Elevated neutrophilic activation in infants hospitalized for bronchiolitis correlates with clinical severity

<bold>INTRODUCTION:</bold> Growing evidence supports the role of neutrophilic inflammation in the pathogenesis of bronchiolitis, but the underlying mechanisms are still unknown. <bold>AIM:</bold> We hypothesized that elevated neutrophilic activation early during bronchiolitis would be associated with severe clinical manifestations. MATERIALS AND METHODS: During 2022-23 bronchiolitis epidemic we enrolled 18 infants aged ≤12 months (age ranged 1-8 months; median 2 months; 7 males), hospitalized with moderate to severe bronchiolitis at the Sapienza University Hospital of Rome, Italy. At admission, we evaluated the expression on neutrophils activation (by flow cytometry) in the peripheral blood of infants. A clinical severity score was assigned to each infant according to respiratory rate, oxygen saturation and the type of oxygen supplementation (low flow, high flow or mechanical ventilation). <bold>Results:</bold> A mean of 4.4% (range 0.03-17.72%) of CD16b and 2.0% (range 0.0-17.48%) of CD16b-CD18 positive neutrophils were detected. There were significant positive associations between neutrophils activation (CD16b p=0.04 and CD16b-CD18 p=0.03) and clinical severity of bronchiolitis. <bold>Conclusions:</bold> Infants with elevated neutrophilic activation early during bronchiolitis develop a severe clinical form, we speculate that neutrophils activation may contribute to bronchiolitis severity.