Low CLL-1 Expression Is a Novel Adverse Predictor in 123 Patients with De Novo CD34+ Acute Myeloid Leukemia

The Immunotherapy of Acute Myeloid Leukemia: A Clinical Point of View

The potential of the immune system to eradicate leukemic cells has been consistently demonstrated by the Graft vs. Leukemia effect occurring after allo-HSCT and in the context of donor leukocyte infusions. Various immunotherapeutic approaches, ranging from the use of antibodies, antibody-drug conjugates, bispecific T-cell engagers, chimeric antigen receptor (CAR) T-cells, and therapeutic infusions of NK cells, are thus currently being tested with promising, yet conflicting, results. This review will concentrate on various types of immunotherapies in preclinical and clinical development, from the point of view of a clinical hematologist. The most promising therapies for clinical translation are the use of bispecific T-cell engagers and CAR-T cells aimed at lineage-restricted antigens, where overall responses (ORR) ranging from 20 to 40% can be achieved in a small series of heavily pretreated patients affected by refractory or relapsing leukemia. Toxicity consists mainly in the occurrence of cytokine-release syndrome, which is mostly manageable with step-up dosing, the early use of cytokine-blocking agents and corticosteroids, and myelosuppression. Various cytokine-enhanced natural killer products are also being tested, mainly as allogeneic off-the-shelf therapies, with a good tolerability profile and promising results (ORR: 20-37.5% in small trials). The in vivo activation of T lymphocytes and NK cells via the inhibition of their immune checkpoints also yielded interesting, yet limited, results (ORR: 33-59%) but with an increased risk of severe Graft vs. Host disease in transplanted patients. Therefore, there are still several hurdles to overcome before the widespread clinical use of these novel compounds.

Tandem bispecific CD123/CLL-1 CAR-T cells exhibit specific cytolytic effector functions against human acute myeloid leukaemia

OBJECTIVES

The treatment of refractory and recurrent acute myeloid leukaemia (AML) is still a challenge with poor response rates and short survival times. In an attempt to solve this problem, we constructed a tandem bispecific chimeric antigen receptor (CAR) targeting CD123 and C-type lectin-like molecule 1 (CLL-1), two different AML antigens, and verified its cytotoxic effects in vitro.

METHODS

We established and cultured K562 cell lines expressing both CD123 and CLL1 antigens. Single-target CAR-T cells specific to CD123 and CLL1 were engineered, alongside tandem CD123/CLL1 bispecific CAR-T cells. Flow cytometry was used to determine cell phenotypes, transfection efficiencies, cytokine release, and CAR-T-cell proliferation, and an lactate dehydrogenase assay was used to detect the cytotoxicity of CD123/CLL-1 bispecific tandem CAR-T cells in vitro.

RESULTS

Two types of tandem CAR-T cells exhibited significant killing effects on CLL-1 + CD123+ leukaemia cell lines and primary AML tumour cells. The killing efficiency of tandem CAR-T cells in the case of single antigen expression is comparable to that of single target CAR-T cells. When faced with dual target tumour cells, dual target CAR-T cells significantly surpass single target CAR-T cells. CD123/CLL-1 CAR-T cells in tandem targeted and killed CD123- and CLL-1-positive leukaemia cell lines and released a large number of cytokines.

CONCLUSIONS

CD123/CLL-1 CAR-T cells in tandem can simultaneously target CD123 and CLL-1 on AML cells, demonstrating a significant ability to kill single antigens and multi-target tumour cells. This suggests that CD123/CLL-1 CAR-T cells exhibit significant advantages in the expression of multiple antigens in a wide range of target cells, which may help overcome the challenges posed by tumour heterogeneity and evasion mechanisms.

