Abstract BACKGROUND: Hantaviruses cause two clinical syndromes; hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). The clinical spectrum in HFRS also often involves respiratory symptoms. As information of the pulmonary pathogenesis in HFRS is limited, we aimed to further study the local airway immune response in the lower airways. METHODS: In 15 hospitalized HFRS patients, bronchoscopy was performed with sampling of endobronchial mucosal biopsies and bronchoalveolar lavage (BAL) fluid. Biopsies were stained for leukocytes, lymphocyte subsets and vascular endothelial adhesion molecules. BAL fluid and blood lymphocyte subsets were determined using flow cytometry. Fourteen healthy volunteers acted as control group. RESULTS: Compared to controls, endobronchial mucosal biopsies from HFRS patients revealed increased numbers of CD8(+) T cells in both epithelium and submucosa (p=0.001), along with an increase in submucosal CD4(+) T cells (p=0.001). In contrast, patients' submucosal neutrophil and eosinophil numbers were reduced (p<0.001). The expression of vascular cell adhesion molecule-1 (VCAM-1) was enhanced in HFRS patients (p<0.001). In HFRS patients, analyses of T cell subsets in BAL fluid showed higher proportions of CD3(+) and CD8(+) T cells (p=0.011 and p=0.025), NK cells (p<0.001) together with an increased expression of activation markers HLA-DR and CD25 on T cells (p<0.001 and p<0.001). CONCLUSIONS: The present findings indicate a local immune response in terms of activated T lymphocytes in the lungs of patients with HFRS. The elevated expression of activation markers and VCAM-1 further implies the importance of cytotoxic lymphocytes in the pathogenesis of pulmonary involvement in HFRS.