Antibody-dependent enhancement of porcine reproductive and respiratory syndrome virus infection downregulates the levels of interferon-gamma/lambdas in porcine alveolar macrophages in vitro

Abstract

Fc gamma receptor-mediated antibody-dependent enhancement (ADE) can promote virus invasion of target cells, sometimes exacerbating the severity of the disease. ADE may be an enormous hurdle to developing efficacious vaccines for certain human and animal viruses. ADE of porcine reproductive and respiratory syndrome virus (PRRSV) infection has been demonstrated in vivo and in vitro. However, the effect of PRRSV-ADE infection on the natural antiviral immunity of the host cells is yet to be well investigated. Specifically, whether the ADE of PRRSV infection affects the levels of type II (interferon-gamma, IFN-γ) and III (interferon-lambdas, IFN-λs) interferons (IFNs) remains unclear. In this study, our results showed that PRRSV significantly induced the secretion of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 in porcine alveolar macrophages (PAMs) in early infection, and weakly inhibited the production of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 in PAMs in late infection. Simultaneously, PRRSV infection significantly increased the transcription of interferon-stimulated gene 15 (ISG15), ISG56, and 2', 5'-oligoadenylate synthetase 2 (OAS2) in PAMs. In addition, our results showed that PRRSV infection in PAMs via the ADE pathway not only significantly decreased the synthesis of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 but also significantly enhanced the generation of transforming growth factor-beta1 (TGF-β1). Our results also showed that the ADE of PRRSV infection significantly reduced the mRNAs of ISG15, ISG56, and OAS2 in PAMs. In conclusion, our studies indicated that PRRSV-ADE infection suppressed innate antiviral response by downregulating the levels of type II and III IFNs, hence facilitating viral replication in PAMs in vitro. The ADE mechanism demonstrated in the present study furthered our understanding of persistent pathogenesis following PRRSV infection mediated by antibodies.

Keywords: IFN-γ; IFN-λs; PAMs; PRRSV-ADE; TGF-β1.

Figure 1

Detection of PRRSV-ADE activity in PAMs. PRRSV RNA (A) and TCID₅₀ (B) in harvested culture supernatants of the PAM cells following PRRSV, PRRSV+PPI, or PRRSV+PNI infection were measured using real-time RT-PCR and viral titration experiments. The bars indicate the RNA copies or TCID₅₀ titers of PRRSV. The error bars indicate the mean ± standard error of the mean (SEM) from three independent experiments. ***p < 0.001.

Figure 2

The effect of PRRSV or PRRSV-ADE on mRNAs of the innate immune cytokines and antiviral protein genes in PAMs. The mRNAs of the innate immune cytokines and antiviral protein genes in treated PAM cells were evaluated using relative quantitative RT-PCR. (A) IFN-γ mRNA, (B) IFN-λ1 mRNA, (C) IFN-λ3 mRNA, (D) IFN-λ4 mRNA, (E) TGF-β1 mRNA, (F) ISG15 mRNA, (G) ISG56 mRNA, and (H) OSA2 mRNA. The bars indicate the relative expression levels of mRNAs of the innate immune cytokines or antiviral protein genes. The error bars indicate the SEM from three independent experiments. ***p < 0.001, **p < 0.01, *p < 0.05, and ns, no significance.

Figure 3

The effect of PRRSV or PRRSV-ADE on proteins of the innate immune cytokines in PAMs. The proteins of the innate immune cytokines in the culture supernatants of the PAM cells treated with the indicated methods were quantified using commercial ELISA Kits. (A) IFN-γ protein, (B) IFN-λ1 protein, (C) IFN-λ3 protein, (D) IFN-λ4 protein, and (E) TGF-β1 protein. The bars indicate the protein concentrations of the innate immune cytokines. The error bars indicate the SEM from three independent experiments. ***p < 0.001, **p < 0.01, *p < 0.05, and ns, no significance.