Myeloid-Derived Suppressor Cells As Novel Biomarkers in Patients with Chronic Lymphocytic Leukemia Treated with Ibrutinib

Introduction: Ibrutinib, a Bruton's tyrosine kinase inhibitor, is a therapeutic agent commonly used for the treatment of chronic lymphocytic leukemia (CLL) patients, with a proved beneficial effect against chemotherapy. Myeloid-derived suppressor cells (MDSCs) are HLA-DR^low/-CD11b⁺CD33⁺ immunomodulatory cells with prominent T-cell suppressive activity. They are increased in neoplastic diseases and their numbers and functionality are correlated with worse disease outcome and poor treatment response. They are further divided in CD15⁺ polymorphonuclear cells (PMN-MDSCs) and CD14⁺ monocytic (M-MDSCs) subpopulations. In this study we aimed to investigate the number and the properties of PMN-MDSCs and M-MDSCs in CLL patients before and after Ibrutinib treatment.

Materials and Methods: From four centers in Greece, 30 Rai stage III/IV CLL patients with indication for treatment, 38 Rai stage 0-II CLL patients with no indication for treatment and 36 age and sex-matched healthy controls were enrolled. Twenty patients from the first group successfully completed the study, i.e. were evaluated pre and after ibrutinib treatment initiation, between six to 24 months under therapy. PMN- and M-MDSCs were quantitated by flow cytometry in the PB mononuclear cell (PBMC) fraction and analysis was performed with the Kaluza software. Their suppressive capacity before and after ibrutinib treatment was assessed with a T-cell suppression assay (2 untreated patients, 3 patients under ibrutinib treatment, 4 healthy controls). More specifically, after three days of culture, the proliferation of T-cells was evaluated in the presence and absence of autologous MDSCs. Statistical analysis was performed with the GraphPad software.

Results: The percentage of M-MDSCs in total CD14⁺ cells was 46,12% ± 18,95% (mean ± standard deviation) in CLL patients pre-treatment and 18,95% ± 12,87% after ibrutinib initiation, while the percentage of PMN-MDSCs in total CD15⁺ cells was 83,20% ± 12,26% in CLL patients pre-treatment and 77,59% ± 19,46% after ibrutinib initiation. When conducting paired analysis in individuals with measurements in both time points, the difference was statistically significant for both sub-populations (Wilcoxon matched-pairs signed rank test, p<0,0001 for M-MDSCs and p=0,0215 for PMN-MDSCs). The difference of both sub-populations was significant (p<0,0001, Kruskal Wallis non-parametric test) among the different groups of the study, i.e. healthy controls, stage 0-II CLL untreated patients, stage III/IV CLL patients pre ibrutinib treatment, CLL stage III/IV CLL patients after ibrutinib initiation. MDSCs were capable of suppressing T-cell proliferation. Importantly, the percentage of suppression tended to be higher in patients pre ibrutinib treatment (61,8% ± 12,1%), compared to patients that were receiving ibrutinib (31,2% ± 12,4%), and healthy controls (22,9% ± 11,7%). However, a larger sample would be needed to extract statistically significant results.

Conclusions: The proportions of MDSC subsets are higher in CLL patients compared to healthy controls. MDSC counts are higher in patients of advanced stage compared to those of early stages. After treatment with Ibrutinib a significant reduction in the populations of MDSCs is observed. The results of our study, highlight the importance of MDSCs in the course of the disease in CLL and their potential role as biomarkers indicating disease severity and monitoring treatment response and relapse.

Papadaki:x4 pharmaceutical company: Honoraria, Membership on an entity's Board of Directors or advisory committees.