Experimental Actinobacillus pleuropneumoniae infection in piglets with different types and levels of specific protection: Immunophenotypic analysis of lymphocyte subsets in the circulation and respiratory mucosal lymphoid tissue

High-dimensional analysis reveals an immune atlas and novel neutrophil clusters in the lungs of model animals with Actinobacillus pleuropneumoniae-induced pneumonia

Due to the increase in bacterial resistance, improving the anti-infectious immunity of the host is rapidly becoming a new strategy for the prevention and treatment of bacterial pneumonia. However, the specific lung immune responses and key immune cell subsets involved in bacterial infection are obscure. Actinobacillus pleuropneumoniae (APP) can cause porcine pleuropneumonia, a highly contagious respiratory disease that has caused severe economic losses in the swine industry. Here, using high-dimensional mass cytometry, the major immune cell repertoire in the lungs of mice with APP infection was profiled. Various phenotypically distinct neutrophil subsets and Ly-6C+ inflammatory monocytes/macrophages accumulated post-infection. Moreover, a linear differentiation trajectory from inactivated to activated to apoptotic neutrophils corresponded with the stages of uninfected, onset, and recovery of APP infection. CD14+ neutrophils, which mainly increased in number during the recovery stage of infection, were revealed to have a stronger ability to produce cytokines, especially IL-10 and IL-21, than their CD14- counterparts. Importantly, MHC-II+ neutrophils with antigen-presenting cell features were identified, and their numbers increased in the lung after APP infection. Similar results were further confirmed in the lungs of piglets infected with APP and Klebsiella pneumoniae infection by using a single-cell RNA-seq technique. Additionally, a correlation analysis between cluster composition and the infection process yielded a dynamic and temporally associated immune landscape where key immune clusters, including previously unrecognized ones, marked various stages of infection. Thus, these results reveal the characteristics of key neutrophil clusters and provide a detailed understanding of the immune response to bacterial pneumonia.

Long-term follow-up of Mycoplasma hyopneumoniae-specific immunity in vaccinated pigs

Mycoplasma hyopneumoniae is the primary agent of enzootic pneumonia in pigs. To minimize the economic losses caused by this disease, M. hyopneumoniae vaccination is commonly practiced. However, the persistence of M. hyopneumoniae vaccine-induced immunity, especially the cell-mediated immunity, till the moment of slaughter has not been investigated yet. Therefore, on two commercial farms, 25 pigs (n = 50) received a commercial bacterin intramuscularly at 16 days of age. Each month, the presence of M. hyopneumoniae-specific serum antibodies was analyzed and the proliferation of and TNF-α, IFN-γ and IL-17A production by different T cell subsets in blood was assessed using recall assays. Natural infection with M. hyopneumoniae was assumed in both farms. However, the studied pigs remained M. hyopneumoniae negative for almost the entire trial. Seroconversion was not observed after vaccination and all pigs became seronegative at two months of age. The kinetics of the T cell subset frequencies was similar on both farms. Mycoplasma hyopneumoniae-specific cytokine-producing CD4⁺CD8⁺ T cells were found in blood of pigs from both farms at one month of age but decreased significantly with increasing age. On the other hand, T cell proliferation after in vitro M. hyopneumoniae stimulation was observed until the end of the fattening period. Furthermore, differences in humoral and cell-mediated immune responses after M. hyopneumoniae vaccination were not seen between pigs with and without maternally derived antibodies. This study documents the long-term M. hyopneumoniae vaccine-induced immune responses in fattening pigs under field conditions. Further research is warranted to investigate the influence of a natural infection on these responses.

