Efficacy of a modified-live virus vaccine in pigs experimentally infected with a highly pathogenic porcine reproductive and respiratory syndrome virus type 1 (HP-PRRSV-1)

A potent CD8 T-cell response may be associated with partial cross-protection conferred by an attenuated Chinese HP-PRRSV vaccine against NADC30-like PRRSV challenge

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most devastating pathogens to the global swine industry. Many commercial PRRSV vaccines, originally designed to provide homologous protection, have shown partial protection against heterologous strains. However, the protective immune mechanisms mediated by these PRRSV vaccines are not fully understood. In this study, we investigated the factors responsible for partial protection conferred by an attenuated Chinese HP-PRRSV vaccine (TJM-F92) against heterologous NADC30-like PRRSV. By analysing peripheral T-cell responses induced by the TJM-F92 vaccine and local and systemic memory responses following challenge with NADC30-like PRRSV (SD17-38 strains) as well as neutralizing antibody response, we found that the TJM-F92 vaccine induced a significant expansion of CD8 T cells but not CD4 T cells or γδ T cells. The expanded CD8 T cells exhibited a phenotype of effector memory T cells and secreted IFN-γ upon restimulation with SD17-38 strains in vitro. In addition, only CD8 T cells in the prior immunized pigs rapidly expanded in the blood and spleen after heterologous challenge, with higher magnitude, compared to the unvaccinated pigs, showing a remarkable memory response. In contrast, no obvious humoral immune response was enhanced in the vaccinated and challenged pigs, and no heterologous neutralizing antibodies were detected throughout the experiment. Our results suggested that CD8 T cells elicited by the TJM-F92 vaccine may be responsible for partial heterologous protection against NADC30-like PRRSV strains and potentially recognize the conserved antigens among PRRSV strains.

Development of a Monoclonal Antibody to Pig CD69 Reveals Early Activation of T Cells in Pig after PRRSV and ASFV Infection

The CD69 molecule, as an early activation marker of lymphocytes, is often used to assess the activation of cellular immunity. However, for pigs, an anti-pig CD69 antibody is not yet available for this purpose after infection or vaccination. In this study, a monoclonal antibody (mAb) against pig CD69 was produced by peptide immunization and hybridoma technique. One mAb (5F12) showed good reactivity with pig CD69 that was expressed in transfected-HEK-293T cells and on mitogen-activated porcine peripheral blood mononuclear cells (PBMCs) by indirect immunofluorescence assay and flow cytometry. This mAb did not cross-react with activated lymphocytes from mouse, bovine, and chicken. Epitope mapping showed that the epitope recognized by this mAb was located at amino acid residues 147–161 of pig CD69. By conjugating with fluorochrome, this mAb was used to detect the early activation of lymphocytes in PRRSV- and ASFV-infected pigs by flow cytometry. The results showed that PRRSV infection induced the dominant activation of CD4 T cells in mediastinal lymph nodes and CD8 T cells in the spleen at 14 days post-infection, in terms of CD69 expression. In an experiment on ASFV infection, we found that ASFV infection resulted in the early activation of NK cells, B cells, and distinct T cell subsets with variable magnitude in PBMCs, spleen, and submandibular lymph nodes. Our study revealed an early event of lymphocyte and T cell activation after PRRSV and ASFV infections and provides an important immunological tool for the in-depth analysis of cellular immune response in pigs after infection or vaccination.

Phenotypic Characterization of a Virulent PRRSV-1 Isolate in a Reproductive Model With and Without Prior Heterologous Modified Live PRRSV-1 Vaccination

