Cell-mediated immune response in gilts vaccinated with a PRRS vaccine

In an environment poor in neutralising antibodies, cell-mediated response could be used to evaluate immune response after using a PRRS vaccine and more or less effectively predict protection against the virus.
The objective of the following study was to evaluate the cell-mediated response generated in gilts vaccinated with a PRRRS vaccine (UNISTRAIN® PRRS, HIPRA) against different PRRSV strains isolated from clinical outbreaks in the field.
EXPERIMENTAL DESIGN:

The study was conducted in six-months-old gilts, negative for PRRSV, coming from an historically disease-free farm. The animals were distributed between two groups: 75% were intramuscularly vaccinated with UNISTRAIN® PRRS (attenuated live PRRS vaccine, European genotype; strain VP-046 BIS) and the remaining 25% were administered 2 ml of intramuscular PBS (control group).
Blood samples were collected from the animals to obtain peripheral blood mononuclear cells (PBMCs) on days 0, 7, 14, 28, 42 and 56 post-vaccination. The samples were sent to CReSA (Centre de Recerca en Sanitat Animal) to evaluate cell-mediated immune response by measuring IFN-γ-SC from the PBMCs (ELISPOT assay) (figure 1). Heterologous cell-mediated response was evaluated using five genotype 1 PRRSV strains (table 1) recovered from clinical outbreaks (DIAGNOS, HIPRA), which represent a wide range of strains that were isolated in different European countries in different years. Table 1. Year and country of isolation of five field strains used to evaluate heterologous cell-mediated immune respons.

   
 
RESULTS: 

PRRSV-specific IFN-γ-SC were first detected against all the strains 14 days post-vaccination (figure 2). In the strains isolated in Spain and the United Kingdom, the response peak was found on day 14 post-vaccination; in the Spanish strain, the level found on day 14 was maintained until the end of the study (D56). In the strains isolated in Hungary, the Slovak Republic and Italy, the response peak was found on day 28, after which it declined.

  
In an environment poor in neutralising antibodies, cell-mediated response could be used to evaluate immune response after using a PRRS vaccine and more or less effectively predict protection against the virus.
The objective of the following study was to evaluate the cell-mediated response generated in gilts vaccinated with a PRRRS vaccine (UNISTRAIN® PRRS, HIPRA) against different PRRSV strains isolated from clinical outbreaks in the field.
EXPERIMENTAL DESIGN:

The study was conducted in six-months-old gilts, negative for PRRSV, coming from an historically disease-free farm. The animals were distributed between two groups: 75% were intramuscularly vaccinated with UNISTRAIN® PRRS (attenuated live PRRS vaccine, European genotype; strain VP-046 BIS) and the remaining 25% were administered 2 ml of intramuscular PBS (control group).
Blood samples were collected from the animals to obtain peripheral blood mononuclear cells (PBMCs) on days 0, 7, 14, 28, 42 and 56 post-vaccination. The samples were sent to CReSA (Centre de Recerca en Sanitat Animal) to evaluate cell-mediated immune response by measuring IFN-γ-SC from the PBMCs (ELISPOT assay) (figure 1). Heterologous cell-mediated response was evaluated using five genotype 1 PRRSV strains (table 1) recovered from clinical outbreaks (DIAGNOS, HIPRA), which represent a wide range of strains that were isolated in different European countries in different years. Table 1. Year and country of isolation of five field strains used to evaluate heterologous cell-mediated immune respons.
 

RESULTS: 

PRRSV-specific IFN-γ-SC were first detected against all the strains 14 days post-vaccination (figure 2). In the strains isolated in Spain and the United Kingdom, the response peak was found on day 14 post-vaccination; in the Spanish strain, the level found on day 14 was maintained until the end of the study (D56). In the strains isolated in Hungary, the Slovak Republic and Italy, the response peak was found on day 28, after which it declined.

