Unmasking of cryptic natural killer (NK) cell recognition sites on chronic lymphocytic leukemia lymphocytes

Natural Killer Cells in Chronic Lymphocytic Leukemia: Functional Impairment and Therapeutic Potential

Immunotherapy approaches have advanced rapidly in recent years. While the greatest therapeutic advances so far have been achieved with T cell therapies such as immune checkpoint blockade and CAR-T, recent advances in NK cell therapy have highlighted the therapeutic potential of these cells. Chronic lymphocytic leukemia (CLL), the most prevalent form of leukemia in Western countries, is a very immunosuppressive disease but still shows significant potential as a target of immunotherapy, including NK-based therapies. In addition to their antileukemia potential, NK cells are important immune effectors in the response to infections, which represent a major clinical concern for CLL patients. Here, we review the interactions between NK cells and CLL, describing functional changes and mechanisms of CLL-induced NK suppression, interactions with current therapeutic options, and the potential for therapeutic benefit using NK cell therapies.

NK Cells in Chronic Lymphocytic Leukemia and Their Therapeutic Implications

Key features of chronic lymphocytic leukemia (CLL) are defects in the immune system and the ability of leukemic cells to evade immune defenses and induce immunosuppression, resulting in increased susceptibility to infections and disease progression. Several immune effectors are impaired in CLL, including T and natural killer (NK) cells. The role of T cells in defense against CLL and in CLL progression and immunotherapy has been extensively studied. Less is known about the role of NK cells in this leukemia, and data on NK cell alterations in CLL are contrasting. Besides studies showing that NK cells have intrinsic defects in CLL, there is a large body of evidence indicating that NK cell dysfunctions in CLL mainly depend on the escape mechanisms employed by leukemic cells. In keeping, it has been shown that NK cell functions, including antibody-dependent cellular cytotoxicity (ADCC), can be retained and/or restored after adequate stimulation. Therefore, due to their preserved ADCC function and the reversibility of CLL-related dysfunctions, NK cells are an attractive source for novel immunotherapeutic strategies in this disease, including chimeric antigen receptor (CAR) therapy. Recently, satisfying clinical responses have been obtained in CLL patients using cord blood-derived CAR-NK cells, opening new possibilities for further exploring NK cells in the immunotherapy of CLL. However, notwithstanding the promising results of this clinical trial, more evidence is needed to fully understand whether and in which CLL cases NK cell-based immunotherapy may represent a valid, alternative/additional therapeutic option for this leukemia. In this review, we provide an overview of the current knowledge about phenotypic and functional alterations of NK cells in CLL and the mechanisms by which CLL cells circumvent NK cell-mediated immunosurveillance. Additionally, we discuss the potential relevance of using NK cells in CLL immunotherapy.

Immune Dysfunctions and Immune-Based Therapeutic Interventions in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is a B-cell malignancy characterized by a wide range of tumor-induced alterations, which affect both the innate and adaptive arms of the immune response, and accumulate during disease progression. In recent years, the development of targeted therapies, such as the B-cell receptor signaling inhibitors and the Bcl-2 protein inhibitor venetoclax, has dramatically changed the treatment landscape of CLL. Despite their remarkable anti-tumor activity, targeted agents have some limitations, which include the development of drug resistance mechanisms and the inferior efficacy observed in high-risk patients. Therefore, additional treatments are necessary to obtain deeper responses and overcome drug resistance. Allogeneic hematopoietic stem cell transplantation (HSCT), which exploits immune-mediated graft-versus-leukemia effect to eradicate tumor cells, currently represents the only potentially curative therapeutic option for CLL patients. However, due to its potential toxicities, HSCT can be offered only to a restricted number of younger and fit patients. The growing understanding of the complex interplay between tumor cells and the immune system, which is responsible for immune escape mechanisms and tumor progression, has paved the way for the development of novel immune-based strategies. Despite promising preclinical observations, results from pilot clinical studies exploring the safety and efficacy of novel immune-based therapies have been sometimes suboptimal in terms of long-term tumor control. Therefore, further advances to improve their efficacy are needed. In this context, possible approaches include an earlier timing of immunotherapy within the treatment sequencing, as well as the possibility to improve the efficacy of immunotherapeutic agents by administering them in combination with other anti-tumor drugs. In this review, we will provide a comprehensive overview of main immune defects affecting patients with CLL, also describing the complex networks leading to immune evasion and tumor progression. From the therapeutic standpoint, we will go through the evolution of immune-based therapeutic approaches over time, including i) agents with broad immunomodulatory effects, such as immunomodulatory drugs, ii) currently approved and next-generation monoclonal antibodies, and iii) immunotherapeutic strategies aiming at activating or administering immune effector cells specifically targeting leukemic cells (e.g. bi-or tri-specific antibodies, tumor vaccines, chimeric antigen receptor T cells, and checkpoint inhibitors).

