Expression of Inflammatory Markers in the Course of Acute Respiratory Viral Infections

Respiratory viral infections caused by influenza, respiratory syncytial virus (RSV), and SARS-CoV-2 remain major causes of morbidity, hospitalization, and mortality among older adults worldwide. Current antiviral therapies have limited effectiveness and generally require administration within the first 48 hours after symptom onset. Increasing evidence suggests that the programmed death receptor-1/programmed death ligand-1 (PD-1/PD-L1) immune checkpoint pathway plays an important role in the host immune response during acute viral respiratory infections. Upregulation of PD-L1 has been associated with impaired antiviral T-cell activity, immune exhaustion, and disease progression in influenza, RSV, and SARS-CoV-2 infections. The AIRE-INT study is a prospective, observational, multicenter, non-interventional study designed to characterize the temporal kinetics of PD-L1 expression and inflammatory biomarkers in adults aged 60 years or older presenting with acute respiratory viral infection. The study will be conducted in primary care centers and urgent care facilities within the Barcelonès Nord and Maresme healthcare regions in Catalonia, Spain. A total of 150 participants will be enrolled, including 75 with confirmed viral respiratory infection and 75 controls with negative rapid antigen tests for influenza, RSV, and SARS-CoV-2. Eligible participants must be aged ≥60 years and present within 24 to 72 hours after the onset of respiratory symptoms compatible with acute viral infection, including fever, cough, nasal congestion, or dyspnea. Participants in the infected group must have a positive rapid antigen test for influenza, RSV, or SARS-CoV-2. Control participants must test negative for all three viruses. Written informed consent will be obtained before enrollment. Participants with severe immunosuppression, chronic immunosuppressive therapy, active oncologic disease, terminal illness, or autoimmune diseases associated with PD-L1 dysregulation will be excluded. Each participant will be followed for up to 60 days and will complete two in-person study visits and one follow-up assessment. Visit 1 will occur between 24 and 72 hours after symptom onset and will include informed consent, collection of demographic and clinical data, assessment of symptoms and medical history, rapid antigen testing, and blood sample collection for PD-L1 and inflammatory biomarker analyses. Visit 2 will occur between 5 and 9 days after symptom onset and will include repeat clinical assessment and blood sample collection to evaluate temporal changes in PD-L1 expression and inflammatory responses during the acute phase of infection. Visit 3 will occur between 30 and 60 days after symptom onset and will consist of clinical follow-up through telephone contact and electronic medical record review to assess symptom resolution, complications, hospitalization, intensive care admission, and mortality. Laboratory analyses will include flow cytometry quantification of PD-L1 expression and evaluation of inflammatory biomarkers, including C-reactive protein (CRP), interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), complete blood count parameters, renal and hepatic function markers, and virus-specific IgG antibodies. Blood samples will be processed according to standardized laboratory procedures at the Hospital Germans Trias i Pujol Microbiology Department. The primary objective is to characterize the temporal profile of PD-L1 expression from symptom onset to infection resolution in older adults with influenza, RSV, or SARS-CoV-2 infection. Secondary objectives include: Describing the evolution of inflammatory serum biomarkers and their association with disease severity. Identifying biomarkers useful for screening, prognosis, and clinical monitoring. Establishing a biological reference framework for future evaluation of PD-L1 inhibitors in respiratory viral infections. The primary outcome measure is the level and temporal evolution of PD-L1 expression and inflammatory biomarkers between study visits. Secondary outcomes include hospitalization, intensive care admission, mortality at 60 days, symptom duration, and clinical progression. Statistical analyses will include descriptive and univariate analyses, longitudinal modeling of biomarker kinetics using locally estimated scatterplot smoothing (LOESS) and nonlinear mixed-effects regression models, and predictive logistic regression models evaluating associations between biomarkers and clinical outcomes. The study is expected to provide important information regarding the kinetics of PD-L1 expression and inflammatory responses during acute respiratory viral infections in older adults. The findings may support the development of prognostic biomarkers and future host-directed therapeutic strategies targeting the PD-1/PD-L1 pathway across multiple respiratory viruses.