Conditioned Natural Killer Cells Promote Differentiation of Stem Cells

Objective: To demonstrate that NK cells conditioned by effectors of tissue microenvironment promote differentiation and resistance of healthy and transformed oral stem cells through cell-cell contact and secreted effectors.

Method: NK cells were cultured with irradiated monocytes and/or anti-CD16 mAb in the presence and absence of IL-2 for 24 hours before their supernatants were removed and the NK cells were washed and cultured with either Oral Squamous Cancer Stem Cell (OSCSCs) as well as human Dental Pulp Stromal Cells (hDPSCs) for 5 days. NK cell function was assessed using 51Cr release assay. Cytokine secretion was quantified with ELISA. 

Result: Addition of either NK cells or supernatant removed from cultured NK cells to OSCSCs or hDPSCs significantly augmented resistance of these cells against NK cell mediated cytotoxicity by triggering differentiation of the stem cells. NK cell differentiated OSCSCs or hDPSCs expressed lower levels of surface CD44 and higher induction of B7H1 and secreted higher levels of cytokines. In agreement, a differentiated primary OSCC line established from an oral cancer patient secreted higher levels of GM-CSF, IL-1b, IL-6, and IL-8, expressed decreased levels of CD44, increased levels of B7H1 and EGFR and increased levels of NFkB and STAT3. 

Conclusion: NK cells by targeting other effectors in the microenvironment may become conditioned to lose cytotoxicity and gain cytokine producing phenotype before they can aid in differentiation of stem cells. These alterations in NK cell effector function will ultimately aid in driving differentiation of a population of surviving healthy as well as transformed stem cells. In cancer patients since the majority of NK cells have lost cytotoxic activity, they may eventually contribute rather than halt the progression of cancer by not only driving the differentiation of tumor cells but more importantly, by allowing the growth and expansion of the pool of cancer stem cells.