Poly I:C adjuvanted inactivated swine influenza vaccine induces heterologous protective immunity in pigs

Abstract

Swine influenza is widely prevalent in swine herds in North America and Europe causing enormous economic losses and a public health threat. Pigs can be infected by both avian and mammalian influenza viruses and are sources of generation of reassortant influenza viruses capable of causing pandemics in humans. Current commercial vaccines provide satisfactory immunity against homologous viruses; however, protection against heterologous viruses is not adequate. In this study, we evaluated the protective efficacy of an intranasal Poly I:C adjuvanted UV inactivated bivalent swine influenza vaccine consisting of Swine/OH/24366/07 H1N1 and Swine/CO/99 H3N2, referred as PAV, in maternal antibody positive pigs against an antigenic variant and a heterologous swine influenza virus challenge. Groups of three-week-old commercial-grade pigs were immunized intranasally with PAV or a commercial vaccine (CV) twice at 2 weeks intervals. Three weeks after the second immunization, pigs were challenged with the antigenic variant Swine/MN/08 H1N1 (MN08) and the heterologous Swine/NC/10 H1N2 (NC10) influenza virus. Antibodies in serum and respiratory tract, lung lesions, virus shedding in nasal secretions and virus load in lungs were assessed. Intranasal administration of PAV induced challenge viruses specific-hemagglutination inhibition- and IgG antibodies in the serum and IgA and IgG antibodies in the respiratory tract. Importantly, intranasal administration of PAV provided protection against the antigenic variant MN08 and the heterologous NC10 swine influenza viruses as evidenced by significant reductions in lung virus load, gross lung lesions and significantly reduced shedding of challenge viruses in nasal secretions. These results indicate that Poly I:C or its homologues may be effective as vaccine adjuvants capable of generating cross-protective immunity against antigenic variants/heterologous swine influenza viruses in pigs.

Keywords: Inactivated swine influenza vaccines; Poly I:C; Swine influenza virus; Vaccine adjuvants.

Fig. 1

Serum HI antibodies in vaccinated/challenged pigs. Mean HI titers in serum of pigs immunized either with PAV or CV were determined at DPV 14 and 35 against MN08 (A) and NC10 virus (B). Data are expressed as mean HI titers of four pigs ± SD. ‘^*’—significantly different from NV group (P < 0.05); ‘^#’—significantly different from NV and CV groups (P < 0.05).

Fig. 2

Serum IgG antibodies in vaccinated/challenged pigs. Challenge virus specific-IgG antibody levels in serum of immunized/challenged pigs were determined by ELISA against MN08 and NC10 at 35 DPV (DPC 0) (A) and at DPC 6 (B). Data are expressed as mean ODs of four pigs ± SD.

Fig. 3

Virus-specific IgG and IgA antibody levels in lungs of pigs. At necropsy on DPC 6, lungs were obtained from the groups of vaccinated and vaccinated challenged pigs and lung lysates were prepared as described in Materials and Methods. IgG (panel A) and IgA (panel B) antibody responses against MN08 and NC10 in lung lysates of these groups of pigs were examined by ELISA. Data are expressed as mean OD at 405 nm of four pigs ± SD.

Fig. 4

Poly I:C adjuvanted vaccine provides protection against antigenic variant and heterologous virus challenge. Groups of immunized and non-immunized pigs were challenged with either MN08 or NC10 SIV. Six days after challenge (DPC 6), gross lung lesions were examined (A). Extensive areas of consolidation were observed in non-vaccinated MN08- or NC10-challenged pigs. Minimal lung lesions were observed in PAV-immunized and MN08- and NC10-challenged pigs whereas pigs immunized with CV and challenged with MN08 and NC10 viruses showed moderate levels of lung consolidation. No lung lesions were observed in mock-inoculated pigs. Arrows indicate influenza specific lesions in the lungs. Representative pictures from different groups are shown. At necropsy on DPC 6, mean gross lung lesions from all the six lung lobes were scored on the basis of percent pneumonic lesions (B). Each bar indicates the mean lung lesion scores of four pigs ± SD. ‘^*’—significantly different from NV group, (P < 0.05).

Fig. 5

Detection of challenge virus shedding in nasal secretions and lungs of pigs. Groups of immunized and non-immunized pigs were challenged with either MN08 or NC10 SIV. At DPC 3 (A) and 6 (B) nasal swabs were collected and challenge virus shedding was determined by titration in MDCK cells. At DPC 6 (C) lungs were harvested and 10% homogenate of lung tissue was prepared. Challenge virus replication was determined by titration in MDCK cells. Values indicate mean virus titers in nasal swabs/lungs of four pigs ± SD except in CV-MN08 group where values are mean virus titer from three pigs ± SD because one pig in this group was euthanized before challenge as it developed respiratory illness after vaccination. ‘^*’—significantly different from NV group, (P < 0.05).