The baseline immunological and hygienic status of pigs impact disease severity of African swine fever

Peste des petits ruminants virus virulence is associated with an early inflammatory profile in the tonsils and cell cycle arrest in lymphoid tissue

Using a systems immunology approach, this study comprehensively explored the immunopathogenesis of peste des petits ruminants (PPR) focussing on strain-dependent differences in virulence. Saanen goats were infected either with the highly virulent (Morocco 2008 [MA08]) or the low-virulent (Ivory Coast 1989 [IC89]) strain of the PPR virus (PPRV). As expected, MA08-infected goats exhibited higher clinical scores, pronounced lymphocyte depletion, and lesions affecting mucosal and lymphoid tissues. CD4 T cells were more affected in terms of depletion and infection in peripheral blood. Transcriptional analyses of the blood and lymphoid tissue demonstrated activation of interferon type I (IFN-I) responses at 3 days post-infection (dpi) only with MA08, but comparable IFN-I expression levels with MA08 and IC89 at 6 dpi. MA08 strain induced strong inflammatory and myeloid cell-related transcriptional responses observed in tonsils but not in mesenteric lymph node. This inflammatory response in the tonsils was associated with an extensive damage and infection of the tonsillar epithelium in the crypts, pointing to a barrier defect as a possible cause of inflammation. An early and prominent downregulation of cell cycle gene networks was observed in all compartments analyzed in MA08-infected animals. This effect can be interpreted as suppressed lymphocyte proliferation that may cause immunosuppression during the first week following MA08 infection. A proteome analysis confirmed synthesis of IFN-I response proteins during infection with both strains, but only MA08 strain additionally upregulated ribosomal and inflammation-related proteins. In conclusion, the present comprehensive investigation delineates strain-dependent differences in early immunopathological processes associated with severe inflammation disease and a blunted lymphocyte proliferation.

IMPORTANCE

Field observations show that the severity of PPR is highly dependent on the viral (PPRV) strains and the host infected, but the mechanisms behind these variations are not well understood. Here we compare immune response in Saanen goats infected with high (MA08) and low (IC89) virulent PPRV strains. Analyses revealed a differential immune response: early activation of IFN-I responses only with MA08 but comparable IFN-I expression levels with MA08 and IC89 at later stages. Additionally, MA08 strain triggered inflammatory and myeloid cell-related responses in the tonsils and marked suppression of lymphocyte proliferation evidenced by cell cycle arrest. CD4 T cells were found to be most affected in terms of depletion in the peripheral blood. Massive infection of the tonsils seems to induce epithelial lesions that promote the inflammatory responses. These results underscore the need to understand strain-specific differences for PPR surveillance and control.

Virulent African swine fever virus infection of porcine monocytes causes SLA I subversion due to loss of proper ER structure/function

African swine fever virus (ASFV) is a large DNA virus of the Asfarviridae family that causes a fatal hemorrhagic disease in domestic swine and wild boar. Infections with moderately virulent strains predominantly result in a milder clinical course and lower lethality. As target cells of ASFV, monocytes play a crucial role in triggering T-cell-mediated immune defense and ASF pathogenesis. We compared the effect of the highly virulent "Armenia2008" (ASFV-A) virus strain with that of the naturally attenuated "Estonia2014" (ASFV-E) on cellular immune activation in vivo and on primary monocytes ex vivo. Specifically, we asked whether antigen presentation of porcine monocytes is impaired upon ASFV-A infection. ASFV-A-infected monocytes are characterized by lower levels of swine leukocyte antigen (SLA) class I on the cell surface than ASFV-E-infected and uninfected monocytes. Despite stable steady-state SLA I mRNA/protein levels and expression of critical components of the antigen processing machinery, a marked decrease in maturation and reduced surface transport of SLA I were observed in ASFV-A-infected monocytes. The intracellular maturation block of SLA I was accompanied by a loss of functional rough ER structures and a pronounced formation of ER-associated aggresomes. This unsolved cellular stress resulted in a shutdown of overall host cell protein translation, mitochondrial dysfunction, and caspase-3-mediated apoptosis. In contrast, no such cellular subversion phenomenon was found in ASFV-E-infected monocytes. Our findings suggest that in domestic pigs infected with highly virulent ASFV-A, sequential subversion events occur in infected monocytes, likely leading to compromised T-cell activation and impaired downstream responses against ASFV.

