Differential Replication and Cytokine Response between Vaccine and Very Virulent Marek's Disease Viruses in Spleens and Bursas during Latency and Reactivation

Abstract

Marek's disease virus (MDV) infection results in Marek's disease (MD) in chickens, a lymphoproliferative and oncogenic deadly disease, leading to severe economic losses. The spleen and bursa are the most important lymphoid and major target organs for MDV replication. The immune response elicited by MDV replication in the spleen and bursa is critical for the formation of latent MDV infection and reactivation. However, the mechanism of the host immune response induced by MDV in these key lymphoid organs during the latent and reactivation infection phases is not well understood. In the study, we focused on the replication dynamics of a vaccine MDV strain MDV/CVI988 and a very virulent MDV strain MDV/RB1B in the spleen and bursa in the latent and reactivation infection phases (7-28 days post-inoculation [dpi]), as well as the expression of some previously characterized immune-related molecules. The results showed that the replication ability of MDV/RB1B was significantly stronger than that of MDV/CVI988 within 28 days post-infection, and the replication levels of both MDV strains in the spleen were significantly higher than those in the bursa. During the latent and reactivation phase of MDV infection (7-28 dpi), the transcriptional upregulation of chicken IL-1β, IL6, IL-8L1 IFN-γ and PML in the spleen and bursa induced by MDV/RB1B infection was overall stronger than that of MDV/CVI988. However, compared to MDV/RB1Binfection, MDV/CVI988 infection resulted in a more effective transcriptional activation of CCL4 in the latent infection phase (7-14 dpi), which may be a characteristic distinguishing MDV vaccine strain from the very virulent strain.

Keywords: Marek’s disease virus; cytokines; lymphoid organs; virus duplication.

Figure 1

MDV/CVI988 and MDV/RB1B DNA replication in spleen and bursa. (A): MDV/CVI988 and MDV/RB1B DNA replication in spleen; (B): MDV/CVI988 and MDV/RB1B DNA replication in bursa; (C): MDV/RB1B DNA replication in spleen and bursa; (D): MDV/CVI988 DNA replication in spleen and bursa. The statistical results are shown as the means ± SD of each experimental group. Statistical significance between experimental groups was assessed by t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significance).

Figure 2

IFN-β, IFN-γ, and PML transcripts in the spleen inoculated with MDV/CVI988 and MDV/RB1B. (A): Specific IFN-β transcripts in the spleen; (B): specific IFN-γ transcripts in the spleen; (C): specific PML transcripts in the spleen. The statistical results are shown as the means ±SD of each experimental group. Statistical significance between each two experimental groups (CVI988 vs. MOCK, RB1B vs. MOCK, CVI988 vs. RB1B) was assessed by t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significance).

Figure 3

IFN-β, IFN-γ, and PML transcripts in the bursa inoculated with MDV/CVI988 and MDV/RB1B. (A): Specific IFN-β transcripts in the bursa; (B): specific IFN-γ transcripts in the bursa; (C): specific PML transcripts in the bursa. The statistical results are shown as the means ± SD of each experimental group. Statistical significance between each two experimental groups (CVI988 vs. MOCK, RB1B vs. MOCK, CVI988 vs. RB1B) was assessed by t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significance).

Figure 4

IL-1β, IL-6, IL-8L1, andCCL4 transcripts in the spleen inoculated with MDV/CVI988 and MDV/RB1B. (A): Specific IL-1β transcripts in the spleen; (B): specific IL-6 transcripts in the spleen; (C): specific IL-8L1 transcripts in the spleen; (D): specific CCL4 transcripts in the spleen. The statistical results are shown as the means ± SD of each experimental group. Statistical significance between each two experimental groups (CVI988 vs. MOCK, RB1B vs. MOCK, CVI988 vs. RB1B) was assessed by t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significance).

Figure 5

IL-1β, IL-6, IL-8L1, andCCL4 transcripts in the bursa inoculated with MDV/CVI988 and MDV/RB1B. (A) Specific IL-1β transcripts in the bursa; (B) specific IL-6 transcripts in the bursa; (C) specific IL-8L1 transcripts in the bursa; (D) specific CCL4 transcripts in the bursa. The statistical results are shown as the means ± SD of each experimental group. Statistical significance between each two experimental groups (CVI988 vs. MOCK, RB1B vs. MOCK, CVI988 vs. RB1B) was assessed by t-test (* p < 0.05; ** p < 0.01; *** p < 0.001; ns, no significance).