IntroductionNatural killer (NK) cells play key roles in innate and adaptive immune responses during early host defense against infectious pathogens and tumors via 2 major mechanisms: contact-dependent cytotoxicity and cytokine production for immune modulation. [1][2][3][4] Target-cell death is primarily mediated via the granule-exocytosis pathway. NK cells are armed by functional cytotoxic granules containing perforin (Prf1) and granzymes, essential effector molecules for NK-cell cytotoxicity as shown in knockout mice, 4,5 and are triggered to mediate effector activity by receptor ligation. Prf1 facilitates the delivery of granzymes into the cytosol of the target cell, and GzmB, the best-characterized granzyme, cleaves several procaspases, BID, inhibitor of caspase-activated DNase, and other intracellular substrates to initiate the classic apoptotic pathways. [6][7][8][9] Many of the studies of Prf1 and GzmB expression in NK cells have suggested the possible involvement of posttranscriptional regulation. Recently, studies using murine NK cells have shown that acquisition of murine NK-cell cytotoxicity requires the translation of a pre-existing pool of Prf1 and GzmB mRNAs. 4 Despite high basal levels of Prf1 and GzmB mRNA, little protein expression is observed under resting conditions in many types of NK cells, whereas expression of both proteins is up-regulated during activation. 4,10,11 These observations are consistent with a posttranscriptional mechanism operating to allow NK cells to be poised for but to prevent translation before activation, such as silencing by microRNAs. 12,13 microRNAs are an abundant class of endogenous small noncoding RNAs (19-22 nt) generated by sequential processing of primary miRNA transcripts by the ribonuclease Drosha in the nucleus and Dicer1 in the cytoplasm, both of which are essential enzymes in the miRNA biogenesis pathway. In mammals, mature miRNAs are integrated into an RNA-inducing silencing complex, including Argonaute 2 (Ago2), a required endonuclease in the RNA interference pathway, and they associate with 3Ј untranslated regions (UTRs) of specific target mRNAs to down-regulate gene expression by targeting mRNAs for translational suppression or mRNA degradation. [13][14][15][16][17] The involvement of miRNA in immune responses and the development of immune cells from hematopoietic stem cells have been widely investigated by manipulation of specific miRNA levels 13,18 or by disruption of molecules involved in biogenesis and activity of all miRNAs, such as Arg, 19 Drosha, 20 and Dicer. [21][22][23][24] Recently, characterization of NK cells from mice with conditional deletion of Dicer and DiGeorge syndrome critical region 8 were reported, with evidence of impairments in NK-cell activation, survival, and function during viral infection. 24 These genetic studies have suggested miRNAs play essential roles in immune cell development and function. 13,14,25 Despite evidence for a broad impact in regulation of immune function, the molecular mechanism, importance, and biologic si...
