Lineage 1 Porcine Reproductive and Respiratory Syndrome Virus Attenuated Live Vaccine Provides Broad Cross-Protection against Homologous and Heterologous NADC30-Like Virus Challenge in Piglets

A lineage 1 branch porcine reproductive and respiratory syndrome virus live vaccine candidate provides broad cross-protection against HP-like PRRSV in piglets

Multiple porcine reproductive and respiratory syndrome virus (PRRSV) subtypes coinfect numerous pig farms in China, and commercial PRRSV vaccines offer limited cross-protection against heterologous strains. Our previous research confirmed that a PRRSV lineage 1 branch attenuated live vaccine (SD-R) provides cross-protection against HP-PRRSV, NADC30-like PRRSV and NADC34-like PRRSV. HP-PRRSV has undergone significant genetic variation following nearly two decades of evolution and has transformed into a subtype referred to as HP-like PRRSV, which also exhibits high pathogenicity. The effectiveness of immunising piglets with the SD-R strain to provide protection against infection with HP-like PRRSV remains uncertain. In the present study, we evaluated the protective effects of SD-R vaccine strains on DLF-challenged piglets. The results revealed that piglets challenged with DLF presented clinical symptoms such as continuous high fever and an obvious decrease in daily weight gain. Importantly, the piglets immunised with SD-R exhibited notable reductions in pathological damage, especially of decreases in DLF-induced thymic atrophy. Moreover, the serum of SD-R-immunised piglets strongly neutralised DLF, and the number of SD-R-vaccinated piglets demonstrating viraemia was greatly reduced. These results suggest that the PRRSV lineage 1 branch live vaccine candidate provides broad cross-protection against HP-like PRRSV in piglets.

Efficacy of a reduced-dosage PRRS MLV vaccine against a NADC34-like strain of porcine reproductive and respiratory syndrome virus

Introduction

After being discovered for the first time in China in 2017, porcine reproductive and respiratory syndrome virus (PRRSV) NADC34-like strains have become the prevalent strain of PRRSV in certain regions of China. Our previous study showed that reduced Ingelvac PRRS MLV vaccination dosages against NADC30-like CF PRRSV had a better protection effect than the normal dosage. However, the protective effect of reduced dosages vaccination of Ingelvac PRRS MLV against NADC34-like PRRSV is unclear. Therefore, this study compared the effectiveness of 0.1 and 1 dosages against a NADC34-like PRRSV infection using commercial PRRSV vaccines, Ingelvac PRRS MLV, which have been widely utilized in China.

Methods

In this study, we immunized piglets with two different dosages of the MLV vaccine and infected piglets within a nasal way with NADC34-like CF PRRSV at 42 days post-vaccination. We observed the changes in growth performance before and after the NADC34-like PRRSV DX strain challenge and the protective effect of different vaccine dosages through multiple assays.

Results

After the challenge, the piglets from the challenge control group displayed clinical signs typical of PRRSV infection, including transient fever, high viremia, mild clinical symptoms, and histopathological changes in the lungs and lymph nodes, which indicates DX is a virulent virus. Without the challenge, the average daily gain of the non-immunized group at 5 weeks after the vaccination is greater than that of the 0.01 dosage group than that of the 1 dosage group, which proved that the commercial MLV vaccine has a negative effect on the growth performance of pigs and this effect may be dose-dependent. After the NADC34-like PRRSV challenge, there was no difference in average daily gain between the immunized pigs and pigs from the challenge control group. From the perspective of clinical score, gross lung lesions, and microscopic lesions, immunization with MLV vaccine can indeed relieve symptoms and lesions caused by the virus, and 0.1 dosage vaccination has a better effect in these aspects. Also, both dosages of MLV immunization shortened viremia with similar effects.

Discussion

Our research suggests that the MLV vaccine can provide piglets with some protection against NADC34-like PRRSV and the 0.1 dosage Ingelvac PRRS MLV vaccination showed greater benefits in our study. Therefore, considering the cost, side effects, and subsequent protective effects, we can adjust the immune dosage appropriately after further investigation to ensure safety, improve production efficiency, and reduce immunization costs.

