Treatment with Venetoclax Positively Modulates Immunological Parameters and T-Cell Features in Patients with Chronic Lymphocytic Leukemia

Background: chronic lymphocytic leukemia (CLL) is characterized by a wide range of immune defects, which contribute to immune evasion of cancer cells and emerge into clinically relevant conditions, such as high risk of infections, occurrence of autoimmune manifestations and lower response to vaccination. Venetoclax is a selective Bcl-2 antagonist that increases leukemic cell apoptosis and has clinical efficacy in treatment-naïve and relapsed patients with CLL. The role of venetoclax in modulating the immune environment has not yet been fully clarified, and available data are limited to the effects on restoration of normal immune cell counts and lowering the number of PD-1+ CD8+ T cells. Aims: the general aim of this study is to evaluate the impact of venetoclax-based treatment on the immune system of patients with CLL, particularly focusing on phenotypic and functional characteristics of T cells. Methods: we included 16 patients with progressive CLL and eligible to therapy with venetoclax +/- rituximab. Peripheral blood samples were collected at the time of treatment start and after 1, 6 and 12 months of therapy. The percentage and the phenotypic features of T-cell populations were assessed by flow cytometry. Peripheral blood mononuclear cells were collected from 17 patients who received the SARS-CoV-2 vaccine after at least 12 cycles of venetoclax plus rituximab (VenR) fixed-duration treatment. T-cell response to SARS-CoV-2 spike peptide pool was evaluated by IFNγ ELIspot. Results: after one month of venetoclax administration we observed a significant decrease in the absolute number of CD3+, CD4+ and CD8+ T cells, which also persisted in the following evaluated timepoints (i.e. 6 and 12 months). In terms of differentiation subsets distribution, venetoclax treatment favoured the accumulation of naïve T cells, which was accompanied by a parallel, significant reduction of the effector memory T-cell fraction, in both the CD4+ and CD8+ compartments and at all analysed timepoints. Venetoclax treatment also resulted in a reduced expression of inhibitory immune checkpoints on T cells. Indeed, in all the CD4+ differentiation subsets (i.e. naïve, central memory, effector memory, terminally differentiated effector memory T cells) we observed a significant decrease in the proportion of LAG3+ and TIGIT+ cells, which was already evident after one month of treatment and persisted in the following timepoints. The downregulation of PD-1 expression appeared later (i.e. after 6 and 12 months of ongoing treatment with venetoclax) and was confined to the CD4+ naïve and central memory T-cell subsets. Within the CD8+ T-cell compartment, we observed a reduced proportion of TIGIT+ cells across all the differentiation subsets and at all analysed timepoints. By contrast, venetoclax therapy did not affect the expression of TIM3 and BTLA on CD4+ and CD8+ T cells. From the functional standpoint, we observed that 8 out of 17 (47%) patients who received the COVID-19 vaccine after at least 12 cycles of VenR fixed-duration treatment were capable to generate a T-cell response toward the SARS-CoV-2 spike protein, as demonstrated by the increased production of IFNγ following cell stimulation with a spike peptide pool. Conclusion: our results show that venetoclax treatment exerts meaningful immunomodulatory effects on T-cell features, shifting the balance toward less differentiated T-cell subsets and strongly attenuating features of exhaustion, such as the expression of checkpoint molecules. Further analysis on a wider cohort of patients, also evaluating the effects of venetoclax treatment on regulatory T cells and innate immunity, are currently ongoing and will be available for the time of data presentation. This study was conducted with the contribution of Abbvie Srl.