Immune-Based Therapeutic Interventions for Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive, clonally heterogeneous, myeloid malignancy, with a 5-year overall survival of approximately 27%. It constitutes the most common acute leukemia in adults, with an incidence of 3-5 cases per 100,000 in the United States. Despite great advances in understanding the molecular mechanisms underpinning leukemogenesis, the past several decades had seen little change to the backbone of therapy, comprised of an anthracycline-based induction regimen for those who are fit enough to receive it, followed by risk-stratified post-remission therapy with consolidation cytarabine or allogeneic stem cell transplantation (allo-SCT). Allo-SCT is the most fundamental form of immunotherapy in which donor cytotoxic T and NK cells recognize and eradicate residual AML in the graft-versus-leukemia (GvL) effect. Building on that, several alternative or synergistic approaches to exploit both self and foreign immunity against AML have been developed. Checkpoint inhibitors, for example, CTLA-4 inhibitors, PD-1 inhibitors, and PD-L1 inhibitors block proteins found on T cells or cancer cells that stop the immune system from attacking the cancer cells. They have been used with limited success in both the AML relapsed/refractory (R/R) and post SCT settings. AML tumor mutational burden is low compared to solid tumors and thus, it is less likely to generate neoantigens and respond to antibody-mediated checkpoint blockade that has shown unprecedented results in solid tumors. Therefore, alternative therapeutic strategies that work independently of the T cell receptor (TCR) specificity have been developed. They include bispecific antibodies, which recruit T cells through CD3 engagement, and in AML have shown an overall response rate ranging between 14 and 30% in early phase trials. Chimeric Antigen Receptor (CAR) T cell therapy is a type of treatment in which T cells are genetically engineered to produce a recombinant receptor that redirects the specificity and function of T lymphocytes. However, lack of cell surface targets exclusively expressed on AML cells including Leukemic Stem Cells (LSCs) combined with clonal heterogeneity represents the biggest challenge in developing CAR therapy for AML. Antibody-Drug Conjugates (ADC) constitute the only FDA-approved immunotherapy to treat AML with Gemtuzumab Ozogamicin, a CD33-specific ADC used in CEBPα-mutated AML. The identification of additional cell surface targets is critical for the development of other ADC's potentially useful in the induction and maintenance regimens, given the ease at which these reagents can be generated and managed. Here, we will review those immune-based therapeutic interventions and highlight active areas of research investigations toward fulfillment of the great promise of immunotherapy to AML.

CLEC12A plays an important role in immunomodulatory function and prognostic significance of patients with acute myeloid leukemia

The physiological function and prognostic significance of C-type lectin domain family 12 member A (CLEC12A) in acute myeloid leukemia (AML) patients are unclear. CLEC12A transcriptional expression in a variety of tumors from several public databases was collected and compared. We found that CLEC12A was highly expressed in AML cell lines and in tissues from AML patients and a higher CLEC12A expression in leukemia stem cells. CLEC12A low expression was associated with poor prognosis in the chemotherapy-only group and high CLEC12A expression may benefit from autologous or allogeneic hematopoietic stem cell transplantation (HSCT). CLEC12A expression was positively correlated with infiltrating levels of type 2 macrophages and monocytes and negatively associated with NK cells and regulatory T cells in AML. CLEC12A high was positively associated with immune checkpoint genes as well as macrophage associated genes. CLEC12A is an ideal chimeric antigen receptor T-cell (CAR-T) therapy target for AML and its expression level was closely linked to treatment response and patients' survival outcome. CLEC12A plays an important immunomodulatory role in AML.

T-cell-based immunotherapy of acute myeloid leukemia: current concepts and future developments

Acute myeloid leukemia (AML) is a heterogeneous disease linked to a broad spectrum of molecular alterations, and as such, long-term disease control requires multiple therapeutic approaches. Driven largely by an improved understanding and targeting of these molecular aberrations, AML treatment has rapidly evolved over the last 3-5 years. The stellar successes of immunotherapies that harness the power of T cells to treat solid tumors and an improved understanding of the immune systems of patients with hematologic malignancies have led to major efforts to develop immunotherapies for the treatment of patients with AML. Several immunotherapies that harness T cells against AML are in various stages of preclinical and clinical development. These include bispecific and dual antigen receptor-targeting antibodies (targeted to CD33, CD123, CLL-1, and others), chimeric antigen receptor (CAR) T-cell therapies, and T-cell immune checkpoint inhibitors (including those targeting PD-1, PD-L1, CTLA-4, and newer targets such as TIM3 and STING). The current and future directions of these T-cell-based immunotherapies in the treatment landscape of AML are discussed in this review.

