Monocytic MDSCs homing to thymus contribute to age-related CD8(+) T cell tolerance of HBV

HBV exposure in children usually causes chronic infection, and HBsAg-specific CD8(+) T cells are rarely detected in this situation. We find that mMDSCs, cross-presenting HBsAg, migrate to the thymus and eliminate HBsAg-specific CD8(+) thymocytes, resulting in a specific tolerance to HBV. Hepatitis B virus exposure in children usually develops into chronic hepatitis B (CHB). Although hepatitis B surface antigen (HBsAg)-specific CD8(+) T cells contribute to resolve HBV infection, they are preferentially undetected in CHB patients. Moreover, the mechanism for this rarely detected HBsAg-specific CD8(+) T cells remains unexplored. We herein found that the frequency of HBsAg-specific CD8(+) T cells was inversely correlated with expansion of monocytic myeloid-derived suppressor cells (mMDSCs) in young rather than in adult CHB patients, and CCR9 was upregulated by HBsAg on mMDSCs via activation of ERK1/2 and IL-6. Sequentially, the interaction between CCL25 and CCR9 mediated thymic homing of mMDSCs, which caused the cross-presentation, transferring of peripheral HBsAg into the thymic medulla, and then promoted death of HBsAg-specific CD8(+) thymocytes. In mice, adoptive transfer of mMDSCs selectively obliterated HBsAg-specific CD8(+) T cells and facilitated persistence of HBV in a CCR9-dependent manner. Taken together, our results uncovered a novel mechanism for establishing specific CD8(+) tolerance to HBsAg in chronic HBV infection.