Interferon: Meaning, Classes and Clinical Application

In this article we will discuss about:- 1. Meaning of Interferon 2. Discovery of Interferon 3. Classes 4. Mechanism of Action 5. Receptors and Signal Transduction 6. Role 7. Commercial Production 8. Clinical Applications.

Meaning of Interferon:

Interferons (INF) are produced by human and animal cells in minute quantities mainly in response to viral infections, and help eliminate such infections. Interferon appears to be body’s first line of defense against viral infection and is considered as nonspecific resistance factor because it does not exhibit specificity towards a particular virus.

That is, an interferon produced in response to one virus is also effective in eliminating the infection caused by another virus. Although interferon is nonspecific toward the viral pathogen it, is specific for the host, i.e., the interferon produced by human cells proves effective only in human cells and not in others.

All the interferons are proteins. The human alpha interferons (IFN-α) are proteins having 165 or 166 amino acids as they are produced by the cells.

Some smaller forms of these interferons have been found in culture medium. B and large, all the alpha interferons are proteins that are not-glycosylated although some IFN-α species are glycoproteins. In contrast, IFN-β, IFN-ϒ and IFN-ω appear to be glycoproteins as produced from natural sources.

Discovery of Interferon:

Interferon was discovered in 1957 by Isaacs and Lindenmann. The experiment was designed to understand the property of viral interference. It had been known for many years that infection by one virus blocks or inhibits infection by another.

When a patient is ill with the measles, it is rare to find that patient infected with chickenpox at the same time or shortly thereafter. The experiment they carried out was a simple and direct one Fig 11.14).

They infected chick chorioallantoic membranes with influenza virus; the supernatant from the infected membranes (cells) was cleared of virus and added to a fresh set of chick membranes; the second set of chick membranes that was treated with the supernatant from the first set was then influenza virus.

The membranes treated with the virus-free supernatant from the first set of infected membranes were resistant to infection by influenza virus. They concluded that after viral infection a substance which could confer upon cells the ability to resist virus infection was secreted into the medium. They named this material interferon.

Since that time a great number of studies have been carried out to characterize and to understand the mechanism of action of this substance interferon. Over the last two decades it was found that the substance originally called interferon represents one member of a large family of substances.

Classes of Interferon:

The interferons represent proteins with antiviral activity that are secreted from cells in response to a variety of stimuli. There are two types of interferons (Table 11.2), Type I and Type II, and interferon-like cytokines. Type I interferons consist of seven classes—IFN-α, IFN-β, IFN-ԑ, IFN-κ, IFN-ω, IFN-δ, and IFN-τ. Type II interferon consists of IFN-ϒ only.

Four interferon-like cytokines have been reported: limitin (found only in mice), IL-28A, IL-28B, and IL-29 found in human and other mammals. IFN-α, IFN-β, IFN-ԑ, IFN-κ, IFN-ω. IL-28A, IL-28B, and IL-29 are found in humans, whereas IFN-δ, IFN-τ, and limitin are not. IFN-τ was described first as ovine trophoblast protein-1 and is found in ungulates where it is required for implantation of the ovum, but there is no direct human homologue.

Human IFN-k, although it exhibits low specific antiviral activity is expressed in human keratinocytes. IL-28A, IL-28B, and IL-29 are found in humans and other mammals and function like Type I interferons.

Human IFN- ԑ has not been characterized in significant detail; however, it appears to play a role in reproductive function in placental mammals. Interferon appears also in reptiles and fish. Interferon-like substances have been reported in virtually all vertebrates. It is not likely that invertebrates produce interferon. Interferons were the first cytokines discovered and the first to be used therapeutically.

Mechanism of Action of Interferon:

The mechanism of action of interferons is interesting in the sense that the interferons produced by host cells in response to viral attack have no direct effect on the viruses, rather they induce an antiviral state in neighbours healthy host cells that prevents viral replication in such cells (Fig. 11.15). Double-stranded RNA viruses are the most potent inducers of interferon synthesis.

When a virus attacks the host cell and releases its genome into the host cytoplasm, the viral genome interacts with the host genome and regulates the synthesis of interferon at the level of transcription. The interferons so synthesized in infected host cells are released and migrate to neighbouring unaffected host cells.

When interferons move to neighbouring uninfected cells, they bind to specific receptor sites on the surface of these cells and stimulate them to produce certain antiviral proteins within them. It is considered that these antiviral proteins are translational inhibitory proteins and block the translation of mRNA molecules of the host cells and, therefore, the viral replication is ultimately blocked in such host cells.

