Attenuation and efficacy of human parainfluenza virus type I (HPIVI) vaccine candidates containing stabilized mutations in the P/C and L genes

Background: Two recombinant, live attenuated human parainfluenza virus type I (rHPIVI) mutant viruses have been developed, using a reverse genetics system, for evaluation as potential intranasal vaccine candidates. These rHPIVI vaccine candidates have two non-temperature sensitive (non-ts) attenuating (att) mutations primarily in the P/C gene, namely (CHNT553A)-H-R84G (two point mutations used together as a set) and C-Delta 170 ( a short deletion mutation), and two ts att mutations in the L gene, namely L-Y942A (a point mutation), and L Delta 1710-11 (a short deletion), the last of which has not been previously described. The latter three mutations were specifically designed for increased genetic and phenotypic stability. These mutations were evaluated on the HPIVI backbone, both individually and in combination, for attenuation, immunogenicity, and protective efficacy in African green monkeys (AGMs). Results: The rHPIVI mutant bearing the novel L Delta 1710-11 mutation was highly ts and attenuated in AGMs and was immunogenic and efficacious against HPIVI wt challenge. The rHPIVI-(CHNLY942A)-H-R84G/Delta 170-L-T553A and rHPIVI-(CHNL Delta 1710-11)-H-R84G/Delta 170-L-T553A vaccine candidates were highly ts, with shut-off temperatures of 38 C and 35 C, respectively, and were highly attenuated in AGMs. Immunization with rHPIVI-(CHNLY942A)-H-R84G/Delta 170-L-T553A protected against HPIVI wt challenge in both the upper and lower respiratory tracts. In contrast, rHPIVI-(CHNL Delta 1710-11)-H-R84G/Delta 170-L-T553A was not protective in AGMs due to over-attenuation, but it is expected to replicate more efficiently and be more immunogenic in the natural human host. Conclusion: The rHPIVI-(CHNLY942A)-H-R84G/Delta 170-L-T553A and rHPIVI-C-R84G/Delta 170HN(T553A)L(Delta 1710-11) vaccine candidates are clearly highly attenuated in AGMs and clinical trials are planned to address safety and immunogenicity in humans.