ALBERT

Description: Background: The introduction of BCR/ABL-specific tyrosine kinase inhibitors (TKIs) more than two decades ago revolutionized chronic myelogenous leukemia (CML) therapy. The majority of CML patients treated with TKIs obtain durable cytogenetic and molecular responses. However, only a subgroup of these patients can successfully discontinue TKI therapy and maintain a treatment-free remission (Laneuville et al. 2011). TKI-resistant leukemia stem cells (LSCs) persist in the majority of patients at low levels over a prolonged period. These quiescent, self-renewing LSCs in the BM are the major cause of relapse after drug discontinuation (Holyoake et al, 2017). The selective elimination of LSCs requires the definition of unique signaling pathways that promote self-renewal of LSCs but not of normal HSCs. Based on the documented expression of CD93 on LSCs (Kinstrie et al, 2015), the aim of the present study was to investigate the role of the cell surface receptor CD93 in the regulation of self-renewal of human and murine CML LSCs and its contribution to disease development and progression. Methods and Results: We found CD93 expression on LSCs and leukemia progenitor cells but not on more differentiated leukemia granulocytes in a murine retroviral lineage-negative Sca-1+ c-kit+ (LSK) transduction/transplantation CML model. Next-generation sequencing analysis revealed that Cd93-/- LSCs have a silenced gene expression signature particularly in genes involved in the regulation of gene expression, stem cell maintenance and proliferation. Out of the 1120 genes differentially expressed between BL/6 and Cd93-/- LSCs, 1108 genes were down-regulated. In contrast, naïve BL/6 and Cd93-/- hematopoietic stem cells (HSCs) did not display a dysregulation in these pathways. Functionally, CD93-deficiency in LSCs resulted in impaired self-renewal, reduced LSC frequencies in vitro (at least by a factor of 100, P

Description: Chronic myeloid leukemia (CML) is a typical stem-cell driven malignancy, driven by leukemia stem cells (LSCs). LSCs are resistant to conventional therapies. This resistance is mediated by cell-intrinsic mechanisms and interactions with the microenvironment. LSCs depend on signals from a specialized microenvironment, a so called niche, to maintain their stem cell characteristics. In CML the bone marrow (BM) as a niche is well-investigated and several therapeutic targets, which aim at LSCs by interrupting their interaction with the BM-niche are under investigation. However, even though splenomegaly is a hallmark of CML the contribution of the splenic microenvironment to CML development has not been studied so far. This project aims to investigate the role of the splenic microenvironment as an independent secondary LSC niche and its contribution to disease development. To induce a CML-like disease in mice we retrovirally transduced FACS-sorted Lineage- Sca-1+ cKit+ BM cells with pMSCV-p210BCR/ABL-IRES-GFP and injected the transduced cells into non-irradiated mice. To find out if the spleen contributes to disease development we induced CML in splenectomized and sham operated mice. Splenectomized mice survived significantly longer compared to sham operated controls (median survival 31 vs. 22 days; p=0.0006) with 20% of the splenectomized mice surviving longer than 90 days. Moreover, the number of LSCs in the BM of splenectomized mice was reduced 3.7-fold (p=0.002). Flowcytometric analysis of the spleen and BM compartments of CML bearing mice revealed that the majority of the leukemic stem and progenitor cells (LSPCs) were located in the spleen (19-fold more LSCs in the spleen; p =0.007). Moreover we found the leukemic compartment in the spleen to be enriched for LSPCs compared to the BM (20 % spleen vs. 10 % BM; % LSPCs of total leukemic cells; p=0.01). To confirm this phenotypic observation functionally we performed a limiting dilution transplantation of leukemic cells from spleen and BM. In line with the phenotypic observation we found a higher frequency of LSCs in the spleen compared to the BM (1/41'703 vs. 1/432'594; p=0.02). We next analyzed the gene expression of LSPCs from spleen and BM. We found that the gene expression profile of splenic LSPCs showed higher expression of stemness-related genes and reduced expression of myeloid differentiation genes compared to BM LSPCs, indicating that the spleen is more supportive of primitive LSPCs. Knowing that the spleen contributes to disease development by providing an alternate niche for LSCs we next analyzed the spleens using confocal microscopy. We found that the LSCs resided exclusively in the red pulp. Previous studies have shown that HSCs reside in direct contact with red pulp macrophages (RPMs) during extramedullary hematopoiesis (Dutta et al., JEM, 2015). In addition we found that in spleens from human CML patients CD34+ leukemia cells localized together with macrophages (p=0.001). Furthermore we could show that RPMs are capable of producing both SCF and G-CSF. To test the role of RPMs as a potential niche component in vitro we co-incubated LSCs and RPMs overnight before plating the LSCs in a colony formation assay. We found that the co-incubation with RPMs improved the colony formation capacity of LSCs (CFUs 166 vs. 138; p=0.0356). To test the role of RPMs in vivo we depleted macrophages in CML mice using clodronate liposomes. This resulted in significantly reduced splenomegaly (867mg vs. 249mg; p

