Marek's disease virus (MDV) causes T cell immunosuppression in chickens during latent infection. Morphological changes specific to apoptosis were demonstrated in peripheral blood mononuclear cells (PBMC) of MDV-infected chickens at 2–3 weeks post-inoculation (p.i.). Analysis of DNA fragmentation in T cell subsets in the peripheral blood revealed that CD4 T cells but not CD8 T cells underwent apoptosis after MDV infection. The proportion of CD4 T cells, but not that of CD8 T cells, in the peripheral blood expanded transiently at 16 days p.i., and rapidly decreased 1 week later. The decrease in CD4 T cells might be mediated by apoptosis, because a rapid reduction in CD4 T cells was observed when these cells underwent apoptosis. Analysis of the T cell-receptor (TCR) repertoire of the peripheral blood showed that Vβ1 but not Vβ2-αβ TCR-bearing cells expanded at 16 days p.i., when the transient expansion of the CD4 T cell population was observed in these chickens. Flow cytometric profiles also showed that the expression of CD8 was down-regulated after infection with MDV, but there was no difference in the expression level of CD4 molecules between normal and infected chickens. Northern blot analysis indicated that the down-regulation of CD8 occurred at the transcriptional level. These results suggest that both apoptosis of CD4 T cells and down-regulation of CD8 molecules could contribute to the immunosuppression caused by MDV.


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