Marek's disease virus immunosuppression alters host cellular responses and immune gene expression in the skin of infected chickens

Unraveling the role of γδ T cells in the pathogenesis of an oncogenic avian herpesvirus

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that causes deadly lymphomas in chickens. In chickens, up to 50% of all peripheral T cells are gamma delta (γδ) T cells. Until now, their role in MDV pathogenesis and tumor formation remains poorly understood. To investigate the role of γδ T cells in MDV pathogenesis, we infected recently generated γδ T cell knockout chickens with very virulent MDV. Strikingly, disease and tumor incidence were highly increased in the absence of γδ T cells, indicating that γδ T cells play an important role in the immune response against MDV. In the absence of γδ T cells, virus replication was drastically increased in the thymus and spleen, which are potential sites of T cell transformation. Taken together, our data provide the first evidence that γδ T cells play an important role in the pathogenesis and tumor formation of this highly oncogenic herpesvirus.IMPORTANCEGamma delta (γδ) T cells are the most abundant T cells in chickens, but their role in fighting pathogens remains poorly understood. Marek's disease virus (MDV) is an important veterinary pathogen, that causes one of the most frequent cancers in animals and is used as a model for virus-induced tumor formation. Our study revealed that γδ T cells play a crucial role in combating MDV, as disease and tumor incidence drastically increased in the absence of these cells. γδ T cells restricted virus replication in the key lymphoid organs, thereby decreasing the likelihood of causing tumors and disease. This study provides novel insights into the role of γδ T cells in the pathogenesis of this highly oncogenic virus.

FAdV-4 Promotes Expression of Multiple Cytokines and Inhibits the Proliferation of aHEV in LMH Cells

Single or mixed infections of multiple pathogens such as avian hepatitis E virus (aHEV) and avian leukosis virus subgroup J (ALV-J) have been detected in numerous laying hens with severe liver injury in China. Thus, aHEV and immunosuppressive viruses are speculated to cause co-infections. In this study, co-infection with aHEV and fowl adenovirus (FAdV) was confirmed by nested RT-PCR and recombinase-aided amplification combined with gene sequencing in two flocks with severe liver injury. Subsequently, the two reference strains, aHEV and FAdV-4, were inoculated into LMH cells to identify their co-infection potential. Confocal microscopy revealed aHEV and FAdV-4 co-infected LMH cells. In addition, the replication dynamics of aHEV and FAdV-4 along with the expression levels of immuno-cytokines were measured. The results indicated colocalization of aHEV and FAdV-4 and inhibition of viral replication in LMH cells. The transcription levels of MDA5, Mx, OASL, and IFN-α were significantly upregulated in LMH cells, whereas those of immune-related factors induced by FAdV-4 were downregulated upon FAdV-4 and aHEV co-infection. These results confirmed the co-infection of aHEV and FAdV-4 in vitro and prompted the antagonistic pathogenic effects of FAdV-4 and aHEV, thereby providing novel insights into the counterbalancing effects of these viruses.

Activated Chicken Gamma Delta T Cells Are Involved in Protective Immunity against Marek's Disease

Gamma delta (γδ) T cells play a significant role in the prevention of viral infection and tumor surveillance in mammals. Although the involvement of γδ T cells in Marek's disease virus (MDV) infection has been suggested, their detailed contribution to immunity against MDV or the progression of Marek's disease (MD) remains unknown. In the current study, T cell receptor (TCR)γδ-activated peripheral blood mononuclear cells (PBMCs) were infused into recipient chickens and their effects were examined in the context of tumor formation by MDV and immunity against MDV. We demonstrated that the adoptive transfer of TCRγδ-activated PBMCs reduced virus replication in the lungs and tumor incidence in MDV-challenged chickens. Infusion of TCRγδ-activated PBMCs induced IFN-γ-producing γδ T cells at 10 days post-infection (dpi), and degranulation activity in circulating γδ T cell and CD8α⁺ γδ T cells at 10 and 21 dpi in MDV-challenged chickens. Additionally, the upregulation of IFN-γ and granzyme A gene expression at 10 dpi was significant in the spleen of the TCRγδ-activated PBMCs-infused and MDV-challenged group compared to the control group. Taken together, our results revealed that TCRγδ stimulation promotes the effector function of chicken γδ T cells, and these effector γδ T cells may be involved in protection against MD.

