The interferons

Some cytokines have already gained approval for medical use. Many more are currently undergoing clinical or pre-clinical trials. Over the next few chapters, the biology and potential medical applications of these cytokines will be discussed in detail. The remainder of this chapter concerns itself with the prototypic cytokine family — the interferons. 

Interferons (IFNs) were the first family of cytokines to be discovered. In 1957 researchers observed that if susceptible animal cells were exposed to a colonizing virus, these cells immediately become resistant to attack by other viruses. This resistance was induced by a substance secreted by virally-infected cells, which was named interferon (IFN). Subsequently it has been shown that most species actually produce a whole range of interferons. Humans produce at least three distinct classes, IFN-a, IFN-)S and IFN-y (Table 4.4). These interferons are produced by a variety of different cell types, and exhibit a wide range of biological effects, including  

No one IFN will display all of these biological activities. Effects are initiated by the binding of the IFN to its specific cell surface receptor present in the plasma membrane of sensitive cells. 

Interferon family Additional name No of distinct IFNs in family Producing cells 

IFN-a Leukocyte IFN, B cell IFN, l5miphoblast IFN 15 L5rniphoc5des, monocytes, macrophages 

No one interferon will display all of these biological activities. Effects are initiated by the binding of the interferon to its specific cell surface receptor present in the plasma membrane of sensitive cells. IFN-a and -P display significant amino acid sequence homology (30 per cent), bind to the same receptor, induce similar biological activities and are acid stable. For these reasons, IFN-a and IFN-P are sometimes collectively referred to as type I interferons, or acid-stable interferons. 

IFN-y is evolutionarily distinct from the other interferons it binds to a separate receptor and induces a different range of biological activities. It is thus often referred to as type II interferon. 

---

Interferons (lENs) (52,53), a family of species-specific vertebrate proteins, confer nonspecific resistance to a broad range of viral infections, affect cell proliferation, and modulate immune responses. AH three principal interferons, a-interferon (lEN-a) produced by blood leucocytes, P-interferon (lEN-P) by fibroblasts, and y-interferon (lEN-y) by lymphocytes, also have antiviral activity. The abiUty of interferons to inhibit growth of transplantable and carcinogen-induced tumor led to research showing the direct antiproliferative and indirect immune-mediated antitumor activities (see Chemotherapeutics, anticancer). IENs have been found to be efficacious in certain malignancies and viral infections, eg, hairy cell leukemia (85% response) and basal cell carcinoma (86% response). However, the interferons do have adverse side effects (54). 

The supernatants containing the interferons were decanted from monolayers, pooled,and tested for freedom from bacteria. Residual arborvirus was inactivated by acid and heat as follows. The liquid was brought to pH 2 by the addition of 0.3N hydrochloric acid in Earle s saline (minus sodium chloride and sodium bicarbonate), kept at 4°C for 24 hours, and then brought back to pH 7 by the addition of 0.3N sodium hydroxide in distilled water. The liquid was then heated at 56°C for 30 minutes. 

At this stage the interferon preparations were assayed and submitted to safety tests for the absence of contaminating viruses. 

Rhesus monkey kidney infected with Semliki Forest arborvirus gave interferon of tltre 1.5 log interferon units/2 ml. (The interferon unit, determined in a volume of 2 ml, is the dilution of interferon which produced a half-maximal score for degree of cytopathic effect In virus-infected tissue culture tubes at the time when the control without interferon first showed the maximal score.)... 

Interim results from the SPRINT-1 phase 2 trial of boceprevir (SCH 503034) have been released. In subjects who received boceprevir plus interferon-a and ribavirin, viral RNA loads were suppressed at week 12 in between 70 and 79% of subjects infected with genotype 1 HCV, compared with only 34% in the interferon-o/ ribavirin standard of care arm (www.sch-plough.com/schering plough/news/release. jsp releaseID = 1064540). However, it is not yet known if this enhanced early response will translate into sustained response. 

Kefford RF. Adjuvant therapy of cutaneous melanoma The interferon debate. Ann Oncol 2003 14 358-365. 

Biomedical research continues to broaden our understanding of the molecular mechanisms underlining both health and disease. Research undertaken since the 1950s has pinpointed a host of proteins produced naturally in the body that have obvious therapeutic applications. Examples include the interferons and interleukins (which regulate the immune response), growth factors, such as erythropoietin (EPO which stimulates red blood cell production), and neurotrophic factors (which regulate the development and maintenance of neural tissue). 

One of the most popular bioassay for interferons is termed the cytopathic effect inhibition assay . This assay is based upon the ability of many interferons to render animal cells resistant to viral attack. It entails incubation of the interferon preparation with cells sensitive to destruction by a specific virus. That virus is then subsequently added, and the percentage of cells that survive thereafter is proportional to the levels of interferon present in the assay sample. Viable cells can assimilate certain dyes, such as neutral red. Addition of the dye followed by spectrophotometric quantitation of the amount of dye assimilated can thus be used to quantitate percentage cell survival. This type of assay can be scaled down to run in a single well of a microtitre plate. This facilitates automated assay of large numbers of samples with relative ease. 

The term cytokine was first introduced in the mid 1970s. It was applied to polypeptide growth factors controlling the differentiation and regulation of cells of the immune system. The interferons and interleukins represented the major polypeptide families classified as cytokines at that time. Additional classification terms were also introduced, including lymphokines (cytokines such as IL-2 and IFN-y, produced by lymphocytes) and monokines (cytokines such as TNF-a, produced by monocytes). However, classification on the basis of producing cell types also proved inappropriate, as most cytokines are produced by a range of cell types (e.g. both lymphocytes and monocytes produce IFN-a). 

The interferon receptor superfamily Cytokine receptor type II family Receptors for IFN-cx, -P, -y, IL-10... 

Some cytokine receptors can directly initiate signal transduction upon binding of ligand. In other cases additional elements are involved. For many receptors, the exact intracellular events triggered upon ligand binding remain to be elucidated. However, the molecular details of signal transduction pathways for others (e.g. the interferons) are now understood... 

All interferon-stimulated genes are characterized by the presence of an associated interferon-stimulated response element (ISRE). Signal transduction culminates in the binding of specific regulatory factors to the ISRE, which stimulates RNA polymerase Il-mediated transcription of the interferon-sensitive genes. The induced gene products then mediate the antiviral, immunomodulatory and other effects characteristically induced by interferons. 

---