Multifarious determinants of cytokine receptor signaling specificity

Abstract

Cytokines play crucial roles in regulating immune homeostasis. Two important characteristics of most cytokines are pleiotropy, defined as the ability of one cytokine to exhibit diverse functionalities, and redundancy, defined as the ability of multiple cytokines to exert overlapping activities. Identifying the determinants for unique cellular responses to cytokines in the face of shared receptor usage, pleiotropy, and redundancy will be essential in order to harness the potential of cytokines as therapeutics. Here, we discuss the biophysical (ligand-receptor geometry and affinity) and cellular (receptor trafficking and intracellular abundance of signaling molecules) parameters that contribute to the specificity of cytokine bioactivities. Whereas the role of extracellular ternary complex geometry in cytokine-induced signaling is still not completely elucidated, cytokine-receptor affinity is known to impact signaling through modulation of the stability and kinetics of ternary complex formation. Receptor trafficking also plays an important and likely underappreciated role in the diversification of cytokine bioactivities but it has been challenging to experimentally probe trafficking effects. We also review recent efforts to quantify levels of intracellular signaling components, as second messenger abundance can affect cytokine-induced bioactivities both quantitatively and qualitatively. We conclude by discussing the application of protein engineering to develop therapeutically relevant cytokines with reduced pleiotropy and redirected biological functionalities.

Keywords: Cytokine engineering; Cytokine signaling; Cytokine structure; Functional specificity; Interleukin; Ligand–receptor affinity; Receptor trafficking.

Figure 1.1

Complex stability and dynamic determination of cytokine-induced bioactivities. Left, extracellular structures of the type I IFN receptor, IFNAR1 and IFNAR2, and four different IFN subtypes are depicted. The four IFN subtypes presented activate STATs to the same extent (as shown by phosphorylated STAT dose–response curves) and exhibit similar antiviral activity (as shown by virus-infected cell abundance, which decreases with increasing IFN doses), but possess starkly different growth arrest potencies (as shown by the number of cells, which decreases with increasing IFN doses). Rather than following STAT activation profiles, the antiproliferative activities of the IFN subtypes closely parallel binding affinities and complex stabilities instead. Center, extracellular structures of IL-4, IL-13, and their shared IL-4 type II receptor comprised of IL-4Rα1 and IL-13Rα1 are shown. IL-4 and IL-13 share a common surface receptor complex, but IL-4 activates STAT6 more efficiently than IL-13 does (as presented in the phosphorylated STAT6 dose–response curve). Greater potency of STAT6 activation by IL-4 is not the result of a more stable complex, since the complex formed by IL-13 is in fact more stable, but rather the result of the differential kinetics of complex formation in response to IL-4 versus IL-13. Right, extracellular structures of Epo and the EpoR homodimer are presented. Epo engages two molecules of EpoR and forms an active surface asymmetric homodimeric complex, leading to STAT5 activation. As is the case for other cytokines that engage homodimeric receptors, Epo-induced STAT5 activation follows a bell-shaped curve, with high doses of Epo activating STAT5 less efficiently than intermediate doses do due to reduced ternary complex formation (our unpublished data).

Figure 1.2

Role of receptor endosomal trafficking in regulating cytokine bioactivities. A canonical cytokine–receptor complex is represented schematically. Upon ligand stimulation the surface complex is formed and leads to the activation of membrane proximal signaling events (e.g., Jak/STAT pathway activity). The complex then traffics to endosomal compartments and follows different subcellular routes depending on its stability. Short-lived complexes dissociate in the acidic pH found in early endosomes and consequently follow a recycling route back to the plasma membrane. Long-lived complexes, on the other hand, persist in early endosomes and continue trafficking through subsequent endosomal compartments where specific signaling molecules are present that help to fine-tune the functional response to a given cytokine.

Figure 1.3

The levels of intracellular signaling proteins impact the nature of the bioactivities induced by cytokines. The cytokine environment surrounding a cell contributes critically to the nature of that cell’s response to a given stimulus. As an example, the IL-6 cytokine complex signals through both STAT1 and STAT3 activation. If a cell has previously been exposed to IL-23 or TNFα, its levels of STAT3 will be upregulated. In this case, stimulation with IL-6 will promote a strong activation of STAT3 and will lead to a potent anti-inflammatory response (left). Conversely, if a cell is primed with type I or II IFNs, its levels of STAT1 will instead be upregulated and IL-6 stimulation will therefore induce STAT1 activation, leading to a potent proinflammatory response.