Chapter 8 The interferon family

1. Pharmaceutical Biotechnology Chapter 8The interferon family Dr. Tarek El-Bashiti Assoc. Prof. of Biotechnology

2. Cytokines • Cytokines are a diverse group of regulatory proteins or glycoproteins whose classification remains somewhat diffuse (Table 8.1). • These molecules are normally produced in minute quantities by the body. • They act as chemical communicators between various cells, inducing their effect by binding to specific cell surface receptors, thereby triggering various intracellular signal transduction events. • Most cytokines act upon, or are produced by, leukocytes (white blood cells), which constitute the immune and inflammatory systems (Box 8.1).

4. Indeed, several immunosuppressive and anti-inflammatory drugs are now known to induce their biological effects by regulating production of several cytokines. • 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-) produced by lymphocytes) and monokines (cytokines such as TNF-, produced by monocytes). • Initial classification of some cytokines was also undertaken on the basis of the specific biological activity by which the cytokine was first discovered (e.g. TNF exhibited cytotoxic effects on some cancer cell lines; CSFs promoted the growth in vitro of various leukocytes in clumps or colonies).

5. This, too, proved an unsatisfactory classification mechanism, as it was subsequently shown that most cytokines display a range of biological activities (e.g. the major biological function of TNF is believed to be as a regulator of both the immune and inflammatory response). • More recently, primary sequence analysis of cytokines coupled to determination of secondary and tertiary structure reveal that most cytokines can be grouped into one of six families (Table 8.2).

8. 2. Cytokines as biopharmaceuticals • Cytokines, in many ways, constitute the single most important group of biopharmaceutical substances. • As coordinators of the immune and inflammatory response, manipulation of cytokine activity can have a major influence on the body’s response to a variety of medical conditions. • Administration of certain cytokines can enhance the immune response against a wide range of infectious agents and cancer cells. • EPO has proven effective in stimulating red blood cell production in anaemic persons.

9. Growth factors have obvious potential in promoting wound healing. • A better understanding of the molecular principles underlining cytokine biology may also provide new knowledge-based strategies aimed at defeating certain viral pathogens. • These pathogens appear to establish an infection successfully, at least in part, by producing specific proteins that thwart the normal cytokine-based immunological response. • The cowpox virus, for example, produces an IL-1-binding protein, and the shope fibroma virus produces a TNF-binding protein.

10. The Epstein–Barr virus, on the other hand, produces a protein homologous to IL-10. • A variety of medical conditions are now believed to be caused or exasperated by overproduction of certain cytokines in the body. • A variety of pro-inflammatory cytokines, including IL-6, -8 and TNF, have been implicated in the pathogenesis of both septic shock and rheumatoid arthritis. • Inhibiting the biological activity of such cytokines may provide effective therapies for such conditions.

11. This may be achieved by administration of monoclonal antibodies raised against the target cytokine, or administration of soluble forms of its receptor that will compete with cell surface receptors for cytokine binding. • Some cytokines have already gained approval for medical use. • Many more are currently undergoing clinical or preclinical trials.

12. 2. The interferons • Interferons were the first family of cytokines to be discovered. • In 1957, researchers observed that susceptible animal cells, if they were exposed to a colonizing virus, immediately became resistant to attack by other viruses. • This resistance was induced by a substance secreted by virally infected cells which was named interferon. • Subsequently, it has been shown that most species actually produce a whole range of interferons. • Humans produce at least three distinct classes, IFN-α, IFN-βand IFN-γ (Table 8.4).

13. These interferons are produced by a variety of different cell types and exhibit a wide range of biological effects, including: • induction of cellular resistance to viral attack; • regulation of most aspects of immune function; • regulation of growth and differentiation of many cell types; • sustenance of early phases of pregnancy in some animal species.

14. 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-α and -β 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-α and IFN-β are sometimes collectively referred to as type I interferons, or acid-stable interferons.

16. Interferon-α • In humans, at least 24 related genes or pseudo-genes exist that code for the production of at least 16 distinct mature IFN-αs. • These can be assigned to one of two families, i.e. type I and II. • Humans are capable of synthesizing at least 15 type I IFN-αs and a single type II IFN-α.

17. Individual members of the IFN-α family each have an identifying name. • In most cases the names were assigned by placing a letter after the ‘α’ (i.e. IFN-αA, IFN-αB, etc.). • However, some exceptions exist which contain a number or a number and letter, e.g. IFN-α7, IFN-α8, IFN-α2B. • Just to ensure total confusion, several are known by two different names, e.g. IFN-α7 is also known as IFN-αJ1.

18. 2. Interferon-β • IFN-β, normally produced by fibroblasts, was the first interferon to be purified. • Humans synthesize a single IFN-β molecule, containing 166 amino acid residues, that exhibits 30 percent sequence homology to IFN-αs. • The mature molecule exhibits a single disulfide bond and is a glycoprotein of molecular mass in excess of 20 kDa.

19. 3 Interferon-γ • IFN-γ is usually referred to as ‘immune’ interferon. • It was initially purified from human periph-eral blood lymphocytes. This interferon is produced predominantly by lymphocytes. Its synthesis by these cells is reduced when they come in contact with presented antigen. Additional cytokines, including IL-2 and -12, can also induce IFN-γ production under certain circumstances. A sin- gle IFN-γ gene exists, located on human chromosome number 12. It displays little evolutionary homology to type I interferon genes. The mature polypeptide contains 143 amino acids with a predicted molecular mass of 17 kDa.