Interferons work in a complex manner and involve a series of molecular events to create antiviral state in the host cells (Fig. 11.16). Interferons bound to specific receptor sites on the surface of uninfected host cells stimulate them to produce at least two enzymes: 2′, 5′-oligoadenylate synthetase and protein kinase.

The 2′ 5′- oligoadenylate synthetase catalyses the synthesis of 2′, 5′-oligoadenyIate, an unusual polymer which activates mRNA endonuclease.

The activated mRNA endonuclease cleaves and thereby inactivates the viral RNA- genome. The protein kinase, however, is activated only if the viral genome is double-stranded RNA. The activated protein kinase catalyzes the phosphorylation of a factor elF-2a.

The factor elF-2a is required tor the initiation of protein synthesis but it becomes inactive when phosphorylated. As a result of the phosphorylation of elF-2a factor, the protein synthesis is stopped and. therefore, the synthesis of viral proteins too ceases.

Receptors and Signal Transduction in Interferon:

The interferons function by binding to a cell surface receptor. There are two classes of receptors: the cell surface receptors specific for IFN-α/β/ω/τ (Type I receptor) on the one hand; a separate receptor binds and responds to IFN- ϒ, the Type II receptor.

After interferon interacts with its receptor, a signal is rapidly transmitted within the cell. One result of this process is the activation or mobilization of specific transcription factors which interact with genes whose promoters contain a specific DNA sequence called an interferon regulatory element.

New mRNAs and their protein products confer upon the cell the properties associated with interferon: antiviral, anti-proliferative stimulation of surface antigens such as the major histocompatibility complex antigens. A large number of these induced genes have been identified and are being studied to understand the detailed mechanisms of these processes.

As noted above, the interferon receptors are of two essential types: the type I receptor for IFN-α, -β, -ω and -τ; the type II receptor for IFF-ϒ. Although not all the pathways linking the receptor to signal transduction have been elucidated, a clear understanding how a type II receptor functions, at-least in one pathway has been elucidated.

Type I Interferon Receptor:

Although much has been learned about the type I receptor (Fig. 11.17), all the functional units still need to be delineated. The cell surface receptor consists of atleast three subunits. No single subunit appears to be able to bind all the type I interferons effectively. However, together the subunits function very well to bind these interferons and initiate the signal transduction cascade.

The signal transduction cascade consists of the following steps through one of the pathways: phosphorylation of two JAK kinases called Jak1 and Tyk2 and phosphorylation of three cytoplasmic transcription factors termed p91 (Stat1α) p84 (Stat1β)andp113 (Stat2); together with an associated transcription factor p48, which does not get phosphorylated, Stat1-α, Stat1-β, and Stat2 translocate to the nucleus to bind to the interferon sequence regulatory elements (ISRE) of the type I interferon-induced genes. Although this appears to be one of the pathways for type I interferon action, other pathways are also involved.

Type II Interferon Receptor:

As stated earlier, it is type II interferon receptor the functions of which, at-least in one pathway, has been elucidated. The type II receptor (Fig. 11.18) for IFN-γ consists of two subunits. Subunit 1 (γ R1) binds the ligand IFN- γ but cannot by itself induce any signal in the cell. Subunit 2 (γ R2) does not alone bind IFN- γ. Together γR1 and γR2 permit IFN- γ to bind to the cell surface receptor and initiate the signal transduction cascade.

The signal transduction cascade consists of several events not necessarily in this order. Phosphorylation of two tyrosine kinases called JAK1 and JAK2, phosphorylation of the receptor, and the subsequent phosphorylation of a transcription factor termed STAT 1α.

After phosphorylation transcription factor STAT1α dimerizes then translocates to the nucleus where it activates all IFN- γ -inducible genes containing a γ activation sequence (GAS).

This apparently simple pathway represents one aspect of the signal transduction cascade with IFN- γ. It is likely that there are other pathways involved, but these have not yet been well delineated.

For example, several groups have reported that phosphohpase C (PLC) is involved and required for activation of T_H1 cells. It is noteworthy that T_H1 cells do not express the IFN- γ R2 chain, whereas T_H2 cells do. Both T_H1 and T_H2 cells express the IFN- γ R1 chain of the receptor.

Overall Role of Interferons:

IFN-α, IFN-β and probably IFN-ω exhibit more potent antiviral activity than IFN- γ. It is felt that these interferons (IFN- α, IFN- β and IFN- ω) induced by viruses are the first and immediate defense against viral infection. When a virus infects a patient, these interferons are produced by the first cells infected. The interferons produced circulate to other cells not yet infected.

