Single‐cell transcriptome atlas revealed bronchoalveolar immune features related to disease severity in pediatric <i>Mycoplasma pneumoniae</i> pneumonia

The mechanisms underlying protective immunity in mild <i>Mycoplasma pneumoniae</i> pneumonia (MPP) and the pathogenesis of severe MPP, characterized by dysregulated immune responses, remain unclear. Here, we performed single-cell RNA sequencing (scRNA-seq) to profile bronchoalveolar lavage fluid (BALF) samples from 13 healthy donors and 24 hospitalized pediatric patients with MPP, covering both mild and severe cases. Severe MPP patients exhibited high levels of exhausted T cells and M1-like macrophages, with the exhaustion of T cells attributed to persistent type I interferon signaling and inadequate assistance from CD4⁺ T cells. Significant cell-cell interactions between exhausted T cells and programmed death-ligand 1⁺ (PD-L1⁺) macrophages were detected in severe patients, potentially mediated through inhibitor molecules (e.g., PD1) and their receptors (e.g., PD-L1), as well as human leukocyte antigen class I molecules and their receptors (e.g., KLRC1/D2), resulting in the dysfunction of anti-MP immune responses. Mild MPP patients were featured by an increased abundance of neutrophils, coupled with enhanced activation, contributing to protective immunity. Together, our study provides a detailed characterization of the BALF immune landscape in MPP patients, revealing distinct immune characteristics between mild and severe cases, which offers a valuable resource for understanding MPP immunopathogenesis and formulating effective therapeutic strategies.