Single-cell analysis of the CD8+ T-cell compartment in multiple myeloma reveals disease specific changes are chiefly restricted to a CD69- subset suggesting potent cytotoxic effectors exist within the tumor bed.

Multiple myeloma (MM) is an incurable disease of the bone marrow (BM) characterized by the uncontrolled proliferation of neoplastic plasma cells. While CD8+ T cells have an established role in disease control, few studies have focused on these cells within the MM tumor microenvironment (TME). We analyzed CD8+ T cells in the BM and peripheral blood (PB) of untreated patients with MM and non-myeloma controls using flow cytometry, mass cytometry and single-cell RNA sequencing, using several novel bioinformatics workflows. Inter-tissue differences were most evident in the differential expression of Granzymes B and K, which were strongly associated with two distinct subsets of CD8+ T cells delineated by the expression of CD69, accounting for roughly 50% of BM-CD8+ T cells of all assessed cohorts. While few differences were observable between health and disease in the BM-restricted CD8CD69+ T-cell subset, the CD8+CD69- T-cell subset in the BM of untreated MM patients demonstrated increased representation of highly differentiated effector cells and evident compositional parallels between the PB, absent in age-matched controls, where a marked reduction of effector cells was observed. We demonstrate the transcriptional signature of BM-CD8+ T cells from patients with MM more closely resembles TCR-activated CD8+ T cells from age-matched controls than their resting counterparts.

A single-day polychemotherapy regimen with proteasome inhibitor combinations for relapsed/refractory myeloma in the era of novel therapies.

PCAB (prednisone, cyclophosphamide, doxorubicin, carmustine) is a single-day regimen previously used for induction and now in relapsed/refractory multiple myeloma (RRMM). We retrospectively analysed the outcomes of 85 patients from five Australian centres. These included 30 patients (35.3%) who received PCAB with one additional agent (bortezomib most frequently). Median age of the patients was 65 years (37-80), with a median of four (1-8) prior lines of therapy. ORR was 37% (CR 4.9%). Median progression free survival and overall survival were 4.4 months (95% CI 3.5-6.7) and 7.4 months (95% CI 6.4-10.2), respectively. Extramedullary disease (EMD) was associated with shorter survival. Grade 3 or 4 cytopenia and febrile neutropenia occurred in 76.2% and 39.1%, respectively, with six (7.1%) treatment-related mortalities. Median inpatient stay was 3.3 days/28-day cycle (IQR 0.6-13), and for patients who died, a median of 20.2% of days alive were spent inpatient (IQR 6.4-39.1%). Three patients were successfully bridged to CAR T-cell therapy using PCAB, despite being penta-exposed and having EMD. PCAB may be considered as a useful salvage therapy amongst other polychemotherapy regimens in late relapse. Further studies is warranted to investigate and define its role as a bridging therapy to novel therapeutics.

FLT3-TKD Measurable Residual Disease Detection Using Droplet Digital PCR and Clinical Applications in Acute Myeloid Leukemia.

The tyrosine kinase domain of the FMS-Like tyrosine kinase 3 (FLT3-TKD) is recurrently mutated in acute myeloid leukemia (AML). Common molecular techniques used in its detection include PCR and capillary electrophoresis, Sanger sequencing and next-generation sequencing with recognized sensitivity limitations. This study aims to validate the use of droplet digital PCR (ddPCR) in the detection of measurable residual disease (MRD) involving the common FLT3-TKD mutations (D835Y, D835H, D835V, D835E). Twenty-two diagnostic samples, six donor controls, and a commercial D835Y positive control were tested using a commercial Bio-rad® ddPCR assay. All known variants were identified, and no false positives were detected in the wild-type control (100% specificity and sensitivity). The assays achieved a limit of detection suitable for MRD testing at 0.01% variant allelic fraction. Serial samples from seven intensively-treated patients with FLT3-TKD variants at diagnosis were tested. Five patients demonstrated clearance of FLT3-TKD clones, but two patients had FLT3-TKD persistence in the context of primary refractory disease. In conclusion, ddPCR is suitable for the detection and quantification of FLT3-TKD mutations in the MRD setting; however, the clinical significance and optimal management of MRD positivity require further exploration.

CD83 is a new potential biomarker and therapeutic target for Hodgkin lymphoma.

