Engagement of TLR3, TLR7 and NKG2D regulate IFN-g secretion but not NKG2D-mediated cytotoxicity by human NK cells stimulated with suboptimal doses of IL-12

GIRART, MARÍA VICTORIA; FUERTES, MERCEDES BEATRIZ; DOMAICA, CAROLINA INÉS; ROSSI, LUCAS EZEQUIEL; ZWIRNER, NORBERTO WALTER

JOURNAL OF IMMUNOLOGY

The American Association of Immunology

Lugar: Bethesda; Año: 2007 vol. 179 p. 3472 - 3479

0022-1767

NK cells express different TLRs such as TLR3, TLR7 and TLR9, but little is known about their role in NK cell stimulation. In this work, we used specific agonists (polyI:C, loxoribine and ODNs) to stimulate human NK cells without or with suboptimal doses of IL-12, IL-15 or IFN-a, and investigated the secretion of IFN-g, cytotoxicity and expression of the activating receptor NKG2D. PolyI:C and loxoribine, in conjunction with IL-12 but not IL-15, triggered secretion of IFN-g. Inhibition of IFN-g secretion by chloroquine suggested that internalization of the TLR agonists was necessary. Also, secretion of IFN-g was dependent on MEK1/ERK, p38 MAPK, p70S6 kinase and NF-kB but not on calcineurin. IFN-a induced a similar effect but promoted lesser IFN-g secretion. However, cytotoxicity (51Cr release assays) against MICA- and MICA+ tumor targets remained unchanged as well as the expression of the NKG2D receptor. Excitingly, IFN-g secretion was significantly increased when NK cells were stimulated with polyI: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-g in response to agonists of TLR3 or TLR7 and IL-12, and this effect can be further enhanced by co-stimulation through NKG2D. Hence, integration of the signaling cascades that involve TLR3, TLR7, IL-12 and NKG2D emerges as a critical step to promote IFN-g-dependent NK cell-mediated effector functions which could be a strategy to promote Th1-biased immune responses in pathological situations such as cancer.