Interferon-delta: the first member of a novel type I interferon family.

We have recently described a novel type I interferon (IFN) co-expressed with IFN-gamma by the trophectoderm of the pig conceptus between day 12 and day 18 of gestation, a development stage that corresponds to implantation in the uterus. This IFN, now officially named IFN-delta, is recognized as the first member of a novel type I IFN family. This paper reviews the main published data on IFN-delta, together with some new data, showing that IFN-delta, while being a true type I IFN, has some very specific structural and biological properties. Sequences related to IFN-delta coding sequence were found in the genome of man and other ungulates but the only other potentially functional gene was found, so far, in the horse. The pig IFN-delta mature protein, with 149 amino acids, is the smallest of all known type I IFNs. It is unusually rich in cysteines (seven residues), and has a very basic isoelectric point. Recombinant IFN-delta expressed in insect cells is glycosylated and has a high antiviral activity on porcine cells, but not on human cells. It has high antiproliferative activity, which is significantly enhanced in the presence of IFN-gamma. This new IFN was shown to bind on pig cells to the same type I receptor as IFN-alpha. IFN-delta and IFN-gamma genes are co-regulated in the pig trophectoderm, whose cells on day 14-16 of development simultaneously secrete both IFN proteins. The biological role of porcine IFN-delta in early pregnancy has been found unrelated to the known antiluteolytic effect of trophoblastic IFN-tau in ruminants.