Effects of oil-based adjuvants on the immune response of pigs after dermal administration of antigen and evaluation of the immunization level after a subsequent Actinobacillus pleuropneumoniae challenge in pigs

Route of vaccine delivery can greatly impact the immunogenicity, efficacy and safety of the vaccine. Four groups of piglets were immunised transdermally (t.d.), intradermally (i.d.) or intramuscularly (i.m.) with the same doses of antigen in combination with a water-in-oil-in-water emulsion adjuvant Montanide™ ISA 201 VG or with a microemulsion adjuvant Montanide™ IMS 1313 VG N ST (Seppic, France). The last group was left without vaccination as a control group. All animals were subsequently exposed to the infection induced by Actinobacillus pleuropneumoniae (App). The immune response was evaluated with respect to the intensity of systemic and mucosal antibody formation, their isotype characterisation and rate of cell-mediated immunity. These findings were compared with the intensity of adverse local reactions and level of protection in experimental challenge. Monitoring of the local reaction at the injection site after each administration showed that microemulsion adjuvant IMS 1313 was less reactogenic than the water-in-oil-in-water emulsion ISA 201. In terms of efficacy, both dermal administrations were less immunogenic than the i.m route. The i.m. injection induced higher anti-App9 IgG and IgM titres. Nevertheless, IgG1 and IgG2 isotypes analysis revealed a close immunological profile between i.m. and i.d. routes. The concentration of IFN-γ from peripheral blood after in vitro restimulation with the specific antigen was only increased in the i.m. group at the day of challenge (D35) and two weeks after (D49). Interestingly, the smallest gross pulmonary lesions were observed in the i.d. vaccinated group (3.4%) compared to the control group (39.4%) and to groups with other routes of administration. Taken together, these results suggest that i.d. administration of vaccines is a promising approach. Even the i.d. vaccine was more reactogenic and slightly less immunogenic than the i.m. vaccine, its protection effectiveness seemed to be superior.

MALDI MSI Reveals the Spatial Distribution of Protein Markers in Tracheobronchial Lymph Nodes and Lung of Pigs after Respiratory Infection

Respiratory infections are a real threat for humans, and therefore the pig model is of interest for studies. As one of a case for studies, Actinobacillus pleuropneumoniae (APP) caused infections and still worries many pig breeders around the world. To better understand the influence of pathogenic effect of APP on a respiratory system-lungs and tracheobronchial lymph nodes (TBLN), we aimed to employ matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-TOF MSI). In this study, six pigs were intranasally infected by APP and two were used as non-infected control, and 48 cryosections have been obtained. MALDI-TOF MSI and immunohistochemistry (IHC) were used to study spatial distribution of infectious markers, especially interleukins, in cryosections of porcine tissues of lungs (necrotic area, marginal zone) and tracheobronchial lymph nodes (TBLN) from pigs infected by APP. CD163, interleukin 1β (IL‑1β) and a protegrin-4 precursor were successfully detected based on their tryptic fragments. CD163 and IL‑1β were confirmed also by IHC. The protegrin-4 precursor was identified by MALDI-TOF/TOF directly on the tissue cryosections. CD163, IL‑1β and protegrin‑4 precursor were all significantly (p < 0.001) more expressed in necrotic areas of lungs infected by APP than in marginal zone, TBLN and in control lungs.

Structural Basis of Ca2+-Dependent Self-Processing Activity of Repeat-in-Toxin Proteins

The Ca²⁺-dependent clip-and-link activity of large repeat-in-toxin (RTX) proteins is an exceptional posttranslational process in which an internal domain called a self-processing module (SPM) mediates Ca²⁺-dependent processing of a highly specific aspartate-proline (Asp-Pro) peptide bond and covalent linkage of the released aspartyl to an adjacent lysine residue through an isopeptide bond. Here, we report the solution structures of the Ca²⁺-loaded SPM (Ca-SPM) defining the mechanism of the autocatalytic cleavage of the Asp414-Pro415 peptide bond of the Neisseria meningitidis FrpC exoprotein. Moreover, deletion of the SPM domain in the ApxIVA protein, the FrpC homolog of Actinobacillus pleuropneumoniae, resulted in attenuation of virulence of the bacterium in a pig infection model, indicating that the Ca²⁺-dependent clip-and-link activity plays a role in the virulence of Gram-negative pathogens.

