Dasatinib Has a Dual Mode of Action: Direct BCR-ABL1 Mediated Anti-Leukemic Effects Are Complemented by Promotion of Th1-Type and NK-Cell Mediated Cellular Immune Responses,

Tyrosine kinase inhibitors (TKIs; imatinib, dasatinib, nilotinib) have dramatically improved outcome of CML. Besides inhibiting target kinases in leukemic cells, off-target kinases in immune effector cells are also affected. We have previously described that dasatinib therapy induces an oligoclonal expansion and mobilization of large granular lymphocytes (LGLs; CD8+ T-cells or NK-cells) in Ph⁺ leukemia patients. Importantly, LGL expansion is associated with improved therapy responses, but the actual mechanisms are unknown. In this study, we explored the function and anti-leukemic properties of LGLs.

Peripheral blood samples from CML patients treated with dasatinib (n=10), imatinib (n=4), or nilotinib (n=7), or healthy controls (n=6) were used to analyze the activation and cytotoxicity of T- and NK-cells. Samples were collected before and after drug intake. The number of LGLs was determined from MGG stained blood smears and compared with granzyme B (GrB) positivity analyzed by flow cytometry.

Th1-type cytokine (TNF-a, IFN-g) production was measured by flow cytometry after stimulation of mononuclear cells (MNCs) with a-CD3/CD28-antibodies. Unpurified and purified NK cells were cultured with K562 cells, and degranulation (CD107 analysis) and cytotoxicity were measured.

As GrB positivity correlated well (r=0.95, p<0.0001, n=17) with the number of LGLs counted from MGG stained blood smears, a GrB specific antibody was used to identify LGLs in further analyses.

At diagnosis CML patients had more GrB⁺CD8⁺ T-cells than healthy controls (38 % vs. 11%, p=0.028). Also GrB⁺CD4⁺ T-cells were slightly increased, but did not differ significantly from healthy controls (3.6% vs. 0.8%, p=0.08). During dasatinib treatment the proportion of GrB⁺CD4⁺ (median at 6 months 28.1%, p=0.03) and GrB⁺CD8⁺ (70.9%, p=0.03) T cells increased significantly, whereas similar increase was not observed during imatinib (1.2% GrB⁺CD4⁺ and 30.0% GrB⁺CD8⁺ T-cells) or nilotinib (4.4% and 41.8%, respectively) therapies.

In patients on dasatinib therapy, GrB⁺CD3⁺cells were more sensitive to CD3/CD28-antibody stimulation and a larger proportion of cells (13.7%) produced Th1-type cytokines (TNF-a+IFN-g) compared to imatinib (2.4%) or nilotinib patients (5.5%) or healthy controls (5%) under same conditions (p=0.015).

As Th-1 cytokine-producing T cells are important in promoting cell-mediated immune responses, we next assessed whether dasatinib also enhances the cytolytic activity of NK cells. When MNC fraction was used as effector population (ratio 20:1), the median percentage of dead K562 cells was 18% in samples taken before dasatinib intake and 32% in samples taken 1h after dasatinib intake (p=0.004). Pre-dasatinib killing did not differ significantly from healthy volunteers (p=0.12). No increase in NK-cytotoxicity was observed after imatinib (11% vs. 8%) or nilotinib (10% vs. 10%) intake. Similar results were also obtained with purified NK-cells: the median percentage of dead K562 cells was 12% pre-dasatinib and 29% in post-dasatinib samples (p=0.06), whereas no differences were noticed with imatinib (30% vs. 28%) or nilotinib (14% vs. 15%) patients. The median percentage of dead K562 cells after incubation with pure NK-cells from healthy volunteers was 20%.

Interestingly, the cytolytic ability of NK-cells differed significantly among dasatinib treated patients. When the patients were divided into two groups based on therapy response, patients who had achieved CMR within 12 months (n=4) had significantly higher cytotoxic capability compared to patients who had not (n=6): 46% vs. 28% of dead K562 cells in post-dasatinib samples (p=0.02).

Dasatinib therapy resulted in increased numbers of GrB⁺ T-cells and generation of a Th1-type cellular immune response. In addition, 1h dasatinib exposure in vivo improved the cytotoxicity of NK-cells. These data support the dual mode of action of dasatinib: potent BCR-ABL1 inhibition in leukemic cells is accompanied by enhancement of cellular immunity, which likely have implications in be the long term control of Ph+ leukemia.

Porkka:Bristol-Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding. Mustjoki:Bristol-Myers Squibb: Honoraria; Novartis: Honoraria.