Aim. To evaluate the activity of the 4-1BB/4-1BBL signaling pathway in patients with influenza A (H1N1) virus-associated pneumonia.Materials and Methods. Here we enrolled 85 patients (41 males and 44 females, median age 48 (36-62) years) with influenza A (H1N1) virus-associated pneumonia. Among the exclusion criteria were unstable hemodynamics, BMI > 30, diabetes mellitus, HIV, tuberculosis, and cancer. Control group consisted of 15 healthy donors. The diagnosis of influenza A / H1N1 was

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... ed by a positive PCR test. Pneumonia was diagnosed according to the Federal Clinical Guidelines «Community-acquired pneumonia in adults». Severity of pneumonia was evaluated by using CURB-65 and SMART-COP scales as well as IDSA/ATS criteria. Plasma concentration of 4-1BB (CD137 or TNFRSF9, an inducible costimulatory receptor expressed on activated T cells and antigen-presenting cells) was determined by flow cytometry.Results. Patients with moderate and severe influenza A (H1N1) virus-associated pneumonia had 1.5- and 2.4 fold-increased concentration of plasma 4-1ВВ as compared with the healthy controls.Conclusion. The 4-1BB/4-1BBL signaling pathway, involved in multiple immune reactions, is associated with the severity of influenza A (H1N1) virus-associated pneumonia.