DISCUSSION AND CONCLUSIONS:

The genetic and antigen variability of PRRSV is considered to be the most important factor in explaining the lack of cross reaction between strains, as heterologous protection is usually inconsistent and incomplete (Lager et al., 1999; Mengeling et al., 1999). On the other hand, the percentage of similarity in the ORF5, or even the complete sequence of the strains, is not a useful parameter for predicting the extent of protection provided by a vaccine against a given strain (Díaz et al., 2006; Prieto et al, 2008).
Although the immunity generated by PRRS vaccine is not fully known, evaluation of neutralising antibodies and cell-mediated response is important and has to be taken into consideration if we want to know how a PRRS vaccine works. The role of cell-mediated immunity for viral elimination or for protecting against a challenge has been discussed and shown in several studies (Díaz et al., 2012; Zuckerman et al., 2007; Lowe et al., 2005; Martelli et al., 2009).
Therefore, in the absence of neutralising antibody production, which is common after the administration of a single dose of any attenuated commercial PRRS vaccine (Díaz et al., 2006; Kim et al., 2008; Zuckerman et al., 2007), the cell-mediated immunity generated by a PRRS vaccine could play an important role in protection against the challenge.
The immunisation of all breeders, and especially gilts, is a key point in the control of PRRS. The main objective in this phase is to obtain good immunisation of gilts, which is why this study was designed using six-month-old gilts.
In this study, despite the wide range of strains used, not only regarding ORF5 variability (88-98% similarity) but also the year of isolation and origin, the results show that vaccination with UNISTRAIN® PRRS induces significant cell-mediated immune response against a wide range of PRRSV strains.
FINAL CONSIDERATIONS 

After more than 25 years confronting PRRS, control of the disease remains a constant challenge for porcine production. The approach to PRRS must be multi-strategic in order to be successful. One of the key aspects of control of this disease is the adaptation programmes for gilts, and their vaccination with UNISTRAIN® PRRS has been shown to be a useful and effective tool.

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Important points of a PRRS vaccine for active immunisation.

  
This article focuses on some of the key aspects of the PRRS vaccine as a tool required for active animal immunisation. From an economic and practical perspective, vaccination is a feasible tool for all kinds of breeders when compared to other immunisation systems.
In recent studies comparing active immunisation with modified live virus (MLV) PRRS vaccines and live virus inoculation (LVI), it was shown that when MLV vaccines are used to stabilise the breeder population after an outbreak, the mean time required to recover production levels and the impact on production are shorter and less severe than when LVI is used. Furthermore, the economic losses associated with an outbreak –quantified according to the piglets that are not bred- are greater with LVI than with MLV PRRS vaccine (Linhares et al., 2013). Finally, the use of MLV ensures correct contact of all the animals with PRRSV; however, when LVI is used the quantity of administered virus cannot be guaranteed, and therefore we cannot ensure proper immunisation of all sows.
CONTROL OF PRRS

Numerous strategies have been described for the control of PRRSV on individual farms. Successful control of the disease depends on a combination of the following measures:
Animal management (sow replacement, unidirectional animal flow, etc.)

Biosecurity (internal and external)

Diagnosis (animals’ immune status, monitoring, etc.)

Active immunisation (PRRS vaccine)

 

 VACCINATION

Although it is known that it does not prevent infection, vaccination is used in order to reduce the clinical onset of the disease and to reduce viral excretion. When it is used in nulliparous sows there is a reduction in viraemia, dead piglets (before and after birth) and congenitally infected piglets (Scortti et al., 2006). Moreover, live piglets have a higher birth weight and a higher survival rate than piglets from non-vaccinated nulliparous sows (Rowland, 2010). The use of MLV PRRS vaccine in multiparous sows infected with PRRSV effectively helps to reduce abortions and time to return to oestrus, increasing the birth rate and number of weaned piglets (Pejsak et al., 2006). During an acute PRRS outbreak or endemic infection, the use of attenuated live PRRS vaccines in growing pigs helps to reduce viral excretion and respiratory syndrome, also increasing the growth rate (Cano et al., 2007). The use of MLV in the field has been shown to be effective in the control of both reproductive and respiratory disease.
Currently, the greatest challenge related to the control of PRRSV is the heterologous protection provided by a given PRRS vaccine against the strains found on farms, as the virus presents high genetic and antigenic variability. Clinical heterologous protection can be defined as the clinical protection conferred by vaccine strains against the different field virus strains.
IMMUNE RESPONSE AGAINST PRRSV

It is very important to understand how immunity against PRRSV is developed in animals after vaccination in order to be able to design the best immunisation strategy. It is well known that PRRSV can strongly modulate the immune response, resulting in unusual characteristics in both humoral and cell component (Mateu and Díaz 2008; Darwich et al., 2010).