The posttranslational Ca²⁺-dependent “clip-and-link” activity of large repeat-in-toxin (RTX) proteins starts by Ca²⁺-dependent structural rearrangement of a highly conserved self-processing module (SPM). Subsequently, an internal aspartate-proline (Asp-Pro) peptide bond at the N-terminal end of SPM breaks, and the liberated C-terminal aspartyl residue can react with a free ε-amino group of an adjacent lysine residue to form a new isopeptide bond. Here, we report a solution structure of the calcium-loaded SPM (Ca-SPM) derived from the FrpC protein of Neisseria meningitidis. The Ca-SPM structure defines a unique protein architecture and provides structural insight into the autocatalytic cleavage of the Asp-Pro peptide bond through a “twisted-amide” activation. Furthermore, in-frame deletion of the SPM domain from the ApxIVA protein of Actinobacillus pleuropneumoniae attenuated the virulence of this porcine pathogen in a pig respiratory challenge model. We hypothesize that the Ca²⁺-dependent clip-and-link activity represents an unconventional strategy for Gram-negative pathogens to adhere to the host target cell surface.

Update on Actinobacillus pleuropneumoniae-knowledge, gaps and challenges

Porcine pleuropneumonia, caused by the bacterial porcine respiratory tract pathogen Actinobacillus pleuropneumoniae, leads to high economic losses in affected swine herds in most countries of the world. Pigs affected by peracute and acute disease suffer from severe respiratory distress with high lethality. The agent was first described in 1957 and, since then, knowledge about the pathogen itself, and its interactions with the host, has increased continuously. This is, in part, due to the fact that experimental infections can be studied in the natural host. However, the fact that most commercial pigs are colonized by this pathogen has hampered the applicability of knowledge gained under experimental conditions. In addition, several factors are involved in development of disease, and these have often been studied individually. In a DISCONTOOLS initiative, members from science, industry and clinics exchanged their expertise and empirical observations and identified the major gaps in knowledge. This review sums up published results and expert opinions, within the fields of pathogenesis, epidemiology, transmission, immune response to infection, as well as the main means of prevention, detection and control. The gaps that still remain to be filled are highlighted, and present as well as future challenges in the control of this disease are addressed.

Frequency of Th17 cells correlates with the presence of lung lesions in pigs chronically infected with Actinobacillus pleuropneumoniae

Porcine contagious pleuropneumonia caused by Actinobacillus pleuropneumoniae (APP) remains one of the major causes of poor growth performance and respiratory disease in pig herds. While the role of antibodies against APP has been intensely studied, the porcine T cell response remains poorly characterized. To address this, pigs were intranasally infected with APP serotype 2 and euthanized during the acute phase [6-10 days post-infection (dpi)] or the chronic phase of APP infection (27-31 dpi). Lymphocytes isolated from blood, tonsils, lung tissue and tracheobronchial lymph nodes were analyzed by intracellular cytokine staining (ICS) for IL-17A, IL-10 and TNF-α production after in vitro stimulation with crude capsular extract (CCE) of the APP inoculation strain. This was combined with cell surface staining for the expression of CD4, CD8α and TCR-γδ. Clinical records, microbiological investigations and pathological findings confirmed the induction of a subclinical APP infection. ICS-assays revealed the presence of APP-CCE specific CD4⁺CD8α^dim IL-17A-producing T cells in blood and lung tissue in most infected animals during the acute and chronic phase of infection and a minor fraction of these cells co-produced TNF-α. APP-CCE specific IL-17A-producing γδ T cells could not be found and APP-CCE specific IL-10-producing CD4⁺ T cells were present in various organs but only in a few infected animals. The frequency of identified putative Th17 cells (CD4⁺CD8α^dimIL-17A⁺) in lung and blood correlated positively with lung lesion scores and APP-specific antibody titers during the chronic phase. These results suggest a potential role of Th17 cells in the immune pathogenesis of APP infection.