Description: Cutaneous T cell lymphoma (CTCL) is a CD4+ T cell malignancy of the skin with heterogeneous outcomes and limited treatment options. Monoclonal antibodies directed against PD-1, such as pembrolizumab, have shown impressive efficacy in multiple advanced malignancies, and are currently tested in clinical trials in patients with CTCL. Initial data indicate that about half of the patients experience treatment response, whereas the other half are non-responders. Non-responders can be further divided into patients with stable disease versus rapid progressors. It is currently unknown why some CTCL patients respond to pembrolizumab while others rapidly progress, and no predictive biomarkers are available. Single-cell analysis approaches to identify biomarkers of response, for example quantifying the expression of PD-1 on tumor cells vs. reactive immune cells, have not enabled stratification of patients. We therefore hypothesized that more complex spatial cellular interactions within the immune tumor microenvironment (iTME) of CTCL could provide insight into the mechanisms of pembrolizumab response and enable prediction. We applied CODEX (CO-Detection by indEXing) highly multiplexed tissue imaging to study the CTCL iTME in matched biopsies before and after pembrolizumab therapy in 7 responders and 7 non-responders (see the Figure). Using 54 markers simultaneously allowed discriminating malignant CD4+ tumor cells from reactive CD4+ T cells and identified 30 different cell clusters with spatial information, including an M2 macrophage cluster that was enriched in non-responders before therapy. Unexpectedly, in pembrolizumab responders compared to non-responders, PD-1 expression levels were higher in multiple clusters of tumor cells and reactive T cells. Computational spatial analysis revealed ten distinct, conserved cellular neighborhoods in the CTCL iTME that changed in composition and frequency during therapy. Interestingly, one cellular neighborhood to be presented dramatically increased after therapy only in responders. Therefore, highly multiplexed spatial analysis of the CTCL iTME allows discovering novel, predictive biomarkers of immunotherapy response and will pave the way for future studies that functionally address the identified cell types and cellular interactions. Disclosures Khodadoust: Corvus Pharmaceuticals: Research Funding. Kim:miRagen: Research Funding; Merck: Research Funding; Medivir: Honoraria, Membership on an entity's Board of Directors or advisory committees; Innate Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Soligenix: Research Funding; Forty Seven Inc: Research Funding; Neumedicine: Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Trillium: Research Funding; Elorac: Research Funding; Galderma: Research Funding; Corvus: Honoraria, Membership on an entity's Board of Directors or advisory committees; Portola Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Seattle Genetics: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Kyowa Hakko Kirin: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Eisai: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Horizon: Research Funding. Nolan:Akoya Biosciences Inc.: Consultancy, Equity Ownership, Patents & Royalties.

Description: The introduction of BCR/ABL-specific tyrosine kinase inhibitors (TKIs) a decade ago revolutionized chronic myelogenous leukemia (CML) therapy. However, disease-initiating leukemia stem cells (LSCs) in CML are resistant to TKIs despite BCR/ABL inhibition. Therefore, CML will ultimately relapse upon drug discontinuation. We have previously shown that blocking CD70/CD27 signaling targets LSCs by inhibiting the activation of the Wnt pathway. Here, we investigated a combination therapy of TKIs and CD70/CD27 blocking monoclonal antibodies in human and murine CML. We demonstrate that TKI-mediated BCR/ABL inhibition down-regulates miR-29, leading to increased expression of specificity protein 1 (SP1), a transcription factor with binding site in the CD70 promoter. In addition, TKI treatment reduced the expression of DNA methyltransferases resulting in de-methylation of the CD70 promoter. These combined effects resulted in CD70 up-regulation on LSCs, enhanced CD70/CD27 signaling and compensatory Wnt pathway activation. Combined BCR/ABL and CD70/CD27 inhibition synergistically reduced Wnt signaling and eradicated leukemia cells in vitro. More importantly, combination therapy effectively eliminated CD34+ CML stem/progenitor cells in murine xenografts and LSCs in a murine CML model. Therefore, TKI-induced CD70 up-regulation triggers CD70/CD27 signaling leading to compensatory Wnt activation. These findings identify an important targetable TKI resistance mechanism of CML LSCs and may lead to new therapeutic strategies to directly target LSCs to overcome treatment resistance. Disclosures No relevant conflicts of interest to declare.

Description: The bone marrow (BM) is responsible for generating and maintaining lifelong output of blood and immune cells. Besides its key hematopoietic function, the BM acts as an important lymphoid organ, hosting a large variety of mature lymphocyte populations, including B-cells, T-cells, NK(T)-cells and innate lymphoid cells (ILCs). Many of these cell types are thought to only transiently visit the BM, but for others, like plasma cells and memory T-cells, the BM provides supportive niches that promote their long-term survival. Interestingly, accumulating evidence points towards an important role for mature lymphocytes in the regulation of hematopoietic stem cells (HSCs) and hematopoiesis in health and disease. In this review, we describe the diversity, migration, localization and function of mature lymphocyte populations in murine and human BM, focusing on their role in immunity and hematopoiesis. We also address how various BM lymphocyte subsets contribute to the development of aplastic anemia and immune thrombocytopenia, illustrating the complexity of these BM disorders, but also the underlying similarities and differences in their disease pathophysiology. Finally, we summarize the interactions between mature lymphocytes and BM resident cells in HSC transplantation and graft-versus-host disease. A better understanding of the mechanisms by which mature lymphocyte populations regulate BM function will likely improve future therapies for patients with benign and malignant hematological disorders.