Immunotherapy in the Context of Hematopoietic Stem Cell Transplantation: The Emerging Role of Natural Killer Cells and Mesenchymal Stromal Cells




Immunotherapy in the context of hematopoietic stem cell transplantation has been dominated for many years by T-cell- and dendritic-cell-based treatment modalities. During the last decade, insight into the biology of natural killer (NK) cells and mesenchymal stromal cells (MSC) has rapidly increased and resulted in NK- and MSC-based therapeutic strategies in clinical practice. This article reviews current knowledge of the biology and clinical aspects of NK cells and MSC.


Immunotherapy in the context of hematopoietic stem cell transplantation (HSCT) has been dominated for many years by T cell– and dendritic cell–based treatment modalities. During the last decade, insight into the biology of natural killer (NK) cells and mesenchymal stromal cells (MSC) has rapidly increased and resulted in NK- and MSC-based therapeutic strategies in clinical practice. This article reviews current knowledge of the biology and clinical aspects of NK cells and MSC.


Part 1: NK cells


Biology of NK Cells


NK cells were first identified in 1975 in mice as a distinct subpopulation of lymphocytes with the capacity of killing tumor cells without prior sensitization. In humans, NK cells represent about 5% to 20% of peripheral blood lymphocytes and are defined as CD56+CD3− lymphocytes, which often have a granular morphology. Extensive research has revealed that NK cells are a heterogenous population of cells with various functions in cytokine production and cytotoxicity. In contrast to T-lymphocytes, NK cells do not recognize foreign antigens, but rather detect changes in self-molecules displayed at the surface of autologous cells. Based on initial observations that syngeneic tumor cells with a deficient expression of major histocompatibility complex (MHC) class I molecules are selectively rejected by NK cells, the concept of the “missing-self” recognition was introduced. This model provided an explanation why virally infected or malignant cells with decreased MHC class I expression are preferentially lysed by NK cells, whereas autologous cells with normal expression of MHC class I antigens are protected.


NK cells express various surface receptors with inhibitory and activatory functions that play a role in various diseases including cancer. The functional response of NK cells is determined by the activation of these receptors in combination with the signaling of various coreceptors and other surface structures on binding to their cognate ligands. Fig. 1 presents a schematic overview of the inhibitory, activatory and coreceptors and the various NK receptor ligands. The inhibitory receptors possess different specificities for HLA class I molecules. The 2 main groups are the killer immunoglobulin (Ig)-like receptors (KIRs), which are receptors for human leukocyte antigen (HLA) class I ligands and the CD94-NKG2A/B, which recognize HLA-E. Although HLA-E shows little polymorphism, many inhibitory KIRs are specific for polymorphic domains of MHC class I. KIR2DL1 (CD158a) recognizes HLA-C alleles with lysine in position 80 (C2 specificity, eg, Cw2,4,5,6; group 2), whereas KIR2DL2 and KIR2DL3 (CD158b1/b2) recognize HLA-C alleles with asparagine in position 80 (C1 specificity, eg, Cw1,3,7,8; group 1). KIR 3DL1 (CD158e1) is specific for HLA-B alleles bearing the Bw4 motif and KIR3DL2 (CD158k) recognizes HLA-A epitopes (A3, A11). HLA class I- and KIR-genes are encoded on distinct chromosomes 6p21.3 and 19q13.4, respectively, and inherited independently. KIR genes are encoded by a set of 15 loci and 2 pseudogenes. These KIR genes are closely linked and inherited as a haplotype, and the variability of KIR gene content can be organized into KIR haplotype A and B. Haplotype group A contains a variable number of inhibitory receptor genes, whereas group B haplotype additionally contains several activating receptor genes. Killer activatory Ig-like receptors (KARs) are also triggered by HLA class I alleles, but additional activatory receptors exist, such as NKG2D, the leukocyte adhesion molecule DNAX accessory molecule (DNAM-1; CD226) and natural cytotoxicity receptors NKp46, NKp30, and NKp44, which recognize so far unknown ligands expressed on hematopoietic cells. The Fc-receptor CD16, binding the Fc portion of IgG, mediates the antibody-dependent cellular cytotoxicity of NK cells. NKD2D and DNAM-1 are receptors for stress-induced ligands, such as MHC class I polypeptide-related sequence A and B (MICA and MICB), UL16-binding proteins, and poliovirus receptor (CD155) and Nectin-2 (CD112), respectively. Other surface molecules contribute to the functional status of NK cells, among them the 2B4, NTB-A, and NKp80 coreceptors, CD18/CD11, CD2 adhesion molecules, and Toll-like receptors. Incubation of NK cells with various cytokines leads to their stimulation and expression of additional surface and intracellular molecules such as perforin, granzymes, Fas ligand, and tumor necrosis factor-related apoptosis-inducing ligand treatment (TRAIL), which enables them to kill a wide spectrum of tumor cells effectively via induction of necrosis or apoptosis.




