Comparison of the efficacy of a commercial inactivated influenza A/H1N1/pdm09 virus (pH1N1) vaccine and two experimental M2e-based vaccines against pH1N1 challenge in the growing pig model

Abstract

Swine influenza A viruses (IAV-S) found in North American pigs are diverse and the lack of cross-protection among heterologous strains is a concern. The objective of this study was to compare a commercial inactivated A/H1N1/pdm09 (pH1N1) vaccine and two novel subunit vaccines, using IAV M2 ectodomain (M2e) epitopes as antigens, in a growing pig model. Thirty-nine 2-week-old IAV negative pigs were randomly assigned to five groups and rooms. At 3 weeks of age and again at 5 weeks of age, pigs were vaccinated intranasally with an experimental subunit particle vaccine (NvParticle/M2e) or a subunit complex-based vaccine (NvComplex/M2e) or intramuscularly with a commercial inactivated vaccine (Inact/pH1N1). At 7 weeks of age, the pigs were challenged with pH1N1 virus or sham-inoculated. Necropsy was conducted 5 days post pH1N1 challenge (dpc). At the time of challenge one of the Inact/pH1N1 pigs had seroconverted based on IAV nucleoprotein-based ELISA, Inact/pH1N1 pigs had significantly higher pdm09H1N1 hemagglutination inhibition (HI) titers compared to all other groups, and M2e-specific IgG responses were detected in the NvParticle/M2e and the NvComplex/M2e pigs with significantly higher group means in the NvComplex/M2e group compared to SHAMVAC-NEG pigs. After challenge, nasal IAV RNA shedding was significantly reduced in Inact/pH1N1 pigs compared to all other pH1N1 infected groups and this group also had reduced IAV RNA in oral fluids. The macroscopic lung lesions were characterized by mild-to-severe, multifocal-to-diffuse, cranioventral dark purple consolidated areas typical of IAV infection and were similar for NvParticle/M2e, NvComplex/M2e and SHAMVAC-IAV pigs. Lesions were significantly less severe in the SHAMVAC-NEG and the Inact/pH1N1pigs. Under the conditions of this study, a commercial Inact/pH1N1 specific vaccine effectively protected pigs against homologous challenge as evidenced by reduced clinical signs, virus shedding in nasal secretions and oral fluids and reduced macroscopic and microscopic lesions whereas intranasal vaccination with experimental M2e epitope-based subunit vaccines did not. The results further highlight the importance using IAV-S type specific vaccines in pigs.

Fig 1. Experimental design.

Vaccinations and sham-vaccinations were done when the pigs were 3 weeks old and repeated at 5 weeks of age. All pigs except the SHAMVAC-NEG group were challenged with IAV at 7 weeks of age. Necropsy was done at 5 days post challenge.

Fig 2. Mean group clinical assessment on the day of IAV challenge and on day post challenge (dpc) 1–5.

A. Presence of cough ranging from 0 = absent to 2 = persisting cough. B. Respiratory score ranging from 0 = normal to 6 = severe respiratory distress when at rest. C. Rectal temperature. Different superscripts ^(A,B) on certain dpc indicate significantly (P < 0.05) different group means.

Fig 3. Mean group log₁₀ IAV RNA genomic copies ±SEM.

A. Nasal swabs were collected at day post challenge (dpc) 1–5 and bronchoalveolar lavage (BAL) fluid was collected on dpc 5. Different superscripts ^(A,B) on certain dpc indicate significantly (P < 0.05) different group means. B. Pen/group based oral fluid samples collected at dpc 1–5.

Fig 4. Macroscopic lung lesions at 5 days post IAV challenge.

A. SHAMVAC-NEG pig 229: There are no visible lung lesions present (Score: 0). B. SHAMVAC-IAV pig 244: Dorsoventral there is diffuse severe dark purple consolidation present (Score: 30.1). C. NvParticle/M2e pig 239: Dorsoventral there is diffuse severe dark purple consolidation present (Score: 33.5). D. NvComplex/M2e pig 232: Dorsoventral there is diffuse severe dark purple consolidation present (Score: 31.1). E. Inact/pH1N1pig 234: There are no visible lung lesions present (Score: 0).