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CD4+ T cells are typically considered as ‘helper’ or ‘regulatory’ populations that support and orchestrate the responses of other lymphocytes. However, they can also develop potent granzyme (Gzm)-mediated cytotoxic activity and CD4+ cytotoxic T cells (CTLs) have been amply documented both in humans and in mice, particularly in the context of human chronic infection and cancer. Despite the established description of CD4+ CTLs, as well as of the critical cytotoxic activity they exert against MHC class II-expressing targets, their developmental and memory maintenance requirements remain elusive. This is at least in part owing to the lack of a murine experimental system where CD4+ CTLs are stably induced. Here, we show that viral and bacterial vectors encoding the same epitope induce distinct CD4+ CTL responses in challenged mice, all of which are nevertheless transient in nature and lack recall properties. Consistent with prior reports, CD4+ CTL differentiation is accompanied by loss of TCF-1 expression, a transcription factor considered essential for memory T cell survival. Using genetic ablation of Tcf7, which encodes TCF-1, at the time of CD4+ T cell activation, we further show that, contrary to observations in CD8+ T cells, continued expression of TCF-1 is not required for CD4+ T cell memory survival. Whilst Tcf7-deficient CD4+ T cells persisted normally following retroviral infection, the CD4+ CTL subset still declined, precluding conclusive determination of the requirement for TCF-1 for murine CD4+ CTL survival. Using xenotransplantation of human CD4+ T cells into murine recipients, we demonstrate that human CD4+ CTLs develop and persist in the same experimental conditions where murine CD4+ CTLs fail to persist. These observations uncover a species-specific defect in murine CD4+ CTL persistence with implications for their use as a model system.

Chronic lymphocytic leukemia (CLL) is a malignancy of CD19+CD5+ B cells that, despite the development of new targeted drugs is considered to be incurable. The only treatment that has shown curative potential in CLL patients is allogeneic stem cell transplantation (allo-SCT). Although most CLL patients are too frail for allo-SCT, this indicates the immune system harbors the potential to cure CLL patients. Novel immunotherapeutic strategies have been developed that aim to recruit immune cells to treat cancer patients. In CLL, most of these strategies show disappointing results, which is probably related to the immunosuppressive tumor micro-environment. This thesis investigates the functionality of two important effector cells for immunotherapy, T cells and natural killer (NK) cells, in both healthy individuals and CLL patients, in order to determine their potential use for immunotherapy in CLL. We show that NK cells of CLL patients are hypo-responsive, but can still be induced to initiate anti-tumor responses via ADCC mechanisms. By comparing virus-specific T cell populations, we were able to unravel the dysfunctional phenotype of T cells in CLL patients in detail, thereby defining what restrictions T cells may have when being used in immunotherapeutic strategies. Finally, we also studied the effects of small molecule inhibitors both in vitro and in vivo in order to analyze their effects on non-malignant lymphocytes. We find that many immunomodulatory effects of CLL on the immune system are reversible, which indicates that successful first-line treatment of CLL patients may open the door for immunotherapy as a consolidation treatment.