NK Cell-Based Immunotherapy for Hematological Malignancies

Abstract

Natural killer (NK) lymphocytes are an integral component of the innate immune system and represent important effector cells in cancer immunotherapy, particularly in the control of hematological malignancies. Refined knowledge of NK cellular and molecular biology has fueled the interest in NK cell-based antitumor therapies, and recent efforts have been made to exploit the high potential of these cells in clinical practice. Infusion of high numbers of mature NK cells through the novel graft manipulation based on the selective depletion of T cells and CD19⁺ B cells has resulted into an improved outcome in children with acute leukemia given human leucocyte antigen (HLA)-haploidentical hematopoietic transplantation. Likewise, adoptive transfer of purified third-party NK cells showed promising results in patients with myeloid malignancies. Strategies based on the use of cytokines or monoclonal antibodies able to induce and optimize NK cell activation, persistence, and expansion also represent a novel field of investigation with remarkable perspectives of favorably impacting on outcome of patients with hematological neoplasia. In addition, preliminary results suggest that engineering of mature NK cells through chimeric antigen receptor (CAR) constructs deserve further investigation, with the goal of obtaining an "off-the-shelf" NK cell bank that may serve many different recipients for granting an efficient antileukemia activity.

Keywords: CAR-NK cells; HLA class I; NK cell alloreactivity; NK cells; acute leukemia; cytokines; hematopoietic stem cell transplantation; immunotherapy; killer immunoglobulin-like receptors; receptors.

Figure 1

Donor selection in haploidentical-hematopoietic stem cell transplantation (haplo-HSCT). (A) Alternative human leucocyte antigen (HLA)-haploidentical donors (e.g., both parents of a pediatric patient) can be available for haplo-HSCT to cure leukemia patients. (B) Various analyses can be performed to define the possible donor natural killer (NK) alloreactivity, killer immunoglobulin-like receptor (KIR) genotype, and NK cell phenotypic repertoire to guide the choice for selecting the optimal donor. (C) A schematic representation of donor A and donor B NK cell repertoires, characterized by presence or absence of NK alloreactivity, respectively. Only donor A has alloreactive NK cells, namely “educated” NK cells expressing only KIR2DL1, the inhibitory KIR (iKIR) specific for HLA-C2 epitope, present in the donor and absent in the recipient. The alloreactive NK cell subset of donor A will be highly efficient in leukemia killing, indicating that donor A can be better than donor B.

Figure 2

Different strategies can be used to induce activation and expansion of natural killer (NK) cells to improve tumor killing. (A) Cytokines, such as interleukin (IL)-15, IL-12, and IL-18 to generate cytokine induced memory-like NK cells (CIML-NK); (B) IL-15 superagonist ALT-803, an IL-15 mutant (IL-15N72D) bound to a soluble, dimeric IL-15Rα Fc fusion protein (IL-15Rα-Fc); (C) IL-2 mutants with high affinity for IL-2Rβγ present on NK cells, but reduced affinity for IL-2Rα expressed on Treg cells; (D) activating NK cell receptor engagement by the use of bispecific or trispecific killer engagers (BiKE or TriKE), capable of binding CD16 on NK cells and one/two tumor antigen(s) (e.g., CD19, CD22, CD33) or CD16 and NKp46 on NK cells and one tumor antigen; (E) tumor-specific antibodies (IgG), capable of inducing the NK-antibody dependent cell-mediated cytotoxicity(ADCC). TA, tumor antigen.

Figure 3

Benefits of chimeric antigen receptor-natural killer (CAR-NK) cells. NK cells of different origin can be genetically modified through the use of CAR constructs able to redirect their specificity against antigens expressed on tumor cells. These NK cells can be further expanded ex vivo to reach clinically meaningful numbers, and further optimized by the activation of their native receptors, including CD16 for the antibody dependent cell-mediated cytotoxicity (ADCC) mechanism. CB, cord blood; iPSC, induced pluripotent stem cells; PBMC, peripheral blood mononuclear cells; FAS-L, FAS-ligand; IL2, interleukin-2; IL15, interleukin-15; TRAIL, tumor necrosis factor (TNF)–related apoptosis-inducing ligand.

Figure 4

Different therapeutic approaches based on the use of natural killer (NK) cells. Adoptive transfer: infusion of unmodified allogeneic NK cells (directly or in combination with different types of immune stimulants) or chimeric antigen receptor (CAR)-modified allogeneic NK cells. Different strategies of haploidentical-HSCT: graft inoculum of “megadoses” of highly purified CD34⁺ cells; infusion of a αβT- and CD19 B cell-depleted graft enriched for hematopoietic stem cells (HSC) and also containing other cell types, including mature (possibly alloreactive) NK cells and γδT lymphocytes; infusion of unmanipulated peripheral blood stem cells (PBSC)/bone marrow (BM) and early (+3 +5 day) post-transplant high-dose cyclophosphamide (PTCy) administration that eliminates donor-derived proliferating cells, including all mature NK cells. Graft versus host disease (GvHD) prophylaxis is given only in the third type of transplant. NK cell reconstitution in the three haploidentical-hematopoietic stem cell transplantation (haplo-HSCT) platforms is depicted, differentiating different stages of maturation. Only in αβT and CD19 B cell-depleted graft are mature NK cells infused and persist in the recipient. BiKE, bispecific killer engagers; TriKE, trispecific killer engagers.