Rheumatoid arthritis (RA) is an autoimmune disease in which your T cells attack the lining (synovium) of your own joints. Many attempts to stop or slow rheumatoid arthritis have failed, but a research team led by Drs. Graeme O'Boyle and John Kirby from the Institute of Cellular Medicine, Newcastle University, the United Kingdom, think they're onto a potential new therapy (O'Boyle et al. 2012). In cell cultures and in "humanized" NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG, 005557) mice, they interfered with the immune system's ability to recruit the synovium-attacking T cells. If their results can be duplicated in humans, they might considerably mitigate RA's debilitating effects.
The T cells in rheumatoid arthritis and other autoimmune diseases express CXCR3, a chemokine receptor associated with cell migration. The O'Boyle/Kirby team thwarted the synovium-attacking T cells with PS372424, a small-molecule agonist for CXCR3 and several other chemokine receptors. The chemokines are found in the synovial fluid of people with rheumatoid arthritis and include CXCL12, CXCL11, and CCL5. Normally, they bind to their respective T cell receptors, activating the T cells and recruiting them to attack the joint synovia. The O'Boyle/Kirby team characterized PS372424's major chemokine-disrupting activities as follows:
- Activates T cell ERK (indicating chemokine receptor signaling and essential for T cell migration), desensitizes CXCR3, and affects Ca2+ flux in a manner similar to the natural CXCR3 ligand, CXCL11
- Inhibits the chemotaxis of activated T cells in culture to CXCL11, CXCL12, and CCL5 (in comparison, the CXCR3 antagonist NBI-74330 prevents migration only to CXCL11)
- Desensitizes the chemokine receptors CXCR3, CXCR4, and CCR5 (in comparison, NBI-74330 desensitizes only CXCR3)
- Inhibits the chemotaxis of numerous T cell subpopulations in culture to chemokines in synovial fluid from people with RA (in comparison, neither a CXCR4 antagonist nor a CXCR3 neutralizing antibody does this)
- Does not inhibit normal chemotactic recruitment of resting and regulatory T (Treg) cells, which both express little CXCR3, suggesting it could modify the composition of inflammatory infiltrate in vivo
- Does not cause any noticeable cytokine burst or harmful side effects in human blood
- Reduces the migration of CD45+ T cells to pouches containing CCL5, CXCL11, CXCL1, or human synovial fluid in a humanized NSG mice (demonstrating that it can limit the migration of human cells to a range of chemokines in the synovial fluid of people with RA)
- Appears to desensitize CXCR3 by causing it to heterodimerize with CCR5 or by mediating its phosphorylation, disrupting its ability to recruit heterotrimeric G proteins and therefore inhibiting its signaling mechanism
In summary, the O'Boyle/Kirby team demonstrated that systemically administered PS372424 engages the chemokine T cell migration system and re-directs chemo-attraction (receptor desensitization) away from inflammatory chemokines expressed in synovial fluid. It does so without affecting the migration of potentially beneficial Tregs. The team's research suggests that agonist-induced chemokine receptor desensitization may mitigate not only rheumatoid arthritis but other inflammatory diseases, including psoriasis, multiple sclerosis and inflammatory bowel disease.