Neutralization of (NK-cell-derived) B-cell activating factor by Belimumab restores sensitivity of chronic lymphoid leukemia cells to direct and Rituximab-induced NK lysis

Natural killer (NK) cells are cytotoxic lymphocytes that substantially contribute to the therapeutic benefit of antitumor antibodies like Rituximab, a crucial component in the treatment of B-cell malignancies. In chronic lymphocytic leukemia (CLL), the ability of NK cells to lyse the malignant cells and to mediate antibody-dependent cellular cytotoxicity upon Fc receptor stimulation is compromised, but the underlying mechanisms are largely unclear. We report here that NK-cells activation-dependently produce the tumor necrosis factor family member 'B-cell activating factor' (BAFF) in soluble form with no detectable surface expression, also in response to Fc receptor triggering by therapeutic CD20-antibodies. BAFF in turn enhanced the metabolic activity of primary CLL cells and impaired direct and Rituximab-induced lysis of CLL cells without affecting NK reactivity per se. The neutralizing BAFF antibody Belimumab, which is approved for treatment of systemic lupus erythematosus, prevented the effects of BAFF on the metabolism of CLL cells and restored their susceptibility to direct and Rituximab-induced NK-cell killing in allogeneic and autologous experimental systems. Our findings unravel the involvement of BAFF in the resistance of CLL cells to NK-cell antitumor immunity and Rituximab treatment and point to a benefit of combinatory approaches employing BAFF-neutralizing drugs in B-cell malignancies.

Primary B-CLL resistance to NK cell cytotoxicity can be overcome in vitro and in vivo by priming NK cells and monoclonal antibody therapy

Despite recent advances with monoclonal antibody therapy, chronic lymphocytic leukemia (CLL) remains incurable. Natural killer (NK) cells are potent antitumoral effectors, particularly against hematological malignancies. Defective recognition of B-CLL leukemic cells by NK cells has been previously described. Here, we deciphered the mechanisms that hamper NK cell-mediated clearance of B-CLL and evaluated the potential of NK cells as therapeutic tools for treatment of CLL. First of all, leukemic B cells resemble to normal B cells with a weak expression of ligands for NK receptors. Conversely, NK cells from B-CLL patients were functionally and phenotypically competent, despite a decrease of expression of the activating receptor NKp30. Consequently, resting allogeneic NK cells were unable to kill leukemic B cells in vitro. These data suggest that patients' NK cells cannot initiate a proper immune reaction due to a lack of leukemic cell recognition. We next set up a xenotransplantation mouse model to study NK-CLL cell interactions. Together with our in vitro studies, in vivo data revealed that activation of NK cells is required in order to control B-CLL and that activated NK cells synergize to enhance rituximab effect on tumor load. This study points out the requirements for immune system manipulation for treatment of B-CLL in combination with monoclonal antibody therapy.

Biology of natural killer cells

Studies of cytotoxicity by human lymphocytes revealed not only that both allogeneic and syngeneic tumor cells were lysed in a non-MHC-restricted fashion, but also that lymphocytes from normal donors were often cytotoxic. Lymphocytes from any healthy donor, as well as peripheral blood and spleen lymphocytes from several experimental animals, in the absence of known or deliberate sensitization, were found to be spontaneously cytotoxic in vitro for some normal fresh cells, most cultured cell lines, immature hematopoietic cells, and tumor cells. This type of nonadaptive, non-MHC-restricted cellmediated cytotoxicity was defined as “natural” cytotoxicity, and the effector cells mediating natural cytotoxicity were functionally defined as natural killer (NK) cells. The existence of NK cells has prompted a reinterpretation of both the studies of specific cytotoxicity against spontaneous human tumors and the theory of immune surveillance, at least in its most restrictive interpretation. Unlike cytotoxic T cells, NK cells cannot be demonstrated to have clonally distributed specificity, restriction for MHC products at the target cell surface, or immunological memory. NK cells cannot yet be formally assigned to a single lineage based on the definitive identification of a stem cell, a distinct anatomical location of maturation, or unique genotypic rearrangements.