TLRs are evolutionarily conserved pattern recognition receptors that play a pivotal role in detecting viral pathogens and initiating innate immune responses (2). TLRs can be categorized according to their location within the cell, with one major group positioned on the plasma membrane, including TLR1, TLR2, TLR4, TLR5, and TLR6, which primarily recognize bacterial and viral envelope components, and those localized within endosomal membranes (e.g., TLR3, TLR7/8, TLR9), which sense nucleic acids derived from internalized viruses or dying cells (28).

Each endosomal TLR specializes in detecting distinct viral nucleic acid patterns. TLR3 recognizes viral double-stranded RNA (dsRNA), a replicative intermediate generated by many RNA viruses, including NDV and AIV (29). This MyD88-independent, TRIF-dependent signaling endows TLR3 with a unique functional role in antiviral immunity by enabling robust type I interferon production while limiting excessive proinflammatory cytokine responses. Such signaling bias is particularly advantageous during viral infection, as it promotes effective antiviral states without exacerbating immunopathology. TLR7 and TLR8 sense GU-rich single-stranded RNA (ssRNA) from viruses such as PRRSV and IAV, while TLR9 detects unmethylated CpG motifs commonly found in the DNA genomes of viruses like ASFV (30).

Downstream signaling pathways of TLRs are mediated by adaptor proteins, primarily MyD88 and TRIF, which determine the nature and kinetics of immune responses (31). Most TLRs, such as TLR7, TLR8, and TLR9, primarily engage the MyD88-dependent signaling route. This activation subsequently triggers NF-κB and IRF7, driving the production of pro-inflammatory cytokines and type I interferons (32). In contrast, TLR3 uniquely signals via the TRIF-dependent pathway, triggering IRF3-mediated interferon production without MyD88 involvement, a critical feature for antiviral responses to dsRNA viruses (33).

Recent comparative studies in pigs and chickens have highlighted both conserved and divergent roles of TLRs (17, 34, 35). For example, chicken TLR7 exhibits rapid upregulation upon ssRNA virus infection and contributes significantly to the early antiviral cytokine burst, whereas porcine TLR7 and TLR9 display tissue-specific expression profiles and temporal differences in response to AIV or ASFV, indicating species-adapted immune regulation (36).

Furthermore, vaccine strategies leveraging TLR signaling have gained attention. Use of TLR3 and TLR7 agonists in poultry and swine vaccines enhances both humoral and cellular immunity. In chickens, co-delivery of poly(I:C) (TLR3 agonist) and imidazoquinolines (TLR7 agonist) with inactivated NDV or AIV antigens has shown synergistic enhancement of immune responses, suggesting combinatorial adjuvant strategies can improve vaccine efficacy (37–39).

In summary, TLRs serve as key antiviral sentinels, with their localization, ligand specificity, and adaptor-mediated signaling underpinning their functional diversity. Cross-species comparison reveals both conserved core pathways and species-specific adaptations, providing insights for the rational design of PRR-targeted therapeutics and adjuvants in livestock immunology.

RIG-I-like receptors (RLRs), comprising RIG-I, MDA5, and LGP2, represent critical cytoplasmic sensors for viral RNA, orchestrating antiviral innate immunity in livestock species such as pigs and poultry (40). While both pigs and chickens rely on RLRs to detect RNA virus infections, there are notable species-specific differences in receptor repertoire and functionality that influence their antiviral responses (27, 41, 42).

In pigs, RIG-I and MDA5 distinctly recognize viral RNA species (15, 25, 42, 43): RIG-I preferentially binds short 5’-triphosphate RNA and blunt-ended dsRNA, commonly generated during negative-strand RNA virus replication, whereas MDA5 senses long dsRNA intermediates typical of positive-strand RNA viruses. This complementary recognition expands the breadth of RNA virus detection in pigs, which is pivotal for mounting timely interferon responses against pathogens like porcine reproductive and PRRSV and CSFV (44). In contrast, chickens notably lack a functional RIG-I ortholog, relying predominantly on MDA5 for cytosolic RNA sensing (45, 46). This evolutionary loss compels poultry to compensate through MDA5-mediated recognition of both short and long viral RNA motifs, which partially explains differential susceptibility patterns to RNA viruses such as AIV compared to pigs.

Taken together, the comparative analysis of RLR-mediated RNA virus recognition and MAVS signaling in pigs and poultry highlights evolutionary divergence shaping innate immune strategies. While pigs exhibit a classical dual RIG-I/MDA5 system facilitating nuanced viral RNA sensing, chickens rely heavily on MDA5 and possibly other PRRs to orchestrate antiviral defenses. Understanding these differences provides insights for tailored antiviral and vaccine strategies in these key livestock species.

