Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract infections (LRIs) in infants and young children and may be accompanied by T helper type (Th) 1/Th2 imbalance. Leukotriene receptor antagonists improve the long-term prognosis of patients with RSV-induced LRIs, but the long-term effects have not yet been investigated. In this study, we measured the long-term effects of pranlukast, a cysteinyl leukotriene receptor 1 antagonist, in hospitalized patients (aged <2 years) with RSV-induced LRIs. Participants were randomized into two groups: the pranlukast group (group P, n = 9) and the control group (group C, n = 11), which received placebo. Both groups were unblinded for prescription at discharge. Group P continued to take pranlukast for 6 months, and group C took nothing after discharge. Serum levels of interferon gamma and interleukin-4 were measured, and Th1 and Th2 cell counts and Th1/Th2 ratio were analyzed by flow cytometry upon hospitalization, at discharge, and 6 months post-discharge. Notably, at 6 months post-discharge, the Th1/Th2 ratio was significantly higher in group C than in group P (p < 0.05). Although these results suggest that long-term pranlukast administration suppresses Th1 response, no side effects or other diseases were noted. Our study included a small number of patients; therefore, large-scale trials are required.