Chemotherapy and hematopoietic stem cell transplantation are effective treatments for most Hodgkin lymphoma patients, however there remains a need for better tumor-specific target therapy in Hodgkin lymphoma patients with refractory or relapsed disease. Herein, we demonstrate that membrane CD83 is a diagnostic and therapeutic target, highly expressed in Hodgkin lymphoma cell lines and Hodgkin and Reed-Sternberg cells in 29/35 (82.9%) Hodgkin lymphoma patient lymph node biopsies. CD83 from Hodgkin lymphoma tumor cells was able to trogocytose to surrounding T cells and, interestingly, the trogocytosing CD83+T cells expressed significantly more programmed death-1 compared to CD83-T cells. Hodgkin lymphoma tumor cells secreted soluble CD83 that inhibited T-cell proliferation, and anti-CD83 antibody partially reversed the inhibitory effect. High levels of soluble CD83 were detected in Hodgkin lymphoma patient sera, which returned to normal in patients who had good clinical responses to chemotherapy confirmed by positron emission tomography scans. We generated a human anti-human CD83 antibody, 3C12C, and its toxin monomethyl auristatin E conjugate, that killed CD83 positive Hodgkin lymphoma cells but not CD83 negative cells. The 3C12C antibody was tested in dose escalation studies in non-human primates. No toxicity was observed, but there was evidence of CD83 positive target cell depletion. These data establish CD83 as a potential biomarker and therapeutic target in Hodgkin lymphoma.

Peripheral Blood Haploidentical Allogeneic Stem Cell Transplantation in Older Adults with Acute Myeloid Leukemia and Myelodysplastic Syndromes Demonstrates Long Term Survival, Results from Australia and New Zealand Transplant and Cellular Therapies.

There is a limited body of evidence for haploidentical hematopoietic stem cell transplantation (haplo-HSCT) in older patients. Previous studies have used a high proportion of bone marrow-derived grafts and a variety of conditioning regimens. In Australia and New Zealand, haplo-HCST is predominantly performed using peripheral blood (PB) with universal use of post-transplantation cyclophosphamide (PTCy). To characterize the outcomes of older recipients undergoing haplo-HSCT for acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). Data were collected through the Australasian Bone Marrow Transplant Recipient Registry (ABMTRR) for patients aged 65 or older receiving a PB haplo-HSCT for AML/MDS between January 2010 and July 2020. A total of 44 patients were included in the analysis. The median follow-up time was 377 days. The median age was 68 (range 65-74) with a median Karnofsky performance status of 90. Thirty patients (68.2%) had AML, whereas 14 (31.8%) had MDS. The median donor age was 40. The most common conditioning regimen was nonmyeloablative fludarabine, cyclophosphamide, and total body irradiation (75%); the remainder of the patients received either melphalan- or busulfan-based regimens, and the majority were reduced intensity, with only 2 patients undergoing myeloablative conditioning. All patients received post-transplantation cyclophosphamide and mycophenolate mofetil, with the majority also receiving tacrolimus (90.5%) and the remainder receiving cyclosporine (9.5%). No patients received anti-thymocyte globulin. Neutrophil engraftment was achieved in 97.6% of patients at a median of 18 days, whereas platelet engraftment was achieved in 92.7% of patients at a median of 28 days. The cumulative incidences of cytomegalovirus (CMV) reactivation and CMV disease were 52.5% and 5.1% at 1 year. The incidence of grade 2-4 acute Graft Versus Host Disease (GVHD) was 18.2%. The incidence of chronic GVHD at 2 years was 40.7%, with extensive chronic GVHD occurring in 17.7% of patients. The incidences of relapse and non-relapse mortality (NRM) at 2 years were 8.8% and 20.7% respectively. The leading causes of death were infection (64.7%) followed by relapse (14.2%). The 2-year overall survival was 74%. Relapse free survival and GVHD free, relapse free survival at 2 years was 70% and 48%. Haplo-HSCT using a peripheral blood graft and PTCy GVHD prophylaxis demonstrates long-term disease control with acceptable rates of NRM for older patients with AML/MDS.

PRESUMPTIVE RECURRENCE OF INTRAOCULAR LYMPHOMA DESPITE CHIMERIC ANTIGEN RECEPTOR T-CELL THERAPY.

