Severe Legionella Pneumonia Mimicking Immune-Related Pneumonitis After Chemoimmunotherapy for Lung Cancer: A Case Report

Haixian LiuZhaolei DingLina Xu^*Tao Guo

• Weifang People's Hospital, Weifang, China

Background: Immune checkpoint inhibitors (ICIs) have significantly improved survival outcomes in patients with various malignancies. However, their associated toxicities, including immune checkpoint inhibitor-related pneumonia (CIP), must not be overlooked. CIP has been reported in 2.6%–33% of patients [1–5], and though not the most common immune-related adverse event, it is among the most serious. Severe (grade 3–4) CIP, if untreated, may compromise subsequent immunotherapy and lead to respiratory failure or death. Legionnaires’ disease, caused by Legionella pneumophila, is a rare but potentially fatal form of atypical pneumonia. With the growing use of ICIs, especially combined with chemotherapy, early CIP and Legionella pneumonia may present with similar radiological features such as ground-glass opacities, complicating differentiation. Early distinction is crucial as management differs: CIP requires corticosteroids, whereas Legionella pneumonia necessitates quinolones. Traditional diagnostic methods for Legionella, including culture and urine antigen testing, have low sensitivity and risk false negatives. Targeted next-generation sequencing (tNGS) has recently emerged as a rapid, sensitive, and cost-effective method, outperforming metagenomic sequencing in clinical utility. Case Summary: We describe a 70-year-old male with squamous cell lung cancer, type 2 diabetes, and COPD who developed severe pneumonia after chemoimmunotherapy (paclitaxel, cisplatin, and tislelizumab). He presented with fever, cough, dyspnea, leukopenia, and myelosuppression. Initial assessment suggested ICI-related pneumonia complicated by bacterial infection. Despite empirical antibiotics and corticosteroids, his condition deteriorated, requiring RICU admission. On February 13, tNGS of sputum identified Legionella pneumophila, Enterococcus faecium, EBV, and HSV-1. The high abundance of L. pneumophila indicated it as the primary pathogen, while EBV and HSV-1 were presumed latent. Treatment was adjusted to moxifloxacin, cefepime, and ganciclovir, resulting in marked improvement, correction of hypoxemia, and partial radiological resolution. The patient was discharged with home oxygen and continues regular oncologic care. Conclusions: In the era of chemoimmunotherapy, diffuse pulmonary ground-glass lesions warrant consideration not only of ICI-related pneumonia but also atypical infections such as Legionella, especially when unresponsive to corticosteroids. Advanced molecular diagnostics such as tNGS should be applied early to clarify pathogen etiology and guide individualized therapy.