Emerging Antiviral Strategies for PRRSV: Beyond Vaccination

Vaccination remains the cornerstone of porcine reproductive and respiratory syndrome (PRRS) control, but the continuous evolution of PRRSV and its ability to evade host immunity mean that vaccination alone may not always provide adequate protection. This has led to growing interest in antiviral strategies that interrupt different stages of the viral life cycle or strengthen the host's natural defense mechanisms. 

Rather than replacing vaccines, these emerging approaches are being explored as complementary tools to reduce viral replication, limit disease progression, and improve overall control of PRRSV. Many of these strategies target host receptors, viral proteins, or cellular pathways that are essential for successful infection. 

Blocking Viral Entry into Host Cells 

PRRSV relies on specific host cell receptors to establish infection. During viral entry, envelope proteins interact with receptors such as heparan sulfate (HS), sialic acid-binding immunoglobulin-like lectin (Sn/CD169), heat shock protein family A member 8 (HSPA8), and CD163 to facilitate attachment, internalization, and viral uncoating^1,2,3. 

Among these receptors, CD163 plays a particularly important role by enabling the release of the viral genome from early endosomes into the cytoplasm^3,4. One of its domains, SRCR5, has been identified as a critical component for successful PRRSV infection. Disrupting interactions between SRCR5 and viral glycoproteins interferes with viral uncoating, limiting infection⁵. 

Building on this understanding, several small molecules have been identified that block the interaction between CD163 and PRRSV proteins, offering potential antiviral activity. Nanobodies directed against the SRCR5-9 domains of CD163 have also demonstrated broad inhibitory activity against multiple PRRSV strains, highlighting host receptor targeting as a promising antiviral strategy^6,7. 

Targeting Viral Proteins to Reduce Replication 

Several relatively conserved PRRSV proteins have emerged as attractive antiviral targets. 

The nonstructural protein nsp9 plays an essential role in viral replication. A nanobody targeting nsp9, known as Nb6, effectively inhibited PRRSV replication in Marc145 cells. To overcome its inability to enter cells, fusion with the transactivating transcriptional activator (TAT) peptide enabled efficient intracellular delivery into Marc145 cells and porcine alveolar macrophages (PAMs), where it inhibited both genotype 1 and genotype 2 PRRSV strains⁷. 

Another strategy involved linking Nb6 to the porcine IgG Fc receptor, creating the Nb6-pFc chimeric antibody. This construct entered PAMs through Fcγ receptor-mediated endocytosis and inhibited PRRSV infection while promoting pro-inflammatory cytokine production⁸. 

Additional attention has focused on nsp4, a viral protease involved in suppressing the host antiviral response by interfering with the NF-κB signaling pathway and reducing IFN-β expression. Nanobodies directed against nsp4 successfully inhibited viral replication and reduced PRRSV-induced cellular damage^7,9. 

The nucleocapsid (N) protein, encoded by ORF7, represents another highly conserved target. Nanobodies directed against this protein interfered with viral particle assembly by blocking N protein self-interaction, resulting in significant inhibition of PRRSV-2 replication in both in vitro and in vivo settings⁷. 

RNA-Based Strategies to Strengthen Antiviral Responses⁷ 

Host-derived microRNAs (miRNAs) have emerged as important regulators of antiviral immunity. Several miRNAs influence PRRSV infection by enhancing interferon responses or regulating host receptors required for viral entry. 

For example, miR-155 suppresses suppressor of cytokine signaling 1 (SOCS1), increasing STAT1 phosphorylation and IFN-β production, thereby reducing viral replication. miR-125b enhances the NF-κB signaling pathway, while miR-23 promotes interferon induction through activation of IRF3 and IRF7. Other miRNAs, including miR-181 and let-7f-5p, have been identified as potential regulators of the PRRSV entry receptors CD163 and MYH9. 

