Engagement of TL3, TLR7 and NKG2D regulates IFN-gamma secretion but not NKG2D-mediated cytotoxicity by human NK cells stimulated with suboptimal doses of IL-12

NK cells express different TLRs, such as TLR3, TLR7, and TLR9, but little is known about their role in NK cell stimulation. In this study, we used specific agonists (poly(I:C), loxoribine, and synthetic oligonucleotides containing unmethylated CpG sequences to stimulate human NK cells without or with suboptimal doses of IL-12, IL-15, or IFN-alpha, and investigated the secretion of IFN-gamma, cytotoxicity, and expression of the activating receptor NKG2D. Poly(I:C) and loxoribine, in conjunction with IL-12, but not IL-15, triggered secretion of IFN-gamma. Inhibition of IFN-gamma secretion by chloroquine suggested that internalization of the TLR agonists was necessary. Also, secretion of IFN-gamma was dependent on MEK1/ERK, p38 MAPK, p70(S6) kinase, and NF-kappaB, but not on calcineurin. IFN-alpha induced a similar effect, but promoted lesser IFN-gamma secretion. However, cytotoxicity (51Cr release assays) against MHC class I-chain related A (MICA)- and MICA+ tumor targets remained unchanged, as well as the expression of the NKG2D receptor. Excitingly, IFN-gamma secretion was significantly increased when NK cells were stimulated with poly(I:C) or loxoribine and IL-12, and NKG2D engagement was induced by coculture with MICA+ tumor cells in a PI3K-dependent manner. We conclude that resting NK cells secrete high levels of IFN-gamma in response to agonists of TLR3 or TLR7 and IL-12, and this effect can be further enhanced by costimulation through NKG2D. Hence, integration of the signaling cascades that involve TLR3, TLR7, IL-12, and NKG2D emerges as a critical step to promote IFN-gamma-dependent NK cell-mediated effector functions, which could be a strategy to promote Th1-biased immune responses in pathological situations such as cancer.Fil: Girart, Maria Victoria. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; ArgentinaFil: Fuertes, Mercedes Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín; ArgentinaFil: Domaica, Carolina Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín; ArgentinaFil: Rossi, Lucas Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín; ArgentinaFil: Zwirner, Norberto Walter. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Hospital de Clínicas General San Martín; Argentin