Dietary sophorolipid accelerates growth by modulation of gut microbiota population and intestinal environments in broiler chickens

BACKGROUND

Gut is a crucial organ for the host's defense system due to its filtering action of the intestinal membrane from hazardous foreign substances. One strategy to strengthen the gut epithelial barrier function is to upregulate beneficial microflora populations and their metabolites. Sophorolipid (SPL), which is a glycolipid bio-surfactant, could increase beneficial microflora and decrease pathogenic bacteria in the gastrointestinal tract. Therefore, herein, we conducted an experiment with broiler chickens to investigate the fortifying effects of SPL on the host's gut defense system by modulating the microbiota population.

METHODS

A total of 540 1-day-old chicks (Ross 308) were used, and they were immediately allotted into three treatment groups (6 replications with 30 chicks/pen) according to their initial body weight. The dietary treatments consisted of CON (basal diet), BAM (10 mg/kg bambermycin), and SPL (10 mg/kg SPL). During the experiment, birds freely accessed feed and water, and body weight and feed intake were measured at the end of each phase. On d 35, birds (one bird/pen) were sacrificed to collect jejunum and cecum samples.

RESULTS

Dietary SPL and BAM supplementation significantly accelerated birds' growth and also significantly improved feed efficiency compared to CON. Intestinal microbial community was significantly separated by dietary SPL supplementation from that of CON, and dietary SPL supplementation significantly increased Lactobacillus spp. and Akkermansia muciniphila. Moreover, birds fed with dietary SPL also showed the highest concentration of cecal butyrate among all treatment groups. Gut morphological analysis showed that dietary SPL significantly increased villus height, ratio of villus height to crypt depth, goblet cell numbers, and the gene expression levels of claudin-1 and mucin 2. Additionally, dietary SPL significantly decreased the mRNA expression level of pro-inflammatory cytokine, interleukin-6, and increased that of anti-inflammatory cytokine, interleukin-10, compared to other treatments.

CONCLUSIONS

Dietary SPL increases the beneficial bacterial population and butyrate concentration, which leads to a strengthened gut barrier function. In addition, the intestinal inflammation was also downregulated by dietary SPL supplementation.

Integrating Genetic and Genomic Analyses of Combined Health Data Across Ecotypes to Improve Disease Resistance in Indigenous African Chickens

Poultry play an important role in the agriculture of many African countries. The majority of chickens in sub-Saharan Africa are indigenous, raised in villages under semi-scavenging conditions. Vaccinations and biosecurity measures rarely apply, and infectious diseases remain a major cause of mortality and reduced productivity. Genomic selection for disease resistance offers a potentially sustainable solution but this requires sufficient numbers of individual birds with genomic and phenotypic data, which is often a challenge to collect in the small populations of indigenous chicken ecotypes. The use of information across-ecotypes presents an attractive possibility to increase the relevant numbers and the accuracy of genomic selection. In this study, we performed a joint analysis of two distinct Ethiopian indigenous chicken ecotypes to investigate the genomic architecture of important health and productivity traits and explore the feasibility of conducting genomic selection across-ecotype. Phenotypic traits considered were antibody response to Infectious Bursal Disease (IBDV), Marek's Disease (MDV), Fowl Cholera (PM) and Fowl Typhoid (SG), resistance to Eimeria and cestode parasitism, and productivity [body weight and body condition score (BCS)]. Combined data from the two chicken ecotypes, Horro (n = 384) and Jarso (n = 376), were jointly analyzed for genetic parameter estimation, genome-wide association studies (GWAS), genomic breeding value (GEBVs) calculation, genomic predictions, whole-genome sequencing (WGS), and pathways analyses. Estimates of across-ecotype heritability were significant and moderate in magnitude (0.22-0.47) for all traits except for SG and BCS. GWAS identified several significant genomic associations with health and productivity traits. The WGS analysis revealed putative candidate genes and mutations for IBDV (TOLLIP, ANGPTL5, BCL9, THEMIS2), MDV (GRM7), SG (MAP3K21), Eimeria (TOM1L1) and cestodes (TNFAIP1, ATG9A, NOS2) parasitism, which warrant further investigation. Reliability of GEBVs increased compared to within-ecotype calculations but accuracy of genomic prediction did not, probably because the genetic distance between the two ecotypes offset the benefit from increased sample size. However, for some traits genomic prediction was only feasible in across-ecotype analysis. Our results generally underpin the potential of genomic selection to enhance health and productivity across-ecotypes. Future studies should establish the required minimum sample size and genetic similarity between ecotypes to ensure accurate joint genomic selection.

