Cloning, expression and biological activity of equine interleukin (IL)-5

The cytokine, interleukin (IL)-5 stimulates eosinophil differentiation, activation and survival and can prime these cells, increasing the response to other mediators. In view of its many effects on eosinophils, IL-5 has been implicated in the pathogenesis of allergic disease in man. Here we report the cloning of equine IL-5 and expression of the recombinant protein by transfection of Chinese hamster ovary (CHO) cells. The cloned cDNA sequence consisted of 405 nucleotides and encoded a protein of 135 amino acids. There is >85% identity with feline, bovine, ovine, canine, and human IL-5 sequences at the nucleotide and protein level. Supernatants containing equine IL-5 were also examined for biological activity. CHO supernatant containing equine recombinant (eqr) IL-5, like the human ortholog (hrIL-5), induced concentration dependent equine eosinophil adherence to autologous serum-coated plastic (9.7±1.5% with a 1:100 dilution of eqrIL-5 and 9.1±1.6% adherence with 1nM hrIL-5; n=4). The eqr protein also caused concentration dependent superoxide production (11.9±2.4nmol {reduced cytochrome (cyt) C}/10 ⁶ cells at a 1:50 dilution, n=4). In contrast, hrIL-5 only caused significant superoxide production when diluted in conditioned CHO medium, an effect that was inhibited by the anti-human mAb, TRFK5 (4.4±0.3 versus 0.3±0.4nmol/10⁶ cells for 0.5nM hrIL-5 in the presence of the isotype matched IgG₁ control (10μM) and TRFK5 (10μM), respectively). TRFK5 also significantly inhibited hrIL-5 induced adherence at concentrations of 0.3μg/ml and above but had no significant inhibitory effect on either superoxide or adherence caused by eqrIL-5. These results demonstrate that equine IL-5 expressed by CHO cells stimulates equine eosinophils, suggesting that this cytokine could play a role in eosinophil recruitment and activation in equine allergic disease. The anti-human and murine moAb TRFK5 does not appear to recognise the equine protein.