Cloning and expression of the ovine CD40 molecule and the inhibition of the mixed lymphocyte reaction by the ovine CD40e–EGFP fusion protein

The CD40 molecule is a member of the tumour necrosis factor receptor (TNFR)-like supergene family and plays a major role as a co-stimulatory molecule in the activation of T cells in response to antigens presented by dendritic cells. In this study, reverse transcription-PCR cloning was used to derive the sequence encoding ovine CD40. The ovine CD40 sequence demonstrated a similarity of 97, 76 and 64% with the bovine, human and murine sequences, respectively, at the nucleic acid level. The cysteine residues characteristic of the TNFR family and N-linked glycosylation sites are conserved. Furthermore, RNA analysis confirmed expression of CD40 mRNA in both ovine dendritic cells from lymphatic drainage and dermal fibroblasts in culture. In addition, cDNA encompassing the extracellular region of ovine CD40 (CD40e) was fused in-frame' with the enhanced green fluorescent protein (EGFP) to generate a fusion protein upon the transfection of Chinese hamster ovary (CHO) cells. Immunoprecipitation with an anti-GFP monoclonal antibody of a 78kD a protein from conditioned medium of CHO transfectants confirmed that the CD40e-EGFP was secreted in the supernatant. All experiments were controlled with a pEGFP-N1 vector-blank construct. Moreover, the biological activity of ovine CD40e-EGFP was demonstrated by its ability to inhibit a two-way mixed lymphocyte reaction. Thus these observations confirm that ovine CD40 blockade inhibits co-stimulation mediated by CD40-CD40L (CD154) interactions as has been reported in murine and human studies.