An Update on the Clinical Evaluation of Antibody-Based Therapeutics in Acute Myeloid Leukemia

PURPOSE OF REVIEW

The advent of several targeted agents has revolutionized the treatment of acute myeloid leukemia (AML) in recent times; however, majority of patients are still not cured. In the ongoing quest for rationally targeted treatment strategies in AML, scientific endeavors have focused on identifying new antigen targets on the leukemic cells for therapeutic exploitation including strategies to directly deliver toxins into the leukemic blasts as well as strategies that harness host immunity to favorably impact clinical outcomes. Gemtuzumab ozogamicin, a CD33 directed antibody-drug conjugate, has provided the proof of concept for the potential efficacy of monoclonal antibody-based therapies in AML. This article provides an overview of immunologically relevant antigen targets expressed on the leukemic cells and synopsizes the clinical results evaluating targeted antibody-based therapeutic approach in AML.

RECENT FINDINGS

AML blasts and leukemic stem cells express several antigens, including CD33, CD47, CD70, CD123, and CLEC12A. The past several years have seen the burgeoning of cell-specific immunotherapy concepts, including checkpoint inhibitors, antibody-toxin conjugates, and bispecific antibodies in the treatment of AML. The first-in-class anti-CD47 antibody magrolimab and anti-CD70 antibody cusatuzumab in combination with hypomethylating agent (HMA) azacitidine, in newly diagnosed AML, and flotetuzumab, a bispecific DART® (dual-affinity retargeting) antibody to CD3ε and CD123 as salvage option in relapsed/refractory AML appear promising. The development of antibody-based immunotherapeutic approach in AML has been encouraging. Ongoing research will define the choice of an appropriate complementary therapeutic agent in antibody-based combination therapy, and whether one or more than one antigen should be simultaneously targeted. Further studies will likely refine the role of antibody-based therapy in post hematopoietic cell transplant setting.

Cytotoxic effect of CLL‑1 CAR‑T cell immunotherapy with PD‑1 silencing on relapsed/refractory acute myeloid leukemia

The activation of chimeric antigen receptor (CAR)-T cells can lead to persistently high levels of programmed cell death 1 (PD-1) antigen and eventually causes the exhaustion of T cells. The effectiveness of CAR-T cells targeting C-type lectin-like molecule-1 (CLL-1) combined with PD-1 silencing therapy for acute myeloid leukemia (AML) was evaluated in the present study. CLL-1 levels in primary AML bone marrow samples was examined using flow cytometric analysis. We designed a CLL-1 CAR-T, containing CLL-1-specific single-chain variable fragment, CD28, OX40, CD8 hinge and TM and CD3-ζ signaling domains. CLL-1 CAR-T with PD-1 silencing was constructed. It was confirmed that CLL-1 is expressed on the surface of AML cells. CLL-1 CAR-T showed specific lysing activity against CLL-1⁺ AML cells. PD-1 silencing enhanced the killing ability of CLL-1 CAR-T. Furthermore, it was found that CAR-T derived from healthy donor T cells was more effective in killing THP-1 cells (a human acute monocytic leukemia cell line) than those from patient-derived T cells. These results indicated that CLL-1 CAR-T and PD-1 knockdown CLL-1 CAR-T could be used as a potential immunotherapy to treat relapsed or refractory AML.