PURPOSE: To present the first reported case of presumptive intraocular recurrence of lymphoma following Chimeric Antigen Receptor (CAR) T-cell therapy despite systemic control by CD19-CAR T cells. METHODS: Observational case report. RESULTS: A 59-year-old man with diffuse, large, B-cell lymphoma subsequently developed secondary central nervous system disease despite chemotherapy. He underwent stem cell transplantation but relapsed again and was scheduled to receive CAR T-cell therapy. He developed vitritis several weeks before treatment, with vitreous biopsy showing non-Hodgkin B-cell lymphoma. He received CAR T-cell therapy following the vitrectomy. He presented 3 months following CAR T-cell therapy with nonspecific right eye floaters and discomfort, with the optical coherence tomography revealing subretinal saw-tooth deposits in the right eye, highly suggestive of lymphoma. This is despite having good systemic control with no other disease elsewhere in the body. He received intravitreal methotrexate to good effect. CONCLUSION: To our knowledge, this is the first case of a vitreoretinal lymphoma nonresponsive to CAR T-cell therapy, despite good central nervous system and systemic control. This is suggestive of anti-CD19 CAR T cells not trafficking into the eye in sufficient numbers to eliminate CD19-expressing neoplastic B cells. We suggest regular ophthalmic follow-up after CAR-T-cell therapy for patients where there is evidence of ocular involvement.

Early diagnosis and resection of an asymptomatic leiomyosarcoma of the inferior vena cava prior to caval obstruction.

Leiomyosarcoma of the inferior vena cava is a rare and aggressive tumor, characterized by a slow growth and usually late diagnosis. The mainstay of therapy is surgical resection with limited role for chemotherapy or radiotherapy; resection modalities and the need for caval reconstruction are still matters of debate. In this case report, we describe an asymptomatic intraluminal leiomyosarcoma of the inferior vena cava diagnosed incidentally prior to caval occlusion during a routine ultrasound examination of the upper abdomen.

Risks of cardiovascular events and effects of routine blood pressure lowering among patients with type 2 diabetes and atrial fibrillation: results of the ADVANCE study.

AIMS: The aim of this study was to investigate serious clinical outcomes associated with atrial fibrillation (AF) and the effects of routine blood pressure lowering on such outcomes in the presence or absence of AF, among individuals with type 2 diabetes. METHODS AND RESULTS: About 11 140 patients with type 2 diabetes (7.6% of whom had AF at baseline) were randomized to a fixed combination of perindopril and indapamide or placebo in the Action in Diabetes and Vascular Disease: preterAx and diamicroN-MR Controlled Evaluation (ADVANCE) study. We compared total mortality and cardiovascular disease outcomes and effects of randomized treatment for 4.3 years on such outcomes between patients with and without AF at baseline. After multiple adjustments, AF was associated with a 61% (95% confidence interval 31-96, P < 0.0001) greater risk of all-cause mortality and comparable higher risks of cardiovascular death, stroke, and heart failure (all P < 0.001). Routine treatment with a fixed combination of perindopril and indapamide produced similar relative, but greater absolute, risk reductions for all-cause and cardiovascular mortalities in patients with AF, compared with those without AF. The number of patients needed to be treated with perindopril-indapamide for 5 years to prevent one cardiovascular death was 42 for patients with AF and 120 for patients without AF at baseline. CONCLUSION: Atrial fibrillation is relatively common in type 2 diabetes and is associated with substantially increased risks of death and cardiovascular events in patients with type 2 diabetes. This arrhythmia identifies individuals who are likely to obtain greater absolute benefits from blood pressure-lowering treatment. Atrial fibrillation in diabetic patients should be regarded as a marker of particularly adverse outcome and prompt aggressive management of all risk factors.

Measurable Residual Disease in Acute Myeloid Leukemia Using Flow Cytometry: A Review of Where We Are and Where We Are Going.

The detection of measurable residual disease (MRD) has become a key investigation that plays a role in the prognostication and management of several hematologic malignancies. Acute myeloid leukemia (AML) is the most common acute leukemia in adults and the role of MRD in AML is still emerging. Prognostic markers are complex, largely based upon genetic and cytogenetic aberrations. MRD is now being incorporated into prognostic models and is a powerful predictor of relapse. While PCR-based MRD methods are sensitive and specific, many patients do not have an identifiable molecular marker. Immunophenotypic MRD methods using multiparametric flow cytometry (MFC) are widely applicable, and are based on the identification of surface marker combinations that are present on leukemic cells but not normal hematopoietic cells. Current techniques include a "different from normal" and/or a "leukemia-associated immunophenotype" approach. Limitations of MFC-based MRD analyses include the lack of standardization, the reliance on a high-quality marrow aspirate, and variable sensitivity. Emerging techniques that look to improve the detection of leukemic cells use dimensional reduction analysis, incorporating more leukemia specific markers and identifying leukemic stem cells. This review will discuss current methods together with new and emerging techniques to determine the role of MFC MRD analysis.

