Nanoscale analysis reveals distinct cytoskeletal architecture and mechanical properties in Chronic Lymphocytic Leukemia cells

Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in the Western world, with an incidence of 5 new cases per 100.000 individuals annually. The disease affects mainly elderly people, however one third of the new cases are diagnosed before the age of 55 years. CLL is an incurable ailment characterised by an intense trafficking and accumulation of mature CD5+ B lymphocytes in peripheral blood, bone marrow (BM) and secondary lymphoid organs (spleen and lymph nodes). It is known that the ability of lymphocytes to recirculate strongly depends on their capability to rapidly rearrange their cytoskeleton and adapt to external cues, however little is known about the differences occurring between CLL and healthy B cells (HB) in this context. Particularly, HB cells can reorganize completely their architecture; an ability depending on their structural plasticity, which facilitates B cells migration and communicative function. To investigate this point, we performed a comparison between primary healthy, leukemic B cells by applying single cells optical (super resolution microscopy), and nanomechanical approaches (Atomic Force Microscopy, Real Time Deformability Cytometry). We demonstrated that CLL cells have a specific nanoscale actomyosin complex organisation and altered mechanical properties in comparison to their healthy counterpart. To evaluate the clinical relevance of our findings we treated the cells in vitro with the BTK inhibitor ibrutinib and we found for the first time that the drug restores the CLL cells mechanical properties towards the healthy phenotype and activates the actomyosin complex in a BCR independent manner. We further validated these results in vivo on CLL cells isolated from patients undergoing ibrutinib treatment. Considering that the majority of CLL patients develop a resistance to ibrutinib, we analysed a resistant case, and we observed a return to the pre-treatment properties. Our results suggest that CLL cells' mechanical properties are linked to their actin cytoskeleton organisation and that could be involved in unknown mechanisms of drug resistance thus becoming a new potential therapeutic target.