Fate of hepatitis D virus-specific CD8+ T cells during bulevirtide monotherapy in patients with chronic hepatitis...

Bulevirtide reduces viremia in chronic hepatitis D virus (HDV) infection, but long-term treatment is required in most patients to prevent relapse. Sustained treatment responses may be fostered by therapy-induced amelioration of HDV-specific CD8+ T-cell responses that are exhausted because of high viremia and antigen loads during chronic HDV infection. We therefore studied the effect of bulevirtide monotherapy on HDV-specific CD8+ T-cell repertoire, phenotype, and functionality.

Twenty-eight patients with HDV infection and cirrhosis starting bulevirtide treatment were followed for 40-120 weeks on treatment. HLA class I typing and HDV sequencing was performed. HDV-specific CD8+ T cells were analyzed longitudinally using optimal epitopes and overlapping peptides spanning the L-HDAg. Ex vivo high-dimensional flow cytometry analysis of peptide/HLA class I tetramer+ CD8+ T cells was performed longitudinally in selected patients.

In 42% of patients, an HDV-specific CD8+ T-cell response was detectable at baseline, but did not substantially increase during treatment. Importantly, a large majority of HDV-specific CD8+ T-cell responses targeted viral epitopes with sequence variations consistent with viral escape mutations. Only one HLA-B∗35-restricted HDV-specific CD8+ T-cell epitope was conserved and continuously targeted throughout therapy. HDV-specific CD8+ T cells targeting this conserved epitope displayed a predominant terminally exhausted phenotype at baseline that shifted longitudinally to a memory-like phenotype with suppression of HDV load.

Albeit based on small patient numbers, antiviral treatment with bulevirtide could ameliorate exhausted HDV-specific CD8+ T cells targeting conserved HDV epitopes. However, as the majority of HDV-specific CD8+ T-cell responses target escaped viral epitopes, this does not translate to a substantial improvement in overall HDV-specific CD8+ T-cell immunity. Our findings may explain in part why long-term bulevirtide treatment is required to prevent viral relapse.

Bulevirtide can control viremia and normalize liver enzymes in a majority of patients with chronic hepatitis delta, but sustained cure with negative HDV RNA even after therapy is difficult to achieve. Restoration of functional HDV-specific CD8+ T cells attributable to reduced antigen loads may prevent viral relapse. We demonstrate here that amelioration of HDV-specific CD8+ T cells does indeed occur, but is limited to the few HDV-specific CD8+ T-cell responses targeting conserved viral epitopes. A majority of HDV-specific CD8+ T cells no longer recognize their target antigens because of mutational viral escape. These findings explain the need for long-term treatment continuation even once HDV RNA is negative, to prevent viral relapse.