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Interventions for prevention of respiratory syncytial virus bronchiolitis and sequelae in guinea pigs Sutton, Troy C.

Abstract

Respiratory syncytial virus (RSV) is the leading cause of infant hospitalization and children who recover from RSV bronchiolitis often develop episodes of recurrent wheezing and "asthma"-like symptoms. Despite considerable research, the mechanisms causing both RSV bronchiolitis and its sequelae are poorly understood. Recent clinical studies suggest a T helper 2 (Th2) immune response may be involved in the pathogenesis of RSV bronchiolitis. By applying the T helper 1 (Thl)/T helper 2 (Th2) paradigm, we examined the effects of two interventions, (administration of "low" dose RSV or CpGODN) designed to shift the Thl/Th2 balance towards a Thl response, on the development of RSV bronchiolitis and post-bronchiolitis sequelae in a guinea pig model of experimental RSV infection. There were four groups of animals: a Sham-Inoculated Group, a Challenge Dose RSV Group (receiving ~10⁴ syncytial forming units (s.f.u.)), an Intervention Dose RSV (~10² s.f.u.) + Challenge Dose RSV Group, and a CpG-ODN + Challenge Dose RSV Group. We compared the four groups (n =11-12) during acute and chronic (days 7 and 60 post-challenge dose inoculation, respectively) RSV infection. Airway hyperresponsiveness (AHR) was measured using an acetylcholine challenge method in a whole body plethysmograph. Airway infiltration by T cells and eosinophils was assessed by point counting stained lung sections. Using a semi-quantitative RT-PCR method we compared lung IFN-ɣ/IL-5 mRNA ratios as an index of the Thl/Th2 balance and, using a RSV immunostaining technique, we assessed the proportion of animals in each group with RSV antigens in the lungs. Results on day 7 showed that the Challenge Dose RSV Group developed AHR, increased levels of airway T cells and eosinophils, a Th2 shift in lung IFN-ɣ/IL-5 mRNA ratio and showed 9/11 animals with RSV positive immunostaining. The Intervention Dose Group differed from the Challenge Dose Group in that it had a significantly lower proportion of animals with RSV positive immunostaining (1/11) and had an intermediate level of eosinophils compared to the Sham-Inoculated and Challenge Dose RSV Groups. The CpG-ODN Group did not develop AHR, a Th2 shift, or increased levels of eosinophils, but did develop an intermediate level of T cells. The CpG-ODN Group, like the Intervention Dose Group, had a significantly lower proportion of animals with RSV positive immunostaining (3/12) compared to the Challenge Dose RSV (positive control) Group, suggesting that both interventions prevented RSV infection and/or enhanced viral clearance. On day 60, the Challenge Dose RSV Group maintained AHR, elevated levels of airway T cells and eosinophils, and a Th2 shift observed on day 7, and 7/12 animals had RSV positive immunostaining. The Intervention Dose Group, on the other hand, did not maintain AHR and increased levels of T cells and eosinophils seen on day 7 and had a Thl/Th2 ratio that was not significantly different from the Sham-Inoculated or Challenge Dose RSV Group. The CpG-ODN Group was unaltered with respect to AHR, airway eosinophils, or a Th2 shift, but did have a decrease in airway T cells to a level similar to that seen in the Sham-Inoculated Group. In both the Intervention Dose Group and CpGODN Group no animals (0/12 and 0/11 respectively) had RSV positive immunostaining, suggesting that the interventions prevented the development of a persistent RSV infection. In conclusion, CpG-ODN immunotherapy protected against the development of RSV bronchiolitis, while the "low" dose intervention did not, and both interventions provided protection against the development of post-bronchiolitis sequelae in guinea pigs.

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