African swine fever virus deploys layered immune evasion that stalls vaccine progress

African swine fever virus, the sole member of the genus Asfivirus in the family Asfarviridae, has assembled one of the most elaborate immune evasion systems described among large DNA viruses. A new review published in Animal Diseases synthesizes recent findings on how the virus, a nucleocytoplasmic large DNA virus with a 170 to 190 kilobase genome encoding 150 to 200 proteins, dismantles both innate and adaptive immune defenses in its porcine hosts. More than half of the viral proteins remain functionally uncharacterized, yet the ones mapped so far reveal a coordinated strategy: the virus suppresses type I interferon production, manipulates inflammatory signaling and programmed cell death, and cripples antigen presentation and lymphocyte activation, all of which sustains intracellular replication and systemic spread.

The disease itself is severe. African swine fever, first reported in Kenya in 1921, has spread from sub-Saharan Africa to Europe, South America and Asia, and its 2018 introduction into China caused enormous economic losses. Infection produces high fever, systemic hemorrhages, and splenic and lymph node enlargement and necrosis in pigs. Alarmingly, the recent emergence between 2024 and 2026 of highly virulent genotype I/II recombinant strains in Asia has worsened the epidemiological threat. These variants cause acute hemorrhagic lesions and near-total case fatality in domestic pigs and, crucially, evade the cross-protection elicited by current genotype II-based live attenuated vaccines, a shift that complicates eradication and underscores the need to understand the molecular basis of immune evasion.

The virus targets porcine monocytes, macrophages and dendritic cells, using these key immune sentinels as its primary replication niche. Viral particles, roughly 260 to 300 nanometers in diameter, enter cells through clathrin-mediated endocytosis and macropinocytosis, and recent work shows the virus can also exploit caveolae-mediated endocytosis and hijack apoptotic bodies for cell-to-cell transmission. Once inside, replication occurs in perinuclear viral factories. The virus is also the only known DNA arbovirus naturally transmitted by soft ticks of the genus Ornithodoros, a trait that enhances its environmental resilience. Its genome is organized into a conserved central region flanked by variable regions enriched in multigene families such as MGF300, MGF360 and MGF505/530, which are critical determinants of virulence, host tropism and immune evasion. Based on B646L sequence divergence, 24 genotypes are recognized, though the standard single-gene assay fails to distinguish mosaic genotype I/II recombinants, requiring multilocus or whole-genome sequencing for accurate typing.

A central evasion strategy involves shielding the viral genome from cytosolic DNA sensors. Rather than releasing naked DNA into the cytoplasm, the virus delivers intact core particles that encapsulate the genome with a prepackaged transcription apparatus, so early gene expression occurs entirely within the core, and late-stage replication is compartmentalized within viral factories. Despite this physical shielding, viral genomic DNA eventually reaches the cytosol and is detected by the sensor cGAS, which catalyzes synthesis of the second messenger 2’3′-cGAMP and initiates STING trafficking from the endoplasmic reticulum to the Golgi, recruiting TBK1 and driving IRF3 phosphorylation and nuclear translocation to trigger interferon and proinflammatory cytokine transcription. RNA intermediates generated during replication are also detected by RIG-I-like receptors, adding a complementary sensing axis.

Against the cGAS-STING pathway, the virus deploys an exceptionally sophisticated antagonistic network operating at three hierarchical levels. Viral proteins act at the source: pQP383R binds cGAS and promotes its palmitoylation to suppress catalytic activity, pI226R induces cGAS autophagic degradation, pMGF505-4R drives cGAS degradation via TOLLIP-mediated selective autophagy, and pEP364R, pC129R and pB175L neutralize the second messenger cGAMP. A second tier disrupts STING itself: pDP71L impedes the STING-TBK1 interaction, pB318L and CD2v block STING translocation from the endoplasmic reticulum to the Golgi, pE184L prevents STING oligomerization, and pMGF505-6R, pMGF505-7R, pMGF505-11R, pL83L and p17 promote STING degradation or block its recruitment of TBK1. A third tier targets the downstream kinase TBK1 and transcription factor IRF3: pI73R binds the mRNAs of both molecules and retains them in the nucleus, pMGF360-14L mediates ubiquitination and degradation of IRF3 through TRIM21, pMGF110-9L degrades TBK1 through a PIK3C2B-mediated autophagic pathway, and pE120R hinders recruitment of IRF3 to TBK1.

The RIG-I-like receptor pathway faces parallel assault. The early protein p30 targets host TRIM21 and the adaptor MAVS to disrupt signaling complex assembly, p17 induces MAVS degradation through SQSTM1-dependent mitophagy, pS183L impairs MDA5 oligomerization and its interaction with MAVS, and pE248R blocks RIG-I association with MAVS while inhibiting TRIM25-mediated ubiquitination of RIG-I. Notably, pMGF505-9R runs counter to this pattern: it binds the E3 ubiquitin ligase RNF125 and induces its autoubiquitination and degradation, relieving RNF125-mediated inhibition of RIG-I, MDA5 and MAVS and thereby promoting RLR pathway activation. This bidirectional regulation suggests the virus finely tunes host immune homeostasis to create a microenvironment favorable to its own replication.

