Interferons

Interferons are the cytokines that can "interfere" with viral growth.  They also have the ability to inhibit proliferation and modulate immune responses.  Four types of interferons have been identified: IFN-a, IFN-b, IFN-w and IFN-g. The first three are Type I IFNs which have relatively high antiviral potency.  IFN-g is the Type II IFN, also called immune IFN.

Type I IFNs are produced by macrophages, neutrophils and other somatic cells in response to infection by viruses or bacteria.  After they are released, they may bind to their receptors that are expressed on most cell types, resulting in the production of over 30 different proteins in the target cell.  Among them, two enzymes play a critical role in the inhibition of viral replication: RNA-dependent protein kinase (PKR) and 2'-5' oligoadenylate synthetase (2-5A synthetase).  Both enzymes can be activated by double-stranded RNA which may be present in some viruses.  The activated PKR can phosphorylate a protein (eIF2) to inhibit protein synthesis.  The 2-5A synthetase produces oligoadenylate that can bind and activate a cellular endonuclease to degrade mRNA. 

IFN-g is produced in activated T_H1 and NK cells, particularly in response to IL-2 and IL-12.  Its production is suppressed by IL-4, IL-10, and TGF-b.  Binding of IFN-g to its receptor increases the expression of class I MHC on all somatic cells.  It also enhances the expression of class II MHC on antigen-presenting cells. IFN-g may also activate macrophages, neutrophils and NK cells.