Beyond miRNAs, additional RNA-based molecules, including small interfering RNA (siRNA), short hairpin RNA (shRNA), artificial miRNAs, and phosphorodiamidate morpholino oligomers (PMOs), have demonstrated antiviral activity against PRRSV. In particular, siRNAs targeting nsp1α, nsp9, and ORF7 effectively suppressed viral replication in host cells. 

Although RNA-based therapies show considerable promise, efficient delivery into target cells remains a significant challenge because extracellular degradation and poor cellular uptake can reduce their effectiveness. Lipid-based delivery systems such as liposomes and lipid nanoparticles are being explored to improve RNA stability and intracellular delivery. 

Key Takeaway 

Emerging antiviral strategies are expanding the options for PRRS control by targeting viral entry, replication, and host immune pathways. Although these approaches are still under development, they highlight new possibilities for complementing vaccination and improving future PRRSV management. 

References 

1. Guo C, Zhu Z, Guo Y, Wang X, Yu P, Xiao S, Chen Y, Cao Y, Liu X. Heparanase upregulation contributes to porcine reproductive and respiratory syndrome virus release. Journal of virology. 2017 Aug 1;91(15):10-128. https://journals.asm.org/doi/pdf/10.1128/jvi.00625-17 

2. Wang L, Li R, Geng R, Zhang L, Chen XX, Qiao S, Zhang G. Heat shock protein member 8 (HSPA8) is involved in porcine reproductive and respiratory syndrome virus attachment and internalization. Microbiology Spectrum. 2022 Feb 23;10(1):e01860-21. https://journals.asm.org/doi/pdf/10.1128/spectrum.01860-21 

3. Rowland RR, Brandariz-Nuñez A. Role of CD163 in PRRSV infection. Virology. 2024 Dec 1;600:110262. https://www.sciencedirect.com/science/article/am/pii/S0042682224002836 

4. Xu Y, Ye M, Sun S, Cao Q, Luo J, Wang Y, Zheng W, Meurens F, Chen N, Zhu J. CD163-expressing porcine macrophages support NADC30-like and NADC34-like PRRSV infections. Viruses. 2022 Sep 16;14(9):2056. https://www.mdpi.com/1999-4915/14/9/2056 

5. Yu P, Wei R, Dong W, Zhu Z, Zhang X, Chen Y, Liu X, Guo C. CD163ΔSRCR5 MARC-145 cells resist PRRSV-2 infection via inhibiting virus uncoating, which requires the interaction of CD163 with calpain 1. Frontiers in Microbiology. 2020 Jan 13;10:3115. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.03115/pdf 

6. Zhu J, He X, Bernard D, Shen J, Su Y, Wolek A, Issacs B, Mishra N, Tian X, Garmendia A, Tang Y. Identification of new compounds against PRRSV infection by directly targeting CD163. Journal of Virology. 2023 May 31;97(5):e00054-23. https://journals.asm.org/doi/pdf/10.1128/jvi.00054-23 

7. He Z, Li F, Liu M, Liao J, Guo C. Porcine reproductive and respiratory syndrome virus: challenges and advances in vaccine development. Vaccines. 2025 Feb 28;13(3):260. https://www.mdpi.com/2076-393X/13/3/260 

8. Zhang L, Wang L, Cao S, Lv H, Huang J, Zhang G, Tabynov K, Zhao Q, Zhou EM. Nanobody Nb6 fused with porcine IgG Fc as the delivering tag to inhibit porcine reproductive and respiratory syndrome virus replication in porcine alveolar macrophages. Veterinary Research. 2021 Feb 17;52(1):25. https://link.springer.com/content/pdf/10.1186/s13567-020-00868-9.pdf 

9. Huang C, Zhang Q, Guo XK, Yu ZB, Xu AT, Tang J, Feng WH. Porcine reproductive and respiratory syndrome virus nonstructural protein 4 antagonizes beta interferon expression by targeting the NF-κB essential modulator. Journal of virology. 2014 Sep 15;88(18):10934-45. https://journals.asm.org/doi/pdf/10.1128/jvi.01396-14