Even when interferon molecules are secreted, the virus neutralizes their impact within infected cells by attacking the JAK-STAT cascade. At the receptor level, p22 promotes IFNAR1 degradation through the autophagic pathway, pB125R induces autophagic degradation of IFNAR2, and pK205R, pH240R, pB318L and CD2v hinder receptor binding to JAK1 or TYK2. During signal transduction, pMGF360-10L recruits HERC5 to degrade JAK1, pI7L blocks STAT1 phosphorylation and homodimerization, pS273R and pI215L induce proteasomal degradation of STAT2, and pB475L inhibits STAT1/STAT2 heterodimerization and nuclear translocation. Downstream, pMGF505-7R and pI215L degrade IRF9 to disrupt ISGF3 complex assembly, pNP419L directly binds the interferon-stimulated response element to block ISGF3 binding to target promoters, and pC315R upregulates IL-6 and promotes STAT3 phosphorylation to competitively suppress the canonical interferon pathway. The redundancy of these antagonists, distributed across early and late expression phases and diverse cellular compartments, forms a resilient blockade that maximizes replication even if individual viral immunomodulators are compromised.

Inflammatory pathways face equally layered manipulation. The virus suppresses NF-κB activation by disrupting IKK complex activity, stabilizing IκBα and blocking p65 nuclear translocation: pB318L blocks NEMO ubiquitination and binding to IKKα, pD345L inhibits IKKα/β enzymatic activity, pMGF300-2R induces degradation of IKKα/IKKβ, pA238L maintains the cytoplasmic inhibitory state of NF-κB, and pMGF360-12L competitively binds KPNA2/3/4 to hinder p65 nuclear import. The NLRP3 inflammasome is regulated bidirectionally: pE364R promotes NLRP3 oligomerization and activation via DDX3X, pB169L exacerbates inflammatory responses, and pI177L facilitates inflammasome assembly and mature IL-1β secretion, while pB318L, pH240R and pMGF505-7R suppress NLRP3 activation. The cysteine protease pS273R cleaves the pyroptotic effector GSDMD atypically, blocking canonical caspase-1-dependent pyroptosis and preserving membrane integrity for viral assembly. In domestic pigs infected with highly virulent strains, this manipulation culminates in cytokine storms, vascular endothelial injury, disseminated intravascular coagulation and multiorgan failure, the central pathological mechanism driving acute death.

Adaptive immunity is undermined at its foundation. The virus downregulates MHC class II molecules on monocytes and conventional dendritic cells and the complement receptor CD21 on B cells, blunting cognate antigen presentation to T helper cells and dampening B-cell costimulatory activation. pEP153R blocks transport of MHC class I from the endoplasmic reticulum to the cell surface, hindering recognition of infected cells by CD8+ cytotoxic T lymphocytes, while viral infection downregulates key cathepsins and the peptide-loading molecules SLA-DMA and SLA-DMB in macrophages, disrupting antigenic peptide loading onto MHC class II. Systemic lymphopenia, a hallmark of evasion, arises through signaling interference and programmed cell death, and persistent viral antigen stimulation combined with upregulation of checkpoint molecules such as PD-1/PD-L1 induces profound T-cell exhaustion. Within infected macrophages, pA238L binds calcineurin and blocks NFAT dephosphorylation and nuclear translocation, impairing NFAT-dependent gene transcription. Humoral defenses are also compromised: antibodies against structural proteins including p72, p54, p30 and CD2v generally become detectable only two to three weeks after infection with virulent strains, by which time systemic dissemination has occurred, and most induced antibodies are nonneutralizing, with some, such as those targeting pA137R, reported to mediate antibody-dependent enhancement of infection in porcine alveolar macrophages in vitro and exacerbate pathogenicity in pigs in vivo.

These evasion mechanisms explain the persistent bottleneck in vaccine development. Single-gene deletion candidates such as I177L, A137R and E184L have shown protective potential under controlled laboratory conditions, but recent clinical evaluations of the I177L-deleted vaccine raised safety concerns including systemic adverse reactions and impaired reproductive performance in breeding boars, alongside a substantial risk of virulence reversion. Multigene deletion strategies are therefore gaining favor: dual deletion of MGF360-9L and MGF505-7R achieves robust attenuation alongside potent interferon responses and T-cell-mediated immunity, and the polygene-deleted candidate HLJ/18-7GD, combining deletions of MGF360/505 family members and CD2v, demonstrated favorable safety and protective efficacy against homologous challenge. Yet the genotype I/II recombinants carry chimeric genomes combining immune evasion loci from genotype I within a virulent genotype II backbone, reducing recognition of cross-reactive T-cell epitopes and neutralizing antibodies induced by conventional genotype II vaccines. The review argues that next-generation design must target strictly conserved, pangenotypic immune evasion hubs, and that structurally informed subunit vaccines using glycoengineering to expose concealed conserved epitopes, combined with optimized delivery systems and adjuvants, represent a promising frontier for achieving broad-spectrum protection.

Subject of Research: Immune evasion mechanisms of African swine fever virus and their implications for vaccine development

Article Title: Host–virus immune crosstalk and immune evasion mechanisms of African swine fever virus: recent progress and translational implications

Article References: He, X., Li, H., Li, P., Wang, G., Sun, Y., Yu, X., Peng, G., Jongkaewwattana, A., He, Q., & Li, W. (2026). Host–virus immune crosstalk and immune evasion mechanisms of African swine fever virus: recent progress and translational implications. Animal Diseases, 6(1), Article 63. https://doi.org/10.1186/s44149-026-00267-1

Image Credits: AI Generated

DOI: 10.1186/s44149-026-00267-1

Keywords: African swine fever virus, immune evasion, type I interferon, cGAS-STING, JAK-STAT, NLRP3 inflammasome, antigen presentation, T-cell exhaustion, vaccine development, swine immunology, cytokine storm, genotype I/II recombinants