Therapeutic response to infliximab correlates with the pharmacokinetics of infliximab and basal expression of TNFα in synovial tissue. Measurements of infliximab blood levels and TNFα expression in joints suggest that TNFα blockade at the site of production – mainly by cells of the macrophage lineage in the joint – is the key to its mode of action. There are close relationships between the dose of infliximab administered, infliximab blood concentrations, the durability of the clinical response, and eventual return of all clinical features following its clearance from blood 1011. Infliximab given repeatedly at a dose of 1 mg/kg was associated with a rapid loss of therapeutic response and accelerated clearance from the blood 10. However, this study also demonstrated the synergy of infliximab when combined with MTX (Figure 1) 10, which is in part explained by a lowered incidence of anti-infliximab antibodies observed with combination therapy. These data indicate that the antibody affects the effector mechanisms and apparently does not terminate the more proximal events that drive the disease process. As approximately 60% to 70% of patients show an initial response to infliximab, it seems likely that, in a subset of the nonresponder population (defined by the ACR response), TNFα is not the key pivotal molecule regulating the cytokine network at that point of the disease course. However, a clear clinical and radiographic response has been noted in patients who do not demonstrate a response as measured by ACR 20 criteria. This conclusion is supported by the documentation of a correlation between a good clinical response to infliximab only when, prior to treatment, there is a significant level of expression of TNFα in synovial biopsies 18.

It was noted in our first trial that, following the administration of infliximab, simultaneous reductions in CRP and IL-6 concentrations were observed in the blood 7. This correlation was clearly demonstrated in a subsequent study, as was the rapid (within a few hours) reduction in serum IL-6 concentrations in infliximab-treated patients, but not in patients receiving placebo (Figure 2) 19. As CRP production by hepatocytes is predominantly regulated by IL-6, the data are consistent with the conclusion that downregulation of IL-6 production in RA joints was a consequence of TNFα blockade. The dominant role of TNFα in the regulation of IL-6 in RA demonstrated in vivo was entirely consistent with the data obtained on the reduction of IL-6 production following the addition of anti-TNFα antibody to RA synovial membrane cell cultures in vitro20. The reduction of IL-1 synthesis in synovial tissue by an anti-TNFα antibody in vitro was the pivotal observation that led investigators to suspect the involvement of a cytokine cascade in RA 21; however, it has been more difficult to verify these observations in vivo. Quantification of immunoreactive IL-1α and IL-1β by image analysis of synovial biopsies both before and 2 weeks after infliximab therapy revealed a reduction in, and linkage between, TNFα and IL-1 synthesis 18. Serologic analysis of extremely low levels of IL-1 with different assays has provided conflicting results, with significant reductions observed in one laboratory and no consistent trend in another laboratory 1922.

Following infliximab therapy, a reduction in serum concentration of IL-1ra (IL-1 receptor antagonist) and soluble TNF receptors has also provided evidence that two major anti-cytokines are regulated by TNFα (Figure 3) 19. The simultaneous reduction in pro-inflammatory and anti-inflammatory molecules provides an interesting example of the dominance of TNFα in the cytokine network and a possible explanation for why anti-TNFα therapy does not restore a long-lasting remission but instead perpetuates the cytokine imbalance, and hence there is relapse of disease upon withdrawal of therapy.

The marked reduction in the swelling and tenderness of joints following infliximab treatment was shown in an early study to be associated with a reduction in the cellularity of the synovium of RA patients 23. In a detailed immunohistologic analysis of serial biopsies before and after infliximab, it was observed that a reduction in CD3+ and CD68+ cells was accompanied by a reduction in the adhesion molecules vascular cell adhesion molecule-1, intercellular adhesion molecule-1, and E-selectin 24. Soluble E-selectin and soluble intercellular adhesion molecule-1 concentrations in blood were similarly reduced by infliximab, but not by placebo, and this reduction was closely correlated with an increase in circulating lymphocytes 25. In further studies, it was found that the expression of the chemokines IL-8 and monocyte chemotactic protein-1 was also reduced in synovial biopsies within two weeks following infliximab therapy (Figure 4) 242627. These data provided the evidence that anti-TNFα therapy regulates the expression of adhesion molecules and chemokines on rheumatoid vasculature. This led to the hypothesis that reversing the migration of circulating leukocytes into inflamed RA joints, and reversing their retention there, might be an important mechanism of action.

Direct evidence of a reduction in leukocyte recruitment in joints was obtained by gamma-camera imaging of ¹¹¹indium-labelled autologous polymorphonuclear cells in the hands and knees of RA patients before and after infliximab therapy (Figure 5) 26. Because inflammatory disease is dependent on the influx of leukocytes, it is likely that this reduction in leukocyte trafficking is an important aspect of the mechanism of action of infliximab.

From the early stages of disease, rheumatoid synovial inflammation is accompanied by a marked increase in angiogenesis. The increase in blood vessel density provides a conduit for the increased trafficking of blood-borne immune and inflammatory cells into joints. This increase in trafficking leads to the formation of vascular pannus tissue that invades and destroys cartilage and bone in the "bare area" of the attachment of synovium to subchondral bone.

The cytokine vascular endothelial growth factor (VEGF) is implicated in new blood vessel formation and is increased in the joints and blood of RA patients 252829. Infliximab therapy reduces circulating VEGF levels and the density of neovasculature in the synovium 2530 (Figure 6). There is direct evidence of a reduction in the number of blood vessels in infliximab-treated patients. A reduction in angiogenesis may be relevant to our understanding of the anti-inflammatory and antidestructive properties of infliximab. In addition, although unproven, the exudative leakage of plasma mediated by VEGF may also be ameliorated by infliximab.

The most compelling evidence for the ability of anti-TNFα therapy to prevent cartilage loss and bone erosions following the onset of disease was obtained by histologic examination of joints in the collagen-induced arthritis mouse model of RA 6. In this model, preservation of chondrocytes and cartilage matrix and the lack of pannus invasion of bone were notable features in response to treatment with infliximab. In RA patients in the ATTRACT trial, protection of cartilage and bone was observed – possibly with healing – as judged by comparison of baseline and 54-week radiographs of hands and feet in patients treated with infliximab 1225. This finding supports the conclusion that mechanisms of tissue destruction in RA are TNFα-dependent. Whether the coadministration of MTX with infliximab plays a part in the mechanism of action needs to be clarified. Because etanercept as monotherapy significantly slowed progression of bone erosions in early RA patients over one year compared with MTX monotherapy, the bone-protective action of anti-TNFα therapy is not in doubt 31. A reduction in matrix metalloproteinase-1 and matrix metalloproteinase-3 following infliximab treatment has been documented, and although the cellular and molecular basis of anti-TNFα in this regard is not yet understood, this implies that a downregulation of matrix-degrading enzymes may be involved 32.

In animal models, IL-1 appears to play a critical role in cartilage destruction; it has been proposed that IL-1 may be a better therapeutic target in RA and that the joint protective effect of anti-TNFα therapy involves regulation of IL-1 production 3334. The activation and function of osteoclasts appear to involve not only IL-1 and TNFα, but also the receptor activator of NFκB ligand (RANKL), also known as TNF-related activation-induced cytokine (TRANCE), and the interaction of RANKL with RANK 3536. Further work is necessary to delineate the relative importance of these mechanisms.