Plasmacytoid dendritic cell sensing of African swine fever virus-infected macrophages results in STING-dependent robust interferon-α production

While several African swine fever virus (ASFV)-encoded proteins potently interfere with the cGAS-STING (cyclic GMP-AMP synthetase-stimulator of interferon genes) pathway at different levels to suppress interferon (IFN) type I production in infected macrophages, systemic IFN-α is induced during the early stages of AFSV infection in pigs. The present study elucidates a mechanism by which such responses can be triggered, at least in vitro. We demonstrate that infection of monocyte-derived macrophages (MDMs) by ASFV genotype 2 strains is highly efficient but immunologically silent with respect to IFN type I, IFN-stimulated gene induction, and tumor necrosis factor production. Additionally, ASFV does not directly activate plasmacytoid dendritic cells (pDCs). However, coculturing pDCs with ASFV-infected MDMs results in a strong pDC response characterized by high levels of IFN-α and tumor necrosis factor. IFN type I, in turn, promoted interleukin-1 receptor antagonist production by macrophages. Similar to the sensing of infected cells by other viruses, pDC activation required integrin-mediated cognate interactions with ASFV-infected MDMs to form an interferogenic synapse. Inhibitor studies indicated that the activation of pDCs requires the STING pathway and the formation of gap junctions. While IL-4-polarized macrophages showed increased susceptibility, IFN-γ-polarized ASFV-infected macrophages induced higher pDC activation. Pretreatment of pDCs with IFN-β and IFN-γ also enhanced IFN-α production in response to ASFV-infected macrophages, highlighting the influence of the immunological microenvironment. These findings suggest that the IFN-α detected during ASFV infection in pigs may be a result of pDC sensing ASFV-infected macrophages.

Transcriptome profiles of organ tissues from pigs experimentally infected with African swine fever virus in early phase of infection

African swine fever, caused by African swine fever virus (ASFV), is a highly contagious and fatal disease that poses a significant threat to the global pig industry. The limited information on ASFV pathogenesis and ASFV-host interactions has recently prompted numerous transcriptomic studies. However, most of these studies have focused on elucidating the transcriptome profiles of ASFV-infected porcine alveolar macrophages in vitro. Here, we analyzed dynamic transcriptional patterns in vivo in nine organ tissues (spleen, submandibular lymph node, mesenteric lymph node, inguinal lymph node, tonsils, lungs, liver, kidneys, and heart) obtained from pigs in the early stages of ASFV infection (1 and 3 d after viremia). We observed rapid spread of ASFV to the spleen after viremia, followed by broad transmission to the liver and lungs and subsequently, the submandibular and inguinal lymph nodes. Profound variations in gene expression patterns were observed across all organs and at all time-points, providing an understanding of the distinct defence strategies employed by each organ against ASFV infection. All ASFV-infected organs exhibited a collaborative response, activating immune-associated genes such as S100A8, thereby triggering a pro-inflammatory cytokine storm and interferon activation. Functional analysis suggested that ASFV exploits the PI3K-Akt signalling pathway to evade the host immune system. Overall, our findings provide leads into the mechanisms underlying pathogenesis and host immune responses in different organs during the early stages of infection, which can guide further explorations, aid the development of efficacious antiviral strategies against ASFV, and identify valuable candidate gene targets for vaccine development.

Impact of swine influenza A virus on porcine reproductive and respiratory syndrome virus infection in alveolar macrophages

