Acute leukemia is the most common malignancy diagnosed in children and represents approximately 30% of pediatric cancer diagnoses [
1]. The vast majority (80%) suffers from acute lymphoblastic leukemia (ALL) while the remainder is diagnosed with acute myeloid leukemia (AML). Among these, approximately 25% of biphenotypic, or bilineage ALL express the myeloid specific antigen CD33. While patients with common B-precursor ALL have excellent prognosis, acute lymphoblastic leukemias with.
Co-expression of CD33 usually have a poor prognosis [
2,
3]. Thus, patients with CD33
+ leukemia mark a high-risk population urgently requiring novel strategies that promote the immune system to overcome the malignancy without adding further life-threatening toxicity.
Through their ability to control human hematologic malignancies and to exhibit antitumoral effects, natural killer (NK) cells represent key players of the innate immune system, capable of immune surveillance [
4]. The modulation of NK cell activity is regulated by a repertoire of activating and inhibitory receptors. Ultimately, the balance of these receptors will determine whether an NK cell will be silent (tolerant), auto-reactive, or cytotoxic (alloreactive) toward the healthy or malignant “self.” NK cell function can be accomplished via I) natural cytotoxicity against tumor target cells upon degranulation of lysosomes containing granzymes and perforin. II) Cytokines such as interferon γ (IFN-γ) and tumor necrosis factor α (TNF-α) help shaping the adaptive immune response and III) via CD16, the potent low-affinity FcγRIII receptor, mediating antibody-dependent cell-mediated cytotoxicity (ADCC) [
5]. The vast majority (> 90%) of circulating NK cells are CD56
dim and express high levels of CD16 [
6]. CD16 induces phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAM), triggering the release of lytic granules such as granzyme and perforin and cytokines such as INF-y and TNF-α [
7,
8]. Different studies have demonstrated the therapeutic potential of manipulating NK cells via CD16 and monoclonal therapeutic antibodies [
9,
10]. In this context, bispecific antibodies represent a novel class of monoclonal antibodies that link surface antigens on tumor cells to effector cell receptors of cytotoxic lymphocytes such as NK cells, thereby creating an antineoplastic effect. These antibodies are characterized by specificity against a target expressed by the malignant cell population or playing a critical role for neoplastic cell development. To more efficiently direct NK cells to leukemic targets, a fully humanized bispecific Killer Cell Engager has been designed recently [
11,
12]. The CD16xCD33 BiKE comprises two antibody fragments, a first recognizing CD16 (FcyRIII) and a second, directed against the myeloid differentiation antigen CD33, which together trigger antibody-dependent cell-mediated cytotoxicity [
11,
12]. Engagement of CD16 signaling against CD33
+ targets is NK cell specific and targets CD33
+ cells exclusively. Thus, the antibody directly triggers NK cell activation through CD16, significantly increasing NK cell cytotoxicity and cytokine production. Moreover, by this means CD16xCD33 BiKE has been shown to potentially overcome the inhibitory effect of KIR signaling and improve NK cell-mediated lysis of AML blasts derived from adults. In the present study, we evaluated whether CD16xCD33 BiKE could enhance NK cell activation against CD33
+ primary childhood ALL and AML cells.