Enhanced IL-6 transcriptional response to adenosine receptor ligands in horses with lower airway inflammation

Accumulation of extracellular adenosine has been closely associated with human asthmatic responses. However, the relevance of adenosine signalling in equine airways has not previously been investigated.

To determine the expression of adenosine receptors (AR) in bronchoalveolar lavage (BAL) cells and assess the reactivity of these cells to AR ligands ex vivo, employing IL-6 as readout of adenosinergic inflammatory signalling.

Eight horses with varying degrees of lower airway inflammation and 10 healthy controls were analysed. Expression of AR-subtypes in each BAL sample was determined by quantitative RT-PCR and compared to that in 13 other tissues. Bronchoalveolar lavage cells were stimulated either with the adenosine analogue NECA, CGS-21680 (A(2A) AR selective agonist) or with a combination of NECA and SCH-58261 (A(2A) AR antagonist) and IL-6 expression assessed.

Bronchoalveolar lavage cells predominantly expressed A(2B) AR, with lower A(2A) AR levels and marginal A(3) AR expression; A(1) AR was not detected. This pattern was similar to that of PBMCs but different from the other tissues tested. No significant differences in AR expression in BAL cells from both groups were detected, although a trend for decreased A(2B) AR in airway-compromised horses was observed. Treatment of BAL cells with the nonselective agonist NECA upregulated IL-6 expression in cells from airway-compromised horses, but levels remained unchanged in control animals. Furthermore, blockage of A(2A) AR with SCH-58261 enhanced IL-6 mRNA induction by NECA in both groups, with higher levels in airway-compromised horses; the amplitude of this response correlated with neutrophil count.

These results demonstrate the presence of an adenosine/IL-6 inflammatory axis in the bronchoalveolar milieu of airway-compromised horses. While A(2B) AR is the predominant proinflammatory AR subtype expressed, A(2A) AR appears to modulate inflammatory signalling (IL-6 expression) by adenosine.

This study supports selective AR targeting as a potential therapeutic approach for the modulation of inflammation in the equine lower respiratory tract.