Toll-Like Receptor (TLR)-2 Mediates Recruitment of Neutrophils During Influenza Infection in a Neonatal Murine Model

Abstract

Abstract Infants less than six months of age are particularly susceptible to influenza infection due to their impaired cytotoxic T cell response. The neonate relies on innate immunity to contain infection without excessive inflammatory response, by using pattern recognition molecules, like collectins and TLRs. TLR2 has been identified as a bacterial recognition receptor. However, little is known about its role in viral recognition and associated pathogenesis, particularly in the neonate. To address this, 3-day old TLR-2−/− and control C57Bl/6 mice were infected with influenza type A virus strain PR/8/34 (H1N1) and tracked for survival. Bronchoalveolar lavage fluid (BALF) was collected and lungs were harvested at post infection day (PID) 1, 3, and 6. BALF was analyzed by flow cytometry for cellular infiltration in the airways. RNA was isolated and analyzed via real-time PCR for viral loads and various cytokines. There was a striking difference in survival between the TLR-2−/− (61%) and control neonates (25%) (p< 0.01). Surprisingly, TLR-2−/− mice had higher viral loads than controls (p < 0.05) on PID 1, which was coupled with lower neutrophil numbers in the BALF (p < 0.02). No significant differences were noted on PID 3 or 6 for neutrophil counts or viral loads. On PID 3, TLR-2−/− mice had more than a 2-fold lower transcription of CXCL-5, a key chemoattractant for neutrophils. Therefore, the resistance of neonatal TLR-2−/− mice to influenza infection was not due to heightened influenza clearance. Rather, there was impaired neutrophil recruitment, which potentially reduced pulmonary inflammation. Future studies using our neonatal murine model could elucidate the mechanism of TLR-2 and CXCL-5 mediated inflammation during neonatal viral infection.