Leukemia stem cells (LSCs) promote the disease and seem resistant to therapy and immune control. Why LSCs are selectively resistant against elimination by CD8 ⁺ cytotoxic T cells (CTLs) is still unknown. In this study, we demonstrate that LSCs in chronic myeloid leukemia (CML) can be recognized and killed by CD8 ⁺ CTLs in vitro. However, Tregs, which preferentially localized close to CD8 ⁺ CTLs in CML BM, protected LSCs from MHC class I–dependent CD8 ⁺ CTL–mediated elimination in vivo. BM Tregs in CML were characterized by the selective expression of tumor necrosis factor receptor 4 (Tnfrsf4). Stimulation of Tnfrsf4 signaling did not deplete Tregs but reduced the capacity of Tregs to protect LSCs from CD8 ⁺ CTL–mediated killing. In the BM of newly diagnosed CML patients, TNFRSF4 mRNA levels were significantly increased and correlated with the expression of the Treg-restricted transcription factor FOXP3 . Overall, these results identify Tregs as key regulators of immune escape of LSCs and TNFRSF4 as a potential target to reduce the function of Tregs and boost antileukemic immunity in CML.

Diffuse malignant mesothelioma (DMM) is one of the prognostically most discouraging cancers with median survivals of only 12–22 months. Due to its insidious onset and delayed detection, DMM is often at an advanced stage at diagnosis and is considered incurable. Combined chemo- and radiotherapy followed by surgery only marginally affect outcome at the cost of significant morbidity. Because of the long time period between exposure to asbestos and disease onset, the incidence of DMM is still rising and predicted to peak around 2020. Novel markers for the reliable diagnosis of DMM in body cavity effusion specimens as well as more effective, targeted therapies are urgently needed. Here, we show that the “don't eat me” signalling molecule CD47, which inhibits phagocytosis by binding to signal regulatory protein α on macrophages, is overexpressed in DMM cells. A two-marker panel of high CD47 expression and BRCA1-associated protein 1 (BAP-1) deficiency had a sensitivity of 78% and specificity of 100% in discriminating DMM tumour cells from reactive mesothelial cells in effusions, which is superior to the currently used four-marker combination of BAP-1, glucose transporter type 1, epithelial membrane antigen and desmin. In addition, blocking CD47 inhibited growth and promoted phagocytosis of DMM cell lines by macrophages in vitro. Furthermore, DMM tumours in surgical specimens from patients as well as in a mouse DMM model expressed high levels of CD47 and were heavily infiltrated by macrophages. Our study demonstrates that CD47 is an accurate novel diagnostic DMM biomarker and that blocking CD47 may represent a promising therapeutic strategy for DMM.

Background: The inflammasome plays an essential role in lower risk MDS and immune subversion, with the up-regulation of immune checkpoint molecules in the progression to higher-risk disease. In this study, we explored the utility of immune-related biomarkers for the diagnosis and prognosis of MDS. Methods: We performed an exploratory, case-control study with 20 randomly selected MDS patients and nine controls with non-inflammatory (n = 3) and inflammatory conditions (n = 6). Patients were stratified in groups of lower (n = 10) and higher risk (n = 10) using IPSS-R. For the exploration of inflammasome and immune checkpoint activities, the expression of caspase-1 (Casp1), programmed cell death protein 1 (PD-1) and its ligand (PD-L1) were assessed in bone marrow samples using immunohistochemistry. Results: In multivariate analysis, we observed significant differences for Casp1 but not PD1/PD-L1 expression in our four conditions (p = 0.003). We found a discordant co-expression of Casp1/PD-L1 in MDS (rho = −0.41, p = 0.07) compared with a concordant co-expression in controls (rho = 0.64, p = 0.06). Neutrophil counts correlated directly with Casp1 (rho = 0.57, p = 0.009) but inversely with PD-L1 expression (rho = −0.58, p = 0.007). Conclusion: We identified characteristic discordant co-expression patterns in lower- (Casp1_high/PD-L1_low) and higher-risk MDS (Casp1_low/PD-L1_high), contrasting with concordant patterns in the non-inflammatory (Casp1_low/PD-L1_low) and inflammatory conditions (Casp1_high/PD-L1_high). Further validation is warranted in larger, prospective studies.

Multiple myeloma (MM) accounts for ∼12% of all hematologic malignancies and remains incurable despite therapeutic advances. Targeting tumor-associated antigens (TAAs) and tumor-specific neoantigens presented on HLA molecules by vaccination or T-cell receptor (TCR)–redirected T cells offers a promising novel immunotherapeutic strategy. However, knowledge of targetable tumor antigens, especially neoantigens, is limited. We applied a mass spectrometry–based immunopeptidomics approach on primary MM samples (n = 8) and MM cell lines (n = 3), integrating whole-exome and RNA sequencing to comprehensively characterize the MM immunopeptidome and systematically search for TAAs and neoantigens. Using our approach, we detected cancer-associated phosphopeptide HLA ligands as well as multiple cancer testis antigen–derived HLA peptides, some shared between tumor samples. Proteogenomic analysis revealed primarily personalized patient-specific immunoglobulin-derived neoantigens generated by variable diversity joining recombination and somatic hypermutation, which were present in all samples. Three patients showed clustering of related major histocompatibility complex class II peptide ligands within the hypermutated region. In addition, we identified non–immunoglobulin-derived neoantigens in both primary MM samples and cell lines. Approximately 30% of the cell line-derived peptides elicited CD8+ T-cell responses in allogeneic HLA-matched healthy donor peripheral blood mononuclear cells (PBMCs). In contrast, none of the patient-derived neoantigens were immunogenic in autologous PBMCs or tumor-infiltrating lymphocytes, reflecting severe immune dysfunction. This study provides, to our knowledge, the first proteogenomic-based identification of neoantigens in MM, extends the observation of presented immunoglobulin-derived neoantigens from B-cell lymphoma to MM, and highlights both actionable antigens and barriers to effective TCR-based immunotherapy.