CDNA CLONING AND EUKARYOTIC EXPRESSION OF FELINE CD9

A monoclonal antibody (vpg15) has been described which can block infec tion with feline immunodeficiency virus (FIV) and which recognizes the feline homologue of CD9. In order to study the role of feline CD9 in infection with FIV we have molecularly cloned a cDNA encoding feline C D9 by R.A.C.E (rapid amplification of cDNA ends). The amino acid seque nce of feline CD9 displays 95.1, 93.8 and 90.7% homology to human, mur ine and bovine CD9, respectively. Although feline CD9 appears most hom ologous to human CD9, it has two important features in common with bov ine and murine CD9: the presence of a histidine residue at position 19 2 which is absent from the corresponding position (194) in human CD9; and the absence of two asparagine residues which are found at position s 51 and 52 of human CD9. Feline CD9 is unique in that it lacks a pote ntial N-linked glycosylation site in the first extracellular loop, a f eature common to CD9 of other species. Despite the high degree of sequ ence homology, significant cross-species variation occurred in the two predicted extracellular loops, notably between amino acids 169 to 180 of the second loop. When feline CD9 was expressed on human and murine cells, it was recognized by both the conformation-dependent feline CD 9-specific antibody, vpg15, and the cross-species reactive anti-human CD9 antibody, FMC56, confirming that the feline CD9 clone encoded a pr otein which was synthesized, transported to the cell surface and expre ssed in a similar conformation to native feline CD9. However, although the vpg15 antibody did not recognize human CD9 when expressed on huma n epithelial cells, it reacted with human CD9 when expressed on murine fibroblast cells. It is possible therefore, that the conformational e pitope recognized by the vpg15 epitope is sensitive to either species- or tissue-specific post-translational modification.