The Functional Role of miR-155 in Natural Killer Cells

MicroRNAs (miRs) are small, non-coding RNA molecules with important regulatory functions. It appears that miRs are involved in lymphocyte, specifically Natural Killer (NK) cell, homeostasis, activation, and over-activation leading to malignancy. The human NK cell is controlled by a milieu of activating and inhibitory signaling that explain its highly regulated role in the immune response. The role of miR-155 in NK cells is complex and has direct effects on a variety of signaling pathways. In terms of external control on miR-155, we note that miR-155 is synergistically induced in primary human NK cells after co-stimulation with IL-12 and IL-18 cytokines. Additionally, miR-155 directly targets SHIP1, a molecule that is an important negative regulator of pathways involving survival and proliferation. Based on this preliminary data, we hypothesize that miR-155 plays a key role in regulation of activating pathways in NK cells. In order to study the role of miR-155 in NK cells, a miR-155 transgene (tg) overexpression was driven via the lck promoter in C57BL/6 mice. Once again, MiR-155 targets and downregulates the inositol phosphatase SHIP1, which is a negative regulator of NK cell activation. Phenotypically, miR-155 tg mice have enlarged spleens with greater absolute number and percentage of NK cells. Using purified NK cells from spleens of miR-155 Tg and wild type (wt) mice, we observed that miR-155 upregulation increases NK cell cytotoxic activity against tumor cells utilizing both spontaneous cytotoxic and antibody dependent cell mediated cytotoxic pathways. NK cells from miR-155 tg mice have similar or lower levels of cytotoxic Perforin and Granzyme proteins compared to wt NK cells. Overexpression of miR-155 in primary mouse NK cells resulted in increased IFN-γ secretion after stimulation with IL-12 and IL-18. These data link miR-155 expression with stimuli that strongly induce IFN-γ in NK cells and suggest that miR-155 functions as a strong positive regulator of IFN-γ production. Mechanistically, we observed downmodulation of SHIP1 that correlates with an increase in PI3K and ERK, which are signaling regulators of NK cell cytotoxicity. Collectively our data indicate that NK cells overexpressing miR-155 exhibit enhanced cytotoxicity which is likely due to an increase in activating signaling. Overall, we have shown significant differences in activation and homeostasis suggesting that miR-155 can play a major role as a positive regulator of NK cell effector function. The study shows that miR-155 plays an important role in activation of NK cell immune function with implications for treatment of cancer.Pelotonia Undergraduate Fellowship ProgramNational Cancer Institute grants CA12454, CA16058, CA95426 and CA68458No embargoAcademic Major: Biochemistr