RESUMEN
Viral respiratory infections are the most frequent cause of hospital admission for infants and young children during winter. However, the mechanisms of illness that are associated with viral lower-respiratory-tract infection (LRI) are unclear. A widely accepted hypothesis attributes the pathogenesis of viral LRI in infants to the induction of innate inflammatory responses. This theory is supported by studies showing that Toll-like receptor 4 is activated by respiratory syncytial virus (RSV), leading to production of inflammatory cytokines. We prospectively examined previously naive infants in Buenos Aires, Argentina, who had either upper- or lower-respiratory-tract symptoms. Infection with human metapneumovirus (hMPV) was second only to RSV in frequency. Both viruses were associated with rhinorrhea, cough, and wheezing; however, hMPV elicited significantly lower levels of respiratory inflammatory cytokines than did RSV. Symptoms in infants infected with influenza virus were different from those in infants infected with RSV, but cytokine responses were similar. These findings suggest that hMPV and RSV either cause disease via different mechanisms or share a common mechanism that is distinct from innate immune activation.