High-frequency and activation of CD4(+)CD25(+) T cells maintain persistent immunotolerance induced by congenital ALV-J infection

Congenital avian leukosis virus subgroup J (ALV-J) infection can induce persistent immunotolerance in chicken, however, the underlying mechanism remains unclear. Here, we demonstrate that congenital ALV-J infection induces the production of high-frequency and activated CD4(+)CD25(+) Tregs that maintain persistent immunotolerance. A model of congenital infection by ALV-J was established in fertilized eggs, and hatched chicks showed persistent immunotolerance characterized by persistent viremia, immune organ dysplasia, severe imbalance of the ratio of CD4(+)/CD8(+) T cells in blood and immune organs, and significant decrease in CD3(+) T cells and Bu-1(+) B cells in the spleen. Concurrently, the mRNA levels of IL-2, IL-10, and IFN-γ showed significant fluctuations in immune organs. Moreover, the frequency of CD4(+)CD25(+) Tregs in blood and immune organs significantly increased, and the frequency of CD4(+)CD25(+) Tregs was positively correlated with changes in ALV-J load in immune organs. Interestingly, CD4(+)CD25(+) Tregs increased in the marginal zone of splenic nodules in ALV-J-infected chickens and dispersed to the germinal center. In addition, the proliferation and activation of B cells in splenic nodules was inhibited, and the number of IgM(+) and IgG(+) cells in the marginal zone significantly decreased. We further found that the mRNA levels of TGF- β and CTLA-4 in CD4(+)CD25(+) Tregs of ALV-J-infected chickens significantly increased. Together, high-frequency and activated CD4(+)CD25(+) Tregs inhibited B cells functions by expressing the inhibitory cytokine TGF-β and inhibitory surface receptor CTLA-4, thereby maintaining persistent immunotolerance in congenital ALV-J-infected chickens.