IL-18 Elicited Suppressor NK Cells with Immunoregulatory Functions

Cancer-induced tolerance involves multiple immunosuppressive pathways, which subvert adaptive immune responses against tumor cells. Extensive phenotypic characterization of NK cells from tumor-bearing mice led us to the observation of a consistent expansion of c-kit-expressing NK cells (Kit⁺NK cells) compromising the NK cell arm of tumor immunosurveillance. In naïve animals, Kit⁺NK cells (CD3⁻NK1.1⁺CD117⁺) represent about 3.3±0.7% of blood, 4.8±1.4% of spleen and 13.2±2.4% of lymph node NK cells. In tumor-bearing animals, percentages and absolute numbers of Kit⁺NK cells increased to levels that mediate inhibitory effects on mature NK cells. Purified Kit⁺NK cells failed to produce IFNγ or GM-CSF in response to IL-2 and could not promote DC maturation in contrast to conventional Kit⁻NK cells. Moreover, adoptive transfer of Kit+ (but not Kit⁻) NK cells into mice injected with CT26 colon carcinomas or B16F10 melanomas promoted the establishment of lung metastases. Micro array comparison of CD3⁻NK1.1⁺Kit⁺ and CD3⁻NK1.1⁺Kit⁻ cells revealed profound differences in their transcriptomes. Two major sets of genes involved in tolerance (B7-H1, CTLA4, Lag3, Hmox1) or tissue repair and bone marrow homeostasis (Cxcl2, CD81, CD63, Csf2) were markedly up-regulated in Kit⁺NK cells. Among these, B7-H1 appeared particularly intriguing as we found that Kit⁺NK cells killed Kit⁻NK cells in a B7-H1/PD-1-dependent manner. Moreover, IL-18 produced by CT26 or B16F10 tumors converts Kit⁻ into Kit⁺NK cells endowed with immunoablative functions in lymph nodes. Upon tumor inoculation, Kit⁺ NK cells, which upregulate B7-H1, accumulate in lymphoid organs in an IL-18R/MyD88-dependent manner and directly kill conventional NK cells, thereby promoting the progression of NK-controlled cancers. Importantly, IL-18R−/− mice lost the development of Kit⁺NK cells during tumor progression. Neutralization of IL-18 by RNA interference in tumors or systemically by IL-18-binding protein (IL-18BP) reduced the expansion of Kit⁺NK cells, stimulated the NK cell-dependent immunosurveillance and significantly reduced tumor growth.

The inflammatory cytokine IL-18 is known to accumulate in cancer-bearing patients but its pathophysiological role still remains unclear. Here, we show that IL-18 is a major tumor-derived immunosuppressant affecting the innate arm of tumor immunosurveillance. In summary, these data suggest that IL-18 promotes the expansion of Kit⁺NK cells endowed with immunosuppressive functions that could act as negative regulator of general NK responses.