The cGAS)- STING pathway has emerged as a pivotal mechanism for the cytosolic detection of DNA viruses in vertebrates, including key livestock species such as pigs and poultry (47). When double-stranded DNA (dsDNA) is detected in the cytoplasm, cyclic GMP-AMP synthase (cGAS) produces the second messenger cyclic GMP-AMP (cGAMP). This molecule then interacts with and activates the adaptor protein STING, located on the endoplasmic reticulum membrane (48). Once activated, STING recruits TANK-binding kinase 1 (TBK1), which in turn phosphorylates interferon regulatory factors IRF3 and IRF7. The activated IRFs drive the transcription of type I interferons and a range of antiviral genes (49).

In pigs, the cGAS-STING axis plays a crucial role in the innate immune response to ASFV, a large cytoplasmic DNA virus with sophisticated immune evasion strategies (50). Recent studies have demonstrated that cGAS and STING are both transcriptionally upregulated following ASFV infection in porcine alveolar macrophages, and genetic ablation or inhibition of either molecule results in impaired type I IFN responses and enhanced viral replication (51). However, ASFV encodes multiple proteins (e.g., MGF505-7R, I215L) that actively disrupt the cGAS-STING signaling cascade by targeting TBK1 and STING, allowing the virus to blunt host interferon responses (52).

In contrast, research on cGAS-STING signaling in poultry remains comparatively limited, yet emerging evidence highlights its relevance in defense against avian DNA pathogens such as Duck Tembusu virus and Riemerella anatipestifer, a bacterium with intracellular DNA sensing implications (53). Notably, functional cGAS and STING orthologs have been identified in ducks and chickens, and their overexpression in vitro enhances IFN-β production upon DNA virus challenge, while knockdown experiments diminish antiviral gene expression (54, 55). The evolutionary conservation of this pathway, despite sequence divergence, suggests that poultry utilize cGAS-STING as a frontline defense mechanism against DNA viruses, although further in vivo validation is warranted.

The NF-κB signaling pathway plays a pivotal role downstream of PRRs in coordinating antiviral innate immune responses in livestock such as pigs and poultry (56, 57). When viral ligands are detected, adaptor proteins such as MyD88 and TRIF initiate downstream signaling events that activate the IκB kinase (IKK) complex. This complex is primarily composed of the catalytic subunits IKKα and IKKβ, along with the regulatory component NEMO (also known as IKKγ) (43, 58, 59). The activated IKK complex phosphorylates the inhibitor proteins IκBs, targeting them for ubiquitination and subsequent degradation. This process releases NF-κB dimers, primarily p65/p50, that then translocate into the nucleus to initiate transcription (60, 61). Upon activation, NF-κB drives the transcription of pivotal proinflammatory mediators, including TNF-α and IL-6. These cytokines play a central role in orchestrating a robust inflammatory response necessary for controlling viral infections (62). Research using porcine alveolar macrophages has shown that NF-κB activation not only triggers the production of inflammatory cytokines but also promotes type I interferon responses, thereby establishing an antiviral state (63). Similar mechanisms have been observed in chickens, although differences in the timing and level of cytokine production may underlie species-specific variations in antiviral defense (64). Moreover, negative regulators such as A20 and CYLD modulate NF-κB signaling to prevent excessive inflammation and tissue damage during viral challenges in both species (65). Deciphering these regulatory pathways is vital for developing strategies that enhance antiviral immunity while minimizing immunopathology in livestock.

Activation of PRRs triggers the phosphorylation and nuclear translocation of interferon regulatory factors 3 and 7 (IRF3/7), which are pivotal transcription factors driving type I interferon (IFN-α/β) production, a cornerstone of antiviral innate immunity in livestock such as pigs and poultry (66, 67). Upon viral RNA or DNA recognition, adaptor proteins such as MAVS, STING, MyD88, or TRIF facilitate the recruitment and activation of kinases TBK1 and IKKϵ, which phosphorylate IRF3/7, enabling their dimerization and nuclear import to initiate IFN gene transcription (68). In pigs, robust activation of IRF3 and IRF7 has been documented during infections with RNA viruses like porcine reproductive and PRRSV and CSFV, correlating with potent type I IFN responses critical for viral control (69). Conversely, chickens predominantly rely on IRF7 due to the absence of RIG-I, and studies suggest differential kinetics and intensity of IRF7 activation compared to pigs, which may underlie species-specific variations in antiviral defenses against pathogens such as AIV (70). The type I interferons subsequently induce a wide array of ISGs, including Mx, OAS, and PKR, which mediate diverse antiviral effects ranging from viral replication inhibition to apoptosis of infected cells (71). Notably, comparative transcriptomic analyses have revealed both conserved and divergent patterns of ISG induction between pigs and poultry, reflecting evolutionary adaptations to their respective viral threats and immune system architectures (72, 73). Together, these findings underscore the centrality of IRF3/7-dependent signaling in orchestrating type I IFN responses in livestock, while highlighting species-specific nuances that could inform tailored antiviral strategies and vaccine development.