Porcine respiratory disease complex represents a major challenge for the swine industry, with swine influenza A virus (swIAV) and porcine reproductive and respiratory syndrome virus (PRRSV) being major contributors. Epidemiological studies have confirmed the co-circulation of these viruses in pig herds, making swIAV-PRRSV co-infections expected. A couple of in vivo co-infection studies have reported replication interferences between these two viruses. Herein, using a reductionist in vitro model, we investigated the potential mechanisms of these in vivo interferences. We first examined the impact of swIAV on porcine alveolar macrophages (AMs) and its effects on AMs co-infection by PRRSV. This was done either in monoculture or in co-culture with respiratory tracheal epithelial cells to represent the complexity of the interactions between the viruses and their respective target cells (epithelial cells for swIAV and AMs for PRRSV). AMs were obtained either from conventional or specific pathogen-free (SPF) pigs. SwIAV replication was abortive in AMs, inducing cell death at high multiplicity of infections. In AMs from three out of four conventional animals, swIAV showed no impact on PRRSV replication. However, inhibition of PRRSV multiplication was observed in AMs from one animal, accompanied by an early increase in the expression of interferon (IFN)-I and IFN-stimulated genes. In AMs from six SPF pigs, swIAV inhibited PRRSV replication in all animals, with an early induction of antiviral genes. Co-culture experiments involving tracheal epithelial cells and AMs from either SPF or conventional pigs all showed swIAV-induced inhibition of PRRSV replication, together with early induction of antiviral genes. These findings highlight the complex interactions between swIAV and PRRSV in porcine AMs, and would suggest a role of host factors, such as sanitary status, in modulating viral propagation. Our co-culture experiments demonstrated that swIAV inhibits PRRSV replication more effectively in the presence of respiratory tracheal epithelial cells, suggesting a synergistic antiviral response between AMs and epithelial cells, consistent with in vivo experiments.

Fitness adaptations of Japanese encephalitis virus in pigs following vector-free serial passaging

Japanese encephalitis virus (JEV) is a zoonotic mosquito-transmitted Flavivirus circulating in birds and pigs. In humans, JEV can cause severe viral encephalitis with high mortality. Considering that vector-free direct virus transmission was observed in experimentally infected pigs, JEV introduction into an immunologically naïve pig population could result in a series of direct transmissions disrupting the alternating host cycling between vertebrates and mosquitoes. To assess the potential consequences of such a realistic scenario, we passaged JEV ten times in pigs. This resulted in higher in vivo viral replication, increased shedding, and stronger innate immune responses in pigs. Nevertheless, the viral tissue tropism remained similar, and frequency of direct transmission was not enhanced. Next generation sequencing showed single nucleotide deviations in 10% of the genome during passaging. In total, 25 point mutations were selected to reach a frequency of at least 35% in one of the passages. From these, six mutations resulted in amino acid changes located in the precursor of membrane, the envelope, the non-structural 3 and the non-structural 5 proteins. In a competition experiment with two lines of passaging, the mutation M374L in the envelope protein and N275D in the non-structural protein 5 showed a fitness advantage in pigs. Altogether, the interruption of the alternating host cycle of JEV caused a prominent selection of viral quasispecies as well as selection of de novo mutations associated with fitness gains in pigs, albeit without enhancing direct transmission frequency.

Infection of domestic pigs with a genotype II potent strain of ASFV causes cytokine storm and lymphocyte mass reduction

The whole-genome sequence of an African swine fever virus (ASFV) strain (HuB/HH/2019) isolated from Hubei, China, was highly similar to that of the Georgia 2007/1 strain ASFV. After infection with strong strains, domestic pigs show typical symptoms of infection, including fever, depression, reddening of the skin, hemorrhagic swelling of various tissues, and dysfunction. The earliest detoxification occurred in pharyngeal swabs at 4 days post-infection. The viral load in the blood was extremely high, and ASFV was detected in multiple tissues, with the highest viral loads in the spleen and lungs. An imbalance between pro- and anti-inflammatory factors in the serum leads to an excessive inflammatory response in the body. Immune factor expression is suppressed without effectively eliciting an immune defense. Antibodies against p30 were not detected in acutely dead domestic pigs. Sequencing of the peripheral blood mononuclear cell transcriptome revealed elevated transcription of genes associated with immunity, defense, and stress. The massive reduction in lymphocyte counts in the blood collapses the body's immune system. An excessive inflammatory response with a massive reduction in the lymphocyte count may be an important cause of mortality in domestic pigs. These two reasons have inspired researchers to reduce excessive inflammatory responses and stimulate effective immune responses for future vaccine development.

