Background/Purpose: Rheumatoid arthritis is a chronic systemic autoimmune disease characterized by inflammation and joint destruction; a consequence of which is the high local concentration of adenosine. Adenosine, acting primarily through a member of the G-protein coupled receptor (GPCR) family of transmembrane receptors, the A_2a receptor ₍A_2aR), mediates anti-inflammatory and immunomodulatory activity. High affinity A_2aR agonists have demonstrated therapeutic efficacy, however, their utility is hindered by lack of adenosine receptor subtype selectivity and inadvertent activation of the A_2aR at non-disease sites. In light of the high concentrations of adenosine found at sites of chronic inflammation we explored the hypothesis that diminished receptor responsiveness to adenosine may play a role in disease expression. GPCR’s are amenable to an agonist-independent mode of activity enhancement known as positive allosteric modulation (PAM). Positive allosteric modulators have no intrinsic agonist activity, yet enhance the activity of the receptor to its agonist. Using a positive allosteric modulator unique to the A_2aR, AEA061, we examined the effects of PAM of the A_2aR on inflammatory cytokine production and chronic arthritis.

 Methods: To assess the effect of PAM of the A_2aR on cytokine production in vitro, mouse splenocytes were stimulated with LPS in the presence of AEA061. To determine whether PAM of the A_2aR inhibits inflammation in vivo, control and A_2aR deficient mice received vehicle or AEA061 i.v. 10 min prior to i.p. injection of LPS. The role of PAM of the A_2aR on progression of chronic arthritis was evaluated in adjuvant-induced arthritis (AIA). Male Lewis rats were immunized by i.d. injection at the base of the tail with M. tuberculosis H37Ra in Freund’s incomplete adjuvant. Paw volumes and clinical severity were assessed for 26 days. When the mean clinical score = 2, AEA061 was administered i.p. once daily. Splenocytes were cultured and cytokine in culture supernatants and in plasma were quantified using multiplex immunoassays.

 Results: PAM of the A_2aR inhibited LPS-stimulated production of TNF-α, MIP-1α, MIP-1β, MIP-2, IL-1α, RANTES and CXCL1 by mouse splenocytes in a dose-dependent manner in vitro (p < 0.05).  AEA061 also reduced plasma TNF-α and MCP-1 levels while increasing IL-10 levels (p< 0.05) in control but not in A_2aR-deficient mice. In AIA rats, AEA061 reduced paw volumes (p<0.01) as well as reduced clinical scores (p<0.01). Splenocytes from AEA061-treated animals produced decreased IL-1 (45%) and increased IL-10 (341%) ex-vivo.

 Conclusion: PAM of the A_2aR to endogenous adenosine inhibits cytokine production in vitro and in vivo in models of acute inflammation and chronic arthritis. This is the first demonstration of the therapeutic utility of PAM of the A_2aR and supports the notion that impaired receptor responsive to adenosine may contribute to the dysregulation of chronic inflammation. PAM of the A_2aR presents a novel, unique and safe approach that focuses the therapeutic benefit at the site of disease, does not employ exogenous agonists and preserves the natural pattern of A_2aR activation by endogenous adenosine while enhancing receptor responsiveness.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/positive-allosteric-modulation-of-the-adenosine-a2a-receptor-alters-inflammatory-cytokine-production-and-alleviates-chronic-arthritis-in-rats/