NK cells in tumor immune evasion: Role of tumor-associated ligands that regulate NK cell function and therapeutical implications

NK cells are lymphocytes of the innate immune system and play a crucial role in tumor immune surveillance, in beneficial graft versus tumor effects upon hematopoietic stem cell transplantations (HSCT) and thereby as effectors in cellular immunotherapies. NK cells are regulated by inhibitory receptors mostly binding to HLA molecules and activating receptors like CD16, NKG2D and NKp30. Aim of this work was to contribute to the molecular understanding of the insufficient control of tumor cells through NK cells. This was examined on the basis of leukemic cell lines with myeloid or lymphatic origin, since these cells, according to their origin, are recognized and killed very differently by donor NK cells upon stem cell transplantation. In this study we could show that the cytotoxic receptor NKp30 contributes decisively to a better lysis of myeloid leukemic cell lines compared to lymphatic leukemic cell lines. Thus, exosomes expressing ligands for NKp30 can help to overcome the resistance of lymphatic leukemia cells against NK cell recognition and killing. Additionally an increased inhibitory signaling via HLA molecules on lymphatic cell lines prevents a more efficient lysis. Moreover, it was shown in the setting of Hodgkin lymphoma (HL) that patients´ NK cells in contrast to healthy NK cells are not able to lyse Hodgkin cell lines. Soluble factors in the patients´ sera were identified to be responsible for that. Of note, patient sera also suppressed cytotoxic activity of NK cells from healthy donors by downregulation of activating receptors. Results indicate that the soluble ligands MICA for NKG2D and BAT3 for NKp30 play an important role. While these ligands trigger cytotoxicity in their membrane bound form, as soluble factors they have inhibitory impact and can downregulate NK cell receptors. Indeed, NK cell derived from HL patients showed a lower expression level of NKG2D than healthy NK cells. After therapy and clearance of the tumor, serum levels of these soluble ligands were comparable to healthy donors again. Furthermore, NK cells in HL patients were successfully reactivated in vivo and ex vivo with a bispecific antibody construct. This construct stimulated NK cells via CD16 and binds CD30 as an antigen on target cells. Thereby, a very efficient lysis of Hodgkin target cells was achieved in experiments. Taken together, the data indicate that the increased expression of tumor associated ligands for NKG2D and NKp30 in the serum of HL patients influences the phenotype and the function of HL NK cells and hence contributes to a tumor immune evasion. However, the inhibition of NK cells is reversible and can be overcome with therapeutical bispecific antibodies. Thus, NK cells seem to be a promising approach for cellular based immunotherapies as an alternative to standard therapies