The molecular basis for differential type I interferon signaling

Abstract

Type I interferons (IFN-1) are cytokines that affect the expression of thousands of genes, resulting in profound cellular changes. IFN-1 activates the cell by dimerizing its two-receptor chains, IFNAR1 and IFNAR2, which are expressed on all nucleated cells. Despite a similar mode of binding, the different IFN-1s activate a spectrum of activities. The causes for differential activation may stem from differences in IFN-1-binding affinity, duration of binding, number of surface receptors, induction of feedbacks, and cell type-specific variations. All together these will alter the signal that is transmitted from the extracellular domain inward. The intracellular domain binds, directly or indirectly, different effector proteins that transmit signals. The composition of effector molecules deviates between different cell types and tissues, inserting an additional level of complexity to the system. Moreover, IFN-1s do not act on their own, and clearly there is much cross-talk between the activated effector molecules by IFN-1 and other cytokines. The outcome generated by all of these factors (processing step) is an observed phenotype, which can be the transformation of the cell to an antiviral state, differentiation of the cell to a specific immune cell, senescence, apoptosis, and many more. IFN-1 activities can be divided into robust and tunable. Antiviral activity, which is stimulated by minute amounts of IFN-1 and is common to all cells, is termed robust. The other activities, which we term tunable, are cell type-specific and often require more stringent modes of activation. In this review, I summarize the current knowledge on the mode of activation and processing that is initiated by IFN-1, in perspective of the resulting phenotypes.

Keywords: STAT transcription factor; cell signaling; interferon; protein-protein interaction; receptor structure-function.

Figure 1.

Structure of IFN-1 ligand·receptor complex. A, ternary structure of the IFNα2 mutant YNS bound to IFNAR1 (domains 1–3, with domain 4 missing) and IFNAR2 (Protein Data Bank (PDB) 3SE3). IFN-1-induced receptor dimerization drives the cross-phosphorylation of associated JAKs and STATs. Although STAT1 and STAT2 are common to all cells, other STAT phosphorylations may be cell type-specific. Activated STATs are imported to the nucleus, where they serve as transcription factors. In addition to phosphorylated STATs, U-STAT1 and U-STAT2 were also observed in the nucleus. B, mouse IFNβ-IFNAR1 structure (PDB 3WCY). C, comparing the IFN-1 ternary complex structure (PDB 3SE3 in green) with their unbound counterparts (PDB IDs: IFNAR1, 3S98; IFNAR2, 1N6U). The alignment was done on SD1. D, sequence conservation of cytokine receptor TMDs (underlined) and their surroundings. Sequence conservation was determined using ConSurf. Highest to lowest conservation is colored from magenta to cyan.

Figure 2.

Affinity-activity relation of IFN-1 signaling. A, antiviral (AV) and antiproliferative (AP) response of WISH cells to IFN-1 treatment. Each dot represents the mean values of six independent antiviral and antiproliferative experiments (5). For clarity, the fraction of antiviral response (y axis) is shown as relative light transmission, while the fraction of antiproliferative response is shown as absorbance upon crystal violet staining (thus antiviral cell survival increases with IFN-1, whereas cell numbers decrease with increased antiproliferative response). B, relative to WT IFNα2, biological potency (EC₅₀, antiviral, antiproliferative) is plotted against interferon receptor-binding affinity of IFNα2 mutants as a measure using surface plasmon resonance.

Figure 3.

Feedback mechanisms regulating IFN-1 signaling. Endocytosis is already observed minutes after interferon receptor activation by IFN-1. Endocytosis is activated by Tyk2 phosphorylation of Ser-535 on IFNAR1, facilitating its polyubiquitination. This in turn exposes a masked linear endocytic motif enabling the recruitment of the AP2 complex and ensuing internalization of the type I IFN receptor. STAT phosphorylation peaks ∼1 h after initiation of IFN-1 induction, and is then suppressed by SOCS1 binding to Tyk2. SOCS1 expression is rapidly up-regulated by IFN-1, enhancing the negative feedback. A third layer of feedback is mediated by USP18, which binds IFNAR2 and obstructs complex formation. USP18 expression is regulated by IFN-1, enhancing the negative feedback with increased expression levels. Obstruction of any one of these feedback mechanisms was shown to cause disease due to enhanced IFN-1 signaling (6).