Multiple cellular communication molecules and pathways, including the NKG2D-MICA system, may be involved in iTreg cells-NK cell cross-talk 38. It was shown that the engagement of NKG2D on activated T cells and NK cells promoted antitumor NK and T-cell responses against epithelial MICA+ tumor cells 39. We also observed stronger killing of MICA+ tumor cells compared with MICA− cells and induction of NKG2D on NK exposed to iTreg cells. However, iTreg cells enhanced NK cell cytotoxicity against tumor cell targets independent of MICA expression on target cells (Fig. 3C). Also, PI3K Inhibitor Library cell line we could not detect any NKG2D

ligands on iTreg cells, which suggests that a mechanism other than direct NKG2D-ligand interaction is involved in the iTreg cell–NK cell cross-talk. Further, it was reported that NK cells can spontaneously lyse certain

transformed cells. However, early in immune responses NK cells are further activated and recruited to tissue sites where they perform effector functions. It was recently reported that NK cells were capable of lysing pathogen-induced Treg cells, which expressed UL16-binding protein 40. In contrast, it was demonstrated that Treg cells in a tumor microenvironment kill NK cells in a granzyme-B-dependent fashion 41. It was even shown that NK cells are able to induce Treg cells, which resulted in immune suppression 42, and underscores the complex cross-talk between these two immune cell subsets. Although we have not yet identified the Opaganib datasheet molecular mechanism of NK activation by iTreg cells, our data suggest that direct contact between both cell types is required. We have also observed that the parallel execution of the perforin and the FasL cytolytic pathway is utilized by iTreg cell-activated NK cells. To our knowledge, this is the first report about enhancement of anti-tumoral NK cell-function

which is mediated by induced regulatory T cells. Without any doubt, still much has to be learned about the interaction of NK cells and regulatory T cells in the tumor microenvironment. A check better understanding of the cellular cross-talk between regulatory T cells and cells of the innate immune system will aid future rationale therapeutic manipulation of this T-cell subset in cancer therapy. This study was conducted according to the principles expressed in the Declaration of Helsinki. The study was approved by the Institutional Review Board (Ethical Committee) of University of Duisburg-Essen. Blood donors provided written informed consent for the collection of samples. Fetal calf serum (Biochrom AG, Berlin, Germany) was heat inactivated for 30 min at 56°C (ΔFCS). RPMI 1640 culture medium, L-glutamine, streptomycin, and penicillin were purchased from Invitrogen (Karlsruhe, Germany).