PF356 CHRONIC LYMPHOCYTIC LEUKEMIA INDUCES SKEWING OF (ANTIGEN‐SPECIFIC) T CELL RESPONSES TOWARDS A DYSFUNCTIONAL SHORT‐LIVED EFFECTOR PHENOTYPE
Authors: Martens A. , van Bruggen J. , Wensveen F. , Arens R. , Eldering E. , van der Windt G. , Kater A. 
Published: June 2019

Background:In recent years, CAR T cells showed to be highly effective in B cell malignancies such as acute lymphocytic leukemia. However, for chronic lymphocytic leukemia (CLL) efficacy of autologous T cell therapies is rather limited. T cells of CLL patients have a dysfunctional, terminally differentiated phenotype, which might be linked with the lack of success of T cell therapies. Recently it was shown that presence of memory T cell subsets in the CAR T cell infusion product associated with complete responses in CLL (Fraietta et al., 2018).Aims:It remains to be elucidated how CLL cells influence T cell skewing and whether this affects T cell responses in vivo. In this context, we functionally investigated acute T cell responses in the TCL1 mouse model, which features similar T cell dysfunction as observed in CLL patients (Gorgun et al., 2009). We applied a set‐up that allows to study the response against acute infection of (1) endogenous CD8+ T cells, which have been exposed to leukemic cells during CLL development, and (2) the antigen‐specific response of naïve, transferred OT‐I T cells.Methods:C57BL/6J (CD45.2+) mice were injected i.p. with 20x106 TCL1 splenocytes or PBS. When CD5+ leukemia reached 70% cells in blood, 50.000 naïve ovalbumin‐specific CD8+ T cells (OT‐I, CD45.1+) were injected i.v. Next, all mice were infected with mCMV‐OVA and sacrificed 7 days later. Splenocytes were analyzed directly or after in vitro ovalbumin stimulation by FACS.Results:(1) Already prior to infection, we found a skewing towards antigen‐experienced cells in the endogenous CD8+ compartment of leukemic mice. After mCMV‐OVA infection, a population of naïve CD8+ T cells persisted in control mice, but in leukemic mice the complete CD8+ compartment differentiated into effector cells. Within the effector compartment, we found a skewing to the short‐lived effector cell (SLEC) phenotype in the leukemic mice, while memory precursor effector cells (MPEC) were reduced. This skewing towards SLEC associated with increased effector‐associated T‐bet expression.(2) Analysis of the OT‐I cells allowed to perform detailed study of the impact of leukemia on an antigen‐specific response. Seven days after infection, spleens of leukemic and control mice contained similar frequencies of differentiated OT‐I cells, that had an effector phenotype. Also here, TCL1‐derived OT‐I cells showed a decreased percentage of MPEC, while SLECs were increased (Figure 1), indicating impaired memory formation. In correlation with skewing towards SLECs in leukemic mice, these OT‐I cells showed reduced expression of memory‐associated transcription factor Bcl6, while T‐bet was also increased. Earlier it was shown that Bcl6 is able to repress glycolysis, essential for T cell effector function (Oestreich et al., 2014). In line with reduced Bcl6 expression, we found increased glucose uptake in TCL1‐derived OT‐I cells. Despite the skewing towards SLEC phenotype, the TCL1‐derived OT‐I cells showed decreased production of IFNγ and TNFα and decreased degranulation (CD107a expression) upon in vitro restimulation.Summary/Conclusion:In this murine model of CLL, both bystander T cells as well as acute antigen‐specific T cell responses are driven towards an SLEC phenotype with signs of functional impairment, and metabolic alterations. This approach may provide crucial insight into the interplay between CLL and T cells, and the underlying failure of cancer immune surveillance.image