The signalling cascade from initial stimulus to downstream effector — and where therapeutic intervention is possible at each node.
Interleukin-5 (IL-5) is the principal cytokine governing eosinophil biology — driving differentiation of eosinophil precursors in the bone marrow, promoting eosinophil priming and activation, and crucially extending eosinophil survival in peripheral blood and tissues through inhibition of apoptosis. IL-5 is produced primarily by Th2 lymphocytes, ILC2s, mast cells, and eosinophils themselves, creating an autocrine amplification loop. In severe eosinophilic asthma, persistent blood and airway eosinophilia drives airway inflammation, smooth muscle hypertrophy, goblet cell metaplasia, and subepithelial fibrosis — all contributing to airway remodelling and progressive functional decline.
Elevated blood eosinophil count (typically above 300 cells/microlitre, and especially above 500) is now established as a clinically actionable biomarker predicting both exacerbation risk and therapeutic responsiveness to IL-5 pathway targeted biologics. Mepolizumab and reslizumab target the IL-5 cytokine directly, preventing its interaction with the IL-5 receptor alpha (IL-5Ralpha) subunit on eosinophils. Benralizumab targets IL-5Ralpha itself, and uniquely exploits antibody-dependent cell-mediated cytotoxicity (ADCC) via its afucosylated Fc region, enabling rapid and near-complete eosinophil depletion within weeks — a pharmacodynamic distinction from direct anti-IL-5 approaches.
Tezepelumab targets thymic stromal lymphopoietin (TSLP), an upstream epithelial alarmin that drives the entire type 2 inflammatory cascade — not only eosinophilic but also allergic and non-type-2 pathways. This broader mechanistic action allows tezepelumab to reduce exacerbations in patients with low or undetectable eosinophil counts where anti-IL-5 agents show less benefit, positioning it as an option for the broadest severe asthma phenotype. The IL-5 pathway is also relevant in eosinophilic granulomatosis with polyangiitis (EGPA) and hypereosinophilic syndrome (HES), where mepolizumab holds licensed indications.
Upstream blockade vs downstream blockade — understanding the distinction is critical for treatment selection and sequencing.
Clinically actionable insights for treatment selection and sequencing
Interleukin-5 (IL-5) is the principal cytokine governing eosinophil biology — driving differentiation of eosinophil precursors in the bone marrow, promoting eosinophil priming and activation, and crucially extending eosinophil survival in peripheral blood and tissues through inhibition of apoptosis.
IL-5 is produced primarily by Th2 lymphocytes, ILC2s, mast cells, and eosinophils themselves, creating an autocrine amplification loop.
In severe eosinophilic asthma, persistent blood and airway eosinophilia drives airway inflammation, smooth muscle hypertrophy, goblet cell metaplasia, and subepithelial fibrosis — all contributing to airway remodelling and progressive functional decline.
Elevated blood eosinophil count (typically above 300 cells/microlitre, and especially above 500) is now established as a clinically actionable biomarker predicting both exacerbation risk and therapeutic responsiveness to IL-5 pathway targeted biologics.
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