Onset and long-term duration of immunity provided by a single vaccination with recombinant a Marek's disease virus with REV-LTR insertion

Abstract

Marek's Disease (MD), caused by Marek's disease virus (MDV), is a highly contagious lymphoproliferative disease in poultry. Despite the fact that MD has been effectively controlled by vaccines, the virulence of field isolates of MDV has continued to evolve, becoming more virulent under the immune pressure of vaccines. Our previous research has confirmed that the recombinant rMDV strain with REV-LTR insertion can be used as a live attenuated vaccine candidate. The aim of this research was to evaluate the onset and duration of immunity of the rMDV strain through two experiments. In both experiments, 1-day-old SPF chickens were vaccinated subcutaneously with the rMDV strain at a dose of 3,000 Plaque Formation Unit (PFU) per chick in 0.2 mL of the MD diluent. Then, in Experimental design 1, the chicks in the groups Vac-3d/CC-3d, Vac-5d/CC-5d, and Vac-7d/CC-7d were challenged separately with 500 PFU vvMDV strain MD5 at 3 days, 5 days, and 7 days after vaccination; in Experimental design 2, the chicks in group Vac-60d/CC-60d, Vac-120d/CC-120d, and Vac-180d/CC-180d were challenged at 60 days, 120 days, and 180 days after vaccination. The clinical symptoms and weight gain of chickens in each group were observed and recorded. The results showed that the rMDV strain with REV-LTR insertion provides protection starting from 3 days of age and achieves good immune effects at 5 days of age after 1-day-old immunization, and the immunization duration can reach for at least 180 days. Given age-related resistance, it can be confirmed that our vaccine can actually provide lifelong immunity. This study provides valuable insights into the onset and duration of immunity of the rMDV strain, which will provide a basis for the development and improvement of MD vaccines.

Keywords: Marek’s disease virus; REV-LTR; duration of immunity; onset of immunity; vaccine.

Figure 1

Weight gain of experimental SPF chickens. The chicks in Vac-3d, Vac-5d and Vac-7d groups were inoculated subcutaneously with rMDV strain in the back and neck at the dose of 3,000 PFU at 1 day of age, whereas chicks in CC-3d-4 were inoculated MD diluent. At 3, 5 and 7 days after vaccination, the chicks in group Vac-3d/CC-3d (A), Vac-5d/CC-5d (B) and Vac-7d/CC-7d (C) were challenged separately via intra-abdominal injection with 500 PFU MD5. The body weight of the chickens in the different groups were measured separately at 14, 28, 42 and 60 days post last challenge (namely 21, 35, 49 and 67 days old). Results represent mean value with error bars representing standard error of the mean. The mean body weights for each group were compared using two-way ANOVA and the significant differences were marked on the top of the columns. Significant differences are indicated by “*” (p < 0.05), “**” (p < 0.01), “****” (p < 0.0001).

Figure 2

Survival curves of rMDV-vaccinated and unvaccinated SFP chickens after challenge of vvMDV MD5 strain. At 3 days (A), 5 days (B) and 7 days (C) post inoculation (dpi), the chicks in group Vac-3d/CC-3d, Vac-5d/CC-5d and Vac-7d/CC-7d were challenged separately via intra-abdominal injection with 500 PFU MD5, and then observed for 60 days. A group of unvaccinated and unchallenged chickens (Control-A) were kept under the same conditions and taken as a healthy control.

Figure 3

Relative ratio of organ weights compared to body weights. At 60 days post-challenge, all the surviving chickens in Vac-3d/CC-3d, Vac-5d/CC-5d, Vac-7d/CC-7d and healthy control Control-A were sacrificed for necropsy after weighing. The hearts, livers, spleens and bursa were collected and weighed. The relative ratio of organ to body weight was determined as follows: $\mathit{\text{Relative}}\mathit{\text{ratio}}=\frac{\text{organ}/\text{body weight of each chicken}}{\text{the mean value of organ}/\text{body weight of the control group}}\times 100\%$. Results represent mean value with error bars representing standard error of the mean. The values for each group were compared using two-way ANOVA and the significant differences were marked on the top of the columns. Significant differences are indicated by “*” (p < 0.05), “***” (p < 0.001), **** (p < 0.0001).

Figure 4

Survival curves of rMDV-vaccinated and unvaccinated old SFP chickens after challenge of vvMDV MD5 strain. At 60 days (A), 120 days (B) and 180 days (C) post inoculation (dpi), the chicks in group Vac-60d/CC-60d, Vac-120d/CC-120d and Vac-180d/CC-180d were challenged separately via intra-abdominal injection with 500 PFU MD5, and then observed for 60 days. A group of unvaccinated and unchallenged chickens (Control-B) were kept under the same conditions and taken as a healthy control.

Figure 5

Weight of chicken carcass of each group at 120, 180 and 240 days of immunization duration. Results represent mean value with error bars representing standard error of the mean. The values for each group were compared using two-way ANOVA and the significant differences were marked on the top of the columns. Significant differences are indicated by “***” (p < 0.01).