Is Hematopoietic Stem Cell Transplantation Required to Unleash the Full Potential of Immunotherapy in Acute Myeloid Leukemia?

From monoclonal antibodies (mAbs) to Chimeric Antigen Receptor (CAR) T cells, immunotherapies have enhanced the efficacy of treatments against B cell malignancies. The same has not been true for Acute Myeloid Leukemia (AML). Hematologic toxicity has limited the potential of modern immunotherapies for AML at preclinical and clinical levels. Gemtuzumab Ozogamicin has demonstrated hematologic toxicity, but the challenge of preserving normal hematopoiesis has become more apparent with the development of increasingly potent immunotherapies. To date, no single surface molecule has been identified that is able to differentiate AML from Hematopoietic Stem and Progenitor Cells (HSPC). Attempts have been made to spare hematopoiesis by targeting molecules expressed only on later myeloid progenitors as well as AML or using toxins that selectively kill AML over HSPC. Other strategies include targeting aberrantly expressed lymphoid molecules or only targeting monocyte-associated proteins in AML with monocytic differentiation. Recently, some groups have accepted that stem cell transplantation is required to access potent AML immunotherapy and envision it as a rescue to avoid severe hematologic toxicity. Whether it will ever be possible to differentiate AML from HSPC using surface molecules is unclear. Unless true specific AML surface targets are discovered, stem cell transplantation could be required to harness the true potential of immunotherapy in AML.

Targeting CD83 in mantle cell lymphoma with anti-human CD83 antibody.

OBJECTIVES: Effective antibody-drug conjugates (ADCs) provide potent targeted cancer therapies. CD83 is expressed on activated immune cells including B cells and is a therapeutic target for Hodgkin lymphoma. Our objective was to determine CD83 expression on non-Hodgkin lymphoma (NHL) and its therapeutic potential to treat mantle cell lymphoma (MCL) which is currently an incurable NHL. METHODS: We analysed CD83 expression on MCL cell lines and the lymph node/bone marrow biopsies of MCL patients. We tested the killing effect of CD83 ADC in vitro and in an in vivo xenograft MCL mouse model. RESULTS: CD83 is expressed on MCL, and its upregulation is correlated with the nuclear factor κB (NF-κB) activation. CD83 ADC kills MCL in vitro and in vivo. Doxorubicin and cyclophosphamide (CP), which are included in the current treatment regimen for MCL, enhance the NF-κB activity and increase CD83 expression on MCL cell lines. The combination of CD83 ADC with doxorubicin and CP has synergistic killing effect of MCL. CONCLUSION: This study provides evidence that a novel immunotherapeutic agent CD83 ADC, in combination with chemotherapy, has the potential to enhance the efficacy of current treatments for MCL.

Targeting CD300f to enhance hematopoietic stem cell transplantation in acute myeloid leukemia.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) significantly reduces the rate of relapse in acute myeloid leukemia (AML) but comes at the cost of significant treatment-related mortality. Despite the reduction in relapse overall, it remains common, especially in high-risk groups. The outcomes for patients who relapse after transplant remains very poor. A large proportion of the morbidity that prevents most patients from accessing allo-HSCT is due to toxic nonspecific conditioning agents that are required to remove recipient hematopoietic stem and progenitor cells (HSPCs), allowing for successful donor engraftment. CD300f is expressed evenly across HSPC subtypes. CD300f has transcription and protein expression equivalent to CD33 on AML. We have developed an anti-CD300f antibody that efficiently internalizes into target cells. We have generated a highly potent anti-CD300f antibody-drug conjugate (ADC) with a pyrrolobenzodiazepine warhead that selectively depletes AML cell lines and colony forming units in vitro. The ADC synergizes with fludarabine, making it a natural combination to use in a minimal toxicity conditioning regimen. Our ADC prolongs the survival of mice engrafted with human cell lines and depletes primary human AML engrafted with a single injection. In a humanized mouse model, a single injection of the ADC depletes CD34+ HSPCs and CD34+CD38-CD90+ hematopoietic stem cells. This work establishes an anti-CD300f ADC as an attractive potential therapeutic that, if validated in transplant models using a larger cohort of primary AML samples, will reduce relapse rate and toxicity for patients with AML undergoing allo-HSCT.

