15353-j-veluchamy

1 General introduction and Scope of this Thesis | 11 infiltrate, comprising regulatory T cells (T-regs) 30 , myeloid derived suppressor cells (MDSCs) 31 , M2 macrophages 32 and immature dendritic cells, severely restricts NK cell functionality and possibly their entry into solid tumors. In chronic viral diseases, such as those associated with human immunodeficiency virus and cytomegalovirus infections, mainly exhausted NK cells with decreased cytokine production and reduced cytolytic activity are observed 33,34 . In cancer, similar observations have been made. In a study with breast cancer patients, the NK cell expression levels of activating receptors (NKG2D, DNAM, CD16 and NKp30) were decreased, whereas inhibitory receptor (NKG2A) expression levels were increased and this apparent dysfunctionality of NK cells was found to directly affect NK cell cytotoxicity 35 . Moreover, the effector subset of NK cells (CD56 dim CD16 + ) from head and neck and breast cancer patients, when tested in vitro , was highly prone to apoptosis, which in part may also explain the low NK cell activity observed in these patients 36 . Impaired NK cell functionality may result from tumor-imposed suppressive mechanisms and presents a major hurdle for NK cell targeted immunotherapies. Therefore, approaches to restore or replace impaired NK cell cytotoxicity may prove essential for an effective host defense against cancer. NK cells in the clinic Novel NK cell based immunotherapeutic strategies are being developed to overcome the functional limitations of the use of cancer patient's autologous NK cells. To increase the number of functional NK cells even in case of a high tumor load, adoptive transfer of autologous NK cells served as a feasible approach, as this ruled out the need for immunosuppression, HLA-matching, and prevented the risk of graft versus host disease (GvHD). These advantages sparked the initiation of large-scale expansion protocols and clinical trials using autologous NK cells as a treatment modality for cancer. Though adoptive transfer of autologous NK cells resulted in an increased number of circulating NK cells in peripheral blood, it failed to produce significant therapeutic effects in hematological malignancies, metastatic melanoma and renal cell carcinoma, likely due to the inhibition by self-HLA molecules 37-39 . Moreover, the expansion efficiency and functional status of autologous NK cells were still limited when compared to allogeneic NK cells, as autologous cells were often obtained from heavily pre-treated patients 40 . In addition to this, it was difficult to track infused autologous NK cells in patients and to study their anti-tumor effects from peripheral blood analyses due to the inability to differentiate ex vivo manipulated and in vivo transferred autologous NK cells from the non-manipulated circulating NK cells. These limitations motivated researchers to shift their focus to allogeneic NK cells to treat cancer. In patients with leukemia undergoing allogeneic Hematopoietic Stem Cell Transplantation (HSCT), NK cells, being the first lymphoid subset to appear after allogeneic HSCT 41 , play a crucial role in controlling host defense against infections and residual cancer cells before T cells are reconstituted 42 . These donor T cells are prime mediators of GvHD 43 and the