TNF-α was identified in 1975 as the factor in serum isolated from endotoxin-treated mice that induced necrosis of a methylcholanthrene-induced murine sarcoma (4
). It soon became apparent that TNF-α had other effects, including the ability to induce signs and symptoms of shock and multiorgan damage (5
) via proinflammatory effects on vascular endothelium (reviewed in ref. 6
). The demonstration that TNF-α played a key role in RA followed from the demonstration of its potential to degrade cartilage (7
) and bone (8
) in vitro. Moreover, it was shown using dissociated RA synovial mononuclear cell cultures that TNF-α and several other proinflammatory cytokines, including IL-1 (9
), IL-6 (10
), GM-CSF (11
), and IL-8 (12
), were spontaneously and chronically produced over a five-day culture period (13
). Importantly, if TNF-α bioactivity was blocked in these cultures, the spontaneous production of both IL-1 protein and IL1B
mRNA was markedly reduced and IL-1 bioactivity was neutralized (13
). This suggested that the presence of many of these cytokines was not random, but that a network or hierarchy controlled their expression. Consistent with this idea, it was subsequently shown that blockade of TNF-α also inhibited the spontaneous production of GM-CSF (11
) (which is important for the induction and maintenance of MHC class II expression on APCs in the synovial fluid and tissue) and the expression of the proinflammatory cytokine IL-6 and the chemokine IL-8 (15
). The pathogenic effects of TNF-α relevant to RA disease are illustrated in Figure .
TNF-α actions relevant to the pathogenesis of RA.
It was soon shown, using immunohistochemistry, that TNF-α and receptors for TNF-α (TNFRs) (16
) were expressed in human rheumatoid joint tissue and, using the collagen-induced arthritis (CIA) model of RA, that administration of an mAb specific for mouse TNF-α after disease onset ameliorated both inflammation and joint damage (18
). Separately, Kollias and colleagues found that transgenic mice expressing the modified human TNFA
gene (with replacement of the 5′-UTR regulatory sequences) spontaneously developed peripheral arthritis. This arthritis was characterized by increased human TNF-α protein, joint inflammation, bone erosion, and cartilage destruction (all hallmarks of RA), and disease could be ameliorated with antibodies specific for human, but not mouse, TNF-α (19
). Together these data provided the rationale for developing therapeutics that block TNF-α.
In 1992, the first open-label trial of a TNF-α blocking agent was initiated at the Kennedy Institute of Rheumatology Division, United Kingdom; 20 patients with active RA were treated with infliximab (Remicade), a chimeric antibody specific for human TNF-α. Treatment with infliximab substantially reduced the signs and symptoms of disease, levels of C-reactive protein (CRP) in the serum, and the erythrocyte sedimentation rate (ESR) (20
). This result was confirmed in other multicenter placebo-controlled trials, together with the observation that therapeutic efficacy was enhanced when infliximab was coadministered with methotrexate. This led eventually to FDA approval of the drug for the treatment of RA (21
). Importantly a subsequent two-year trial indicated that this therapy led to retardation or arrest of both joint space narrowing and bone erosion (23
). In addition to infliximab, two other drugs that function as TNF-α blockers are licensed: etanercept (Enbrel), which is a fusion protein comprising human soluble TNFR linked to the Fc component of human IgG1, and adalimumab (Humira), which is a fully human antibody specific for human TNF-α (Table ).
Biological therapeutics that target cytokines in RA
TNF-α is now recognized as mediating a wide variety of effector functions relevant to the pathogenesis of RA, including endothelial cell activation and chemokine amplification, leading to leukocyte accumulation (24
) and probably attendant cardiovascular comorbidity (25
); osteoclast and chondrocyte activation, promoting articular destruction; nociceptor sensitization; impaired cognitive function; and metabolic syndrome (26
). These are all recognized components of the RA disease spectrum and explain the broad effects of TNF-α blockade in patients. Further, therapies targeting TNF-α are now also recognized to be effective in multiple other chronic inflammatory diseases, including juvenile RA (JRA), Crohn disease, psoriasis, psoriatic arthritis, and ankylosing spondylitis (reviewed in refs. 24
More recently, attention has turned to the utility of additional members of the TNF superfamily as therapeutic targets (Table ). Lymphotoxin-β (LT-β) is detected in synovial membrane cells (of note, LT-β is a cell-associated molecule rather than being secreted from the cell), and its known effector functions, particularly its role in supporting the formation of germinal centers and higher lymphoid organization, suggest that it has a role in the pathogenesis of RA (28
). LTα1β2 and ligand for herpesvirus entry mediator (LIGHT) are both ligands of the receptor for LT-β and have been implicated in the progression of RA. Consistent with a role for LT-β receptor signaling, treatment of individuals with RA with an inhibitor of these two ligands, baminercept (a fusion protein comprising the extracellular domain of the human LT-β receptor linked to the Fc component of human IgG1) resulted in significant clinical improvement in a proof-of-concept study (29
). Other members of the TNF superfamily regulate B cell biology in RA. B lymphocyte stimulator (BLyS; also known as BAFF) and a proliferation-inducing ligand (APRIL) derived from fibroblast-like synoviocytes (FLSs) and mononuclear lineages in the synovium regulate B cell maturation, differentiation, and activation (30
) and are thought likely to promote the production of autoantibodies, including rheumatoid factor and antibodies specific for cyclic citrullinated peptide. In a randomized, placebo-controlled phase I clinical trial, treatment of individuals with RA with an inhibitor of BLyS and APRIL, atacicept (a fusion protein comprising the extracellular portion of human transmembrane activator and CAML interactor [TACI], to which both BLyS and APRIL bind, linked to the Fc component of human IgG1) was well tolerated, with preliminary effects on autoantibody expression noted (31
). However, results of a clinical trial in RA with belimumab (an mAb that binds BLyS but not APRIL) were less impressive (32
Finally, there is intense interest in modulating the RANKL/RANK/osteoprotegerin (OPG) pathway. RANKL is expressed by mesenchymal cells such as FLSs and activated synovial T cells in either its membrane-bound or soluble form. It is upregulated in rodent models of RA and in RA synovium. Together with M-CSF, RANKL has a key role in osteoclast formation through its regulation of osteoclast differentiation, maturation, and induction of resorptive activity (reviewed in ref. 33
). Expression of RANKL is regulated by TNF-α as well as other inflammatory cytokines and noncytokine mediators such as PGE2
). RANKL effector function is modulated by OPG, a soluble decoy receptor also expressed by mesenchymal cells that is present at increased levels in RA synovium (reviewed in ref. 35
). Clinical trials assessing the effects of neutralizing RANKL using a fully human RANKL-specific mAb (denosumab) are ongoing in osteoporosis, metastatic bone disease, and RA, and the phase II clinical trials in RA seem promising with respect to suppressing erosive progression (36
). Together, these data indicate that the TNF superfamily represents a rich source of therapeutic targets with future potential in the treatment of RA.