Distinguishing human peripheral blood CD16+ myeloid cells based on phenotypic characteristics.

Myeloid lineage cells present in human peripheral blood include dendritic cells (DC) and monocytes. The DC are identified phenotypically as HLA-DR+ cells that lack major cell surface lineage markers for T cells (CD3), B cells (CD19, CD20), NK cells (CD56), red blood cells (CD235a), hematopoietic stem cells (CD34), and Mo that express CD14. Both DC and Mo can be phenotypically divided into subsets. DC are divided into plasmacytoid DC, which are CD11c- , CD304+ , CD85g+ , and myeloid DC that are CD11c+ . The CD11c+ DC are readily classified as CD1c+ DC and CD141+ DC. Monocytes are broadly divided into the CD14+ CD16- (classical) and CD14dim CD16+ subsets (nonclassical). A population of myeloid-derived cells that have DC characteristics, that is, HLA-DR+ and lacking lineage markers including CD14, but express CD16 are generally clustered with CD14dim CD16+ monocytes. We used high-dimensional clustering analyses of fluorescence and mass cytometry data, to delineate CD14+ monocytes, CD14dim CD16+ monocytes (CD16+ Mo), and CD14- CD16+ DC (CD16+ DC). We sought to identify the functional and kinetic relationship of CD16+ DC to CD16+ Mo. We demonstrate that differentiation of CD16+ DC and CD16+ Mo during activation with IFNγ in vitro and as a result of an allo-hematopoietic cell transplant (HCT) in vivo resulted in distinct populations. Recovery of blood CD16+ DC in both auto- and allo-(HCT) patients after myeloablative conditioning showed similar reconstitution and activation kinetics to CD16+ Mo. Finally, we show that expression of the cell surface markers CD300c, CCR5, and CLEC5a can distinguish the cell populations phenotypically paving the way for functional differentiation as new reagents become available.

Inverse relationship between oligoclonal expanded CD69- TTE and CD69+ TTE cells in bone marrow of multiple myeloma patients.

CD8+CD57+ terminal effector T (TTE) cells are a component of marrow-infiltrating lymphocytes and may contribute to the altered immune responses in multiple myeloma (MM) patients. We analyzed TTE cells in the bone marrow (BM) and peripheral blood (PB) of age-matched controls and patients with monoclonal gammopathy of undetermined significance (MGUS), smoldering MM (SMM), and newly diagnosed (ND) MM using flow cytometry, mass cytometry, and FlowSOM clustering. TTE cells are heterogeneous in all subjects, with BM containing both CD69- and CD69+ subsets, while only CD69- cells are found in PB. Within the BM-TTE compartment, CD69- and CD69+ cells are found in comparable proportions in controls, while CD69- cells are dominant in MGUS and SMM and predominantly either CD69- or CD69+ cells in NDMM. A positive relationship between CD69+TTE and CD69-TTE cells is observed in the BM of controls, lost in MGUS, and converted to an inverse relationship in NDMM. CD69-TTE cells include multiple oligoclonal expansions of T-cell receptor/Vß families shared between BM and PB of NDMM. Oligoclonal expanded CD69-TTE cells from the PB include myeloma-reactive cells capable of killing autologous CD38hi plasma cells in vitro, involving degranulation and high expression of perforin and granzyme. In contrast to CD69-TTE cells, oligoclonal expansions are not evident within CD69+TTE cells, which possess low perforin and granzyme expression and high inhibitory checkpoint expression and resemble T resident memory cells. Both CD69-TTE and CD69+TTE cells from the BM of NDMM produce large amounts of the inflammatory cytokines interferon-γ and tumor necrosis factor α. The balance between CD69- and CD69+ cells within the BM-TTE compartment may regulate immune responses in NDMM and contribute to the clinical heterogeneity of the disease.

Targeting the niche: depleting haemopoietic stem cells with targeted therapy.

Haemopoietic stem cell transplantation is an expanding procedure worldwide but is associated with significant morbidity and mortality. Depletion of resident haemopoietic stem and progenitor cells (HSPC) is required for both autologous and allogeneic haemopoietic stem cell transplantation. Current conditioning protocols utilise chemotherapy or radiation to effectively reduce HSPC but are toxic in both the short and long term. The initial trials to use monoclonal antibodies to target HSPC were limited with marginal efficacy but platforms including antibody drug conjugates and chimeric antigen receptor T cells have made targeted conditioning strategies achievable. In this review we summarise the work developing targeted conditioning that may replace or reduce alkylating agents and total body irradiation. The prospect of conditioning with significantly reduced toxicity will improve outcomes and open transplantation to patients unable to tolerate current conditioning protocols.

