Dual functions of natural killer cells in selection and differentiation of stem cells; role in regulation of inflammation and regeneration of tissues

Abstract

Accumulated evidence from our laboratory indicates that conditioned or anergized NK cells have the ability to induce resistance of healthy stem cells and transformed cancer stem cells through both secreted factors and direct cell-cell contact by inducing differentiation. Cytotoxic function of NK cells is suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. Furthermore, decreased peripheral blood NK cell function has been documented in many cancer patients. We have previously shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells (OSCCs). In addition, human embryonic stem cells (hESCs), human mesenchymal stem cells (hMSCs), human dental pulp stem cells (hDPSCs) and induced human pluripotent stem cells (hiPSCs) were all significantly more susceptible to NK cell mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or gene deletion of COX2 significantly augmented NK cell function. Furthermore, the induction of resistance of the stem cells to NK cell mediated cytotoxicity and their subsequent differentiation is amplified when either the stem cells or the NK cells were cultured in the presence of monocytes. Therefore, we propose that the two stages of NK cell maturation namely CD16+CD56dimCD69- NK cells are important for the lysis of stem cells or poorly differentiated cells whereas the CD16dim/-CD56dim/+CD69+NK cells are important for differentiation and eventual regeneration of the tissues and the resolution of inflammation, thus functionally serving as regulatory NK cells (NK(reg)). CD16 receptor on the NK cells were found to be the receptor with significant potential to induce NK cell anergy, however, our recent data indicated that NKp46 but not NKp30 or NKp44 were also able to induce significant anergy in NK cells, although the levels were less when compared to CD16 receptor triggering. The concept of split anergy in NK cells and generation of NK(reg) and its contribution to cell differentiation, tissue repair and regeneration and in tumor resistance will be discussed in this review.

Keywords: NFκB; NK; Regulation.; apoptosis; cancer stem cells; differentiation.

Fig 1

Hypothetical model of conditioning of NK cells by immune inflammatory cells and the effectors of connective tissue to modify NK cell phenotypic and functional properties in order to support differentiation of the cells and the resolution of inflammation. Hypothetical model of NK cell conditioning in the tumor microenvironment as well as in non-transformed immune inflammatory microenvironment is shown in this figure. Significant infiltration of immune effectors right beneath the epithelial layer can be seen in the connective tissue area where the immune inflammatory cells are likely to condition NK cells to lose cytotoxicity and gain the ability to secrete cytokines, a term which we have previously coined as split anergy in NK cells, and to support differentiation of the basal epithelial layer containing stem cells. NK cells are likely to encounter and interact with the other immune effectors such as monocytes or other myeloid-derived suppressor cells (MDSCs), and in tumor microenvironment with the tumor-associated macrophages (TAMs), or with connective tissue-associated fibroblasts (CAF) in order to be conditioned to form regulatory NK cells (NK_reg). NK cells may also directly interact with the stem cells at the base of the epithelial layer, in which case by eliminating their bound stem cells, they can become conditioned to support differentiation of other stem cells. In addition, bacteria through the binding to Toll like receptors can further aid in the generation of NK_reg. cells. All the above mentioned mechanisms may be operational during inflammatory processes in the tumor microenvironment or in healthy non-transformed inflammatory microenvironment. NK cell-differentiated epithelial cells will no longer be killed or induce cytokine secretion by the NK cells, therefore, resulting in the resolution of inflammation.

Fig 2

The majority of the immune effectors in healthy and periodontal gingiva are of activated phenotype. Immune infiltrates from healthy and periodontal patient gingiva were dissociated, and the levels of CD69 expression on CD45+ immune effectors were determined using flow cytometry.