Interferons (/ntrfrn/) are a family of signalling proteins produced and released by host cells in response to the presence of a variety of viruses. A virus-infected cell will typically release interferons, causing neighbouring cells to boost their antiviral defences.
IFNs are part of the cytokine family of proteins, which are utilised to communicate between cells in order to activate the immune system’s defensive defences and help destroy pathogens.
Interferons get their name from their ability to shield cells from virus infections by “interfering” with viral reproduction. IFNs have a variety of different tasks, including activating immune cells such as natural killer cells.
Types of interferon
Based on the type of receptor through which they signal, human interferons have been classified into three major types.
- Interferon type I: All type I IFNs bind to a specific cell surface receptor complex known as the IFN-α/β receptor (IFNAR) that consists of IFNAR1 and IFNAR2 chains. The type I interferons present in humans are IFN-α, IFN-β, IFN-ε, IFN-κ and IFN-ω. In general, type I interferons are produced when the body recognizes a virus that has invaded it. They are produced by fibroblasts and monocytes. However, the production of type I IFN-α is inhibited by another cytokine known as Interleukin-10. Once released, type I interferons bind to specific receptors on target cells, which leads to expression of proteins that will prevent the virus from producing and replicating its RNA and DNA. Overall, IFN-α can be used to treat hepatitis B and C infections, while IFN-β can be used to treat multiple sclerosis.
- Interferon type II (IFN-γ in humans): This is also known as immune interferon and is activated by Interleukin-12. Type II interferons are also released by cytotoxic T cells and type-1 T helper cells. However, they block the proliferation of type-2 T helper cells. The previous results in an inhibition of Th2 immune response and a further induction of Th1 immune response. IFN type II binds to IFNGR, which consists of IFNGR1 and IFNGR2 chains.
- Interferon type III: Signal through a receptor complex consisting of IL10R2 (also called CRF2-4) and IFNLR1 (also called CRF2-12). Although discovered more recently than type I and type II IFNs, recent information demonstrates the importance of Type III IFNs in some types of virus or fungal infections.
In general, type I and II interferons are responsible for regulating and activating the immune response. Expression of type I and III IFNs can be induced in virtually all cell types upon recognition of viral components, especially nucleic acids, by cytoplasmic and endosomal receptors, whereas type II interferon is induced by cytokines such as IL-12, and its expression is restricted to immune cells such as T cells and NK cells.
All interferons share several common effects: they are antiviral agents and they modulate functions of the immune system. Administration of Type I IFN has been shown experimentally to inhibit tumor growth in animals, but the beneficial action in human tumors has not been widely documented. A virus-infected cell releases viral particles that can infect nearby cells. However, the infected cell can protect neighboring cells against a potential infection of the virus by releasing interferons. In response to interferon, cells produce large amounts of an enzyme known as protein kinase R (PKR). This enzyme phosphorylates a protein known as eIF-2 in response to new viral infections; the phosphorylated eIF-2 forms an inactive complex with another protein, called eIF2B, to reduce protein synthesis within the cell. Another cellular enzyme, RNAse L—also induced by interferon action—destroys RNA within the cells to further reduce protein synthesis of both viral and host genes. Inhibited protein synthesis impairs both virus replication and infected host cells. In addition, interferons induce production of hundreds of other proteins—known collectively as interferon-stimulated genes (ISGs)—that have roles in combating viruses and other actions produced by interferon. They also limit viral spread by increasing p53 activity, which kills virus-infected cells by promoting apoptosis. The effect of IFN on p53 is also linked to its protective role against certain cancers.