Live attenuated influenza vaccine provides superior protection from heterologous infection in pigs with maternal antibodies without inducing vaccine-associated enhanced respiratory disease

Abstract

Control of swine influenza A virus (IAV) in the United States is hindered because inactivated vaccines do not provide robust cross-protection against the multiple antigenic variants cocirculating in the field. Vaccine efficacy can be limited further for vaccines administered to young pigs that possess maternally derived immunity. We previously demonstrated that a recombinant A/sw/Texas/4199-2/1998 (TX98) (H3N2) virus expressing a truncated NS1 protein is attenuated in swine and has potential for use as an intranasal live attenuated influenza virus (LAIV) vaccine. In the present study, we compared 1 dose of intranasal LAIV with 2 intramuscular doses of TX98 whole inactivated virus (WIV) with adjuvant in weanling pigs with and without TX98-specific maternally derived antibodies (MDA). Pigs were subsequently challenged with wild-type homologous TX98 H3N2 virus or with an antigenic variant, A/sw/Colorado/23619/1999 (CO99) (H3N2). In the absence of MDA, both vaccines protected against homologous TX98 and heterologous CO99 shedding, although the LAIV elicited lower hemagglutination inhibition (HI) antibody titers in serum. The efficacy of both vaccines was reduced by the presence of MDA; however, WIV vaccination of MDA-positive pigs led to dramatically enhanced pneumonia following heterologous challenge, a phenomenon known as vaccine-associated enhanced respiratory disease (VAERD). A single dose of LAIV administered to MDA-positive pigs still provided partial protection from CO99 and may be a safer vaccine for young pigs under field conditions, where dams are routinely vaccinated and diverse IAV strains are in circulation. These results have implications not only for pigs but also for other influenza virus host species.

Fig 1

Serum antibody levels due to maternally derived antibodies and/or responses to vaccination. Reciprocal geometric mean HI titers against TX98 H3N2 antigen (A) and CO99 antigen (B) are shown for multiple time points prior to challenge. MDA, groups with maternally derived antibodies induced by immunizing dams with the TX98 vaccine. Treatment groups were nonvaccinated (NV), vaccinated at 0 dpv and 14 dpv with TX98 WIV, or vaccinated intranasally with TX98 LAIV at 0 dpv only. Mean ODs in whole-virus ELISAs at 49 dpv (0 days postinfection) are shown for serum IgGs against TX98 antigen (C) and CO99 antigen (D). Mean ODs in whole-virus ELISAs at 5 days postchallenge are shown for IgG against TX98 antigen for groups challenged with TX98 (E) and for IgG against CO99 antigen for groups challenged with CO99 (F). Open bars designate groups without MDA, and solid bars designate groups with MDA. Statistically significant differences between MDA statuses within a vaccine group are marked with asterisks, and differences between vaccine treatment groups with matched MDA statuses are identified by connecting lines (P < 0.05).

Fig 2

Antibody levels in bronchoalveolar lavage fluid at 5 days postinfection. Mean ODs in whole-virus ELISAs are shown for IgGs against TX98 antigen (A) and CO99 antigen (B) and for IgAs against TX98 antigen (C) and CO99 antigen (D). Groups challenged with TX98 are represented in panels A and C, whereas groups challenged with CO99 are represented in panels B and D. Open bars designate groups without MDA, and solid bars designate groups with MDA. Statistically significant differences between MDA statuses within a vaccine group are marked with asterisks, and differences between vaccine treatment groups with matched MDA statuses and challenge virus strains are identified by connecting lines (P < 0.05).

Fig 3

(A) Nasal shedding of challenge virus in nasal swabs at 3 (A) and 5 (B) dpi. Piglets were vaccinated in the presence or absence of circulating MDA against TX98. At vaccination, piglets received no vaccine (NV), two intramuscular doses of TX98 WIV, or one intranasal dose of TX98 LAIV. Forty-nine days after the initial vaccine dose, piglets were challenged by intratracheal inoculation with TX98 or CO99. Nasal swab specimens were collected at 3 and 5 dpi and titrated by TCID₅₀ assay on MDCK cells. Statistically significant differences between MDA statuses within a vaccine group are marked with asterisks, and differences between vaccine treatment groups with matched MDA statuses and challenge virus strains are identified by connecting lines (P < 0.05).

Fig 4

Adjuvanted TX98 WIV administered to MDA-positive piglets enhances the severity of subsequent infection with heterologous H3N2 strain CO99, whereas the TX98 LAIV vaccine partially cross-protects pigs. MDA-positive pigs suckled colostrum from TX98-vaccinated sows, and MDA-negative pigs suckled colostrum from naïve sows. WIV was delivered intramuscularly at 2 and 4 weeks of age, while LAIV was delivered intranasally at 2 weeks of age only. At 8 weeks of age (49 dpv), pigs were challenged intratracheally with TX98 or CO99. At 5 dpi, pigs were euthanized, BALF samples were collected, and necropsy was conducted. (A) BALF samples were titrated by TCID₅₀ assay on MDCK cells. (B) Macroscopic lesions were scored as the percentage of total lung surface area involved. Microscopic pneumonia (C) and tracheal damage (D) were scored as described in Materials and Methods. Statistically significant differences between MDA statuses within a vaccine group are marked with asterisks, and differences between vaccine treatment groups with matched MDA statuses and challenge virus strains are identified by connecting lines (P < 0.05).

Fig 5

Photographs of macroscopic lung pathology in pigs positive for MDA at the time of vaccination, shown 5 days after heterologous challenge with CO99. Photographs of ventral surfaces of lungs are representative of three vaccine treatment groups: nonvaccinated challenge controls (A), TX98 LAIV-vaccinated pigs (B), and TX98 WIV-vaccinated pigs (C).