Evaluation of the cross-protective effect of VR2332 modified live virus vaccine against a recombinant NADC34-like porcine reproductive and respiratory syndrome virus

In recent years, NADC34-like strains of porcine reproductive and respiratory syndrome virus have gradually emerged as mainstream strains on Chinese pig farms. These strains have high mutation rates and can recombine with local strains, representing great challenges to prevention and control efforts. Previously, a new recombinant NADC34-like subtype strain was isolated in our laboratory. Herein, we evaluated the cross-protective effect of the VR2332 modified live virus (MLV) against the novel NADC34-like recombinant strain using the immune challenge protection test in piglets and sows. The results revealed that immunization with the vaccine in piglets significantly reduced viremia, lung damage and stimulated the production of PRRSV-N antibodies. In the sow challenge experiment, one abortion and one death were recorded in the positive control group, and the survival rate of offspring was only 25%. However, there were no sow deaths or abortions in the immunization group during the experiment, and the average piglet survival rate was high at 76.5%. In general, the VR2332 MLV confers a certain extent of cross-protection against the NADC34-like recombinant strain, providing an effective reference and guidance for prevention and control efforts and clinical vaccine use.

Evaluation of cross-protection of a reduced-dose PRRS MLV vaccine against the NADC30-like PRRSV challenge

Introduction

At present, the NADC30-like strain has become the prevalent strain of PRRSV in China. Many studies have found that existing commercial vaccines are ineffective or provide only limited protection. No study has investigated the cross-protection of different dosages of commercial MLV vaccines against NADC30-like PRRSV. Therefore, this study assessed the effectiveness of various dosages against a NADC30-like PRRSV infection using commercial PRRSV vaccines, Ingelvac PRRS MLV, which have been widely utilized in China.

Methods

In this study, we immunized piglets with four different dosages of the MLV vaccine and infected piglets within a nasal way with NADC30-like CF PRRSV at 28 days post-vaccination. We observed the status of pigs before and after the challenge of NADC30-like PRRSV CF strain and reflected the protective effect of different dosages of MLV vaccine through multiple assays.

Results

Compared to those piglets immunized with 1 dosage, the piglets immunized with 0.01 dosage had better performance, such as the highest average daily gain before the challenge, lesser lesions and viremia after the challenge, low clinical score, and stable temperature during the study. However, the piglets immunized with 0.01 dosage still showed viremia, viruses were detected in their lungs, tonsils, and inguinal lymph nodes, and pathological lesions occurred in their lung. Immunohistochemistry staining of the lung of vaccinated piglets revealed a similar viral load to that of unvaccinated piglets, suggesting that immunization could not completely remove the virus from the vaccinated piglets' tissues.

Discussion

Our research suggests that the MLV vaccine could provide limited protection against the NADC30-like PRRSV infection, and lowering the dosage to 0.01 may produce better protective efficacy. In the context of identifying the immunological target, comprehending the virulence of the virus in the field, and guaranteeing safety, we might be able to reevaluate vaccination dosages to achieve higher economic value.

Porcine reproductive and respiratory syndrome developments: An in-depth review of recent findings

The porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) belonging to the Arteriviridae family is the cause of PRRS disease. After being discovered for the first time in the United States in 1987, this illness quickly expanded to Canada. The disease was initially discovered in late 1990 in Germany, from where it quickly spread throughout Europe. The consequences of PRRSV lead to a number of epidemiological issues, including a sickness with a delayed immune response that permits extended viremia, which facilitates viral transmission. The virus penetrates the nasal epithelium, tonsils, lung macrophages, and uterine endometrium through the oronasal and genital pathways. Abortions performed late in pregnancy and premature or delayed deliveries resulting in dead and mummified fetuses, stillborn pigs, and weakly born piglets are indicative of reproductive syndrome. In the meanwhile, dyspnea, fever, anorexia, and lethargic behavior are signs of respiratory syndrome. The virus can be isolated from the tissue or serum of animals that have been infected to confirm the diagnosis. Pig movements and potential airborne dissemination are two ways that the virus can enter new herds and propagate through nose-to-nose contact or aerosols. Various supportive therapies may enhance infant survival, and antibiotics may or may not lessen the impact of secondary bacterial infections. The absence of simple diagnostic tests, the virus's airborne transmission, the occurrence of subclinical infections, and the virus's persistence in infected populations have all contributed to the failure of control efforts for PRRS.

Protective efficacy of a candidate live attenuated vaccine derived from the SD-R strain of lineage 1 porcine reproductive and respiratory syndrome virus against a lethal challenge with HP-PRRSV HuN4 in piglets

IMPORTANCE

Both highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) and NADC30-like PRRSV have caused tremendous economic losses to the Chinese pig industry. In this study, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative HP-PRRSV strain (HuN4). The control piglets in the challenge experiment displayed obvious clinical symptoms of PRRSV infection, with a mortality rate up to 40%. In contrast, all the piglets in the vaccinated challenged group survived, and only some pigs had transient fever. The daily gain of SD-R immunized group piglets was significantly increased, and the pathological changes were significantly reduced. In addition, the viral replication levels in the serum of the immunized group were significantly lower than those of the challenged control group. The live attenuated vaccine SD-R strain can provide protection against HP-PRRSV challenge, indicating that the SD-R strain is a promising vaccine candidate for use in the swine industry.

