Numerous types of cells, including T and B lymphocytes, macrophages and dendritic cells participate in immune responses and have been recognized as immune cells for many years. OCPs are formed in the bone marrow from myeloid precursors, which can also give rise to macrophages and dendritic cells when OCPs are cultured with M-CSF or GM-CSF plus IL-4 (25
). It is still not clear at exactly which stage OCPs lose their potential for differentiation along multiple pathways or when “trans-differentiation” occurs in vivo. Like macrophages, OCPs express Fcγ receptors (6
). Their involvement in immune responses and their origin from circulating monocytes has been recognized for many years (27
), although their relationship to macrophages has been controversial (28
). However, their roles as immune cells or immune response modulators has become clearer more recently (3
). OCPs enter the bloodstream and circulate like other hematopoietic cells. They can be detected in the blood and spleens of mice and in the bloodstream of humans using antibodies to CD11b, c-kit, Gr-1, c-fms (the receptor for c-Fos) and RANK (30
). Their numbers are increased in the blood of humans (30
) and mice (30
) with various forms of inflammatory arthritis in which TNF production is increased. Transgenic mice over-expressing TNF (TNF-Tg) have high serum TNF levels and develop a form of inflammatory erosive arthritis similar to rheumatoid arthritis. Treatment of these mice with anti-TNF therapy reversed the increase in OCP numbers in their blood, suggesting that TNF induces OCP egression from the blood.
OCPs express CXCR4, the receptor for stromal cell-derived growth factor (SDF-1). SDF-1 regulates the movement of hematopoietic cells from bone marrow into and from the bloodstream in a dose-dependent manner (reviewed in (34
)). Expression levels of SDF-1 by bone marrow stromal cells are reduced in TNF-Tg mice (35
) and we believe that this is one of the mechanisms whereby TNF promotes OCP egression from the marrow into the bloodstream. Another mechanism may be related to TNF increasing the expression of c-Fms by OCPs thereby increasing the bone marrow OCP pool (36
). SDF-1 and TNF levels are increased in the joints of TNF-Tg mice and of patients with inflammatory arthritis. Thus, OCPs could be attracted to sites of inflammation in and around affected bones where SDF-1 concentrations are increased (34
). Why SDF-1 levels are decreased in bone marrow of these mice and increased in their joints in response to increased TNF production remains unexplained, but may be TNF concentration dependent.
OCPs not only respond to TNF, they and osteoclasts also secrete TNF and other cytokines, such as IL-6 and IL-1 (37
). Secretion of these cytokines is increased in response to TNF (39
). Thus, TNF could induce an auto-amplifying cycle at sites of inflammation in and around bones to enhance osteoclast formation (34
) directly through autocrine and indirectly through paracrine mechanisms. These mechanisms could enhance TNF’s established action to increase osteoclast formation indirectly by promoting RANKL expression by accessory cells, such as T cells and synoviocytes (8
TNF expressed by OCPs could also increase the activity of osteoclasts by a mechanism involving activation of c-Fos in OCPs (34
). c-Fos is activated downstream of NF-κB in OCPs in response to RANKL and TNF to regulate osteoclast differentiation (34
). We found that when we over-expressed c-Fos in OCPs and treated the cells with TNF, the osteoclasts derived from these cells had increased resorptive activity (34
). Thus, at sites in bone where TNF levels are increased, TNF could increase bone resorption by a number of mechanisms: indirectly by increasing RANKL expression by accessory cells and directly by increasing not only osteoclast formation, but also osteoclastic resorptive activity.
Although TNF mediates bone loss in a variety of pathologic states, like RANKL it can also directly limit osteoclast formation. However, the inhibitory mechanisms involved are different from those described earlier and activated by RANKL. Both RANKL and TNF activate the canonical NF-κB pathway in OCPs to induce osteoclast formation directly by inducing the phosphorylation and subsequent degradation of inhibitory kappa kinase β (IKKβ) (reviewed in (40
)). Consequently, NF-κB p65/p50 dimers are released from IKKβ and translocate to the nucleus where they induce expression of osteoclastogenic genes. RANKL, but not TNF, also activates the non-canonical or alternative NF-κB pathway in OCPs. This pathway is activated by phosphorylation and degradation of another inhibitory NF-κB protein, p100, which binds to the NF-κB protein, RelB, and prevents it translocating to nuclei. Upon activation of the alternative pathway, p100 is processed in the proteasome and a p52 fragment of it is released. P52 binds to RelB and p52/RelB dimers can then translocate to the nucleus to induce gene expression. RANKL induces more osteoclasts than TNF in vitro
), but the reason for this has been unexplained. We found that TNF, but not RANKL up-regulates NF-κB p100 expression in OCPs (41
) and hypothesized that p100 might limit osteoclast formation by TNF. We treated OCPs from NF-κB p100-deficient mice with TNF and found that it induced similar numbers of osteoclasts from these as RANKL, suggesting that induction of p100 expression limits TNF-mediated osteoclast formation. Up-regulation of NF-κB p100 expression or prevention of its degradation may be a mechanism to limit excessive bone resorption in a variety of bone diseases. Recently, we have found that osteoclasts can increase their activity through another autocrine mechanism in response to RANKL. For example, they increase their secretion of VEGF-C, a member of the VEGF family of angiogenic proteins in response to RANKL and TNF. VEGF-C in turn increases the resorptive activity of osteoclasts directly in vitro, although it does not increase the formation of osteoclasts from OCPs (35
). Osteoclasts in inflamed joints of TNF-transgenic mice have increased expression of VEGF-C (42
), supporting our in vitro findings. A major function of VEGF-C is to promote the formation of lymphatic channels. We have found that the numbers and size of lymphatic vessels around affected joints in these TNF-Tg mice is increased (42
). This increase could enhance the immune responses around these joints by permitting an increase in the rate of flow of lymph and other inflammatory mediators from affected joints. It will be important to determine if inhibition of osteoclasts has beneficial or detrimental effects on this and other recently discovered functions of OCPs and osteoclasts in inflammatory arthritis and other conditions associated with increased cytokine-induced osteoclast formation and activity and to fully investigate the function of VEFG-C produced by osteoclasts.