Cytokine Therapy:Interferons against viral infection
The language of cells is one that we are still learning. For any group of individuals to work efficiently, communication is essential. Of late, there has been a lot of conversation about immunity. Yet there remains a cloud of obscurity around how it is that some individuals mount a successful immunological response and come out stronger, where some others fail. One way to understand this phenomenon better is to learn how cells communicate among themselves.
Cytokines and their families
Picture this: a viral particle has been identified by one of the cells patrolling the body for intruders. This cell, say A, is now responsible for communicating to another set of guard cells, say Z, who can capture the intruder and imprison it. This communication is done by A releasing certain molecules which carry the message of urgency to Z.
The molecules which communicate among the cells of the immune system are called cytokines, as in they cause cells to move. Owing to the fact that there are numerous different kinds of cytokines, it was thought to be wise to classify them according to the similarities in their structure and function. In this way, cytokines came to be classified into six families. One among them being the topic of our discussion today: interferons.
Figure 1: Upon receiving an external stimulus, a cytokine producing cell secretes cytokines into the extra-cellular environment. These cytokines then travel towards and bind to specific receptors on target cells which leads to activation of desired biological response.
Discovery of interferons
The interferons were the first cytokine family to be discovered, making them all the more significant in the study of cell signalling. It so happened that two groups of scientists working in laboratories half the world away from each other, discovered interferons at the same time. This was in the 1950s.
One was a group of Japanese virologists, who were using rabbit skin and testes tissue culture to develop a vaccine for smallpox. They found that a UV-inactivated cowpox virus when applied to their tissue cultures led to a localized inhibition of viral growth when the same virus was encountered again. This meant that an immunological response had developed from the weakened viral inoculum, making the technique a potential vaccine strategy. The inhibition of viral growth, they postulated, was done by the production of inhibitory factors which were not antibodies. The papers that this group of scientists had published were in French, which prevented their findings to reach the broader scientific community of the time. But it was later understood that these viral inhibitory factors that Nagano and Kojima had reported were in fact interferons.
In a separate study done in London, Isaacs and Lindenman were growing influenza virus on chick egg chorio-allantoic membranes to develop another type of vaccine. In this case, they were exposing the egg membrane to heat-inactivated forms of the influenza virus. This resulted in interference in the growth of the virus on the sensitized membranes, leading them to call the inhibitory factors as interferons. Although by the late 1950s interferons had been discovered, it wasn’t until 1978 that their biochemical and crystallographic analysis was possible owing to the low concentrations in which interferons are produced.
It is now understood that interferons are proteins that interact with receptors on cell surfaces and signal the production of certain enzymes called ribonucleases which can destroy viral and cellular RNA, causing disruption of protein synthesis. In an infected state, destroying viral RNA is a fight against the infection. But interferon administration in a non-infected state simply disrupts the regular protein synthesis which is very much important to keep cellular functions working ceaselessly. There are two major types of interferons, Type I and Type II, of which the former can be sub-divided into two subgroups: interferon α and interferon β.
Signal transduction by Interferons
Interferons α and β are secreted by various different cells upon being infected by viruses. The secreted Type I interferons interact with receptors found on the surface of other cell types. Type I interferons exert their anti-viral effects by binding to IFNAR (IFN alpha receptor). This activates IFNAR to recruit and activate the JAK-STAT pathway. This induces expression of various genes that block viral replication:
· Protein kinase R- which inhibits viral and cellular protein synthesis by inhibiting elongation factor eIF2α
· 2’,5’- oligoadenylate synthetase- which activates ribonuclease RNase L that degrades viral mRNA
· Mx proteins-which inhibit both the transcription of viral genes into mRNA and the assembly of viral particles.
Interferons in clinical therapy
In 1978, Sidney Pestka and his colleagues succeeded in purifying human interferon. Upon purification, the amino acid sequence was determined. Eventually, the production of human interferons through recombinant DNA technology became a real possibility. By 1979, pharmaceutical giant Roche was negotiating collaborations with biotech trailblazer Genentech to clone the human interferon in vitro. In June 1986, recombinant interferon alfa became the first genetically engineered drug to get market approval from the FDA to treat hairy cell leukemia. The FDA approved drug was developed by Hoffman-La Roche in partnership with Genentech and sold under the name Roferon. It then went on to get approval for treating AIDS related Kaposi’s sarcoma, Hepatitis B, and Hepatitis C.
The recombinant product is identical to one of approximately 15 subtypes of interferon alpha produced by human leukocytes, and it is produced in bacteria using rDNA technology. The biological activities of interferon alfa-2a are species restricted as they are expressed in a limited number of species other than humans. Pre-clinical evaluation of interferon alfa-2a involved in vitro experiments with human cells and some in vivo studies. Using human cells in culture, it has been shown to have anti-proliferative and immunomodulatory activities that are similar to the interferon alpha subtypes produced by human leukocytes.
The world has come a long way in employing these inhibitory factors to fight back several viral infections. Now, recombinantly produced interferons (IFN) are made by the biotechnology industry at costs which make their clinical use practical. Although a clear understanding of the exact mechanism of action still remains a challenge, the current knowledge that the industry has about interferons make them a viable option for treating viral infections and related cancers. The discovery and subsequent studies into interferons have opened up several possibilities for treating difficult diseases, and they remain a promising player in the world of cytokine-based therapies.
Reference:
1. How to produce ‘marketable and profitable results for the company’: from viral interference to Roferon A, Carsten Timmermann, History and Philosophy of the Life Science, 2019
2. Kuby, Immunology, 8th Edition
3. Roferon Monograph, 2008
4. www.rcsb.org
