Interferons
Type I interferons (IFNa and IFNβ) are produced by all cell types and are particularly important in cellular defense against viral infection. Expression of type I IFNs is triggered by virus infection, PAMPs, and proinflammatory cytokines.
Type I IFNs are expressed by a virus-infected cell to inhibit intracellular viral protein synthesis by inhibiting the translation of viral proteins, degrading viral mRNAs, and inhibiting RNA synthesis (Gibbert et al., 2013). They are also secreted to stimulate type I IFN expression by adjacent cells. This protects neighboring cells from viral infection and inhibits virus spread.Type II IFNs (e.g. IFNγ) are produced by T helper 1 (Th1)-type T cells and conventional innate lymphoid cells (ILCs, aka conventional natural killer [cNK] cells), which are components of a cell-mediated immune response. IFNγ induces inflammatory cells to express Fc and C3b receptors, which enhance phagocytosis of organisms opsonized by antibodies and C3b, respectively. IFNγ is the most important mediator for activating macrophages and stimulates nitric oxide (NO) synthetase so that macrophages can produce NO. IFNγ also induces the synthesis of antiprotozoal enzymes (Zhao et al., 2009). IFNγ stimulates ILC subsets to infiltrate into an area infected by a virus and kill the virus-infected cells. Additionally, IFNγ enhances major histocompatibility (MHC) class II Ag processing and recognition.
Type III IFNs (e.g. IFNλ) also have important antiviral functions, but receptors for these cytokines are predominantly found on epithelial cells. Type III IFNs may also play a role in the differentiation of hematopoietic cells (Rauch et al., 2013).