PCV2 vaccination induces IFN-γ/TNF-α co-producing T cells with a potential role in protection. Hanna C Koinig, Stephanie C Talker, Maria Stadler, Andrea Ladinig, Robert Graage, Mathias Ritzmann, Isabel Hennig-Pauka, Wilhelm Gerner and Armin Saalmüller. Veterinary Research 2015, 46:20. DOI 10.1186/s13567-015-0157-4
What are they studying?
Vaccines are widely used across the world for PCV2 control. However, it is not fully understood how PCV2 vaccines induce a protective immune response and which immune parameters correlate with protection. Viraemia is frequently detected in seropositive animals, proving that antibodies are not protective per se. Neutralizing antibodies seem to be important for the control of the virus. The cellular immune response for protection has been even less studied. The goal of the study was to find a correlate of protection against PCV2.
How is it done?
The experimental study was conducted with 24 piglets at the isolation unit of the University of Veterinary Medicine in Vienna. At the beginning of the trial 24 day-old piglets were divided into 4 treatment groups (6 animals per treatment group):
- Group 1 non-vaccinated, non-infected control group
- Group 2 infected intranasally at 49 days of age with PCV2
- Group 3 vaccinated with a PCV2 subunit vaccine adjuvanted with an aqueous polymer at 25 days of age
- Group 4 vaccinated with a PCV2 subunit vaccine adjuvanted with an aqueous polymer at 25 days of age and infected intranasally at 49 days of age.
Blood samples were taken at 21 days of age (arrival), at 25 days of age (before vaccination), at 49 days of age (before challenge) and at 53, 60, 67, 74 and 81 days of age. All animals were euthanized at 81 days of age.
What are the results?
No clinical signs of PCV2 were observed in any of the studied animals. Only animals of treatment group 2 (non-vaccinated, infected) became viraemic after infection. On the one hand, this shows that the infection model was successful. It also shows that the vaccine was effective in controlling the virus in treatment group 4 (vaccinated, infected).
A test that distinguishes between IgM and IgG was used for serology. 12 out of 24 pigs were seropositive at arrival. These were most likely maternally-derived antibodies. At 49 days of age 5 out of 12 vaccinated animals were seropositive. This seroconversion was considered to be vaccine-induced. After infection all animals of treatment group 2 and 4 seroconverted. However, animals in treatment group 4 (vaccinated, infected) seroconverted faster compared to treatment group 2 (non-vaccinated, infected). Also, they predominantly responded with IgG compared to a higher IgM response in treatment group 2 (non-vaccinated, infected).
Clearance of virus in infected pigs was earlier described as concurrent with the appearance of IFNγ secreting cells as an indicator for cellular immunity against PCV2. This study shows that the vaccine used induced multifunctional T-helper cells which for other pathogens seem to be a good correlate of protective cellular immune response. In treatment group 2 (non-vaccinated, infected) the appearance of the cells coincided with the switch in antibody types from IgM to IgG. This supports the assumption that multifunctional T-helper cells are critical for protective immune response against PCV2. Vaccination also induced memory T-cells.
What implications does this paper have?
The results of this study show that antibodies are not a good correlate of protection for PCV2. Although the vaccine did not induce a clear seroconversion, vaccinated animals were protected after vaccination. The fact that vaccinated pigs responded primarily with IgG after infection (secondary response) whereas non-vaccinated pigs responded initially with IgM (primary response) could be used in the field as an indicator of whether pigs have been vaccinated with the vaccine used in this study. But to do this it is critical to also determine the time of exposure to PCV2.
Vaccination did induce memory T cells. This supports and explains why single vaccination results in a relatively long duration of immunity.
The finding of multifunctional T helper cells as a potential correlate of protection after vaccination can help to understand how PCV2 vaccines work: the vaccine induces a strong cellular immune response, which supports and triggers an antibody response upon exposure.
The view from the field by Enric Marco
Problems caused by PMWS (Post-Weaning Wasting Syndrome), which lead to many economic losses in the sector in the first decade of this century, were solved with the arrival on the market of vaccines against PCV2. No one could have imagined that a vaccine would be so effective! As some producers declared: "After Aujeszky's disease vaccination, this is possibly the most effective vaccine to reach the market." However, circovirosis remains a disease that continues to generate questions, and one of them is how to interpret the serological tests routinely used to know the dynamics of the disease and to check that the prophylactic measures established are working properly. The first surprise with the arrival of vaccines against PCV2 was the realization that not all of them generated an immune response after application, or at least not sufficiently consistent, and this was the type of arguments used to reinforce some products in detriment of others. However, over time we all learned to detract from this information when we realized that, in spite of not having a consistent post-vaccination serological reaction, vaccinated animals behaved in a completely different way from unvaccinated animals under field conditions. The article explains why vaccines offer protection, specifically because they are able to generate a powerful cellular immunity, capable even of limiting replication totally (as in the article) or partially, as it usually happens. And also, it provides important information that allows us to interpret the results of the lab tests commonly used, such as the detection of specific IgG and IgM. For vaccinated animals, the vaccine may generate a weak and transient elevation of IgM (at 3 weeks post-vaccination) and IgG, albeit in a small proportion of animals. However, when the vaccinated pigs become infected with the field virus, the increase of IgG is much clearer. Ultimately, such tests are useful to monitor the dynamics of the infection, but they are not a useful tool to check whether vaccination has been properly applied.
Unfortunately, we still have no techniques available that provide an easy and inexpensive way to monitor the cellular immune response, specifically the one responsible for the production of INF-γ that, in this case, appears to be directly related to the reduction in viral load.