Abstract A115: Tyrosine kinase inhibitor therapy modulates immune checkpoints and TCR repertoire diversity in chronic myeloid leukemia
Authors: Dufva Olli , Kasanen Tiina , Missiry Mohamed El , Klievink Judith , Lähteenmäki Hanna , Mustjoki Satu 
Published: November 2016

Abstract
The potential of immune checkpoint inhibition in many hematological malignancies such as chronic myeloid leukemia (CML) has not been characterized in detail. CML is a hematopoietic stem cell disease driven by the BCR-ABL fusion gene. Tyrosine kinase inhibitor (TKI) therapy is able to induce remissions in the majority of patients. Despite treatment responses, leukemic stem cells persist in the bone marrow (BM). Enhancing antitumor immune responses has been suggested as a potential strategy of enabling treatment discontinuation and disease control in CML. We hypothesized that the bone marrow may represent a distinct immunological milieu in CML patients. Furthermore, TKI therapy has been shown to induce immunological changes. Therefore, we evaluated the expression of immune checkpoint molecules in paired samples from BM and peripheral blood (PB) in chronic phase CML patients at diagnosis and during treatment with TKIs. In addition, we monitored changes in the BM T cell receptor (TCR) repertoire induced by TKI therapy and disease resolution.
We performed multicolor flow cytometry on frozen PB and BM samples from chronic phase CML patients at diagnosis and during treatment with the tyrosine kinase inhibitors imatinib, dasatinib and nilotinib as well as healthy controls. We analyzed the expression of PD-1, CTLA-4, LAG-3, ICOS and TIM-3 on T cells and PD-L1, PD-L2, CD80 and CD86 on antigen-presenting cells and CD34+ leukemic progenitor cells. TCR repertoire was assayed by deep sequencing of the CDR3 region using the immunoSEQ assay.
At diagnosis, CD8+ T cells in the bone marrow exhibited a more exhausted phenotype compared to peripheral blood as measured by PD-1 positivity (26.1 vs. 12.7%, p = 0.001). This difference was recapitulated in all T cell subsets, including effector memory (TEM), terminally differentiated (TEMRA), central memory (TCM) and naïve, with the highest PD-1 positivity in TEM and TEMRA cells. In addition, dendritic cells as well as leukemic CD34+ progenitor cells expressed higher levels of PD-L1 and CD86 in BM compared to PB at diagnosis. Interestingly, TKI therapy led to a reduction of PD-1-positive CD8+ T cells in the BM after 1 and 6 months of treatment. This was accompanied by a concomitant decrease in PD-L1 and CD86 positivity on dendritic cells. Finally, treatment with dasatinib led to a reduction in TCR repertoire diversity and increased clonality at 6 months, whereas imatinib or nilotinib did not alter the repertoire diversity.
In conclusion, BM and PB seem to exhibit different immunological milieus in terms of expression of immune checkpoint molecules in CML. During TKI therapy both PD-1 and PD-L1 levels decrease, suggesting at least partial reversal of immune cell exhaustion. Treatment with the TKI dasatinib is able to drive the TCR repertoire towards a more clonal composition, which has been associated with good responses to checkpoint blockade. Thus, targeted therapies such as TKIs are able to modulate immune checkpoints and T cell activity in unexpected ways, providing a potential strategy for enhancing immunotherapies in combination treatment regimens.
Citation Format: Olli Dufva, Tiina Kasanen, Mohamed El Missiry, Judith Klievink, Hanna Lähteenmäki, Satu Mustjoki. Tyrosine kinase inhibitor therapy modulates immune checkpoints and TCR repertoire diversity in chronic myeloid leukemia [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A115.