IFNα signaling through PKC-θ is essential for antitumor NK cell function

Abstract

We have previously shown that the development of a major histocompatibility complex class I (MHC-I)-deficient tumor was favored in protein kinase C-theta knockout (PKC-theta-/-) mice compared to that occurring in wild-type mice. This phenomenon was associated with scarce recruitment of natural killer (NK) cells to the tumor site, as well as impaired NK cell activation and reduced cytotoxicity ex vivo. Poly-inosinic:cytidylic acid (poly I:C) treatment activated PKC-theta in NK cells depending on the presence of a soluble factor produced by a different splenocyte subset. In the present work, we sought to analyze whether interleukin-15 (IL-15) and/or interferon-alpha (IFNalpha) mediate PKC-theta-dependent antitumor NK cell function. We found that IL-15 improves NK cell viability, granzyme B expression, degranulation capacity and interferon-gamma (IFNgamma) secretion independently of PKC-theta. In contrast, we found that IFNalpha improves the degranulation capability of NK cells against target cancer cells in a PKC-theta-dependent fashion both ex vivo and in vivo. Furthermore, IFNalpha induces PKC-theta auto-phosphorylation in NK cells, in a signal transduction pathway involving both phosphatidylinositol-3-kinase (PI3K) and phospholipase-C (PLC) activation. PKC-theta dependence was further implicated in IFNalpha-induced transcriptional upregulation of chemokine (C-X-C motif) ligand 10 (CXCL10), a signal transducer and activator of transcription-1 (STAT-1)-dependent target of IFNalpha. The absence of PKC-theta did not affect IFNalpha-induced STAT-1 Tyr701 phosphorylation but affected the increase in STAT-1 phosphorylation on Ser727, attenuating CXCL10 secretion. This connection between IFNalpha and PKC-theta in NK cells may be exploited in NK cell-based tumor immunotherapy.