Integration of gene expression profile data to screen and verify immune-related genes of chicken erythrocytes involved in Marek's disease virus

Chicken erythrocytes participated in immunity, but the role of erythrocytes in the immunity of Marek's disease virus (MDV) has not been reported related to the immunity genes. The purpose of this study was to screen and verify the immune-related genes of chicken erythrocytes which could be proven as a biomarker in MDV. The datasets (GPL8764-Chicken Gene Expression Microarray) were downloaded from the GEO profile database for control and MDV infected chickens to obtain differentially expressed genes (DEGs) through bioinformatics methods. Kyoto Encyclopedia of Genes and Genomes (KEGG) was performed to find enriched pathways, including Gene Ontology (GO). Based on enriched pathways, the top 19 immune-related genes were screened-out and process further to construct the protein-protein interaction (PPI) networks. The screened genes were validated on RT-PCR and qPCR. Results suggested that the mRNA transcription of Toll-like receptors 2, 3, 4, 6 (TLR2, TLR3, TLR4, TLR6), major histocompatibility complex-II (MHCII), interleukin-7 (IL-7), interferon-βeta (IFN-β), chicken myelomonocytic growth factor (cMGF) and myeloid differentiation primary response 88 (MyD88) were significantly up-regulated. The expression of toll-like receptor 5, 7 (TLR5, TLR7) interleukin-12 (IL-12 p40), interleukin-13 (IL-13), and interferon-αlpha (IFN-α) were significantly down-regulated in the erythrocytes of the infected group (P < 0.05). In contrast, the expression of toll-like receptor-1, 15, 21 (TLR1, TLR15, TLR21), major histocompatibility complex I (MHCI) and Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) were not significant. In conclusion, it has been verified on qRT-PCR results that 19 immune-related genes, which included TLRs, cytokines and MHC have immune functions in MDV infected chickens.

Avian oncogenic herpesvirus antagonizes the cGAS-STING DNA-sensing pathway to mediate immune evasion

The cellular DNA sensor cGMP-AMP synthase (cGAS) detects cytosolic viral DNA via the stimulator of interferon genes (STING) to initiate innate antiviral response. Herpesviruses are known to target key immune signaling pathways to persist in an immune-competent host. Marek’s disease virus (MDV), a highly pathogenic and oncogenic herpesvirus of chickens, can antagonize host innate immune responses to achieve persistent infection. With a functional screen, we identified five MDV proteins that blocked beta interferon (IFN-β) induction downstream of the cGAS-STING pathway. Specifically, the MDV major oncoprotein Meq impeded the recruitment of TANK-binding kinase 1 and IFN regulatory factor 7 (IRF7) to the STING complex, thereby inhibiting IRF7 activation and IFN-β induction. Meq overexpression markedly reduced antiviral responses stimulated by cytosolic DNA, whereas knockdown of Meq heightened MDV-triggered induction of IFN-β and downstream antiviral genes. Moreover, Meq-deficient MDV induced more IFN-β production than wild-type MDV. Meq-deficient MDV also triggered a more robust CD8+ T cell response than wild-type MDV. As such, the Meq-deficient MDV was highly attenuated in replication and lymphoma induction compared to wild-type MDV. Taken together, these results revealed that MDV evades the cGAS-STING DNA sensing pathway, which underpins the efficient replication and oncogenesis. These findings improve our understanding of the virus-host interaction in MDV-induced lymphoma and may contribute to the development of novel vaccines against MDV infection.

