N6-Cyclohexyl-adenosine but not adenosine is a modulator of teleost fish innate immune activities

The possible immuneregulatory effects of two purine nucleotides, adenosine and cyclohexyladenosine, on gilthead seabream (Sparus aurata L.) leucocytes were investigated. Leucocytes isolated from the head-kidney were incubated with five different concentrations (ranging from 0 to 100 microM) of adenosine or cyclohexyladenosine for 30, 180 and 360 min and the effect on leucocyte viability and some of the main innate cellular immune responses was evaluated. Adenosine did not significantly affect the viability or the innate immune parameters of seabream leucocytes at any of the assayed concentrations or incubation times. High concentrations of cyclohexyladenosine, on the other hand, caused a significant drop in the respiratory burst activity of head-kidney leucocytes at all the times assayed. In addition, phagocytosis of yeast cells was significantly rapidly depressed (30 min) after incubation with 100 microM of this analogue. The present results demonstrate that cyclohexyladenosine at high concentrations is capable of down-regulating certain innate immune responses in seabream leucocytes, suggesting that telesot fish immune cells, like their mammal counterparts, possess receptors for purine nucleotides.