In addition to knockout models, which may help to identify potential targets, blocking studies with neutralizing antibodies or small molecules could provide insight into the overlapping and distinct effects of chemokines and their receptors. Furthermore, animal models can be used to assess the role of several pathogenic factors at various stages of disease, and thus may serve as a sophisticated tool with which to study the relevance of individual chemokines and chemokine receptors in vivo.
Despite the availability of multiple highly specific compounds, species specificity of small molecules and neutralizing antibodies complicates the use of these compounds in animal models. Nevertheless, various chemokines and chemokine receptors have been targeted successfully in animal models of arthritis. Studies using this approach suggest that redundant recruitment mechanisms do not necessarily exclude the possibility that biological and clinical effects may occur after specific chemokine blockade.
For instance, a study using specific blockade of CXCL8/IL-8, an important stimulant of neutrophil accumulation in acute inflammation, showed that it is possible to block neutrophil migration selectively [20
]. In that study a highly specific neutralizing antibody against IL-8 was administered in several types of acute inflammatory disease, including lipopolysaccharide/IL-1 induced arthritis. Anti-IL-8 treatment prevented neutrophil infiltration and resulting tissue damage, despite the fact that CXCL8/IL-8 is also a known ligand for CXCR1, which may be present at high concentrations in the synovial compartment.
Similarly, injection of a specific neutralizing monoclonal antibody against rat CCL2/MCP-1 in rats with collagen-induced arthritis resulted in reduced ankle swelling, in association with decreased macrophage numbers in the joints [21
]. Paw swelling of the hindfeet in the antirat MCP-1 treated rats was decreased to about 70% of that in untreated rats. Moreover, destruction of the joints was significantly reduced. This was confirmed in the MRL/lpr mouse model of RA. In MRL/lpr mice that spontaneously develop chronic inflammatory arthritis, daily injection of the antagonist MCP-1(9–76) prevented the onset of arthritis whereas controls treated with native MCP-1 had enhanced arthritis symptoms [22
]. Of importance for clinical use, there was also a marked reduction in symptoms and histopathology if the antagonist was given only after the disease had already developed. The protective effect on cartilage and bone destruction might be explained in part by the fact that CCL2/MCP-1 is able to stimulate matrix metalloprotease-3 [23
The CXCR4–CXCL12/SDF-1α complex appears to be another interesting target. Because CXCL12/SDF-1α only recognizes a single receptor (i.e. CXCR4), which itself is only recognized by CXCL12/SDF-1α, and because the deletion of CXCR4 has dramatic effects on the phenotype, targeting this molecule in animal models is expected to have significant effects. Blockade of CXCR4 with a synthetic, nonpeptide antagonist that does not crossreact with other chemokine receptors exerted clear beneficial effects, both histopathologically and clinically, in murine collagen-induced arthritis [24
]. Clinical improvement was also achieved when treatment was initiated at the time of disease onset. Apparently, this effect was solely due to inhibition of migration of CXCR4+
cells through the interference with the chemotactic activity of CXCL12/SDF-1α.
CCR5 attracted much attention as a potential therapeutic target for treatment of HIV infection. A nonpeptide antagonist of this chemokine receptor, namely TAK-779, has also been tested in murine collagen-induced arthritis [25
]. Subcutaneous treatment with the CCR5 antagonist initiated a few days before clinical signs of arthritis developed markedly reduced the incidence and severity of the disease, in association with significantly decreased leucocyte migration to the joints.
Taken together, these studies in animal models of arthritis suggest that specific chemokine (receptor) blockade may result in clinically meaningful effects, despite the large number of chemokine family members and their existing overlapping functions. It should be stated, however, that the data are still limited. It remains to be shown whether long lasting effects can be achieved, because it is conceivable that compensatory feedback systems need more time to become effective.