  

INNATE RESPONSE

Innate response generically recognises and responds to pathogens. PRRSV can act as an antagonist of the pig’s defence mechanisms in this initial phase. It can also interfere in the correct presentation of the antigen and T-cell activation. PRRSV modulation of the immune system is variable and depends on each strain.
HUMORAL ADAPTIVE RESPONSE

The adaptive response is specific to each antigen and is characterised by immunological memory. Humoral response to PRRSV is characterised by the early appearance of largely non-neutralising antibodies. Neutralising antibodies usually appear 2-4 weeks post-infection, but are occasionally not even detected (Díaz et al., 2012). Variability of both the quantity and time of appearance of neutralising antibodies has been described.
CELL ADAPTIVE RESPONSE

When cell-mediated response is evaluated using the ELISPOT assay (quantity of IFN-γ secreting cells (IFN-γ-SC)), it takes place 2-3 weeks post-infection. Its evolution is slow, erratic and of a low level compared to other porcine pathogens. The cell response generated against PRRSV appears to be a strain-dependent phenomenon (Díaz et al., 2012). In summary, adaptive cell response is weak and characterised by late, reduced production of neutralising antibodies and a poor cell-mediated response. In an environment poor in neutralising antibodies, cell-mediated response could be used to evaluate immune response after using PRRS vaccine and more or less effectively predict protection against the virus (Díaz et al., 2006, 2012; Lowe et al., 2005; Zuckerman et al., 2007).

Heterologous protection in PRRS vaccination

New post in prrscontrol.com. 

  
Despite the fact that live attenuated vaccines have demonstrated their efficacy both under experimental conditions and in field trials, their capacity to confer cross-protection against heterologous PRRSV strains is still one of the main concerns for swine farmers and veterinarians.

Unfortunately, homology between field virus and vaccine strain is not predictive for the vaccine’s efficacy. However, UNISTRAIN® PRRS has demonstrated clinical protection against both genotype I and II highly pathogenic PRRS challenges. Thus, heterologous protection provided by UNISTRAIN® PRRS is a key point for the control of PRRS in pigs.
One hallmark of PRRS in pigs is the extremely high genetic diversity of the virus. Immune cross-protection of commercial vaccines against this wide range of different PRRS field strains is one of the most controversial and widely-discussed issues between farmers, swine practitioner veterinarians and academic researchers.
According to different studies vaccine efficacy cannot be predicted by PRRS virus strain homology between vaccine and virus infection1 and 2. Despite the fact that mechanisms of immunogenicity and development of clinical protection through PRRS vaccines have not been clearly elucidated yet, some commercial vaccines have shown different degrees of heterologous protection even against field strains of different genotypes. The specific ability of the virus used in immunization to induce cellular immunity could be an important factor for heterologous protection.
VP-046-BIS PRRS strain, the vaccine virus of UNISTRAIN PRRS, has demonstrated a strong cellular heterologous immunogenicity both when it is administered via intramuscular route and intradermally. At the same time, UNISTRAIN PRRS has demonstrated partial clinical protection against highly pathogenic PRRS in pigs of genotype I subtype 3.
Moreover, VP-046-BIS PRRs strain also showed field clinical protection against genotype II PRRS virus infection and against highly pathogenic Chinese-like PRRS virus under experimental conditions3.
Although PRRS vaccines cannot provide full and universal protection against PRRS virus infection, using a vaccine with a wide heterologous clinical protection against multiple field virus strains from both genotypes is a key point for the control of PRRS in pigs.
References:
1. Opriessnig T. et al., 2005. Journal of Swine Health and Production. 13(5): 246-53. 