Reproductive disorders induced by porcine reproductive and respiratory syndrome virus (PRRSV) cause high economic losses in the pig industry worldwide. In this study, we aimed to phenotypically characterize a virulent PRRSV-1 subtype 1 isolate (AUT15-33) in a reproductive model. Furthermore, the protective effect of a heterologous modified live virus vaccine (ReproCyc® PRRS EU) was evaluated. In addition, PRRSV AUT15-33 was genotypically compared to other well-characterized isolates. Sixteen gilts were equally divided into four groups: a vaccinated and infected group (V–I), a vaccinated and non-infected group (V–NI), a non-vaccinated and infected group (NV–I), and a non-vaccinated and non-infected (NV–NI) group. After PRRSV infection on gestation day 84, all gilts were clinically examined on a daily basis, and blood samples were taken at five timepoints. Necropsy was performed 3 weeks after infection. The fetal preservation status was assessed, and PRRSV RNA concentrations were measured in the blood and tissue samples from all gilts and fetuses. After infection, all four gilts in the NV–I group were viremic throughout 17 days post-infection (dpi), whereas two gilts in the V–I group were viremic at only one timepoint at 6 dpi. The viral load was significantly higher in gilt serum, tracheobronchial lymph nodes, uterine lymph nodes, maternal endometrium, and fetal placenta of NV–I gilts compared to the V–I ones (p < 0.05). Moreover, the preservation status of the fetuses derived from NV–I gilts was significantly impaired (55.9% of viable fetuses) compared to the other groups (p < 0.001). Upon comparison with other known isolates, the phylogenetic analyses revealed the closest relation to a well-characterized PRRSV-1 subtype 1 field isolate from Belgium. In conclusion, the high virulence of AUT15-33 was phenotypically confirmed in an experimental reproductive model. The vaccination of the gilts showed promising results in reducing viremia, fetal damage, and transplacental transmission of the PRRSV-1 strain characterized in this study.

Efficacy of a Modified Live Virus Vaccine against Porcine Reproductive and Respiratory Syndrome Virus 1 (PRRSV-1) Administered to 1-Day-Old Piglets in Front of Heterologous PRRSV-1 Challenge

PRRSV is one of the most important viruses in the global swine industry and is often controlled by the use of modified live virus (MLV) vaccines. This study assessed the impact of a PRRSV-1 MLV vaccine applied to 1-day-old piglets challenged on day 28 of life with a PRRSV-1 field isolate (AUT15-33). Twenty-one piglets were vaccinated within 24 h of birth (T02), whereas 20 piglets were left unvaccinated (T01). Necropsy was performed two weeks post-challenge. Comparing the two groups, T02 piglets showed significantly higher (p = 0.017) average daily weight gain. In addition, significantly lower (p < 0.0001) PRRSV RNA loads were measured in serum of T02 piglets at all investigated time points. All T01 piglets were viremic and shed virus in nasal swabs, whereas only 71.4% and 38.1% of the T02 group were viremic or shed virus, respectively. Piglets from T02 had significantly higher numbers (p < 0.0001) of IFN-γ producing lymphocytes compared to T01. At necropsy, differences in gross and histologic lung lesions were statistically significant (p = 0.012 and p < 0.0001, respectively) between the two groups. Hence, this MLV vaccine administered to 1-day-old piglets was able to protect piglets against PRRSV infection at weaning.

Challenge of Naïve and Vaccinated Pigs with a Vaccine-Derived Recombinant Porcine Reproductive and Respiratory Syndrome Virus 1 Strain (Horsens Strain)

In July 2019, a vaccine-derived recombinant Porcine reproductive and respiratory syndrome virus 1 strain (PRRSV-1) (Horsens strain) infected more than 40 Danish sow herds, resulting in severe losses. In the present study, the pathogenicity of the recombinant Horsens strain was assessed and compared to a reference PRRSV-1 strain using a well-characterized experimental model in young SPF pigs. Furthermore, the efficacies of three different PRRSV-1 MLV vaccines to protect pigs against challenge with the recombinant strain were assessed. Following challenge, the unvaccinated pigs challenged with the Horsens strain had significant increased viral load in serum compared to all other groups. No macroscopic changes were observed at necropsy, but tissue from the lungs and tonsils from almost all pigs were PRRSV-positive. The viral load in serum was lower in all vaccinated groups compared to the unvaccinated group challenged with the Horsens strain, and only small differences were seen among the vaccinated groups. The findings in the present study, combined with two other recent reports, indicate that this recombinant “Horsens” strain indeed is capable of inducing infection in growing pigs as well as in pregnant sows that is comparable to or even exceeding those induced by typical PRRSV-1, subtype 1 strains. However, absence of notable clinical signs and lack of significant macroscopic changes indicate that this strain is less virulent than previously characterized highly virulent PRRSV-1 strains.