Ligand Density Controls C-Type Lectin-Like Molecule-1 Receptor-Specific Uptake of Polymer Nanoparticles

The newest generation of drug delivery systems (DDSs) exploits ligands to mediate specific targeting of cells and/or tissues. However, studies investigating the link between ligand density and nanoparticle (NP) uptake are limited to a small number of ligand-receptor systems. C-type lectin-like molecule-1 (CLL1) is uniquely expressed on myeloid cells, which enables the development of receptors specifically targeting treat various diseases. This study aims to investigate how NPs with different CLL1 targeting peptide density impact cellular uptake. To this end, poly(styrene-alt-maleic anhydride)-b-poly(styrene) NPs are functionalized with cyclized CLL1 binding peptides (cCBP) ranging from 240 ± 12 to 31 000 ± 940 peptides per NP. Unexpectedly, the percentage of cells with internalized NPs is decreased for all cCBP-NP designs regardless of ligand density compared to unmodified NPs. Internalization through CLL1 receptor-mediated processes is further investigated without confounding the effects of NP size and surface charge. Interestingly, high density cCBP-NPs (>7000 cCBP per NP) uptake is dominated by CLL1 receptor-mediated processes while low density cCBP-NPs (≈200 cCBP per NP) and untargeted NP occurred through non-specific clathrin and caveolin-mediated endocytosis. Altogether, these studies show that ligand density and uptake mechanism should be carefully investigated for specific ligand-receptor systems for the design of targeted DDSs to achieve effective drug delivery.

The small-molecule compound AC-73 targeting CD147 inhibits leukemic cell proliferation, induces autophagy and increases the chemotherapeutic sensitivity of acute myeloid leukemia cells

CD147 is a transmembrane glycoprotein with multiple functions in human healthy tissues and diseases, in particular in cancer. Overexpression of CD147 correlates with biological functions that promote tumor progression and confers resistance to chemotherapeutic drugs. In contrast to solid tumors, the role of CD147 has not been extensively studied in leukemia. Understanding whether CD147 represents a new hematologic target and whether its inhibitor AC-73 may be used in leukemia therapy may reveal an alternative treatment strategy in patients with acute myeloid leukemia (AML). We analyzed CD147 expression and function in hematopoietic progenitor cells from normal cord blood, in several leukemic cell lines and in primary leukemic blasts obtained from patients with AML. We investigated the effects of AC-73, used alone or in combination with arabinosylcytosine (Ara-C) and arsenic trioxide (ATO), on leukemic cell proliferation. We demonstrated that CD147 overexpression promotes leukemic cell proliferation. We showed that AC-73 exhibits a potent growth inhibitory activity in leukemic cells, by inhibiting the ERK/STAT3 activation pathway and activating autophagy. We demonstrated that AC-73 exerts an anti-proliferative effect additive to chemotherapy by enhancing leukemic cell sensitivity to Ara-C-induced cytotoxicity or to ATO-induced autophagy. We also reported CD147 expression in the fraction of leukemic blasts expressing CD371, a marker of leukemic stem cells. Altogether, our study indicates CD147 as a novel potential target in the treatment of AML and AC-73 as an anti-proliferative drug and an inducer of autophagy in leukemic cells to use in combination with chemotherapeutic agents.

Prognostic and therapeutic role of CLEC12A in acute myeloid leukemia

CLEC12A has recently been identified as an antigen, expressed on leukemic stem cells and leukemic blasts. Given the fact that this expression profile seems stable throughout diagnosis, treatment and relapse on leukemic blasts and leukemic stem cells, CLEC12A can be considered a highly potent and reliable marker for the detection of measurable residual disease and therefore applicable for risk stratification and prognostication in AML. Low CLEC12A expression on leukemic blasts seems to be independently associated with lower likelihood of achieving complete remission after 1 cycle of induction chemotherapy, shorter event free survival, as well as overall survival, indicating potential prognostic properties of CLEC12A expression itself. Lack of expression on the normal hematopoietic stem and progenitor cells, in contrast to CD123 and CD33, might result in less toxicity regarding cytopenias, making CLEC12A an interesting target for innovating immunotherapies, including monoclonal and bispecific antibodies, antibody-drug conjugates and CAR-T cells therapy.