CD8⁺ T-cell recognition of human cytomegalovirus latency-associated determinant pUL138

Recent studies have shown that long-term persistence of human cytomegalovirus (HCMV) in mononuclear cells of myeloid lineage is dependent on the UL138 open reading frame, which promotes latent infection. Although T-cell recognition of protein antigens from all stages of lytic HCMV infection is well established, it is not clear whether proteins expressed during latent HCMV infection can also be recognized. This study conducted an analysis of T-cell response towards proteins associated with HCMV latency. Ex vivo analysis of T cells from healthy virus carriers revealed a dominant CD8⁺ T-cell response to the latency-associated pUL138 protein, which recognized a non-canonical 13 aa epitope in association with HLA-B*3501. These pUL138-specific T cells displayed a range of memory phenotypes that were in general less differentiated than that previously described in T cells specific for HCMV lytic antigens. Antigen-presentation assays revealed that endogenous pUL138 could be presented efficiently by HCMV-infected cells. However, T-cell recognition of pUL138 was dependent on newly synthesized protein, with little presentation from stable, long-lived protein. These data demonstrate that T cells targeting latency-associated protein products exist, although HCMV may limit the presentation of latent proteins, thereby restricting T-cell recognition of latently infected cells.