Interferons

Interferons (IFNs) are a class of natural proteins produced by the cells of the immune systems of most animals in response to challenges by foreign agents such as viruses, bacteria, parasites and tumor cells. Interferons belong to the large class of glycoproteins known as cytokines.

Summary
There are three major classes of interferons: alpha (α), beta (β), and gamma (γ). They generally have several effects: antiviral and antioncogenic properties, macrophage and natural killer lymphocyte activation, and enhancement of major histocompatibility complex glycoprotein classes I and II.

Interferon-α is secreted by leukocytes (B-cells and T-cells). Interferon-β is secreted by fibroblasts, and interferon-γ is secreted by T-cells and natural killer lymphocytes.

All classes of interferon production are very important in the course of RNA virus infections. However, their presence also accounts for some of their symptoms, such as sore muscles and fever. They are emitted when abnormally large amounts of dsRNA are found in a cell. dsRNA is normally present in very low quantities. The dsRNA acts like a trigger for the production of interferon. The gene that codes for this cytokine is switched on in an infected cell, and is secreted to surrounding cells.

As the original cell dies from the cytolytic RNA virus, these thousands of viruses will infect nearby cells. However, these cells have received the interferon, which essentially warns these other cells that there's a wolf in the pack of sheep. They then start producing large amounts of a protein known as protein kinase R (P.K.R. or PKR). If a virus chooses to infect a cell that has been “pre-warned” by interferon, it is like charging into a hail of bullets for the virus.

The P.K.R. is activated by the dsRNA, and begins transferring phosphate groups (phosphorylating) a protein known as eIF2, a eukaryotic translation initiation factor. Upon phosphorylation, eIF2 has a reduced ability to initiate translation, the production of proteins coded by cellular mRNA. This prevents viral replication, but also inhibits normal cell ribosome function, killing both. All RNA within the cell is also degraded, preventing the mRNA from being translated by eIF2 if some of the eIF2 failed to be phosphorylated.

Discovery
While aiming to develop an improved vaccine for smallpox, two Japanese virologists, Yasu-ichi Nagano and Yasuhiko Kojima working at the then Institute for Infection Disease at the University of Tokyo, noticed that rabbit-skin or testis previously inoculated with UV-inactivated virus exhibited inhibited viral growth when re-infected at the same site with live virus. They hypothesised that this was due to some “facteur inhibiteur” (inhibitory factor), and began to characterise it by fractionation of the UV-irradiated viral homogenates using an ultracentrifuge.

They published these findings in 1954 in the French journal now known as “Journal de la Société de Biologie”. While this paper demonstrated that the activity could be separated from the virus particles, it could not reconcile the antiviral activity demonstrated in the rabbit skin experiments, with the observation that the same supernatant led to the production of antiviral antibodies in mice. A further paper in 1958, involving triple-ultracentrifugation of the homogenate demonstrated that the inhibitory factor was distinct from the virus particles, leading to trace contamination being ascribed to the 1954 observations.

Meanwhile, the Scottish virologist Alick Isaacs and the Swiss researcher Jean Lindenmann, at the National Institute for Medical Research in London, noticed an interference effect caused by heat-inactivated influenza virus on the growth of live influenza virus in fragments of chick chorioallantoic membrane. They published their results in 1957; in this paper they coined the term ‘interferon’, and today that specific interfering agent is known as a ‘Type I interferon’.

Nagano’s work was never fully appreciated in the scientific community; possibly because it was printed in French, but also because his in vivo system was perhaps too complex to provide clear results in the characterisation and purification of interferon. As time passed, Nagano became aware that his work had not been widely recognised, yet did not actively seek reevaluation of his status in field of interferon research. As such, the majority of the credit for discovery of the interferon goes to Isaacs and Lindenmann, with whom there is no record of Nagano ever having made personal contact.

Types
In humans, there are 3 major classes of interferon (IFN):


 * 1) The human type I IFNs consists of 13 different alpha isoforms (subtypes with slightly different specificities) - IFNA(1,2,4,5,6,7,8,10,13,14,16,17,21), and single beta - IFNB1, omega - IFNW1, epsilon - IFNE1 and kappa - IFNK isoforms.  Homologous molecules are found in many species, including rats and mice (and most mammals) and have been identified in birds, reptiles, amphibians and fish species. In addition to these IFNs, IFN zeta (limitin) in mice,IFN nu in cats, IFN tau in ruminants and IFN delta in pigs have been identified. All type I IFNs bind to a specific cell surface receptor complex known as IFNAR consisting of IFNAR1 and IFNAR2 chains.
 * 2) The type II IFNs consists of IFN gamma - IFNG, its sole member.  The mature IFNG ligand is an anti-parallel homodimer, and it binds to the IFNG receptor (IFNGR) complex, which is made up of two of each IFNGR1 and IFNGR2 subunits.
 * 3) The recently discovered 3rd class consists of IFN-lambda with 3 different isoforms - IL29. IL28A, IL28B and signal through a receptor complex consisting of IL10R2 and IFNLR1.