γδ T lymphocytes are recruited into the inflamed uterus of bitches suffering from pyometra

Very little is known about the occurrence of immune system cells in the canine uterus. The aim of this study was to generate information about lymphocyte subsets that are present in the healthy canine uterus and that are recruited under inflammatory conditions caused by pyometra. Using immunohistochemistry and flow cytometry, a significant influx of γδ T lymphocytes was found in pyometra samples mainly due to recruitment of γδ(+)/CD8(-) T lymphocytes. The relative expression of genes encoding selected cytokines/chemokines was evaluated in samples from healthy and pyometra-affected uteri. Expression of pro-inflammatory cytokines (including IL-1β, TNF-α, IL-8, IL-17 and IFN-γ) and chemokines (including CXCL10, CCL4 and CCL5) was upregulated in pyometra samples confirming the presence of inflammation. In contrast, the expression of the homeostatic chemokine CCL25 and of the anti-inflammatory cytokine IL-10 was downregulated and unchanged, respectively.

New insights in cellular immune response in colostrum-deprived pigs after immunization with subunit and commercial vaccines against Glässer's disease

Four groups of colostrum-deprived pigs were immunized with Porcilis Glässer® (PG) or with subunit vaccines developed by us (rTbpA, NPAPT(M) or NPAPT(Cp)) against Glässer's disease, and they were challenged with 3×10(8)CFU of Haemophilus parasuis. A strong reduction in CD3(+)γδTCR(+) cells was seen in non-immunized control and scarcely protected (rTbpA) groups, suggesting that these cells could represent a target of H. parasuis infection. A significant increase in CD172α(+)CD163(+) cells was detected in all groups but PG, while a reduction in SLAIIDR(+) molecules expression was observed after challenge in control animals. Significant increases in CD3ε(+)CD8α(+)CD8β(+) and B cells were detected respectively in control and NPAPT groups, and in scarcely (rTbpA) and well-protected (NPAPT(M) and NPAPT(Cp)) groups. Finally, a greater response in CD4(+)CD8α(-) cells was observed in NPAPT(Cp) compared to NPAPT(M) and PG groups. These results state the potential of NPAPT antigen for developing effective vaccines against Glässer's disease.

CD4+ and γδTCR+ T lymphocytes are sources of interleukin-17 in swine

In the veterinary field, only limited information is available about interleukin-17A (IL-17), despite the fact that this cytokine plays an important role during pro-inflammatory immune responses and induces the production of chemotactic factors for neutrophils. The aim of this study was to characterize porcine IL-17-producing cells. We tested the cross-reactivity of five anti-human IL-17 monoclonal antibodies because such antibodies against porcine IL-17 are currently unavailable. Whole blood cells (WBCs) were stimulated with phorbol-myristate-acetate (PMA) and ionomycin and subsequently analyzed by flow cytometry. The antibody clone SCPL1362 was found to cross-react with porcine IL-17, whereas the other four antibodies tested did not recognize this cytokine. Using this antibody, we characterized porcine WBC-secreting IL-17 after PMA and ionomycin stimulation. All IL-17-producing WBCs were positive for the T lymphocyte marker CD3. Myeloid cells (CD172α(+)) and B lymphocytes (CD79α(+)) were IL-17 negative. The major subset of IL-17 positive T lymphocytes was the CD4(+) lymphocytes (about 60% of all IL-17 positive WBCs). The remaining IL-17 positive WBCs were γδTCR(+) lymphocytes. CD8 positive and CD8 negative cells were found within both CD4(+) and γδTCR(+) cells producing the cytokine. Moreover, IL-17 positive cells were mostly CD45RA negative, therefore activated cells or memory cells. Flow cytometry data were confirmed using sorted cells. Both sorted CD4(+) and γδTCR(+) cells produced IL-17 at mRNA level after PMA and ionomycin stimulation while double negative CD4(-)γδTCR(-) cells were negative for IL-17. We can conclude that only two subpopulations of porcine WBCs are sources of IL-17 after non-specific stimulation: CD3(+)CD4(+) and CD3(+)γδTCR(+).

