence of appropriate human studies (44), especially given the apparently contradictory observations in murine monocyte subpopulations (45).

There is more information on the function of murine monocyte subpopulations than on their human counterparts, since their presence and trafficking into inflamed tissues have been analyzed in a number of models. Similar to the major subset of “classic” human CD14+ monocytes, murine Ly6C(Gr-1)high monocytes are M-CSFR+CD11b+CCR2+CX3CR1lowCD62L+, whereas, similar to the “nonclassic” human CD14low CD16+ monocytes, murine Ly6C(Gr-1)low monocytes are M-CSFR+CD11b+CCR2−CX3CR1highCD62L− (43, 133). Members of the former population have been referred to as “inflammatory” monocytes, while the latter have been termed “resident” monocytes (43).

The functional data are difficult to reconcile with this scheme comparing the respective human and murine subpopulations, since the murine Ly6Chigh monocytes and the human CD16+ monocytes are the main producers of TNFα (45). The early recruitment of the less mature Ly6Chigh monocytes into inflamed tissue has been shown in a variety of murine models (43, 131, 133, 136–140). The more mature Ly6Clow subset has been proposed to give rise to locally generated tissue macrophages in the steady state (43). However, there is also recent evidence that this population can migrate into inflamed tissue, depending on the model used (45). In tissue, monocyte/macrophages are subjected to various stimuli that impact their functional heterogeneity or polarization. Interestingly, in the context of the above discussion of the “CSF network” and dysregulated myelopoiesis, overexpression of TNF in the mouse leads to the mobilization of cycling osteoclast precursors from bone marrow into blood, with M-CSF being implicated in this process (83).

A subpopulation of human blood monocytes has been observed to proliferate in vitro in response to M-CSF and GM-CSF (141–143). It has been suggested that this proliferative subpopulation has a proinflammatory phenotype, since it could be the subpopulation that has most recently emigrated from bone marrow and that could also give rise to any proliferative macrophages in inflamed tissue (142). It has been reported that this relatively immature subpopulation is distinct from the CD14lowCD16+ population and contains osteoclast precursors (143). More information on human blood monocyte maturation and the possible trafficking of different populations during chronic inflammation is needed. We speculate that in the studies that have shown differences in RA blood monocyte numbers or properties (1, 34, 40–42) and reduced numbers in response to DMARD therapy (118, 119, 123), there may have been preferential regulation of perhaps a less mature (and possibly proliferative) subpopulation (Figure 1). Therefore, future studies along the same lines with blood monocytes in RA and other inflammatory and autoimmune conditions should perhaps take subpopulations into account; further characterization of the properties of such subpopulations would assist investigators in carrying out such studies.

Synovial macrophage heterogeneity.

Macrophages appearing at sites of inflammation are considered to be “young” in the sense that they arise from recently divided precursors (35, 36). As noted above, in RA the numbers of sublining macrophages and not those at the synovial surface have been shown to correlate best with disease severity, and the former population was also preferentially depleted early upon successful DMARD therapy (11, 15, 16). We are proposing that sublining macrophages may be a more dynamic population, perhaps as progeny of an immature (possibly “inflammatory”) monocyte subpopulation, than the longer-lived intimal macrophages. During an inflammatory reaction, they would therefore perhaps be more subject in the short term to factors controlling trafficking, and possibly retention and survival, and therefore more vulnerable to DMARD therapy. This speculative concept, which needs validation, is illustrated in Figure 1. Consistent with this idea, anti-TNF treatment in RA has been shown to have a rapid and pronounced effect on the infiltration of myeloid-related protein–positive macrophages and not on the number of resident tissue macrophages (144), and reduced sublining macrophage numbers have been observed as soon as 24–48 hours after such blockade (73, 75).

Interestingly, one long-term study, in which RA patients were followed up through remission and the macrophage content of synovial biopsy specimens was observed over time, showed that DMARDs reduced the numbers of both sublining and intimal macrophages (145). There is evidence of heterogeneity in RA synovial macrophages, as judged by marker expression (146–149). One interpretation of the biopsy findings mentioned above (145) is that, as proposed earlier, intimal macrophages are long-lived and possibly derive from the more transient sublining population. However, it is also possible that they are independent of each other. Obviously, further information is needed on the relationships between monocyte subpopulations and synovial macrophage populations, as well as the relationship between the latter populations themselves. Given the data demonstrating that the number of sublining macrophages is a good RA biomarker, if there is such a relationship between a peripheral blood monocyte subset and sublining macrophages, then the former may turn out to be a more convenient biomarker of RA severity and response to treatment. For other analogous diseases, the same considerations regarding the possible links between monocyte subpopulations and inflamed tissue macrophage subpopulations should be taken into account.


  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements

We have summarized the data supporting the concept that the number of synovial sublining macrophages is a biomarker of disease severity in RA and can be used to assess the benefit of DMARD therapy. In addition, we have proposed a model implicating not only TNF but also GM-CSF and M-CSF in the control of these numbers (Figure 1). Current clinical trials targeting these CSFs will provide the ultimate test of their relevance (87). The possibility that sublining macrophages in the inflamed RA synovium are a relatively dynamic population, perhaps derived from a less mature, “inflammatory” monocyte subpopulation, may explain their relationship to disease severity and DMARD efficacy (Figure 1). Blood monocyte subpopulations should be monitored carefully, since an “inflammatory” subpopulation may turn out to be a more amenable biomarker than sublining macrophages whose analysis requires a biopsy, as well as a more relevant one than the bulk of the monocytes. How both the disease and DMARDs regulate sublining macrophage numbers has implications for future therapy.

The focus of this review has been mainly on the control of macrophage lineage populations both locally and systemically in inflammatory and autoimmune conditions, particularly RA. By no means, of course, is it assumed that other aspects of RA pathogenesis and DMARD action are not relevant. For example, the activation of synovial macrophages and other cell types, such as lymphocytes and synovial fibroblasts, to produce proinflammatory mediators in addition to TNF (1, 66); the control of the numbers and function of other myeloid populations, such as neutrophils, platelets, and mast cells (119); and the other actions of chemical and biologic DMARDs (3, 68, 70, 110, 112–114) are obviously all worthy of consideration but are beyond the scope of this review. The same qualification needs to be made for the pathogenesis of and treatment protocols for other diseases with some overlapping features, such as multiple sclerosis, lung inflammation, nephritis, inflammatory bowel disease, psoriasis, and atherosclerosis. In conclusion, for RA and other similar conditions, there are a number of clinically relevant questions related to the control of macrophage lineage populations that need to be addressed, such as whether regulation of tissue macrophage survival, in addition to altered cell migration and retention, is significant, whether there are critical systemic changes in monocyte subpopulations, and whether the CSFs have any importance as therapeutic targets.


  1. Top of page
  2. Introduction
  3. Synovial macrophage number as an RA biomarker
  4. Mechanisms controlling RA synovial macrophage numbers
  5. Effect of treatments on macrophage lineage numbers
  6. Macrophage lineage heterogeneity
  7. Conclusions
  8. Acknowledgements
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