Bim-mediated deletion of antigen-specific CD8+ T cells in patients unable to control HBV infection

Abstract

HBV-specific CD8⁺ T cells are critical for a successful immune response to HBV infection. They are markedly diminished in number in patients who fail to control the virus, but the mechanisms resulting in their depletion remain ill defined. Here, we dissected the defective HBV-specific CD8⁺ T cell response associated with chronic HBV infection by gene expression profiling. We found that HBV-specific CD8⁺ T cells from patients with different clinical outcomes could be distinguished by their patterns of gene expression. Microarray analysis revealed that overlapping clusters of functionally related apoptotic genes were upregulated in HBV-specific CD8⁺ T cells from patients with chronic compared with resolved infection. Further analysis confirmed that levels of the proapoptotic protein Bcl2-interacting mediator (Bim) were upregulated in HBV-specific CD8⁺ T cells from patients with chronic HBV infection. Blocking Bim-mediated apoptosis enhanced recovery of HBV-specific CD8⁺ T cells both in culture and directly ex vivo. Consistent with evidence that Bim mediates apoptosis of CD8⁺ T cells expressing low levels of CD127 (IL-7R), the few surviving HBV-specific CD8⁺ T cells were CD127^hi and had elevated levels of the antiapoptotic protein Mcl1, suggesting they were amenable to IL-7–mediated rescue from apoptosis. We therefore postulate that Bim-mediated attrition of HBV-specific CD8⁺ T cells contributes to the inability of these cell populations to persist and control viral replication.