Porcine mononuclear phagocyte subpopulations in the lung, blood and bone marrow: dynamics during inflammation induced by Actinobacillus pleuropneumoniae

Mononuclear phagocytes (MP) are cells of nonspecific immunity, playing an essential role in defense against bacterial pathogens. Although various MP subpopulations have been described in the pig, relations among these populations in vivo are unknown to date. The present study was aimed at describing porcine MP subpopulations infiltrating inflamed tissue of pigs under in vivo conditions. Actinobacillus pleuropneumoniae (APP) infection was used to induce an inflammatory response. CD172α, CD14, CD163, MHCII and CD203α cell surface molecules were used to identify MP by flow cytometry. Changes in MP subpopulations in the peripheral blood (PB) and bone marrow (BM) compartments along with the analysis of MP appearing in the inflamed lungs were assessed to elucidate the possible origin and maturation stages of the infiltrating MP. The MP population migrating to the inflamed lungs was phenotype CD14⁺ CD163⁺ CD203α^+/− MHCII^+/−. Concomitantly, after APP infection there was an increase in the PB MP CD14⁺ CD163⁺ CD203α⁻ MHC II⁻ population, suggesting that these cells give rise to inflammatory monocytes/macrophages. The CD203α and MHCII molecules appear on these cells after leaving the PB. In healthy animals, the BM MP precursors were represented by CD14⁻ CD163⁻ cells maturing directly into CD14⁺ CD163⁻ that were then released into the PB. After infection, an altered maturation pathway of MP precursors appeared, represented by CD14⁻ CD163⁻ CD203α⁻ MHCII⁻ MP directly switching into CD14⁺ CD163⁺ CD203α⁻ MHCII⁻ MP. In conclusion, two different MP maturation pathways were suggested in pigs. The use of these pathways differs under inflammatory and noninflammatory conditions.

Comparison of different doses of antigen for intradermal administration in pigs: the Actinobacillus pleuropneumoniae model

The intensity of antibody responses and level of protection against challenge infection induced by Actinobacillus pleuropneumoniae (APP) were compared in piglets vaccinated intramuscularly with different vaccination doses of a subunit vaccine. Secondary antibody responses to APP exotoxins induced by the intradermally administered diluted vaccines did not differ from those induced by undiluted vaccines administered either intradermally or intramuscularly. The level of protection measured by the clinical course of challenge infection to the extent of lung lesions was significantly higher in animals vaccinated with a three-time diluted vaccine in comparison with animals that were administered intramuscularly or intradermally with the most concentrated or the most diluted vaccines.

Blood cellular immune response in pigs immunized and challenged with Haemophilus parasuis

The cellular immune response to an experimental infection by Haemophilus parasuis, the etiological agent of Glässer's disease in pigs, was characterized studying changes in peripheral blood mononuclear cells (PBMC) in colostrum-deprived pigs. Five groups were studied, four of those were previously immunized with different formulations and the fifth was maintained as non-immunized control. All groups were challenged with 5 x 10(9) CFU of H. parasuis serotype 5. The non-commercial bacterin conferred a complete protection, while the OMP-vaccine and the exposure to a subletal dose of 10(5) CFU of H. parasuis protected only partially, and the recombinant Tbp B-vaccine induced no protection. PBMC were analyzed using monoclonal antibodies against porcine CD45(+), CD3(+), CD4(+), CD8alpha(+), CD25(+), CD4(+) naïve, alphaIgM(+) and SWC3(+) cells in single-colour fluorescence, and CD4(+)/CD8alpha(+) and CD8alpha(+)/CD8beta(+) combinations in two-colour fluorescence. The different groups showed no significant changes in PBMC subsets following vaccination, and only minor changes were encountered after challenge, consisting mainly of significant increases (P<0.05) in the relative proportions of monocytes and granulocytes (SWC3(+)) and B cells (alphaIgM(+)), as well as a significant reduction in CD3(+) cells (P<0.05). These changes were similar for the five groups compared, except for the significant increase of CD25(+) cells, which was only observed for the bacterin-vaccinated group. These results suggest an increase of trafficking of inflammatory cells and the onset of the adaptive antibody response against H. parasuis infection; in addition, the blood cellular response developed by the different groups was not relevant to protection.