2. Prieto C. et al., 2008. Veterinary Journal. 175(3): 356-63.

3. Roca M. et al., 2012. Veterinary Journal. 193(1): 92-6.

Five key aspects of the immune response against PRRS virus infection in swine

Although the porcine reproductive and respiratory syndrome caused by PRRS virus was recognized over 20 years ago, some fundamental aspects of the immune response triggered after infection have not yet been elucidated.It seems clear that PRRS virus suppresses innate immunity, however the interactions between innate and adaptive immune responses still need to be elucidated:

  

One of the main limitations of the studies of PRRS virus infection in pigs at the cellular and molecular level, that partly explains the lack of basic knowledge of the immune response to PRRS virus, is the inability to establish a mouse model of infection, due to the fact that PRRS virus replication in common laboratory rodent species is inefficient1. On the other hand, infection with PRRS virus is widespread in the swine population, making it difficult to find uninfected animals with which to perform experiments aimed at investigating how the virus affects the immune system of the pig. Furthermore, pigs reared under conventional conditions are usually colonized by microorganisms that may eventually cause disease during an experimental infection as a result of the effect of PRRS virus on the immune system, thus invalidating the conclusions of the study.
Despite the above, some important aspects of the immune response developed by pigs to PRRS virus have been described, allowing an understanding of the behavior of the infection in vivo.
PRRS virus infects cells that have regulatory control: Unlike the influenza virus, which infects the respiratory tract and lungs of pigs, causing lesions as a direct result of its cytolytic effect on the epithelium, PRRS virus not only infects cells of the lung, causing pneumonia, but also infects cells of hematopoietic and lymphoid tissues. This triggers another type of disease mechanism and clinical manifestations, as a consequence of immune dysregulation. 

 

PRRS virus interferes with interferon induction: Interferons (INFs) are cytokines of primary importance in the innate immune response to virus infections. Expression of IFNs is induced upon recognition of viral molecular patterns by cellular cytoplasmic and endosomal receptors, or by cytokines such as IL-12. Based on the type of receptor through which they signal, interferons have been classified into types I, II and III, all being antiviral agents and immune modulators. It has been demonstrated that PRRS virus inhibits the synthesis of type I IFN-α in pigs2. IFN-α, like other type I IFNs, activates molecules which prevent the virus from producing and replicating its RNA. PRRS virus also fails to induce porcine plasmacytoid dendritic cells to produce IFN-α in vivo. Finally, the PRRS virus non–structural proteins Nsp1, Nsp2, and Nsp11, and the structural protein N (nucleocapsid protein) have been found to be antagonists of IFN induction.

 

PRRS virus fails to develop sterilizing immunity: Although anti-PRRS virus antibodies can be detected as early as 1 week after infection, and viral neutralizing antibodies (VNab) are detected at 4 weeks after infection, viremia can persist even in the presence of VNab, and can even be resolved before VNab appear. As a consequence, PRRSV infections are not resolved rapidly in piglets, and the carrier state may exist for up to 150 days. Nevertheless, the VNab are considered to be important in protecting swine against PRRS virus infection, although their mechanism of action has not yet been described. 

 

Different PRRS virus strains show differences in immune dysregulation: It is well known that PRRS viruses displays great genetic variability. Apart from the two major genotypes (types I and II), several subtypes and lineages with varying pathogenicity and biological characteristics have been described. It has been shown that PRRSV field isolates have a variable suppressive effect on IFN-α induction in PAM cultures. A2MC2, which is a novel PRRS virus strain, resulted in a higher level of neutralizing antibody after infection than an MLV vaccine strain that is highly homologous in sequence. 

  

PRRS virus affects both innate and adaptive immunity: It is known that PRRS virus is able to infect both the fetus in utero and the newborn piglet. This early infection interferes with innate immunity, by altering the function of immune subsets of cells and the induction of IFN. This not only inhibits the early host defense, but also alters the development of adaptive immunity in the grower? (growing? fattening?) pig. Besides this, the immune system of the young piglet is immature compared to the adult pig. When young, the piglet depends primarily on innate immunity and antibodies received during lactation (Figure 1). If the piglet cannot properly develop adaptive immunity as a consequence of PRRS virus infection, the infection is not controlled and the disease is more severe.