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A "Leaky" Vaccine with Debatable Efficacy and Safety

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.

Commercial PRRS Modified-Live Virus Vaccines

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) presents one of the challenging viral pathogens in the global pork industry. PRRS is characterized by two distinct clinical presentations; reproductive failure in breeding animals (gilts, sows, and boars), and respiratory disease in growing pigs. PRRSV is further divided into two species: PRRSV-1 (formerly known as the European genotype 1) and PRRSV-2 (formerly known as the North American genotype 2). A PRRSV-2 modified-live virus (MLV) vaccine was first introduced in North America in 1994, and, six years later, a PRRSV-1 MLV vaccine was also introduced in Europe. Since then, MLV vaccination is the principal strategy used to control PRRSV infection. Despite the fact that MLV vaccines have shown some efficacy, they were problematic as the efficacy of vaccine was often unpredictable and depended highly on the field virus. This paper focused on the efficacy of commercially available MLV vaccines at a global level based on respiratory disease in growing pigs, and maternal and paternal reproductive failure in breeding animals.

A chimeric porcine reproductive and respiratory syndrome virus (PRRSV)-2 vaccine is safe under international guidelines and effective both in experimental and field conditions

Vaccination is currently the most effective strategy to control porcine reproductive and respiratory syndrome (PRRS). New-generation PRRS vaccines are required to be safe and broadly cross-protective. We have recently created the chimeric PRRS virus K418DM which proved to be a good vaccine candidate under field conditions. In the present study, we designed safety and efficacy tests under experimental and field conditions for further evaluation of K418DM1.1, a plaque-purified K418DM. In the homologous challenge study, K418DM1.1 induced high serum virus neutralization (SVN) antibody titers (i.e., 4.2 log₂ ± 1.7) at 21 days post-challenge (dpc) and provided protection as demonstrated by the significantly lower levels of viremia at 3 and 7 dpc and significantly lower microscopic lung lesion scores compared to the unvaccinated group. K418DM1.1 was also protective in the heterologous challenge study, with vaccinated pigs showing significantly lower levels of viremia at 14 dpc compared to the unvaccinated pigs. A field study was performed to evaluate the efficacy of K418DM1.1 against heterologous exposure and vaccinated pigs presented significantly lower viremia than unvaccinated pigs. According to the safety test for the examination of virulence reversion, no infectivity was observed in tissue homogenate filtrate both in the vaccinated and comingled groups. Thus, the risk of virulence, as well as transmission, appeared negligible. These overall results indicate that K418DM1.1 is a good vaccine candidate based on its safety and protective efficacy.

Modeling Economic Effects of Vaccination Against Porcine Reproductive and Respiratory Syndrome: Impact of Vaccination Effectiveness, Vaccine Price, and Vaccination Coverage

Porcine reproductive and respiratory syndrome (PRRS) causes substantial financial losses in pig farms and economic losses to societies worldwide. Vaccination against PRRS virus (PRRSV) is a common intervention in affected farms. The aim of this study was to assess the economic impact and profitability of potential new PRRS vaccines with improved efficacy at animal, herd, and national level. Two vaccination strategies were modeled; (i) mass vaccination of sows only (MS) and (ii) mass vaccination of sows and vaccination of piglets (MSP), comprising different scenarios of vaccine effectiveness, vaccine price, and vaccination coverage. A farrow-to-finish farm with 1,000 working sows from a pig-dense region in Germany served as an example farm. Financial benefits were obtained from gross margin analyses and were defined as difference in gross margin between a PRRSV-infected farm without vaccination (baseline) and with vaccination (intervention). Financial benefits were highest if both sows and piglets (MSP) were vaccinated. In these scenarios, median annual net benefits per working sow ranged from €170 to 340. If sows only were vaccinated (MS), estimated benefits attributable to vaccination were between €148 and 270. Decisive variables for the estimation of national level benefits were the number of farmers switching from existing to a better protecting vaccine, the number of previously non-vaccinating herds adopting the new vaccine, and the effectiveness of the new vaccine relative to those already available. Benefits were greatest when the new vaccine was adopted by previously non-vaccinating herds. The analyses showed that vaccination against PRRS was beneficial for all modeled scenarios. The magnitude of benefits derived from vaccination was more susceptible to changes in vaccination effectiveness than to vaccine price changes. This study provides evidence to support future vaccine development. The estimates indicate that the introduction of more efficient vaccines might lead to substantial financial benefits, is of socio-economic importance and that new vaccines might significantly contribute to the reduction of disease burden.