CD83: Activation Marker for Antigen Presenting Cells and Its Therapeutic Potential.

CD83 is a member of the immunoglobulin (Ig) superfamily and is expressed in membrane bound or soluble forms. Membrane CD83 (mCD83) can be detected on a variety of activated immune cells, although it is most highly and stably expressed by mature dendritic cells (DC). mCD83 regulates maturation, activation and homeostasis. Soluble CD83 (sCD83), which is elevated in the serum of patients with autoimmune disease and some hematological malignancies is reported to have an immune suppressive function. While CD83 is emerging as a promising immune modulator with therapeutic potential, some important aspects such as its ligand/s, intracellular signaling pathways and modulators of its expression are unclear. In this review we discuss the recent biological findings and the potential clinical value of CD83 based therapeutics in various conditions including autoimmune disease, graft-vs.-host disease, transplantation and hematological malignancies.

Examination of CD302 as a potential therapeutic target for acute myeloid leukemia.

Acute myeloid leukemia (AML) is the most common form of adult acute leukemia with ~20,000 new cases yearly. The disease develops in people of all ages, but is more prominent in the elderly, who due to limited treatment options, have poor overall survival rates. Monoclonal antibodies (mAb) targeting specific cell surface molecules have proven to be safe and effective in different haematological malignancies. However, AML target molecules are currently limited so discovery of new targets would be highly beneficial to patients. We examined the C-type lectin receptor CD302 as a potential therapeutic target for AML due to its selective expression in myeloid immune populations. In a cohort of 33 AML patients with varied morphological and karyotypic classifications, 88% were found to express CD302 on the surface of blasts and 80% on the surface of CD34+ CD38- population enriched with leukemic stem cells. A mAb targeting human CD302 was effective in mediating antibody dependent cell cytotoxicity and was internalised, making it amenable to toxin conjugation. Targeting CD302 with antibody limited in vivo engraftment of the leukemic cell line HL-60 in NOD/SCID mice. While CD302 was expressed in a hepatic cell line, HepG2, this molecule was not detected on the surface of HepG2, nor could HepG2 be killed using a CD302 antibody-drug conjugate. Expression was however found on the surface of haematopoietic stem cells suggesting that targeting CD302 would be most effective prior to haematopoietic transplantation. These studies provide the foundation for examining CD302 as a potential therapeutic target for AML.

CD300f epitopes are specific targets for acute myeloid leukemia with monocytic differentiation.

Antibody-based therapy in acute myeloid leukemia (AML) has been marred by significant hematologic toxicity due to targeting of both hematopoietic stem and progenitor cells (HSPCs). Achieving greater success with therapeutic antibodies requires careful characterization of the potential target molecules on AML. One potential target is CD300f, which is an immunoregulatory molecule expressed predominantly on myeloid lineage cells. To confirm the value of CD300f as a leukemic target, we showed that CD300f antibodies bind to AML from 85% of patient samples. While one CD300f monoclonal antibody (mAb) reportedly did not bind healthy hematopoietic stem cells, transcriptomic analysis found that CD300f transcripts are expressed by healthy HSPC. Several CD300f protein isoforms exist as a result of alternative splicing. Importantly for antibody targeting, the extracellular region of CD300f can be present with or without the exon 4-encoded sequence. This results in CD300f isoforms that are differentially bound by CD300f-specific antibodies. Furthermore, binding of one mAb, DCR-2, to CD300f exposes a structural epitope recognized by a second CD300f mAb, UP-D2. Detailed analysis of publicly available transcriptomic data indicated that CD34+ HSPC expressed fewer CD300f transcripts that lacked exon 4 compared to AML with monocytic differentiation. Analysis of a small cohort of AML cells revealed that the UP-D2 conformational binding site could be induced in cells from AML patients with monocytic differentiation but not those from other AML or HSPC. This provides the opportunity to develop an antibody-based strategy to target AMLs with monocytic differentiation but not healthy CD34+ HSPCs. This would be a major step forward in developing effective anti-AML therapeutic antibodies with reduced hematologic toxicity.