Disease Course of Korean African Swine Fever Virus in Domestic Pigs Exposed Intraorally, Intranasally, Intramuscularly, and by Direct Contact with Infected Pigs

African swine fever (ASF) is a fatal contagious disease affecting swine. The first Korean ASF virus (ASFV) isolate (Korea/Pig/Paju1/2019) was used to compare the disease course of ASFV in pigs inoculated via the four routes. In the challenge experiment, domestic pigs were infected via the intraoral (IO) and intranasal (IN) routes with a 106 50% hemadsorbing dose (HAD50) and an intramuscular (IM) injection of 103 HAD50. In the direct contact (DC) group, five naïve pigs were brought into direct contact with two IM-ASFV-infected pigs. IO-, IN-, and IM-inoculated pigs showed similar disease courses, whereas DC pigs had comparable ASF syndrome after a 7-day latent period. The disease course in the DC route, one of the most common routes of infection, was not significantly different from that in the IO and IN routes. IM and DC groups differed in terms of the severity of fever and hemorrhagic lesions in the lymph nodes and spleen, indicating that the IM route, suitable for early vaccine development trials, is not appropriate for studying the ASFV infection mechanism, including early stage of infection, and IO and IN challenges with a designated dose can be alternatives in trials for assessing ASFV pathogenicity and vaccine efficacy investigations.

A sensitive luciferase reporter assay for the detection of infectious African swine fever virus

African swine fever virus (ASFV) is a complex DNA virus causing severe hemorrhagic disease in domestic pigs and wild boar. The disease has spread worldwide, with important socio-economic consequences. Early virus detection and control measures are crucial as there are no effective vaccines nor antivirals on the market. While the diagnosis of ASFV is fast and based primarily on qPCR, the detection of infectious ASFV is a labor-intensive process requiring susceptible macrophages and subsequent antibody-based staining or hemadsorption. The latter cannot detect ASFV isolates devoid of functional CD2v (EP402R) expression. Here, we report the development of a plasmid-based reporter assay (RA) for the sensitive detection and titration of infectious ASFV. To this end, we constructed a plasmid for secreted NanoLuc luciferase (secNluc) expression driven by the ASFV DNA polymerase gene G1211R promoter. Infection of plasmid-transfected immortalized porcine kidney macrophages (IPKM) followed by measurement of secNluc from cell culture supernatants allowed reliable automated quantification of infectious ASFV. The RA-based titers matched the titers determined by conventional p72-staining or hemadsorption protocols. The novel assay is specific for ASFV as it does not detect classical swine fever virus nor porcine reproductive and respiratory syndrome virus. It is applicable to ASFV of different genotypes, virulence, and sources, including ASFV from sera and whole blood from infected pigs as well as non-hemadsorbing ASFV.

Porcine respiratory disease complex: Dynamics of polymicrobial infections and management strategies after the introduction of the African swine fever

A few decades ago, porcine respiratory disease complex (PRDC) exerted a major economic impact on the global swine industry, particularly due to the adoption of intensive farming by the latter during the 1980's. Since then, the emerging of porcine reproductive and respiratory syndrome virus (PRRSV) and of porcine circovirus type 2 (PCV2) as major immunosuppressive viruses led to an interaction with other endemic pathogens (e.g., Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Streptococcus suis, etc.) in swine farms, thereby exacerbating the endemic clinical diseases. We herein, review and discuss various dynamic polymicrobial infections among selected swine pathogens. Traditional biosecurity management strategies through multisite production, parity segregation, batch production, the adoption of all-in all-out production systems, specific vaccination and medication protocols for the prevention and control (or even eradication) of swine diseases are also recommended. After the introduction of the African swine fever (ASF), particularly in Asian countries, new normal management strategies minimizing pig contact by employing automatic feeding systems, artificial intelligence, and robotic farming and reducing the numbers of vaccines are suggested. Re-emergence of existing swine pathogens such as PRRSV or PCV2, or elimination of some pathogens may occur after the ASF-induced depopulation. ASF-associated repopulating strategies are, therefore, essential for the establishment of food security. The "repopulate swine farm" policy and the strict biosecurity management (without the use of ASF vaccines) are, herein, discussed for the sustainable management of small-to-medium pig farms, as these happen to be the most potential sources of an ASF re-occurrence. Finally, the ASF disruption has caused the swine industry to rapidly transform itself. Artificial intelligence and smart farming have gained tremendous attention as promising tools capable of resolving challenges in intensive swine farming and enhancing the farms' productivity and efficiency without compromising the strict biosecurity required during the ongoing ASF era.

