Concomitant Swine Influenza A Virus Infection Alters PRRSV1 MLV Viremia in Piglets but Does Not Interfere with Vaccine Protection in Experimental Conditions

Abstract

Modified-live vaccines (MLVs) against porcine reproductive and respiratory syndrome viruses (PRRSVs) are usually administrated to piglets at weaning when swine influenza A virus (swIAV) infections frequently occur. SwIAV infection induces a strong interferon alpha (IFNa) response and IFNa was shown to abrogate PRRSV2 MLV replication and an inherent immune response. In this study, we evaluated the impacts of swIAV infection on the replication of a PRRSV1 MLV (MLV1), post-vaccine immune responses and post-challenge vaccine efficacy at both the systemic and pulmonary levels. Piglets were either swIAV inoculated and MLV1 vaccinated 6 h apart or singly vaccinated or mock inoculated and mock vaccinated. Four weeks after vaccination, the piglets were challenged with a PRRSV1 field strain. The results showed that swIAV infection delayed MLV1 viremia by six days and post-vaccine seroconversion by four days. After the PRRSV1 challenge, the swIAV enhanced the PRRSV1-specific cell-mediated immunity (CMI) but the PRRSV1 field strain viremia was not better controlled. High IFNa levels that were detected early after swIAV infection could have been responsible for both the inhibition of MLV1 replication and CMI enhancement. Thus, whereas swIAV infection had a negative impact on humoral responses post-vaccination, it did not interfere with the protective effectiveness of the PRRSV MLV1 in our experimental conditions.

Keywords: interference; interferon; modified-live vaccine; porcine reproductive and respiratory syndrome; swine influenza A virus.

Figure 1

Description of the experiment design.

Figure 2

Clinical data recorded during the post-vaccination period (D0 to D28 pv). (a) Rectal temperature; (b) Average daily weight gain of piglets. All data are reported as the mean (±SD) of the results obtained from piglets in each group. Different letters (a–c) indicate that the considered group (specified by its color) was significantly different from the CTRL group (a), from the UNVAC group (b) or from the VAC group (c) with p < 0.05.

Figure 3

Swine influenza A virus (SwIAV) genomic loads quantified by M gene RT-qPCR (a) in nasal swab supernatants or (b) in bronchoalverolar lavage fluid (BALF) collected from piglets in the SIVAC group. Data are reported as the mean (±SD) of the group.

Figure 4

Interferon alpha (IFNa) levels quantified in (a) serum or (b) BALF. Data are reported as the mean (±SD) of the results obtained from all piglets in each group. Different letters (b,c) indicate that the considered group (specified by its color) was significantly different from the UNVAC group (b) or from the VAC group (c) with p < 0.05.

Figure 5

Relative gene expression assessed by RT-PCR in BALCs from the SIVAC and VAC groups for (a) IFNg or (b) IL10 using expression results obtained in the UNVAC group as the reference. Data are reported as the mean (±SD) of the results obtained from all piglets in each group. Letter (c) indicates that the SIVAC group was significantly different from the VAC group with p < 0.05.

Figure 6

Porcine reproductive and respiratory syndrome virus (PRRSV) genomic loads in (a) serum or in (b) BALF. All data are reported as the mean (±SD) of the results obtained from all piglets in each group. Letter (c) indicates that the SIVAC group was significantly different from the VAC group with p < 0.05. AUC: area under the curve.

Figure 7

Anti-PRRSV antibody response assessed by ELISA in (a) serum or in (b) BALF. Data are reported as the mean (±SD) of results the obtained from all piglets in each group. Different letters (b,c) indicate that the considered group (specified by its color) was significantly different from the UNVAC group (b) or from the VAC group (c) with p < 0.05.

Figure 8

Anti-PRRSV cell-mediated immune response to the vaccine strain assessed by IFNg ELISPOT in (a) PBMC or in (b) BALCs. Data are reported as the mean (±SD) of the results obtained from all piglets in each group. Different letters (b,c) indicate that the considered group (specified by its color) was significantly different from the UNVAC group (b) or from the VAC group (c) with p < 0.05. There is no significance c found in these graphs.

Figure 9

Clinical data recorded during the post-challenge period (D28 to D70 pv). (a) Rectal temperature; (b) Average daily weight gain of piglets. All data are reported as the mean (±SD) of the results obtained from piglets in each group. Different letters (a–c) indicate that the considered group (specified by its color) was significantly different from the CTRL group (a), from the UNVAC group (b) or from the VAC group (c) with p < 0.05. There is no significance c found in these graphs.

Figure 10

PRRSV1 Finistere strain genomic loads in (a) serum or in (b) BALF. Data are reported as the mean (±SD) of the results obtained from all piglets in each group. Different letters (b,c) indicate that the considered group (specified by its color) was significantly different from the UNVAC group (b) or from the VAC group (c) with p < 0.05. AUC: area under the curve. There is no significance c found in these graphs.

Figure 11

Anti-PRRSV cell-mediated response to the Finistere strain assessed by IFNg ELISPOT in peripheral blood mononuclear cells (PBMC). Data are reported as the mean (±SD) of the results obtained from all piglets in each group. Different letters (a–c) indicate that the considered group (specified by its color) was significantly different from the CTRL group (a), from the UNVAC group (b) or from the VAC group (c) with p < 0.05.