Marek’s disease virus (MDV) is an avian oncogenic herpesvirus that causes a fatal disease in poultry worldwide. Chickens infected with MDV become more susceptible to secondary viral or bacterial infections. However, the mechanisms of MDV-induced immunosuppression and tumorigenesis remain largely unknown. The cGAS-STING pathway is crucial for innate immune responses against both microbial pathogens and intrinsic tumors. Here we identified the MDV oncoprotein, Meq, as an inhibitor of the cGAS-STING DNA-sensing pathway. Mechanistically, Meq interacted with STING and IRF7, and impaired the recruitment of TBK1 and IRF7 to the STING complex, thus inhibiting IRF7 activation and IFN-β induction. Loss of Meq potently enhanced innate immune response, while impaired the replication and oncogenesis of MDV in chickens. Our findings reveal an important mechanism of immune evasion of MDV, instructing us on the virus-host interaction in MDV-induced lymphoma and potential new means to develop MDV vaccine.

Virus-Induced Immunosuppression in Chickens

A healthy immune system is a cornerstone for poultry production. Any factor diminishing the immune responses will affect production parameters and increase cost. There are numerous factors, infectious and noninfectious, causing immunosuppression (IS) in chickens. This paper reviews the three viral diseases that most commonly induce IS or subclinical IS in chickens: Marek's disease virus (MDV), chicken infectious anemia virus (CIAV), and infectious bursal disease virus (IBDV), as well as the interactions among them. MDV-induced IS (MDV-IS) affects both humoral and cellular immune responses. It is very complex, poorly understood, and in many cases underdiagnosed. Vaccination protects against some but not all aspects of MDV-IS. CIAV induces apoptosis of the hemocytoblasts resulting in anemia, hemorrhages, and increased susceptibility to bacterial infections. It also causes apoptosis of thymocytes and dividing T lymphocytes, affecting T helper functions, which are essential for antibody production and cytotoxic T lymphocyte (CTL) functions. Control of CIAV is based on vaccination of breeders and maternal antibodies (MAbs). However, subclinical IS can occur after MAbs wane. IBDV infection affects the innate immune responses during virus replication and humoral immune responses as a consequence of the destruction of B-cell populations. Vaccines with various levels of attenuation are used to control IBDV. Interactions with MAbs and residual virulence of the vaccines need to be considered when designing vaccination plans. The interaction between IBDV, CIAV, and MDV is critical although underestimated in many cases. A proper control of IBDV is a must to have proper humoral immune responses needed to control CIAV. Equally, long-term control of MDV is not possible if chickens are coinfected with CIAV, as CIAV jeopardizes CTL functions critical for MDV control.

Differential expression of type I interferon mRNA and protein levels induced by virulent Marek's disease virus infection in chickens

Marek's disease virus (MDV), an α-herpesvirus targeting avian species, causes fatal Marek's disease (MD) in chickens. The host interferon (IFN) responses play a key role in resisting viral infection. However, host IFN responses following MDV infection in the chicken central immune organs (thymus and bursa of Fabricius), which contain numerous MDV target cells, is poorly understood. In this study, we performed animal experiments in specific pathogen-free chickens infected with two virulent MDV strains (BS/15 and Md5) or without infection as negative controls. Specifically, the type I IFN (IFN-α and IFN-β) transcriptional and proteomic expression levels at 7, 10, 14, 17, and 21 days post infection (dpi) were detected and analyzed. Our results indicated that the mRNA and protein expression levels of IFN-α and IFN-β in the thymus and bursa of Fabricius were mainly downregulated in cytolytic infection (such as 10 dpi) and reactivation (such as 17 dpi) stages, but not the latent (such as 14 dpi) stage of MDV infection, which was determined by comprehensively analyzing the MDV viral load and immune organ damage caused by MDV infection. These data suggest that MDV could inhibit the expression of host type I IFNs, which may be involved in the MDV-induced host immunosuppression and contribute to the immune escape of MDV from host immunity. Furthermore, we found that the downregulated expression of the host type I IFNs induced by BS/15 and Md5 infection was significantly different, which we speculated may be related to the diverse virulence and pathogenicity of MDV strains. In conclusion, our study demonstrated that MDV mostly inhibited the expression of type I IFNs in infected hosts, which may be associated to its pathogenesis.