Upon secretion, type I interferons (IFN-α/β) bind to their specific surface receptors, initiating the Janus kinase (JAK), signal transducer and activator of transcription (STAT) signaling pathway (74, 75). In both swine and poultry, receptor engagement activates the associated kinases JAK1 and TYK2, which in turn phosphorylate STAT1 and STAT2 (76, 77). The phosphorylated STAT proteins heterodimerize and recruit interferon regulatory factor 9 (IRF9), forming the interferon-stimulated gene factor 3 (ISGF3) complex. Once assembled, ISGF3 translocates into the nucleus and binds to interferon-stimulated response elements (ISREs) in target gene promoters, driving the transcription of interferon-stimulated genes (ISGs) that mediate antiviral defense (78, 79).

The ISGs induced through JAK-STAT signaling constitute a broad antiviral arsenal, including canonical effectors such as Mx1, OAS, and ISG15, which execute diverse functions from inhibition of viral replication and assembly to modulation of host cell apoptosis and immune regulation (80, 81). In pigs, the robust induction of these ISGs has been closely associated with effective control of viruses like porcine reproductive and PRRSV and CSFV, where defects or viral antagonism of JAK-STAT signaling correspond with increased viral persistence and pathogenicity (78, 82, 83).

Comparatively, poultry also rely on this pathway for mounting antiviral defenses, although subtle differences in STAT protein isoforms and regulatory mechanisms may influence the magnitude and kinetics of ISG expression (84, 85). For example, variations in the expression levels of STAT2 and IRF9 in chickens relative to pigs have been linked to differential susceptibility to viruses such as AIV, suggesting evolutionary adaptation of the JAK-STAT axis to distinct viral landscapes (84, 86). Moreover, viral pathogens in poultry have evolved specific antagonistic strategies targeting JAK-STAT components, underscoring the critical role of this pathway in antiviral immunity.

To conclude, the JAK-STAT pathway plays a central role in converting interferon signals into a well-orchestrated antiviral gene response in livestock. In particular, key distinct features include species-specific PRR repertoires (such as the presence or absence of RIG-I), differential STAT isoform usage and signaling efficiency, distinct kinetic profiles of interferon-stimulated gene expression, and virus-specific immune evasion strategies targeting PRR signaling pathways.

While the core PRR signaling architecture is broadly conserved across vertebrates, species-specific variations in receptor repertoire, signaling strength, and regulatory mechanisms critically shape antiviral immunity in livestock. In pigs, the recognition of viral pathogens predominantly involves several key PRRs that detect distinct viral components to initiate innate immune responses (15). Porcine reproductive and PRRSV, a major RNA virus afflicting swine populations globally, is primarily sensed by endosomal TLR7 and cytoplasmic RIG-I-like receptors, notably RIG-I (87, 88). Stimulation of these receptors generally triggers the production of type I interferons along with proinflammatory cytokines, both of which are crucial for suppressing viral proliferation (89). However, PRRSV has evolved sophisticated immune evasion strategies that suppress interferon signaling pathways. For example, PRRSV nonstructural proteins such as nsp1 and nsp11 actively interfere with IRF3 activation and degrade key signaling molecules, effectively dampening the antiviral IFN response and facilitating viral persistence (90, 91).

ASFV encodes multiple immune evasion proteins that directly target key components of host innate immune signaling. For instance, the viral protein pE248R interferes with cGAS-STING signaling by disrupting STING activation, whereas MGF360 and MGF505 family members suppress type I interferon production by inhibiting IRF3 phosphorylation and nuclear translocation (Figure 2) (50, 92, 93).

These dual examples highlight how porcine viruses have adapted to both utilize and evade PRR-mediated immune recognition, underscoring the dynamic interplay between host sensing mechanisms and viral countermeasures.

In poultry, the detection of viral pathogens is mediated by a repertoire of PRRs that exhibit both conserved and divergent features compared to their mammalian counterparts. Notably, ducks and chickens, two key representatives of poultry livestock, show distinct immune receptor expression profiles that significantly influence their antiviral innate immune responses (94, 95).