Protective Efficacy of a Candidate Live-Attenuated Vaccine Derived from the SD-R Strain against NADC34-like Porcine Reproductive and Respiratory Syndrome Virus

NADC34-like porcine reproductive and respiratory syndrome virus (PRRSV) strains were first detected in China in 2017 and became major circulating strains in 2021. Our previous study showed that the live-attenuated vaccine candidate SD-R strain could provide broad cross-protection against different NADC30-like PRRSVs (sublineage 1.8). However, the protective effect of SD-R against NADC34-like PRRSV is unclear. Here, a novel NADC34-like PRRSV, LNTZJ1341-2012, was isolated from a pig farm experiencing disease in 2020. Sequence analysis revealed that LNTZJ1341-2012 belonged to PRRSV-2 sublineage 1.5, exhibited the same Nsp2 amino-acid deletion characteristics as IA/2014/NADC34, and had not recombined with other strains. Additionally, a good challenge model was established to evaluate the protection afforded by the candidate SD-R vaccine against infection with a representative NADC34-like strain (LNTZJ1341-2012). The control piglets in the challenge experiment displayed clinical signs typical of PRRSV infection, including transient fever, high viremia, mild clinical symptoms, and histopathological changes in the lungs and submaxillary lymph nodes. In contrast, SD-R vaccination significantly reduced serum and lung tissue viral loads, and vaccinated piglets did not show any clinical symptoms or histopathological changes. Our results demonstrated that LNTZJ1341-2012 is a mildly virulent NADC34-like PRRSV and that the live-attenuated vaccine SD-R can prevent the onset of clinical signs upon challenge with the NADC34-like PRRSV LNTZJ1341-2012 strain, indicating that SD-R is a promising vaccine candidate for the swine industry.

Influence of PRRSV-1 vaccination and infection on mononuclear immune cells at the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most devastating viruses for the global swine industry. Infection during late gestation causes reproductive failure but the local immune response in utero remains poorly understood. In this study, an experimental PRRSV-infection model with two different PRRSV-1 field isolates was used to investigate the immune cell phenotypes at the maternal-fetal interface during late gestation. In addition, phenotypic changes induced by a modified live virus (MLV, ReproCyc® PRRS EU) vaccine were studied. Vaccinated (n = 12) and non-vaccinated pregnant gilts (n = 12) were challenged with either one of the PRRSV-1 field isolates (low vs. high virulent, LV or HV) or sham-inoculated at day 84 of gestation. Twenty-one days post infection all gilts were euthanized and the fetal preservation status for all fetuses per litter was assessed. Leukocytes from the maternal-fetal interface were isolated and PRRSV-induced changes were investigated using ex vivo phenotyping by flow cytometry. PRRSV load in tissue from the maternal endometrium (ME) and fetal placenta (FP) was determined by RT-qPCR. In the ME, a vast increase in CD8β T cells with CD8αposCD27dim early effector phenotype was found for fetuses from the non-vaccinated LV and HV-challenged gilts, compared to non-treated and vaccinated-only controls. HV-challenged fetuses also showed significant increases of lymphocytes with effector phenotypes in the FP, including NKp46pos NK cells, CD8αhigh γδ T cells, as well as CD8αposCD27pos/dim CD4 and CD8 T cells. In vaccinated animals, this common activation of effector phenotypes was more confined and the fetal preservation status significantly improved. Furthermore, a negative correlation between the viral load and CD163highCD169pos mononuclear phagocytic cells was observed in the FP of HV-infected animals. These results suggest that the strong expansion of effector lymphocytes in gilts that were only infected causes immune-pathogenesis rather than protection. In contrast, the attenuated MLV seems to dampen this effect, yet presumably induces memory cells that limit reproductive failure. This work provides valuable insights into changes of local immune cell phenotypes following PRRSV vaccination and infection.

Recent advances in the study of NADC34-like porcine reproductive and respiratory syndrome virus in China

Since porcine reproductive and respiratory syndrome virus (PRRSV) was first described in China in 1996, several genetically distinct strains of PRRSV have emerged with varying pathogenicity and severity, thereby making the prevention and control of PRRS more difficult in China and worldwide. Between 2017 and 2021, the detection rate of NADC34-like strain in China increased. To date, NADC34-like strains have spread to 10 Chinese provinces and have thus developed different degrees of pathogenicity and mortality. In this review, we summarize the history of NADC34-like strains in China and clarify the prevalence, genomic characteristics, restriction fragment length polymorphisms, recombination, pathogenicity, and vaccine status of this strain in China. In so doing, this study aims to provide a basis for the further development of prevention and control measures targeting the NADC34-like strain.