Cross-protection against African swine fever virus upon intranasal vaccination is associated with an adaptive-innate immune crosstalk

African swine fever virus (ASFV) is causing a worldwide pandemic affecting the porcine industry and leading to important global economic consequences. The virus causes a highly lethal hemorrhagic disease in wild boars and domestic pigs. Lack of effective vaccines hampers the control of virus spread, thus increasing the pressure on the scientific community for urgent solutions. However, knowledge on the immune components associated with protection is very limited. Here we characterized the in vitro recall response induced by immune cells from pigs intranasally vaccinated with the BA71ΔCD2 deletion mutant virus. Vaccination conferred dose-dependent cross-protection associated with both ASFV-specific antibodies and IFNγ-secreting cells. Importantly, bulk and single-cell transcriptomics of blood and lymph node cells from vaccinated pigs revealed a positive feedback from adaptive to innate immunity. Indeed, activation of Th1 and cytotoxic T cells was concomitant with a rapid IFNγ-dependent triggering of an inflammatory response characterized by TNF-producing macrophages, as well as CXCL10-expressing lymphocytes and cross-presenting dendritic cells. Altogether, this study provides a detailed phenotypic characterization of the immune cell subsets involved in cross-protection against ASFV, and highlights key functional immune mechanisms to be considered for the development of an effective ASF vaccine.

African swine fever (ASF) pandemic is currently the number one threat for the porcine industry worldwide. Lack of treatments hampers its control, and the insufficient knowledge regarding the immune effector mechanisms required for protection hinders rational vaccine design. Here we present the first comprehensive study characterizing the complex cellular immune response involved in cross-protection against ASF. We show that, upon in vitro reactivation, cells from immune pigs induce a Th1-biased recall response that in turn enhances the antiviral innate response. Our results suggest that this positive feedback regulation of innate immunity plays a key role in the early control of ASF virus infection. Altogether, this work represents a step forward in the understanding of ASF immunology and provide critical immune components that should be considered to more rationally design future ASF vaccines.

Assessment of the Impact of a Toll-like Receptor 2 Agonist Synthetic Lipopeptide on Macrophage Susceptibility and Responses to African Swine Fever Virus Infection

Toll-like receptor 2 (TLR2) ligands are attracting attention as prophylactic and immunopotentiator agents against pathogens, including viruses. We previously reported that a synthetic diacylated lipopeptide (Mag-Pam2Cys_P48) polarized porcine macrophages towards a proinflammatory antimicrobial phenotype. Here, we investigated its role in modulating monocyte-derived macrophage (moMΦ) responses against African swine fever virus (ASFV), the etiological agent of one of the greatest threats to the global pig industry. Two ASFV isolates were compared: the attenuated NH/P68 and the virulent 26544/OG10. No effect on virus infection nor the modulation of surface markers’ expression (MHC I, MHC II DR, CD14, CD16, and CD163) were observed when Mag-Pam2Cys_P48 treated moMΦ were infected using a multiplicity of infection (MOI) of 1. Mag-Pam2Cys_P48 treated moMΦ released higher levels of IL-1α, IL-1β, IL-1Ra, and IL-18 in response to infection with NH/P68 ASFV compared to 26544/OG10-infected and mock-infected controls. Surprisingly, when infected using a MOI of 0.01, the virulent ASFV 26544/OG10 isolate replicated even slightly more efficiently in Mag-Pam2Cys_P48 treated moMΦ. These effects also extended to the treatment of moMΦ with two other lipopeptides: Mag-Pam2Cys_P80 and Mag-Pam2Cys_Mag1000. Our data suggested limited applicability of TLR2 agonists as prophylactic or immunopotentiator agents against virulent ASFV but highlighted the ability of the virulent 26544/OG10 to impair macrophage defenses.