Fig. 1


The inhibitory, activatory, and coreceptors expressed on NK cells and their corresponding ligands in parentheses.


In summary, the output signal (ie, the functional status of NK cells) is regulated by various input signals and the large numbers of inhibitory, activatory, and modulating coreceptors allow cross talk of NK cells with other immune or tissue cells.


NK Cells and Cancer


Most evidence for a role of NK cells in tumor surveillance in humans comes from a clinical long-term follow-up study, in which the inherent NK activity of 3625 individuals was measured longitudinally and the results compared with the incidence of cancer. Individuals who had a high spontaneous NK activity had a lower risk of developing cancer, whereas individuals with lower activity had a higher incidence of malignant diseases. It is thought that the main target of NK cell activity is within the hematopoietic system, as shown by the hybrid resistance model, in which NK cells reject allogeneic bone marrow but not skin or solid organs. In patients with leukemia, functional impairment of NK cells has prognostic significance and NK cell–mediated cytotoxicity against autologous blasts tested either in vitro or in a xenogeneic in vivo model correlated with the duration of remission. In addition, leukemic blasts can shed MICA, a ligand of the activating NK receptor NKG2D, which can negatively affect the cytotoxicity of NK cells. A decreased number and impaired function of NK cells have also been described in children with acute leukemia at the time of diagnosis and in relapse. The low cytotoxicity of NK cells against autologous leukemic blasts could be restored in vitro after stimulation with interleukin 2 (IL-2). The role of NK cells in tumor surveillance of solid tumors is less clear. It has been shown that the infiltration of tumors with NK cells can be a positive prognostic marker in carcinomas. In vitro activation of NK cells obtained from children with malignant solid tumors with interferon α (IFN-α) and IL-2 also enhanced their cytotoxicity against solid tumor cell lines. Therefore, strategies to augment the antitumor activity of the NK cell system in the autologous or allogeneic setting could be beneficial in the treatment of pediatric patients with acute leukemia or solid tumors.


NK Cells in the Context of Allogeneic Stem Cell Transplantation


The concept of alloreactive NK cells


This concept is based on the observation that NK cells attack lymphohematopoietic target cells that express HLA class I molecules for which they do not express the corresponding inhibitory receptor as would be predicted based on the missing self model described earlier. As depicted in Fig. 2 A, NK cells are in a permanent activated status via binding of natural cytotoxic receptors (NCRs) to their yet unknown ligands expressed on normal or malignant hematopoietic cells. Only in the presence of a corresponding ligand for the inhibitory receptor, such as self HLA-Cw alleles, -Bw4 alleles and some HLA-A alleles, the cytotoxic function of NK cells is inhibited and the target cells are resistant to NK-mediated lysis. In contrast, if the target cell does not express the corresponding inhibitory ligand for the KIR, NK cells lyse their target cells because of the lack of inhibition. This situation is often encountered in HLA-mismatched but also in HLA-matched allogeneic stem cell transplantation because of the disparity of the donors’ KIR repertoire and the HLA class I type of the recipients ( Fig. 2 B and discussed earlier). Therefore, the term alloreactive NK cells is used to describe this situation. However, regardless of donor-recipient KIR-HLA matching status (discussed in greater detail later), additional clinical situations can be envisioned, which are shown in Fig. 2 C and D: Tumor cells can have a reduced expression or complete lack of HLA class I molecules, which can be encountered in leukemic blasts or certain tumors, such as neuroblastoma ( Fig. 2 C). This constellation leads to killing of the tumor cells and the intensity of killing is dependent on the amount of residual HLA molecules expressed on the surface of the target cells. Another situation seen clinically in the early phase of immunoreconstitution of NK cells after pediatric haploidentical transplantation is the absence of killer inhibitory or killer activatory receptors for HLA class I alleles (KIRs or KARs) on the reconstituting NK cells. This situation either results in NK cell killing of their target independent of the expression of the amount, or the specificity of the HLA class I molecules on the tumor target cells, or a hyporesponsive status of NK cells via yet unknown mechanisms ( Fig. 2 D).


Oct 3, 2017 | Posted by in PEDIATRICS | Comments Off on Immunotherapy in the Context of Hematopoietic Stem Cell Transplantation: The Emerging Role of Natural Killer Cells and Mesenchymal Stromal Cells

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