Serum migration-inhibitory activity in infants with respiratory syncytial virus bronchiolitis

Using an unidirectional leucocyte migration-inhibition (LMI) assay, we investigated the migration-inhibitory activity (LIF) in the sera of 25 infants who developed bronchiolitis following respiratory syncytial virus (RSV) infection. Blood samples for serum-LIF activity detection were obtained from patients the day after admission and two weeks later. The LMI assay revealed serum-LIF activity in 17 (68%) blood samples taken during the acute phase of the disease. In contrast, no inhibition was found in the convalescent blood samples with the exception of 5 (20%). This difference was statistically significant (chi2 = 9.82; P less than 0.005). As positive serum-LIF test is a good in vitro correlate of cell-mediated immunity, our results suggest that cell-mediated hypersensitivity reactions occur during the clinical course of RSV bronchiolitis. These findings are consistent with the hypothesis that alterations of RSV-specific cell-mediated immune mechanisms may be involved in the pathogenesis of this illness.