Innate immune protection from pneumonia virus of mice induced by a novel immunomodulator is prolonged by dual treatment and mediated by macrophages.

Respiratory syncytial virus (RSV) is responsible for a large proportion of acute lower respiratory tract infections, specifically in children. Pneumonia virus of mice (PVM) causes similar lung pathology and clinical disease in rodents, and is therefore an appropriate model of RSV infection. Previously, we demonstrated that a single intranasal dose of P-I-P, a novel immunomodulator composed of the toll-like receptor 3 agonist poly(I:C), an innate defense regulator peptide and a polyphosphazene, confers protection in Balb/c mice for up to 3 days from lethal PVM-15 infection. In the present study a dual intranasal treatment with P-I-P was shown to extend the duration of the protection conferred by P-I-P from PVM-15 challenge. Balb/c mice treated twice with P-I-P showed higher survival rates and milder clinical signs when compared to animals that received a single P-I-P dose. While the mice treated with two consecutive doses of P-I-P experienced some weight loss, they all recovered. The dual P-I-P treatment mediated infiltration of several innate immune cells into the BALF and lung, including alveolar macrophages, neutrophils, and γδ T cells. Partial depletion of alveolar macrophages decreased survival rates and exacerbated clinical signs of mice subjected to the P-I-P dual treatment regime followed by PVM-15 challenge. This suggests that the alveolar macrophage is at least partially responsible for the protection elicited by this novel prophylactic treatment strategy.