Recruited Monocyte-Derived Macrophages Drive T Cell Inflammation in Immune Checkpoint Inhibitor-Mediated Pneumonitis

Background: Immune checkpoint inhibitor-mediated pneumonitis (CIP) constitutes a major toxicity that limits the clinical application of cancer immunotherapy, whereas its underlying mechanisms remain incompletely understood. Current management relies on nonspecific immunosuppressants, lacking precision therapies. Although single-cell RNA sequencing (scRNA-seq) of bronchoalveolar lavage fluid (BALF) has implicated T cell activation and inflammatory myeloid responses in CIP pathogenesis, critical gaps persist regarding interstitial lung immunity and mechanisms governing monocyte/macrophage-T cell co-enrichment and crosstalk. We aimed to delineate the lung immune circuits that drive CIP and to identify targetable monocyte/macrophage-T cell pathways that could be leveraged for precision intervention. Methods: To elucidate CIP pathogenesis, we performed integrated scRNA-seq analysis of BALF from CIP⁺ and CIP^- patients with validation in a prospective cohort using flow cytometry, enzyme-linked immunosorbent assay, Western blotting, and quantitative polymerase chain reaction. Mechanistic studies were performed using an established tumor-bearing forkhead box P3-diphtheria toxin receptor-green fluorescent protein (Foxp3-DTR-GFP) mouse model of programmed death-1 inhibitor-induced CIP via micro-computed tomography, histopathology, scRNA-seq, flow cytometry, multiplex immunofluorescence, Western blotting, Transwell migration assays, and pharmacologic interventions. Results: In CIP⁺ patient BALF and mouse lung tissues, CD8⁺ T cells expressing cytotoxic effectors and C-X-C chemokine receptor 3 (CXCR3) expanded concomitantly with distinct C-C chemokine receptor 2 (CCR2)⁺ monocyte-derived macrophages (MoMΦ) exhibiting a highly inflammatory phenotype, while tissue-resident macrophages were markedly reduced. Mouse models revealed that expanded CCR2⁺ MoMΦ originating from circulation replenished the depleted niche of lung-resident interstitial macrophages. Mechanistically, integrated in silico prediction and experimental validation demonstrated that CCR2⁺ MoMΦ recruited CD8⁺ T cells via the C-X-C motif chemokine ligand 9/10 (CXCL9/10)-CXCR3 axis. Conversely, CD8⁺ T cells drove CCR2⁺ MoMΦ expansion and pro-inflammatory phenotype via the interferon-γ (IFN-γ) axis, suggesting the existence of a positive feedback loop between these cell types. Pharmacological targeting of CCR2/CCR5 or CXCR3 signaling attenuated pneumonitis, reduced pulmonary CCR2⁺ MoMΦ infiltration, diminished pathogenic T cell activation and cytotoxicity, and improved survival without compromising antitumor immunity. Conclusions: Our findings establish CCR2⁺ MoMΦ and the IFN-γ-CXCL9/10-CXCR3 axis as core drivers of CIP pathogenesis and validate their therapeutic targeting potential. This work provides a scientific foundation for developing CIP-specific prevention and treatment strategies.