Structure of the interleukin-5 receptor complex exemplifies the organizing principle of common beta cytokine signaling.

Cytokines regulate immune responses by binding to cell surface receptors, including the common subunit beta (ßc), which mediates signaling for GM-CSF, IL-3, and IL-5. Despite known roles in inflammation, the structural basis of IL-5 receptor activation remains unclear. We present the cryo-EM structure of the human IL-5 ternary receptor complex, revealing architectural principles for IL-5, GM-CSF, and IL-3. In mammalian cell culture, single-molecule imaging confirms hexameric IL-5 complex formation on cell surfaces. Engineered chimeric receptors show that IL-5 signaling, as well as IL-3 and GM-CSF, can occur through receptor heterodimerization, obviating the need for higher-order assemblies of ßc dimers. These findings provide insights into IL-5 and ßc receptor family signaling mechanisms, aiding in the development of therapies for diseases involving deranged ßc signaling.

The GM-CSF/IL-3/IL-5 cytokine receptor family: from ligand recognition to initiation of signaling.

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM-CSF receptor ternary complex and the IL-5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure-function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure-based approaches for the discovery of novel and disease-specific therapeutics. In addition, recent biochemical evidence has suggested that the GM-CSF/IL-3/IL-5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.

Interleukin-5 and IL-5 receptor in health and diseases.

While interleukin-5 (IL-5) is initially identified by its ability to support the growth and terminal differentiation of mouse B cells in vitro into antibody-secreting cells, recombinant IL-5 exerts pleiotropic activities on various target cells including B cells, eosinophils, and basophils. IL-5 is produced by both hematopoietic and non-hematopoietic cells including T cells, granulocytes, and natural helper cells. IL-5 exerts its effects for proliferation and differentiation via receptors that comprise an IL-5-specific α and common ß-subunit. IL-5Rα expression in activated B cells is regulated by a complex of transcription factors including E12, E47, Sp1, c/EBPß, and Oct2. IL-5 signals are transduced through JAK-STAT, Btk, and Ras/Raf-ERK signaling pathways and lead to maintenance of survival and functions of B cells and eosinophils. Overexpression of IL-5 in vivo significantly increases eosinophils and B cells in number, while mice lacking a functional gene for IL-5 or IL-5 receptor display a number of developmental and functional impairments in B cells and eosinophil lineages. In humans, the biologic effects of IL-5 are best characterized for eosinophils. The recent expansion in our understanding of eosinophil development and activation and pathogenesis of eosinophil-dependent inflammatory diseases has led to advance in therapeutic options. Intravenous administration of humanized anti-IL-5 monoclonal antibody reduces baseline bronchial mucosal eosinophils in mild asthma; providing important implications for strategies that inhibit the actions of IL-5 to treat asthma and other allergic diseases.

Clarification of the role of N-glycans on the common beta-subunit of the human IL-3, IL-5 and GM-CSF receptors and the murine IL-3 beta-receptor in ligand-binding and receptor activation.

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3 and IL-5 are related cytokines that play key roles in regulating the differentiation, proliferation, survival and activation of myeloid blood cells. The cell surface receptors for these cytokines are composed of cytokine-specific alpha-subunits and a common beta-receptor (betac), a shared subunit that is essential for receptor signaling in response to GM-CSF, IL-3 and IL-5. Previous studies have reached conflicting conclusions as to whether N-glycosylation of the betac-subunit is necessary for functional GM-CSF, IL-3 and IL-5 receptors. We sought to clarify whether betac N-glycosylation plays a role in receptor function, since all structural studies of human betac to date have utilized recombinant protein lacking N-glycosylation at Asn(328). Here, by eliminating individual N-glycans in human betac and the related murine homolog, beta(IL-3), we demonstrate unequivocally that ligand-binding and receptor activation are not critically dependent on individual N-glycosylation sites within the beta-subunit although the data do not preclude the possibility that N-glycans may exert some sort of fine control. These studies support the biological relevance of the X-ray crystal structures of the human betac domain 4 and the complete ectodomain, both of which lack N-glycosylation at Asn(328).