Depression of Vaccinal Immunity to Marek's Disease by Infection with Chicken Infectious Anemia Virus

Marek’s disease (MD) has been occurring with increasing frequency in chickens in recent years. To our knowledge, however, there has been no report of the very virulent plus (vv+) MD virus (MDV) field isolate in China. Studies have shown that dual infection with immunosuppressive viruses such as chicken infectious anemia virus (CIAV) occurs frequently in chickens developing MD. In this study, we performed a designed set of in vivo experiments, which comprised five different groups of chickens, including the group of CVI988/Rispens-vaccinated chickens, the groups of CVI988/Rispens-vaccinated chickens infected with MDV or CIAV or both viruses (MDV and CIAV), and the group of MDV-challenged chickens. The effects of CIAV dual infection on the immunization of commercial MDV vaccine CVI988/Rispens were evaluated. The results show that infection of the SD15 strain of CIAV significantly reduced the weight and antibody titers to avian influenza virus (AIV)/Newcastle disease virus (NDV) inactivated vaccines of chickens immunized with the CVI988/Rispens, and resulted in the atrophy of thymus/bursa and the enlargement of spleen. The CVI988/Rispens vaccination conferred good immune protection for chickens challenged with 2000 PFU of the GX0101 strain of MDV. However, dual infection with SD15 significantly reduced the body weight, antibody titers induced by AIV/NDV inactivated vaccines and protective index of CVI988/Rispens, and resulted in the aggravation of the immunosuppression, mortality, and viremia of GX0101 in CVI988/Rispens-immunized/GX0101-challenged chickens. Overall, CIAV infection significantly reduced the protective effects of the CVI988/Rispens vaccine against MDV, implying that concurrent infection with CIAV may be a major contributor in the frequent attacks of MD in China in recent years.

Early Immune Responses to Marek's Disease Vaccines

Marek's disease virus (MDV), a highly cell-associated lymphotropic α-herpesvirus, is the causative agent of Marek's disease (MD) in domestic chickens. MDV replicates in chicken cells and establishes a latent infection within CD4⁺ T cells. Although MD vaccines have been in use for several decades, the exact mechanism of vaccine-induced protection is unclear. It is believed that the innate immune system plays a role in vaccine-induced immunity against pathogenic strains of MDV. To shed light on the possible function of the innate immunity in vaccine-mediated protection, we investigated the effect of vaccination, Rispens/CVI988, on the activation of cellular components of the innate immune system by analyzing the expression pattern of select immune-related genes in the cecal tonsils (CT) and duodenum of two MD-susceptible and MD-resistant chicken lines at 3, 5, and 10 days postvaccination (dpv). The differential expression patterns of the tested genes within the CT and duodenum of vaccinated birds revealed the activation of the innate immune system in both the susceptible and resistant lines. Stronger innate immune response was induced within the CT of the vaccinated birds of the susceptible line at 5 dpv. Upregulation of some of the tested genes at 10 dpv was likely due to the activation and response of the adaptive immune system to vaccination. Immunohistochemical analysis showed no increase in the number of CD3⁺ T cells in the CT and duodenum of the vaccinated birds of either line at 5 dpv. There was, however, an increase in the macrophage populations within the duodenum of the vaccinated birds of both the susceptible and resistant lines at 5 dpv. The vaccine strain antigen was detected in the CT and duodenum of the susceptible line, but not the resistant line at 5 dpv.