An Uncommon Case of Non-Specific Interstitial Pneumonia (NSIP) Associated With Idiopathic Hypereosinophilic Syndrome (HES) Reversed by Mepolizumab.

Hypereosinophilic Syndrome (HES) represents a heterogeneous and complex group of disorders characterised by persistent blood and tissue eosinophilia, leading to progressive tissue damage and organ dysfunction. This spectrum includes both hematologic variants and non-hematologic forms, either secondary to identifiable causes or idiopathic in nature. In this article, we describe a rare clinical presentation of HES manifesting primarily with pulmonary involvement, diagnosed as Non-Specific Interstitial Pneumonia (NSIP). Remarkably, the interstitial lung disease showed near-complete reversibility following targeted inhibition of the IL-5 pathway with mepolizumab, highlighting the potential role of Th2-driven eosinophilic inflammation in the pathogenesis of certain forms of interstitial lung disease.