A series of biochemical reactions are induced in these uninfected cells that makes them resistant to virus infection. This occurs relatively rapidly (within 12-24 hours) much before the humoral immune response (the antibody system) can be mobilized. IFN-τ is required for implantation of the fertilized ova in ruminant species only. It does not appear to be present in other species (humans, mice, for example).

In contrast to these interferons, IFN- γ is not induced by viral infection. It is induced normally by exposure of the patient to an antigen after a previous primary exposure. IFN- γ is produced by a subset of T-lymphocytes in the course of many immune responses. Its major roles are therefore confined to modulation of the immune system.

It is a lymphokine involved in intercellular communication in the immune system. It can stimulate cytotoxic activity of T-cells, natural killer cells and macrophages. It can modulate the antibody response of B- cells. IFN- γ may be involved in T-cell development in the thymus. Although it exhibits antiviral activity, its antiviral activity is probably not its primary role in vivo. T_H2, but not T_H1 cells, respond to IFN- γ.

Commercial Production of Interferon:

The large-scale production of interferon has increased multi-fold now a days. Until 1980, the sole source of human interferon was the human cells. The WBCs from donors were deliberately infected with viruses that stimulated them to produce interferon. Blood taken from 90,000 donors provided only one gram (1.0 gm) of interferon in a form which was, at best, only 1% pure. Interferon was thus extremely costly.

But, at last, the technique of genetic engineering came to forefront and the genetic engineers successfully integrated interferon gene into a plasmid and introduced it into E. coli and Methylophilus methylotrophus bacteria.

They have now obtained the entire family of human interferon in large quantities and the world’s supply of interferon has increased dramatically. Its cost is also reduced by about 90% and, most importantly, very pure interferon became available.

The in vitro synthesis of interferon and its collection in pure form is so emphasized due to the following reasons:

(i) The interferons are being used to treat various viral diseases. It has been used for quite a while in Russia for the treatment of common cold and influenza diseases, and has also been tried against hepatitis and herpes zoster,

(ii) Interferons are hailed as the new hope for cancer sufferers and, indeed, there are hopeful signs that it can knock out cancer. Encouraging results have been found where interferon seems to benefit patients suffering from cancer of skin, bone, breast and blood. It is assumed that the interferon kills the cancerous cells by increasing host immunity,

(iii) Interferons play a direct role in regulating the immune system and suppress the multiplication of both β and γ-lymphocytes in vitro. They are also considered to inhibit antibody molecules in vitro.

Clinical Applications of Interferon:

The interferons have been found to have a wide array of clinical uses, which are the following:

1. Although discovered as an antiviral agent, the first approved clinical use for alpha interferon has been its use for the treatment of a malignancy, hairy cell leukemia. It has been highly effective in the treatment of this disease. Patients with hairy cell leukemia can live normal lives on interferon treatment.

2. Interferon has also been approved for the treatment of another malignancy, Kaposi’s sarcoma, a cancer associated with the acquired immune deficiency syndrome (AIDS).

3. It has been shown to be effective in the treatment of chronic myelogenous leukemia as well as in the treatment of a variety of other tumours to one extent or another. It is the most effective treatment for metastatic malignant melanoma. However, the side effects of interferon limit the dose that can be given to patients. This dose limitation due to side effects will need to be overcome if IFN-α and other interferons are to have a major role in treating malignancies.

4. Although interferon has been found to be effective in preventing rhinovirus infection, it has not yet been approved for the prevention of the common cold. Approximately 50% of colds are produced by rhinoviruses. However, another 50% of colds are produced by the influenza, parainfluenza, corona, and other viruses. Not all of these are effectively blocked by introducing interferon intranasally prior to infection. Thus, only about 90% of the colds can be prevented by interferon.

5. It was shown that chronic hepatitis B and C can be treated with IFN-α. Since no other treatment for these diseases exist, interferon is the only acceptable treatment at this time. Furthermore, IFN-α has been approved for the treatment of human papilloma virus disease, specifically, genital warts or condyloma acuminata.

Though it has been found that IFN- α is remarkably useful in the treatment of another human papilloma virus infection, laryngeal papillomatosis, it has not yet been approved for use in treatment of this disease. IFN- α is effective for the treatment of haemangiomas as it is a potent inhibitor of angiogenesis. Reports indicate that it is useful for the treatment of diabetic retinopathy and Crohn’s disease (regional enteritis).