Mechanism of activation of natural killer and CD8 T cells by interleukin 15 in the prostate cancer microenvironment

Abstract

BackgroundThe prostate cancer microenvironment profoundly inhibits effector immune-cell activity. We have shown that interleukin (IL) 15, unlike other cytokines proposed for anti-tumour immunotherapy such as IL12 and IL21, can increase natural killer (NK) cell and CD8 T-cell activity within the prostate cancer microenvironment. To investigate mechanisms of this activation, we studied the effects of IL15 on inhibitory and activatory receptors on NK cells and their corresponding ligands on prostate cancer cells. NK receptors known as killer-Ig-like receptors (KIRs) have a crucial role in NK-mediated killing of tumour cells. NK-receptor ligands on tumour cells can attenuate NK activity by binding their corresponding inhibitory receptors. Conversely, increased HLA-related ligand expression can increase the recognition and killing of tumour cells by T cells. MethodsThe prostate cancer cell-lines PC3 and LNCaP were incubated with non-adherent peripheral blood mononuclear cells (PBMCs) at effector-to-target ratios of 8:1 and cytokines IL2 or IL15. After 1 week, NK cells in the co-cultures were stained with antibodies to inhibitory KIRs (KIR2DL1, KIR2DL2, KIR3DL1) and activatory KIRs (NKp44, NKG2D, NKp46, DNAM1). Tumour cells in the co-cultures were stained with antibodies to inhibitory NK-receptor ligands HLA-class1-Bw4 and HLA-G, and activatory receptor ligands Nectin-2, and MICA/B. Antibody to HLA-ABC was also used. Staining was followed by flow cytometric analysis. Groups were compared with one-way ANOVA and post-hoc Newman-Keuls (five repeated experiments). FindingsIL15, but not IL2, inhibited expression of KIR2DL1 and KIR3DL1 by up to 50% (p<0·001) on NK cells in PBMC-prostate cancer cell co-cultures. NKG2D was increased up to 40% (p<0·001) in these cells with IL15 but not IL2. On tumour cells, IL15 decreased HLA-Class-1-Bw4 and HLA-G by 60% (p<0·001) and 75% (p<0·001), respectively. No significant effects were seen on the activatory-ligands whereas HLA-ABC expression was increased by over 65% (p<0·001). InterpretationOne mechanism by which IL15 increases prostate cancer killing by NK cells in PBMC-prosate cancer co-cultures is through upregulating activatory NKG2D and decreasing inhibitory KIR2DL1 and KIR3DL1 receptors. In addition, inhibitory ligands on prostate cancer cells are downregulated, suggesting strong shifts towards NK activation versus inhibition in the co-cultures. Increased HLA-ABC also favours stronger cytotoxic-T-cell recognition. FundingHeathside Trust, UK Medical Research Council, National Institute for Health Research, Prostate cancer UK. The research was funded and supported by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy's and St Thomas' NHS Foundation Trust and King's College London.