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Prediction of the range or severity of side effects noted after administration of any IFN preparation is impossible. Careful monitoring of the patients, particularly in the earliest stages of treatment, soon reveals the onset of any side effects which might warrant suspension of treatment.
In the last few years additional members of the IFN family has been discovered. Amino acid sequence analysis of a protein called trophoblastin — which is found in many ruminants — revealed that it was closely related to IFN-a. This result was surprising because, in sheep and several other ruminants, the primary function (and until recently the only known function) of trophoblastin is to sustain the corpus luteum during the early stages of pregnancy. The 172 amino acid protein is produced by the trophoblast (an outer layer of cells which surround the cells that constitute the early embryo) for several days immediately preceding implantation. In many ruminants, therefore, trophoblastin plays an essentially similar role to hCG in humans (Chapter 8).
If amino acid analysis hinted that trophoblastin was in fact an IFN, functional studies have proved it. These studies show that trophoblastin:
• displays the same anti-viral activity as type I IFNs;
THE CYTOKINES—THE INTERFERON FAMILY 219
• displays anti-proliferative activity against certain tumour cells, in vitro at least;
• binds the type I IFN receptor.
Trophoblastin has therefore been named interferon-tau (IFN-t), and is classified as a type I IFN. There are at least three or four functional IFN-t genes in sheep and cattle. The molecule displays a molecular mass of 19 kDa and an isoelectric point of 5.5-5.7, in common with other type 1 interferons. Interestingly, the molecule can also promote inhibition of reverse transcriptase activity in cells infected with the HIV virus.
IFN-t is currently generating considerable clinical interest. While it induces effects similar to type I IFN, it appears to exhibit significantly lower toxicity. Thus it may prove possible to safely use this IFN at dosage levels far greater than the maximum dosage levels applied to currently used type I IFNs; however, this can only be elucidated by future clinical trials.
Interferon-omega (IFN-o) represents an additional member of the IFN (type I) family. This 170 amino acid glycoprotein exhibits 50-60% amino acid homology to IFN-as, and appears even more closely related to IFN-t.
IFN-o genes have been found in man, pigs and a range of other mammals, but not in dogs or rodents. The IFN induces its antiviral, immunoregulatory and other effects by binding the type I IFN receptor, although the exact physiological relevance of this particular IFN remains to be elucidated. Recently, a recombinant form of feline IFN-o has been approved within the EU for veterinary use. Its approved indication is for the reduction of mortality and clinical symptoms of parvoviral infections in young dogs. The recombinant product is manufactured using a novel expression system, which entails direct inoculation of silkworms with an engineered silkworm nuclear polyhedrosis virus housing the feline IFN-o gene. Overviewed in Figure 3.10, the production process begins by (automatically) inoculating silkworms (typically 8000 worms/ batch) with the recombinant virus. Infection of silkworm cells results in high-level IFN-o production, which is subsequently acid-extracted from silkworm body parts. After clarification, the product is purified by a two-step affinity chromatographic process (dye affinity chromatography, using blue sepharose, followed by a metal affinity step, using a copper-chelated sepharose column). A final gel filtration step is undertaken before addition of excipients (sodium chloride, D-sorbitol and gelatin). After filling, the product is freeze-dried.
Interferons represent an important family of biopharmaceutical products. They have a proven track record in the treatment of selected medical conditions, and their range of clinical applications continue to grow. It is also likely that many may be used to greater efficacy in the future by their application in combination with additional cytokines.
While it is premature to speculate upon the likely medical applications of IFN-t, the reduced toxicity exhibited by this molecule will encourage its immediate medical appraisal. The classification of t as an IFN also raises the intriguing possibility that other IFNs may yet prove useful in the treatment of some forms of reproductive dysfunctions in veterinary and human medicine.
Abbas, A. (2003). Cellular and Molecular Immunology. W. B. Saunders, London.
Aggarwal, B. (1998). Human Cytokines. Blackwell Scientific, Oxford.
Estrov, Z. (1993). Interferons, Basic Principles and Clinical Applications. R. G. Landes, Florence, KY. Fitzgerald, K. (2001). The Cytokine Facts Book. Academic Press, London.