Terminal differentiation program of CD8 T cells guides immunotherapeutic options for CLL patients

Abstract Chronic lymphocytic leukemia (CLL) remains incurable and autologous T-cell-based therapies induce durable remissions in only a minority of patients. Although poor T-cell performance in CLL has been attributed to exhaustion, T cells from CLL patients retain cytokine production and lack key transcriptional markers of exhaustion, suggesting an alternative state of differentiation. Here, we performed an integrated phenotypic, epigenetic and functional analysis of circulating CD8⁺ T cells from treatment-naive CLL patients to define the nature and therapeutic relevance of terminal differentiation in CLL. CD8⁺ T cells from treatment-naive CLL patients were enriched for a terminally differentiated, CD27⁻CD28⁻CD57⁺KLRG1⁺ population with several senescence-associated features, including increased cell size, granularity, β-galactosidase activity, and reduced basal autophagy. However, these cells lacked key hallmarks of irreversible senescence. Instead, CLL T cells mounted robust but transient proliferative bursts and developed activation-associated γH2AX signals followed by impaired persistence and increased cell death upon repeated activation. Especially CD57⁺ T cells were vulnerable and incompatible with the experimental steps required for CAR T-cell manufacturing. The potent, acute effector function of these cells could, however, be harnessed by bispecific T-cell engagers, which induced effective cytolysis of both cell lines and primary CLL targets by these T cells. Collectively, our data support a model in which CD8+ T-cell dysfunction in CLL reflects accumulation of short-lived, terminally differentiated effector cells rather than canonical exhaustion or irreversible senescence. This state limits durable persistence required for CAR T-cell approached while preserving acute cytotoxicity relevant for bispecific antibody-based therapies in CLL.