Immunopathogenic Mechanisms in Connective Tissue Disease-Associated Interstitial Lung Disease: Incessant Loop of Immunity to Fibrosis

Connective tissue disease-associated interstitial lung disease (CTD-ILD) represents a significant cause of morbidity and mortality. It is characterized by the progressive convergence of chronic inflammation, immune dysregulation, and fibrotic remodeling in the lung parenchyma. While often conceptualized through a model of idiopathic pulmonary fibrosis (IPF), CTD-ILD is fundamentally an immune-driven pathology with distinct inflammatory mechanisms in which adaptive immunity plays a profound role. This narrative review explores the "inflammation-immunity-fibrosis continuum" in CTD-ILD, elaborating the intricate cellular and molecular pathways that distinguish it from IPF. We highlight the central role of persistent T-cell responses and B-cell dysregulation, which often occur within organized tertiary lymphoid structures in the lung. This review examines how these immune processes are propagated by multiple cytokine pathways, including the TGF-β/SMAD, JAK/STAT, and phosphodiesterase-4 signaling pathways, which serve as crucial links between inflammation and fibrosis. This distinct immune mechanism in CTD-ILD explains why immunomodulatory agents are a cornerstone of CTD-ILD treatment, in contrast to their limited efficacy in IPF, and emphasizes the current paradigm of combining immunosuppression with antifibrotic drugs to target the dual drivers of the disease.