The live attenuated influenza virus vaccine (LAIV) is preferentially recommended for use in persons 2 through 49 years of age but has not been approved for children under 2 or asthmatics due to safety concerns. Therefore, increasing safety is desirable. Here we describe a murine LAIV with reduced pathogenicity that retains lethality at high doses and further demonstrate that we can enhance safety in vivo through mutations within NS1. This model may permit preliminary safety analysis of improved LAIVs.
Influenza A virus is a respiratory pathogen that infects through the upper airway and leads to pathology via replication in the lower airway (1). The temperature gradient between these two areas in people enabled the development of the cold-adapted, live attenuated influenza virus vaccine (LAIV [FluMist]) that replicates in the cooler upper respiratory tract to trigger a protective immune response but cannot damage the lower respiratory tract due to the elevated temperatures restricting replication (2). This temperature-sensitive (ts) attenuated (att) phenotype is imparted by five mutations within the viral replicative machinery: namely, PB2 N265S, PB1 K391E, D581G, and A661T, and NP D34G (3, 4). Although this vaccine has an overall acceptable safety profile, it is not approved for use in children under 2 years of age due to concerns about elevated hospitalizations due to wheezing (5, 6). For this reason, it is also not approved for use in asthmatics. Therefore, development of vaccines with increased safety over LAIV is desirable. Currently, no mouse model exists for the adequate assessment of the safety of the LAIV. In experimental animal studies with LAIV, elevated doses of LAIV do not elicit pathology, rendering determination of safety impossible (7-10). Here, we describe a model with which we can assess alterations in vaccine safety.The parental strain for our vaccine is a well-characterized, murine-lethal strain of influenza A virus (A/Puerto Rico/8/34 H1N1, PR8). We introduced four ts att mutations from LAIV into PR8 (NP D34G is natively present) via site-directed mutagenesis (Agilent) and rescued this virus using plasmid-based reverse genetics techniques (11). This ts att virus (referred to henceforth as "PR8 LAIV") has been previously characterized in cell culture, but its phenotype in mice was not demonstrated (8). PR8 wild-type (WT) virus has a 50% lethal dose (LD 50 ) in C57BL/6 (B6) mice of 10 to 25 PFU (12, 13). Thus, we sought to ascertain the LD 50 of PR8 LAIV (Fig. 1). Groups of mice (n ϭ 5) were intranasally