Recombinant Porcine Interferon Alpha Enhances Immune Responses to Killed Porcine Reproductive and Respiratory Syndrome Virus Vaccine in Pigs

In this study, the immunoadjuvant effects of recombinant porcine interferon alpha (rPoIFNα) on the killed virus vaccine (KV) of porcine reproductive and respiratory syndrome virus (PRRSV) in pigs were investigated. The experimental pigs were divided into six groups, including normal control group, rPoIFNα control group, PRRSV KV control group, KV+40,000 U rPoIFNα immunization group, KV+400,000 U rPoIFNα immunization group, and KV+4,000,000 U rPoIFNα immunization group. The experimental pigs were boosted immunized on the 28th day after the initial immunization, and the heparinized blood and serum samples were collected at different time points of these two immunizations to detect and evaluate the immune responses of pigs after immunization by ELISA assay, neutralization assay, flow cytometry, and so on. The results showed that the proportion of the levels of PRRSV-specific antibodies, neutralizing antibodies, stimulation index, IL-4, IFN-γ, and lymphocytes within the groups immunized with KV+rPoIFNα were significantly higher than that group immunized with KV alone. The humoral and cellular immune responses in pigs were markedly enhanced by rPoIFNα after the coadministration with KV vaccine. Therefore, we tentatively think that rPoIFNα is a potential immune promoter with prospects for future applications in the pig industry.

An engineered anti-idiotypic antibody-derived killer peptide (KP) early activates swine inflammatory monocytes, CD3+CD16+ natural killer T cells and CD4+CD8α+ double positive CD8β+ cytotoxic T lymphocytes associated with TNF-α and IFN-γ secretion

This study evaluated the early modulation of the phenotype and cytokine secretion in swine immune cells treated with an engineered killer peptide (KP) based on an anti-idiotypic antibody functionally mimicking a yeast killer toxin. The influence of KP on specific immunity was investigated using porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) as ex vivo antigens. Peripheral blood mononuclear cells (PBMC) from healthy pigs were stimulated with KP and with a scramble peptide for 20 min, 1, 4 and 20 h or kept unstimulated. The cells were analyzed using flow cytometry and ELISA. The same time-periods were used for KP pre-incubation/co-incubation to determine the effect on virus-recalled interferon-gamma (IFN-γ) secreting cell (SC) frequencies and single cell IFN-γ productivity using ELISPOT. KP induced an early dose-dependent shift to pro-inflammatory CD172α⁺CD14^+high monocytes and an increase of CD3⁺CD16⁺ natural killer (NK) T cells. KP triggered CD8α and CD8β expression on classical CD4^-CD8αβ⁺ cytotoxic T lymphocytes (CTL) and double positive (DP) CD4⁺CD8α⁺ Th memory cells (CD4⁺CD8α^+low CD8β^+low). A fraction of DP cells also expressed high levels of CD8α. The two identified DP CD4⁺CD8α^+high CD8β^+low/+high CTL subsets were associated with tumor necrosis factor alpha (TNF-α) and IFN-γ secretion. KP markedly boosted the reactivity and cross-reactivity of PRRSV type-1- and PCV2b-specific IFN-γ SC. The results indicate the efficacy of KP in stimulating Th1-biased immunomodulation and support studies of KP as an immunomodulator or vaccine adjuvant.