While there are evidence to suggest other signaling mechanisms exist, the JAK-STAT signaling pathway is the best-characterised and commonly accepted IFN signaling pathway.

Principles
In a majority of cases, the production of interferons is induced in response to microbes such as viruses and bacteria and their products (viral glycoproteins, viral RNA, bacterial endotoxin, flagella, CpG DNA), as well as mitogens and other cytokines, for example interleukin 1, interleukin 2, interleukin-12, tumor necrosis factor and colony-stimulating factor, that are synthesised in the response to the appearance of various antigens in the body. Their metabolism and excretion take place mainly in the liver and kidneys. They hardly pass the placenta and the blood-brain barrier.

Interferon-alpha and -beta are produced by many cell types, including T-cells and B-cells, macrophages, fibroblasts, endothelial cells, osteoblasts and others, and are an important component of the anti-viral response. They stimulate both macrophages and NK cells. Interferons -alpha and -beta are also active against tumors.

Interferon-gamma is involved in the regulation of the immune and inflammatory responses; in humans, there is only one type of interferon-gamma. It is produced in activated T-cells and natural killer cells. Interferon-gamma has some anti-viral and anti-tumor effects, but these are generally weak; however, interferon-gamma potentiates the effects of interferon-alpha and interferon-beta. However, interferon-gamma must be released at the site of a tumor in very small doses; at this time, interferon-gamma is not very useful for treating cancer.

Interferon-gamma is also released by Th1 cells, and recruits leukocytes to a site of infection, resulting in increased inflammation. It also stimulates macrophages to kill bacteria that have been engulfed. The interferon-gamma released by Th1 cells is also important in regulating the Th2 response. As interferon-gamma is vitally implicated in the regulation of immune response, its production can lead to autoimmune disorders.

Interferon-omega is released by leukocytes at the site of viral infection or tumors.

Pharmacological uses
Interferon was scarce and expensive until 1980 when the interferon gene was inserted into bacteria using recombinant DNA technology, allowing mass cultivation and purification from bacterial cultures.

Interferon beta-1a is produced in mammalian cells.

Several different types of interferon are now approved for use in humans, and interferon therapy is used (in combination with chemotherapy and radiation) as a treatment for many types of systemic cancer. When used in the systemic therapy, IFN-&alpha; and IFN-&gamma; are mostly administered by an intramuscular injection. The injection of interferons in the muscle, in the vein, or under skin is generally well-tolerated. The most frequent side-effects are flu-like symptoms: increased body temperature, feeling ill, fatigue, headache, muscle pain, convulsion, dizziness, hair thinning, and depression. Erythema, pain and hardness on the spot of injection are also frequently observed. All known effects are usually reversible and disappear a few days after the therapy has been finished. However, there are some serious side effects and the patient is advised to read the accompanying pamphlet.

Interferon-alpha was approved by the United States Food and Drug Administration (FDA) on February 25 1991 as a treatment for hepatitis C. Several different forms of interferon alpha, including interferon-alpha-2a, interferon-alpha-2b, and interferon-alfacon-1 are approved for the treatment of viral hepatitis. Interferon-alfa-2b is also used for chronic myelogenous leukemia.

More than half of hepatitis C patients treated with interferon respond, with better blood tests and better liver biopsies. There is some evidence that giving interferon immediately following infection can prevent hepatitis C; however, people infected by hepatitis C often do not display symptoms of HCV until months or years later.

More recently, the FDA approved pegylated interferon-alpha, in which polyethylene glycol is added to make the interferon last longer in the body. (Pegylated interferon-alpha-2b was approved in January 2001; pegylated interferon-alpha-2a was approved in October 2002.) The pegylated form is injected once weekly, rather than three times per week for conventional interferon-alpha. Used in combination with the antiviral drug ribavirin, pegylated interferon produces sustained cure rates of 75% or better in people with genotype 2 or 3 hepatitis C (which is easier to treat) but still less than 50% in people with genotype 1 (which is most common in the U.S. and Western Europe).

Interferon-beta (Interferon beta-1a and Interferon beta-1b) is used in the treatment and control of the neurological disorder multiple sclerosis. By an as-yet-unknown mechanism, interferon-beta inhibits the production of Th1 cytokines and the activation of monocytes.

Administered intranasally in very low doses, interferon is extensively used in Eastern Europe and Russia as a method to prevent and treat viral respiratory diseases such as cold and flu. It is claimed that the treatment can lower the risk of infection by as much as 60-70%. Mechanisms of such action of interferon are not well understood; it is thought that doses must be larger by several orders of magnitude to have any effect on the virus. Consequently, most Western scientists are sceptical of these claims.