Ducks are known for their exceptional resilience to IAV, a trait largely attributed to the efficient recognition of viral dsRNA intermediates by MDA5 (27, 95). Unlike mammals, ducks lack the RIG-I gene, and MDA5 compensates by acting as the dominant cytosolic sensor of IAV, leading to downstream activation of MAVS, TBK1, and IRF7, and robust induction of type I interferons (95). This mechanism contributes to the early control of viral replication and explains, in part, the relatively asymptomatic IAV infection course observed in ducks compared to chickens (96). Beyond the presence of RIG-I, ducks display distinct immune regulatory features that contribute to their natural resistance to AIV. Duck RIG-I and MDA5 signaling is characterized by rapid induction of type I interferons with relatively restrained proinflammatory cytokine production, thereby limiting immunopathology. In addition, duck-specific differences in negative regulatory pathways, interferon feedback control, and antiviral ISG expression patterns have been proposed to promote effective viral clearance while avoiding excessive tissue damage (73, 84). Additional contributing factors may include differential expression of antiviral ISGs, enhanced basal interferon readiness, and evolutionary adaptation to aquatic virus reservoirs, all of which likely cooperate with PRR-mediated sensing to confer heightened AIV resistance in ducks.

In contrast, chickens exhibit a reduced PRR repertoire, most notably the absence of functional RIG-I and TLR8 orthologs (84). This structural limitation is associated with a heightened susceptibility to certain RNA viruses and necessitates compensatory reliance on alternative sensors such as TLR3 and TLR7, which are localized to endosomes and recognize viral dsRNA and GU-rich single-stranded RNA, respectively (84). For instance, infection with NDV triggers robust expression of TLR3 and TLR7 in chicken splenocytes and macrophages, leading to the production of type I IFNs and inflammatory cytokines (97). Despite lacking RIG-I, chickens utilize MDA5 to some extent in sensing NDV and IAV, though the magnitude and timing of the response differ markedly from that in ducks.

Interestingly, the functional absence of RIG-I in chickens correlates with delayed and reduced interferon responses following RNA virus infection, a factor that may partially explain their greater clinical vulnerability. Conversely, ducks retain both RIG-I and MDA5, allowing for a more layered and robust antiviral signaling cascade. The evolutionary divergence of PRRs in poultry species thus underscores an important axis of differential viral susceptibility and offers insights into species-specific immune tuning (27).

Altogether, poultry innate immune recognition mechanisms reflect a complex interplay between conserved PRRs and species-specific adaptations. A clearer understanding of these differences not only informs comparative immunology but also supports the rational design of immunomodulatory interventions and genetic selection strategies in poultry farming.

To achieve successful infection and replication, many viruses employ diverse strategies to evade the host’s innate immune defenses. These tactics frequently target pattern recognition receptor (PRR) signaling, a central element of early antiviral immunity responsible for initiating interferon production. In both pigs and poultry, various viruses exploit vulnerabilities in these signaling networks to impair immune detection and response (44, 98).

A primary mode of evasion is the direct targeting of PRRs, either by suppressing their expression or promoting their degradation, thereby hindering the host’s ability to sense viral nucleic acids. For example, the NS1 protein of IAV in ducks and chickens can antagonize the activity of RIG-I or MDA5 by blocking their activation or interfering with their interactions within the signaling complex (95). Similarly, PRV and porcine reproductive and PRRSV encode proteins such as UL13 and nsp1α that promote degradation of RIG-I or cGAS in pigs, thereby attenuating the initial antiviral signaling triggered by viral RNA or DNA (89, 99).

Beyond receptor-level interference, viruses often disrupt key downstream signaling events, particularly those involving the activation of IRF transcription factors (44). In pigs, PRRSV nsp1β has been shown to inhibit TBK1-mediated phosphorylation of IRF3, a critical step for IFN gene induction. Porcine circovirus type 2 (PCV2) similarly impairs antiviral signaling by targeting both TBK1 and IKKϵ (100, 101). In poultry, the V protein of NDV interferes with the MDA5-MAVS axis by sequestering MDA5 or LGP2, preventing MAVS activation and downstream signaling events that lead to IFN production (21).

Viruses also manipulate host protein synthesis and degradation as an additional layer of immune evasion. In swine, PRRSV nsp2 and nsp11 have been reported to degrade STAT2 or inhibit host mRNA translation, thereby dampening the expression of ISGs (102). Influenza A virus employs a similar tactic: its NS1 protein inhibits host mRNA processing, effectively shutting down host protein synthesis and reducing the availability of antiviral effectors (103, 104). In avian species, certain viruses exploit the ubiquitin, proteasome system to degrade signaling intermediates such as MAVS and TRAF3, further impairing immune activation.

These multifaceted strategies reflect a dynamic evolutionary arms race between viruses and their hosts. Despite differences in PRR repertoires between pigs and poultry, many viruses have evolved both conserved and host-specific mechanisms to subvert innate immune detection. Elucidating these evasion tactics not only enhances our understanding of viral pathogenesis but also provides critical insights for the development of more effective vaccines and antiviral therapies in livestock.