The PRRSV-Specific Memory B Cell Response Is Long-Lived in Blood and Is Boosted During Live Virus Re-exposure

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen of swine health and well-being worldwide largely due to an insufficient understanding of the adaptive immune response to infection leading to ineffective PRRSV control. The memory and anamnestic response to infection are critical gaps in knowledge in PRRSV immunity. The lack of effective tools for the evaluation of the memory response previously hindered the ability to effectively characterize the porcine memory response to infection. However, the creation and validation of a PRRSV nsp7-specific B cell tetramer now facilitates the ability to detect very rare memory B cells and thus define the memory response of the pig. Here, we describe the PRRSV nsp7-specific B cell response following vaccination and challenge in six key secondary lymphoid organs including the identification of PBMCs as the tissue of interest for the memory immune response in pigs. Following live virus challenge of immune animals, an anamnestic response of nsp7-specific memory B cells and neutralizing antibodies was observed. This characterization of the functional humoral immune response to PRRSV answers key questions involved in regional specialization of the immune response following intramuscular inoculation of PRRSV MLV.

Impact of PRRSV strains of different in vivo virulence on the macrophage population of the thymus

The emergence of "highly pathogenic" isolates of porcine reproductive and respiratory syndrome virus (HP-PRRSV) has raised new concerns about PRRS control. Cells from the porcine monocyte-macrophage lineage represent the target for this virus, which replicates mainly in the lung, and especially in HP-PRRSV strains, also in lymphoid organs, such as the thymus. This study aimed at evaluating the impact of two PRRSV strains of different virulence on thymic macrophages as well as after heterologous vaccination. After experimental infection with PR11 and PR40 PRRSV1 subtype 1 strains (low and high virulent, respectively) samples from thymus were analysed by histopathology and immunohistochemistry for PRRSV N protein, TUNEL, CD172a, CD163, CD107a and BA4D5 expression. Mortality was similar in both infected groups, but lung lesions and thymus atrophy were more intense in PR40 group. Animals died at 10-14 dpi after PR11 or PR40 infection showed the most severe histopathological lesions, with a strong inflammatory response of the stroma and extensive cell death phenomena in the cortex. These animals presented an increase in the number of N protein, CD172a, CD163 and BA4D5 positive cells in the stroma and the cortex together with a decrease in the number of CD107a positive cells. Our results highlight the recruitment of macrophages in the thymus, the increase in the expression of CD163 and the regulation of the host cytotoxic activity by macrophages. However, no marked differences were observed between PR11- and PR40-infected animals. Heterologous vaccination restrained virus spread and lesions extent in the thymus of PR40-infected animals.

Modulation of the somatotropic axis, adiponectin and cytokine secretion during highly pathogenic porcine reproductive and respiratory syndrome virus type 1 (HP-PRRSV-1) infection

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to be clinically responsible for reproductive failure in sows and post-weaning respiratory disease in growing piglets. During the last years, highly pathogenic PRRSV isolates have been discovered. In Italy, a PRRSV-1 subtype 1 strain (namely PR40/2014) characterized by high pathogenicity was isolated and experimental infection was characterized in terms of virological/clinical features and immune modulation (Canelli et al., 2017; Ferrari et al., 2018). The present study was performed in 4-week-old pigs experimentally infected with the highly pathogenic PRRSV1_PR40/2014 (HP-PR40) or with the conventional PRRSV1_PR11/2014 (PR11). The aim was to evaluate the interrelation between plasmatic hormones and cytokines in infected pigs compared to uninfected controls in order to address potential effects on the course of an experimental infection. The time-related changes of growth hormone (GH), insulin-like growth factor-1 (IGF-1), adiponectin, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) levels appear to be modulated by the infection depending on the PRRSV isolate (HP-PR40 vs. PR11). In particular, in HP-PR40 infected animals, the association between high GH levels and viremia may testify the need to block the anabolic action of GH in order to shift available energy towards the immune response. This need appeared to be delayed in PR11 animals, given the lower pathogenicity of the isolate. Adiponectin, IL-6 and TNF-α course supports the hypothesis of GH resistance mechanisms to guarantee homeostasis in HP-PR40 animals and underlines the key role of energy availability in events leading to an effective response to the virus.