Cytokine responses and inducible nitrous oxide synthase expression patterns in neonatal chicken brain microglia infected with very virulent Marek's disease virus strain YL040920

Purified and enriched brain microglia from neonatal chickens were infected with live Marek's disease virus (MDV)—both the very virulent (vv) YL040920 strain and the attenuated vaccine strain CVI988/Rispens in vitro. Although YL040920-infected microglia showed lower viral DNA loads compared with those infected with CVI988/Rispens at the same infectious dose (400 plaque-forming units for each), no significant differences in IFN-γ and IL-12p35 transcription were detected between the two MDV strains. Chicken microglia infected with live or fixed YL040920 expressed dramatically higher levels of IL-12p40, IL-8, and macrophage inflammatory protein-1β (MIP-1β) transcripts compared with those infected with CVI988/Rispens. On the other hand, CVI988/Rispens induced significantly higher levels of IFN-β transcription than YL040920, especially the live virus. Inducible nitric oxide (NO) synthase (iNOS) transcription and NO production correlated with levels of both YL040920 and CVI988/Rispens live strain infection. Moreover, fixed MDVs induced higher levels of iNOS/NO than live viruses, especially with CVI988/Rispens. This study demonstrates that chicken microglial cells can become infected with live YL040920 and CVI988/Rispens and that microglia represent cellular sources of IL-12p40, IL-12p35, IFN-γ, IFN-β, IL-8, MIP-1β, iNOS mRNA, and NO expression after MDV infection in vitro. Transcription levels of IL-12p35 and IFN-γ were associated with MDV DNA replication, whereas transcription levels of IL-12p40, IFN-β, IL-8, and MIP-1β were associated with both MDV DNA replication and expression of viral specific genes. The transcription of iNOS was responsible for expression of viral specific genes, whereas it was suppressed by viral DNA replication during infection. Although YL040920, compared with CVI988/Rispens, induced similar levels of the typical Th1-type cytokine IFN-γ in microglia, vvMDV induced significant increases in other cytokines [IL-12 (p40 and 12p35), IL-8, and MIP-1β]. More detailed investigation, as well as in vivo testing of the effects of vvMDV infection on Th1 responses, iNOS expression, and NO production in the brain of chickens should be undertaken.