Molecular characterization of tumor necrosis factor receptors 1 and 2 of the goldfish (Carassius aurutus L.)

Tumor necrosis factor alpha (TNFα) is a pleotropic cytokine that mediates its effects by binding to one of two TNF receptors, TNF-R1 or TNF-R2. We have recently identified the cDNA sequences of both goldfish TNF-R1 and TNF-R2. In silico analyses revealed conserved cysteine rich domains, predicted docking sites for TNFR-specific downstream signaling factors for both receptors and a conserved death domain for TNF-R1. The expression of these receptors in tissues and various immune cell types was investigated by Q-PCR. The TNF-R2 expression was substantially higher than that of TNF-R1 in all tissues. Both receptors were most robustly expressed in monocytes where as the mRNA levels of TNF-R1 were the lowest in mature macrophages and those of TNF-R2 in peripheral blood leukocytes. Treatment of goldfish macrophages with recombinant goldfish (rg) TNFα-2, rgIFNγ or rgTGFβ differentially altered the expression of both TNF receptors. The rgTNFα-2 up-regulated the expression of TNF-R2 but down-regulated the expression of TNF-R1. The rgIFNγ increased the expression of both TNF receptors while rgTGFβ caused a time-dependent decreases in mRNA of goldfish TNF-R1 and TNF-R2. In vitro binding studies using recombinant TNFα-1 and TNFα-2 revealed that either isoform was capable of interacting with the recombinant forms of the extracellular domains of either TNF-R1 or TNF-R2. Functional significance of these ligand–receptor interactions was confirmed by experiments showing that goldfish TNF-R1 and TNF-R2 down-regulated the rgTNFα-1 or rgTNFα-2-primed respiratory burst response of goldfish macrophages.